Flexography Printing of Silver Based Conductive Inks on Packaging Substrates

Transcription

1 Western Michigan University ScholarWorks at WMU Dissertations Graduate College Flexography Printing of Silver Based Conductive Inks on Packaging Substrates Ramesh Chandra Kattumenu Western Michigan University Follow this and additional works at: Part of the Chemical Engineering Commons, and the Engineering Science and Materials Commons Recommended Citation Kattumenu, Ramesh Chandra, "Flexography Printing of Silver Based Conductive Inks on Packaging Substrates" (2008). Dissertations This Dissertation-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Dissertations by an authorized administrator of ScholarWorks at WMU. For more information, please contact

2 FLEXOGRAPHY PRINTING OF SILVER BASED CONDUCTIVE INKS ON PACKAGING SUBSTRATES by Ramesh Chandra Kattumenu A Dissertation Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements for the Degree of Doctor of Philosophy Department of Paper Engineering, Chemical Engineering and Imaging Advisor: Margaret Joyce, Ph.D. Western Michigan University Kalamazoo, Michigan December 2008

3 UMI Number: INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. UMI UMI Microform Copyright 2009 by ProQuest LLC. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 E. Eisenhower Parkway PO Box 1346 Ann Arbor, Ml

4 Copyright by Ramesh Chandra Kattumenu 2008

5 ACKNOWLEDGMENTS I would like to express my gratitude to the faculty, family and friends for their support and assistance throughout the course of my work. I am very thankful to my advisor Dr. Margaret Joyce for her guidance and support during the course of my journey as a doctoral student. I owe my sincere gratitude to Dr. Paul D. Fleming and Dr. Bradley Bazuin for their insight and ideas that made this work more enjoyable. The vast knowledge and ideas put forth by my dissertation committee in reviewing my work helped me immeasurably in looking at things more scientifically. My special thanks to Dr. Marian Rebros and Dr. Erika Hrehorova for their ideas and support during this work. My sincere thanks go to the vast group of colleagues and friends at Western Michigan University who made this experience very enjoyable. Finally, I would like to thank my wife Samyuktha who always walked beside me during my sunny days and rainy days. Our experiences during this long graduate journey have brought us closer and helped us in chalking out ii

6 Acknowledgments continued plans for our future. I would like to thank to my parents for their love and support. Their constant encouragement reminded me of my goal during my stay in school as a graduate student. Ramesh Chandra Kattumenu iii

7 TABLE OF CONTENTS ACKNOWLEDGMENTS LIST OF TABLES LIST OF FIGURES ii viii xi CHAPTER 1. INTRODUCTION 1 2. LITERATURE REVIEW RFID Technology Introduction Radio Frequency Identification Tag RFID Tags - Classification Choice of Operating Frequency Markets for RFID Technology RFID Benefits RFID Drawbacks Considerations for Item - Level Tagging 14 iv

8 Table of Contents continued CHAPTER 2.2 Printed Electronics Introduction Material Aspects Printing Processes and Their Potential for RFID Printing Printing Technologies Flexography as a Manufacturing Platform for Printing Electronics Basic Principle of Flexography Printing Substrates for Electronic Printing Conductive Inks Printing Plates for Flexography Anilox Roll PROBLEM STATEMENT DESIGN OF EXPERIMENTS MATERIALS AND PROCESSES Substrates Properties of Substrates 53 v

9 Table of Contents continued CHAPTER Roughness and Compressibility of Substrates Porosity and Permeability Coefficient Mercury Intrusion Porosimetry Contact Angle and Surface Energy Conductive Inks Print Trials - Flexography Trial Trial Characterization of Printed Samples Print Quality Evaluation Ink Film Thickness Electrical Characterization of Printed Resistors Printed Antenna Radio Frequency Testing RESULTS AND DISCUSSIONS Data Analysis Procedure Analysis of Printed Traces on Paper Substrates Effect of Different Factors on Sheet Resistance 99 vi

10 Table of Contents continued CHAPTER Effect of Different Factors on Line Raggedness Effect of Different Factors on Line Width Analysis of Printed Traces on Paperboard Substrates Effect of Different Factors on Sheet Resistance Effect of Different Factors on Line Width Effect of Different Factors on Line Raggedness Ink Film Thickness Analysis Printed Antenna Radio Frequency Testing Trial Trial Simulation Results CONCLUSIONS REFERENCES 130 vii

11 LIST OF TABLES 1. Active and Passive Tags 7 2. RFID Frequency Ranges and Application 8 3. RFID Global Sale Forecasts ( ) Comparison of Traditional Printing and Electronics Printing Comparison of Conventional and Printed Electronics Processes Specifications for Major Printing Process Contact Pressures for Printing Processes Markets that use Flexography Printing Printing Application and Appropriate Line Screen and Volume Project Flow Chart Design Matrix Determining the Order of Experiments Substrates used for Printing Properties of Paper and Paperboard Parameters Measured with MIP Contact Angle and Surface Energy of Substrates Conductive Inks used for Printing 63 viii

12 List of Tables continued 17. Properties of Silver Inks Dryer Temperature at the Print Stations Print Trial Summary Symbols used During Print Trial Print Trial - 2 Summary Substrate Surface and Physical Properties Characterization of Printed Traces Calculated Resistance and Sheet Resistivity of Printed Traces Best Subsets Regression Analysis for Ink Film Thickness Factors for ANOVA Analysis Results Obtained for Traces Printed with WB Ink Results Obtained for Traces Printed with SB Ink Analysis of Variance for Sheet Resistance Tukey's Pair Wise Comparison Test - Sheet Resistance Analysis of Variance for Line Raggedness Analysis of Variance for Line Width Tukey's Pair Wise Comparison Test - Line Width Factors for ANOVA Analysis 108 ix

13 List of Tables continued 35. Analysis of Variance for Sheet Resistance Tukey's Pair Wise Comparison Test - Sheet Resistance Analysis of Variance for Line Width Tukey's Pair Wise Comparison Test - Line Width Analysis of Variance for Line Raggedness Alien Printed Antenna Performance with IC 119 x

14 LIST OF FIGURES 1. Inductively Coupled and Microwave Transponders 6 2. Simple Band Picture of Insulator, Semiconductor and Conductor Applications of Conducting Polymers in Microelectronics Comparison of Conductivity of Different Materials Overview of Printing Technologies Printing Unit Cylinders with Flexible Layers Diagram of a Flexographic Plate Image Operating Principle of Flexography Process Rubber Plate Making Process Laser Engraving of a Rubber Plate Comparison of a Photopolymer and Laser Engraved Plate Dot Gain and Ink Film Thickness Based on Anilox Cell Volume Percentage Film Transfer Versus Film on Plate Cumulative Intrusion Curves for All Substrates Pore Size Distribution of Paper Substrates 59 xi

15 List of Figures continued 16. Pore Size Distribution of Board Substrates Change in Contact Angle of Substrates with Water Change in Contact Angle of Substrates with Methylene Iodide Printing Plate Design for Flexo Trial Printing Plate Design for Flexo Trial Dimensions of a Simple Bar for Calculating Sheet Resistance Four Point Measurement Technique Laboratory Antenna Measurement Range Printed Line Traces on Substrates PL P2 and P Printed Line Traces on Substrates Bl, B2, B3 and B3bs Scatter Plots - Ink Film Thickness, Roughness and Surface Energy Printed Line Trace on Substrate B3bs Main Effects Plot for Sheet Resistance Interaction Effects Plot for Sheet Resistance WB Traces on P2,15BCM Anilox, 80%, 90% and 100% Tone Step Main Effect and Interaction Effect Plot for Line Raggedness Main Effects Plot for Line Width 106 xii

16 List of Figures continued 33. Interaction Effects Plot for Line Width Main Effects Plot for Sheet Resistance Interaction Plot for Sheet Resistance Ill 36. Main Effects Plot for Line Width Interaction Effects Plot for Line Width Main Effects and Interaction Effect Plot for Line Raggedness Ink Film Thickness at 90% Tone Step with 18.6 urn Anilox Band Alien Antenna Performance Data Set - Trial Antenna Performance at 18.6 um Anilox Cell Volume Antenna Performance for WB Ink Simulation Resistance Results for WB Ink Simulation Resistivity Result for WB Ink 125 xiii

17 CHAPTER 1 INTRODUCTION Printing technologies for the production of flexible electronics have gained much interest due to its potential as a means to reduce the complexity and costs of present solid state technology 1. Both screen printing 2 and inkjet printing 3 ' 4 ' 56 have been explored but due to the limitation in speed and volume flexography and gravure, are currently being implemented. High volume printing techniques are of great importance in order to produce rolls of flexible electronics printed directly on paper substrates, resembling the newspaper production of the paper printing industry 7, with the main idea of reducing the cost in addition to new applications that require large area and flexibility. This is mainly driven by desire to lower the cost of printed electronics including large displays, sensors and RFID (radio frequency identification). The direct printing of RFID tags onto paper substrates or packages directly would prove to be a great cost saving factor and would benefit the retail and supply chain management operations. 1

18 Printing techniques such as screen printing 89 ' 10 has been widely adapted for low resolution printing for switch pads. Offset lithography has been used to fabricate electronic components such as LED 11, sensors 12 and circuits 13 using functional inks. Flexography 14 and gravure 15 printing have been used for sensors, solar cells transistors, and RFID system components, such as conductors or RFID tag antennae. Flexible polymer substrates, including polyester, polycarbonate and polyimide have been used for the high volume printing of electronic devices. The packaging sector is one of the main areas where the direct printing of RFID tags can provide a great cost advantage to retailers and supply chain management logistics. Due to the non-uniformity and hygroscopic properties of paper and paperboard, there are many challenges to yet be met for printing functional electronic devices to these materials. Hence, the requirements for printing electronics on paper are much higher than for conventional printing. However, in comparison to film, paper and board offer the benefits of opacity and stiffness. For conventional printing of the text and graphics, image quality must meet the optical requirements of an observer, while for printed electronics, although visually acceptable, may fail due to numerous problems 2

19 including open circuits, short circuits or inadequate overlapping areas or geometries. The components of an RFID tag antenna and circuit must be in proper registration to ensure required contact. Printed material gaps and line widths must be held to very close tolerances. The present work is divided into several chapters. At the beginning an introduction to RFID is presented with tag classification, choice of operating frequency, markets and price considerations. Various materials used in the manufacture of RFID tags and other printed electronics are discussed. The effects of various substrate and ink properties along with printing process parameters are considered in the analysis of printed traces. 3

20 CHAPTER 2 LITERATURE REVIEW 2.1 RFID Technology Introduction Radio Frequency Identification (RFID) is a technology that uses electromagnetic radiation in the radio frequency range to identify an object 16. This is a wireless data transmission and identification technology that has been in use for many years and is a unique and powerful identification tool for a wide range of applications such as contact less payment, transportation, personal access, industrial and business applications 17. Today, this technology has been catapulted into the technological forefront by integrating it into the field of supply chain management by providing improved speed, accuracy, efficiency and security of information sharing across the supply chain Radio Frequency Identification Tag A basic RFID system consists of three main components 19 The transponder that is located on the object to be identified. 4

21 A reader with an antenna that communicates with the transponder. Network to analyze the transmitted and received information. An RFID tag, also called a transponder, is an object that can be attached to a product, animal or person for the purpose of identification using radio waves. The transponder consists of a coupling element or antenna and an electronic microchip carrying the data about the tagged item. Figure 1 shows two types of transponders- the inductively coupled transponder with loop antenna on the left and for higher operating frequencies and greater range of use, a dipole antenna on the right 19. In passive RFID tags, the transponder usually does not possess its own voltage supply. When the transponder is within the interrogation zone of a reader it is activated and the power required for activation and operation is supplied through the wireless link to the reader. When not in the vicinity of an interrogation zone, a passive tag is inactive. 5

22 Coupling Element (Coil, Antenna^ Housing Chip J Figure 1: Inductively Coupled and Microwave Transponders 19 An RFID reader contains a transmitter and a receiver. The transmitter sends out a wireless signal providing power and commands for a passive tag. When not sending commands, the receiver converts the radio waves returned from the tag back to a digital form, interprets the returned signal and has an additional interface to forward the information to computer systems for further processing 20. There are several factors that determine the size and shape of an antenna such as the frequency range at which it operates the required read range and the product to which the tag will be applied RFID Tags- Classification A common way of categorizing tags is by their source of power and is also one of the main determining factors for the cost and longevity of a tag 22. Different types of tags are in use today and these include active, passive and semi-active tags. Active tags are powered by their own internal power 6

23 supply. These use an on-board battery to power communications, processor, memory and possible sensors. When a reader is querying information these tags transmit data and due to their on-board power supply the active tags are more reliable, can transmit at higher power levels allowing greater range and can transmit data in 'RF challenged' environments. These tags are typically larger and significantly more expensive than passive tags due to the presence of a battery on board. With the increase in size the capabilities of the tag increases 23. Table 1 displays the advantages and disadvantages of active and passive tags 24. Tag Advantages Table 1: Active and Passive Tags Disadvantages Examples Intermodal transport Active Tags Long read ranges Large size, limited operational life, high cost containers, airport ground support equipment (GSE), highway toll tags Passive Tags Small size, long operational life, low cost Short read ranges, more sensitive, close proximity needed Access control and security, library books, identifying widgets Passive tags do not have an internal power supply. The incoming radio frequency induces an electrical current in the antenna which is used to power 7

24 the onboard integrated circuit. Passive tag responses are transmitted by backscattering the carrier signal from the reader. Thus, the antenna of a passive tag is used to power up the circuit and transmit the backscatter signal Choice of Operating Frequency Based on the applications and the environment through which a RF signal must pass, different configurations and frequencies are employed. The frequency range in which an RFID tag operates is usually broken down into low, high, ultrahigh and microwave frequency ranges 25 as shown in Table 2. Name Table 2: RFID Frequency Ranges and Applications Frequency Read Typical Applications Range Low Frequency khz 50 cm Pet identification, items with (LF) highwater content High Frequency 3-30 MHz 3 m Building access control (HF) Ultrahigh 300MHz- 9m Boxes and Pallets Frequency (UHF) 3GHz Microwave >3GHz >10m Vehicle identification Frequency 8

25 It is crucial to choose the appropriate frequency to retain readability throughout the life of the product 26. The useful carrier frequencies for RF tags range from a khz to GHz. A robust frequency is 125 KHz which can be read through metal, water or practically any other surface but the read range is short. Such tags cost between 2 and 10 dollars per unit and therefore are typically only used on larger and more expensive items. An application example is in the automobile immobilizers where the car key has a passive RFID tag incorporated into it that the steering column authenticates 27 reducing auto theft by as much as 50% 28. The MHz RFID tags are less expensive and have a smaller read range of about 3 feet. The most common frequency band that is being used today is MHz, or UHF (Ultra-High Frequency) RFID. Active UHF tags can read at distances of up to 20 feet in open air; however, they can not operate in water or either on or through metal. These frequency ranges are currently being implemented by Wal- Mart RFID systems are regulated or restricted as radio devices and hence must follow governmental restrictions with other wireless applications such as amateur radio, radio station, emergency services or television transmissions. The different frequency ranges for RFID have different read range properties. 9

26 2.1.5 Markets for RFID Technology First patented in 1973 by Mardio Cardullo 31 this technology has recently seen tremendous growth and is becoming commercially viable. Significant focus from the business world is due to mandates placed by Wal-Mart 3233 and the United States Department of Defense (DOD) 1334 requiring the use of passive RFID systems. The major emphasis placed by Wal-Mart is similar to the one it had on barcodes in The DOD is using RFID to trace military supply shipments. Active RFID tags have been placed on more than 270,000 cargo containers with passive RFID tags placed on packaged items inside the containers to track shipments throughout more than 40 countries 35 ' 36. The airline carrier, Delta Air Lines, intends to spend around $25 million to roll out an RFID system to track baggage, reduce loss and make it easier to route bags if customers change their flight plans 37. In order to stop losses due to theft, RFID concepts are being used in corporate PCs, networking equipment and handheld devices. The use of RFID in these applications seems reasonable and non-intrusive. The credit card company VISA is promoting contactless RFID enabled credit and debit cards to conduct transactions without having to use cash or coins 38. The tire manufacturer Michelin has begun fleet testing of tires with RFID tags inserted into them with unique number that will be 10

27 associated with the car's VIN (Vehicle Identification Number) for tracking purposes 39. But concerns regarding privacy issues are yet to be resolved. In addition to the above mentioned applications, consumer market RFID technology has attracted considerable interests from companies such as Gillette 12 ' 40, Georgia-Pacific 41, Intel 42 ' 43 and Texas Instruments 44 to name a few. It is estimated that revenue generated with RFID tags will increase from $300 million in 2004 to $2.8 billion in 2009 according to a report by market research company In-Stat 45. A recent market research report by IDTechEx showed that the market value of RFID tags by 2018 will be more than five times the current market value 46. Table 3 below shows the trend for the increase in tag production from Table 3: RFID Global Sale Forecast ( ) Number (Billions) Item Pallet/case Other Total RFID Benefits While the bar code is prevalent, RFID offers benefits that exceed those of the barcode 47. No physical contact or line of light is needed between the reader 11

28 and the tag. Due to this, the process of scanning is quicker and hence goods are able to move in and out through facilities faster, which increase productivity 48. Industry experts have estimated that the distribution capacity can be increased by as much as 10 to 20 percent with RFID systems. RFID can be used to reduce the error rate in processes that are labor intensive. This would help in increasing the accuracy level from 90 to 100% 49. In this regard, Delta Air Lines Inc implemented RFID to test 40,000 pieces of luggage from check-in to plane loading. They reported accuracy levels up to 99.9% illustrating the success of the RFID process 50. An RFID system can be used to better monitor the expiration date of returns because the products are more transparent to the supplier. It can be used as an anti-counterfeiting measure in the pharmaceutical industry because it can be difficult to reproduce uniquely identified tags. Another benefit is fewer stock outs for retailers. Based on checkout data, retailers can replenish the shelves and inform the distribution and supply centers to replenish the inventory so that customers will be able to find what they want without needing to inquire RFID Drawbacks Apart from the various benefits of using RFID there are also several issues to be tackled for RFID implementation in the future. One drawback would be 12

29 privacy and security concerns. A tag on an item can be detected by other RFID scanners at other locations thereby giving a personal shopping history to the merchants 51. This is similar to the current issue of consumer behavior tracking on the Internet using cookies, which tells subsequent sites visited by a user where the user has previously visited on the Internet. These security and privacy issues are more complicated in open-environments 52 compared to closed environments where information is encrypted with some write-once only 'locks'. With packages traveling between two or more companies, encryption becomes even more difficult since the contents have to be read by the manufacturer, supplier and the retailer. Unified Global standards- A lack of uniform standards for RFID persists. It is difficult to understand how the standards will vary from one part of the world to another. Currently, United States and Europe operate in a similar segment of the UHF spectrum, however in other regions of the world, the UHF spectrum has been set aside for other uses such as mobile telephones. Although Electronic Product Code (EPC) global is a major player with respect to standards, the DOD has sought to follow the guidelines of the International Standards Organization (ISO) 53. To date EPC global and ISO have proposed standards that are not compatible. 13

30 Tag Cost- One way for adopting RFID tags at item level is to bring the cost down to $0.01-$0.05 level. This can be achieved by permanently affixing RFID tags to goods for applications in logistics, anti-counterfeiting, transaction processing and brand protection. Hence, packaging manufacturers will be able to deliver these tags at much favorable prices than Consumer Packed Goods (CPG) firms buying millions of tags at a time Considerations for Item-Level Tagging Based on the above considerations, implementation of RFID technology at lower cost is realizable and can replace barcodes only when tags can be placed on individual items in a manner analogous to optically scanned barcodes. For such widespread use, tags must cost less than l-2<t to be economically viable. In order to cross this cost barrier, emphasis is being placed on the development of printing technologies, since they are 2-3 orders of magnitude lower in cost than silicon technology per unit area. Based on current standards, the 'electronic barcode' will likely consist of 96 bits of information 54. Pallet level tagging is being deployed at price points > 10(t. For item-level tagging, typical read ranges are expected to be less than lm. Hence, 13.56MHz or lower seems to be the "sweet spot" though there is a push for 900MHz frequency. The limiting factor that determines the operating capacity 14

31 at 13.56MHz is the antenna size. As a result, printed electronics may fit well into this from an economic perspective 55. In the conventional approach silicon based tags in the form of small chips are connected to an external antenna using various attachment technologies (pick and place and lower cost technologies such as fluidic self-assembly 56 ). As manufacturers are pushing towards higher frequencies and lower costs, conventional tags work well for pallet/case level tracking but are not suitable for item-level tagging in water or metal-contaminated environments. Using organic materials applied by vacuum sublimation, carrier mobilities greater than 10cm 2 /V-S have been realized resulting in circuits operating at several MHz 57. Though the performance of devices by printing is still lower, progress is being made in this area. RFID tags by printing, an additive and high throughput process eliminates the need for photolithography, vacuum processing and hence is expected to be much lower in cost than silicon based techniques per unit area. 2.2 Printed Electronics Introduction In the past, the growing need for low cost, high volume information led to the development of printing technologies such as the invention of a mold, 15

32 allowing the manufacture of movable type in the 1450s 58. Printing processes could be efficient and cost effective ways of producing electronic components such as printed circuits (by printing conductive elements directly on the substrate without the etching stage), displays (such as Organic Light Emitting Devices), RFID antennas, batteries, etc. Robustness is an added advantage with printed electronics 59. The trend is being driven by the demand for low cost, large area, flexible and lightweight devices. Organic and polymeric semiconducting materials have been widely used offering advantages of flexibility, compatibility with a wide variety of substrates, and ease of processing. These materials can be processed at a lower temperature (less than 120 C) compared to high temperature (900 C) and vacuum needed for inorganic semiconductors 60. However, with the advantages of low cost printed electronic circuits, come challenges such as limited resolution, high registration requirements, and a high uniformity of the printed layers. Another challenging aspect is obtaining the proper ink characteristics for a desired printing process without reducing the functional properties of materials. Table 4 compares the requirements for traditional graphic printing to the requirements for printing electronics 61. The unique benefits to printing electronics rather than using solid state technology are given below. 16

33 High speed and high volume Environment friendly Negligible waste Short time cycle from design to manufacturing Applicability in novel products Low end integrated electronics Flexible substrates Reduced logistic costs Table 4: Comparison of Traditional Printing and Electronics Printing' Requirement Traditional Electronics Resolution Register Edge Sharpness Layer Thickness Homogeneity Adhesion of Layers Solvents of Inks Purity of Solutions Visual Properties Electronic Properties 15um-100 um Low High High Not Important Important Cost Issue Not Important Very Important Not Important «20um High Very High Less Very Important Important Functional Issue Very Important Not Important Very Important 17

34 Printed organic circuits require at least three functional components: a conductor, a semiconductor and a non-conductor (dielectric). A combination of these can be used to form capacitors, field effect transistors (FET), resistors etc. These components can be combined to form functional circuits and ultimately complete electronic systems Material Aspects The materials comprising the entire spectrum of conductivity, conductors, semiconductors and insulators are all needed for the processing of an integrated circuit. Conductors are used for device interconnection and contacts and as functional elements of inductors and capacitors. The electrical properties of a material arise from their electronic structure. Figure 2 shows the energy levels in the conduction and valence bands of an insulator, semiconductor and a metal. The energy spacing between the highest occupied molecular orbital called the valence band and the lowest unoccupied molecular orbital called the conduction band is called the band gap 62. In metals atomic orbitals overlap with equivalent orbitals of neighboring atoms to form molecular orbitals. In a given range of energies, these molecular orbitals form a continuous band of energies making them conductive. In semiconductors 18

35 there is a small band gap that electrons may bridge the gap due to various forms of excitation (i.e. thermal, photon). Increasing energy Insulator Wide Band Gap Semiconductor Narrow Band Gap Metal No Gap Energy Levels in Conduction Band Energy Levels in Valence Band Figure 2: Simple Band Picture of Insulator, Semiconductor and Conductor The most basic of all electronic components is the conductor which acts as a conduit for current through a circuit. For printed electronics, conductors are formed using inks. For field effect transistors conductive inks form the source, gate and drain electrodes. Conducting polymers are considered as an attractive option for printed electronics due to their unique combination of properties 63 as shown in Figure 3. 19

36 Electrostatic discharge Electromagnetic,,,.,. (ESD) interference Metallization,,. /T^ m,.,,. protection (EMI) shielding \ t / Lithography- ' / e.g. charge < Conducting ^ Interconnection dissipators, Polymers ^ technologies/wiring conducting resists x Corrosion protection of metals Devices " e &> dlodes ' transistors Figure 3: Applications of Conducting Polymers in Microelectronics 63 The conducting properties of different materials are shown in Figure 4 that range from quartz, diamond as insulators to copper, silver as very good conductors. < Insulators A Conjugated Polymers Semi-conductors > Metals s/m lo" 16 Conductivity Quartz io Diamond Glass Silicon 10 Germanium Copperilron/Silvei Figure 4: Comparison of Conductivity of Different Materials 20

37 2.2.3 Printing Processes and Their Potential for RFID Printing Conventional patterning of electronics to obtain a specific design has been carried out on silicon wafers by photolithography, where images and patterns were transferred from a mask to the surface of a silicon wafer 64. Recent technological advances in the fields of materials, printing and electronics have led to the evolution of the field of printable electronics. Table 5 gives a comparison of electronics manufactured by conventional processes and by printing technologies. Table 5: Comparison of Conventional and Printed Electronics Processes 16 Solid- state Organic and Printed (Conventional) Electronics Process Production Speed Capital Cost Materials Cost Substrate Environmental Economic Run Length Batch Slow Extremely high Well defined Moderate in high volume Rigid Silicon Acceptable Large Continuous Potentially fast Low to moderate Developmental Low to moderate Various, Flexible Friendly Small to very large The primary goal of printing organic electronics is to create structures that are functionally similar to conventional electronics, but at a far greater 21

38 production speed, lower cost and with less manufacturing complexity. With the introduction of printing technologies, manufacturing can be done at room temperature in ambient conditions, compared to high vacuum, high purity circuits by conventional methods. The motivations to implementing printing technologies are the following advantages: Established technology Low cost manufacturing High speed fabrication Low temperature process Flexible substrates Reduction in waste products The transition to continuous roll to roll process printing of materials in ambient conditions is very appealing and it becomes necessary to use either a suspension or solution of material (ink), which can be layered to approximate the bulk properties of a solid. The use of available printing technologies and properly designed inks allows for the patterning of electronic structures in ambient conditions. 22

39 2.2.4 Printing Technologies An overview of printing technologies and specifications for various processes are shown in Figure 5 and Table Printing technologies can be classified into conventional and non-impact printing technologies. Conventional printing (with master) is based on information carrying medium with a plate and the non-impact technology does not require a stable, physical, fixed image carrier 66. These systems are controlled digitally. 0 Printing Techniques ) f Conventional J f Non-Impact J I Screen Printing f f Electrophotographyy L/i Flexography Ionography I ( Lithography J ' ( Ink-Jet ) L f Gravure J ' f Thermograp Figure 5: Overview of Printing Technologies 23

40 Table 6: Specifications for Major Printing Process 1 Lithography Flexography Gravure Screen Ink-jet Printing Form Flat Plate) (Al Relief (Polymer plate) Engrave d Cylinder Stencil Mesh Digital Substrates Papers, boards, polymers Papers, boards, polymers Coated papers & boards, polymers All substrates All substrates Lateral Resolution [ ^m] Ink Film Thickness [(am] Viscosity of Ink [Pa. s] Pigment Particle Size [fim] Material Volume Shear Rate Web Speed [ft/min] High Medium High High Medium High High High Medium Low Low N/A N/A 24

41 Conventional Printing Technology These technologies have a printing plate where information is generated by the surface of the substrate being partially coated with ink. The 4 main processes are flexography, gravure, lithography and screen printing. In these processes the contrast between the printed and non-printed areas is highlighted with the help of printing plates. For each printing job, the plate has to be made and mounted since the image information stored in a master cannot be changed. In order to transfer the ink to the substrate from the master or other substances, process specific contact pressures are applied as shown in Table 7. The configuration for the various layers in printing is shown in Figure 6. Table 7: Contact Pressures for Printing Processes 63 Process Pressure, MPa Letterpress Printing Flexographic Printing Offset Printing Gravure Printing

42 Letterpress printing Flexographic printing Lithographic printing loffsetl Gravure printing Figure 6: Printing Unit Cylinders with Flexible Layers 116 Flexography is a relief process where the image elements are raised above the nonimage areas. Flexographic inks are referred to as fluid inks due to their low viscosity and the plates are made from mostly photopolymers 65. Hence, only a slight pressure is required for the transfer of ink to the substrate. In the Gravure process the image element areas are recessed. Higher printing pressures are required, due to the hardness of the gravure cylinder. A smooth compressible substrate is required in order to transfer the ink from the recessed cells to the substrate. Lithography, also known as planography requires very highly viscous inks and are generally referred to as pastes. Image transfer is done by the difference in wetting of the image and non-image area. These inks are 26

43 specifically designed such that they do not chemically dissolve the image or non-image areas during printing. Screen Printing inks are comprised of polymer thick films (PTF) that are widely used in the electronic industry. It is prevalent in the electronic industry because it allows relatively thick layers of ink to be fabricated Non-Impact Printing Technology The predominant non-impact printing technologies are inkjet and electrophotography in comparison to tonography, thermography etc. Inkjet has been the fastest developing printing process in the last decade 67. This digital, non-impact printing process requires a complex ink formulation that can be water-based, solvent-based, hot melt or UV cured. The viscosities are around 10 MPa.s. The inkjet process may be of two types: continuous ink jet and drop on demand (DOD) inkjet. In a continuous process, the droplet is generated when the pressure inside the reservoir increases due to vibration. The droplets pass through a charged electrode and get deflected into a gutter to be recycled or be printed. Resolution of 300dpi could be achieved with the continuous process 68. In DOD process one single ink droplet can be jetted from the reservoir through a nozzle when the pressure within the reservoir 27

44 increases, either due to the vibration of a piezo element or to a bubble resulting from the rapid evaporation of the heated ink system. The size of DOD droplets may be under 21 um which allows excellent resolution. Electrophotography is the second main digital printing process. This process has not found many advantages in printed electronics because the 'toner' must be charged and transferred electrostatistically, which may raise issues when dealing with conductive elements Flexography as a Manufacturing Platform for Printing Electronics Flexography (flexo) is one of the most basic forms of printing. With this printing process various substrates such as thin, flexible, and solid films, virtually all papers, thick cardboard, rough surfaced packaging materials and fabrics can be used. The resolution that can be achieved with this process has improved up to 1751pi.Table 8 gives the major markets for flexographic printing and the percentage printed in the USA (based on 2002 data) as published in a report by Flexographic Technical Association 69. Antennas in other areas of RFID have been printed successfully with the flexographic technology using adaptable conductive inks 14. There is reason to believe that the same kind of inks can be used in conjunction with traditional package 28

45 printing processes to ease the transition to item level RFID implementation and inline processing. Table 8: Markets that use Flexography Printing Market Segments Application Flexography Market in the USA (%) Corrugated Boxes Flexible Packaging Shipping containers, club stores, bulk packaging etc Pet food, bread bags, disposable diaper Label Folding cartons Envelopes Paper Bags Newspapers packaging, frozen food bags etc Label printing Frozen food packages and other assorted consumer packages Simple and medium quality envelopes for business and direct Most fast foods and grocery bags, both paper and plastic Consumer use

46 2.2.6 Basic Principle of Flexography Printing Flexography relies on raised flexible plates to carry the image similar to a rubber stamp as shown in Figure 7. With the combination of resilient plates and low viscosity inks, this process has the capability to print on flexible substrates such as plastic and rough surfaces such as packaging. The flexo process consists of 4 components namely 66 1) fountain roll 2) ink-metering (anilox) roll 3) plate cylinder and 4) impression cylinder. The flexographic image carrier uses a raised image. This image carrier is adhered to a cylinder. The fountain roll, usually covered with rubber, picks up ink from a reservoir and delivers a relatively heavy flow of ink. The anilox roll, usually chrome or ceramic plated has millions of small cells on its surface to carry precise volumes of ink transferred to the plate. The number of cells varies from 80 to 1200 cells per inch. Ink is delivered to the plate in a controlled manner from the anilox roll that has a doctor blade to wipe off excess ink on the roll. Due to this, a thin controlled layer of ink is transferred to the plate, which is then transferred to the substrate backed by an impression roller. The pressure between the anilox roll and the plate and that in between the plate and the substrate is mechanically controlled. Figure 8 shows the basic operating principle of a flexography printing. 30

47 Printed Ink Raised Image Relief Height Flexographic Plate Figure 7: Diagram of a Flexographic Plate Image 1 ' Printing Rate cylinder. ^ ~ Printin S substr9te P,ate / / Impression cylinder (hard! (soft) ^ --**-- '/- ' FuiSlic.T'ntingplaievvith raised TiHURrie-iienrs / mm^ Ink supply (chambered doctor blade system) Cells of the anilox roller filled with ink Inked up image element Figure 8: Operating Principle of Flexography Process

48 Flexographic print quality is affected by many factors namely, substrates, inks, plate material, anilox roll and press conditions etc. Substrates properties such as smoothness, porosity, wettability, and ink receptivity play a major role. Substrate smoothness is less critical for flexography than gravure. For rough surfaces the pressure applied by the plate on the substrate must be high or a greater amount of ink must be metered to transfer into the valleys of the rough substrate. Due to the application of higher pressure there is a tendency of the image to grow in size and reduce resolution. Absorbency of the substrate dictates the amount of ink that penetrates the surface of the substrate. Inks used in flexography are fluid inks with a relatively low viscosity. This is necessary so that the ink will flow into the cells of the anilox roll. The inks must be designed to be dispersible over the lifetime of the product. This is done by appropriately selecting the dispersing agents and binders to obtain optimum properties. Process parameters, such as the type of plate, anilox roll type, design and speed, affect the print quality Substrates for Electronic Printing There are many requirements for the substrate that would be used for printing electronics. The surface energy of the substrates must be in the appropriate range to promote proper adhesion and spreading of the 32

49 conductive inks to the surface. The properties of the substrate should be such that they do not interfere with the electrical properties of the inks. The substrate has to be non-conductive, so as not to interfere with the electrical properties, should dissipate static build-up, must be flexible so that they can conform to the package shape, have controlled smoothness and porosity to enable conductivity at lower ink film thicknesses. Different substrates such as paper polymers such as polyester, polyethylene and polyimide may be used for electronic printing 70. The purpose of printing circuits on paper and board is that the total cost in manufacturing would be less; printing can be done at ambient conditions, the product is recyclable and the process can be implemented inline. Different types of paper such as coated papers, cellulose based (Gloss Art) papers; synthetic papers (polyethylene based) have been used 67. The properties of paper such as the smoothness, porosity, stability, dielectric constant, moisture content etc. have to be taken into consideration Conductive Inks The traditional function of an ink is to convey visual information. The presence of conductive components in inks makes them suitable for electronic applications. Conductive components can be metals such as silver or copper in a matrix, or nonmetals such as carbon or conducting polymers 71. For the 33

50 movement of electrons, pathways of low resistance have to be present in the conductive ink. For example polymers dispersed in a conductive matrix rely on conduction along the polymer backbone. Conductive inks with polymers can be used in place of traditional coils used for antennas 72. This considerably reduces the cost when there is a lot of research being pushed to reduce the cost of a RFID tag to less than 5<t by The use of these inks would also increase the flexibility of usage of a package compared to the circuits that have multiple layers of copper that are stiffer and will crack when the package is flexed 70. Moreover compared to traditional electronic tags conductive inks are more environmentally friendly because of recyclability. Conductivity can be increased by adding conductive substances to an integrated ink system. Due to the addition of materials, electron conduction will increase due to the formation of continuous conduits or pathways. This point is called the percolation threshold where the conductivity increases drastically. This increase eventually levels out at a certain volume fraction of conductive particles. The most conductive inks are based on dispersions of silver or carbon. The silver flakes are designed so that they float on the surface upon drying to promote high surface conductivity. Silver based inks 34

51 can be used in printing RFID antennas 74 but not ICs due to their high percolation threshold 75 and poor adhesion resulting from high loading Printing Plates for Flexography Printing plates are made from rubber or photopolymer materials. Depending on the type of plate, they are imaged using a variety of methods. Suitable plates have to chosen to be used with different substrates and inks. Plates are either attached to the plate cylinder by double sided adhesive tape or are produced in cylindrical form Rubber Plates Traditional plates are made from rubber and look such as a rubber stamp. This plate manufacturing process has many steps 16. Photoengraving is done on a magnesium base. A matrix is then made from this base to form the right reading mold. Rubber is placed over the matrix and coated with a photoresist. This is imaged by exposing it to ultraviolet light. The unexposed part is washed with acid until the required depth of etching is achieved. Figure 16 shows the process for the manufacture of a rubber plate. Lasers are also used to engrave images on the plates. A molded rubber plate is limited to printing 35

52 of 100 lines/inch (lpi) or less and a laser engraved plate can resolve up to 133 lpi. Plate cylinders are often made using the laser engraving process. Figure 9 shows rubber plate making process and Figure 10 shows the process for making a rubber plate. -jjjj:j.-:j^v,'.-jj^1 i?l SBS<r.!7!a-M 1 E'.i Original Engraving Matrix Mold Siili\zii>vSAjr-} Rubber Plate Figure 9: Rubber Plate Making Process 1 ' 36

53 Laser "S^ Rubber Plate Figure 10: Laser Engraving of a Rubber Plate Photopolymer Plates These plates have become the dominant type of plate used in flexography because of their, uniformity, durability and ability to hold fine details 76. In this process, a photosensitive resin composition having at least one photopolymerizable olefinic double bond and a photopolymerizable initiator 77 is used. When exposed to light, the cross linked polymers are not soluble and become hard. The unexposed areas retain their solubility and therefore are washable. Figure 11 shows a comparison of a photopolymer plate to a laser engraved rubber plate. 37

54 Chemical Process Direct Laser Engraving J ill J v_y \_ Figure 11: Comparison of a Photopolymer and Laser Engraved Plate Anilox Roll The anilox roll plays a major role in reproducing consistent print quality with predictable ink densities. Similar to a gravure cylinder the anilox roll is engraved. Mechanically engraved rolls are used for low cost printing and laser engraved rolls area used for high quality printing. Volume is the most important attribute of an anilox roll. It determines the amount of ink on the roll. The volume of a cell in an anilox roll is measured by the unit- billion of cubic microns per square inch (BCM). Typical volumes can be less than 1BCM and sometimes over 20 BCM and typical anilox line counts range from under 100 lpi to over 1000 lpi. A banded anilox test will give the correct volume needed for the process. As shown in Figure 12 below dot gain decreases for thinner ink films because there is less ink to be transferred from the printing plate. 38

55 ^tful Thick Ink Thin Film 3 20% Dot Gain 12% Dot Gain Ink Film Figure 12: Dot Gain and Ink Film Thickness Based on Anilox Cell Volume 89 If the cells are too deep there are disadvantages such as improper ink transfer, difficulties in cleaning etc. An optimum depth to opening ratio is thought to be approximately 28% for acceptable ink release 78. Table 9 gives a rough outline of printing applications matched with appropriate line screens and cell volumes. Table 9: Printing Application and Appropriate Line Screen and Volume 7 ' Application Heavy Line and Solids Line and Type Vignettes Process Approximate Anilox Line Screen Ruling (lpi) Approximate Anilox Volume, (BCM)

56 Ink Transfer Mechanism Anilox rolls made of steel have cells engraved in them to accept and transfer ink from the inking pan to the printing plate. The cells are flanked by ribs or ridges projecting from the walls that may be of wear resistant material. The outer surface region of the anilox roll is formed by ridges of chromium dioxide, aluminum oxide ceramic or the like and the cells are formed with a softer material for example copper. The depth of the engraved cells determines the amount of ink that can be accepted by the anilox roll. For the anilox roll to transfer a desired volume of ink, the cells are formed with a predetermined volume. The actual amount of ink that is transferred is less than that is theoretically possible. Moreover the surface of the anilox roll changes due to wear during the operation bringing down the volume of ink during printing. A layer of copper coating to the inner walls of the anilox cells render them hydrophobic or oleophilic with the capability of accepting and retaining ink. The physical and chemical properties of both the ink and substrate determine the ink-substrate interaction during printing The printing press speed should be adjusted in accordance to ink drying ability and film formation to give a smooth and continuous film. Ink from the inking chamber is 40

57 transferred to the anilox cells due to the hydraulic pressure that is generated when the anilox roll spins. The hydraulic pressure increases to a point where the anilox cells are filled with the required volume of ink. Different ink systems like solvent based and ultra violet based inks have been widely used for flexography printing in addition to water based inks. These ink systems have higher viscosity than water based inks and pose difficulties in filling the anilox cells to the desired volume. The pressure is not enough to force the thicker, more viscous inks to fill the anilox cells to the desired volume. The volume of ink delivered to the printing plate is determined by the number of cells per inch and the anilox cell volume. Ink transfer between the anilox roller and the plate cylinder takes place due to the pressure difference between the anilox roll and printing cylinder. The contact pressure between the anilox roll and the printing cylinder is low and is adjusted based on the substrates and print quality. Since the amount of ink present in the anilox roll is very less and the base paper is never ideally smooth it is difficult to apply an even distribution of ink across the entire paper surface. The pressure in the application area and the amount of ink that is transferred from the anilox cells is normally higher than the desired volume of ink to be transferred to the paper. For ink transfer 41

58 and print quality the contact mechanics at the nip between the printing plate and the substrate is important. When a printing surface having a uniform ink layer makes contact with a substrate, the initial ink film thickness is equal to the depth of the deepest surface that can be coated with the ink 82. Earlier studies on ink transfer done by Walker and Fetsko 83 suggested that at low ink levels there was an incomplete contact and the fraction of ink transferred was insufficient to satisfy the ink immobilizing capacity of the paper and at higher ink levels there is complete contact and ink is immobilized during impression. The remaining ink split between the paper and plate depending on the interaction between the ink and paper and press geometry. Ink film split studies done by Taylor and Zettlemoyer 84 suggested that the viscosity variations within the ink film promoted ink film split. The variations in viscosity caused by high shear and temperature at the nip make the film weaker closer to the surface and hence less ink is transferred. Three regions can be identified for ink-substrate interaction when film transfer is plotted against ink film on the printing plate as shown in Figure In region I, ink transfer percentage increases with ink film thickness but the pores and surface have an incomplete coverage. In this region surface 42

59 structure of the substrate has an impact on print quality and ink transfer. Uniform ink film is transferred to a smooth surface from the printing plate 86. Regk Film Transfer % Film on Plate Figure 13: Percentage Film Transfer Versus Film on Plate 77 In region II, the percentage of ink transfer decreases with increase in ink film thickness and ink transfer is dictated by the absorptive and volumetric properties of the substrate. Non uniform distribution of porosity on a substrate causes a non uniform ink film to be transferred. In region III, percent ink film transfer remains constant and ink transfer is dictated by the 43

60 properties of the ink. At the nip the ink film experiences temperature and shear gradients that affect the viscosity of the ink in these regions and hence the ink split varies. Also other factors like printing pressure, printing speed and roughness of the substrate play an important role in ink transfer. Several studies performed on printing pressure concluded that increasing the contact pressure and dwell time at the nip increased the amount of ink transferred to the substrate For higher viscous inks and rougher substrates higher pressure is needed to cover the surface profile to achieve a uniform print density

61 CHAPTER 3 PROBLEM STATEMENT Printed Radio Frequency Identification Technology (RFID) is a wireless technology of great research interest due to its potential to replace bar code technology. The main interest in replacing bar code technology is to affix RFID tags onto goods for applications in logistics, anti-counterfeiting, transaction processing and supply chain management to enable items to be tracked without requiring a line of sight or the manual scanning of items. To realize all these applications, a price barrier of <5 cents/tag is needed. This price point can be achieved by implementing high volume in-line printing processes to print the RFID tags directly to packaged products. The proposed research focused on the flexographic printing of RFID tag components to cellulose based substrates. The research sought to identify the key substrate and ink properties required for producing a low cost tag. The following tag attributes were studied. Substrate properties Conducting ink properties and formulations Antenna design 45

62 Printing process parameters With the evaluation of these parameters for RFID tag efficiency the following research questions were addressed. How does the substrate influence conductivity of a printed trace? How does ink film thickness impact the conductivity of a printed trace? What is the effect of substrate and ink interactions in determining tag efficiency? What is the effect of trace width on conductivity? There are many factors that influence the line print quality and electrical behavior of printed conductive traces. The various tasks will be performed in evaluating the properties of substrates, conductive inks, antenna design, and process parameters. Table 10 shows the project flow chart followed in the course of conducting this research. Task 1 identified suitable paper and board substrates for printing of conductive traces and RFID antenna. Different grades of label paper and coated board were evaluated. Four paper substrates supplied by Dunn Paper, Port Huron, MI and three board types from Graphic Packaging, Kalamazoo, MI were obtained and characterized. The properties tested were roughness, 46

63 porosity, permeability, surface energy and compressibility. Smoothness was characterized using Parker Print Surf Tester and Emveco Profilometer. Porosity and air permeability were tested using Micromeritics Autopore IV mercury porosimetry and Parker Print Surf Tester, respectively. Table 10: Project Flow Chart Need for the research Background research Identify the research problem Identify and gather materials for substrates and inks Create the design of experiment Evaluation of substrate and ink properties Data analysis Press trial Data analysis Draw conclusions Optimize parameters Task 2 focused on characterizing the properties of conducting inks for the printing of the antennae portion of the tag. The inks were supplied by 47

64 Acheson Colloids, Port Huron, MI. A set of three inks (solvent based, water based and UV based) inks were evaluated. The ink properties tested were particle size, rheology and surface tension. Rhoelogical studies were performed using a Brookfield viscometer. Surface tension was measured using a First Ten Angstrom dynamic contact angle instrument. In task 3, antennae were flexographically printed using the characterized inks and substrates. A Comco Commander narrow-web inline flexographic printing press located in the pilot plant of Western Michigan University was used. Printing plate materials were supplied from Southern Graphics System. The plates were imaged with lines of different trace widths and lengths along with three different antenna designs namely a bow tie, Alien Tech and TI design. The print attributes of the test targets were evaluated using an ImageXpert image analyzer. Characteristics of line width, length, raggedness were evaluated. The thickness of the ink films were measured using a Nikon optical microscope. The resistance of the printed traces was measured using a digital multimeter (Keithley Instruments Inc) in a 4-wire sensing mode. Task 4 focused on the evaluation of the printed traces with the selected inks, substrates, antenna design for conductivity, resolution and read range. The best substrate, ink and design and process parameters were determined. 48

65 CHAPTER 4 DESIGN OF EXPERIMENTS A design of experiment (DOE) was performed using Minitab software. The response variables studied were line print quality and electrical behavior. A full factorial DOE with multi levels of factors was performed. The DOE matrix is displayed in Table 11. An Analysis of Variance (ANOVA) analysis was performed on the measured data. The overall objective of the DOE was to determine the effect of substrate, ink and tone step on the final conductivity and line quality of printed traces. Table 11: Design Matrix Determining the Order of Experiments Standard Order Run Order Substrate P3 P5 P6 P6 P5 P4 P7 P5 P7 P2 PI P8 Ink UV uv SB UV WB SB SB UV SB WB WB SB Tone Step 100% 100% 90% 100% 100% 80% 90% 90% 80% 100% 100% 100% 49

66 Table 11- Continued Standard Order Run Order Substrate P2 P6 P3 P2 P5 P2 P4 P3 P6 P7 P6 PI P8 PI P7 P5 P3 P6 P2 PI PI P7 P5 PI P4 P4 P3 P8 P4 P7 Ink WB UV SB UV SB WB SB WB WB WB WB UV WB SB WB WB SB SB SB SB SB WB UV WB UV WB WB WB UV SB Tone Step 90% 90% 100% 100% 90% 80% 100% 90% 90% 90% 80% 90% 80% 100% 80% 90% 90% 100% 90% 90% 80% 100% 80% 90% 90% 100% 100% 100% 80% 100% 50

67 Table 11- Continued Standard Order Run Order Substrate P4 P8 P7 P3 P6 PI PI P8 PI P4 P3 P7 P8 P5 P6 P7 P3 P2 P6 P2 P2 P8 P4 P4 P8 P5 P3 P2 P5 P8 Ink WB UV UV SB WB UV WB SB UV SB WB UV WB SB SB UV UV UV UV UV SB UV UV WB UV SB UV SB WB SB Tone Step 80% 80% 90% 80% 100% 80% 80% 80% 100% 90% 80% 100% 90% 80% 80% 80% 90% 80% 80% 90% 80% 90% 100% 90% 100% 100% 80% 100% 80% 90% 51

68 CHAPTER 5 MATERIALS AND PROCESSES 5.1 Substrates The substrates used for this work included four label stock papers provided by Dunn Paper (Port Huron, Michigan, USA) and three packaging paperboards substrates provided by Graphic Packaging Inc (Kalamazoo, Michigan, USA). The substrates along with their intended applications are listed in Table 12. Substrate Table 12: Substrates used for Printing Paper/Board Substrate ID Application DunUltra IIMB3 FWS Paper PI Labels Coated Thermal Transfer 70gsm Paper P2 Labels Duncote gsm Paper P3 Labels CIS Release 75gsm Paper P4 Labels Bacon Board Board Bl Packaging Polyethylene Coated Board Board B2 Packaging Coated News Board Board B3 Packaging Coated News Board Back Side Board B4 Packaging 52

69 5.2 Properties of Substrates The substrates were characterized in terms of roughness, compressibility, porosity, caliper, wettability and surface energy. The tests were performed according to TAPPI standards- T 555 om-04, T 558 om- 06, T 411 om Roughness and Compressibility of Paper and Paperboard Surface roughness plays an important role in determining the printability of paper and paperboard. Two methods namely the Parker Print-Surf (PPS) method and Emveco profilometer were used. The PPS method is an air leak method intended to simulate nip pressures and backing substrates found in printing processes 91. The principle is based on determining the resistance to flow of air between the substrate and a metal band in contact with it. The rate of air flow is related to the surface roughness of paper and paperboard. This method measures the deviation of specimen surface from a plane affected by depth; width and number of departures form that plane. This is an indirect measurement of surface roughness. An average of 20 measurements at different places is reported. The second method using the stylus method is a direct method to measure surface roughness. In this method roughness profiles are generated by a very 53

70 light stylus tip (25.4 am) riding on the surface contours of the specimen while generating vertical position readings at pre-determined intervals (0.127mm) for a total of 500 sample points 92. An average value of the measurements carried out in both machine and cross-machine direction are reported in Table 13. The compressibility of the substrates was calculated by taking the ratio of roughness values obtained at two different clamping pressures according to equation l 91. Compressibility Coefficient = PPS 1000/PPS 500 [1] Where PPS 1000 and PPS 500 are roughness values at clamping pressures of loookpa and 500kPa PPS Porosity and Permeability Coefficient Porosity can be defined as a measure of the fluid storage capacity of a porous material 93. Permeability describes how easily a fluid is able to move through the porous material. While porosity is an important geometrical property, permeability is an important physical property of a porous medium. Porosity, permeability and roughness are important properties for print quality 94. Porosity was measured using PPS tester at a clamping pressure of loookpa. 54

71 Table 13: Properties of Paper and Paperboard Property/Substrate Emveco Roughness, um Compressibility Coefficient Thickness, um PPS Porosity, ml/min Permeability -6 Coefficient xlo, um 2 Avg MD St. Dev Avg CD St. Dev Avg St. Dev Avg St. Dev Avg St. Dev Avg St. Dev PI P P P Bl <0.01 <0.01 <0.02 <0.02 B <0.01 <0.01 <0.02 <0.02 B B3bs <0.01 <0.01 <0.02 <

72 The permeability coefficient was calculated using Darcy's law. For each substrate porosity measurements from the PPS tester and thickness measurements from a caliper instrument for each sample were used with standard constants for the PPS equipment, to calculate permeability coefficient K using equation K = *Q* L Where K is permeability coefficient [urn 2 ], Q is flow rate of air in [ml min 1 ] L is thickness of tested sample [m] Mercury Intrusion Porosimetry In addition to measuring porosity by PPS tester, Mercury Intrusion Porosimetry (MIP) was also used to estimate porosity. MIP provides information about the total pore volume and also pore size distribution 95. In this method the sample is immersed in mercury and the volume of mercury that intrudes by increasing pressure is measured. Both the volume of mercury intruded and pressure applied are measured simultaneously. Mercury penetrates into the pores that have capillary pressures equivalent to the applied pressure. The applied pressure is inversely proportional to the pore radius according to the Washburn equation 9697 in equation 3. 56

73 AP = -(2y/R)cose [3] Where P is applied pressure y is surface tension of the liquid R is pore radius and 9 is contact angle The contact angle of mercury on various materials was found by Ritter and Drake 98 to be between 135 and 142 with an average value of 140 and a value of 480mN/m for surface tension in equation [3], the following relation is obtained between applied pressure and pore radius and the constant has units Pa.m 99 ' 100 R = 0.735/P [4] Where R is pore radius in um P is applied pressure in MPa A Micromeritics AutoPore IV was used to measure the pore size distribution of the substrates. Since the substrates had coating only on one side, the back side was taped with a transparent tape after drying the samples in an oven overnight at 105 C. Parameters measured with MIP software have been integrated and are displayed in Table

74 Table 14: Parameters Measured with MIF Substrate PI P2 P3 P4 Bl B2 B3 Average Pore Diameter, nm Total Pore Area, m Porosity, % Data from the MIP measurements were obtained as the integral (cumulative) volume of mercury intruded with increasing pressure. Curves plotted between cumulative intrusion volume and applied pressure or pore diameter are called porosimetry curves or intrusion-extrusion curves. Intrusion curves for the substrates with both pressure and pore diameter are shown in Figure 14. The area under the curves provides the total porosity of each substrate. Porosimetry data for all paper substrates and board substrate Bl indicate a bimodal size distribution. For these substrates intrusion commences into a new range of cavities at the point where there is a large increase in the slope of the curve. Board substrates B2 and B3 represent a wide and continuous pore size distribution 101. Pore size distribution of paper and board substrates are depicted in Figures 15 and 16, respectively. The paper substrates have a bimodal distribution with peaks around ljum and 290 am. Board substrates have a greater distribution ranging from 0.07 urn to 290 um. 58

75 DPI - f S«!BI«* B «t P2 *P "***. :«5* w ::XSStfSs ss555 1 u Pi -Bl B2 XB3 0.4 m m x S i I _ ~*~ - _.-- -» Pressure, Psia Figure 14: Cumulative Intrusion Curves for All Substrates PI P2 P3 P4 Pore Diameter, (im Figure 15: Pore Size Distribution of Paper Substrates 59

76 Pore Diameter, ^m Figure 16: Pore Size Distribution of Board Substrates Contact Angle and Surface Energy Measuring of wetting properties is very important to the understanding of paper/ink interactions. Physicochemical properties such as work of adhesion and surface free energy are important in addition to physical properties of substrates in determining printability 102. Contact angle is used as a measure of wettability of a solid by a liquid. Surface energy of the substrates can be estimated from contact angle measurements. A FTA dynamic contact angle analyzer FTA200 was used for the measurement of the contact angle of liquids on the substrates. Water and methylene iodide were used as test liquids. In addition to contact angle, 60

77 change in drop volume was also recorded to determine the amount of spreading versus absorption of the test liquids. The change in contact angle of the substrates with time is shown in Figure 17 (water) and Figure 18 (methylene iodide).the equilibrium value of contact angle for each substrate was taken when a constant value was reached. The time to reach this value varied among the substrates and was less than 30 seconds. The change in drop volume on P2 for both the test liquids was negligible, indicating that spreading rather than absorption occurred. If there was spreading of the test liquids on the substrates it took place within 5 seconds from the time measurements were taken. The surface energy of the substrates was estimated using the Owens-Wendt method 103. Constant values of contact angles were used to estimate the surface energy of the substrates as shown in Table 15. Paper substrates have surface energy values greater than 40mN/m while board substrates have values less than 40mN/m. Spreading on P2 is due to its higher surface energy and can be evidenced by lower contact angles with the test liquids. There was no spreading on substrate Bl which had the lowest surface energy and highest contact angle among all substrates. 61

78 PI oc D Contact Angle, v ^ _ 1 ' o o 40 - ^ \ "'" - -,.. ^ "^"'^" " ravm '"""", " K -"" '""' ~ W ~- - w-,,,.,. _ "- ~-~ -» ~ ^ _ i P2 P3 P4 ~B1 B2 B3 B3bs Time, sec Figure 17: Change in Contact Angle of Substrates with Water -PI P2 P3 -P4 -Bl -B2 B3 B3bs Time sec Figure 18: Change in Contact Angle of Substrates with Methylene Iodide 62

79 Table 15: Contact Angle and Surface Energy of Substrates Contact Angle \ Substrates PI P2 P3 P4 Bl B2 B3 B3bs Water (Degree) Methylene Iodide (Degree) Surface Energy, mn/m Conductive Inks Three conductive silver based inks designed to be applied using high speed flexographic printing (Acheson Industries Ltd, Port Huron, Michigan, USA) were used for printing RFID antenna patterns onto the paper and board substrates. The water based, solvent based and UV based inks used are shown in Table 16. Ink Table 16: Conductive Inks used for Printing Preferred Printing Binder Solvent Method Ink ID Acheson PM-500 Acrylic Copolymer Water Flexography, Gravure WB Electrodag PD-056 Vinyl Resin PM Acetate Gravure SB Electrodag PD-054 Urethane Acrylate N/A Flexography/Gravure UV 63

80 Electrodag is a registered trademark of Acheson Industries, Inc. Typical properties of the silver inks as specified by the manufaturer are displayed in Table 17. Property/Ink Solids Content, % Table 17: Properties of Silver Inks WBInk SB Ink 63.6 UVInk 100 PH N/A N/A Viscosity, mpa.s Density, Kg/L Theoretical Coverage, m 2 25 um Sheet Resistance, lum Silver Particle Size, um <3 <7 <7 Acheson PM-500 is a dispersion of a silver pigment in a specially selected emulsion that rapidly dries to form a flexible conductive coating. It is designed to be applied using high speed flexography printing on a variety of paper and polymer substrates. Typical applications include printing of RFID antenna, electronic circuits and medical sensors. Electrodag PD-056 is a suspension of silver pigment in a thermoplastic resin that rapidly dries to form a flexible conductive coating to provide controlled 64

81 electrical properties. Typical applications include smart labels/rfid, electronic circuitry, medical sensors and EMI shielding. Electrodag PD-054 is an electrically conductive low viscous silver ink designed to be UV cured and is ideal for applications that require low resistivities, fast curing, low VOC and solvent or heat sensitive substrates. Typical applications include conductive label applications. 5.4 Print Trials - Flexography Trial -1 Printing trials were carried out on a Comco Commander narrow-web inline flexographic printing press (Mark Andy Inc, St.Louis, MO) at Western Michigan University. Prior to printing a plate image as shown in Figure 19 was created in Adobe Illustrator and the printing plate was engraved by Southern Graphics, Battle Creek, MI. Three antenna designs namely Alien Technologies design, Texas Instruments design and bow tie antenna design pattern were chosen to test the performance at different frequencies. Included in the design were fine lines to be printed at different angles to the print direction. The lines were designed to be printed with varying thickness and lengths to test the reproducibility of fine lines for transistor and circuit 65

82 printing. The printing plate was designed to have three repetitions of the same image to be printed at different tints (100%, 90%, and 80%). The printing plate used was a Dupont DPL photopolymer material using a CDI digital plate setter of 120 lpi (47 lines/cm) screen ruling. The plate was mounted on to the plate cylinder using a medium sticky back with a thickness of 15mils (0.381 mm). An enclosed doctor blade system was used to limit the evaporation of solvent and prevent the drying of inks in the recessed cells. Anilox rolls having different cell volumes and screen ruling were chosen to print the traces. The conventional unit for cell volume measured in BCM is a mixed unit and is not obviously related to cell dimensions. A consistent set of units for cell volume per unit area (lev) is used, because it gives a measure of the actual dimensions of a cell. The two are related by equation 5: lev (urn) = 1.551cv (BCM) [5] Anilox rolls having cell volumes 15 BCM (23.3um), 45 BCM (69.75um) and 75 BCM (116.25um) at 60 lpi, 100 lpi and 180 lpi screen ruling respectively, were chosen to print the conductive inks at different ink thickness. The resulting ink thickness was too high to be dried on the print station and led to printing 66

83 problems. To rectify the ink tracking problem of higher ink thickness an anilox cell volume of 8 BCM (12.4um) and 2001pi screen ruling, supplied by Harper Corporation of America, Charlotte, NC was chosen. IIP W *[j0* jjjjjjjn* Figure 19: Printing Plate Design for Flexo Trial 1 67

84 The three inks SB, WB and UV inks used for printing had to be dried on the press station before collecting samples. To assist in drying of the inks hot air dryers on all the three color stations were used and the temperatures maintained are listed in Table. 18. A XericWeb (Neenah, WI) infrared (IR) dryer system was installed just after the first inking station to assist in additional drying. This system included 9 IR quartz tube heaters with a dryer effective length of 15 inches and suitable for a web width of 10 inches. To assist in curing UV inks the ultraviolet light system attached to the print unit on press was used. Table 18: Dryer Temperatures at the Print Stations Dryers F Ink Tl T2 T3 UV - - SB - - WB The printing trial carried out at the pilot plant was run as per the trial plan shown in Table 19 and Table 20. The impression pressure was adjusted throughout the trial for different substrates to compensate for differences in substrate thickness. The pressure that best balanced ink transfer, print 68

85 uniformity and print quality was used for each substrate. The trial was set up to print first with SB ink followed by UV and WB ink to avoid problems with ink drying in the recessed cells of the anilox roll. Table 19: Print Trial Summary Speed Run# Substrate Ink Type Dryers F ft/min IR Tl T2 T3 1 B3 SB Y B4 SB Y B2 SB Y Bl SB Y P4 SB Y P3 SB Y P2 SB Y PI SB Y PI UV Y P2 UV Y P3 UV Y P4 UV Y Bl UV Y B2 UV Y B3 UV Y

86 Table 19- Continued 16 B4 UV Y B4 WB Y B3 WB Y B2 WB Y Bl WB Y P4 WB Y P3 WB Y P2 WB Y P3 SB Y P3 SB Y P3 SB Y B3 SB Y B3 SB Y B3 SB Y Table 20: Symbols used During Print Trial Substrate Name DunUltra IIMB3 FWS Coated Thermal Transfer Duncote 3 67gsm 2.5 MIL CIS Release Bacon Board PECNB CNB Symbol subol sub02 sub03 sub04 sub05 sub06 sub07 70

87 Table 20- Continued Name CNB(back side) Symbol sub07b Plate 1201pi 100_90_80 black 1201pi 80_100_90 black 1201pi 90_80_100 black 1501pi 100_90_80 black 1501pi 80_100_90 black 1501pi 90_80_100 black PI PII PHI PIV PV PVI Ink Acheson PD-054/UV Acheson PD-056/SB Acheson PM-500A/WB a b c The last substrate printed with one particular ink was used as the first substrate for the next ink system to avoid delays. During printing care was taken to avoid drying of inks in the inking pan. The press was run at regular speeds until a proper impression pressure setting was achieved. The impression pressure was adjusted for an acceptable print that was evaluated based on visual inspection and resistivity measurement done on the press. Upon reaching the appropriate impression pressure for each substrate, printed samples were collected and stored in an environmentally controlled room at TAPPI conditions of 23 C and 50% RH (ASTM). 71

88 5.4.2 Trial - 2 A second printing trial was performed with the same substrates and ink to increase the performance of printed antenna. During this trial changes were made to the anilox roll and printing plate design. The specifications of the anilox roll were adjusted to deposit a higher ink thickness than obtained during the first trial. A banded anilox roll supplied by Praxair of Danbury, CT with cell volumes 11 (17.05um), 12 (18.6um) and 15 BCM (23.25um)at 2001pi screen ruling was used. The plate design did not include the TI design, instead blocks of lines and gaps with various widths was added in order to better study the printability and capability of the test method. The print pattern was designed as a 90% tone step at 150 Ipi screen ruling. The plate design used during this trial is shown in Figure 20. During print trial - 2, dryers on all the print stations were used for all ink types. Trial - 2 included a run-to-fail test to determine the maximum printing speed that could be achieved for each ink type without print problems. The run-to-fail tests were performed on Bl, which was the least absorptive among all substrates. Table 21 shows the print trial - 2 summary. 72

89 90% 150 Ipi 90% 150 Ipi 90% 150 Ipi Figure 20: Printing Plate Design for Flexo Trial 2 73

90 Table 21: Print Trial-2 Summary Run# l Sample B3 B4 B2 Bl P4 P3 P2 PI Bl P2 P3 P4 Bl B2 B3 B4 B4 Bl B2 Bl P4 Ink Type SB SB SB SB SB SB SB SB SB uv uv uv uv uv uv uv uv uv WB WB WB IR Y/N Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Tl Dryers F T2 T3 Speed ft/min Run to fail test, ft/min

91 Table 21- Continued Run to Run# Sample Ink Type IR Dryers F Speed ft/min fail test, Y/N Tl T2 T3 ft/min 22 P3 WB Y P2 WB Y P3 WB Y P3 WB Y P3 WB Y Bl WB Y Before further testing, a few selected samples were subjected to a heat treatment of 105 C for 1, 5, 10 and 20 minutes and 14 hours in a hot air oven. The resistance of the samples was measured after each time period. This was done to observe if there was an improvement in electrical conductivity of printed traces and if additional curing was required. With an additional post curing of 20 min. at 105 C, the resistance of the printed lines reached an asymptotic value. Therefore, all samples were subjected to this treatment before any additional measurements were conducted. For print trial -2 a postcuring of 10 min. at 105 C was required. The change in time required for post curing is due to fact that all the driers were switched on. The rate of post 75

92 curing was found to depend on the drying temperatures and number of print stations available on press. Since the Comco Commander Press was only a three color unit and was limited to 93 C drying temperatures in the second and third units, post curing was required. 5.5 Characterization of Printed Samples After printing, straight lines that were included in the print design were cut out from the printed substrates to measure line dimensions (line width, length and height), line raggedness, DC resistance, AC impedance and antenna performance Print Quality Evaluation The quality of printed images was quantified using an ImageXpert (KDY Inc, NH) image analysis system. The camera based system uses a calibrated high precision optical measurement system, which provided a full suite of image analysis algorithms to characterize the quality of printed output 104. Two cameras were mounted above a manual X-Y stage. A manual adjustable stage enabled the samples to be arranged under the cameras. The calibrated cameras were used for image capture. The ImageXpert (IX) image quality measurement software (IX 10.0b63) processes the data and provides 76

93 quantitative, objective image quality measurements that include dot quality, line quality, text quality, edge raggedness, halftone quality, resolution etc 105. The system was calibrated with a high precision ceramic target that has photolithographically deposited chrome features on the surface of a finepolished ceramic material. The basic image elements that were examined were printed edges and lines. They should be thought of as transition points or zones from unprinted media. These transition zones are described by different measurements that include 'blur' and 'raggedness', which ultimately describe perceived visual sharpness 106. The most significant variable in measuring edges is the threshold setting that determines the actual transition from dark to light or vice versa. IX images are captured in gray scale mode and it reports lightness/darkness in terms of a gray value ranging from 0 to 255. A threshold gray level value is determined by measuring a line that is laser-engraved on a ceramic medium. The threshold setting is varied until the line measures mm in width (±0.005mm). The threshold setting obtained is thus a standard gray value calculation. Some components of line quality that were measured were line width and edge raggedness. Line width is the average distance between the 2 edges of the line. Raggedness is determined by the displacement of the black- 77

94 white boundary line from the ideal boundary line. The ideal boundary line is determined by calculating the best fit line through the boundary points Ink Film Thickness The ink film thickness of the printed lines was measured using a Nikon optical microscope at 1000X magnification. The samples were placed in spring clips which enabled them to be vertically viewed under the microscope. The samples were embedded in an epoxy resin and allowed to solidify for at least 18 hours. These samples were then ground and polished using an automated grinder/polisher from Leco Corporation to achieve a smooth surface. The cross-section of the ink-substrate interface normal to the printed line was used to measure the ink film thickness. Measured values are an average of 10 measurements Electrical Characterization of Printed Resistors Resistivity is an important property of insulators. This property helps to determine various other characteristics of an insulating or conducting material such as the dielectric breakdown, dissipation factor, moisture content, mechanical continuity to name a few 108. By using Ohm's Law, the resistivity of the material can be measured by sourcing a known voltage, 78

95 measuring the value of current obtained and then calculating the resistance 109. Based upon the values obtained for the area and length of the sample, the resistivity can be calculated. The resistivity of any sample is dependent on the applied voltage, electrification time, humidity and several other factors. By varying the voltage, one can determine the samples dependence on potential differences. On the other hand, the longer the electrification time, the higher the resistivity because the material continues to charge exponentially. Hence, most of the resistivity graphs obtained exhibit a positive slope showing a steady increase in resistivity with passing time. Therefore, to make a valid comparison between each test, the applied voltage, humidity and time, must be held constant. In order to understand the correct working of any electrical device, especially in the realm of printed electronics, it is necessary to characterize the device based upon surface/sheet resistivity, volume resistivity and its dielectric properties. The resistance of any material is determined by the combination of four factors, the resistivity of the material, Q, the length, L, thickness, T, and the width, W, as shown in Figure

96 If -If i t v ^^^ ^A* " ^ \ r ^L^*~-~>- *-\^ "'^"' Nht' "^"Stm*. 1 L **^L».. Figure 21: Dimensions of a Simple Bar for Calculating Sheet Resistance Antenna printed material tests were conducted using direct current (DC) or zero frequency and at a low frequency alternating current (AC) of 1 khz. DC resistance was measured using a Keithley 2400 multimeter and an Agilent 4338 Milliohmmeter was used to perform AC impedance measurements. This instrument performs in a 4-point probe measurement where four probes spaced equally apart from one another are brought in contact with the surface of the material 110 as shown in Figure 22. > < - -x- Figure 22: Four Point Measurement Technique 80

97 A current is driven through the two outer probes (If) and the potential difference (AV) developed between the two inner probes is measured. Complex impedance is computed in terms of resistance and reactance as shown in equation Z=R + jx [6] Where R is electrical resistance in Q, X is reactance in Q, and j=v~i The electrical resistance R, was compared directly with DC impedance, thereby, providing verification and validation of DC measurements. For all ink/substrate conditions, the reactance (jx) was negligible when compared to the resistance. Therefore, the low frequency reactance wasn't considered for the low frequency conduction model. Sheet resistivity, Rs, measured in ohms/square was calculated from the resistance measurements with the help of equation Rs = R/(L/W) [7] Where: R is Resistance [Q] L is Length [mm] W is Width [mm] 81

98 The sheet resistivity multiplied by the ink film thickness gives bulk resistivity, pdc. Bulk resistivity was calculated using equation pe>c=r(w.t/l) [8] Where pocis in Q- mil, R is line resistance in Q, W is line width in mm L is line length in mm and T is ink film thickness in mil Simulation In order to accurately depict the model of a printed resistor, the variations in width and thickness were taken into account. Employing a Monte-Carlo simulation model, a random variable was introduced along with the variance and added to the mean width and thickness to obtain a statistical average for the resistance of the printed sample. By considering sections of AL along the length of the printed trace, the average resistance as a summation of DC resistances along every section of AL was computed, since the sum of impedances of a series of resistors is the total impedance of the printed sample. Uniform and Gaussian distributions have been employed to generate the random variables. It is not clear at this point whether these are the best 82

99 distributions to be used for depicting this resistor model. Matlab code was used for obtaining the statistical average of the resistance along with its variance Printed Antenna Radio Frequency Testing Antenna testing was conducted at multiple levels of radio frequency (RF) continuous wave (CW), reflected impedance and RFID dynamic time waveform capture. The testing provides insight into the ability and usefulness of various combinations of substrates and inks for printed antenna. The initial RF testing was performed using a laboratory antenna measurement range. This testing was intended to provide both antenna beam pattern and comparative antenna performance measures for various printed antennae provided. The measurement range is shown in Figure. 23. The antenna range consists of a fixed reference antenna at a defined distance to a test antenna placed on a movable pedestal. An RF network analyzer was used to source a continuous wave RF signal at a known frequency and phase to the reference antenna. The antenna transmits the RF wave to the test antenna, where it was received as an input to the RF network analyzer. The network analyzer determined the incoming signals magnitude and relative phase as compared to the transmitted signal. The results were captured over 83

100 an HP-IB network connection to a personal computer, where operational software controlled the test and data collection. The software and PC were also in control of the movable pedestal mounted on top of a tripod. The pedestal was programmed to rotate 360 degrees in the horizontal plane and tilt the plane by +/- 45 degrees. This allowed a three-dimensional pattern of the antenna performance to be collected for viewing and analysis. Figure 23: Laboratory Antenna Measurement Range 84

101 In performing the antenna tests, two 10 foot RF cables (nominally 10 db loss per 100 feet) were used, where one cable was a high performance cable and the other was a custom built cable using CA240 cable and crimp connectors in order to properly connect the network analyzer (SMA) to the reference antenna (type-n). In addition, the antenna pedestal cable connections used a special connector and fitting to allow continuous contact through the 360 degree range, each with some additional loss. If it is assumed that there is 1 to 2 db loss in the pedestal, each RF cable has 1 db of loss and that the mating connections on the transmitting antenna provides another 1 db loss, a total of 4 to 5 db of loss is readily expected. Based on the above measurements and analysis, the open laboratory range appears to provide acceptable performance for antenna measurements. The theoretical range of an antenna from Friis transmission equation is given as equation R = X/47i(P t.gt.gr/pr) 1/2 [9] Where X is the wavelength of the RF carrier, Pr,tis the received (transmitted )signal power Gr,i is the effective antenna gain R is the distance between transmitter and receiver The network analyzer provides a reference transmitted signal power and 85

102 measures the received signal magnitude. The gain of the transmitting antenna is specified and the effective gain can be measured, the signal frequencies and thereby wavelength are known, and the antenna range distance can be measured. 86

103 CHAPTER 6 RESULTS AND DISCUSSIONS Conductive traces printed by flexography are evaluated for sheet resistance and antenna performance. Factors such as substrate properties, ink types, printing conditions, heat treatment are studied to find their influence on electrical testing. Table 22 displays substrate properties measured before print trial 1. Table 22: Substrate Surface and Physical Properties Substrates Roughness, Thickness, PPS Permeability Surface [Jim fjim Porosity, Coefficient, Energy, ml/min -6 2 xlo [xm mn/m PI 0.99± ± ± P2 1.24± ± ± ± P3 1.73± ± ± ± Bl ±1 <.01 < B2 1.39± ±1 <.01 < B ±1 <.01 < B3bs 4.56± ±1 <.01 < Although not reported, measured values of PPS roughness followed the same trend. The compressibility coefficient K was calculated using equation 2. 87

104 Substrates that have very low porosity and permeability values indicate that the samples are non-permeable. Printed lines were characterized for line width, length, raggedness and ink film thickness. These values are displayed in Table 23. Increase in length and width from the specified 50mm and 1mm were measured and the change in area was calculated from these measurements. Table 24 displays the resistance measured across the printed line and the calculated sheet resistivity values. A section of printed line images of all substrates are shown in Figures 24 and 25. Table 23: Characterization of Printed Traces Substrate Width (mm) Length (mm) Change in Area (%) Raggedness (mm) Ink Film Thickness,(nm) PI 1.14± ± ± ±0.33 P ± ± ±0.51 P3 1.20± ± ± ±0.90 Bl 1.18± ± ± ±0.65 B2 1.33± ± ± ±0.50 B3 1.39± ± ± ±1.43 B3bs 1.39± ± ± ±

105 Table 24: Calculated Resistance and Sheet Resistivity of Printed Traces Substrate PI P2 P3 Bl B2 B3 B3bs Resistance (O) ±1.91 ±0.64 ±0.18 ±1.71 ±1.95 ±0.28 ±1.90 Sheet Resistivity (O/sq) ±0.05 ±0.02 ±0.01 ±0.06 ±0.15 ±0.03 ±0.10 yam* m* ill Figure 24: Printed Line Traces on Substrates PI, P2 and P3 Figure 25: Printed Line Traces on Substrates Bl, B2, B3 and B3bs Substrates were characterized in terms of their surface and physical properties, which are known to impact conductivity of the printed traces 113. In particular the role of roughness and surface energy of the substrates were examined to study their effect on electrical properties of printed traces. 89

106 Understanding these effects is crucial for the optimization of the electrical performance of conductive inks printed on paper substrates. This study includes the selection of different commercial grade papers and paper boards. The substrates selected have applications in different areas such as flexible packaging, barcode printing, dry good packaging application etc. Based on the application of these grades in different areas of packaging, the properties vary. The effects of substrate properties on ink film thickness and sheet resistivity were evaluated. Average ink film thickness values of the printed traces are displayed in Table 23. These measured values range from 1.33um to 5.70um. Though the ink, anilox roll and plate used for printing the traces was the same, the ink film thicknesses varied. The variation in ink film thickness is due to the effect of substrate properties, including roughness, surface energy, permeability and compressibility of substrate and printing plate. The amount of ink transferred to the substrate depends on roughness of the substrate, fraction of ink immobilized in the paper mass and fraction of free ink film transferred as proposed by the Walker-Fetsko 114 model shown in Equation 10. y = (\~e- b lb(\-e- xlh )+f[x-b(\-e-* lb ) [10] Where: y is the amount of ink transferred per unit area of print 90

107 k is constant (related to printing smoothness of paper) x is ink film thickness originally on plate b is maximum capacity of the paper surface to immobilize ink f is the fraction of free ink transferred in the contact areas The roughness of the substrate influences ink transfer, so that a rougher substrate accepts more ink, as was studied by De Grace and Mangin 115. However factors such as impression pressure and printing speed play a greater role for ink transfer in addition to surface profile and compressibility that affect the thickness of the ink film transferred 116. The surface energy of the plate has little or no influence on ink transfer ' 119 A multiple regression model involving the effect of substrate properties on ink film thickness was examined. The best subsets procedure 120 was used to fit a regression model equation in determining the effects of substrate properties namely PPS porosity, surface energy, surface roughness and compressibility coefficient on ink film thickness. The best subsets regression method involved examining models with 1-4 variables and selecting the model that has the highest coefficient of determination and Mallows C-p value less than the number of variables. Table 25 shows the best subset regression model obtained with a 2 variable model involving roughness and surface energy. 91

108 Data reported in Table 25 shows only the best 2 models for each variable along with their R-Sq and Mallows C-p test statistic. The regression model Equation 11 shows the effect of surface roughness and surface energy on ink film thickness. Table 25: Best Subsets Regression Analysis for Ink Film Thickness Variables 1 R X p SE R-Sq 95.0 R-Sq(adj) 93.9 Mallows C-p 0.8 S X X X X X X X X Where R is surface roughness, P is porosity and SE is surface energy The regression equation is Ink Film Thickness = R SE Analysis of Variance Source DF SS MS F P Regression Residual Error Total Where: T is Ink Film Thickness [um] R is Roughness [um] T = R SE [11] 92

109 SE is Surface Energy [mn/m] The coefficient of determination that measures the variability in ink film thickness explained by the model and was found to be 95.8%. The regression equation is significant at a confidence interval of 95%. From equation 11 ink film thickness increased with roughness and decreased with surface energy. The effect of roughness on ink film thickness is higher than surface energy from the respective coefficients in equation 11 having p-values and 0.02 respectively. The rougher substrate B3bs has the highest ink film thickness and the smoothest substrate PI has the lowest ink film thickness. The smoother substrate had a lower contact angle values measured with both water and methylene iodide. The change in drop volume of the test liquids on the substrates was negligible indicating spreading. Due to this the substrates with higher surface energy had a lower ink film thickness. Figure 26 shows the scatter plots of roughness and surface energy with ink film thickness. A change in width of 14% - 39% and a maximum change in length of 1.20% were observed for the printed traces. The change in length and width of the printed traces is due to the pressure at the nip between plate and substrate and compressibility of the substrate and plate. The change in area of the printed traces increased with thickness of the ink film deposited due to the 93

110 availability of more ink that could have spread before it is absorbed by the substrate or dried due to evaporation of solvent. 6- s X. u J3 H 3- E s Roughness, urn Surface Energy, mn/m Figure 26: Scatter Plots-Ink Film Thickness, Roughness and Surface Energy The sheet resistivity of the substrates calculated was within the range Q/sq Q/sq at lum ink film thickness specified by the ink manufacturer Acheson Colloids Inc (Port Huron, MI). Surface sheet resistivity values obtained by high volume printing process using flexography are comparable to reported values of 0.58Q/sq at lum by screen printing 121. The printing of conductive inks by flexography has limitations in terms of changes in viscosity of ink in the anilox roller due to evaporation of solvent when compared to screen printing inks that are more stable such as conductive 94

111 pastes. The sheet resistivity of conductive traces should decrease with increase in ink film thickness, due to the large number of conductive particles present for conduction. The correlation between sheet resistivity and ink film thickness was very low as calculated using Pearson correlation. The printed ink contains silver particles, which are responsible for conductivity along the trace. With higher ink film thickness there are more silver flakes present per unit area of the printed trace. This results in better contact between the silver flakes leading to less resistivity. Substrate B3bs has the highest ink film thickness but due to the discontinuities in the ink film trace, as shown in Figure 27 its sheet resistivity is high. After printing there is a possibility that the silver flakes in the ink sometimes may not be in close contact to induce conductivity along a continuous printed trace. Hence, the electrical conductivity of the printed traces depends on the even distribution of silver flakes on the substrate, which varies with the amount of ink deposited. Though roughness and ink film thickness affect sheet resistivity, there might be a limitation on the amount of conductivity that can be gained which depends on the distribution of silver flakes in the ink layer. 95

112 Figure 27: Printed Line Trace on Substrate B3bs 6.1 Data Analysis Procedure Analysis of the data collected was examined using Minitab 14 statistical software program. An analysis of variance (ANOVA) was performed using a multilevel factorial design. A full factorial experiment was designed to study the effects of different factors at levels shown in Table 28 on response variables sheet resistance, line raggedness, line width and ink film thickness. Table 26: Factors for ANOVA Analysis Factors Substrate Level 4 Values P1,P2,P3,P4 Ink Cell Volume Tone Step SB,WB 18.6 um, 23.3 urn 80%, 90%, 100% 96

113 The test of null hypothesis that the factor level means are equal was analyzed for probability values (p-value) against a level of significance (a level) of Each response variable was analyzed for the effect of different factors illustrated by an ANOVA table, main effects plot, and interaction effects plot and factors that have a significant effect on the response variables are reported. In conjunction with the ANOVA tests, factors that had a significant effect on the response variable were further tested (Tukey's test) for multiple comparisons to find out the significant difference within each factor. The ANOVA analysis was run for all the raw data that included replicate measurements that were taken while performing the experiments. 6.2 Analysis of Printed Traces on Paper Substrates Tables 26 and 27 are results that were observed by printing conductive traces during printing on paper substrates with WB and SB inks at 80%, 90% and 100%. Printed traces were evaluated for line width gain, edge raggedness, ink film thickness and sheet resistance for WB and SB ink systems printed with the anilox roll bands with 18.6 um (12BCM) and 23.3um (15BCM) anilox cell volumes at 80%, 90% and 100% tone steps. The lower conductivity of traces printed with um (11 BCM) anilox band (compared to 18.6um and 23.3um anilox bands), due to inadequate ink coverage and poor print quality data from these runs were excluded from ANOVA analysis. 97

114 Substrates PI P2 P3 P4 Substrates PI P2 P3 P4 Table 27: Results Obtained for Traces Printed with WB Ink Anilox Band Tone Step Width Gain, % Ink Film Thickness, Sheet Resistivity, Edge Raggedness, Width Gain, % Ink Film Thickness, Sheet Resistivity, Edge Raggedness, Width Gain, % Ink Film Thickness, Sheet Resistivity, Edge Raggedness, Width Gain, % Ink Film Thickness, Sheet Resistivity, Edge Raggedness, Anilox (18.( mm) 80% % % Anilox (23.3um) 90% % Table 28: Results Obtained for Traces Printed with SB Ink Anilox Band Tone Step Width Gain, % Ink Film Thickness, Sheet Resistivity, Edge Raggedness, Width Gain, % Ink Film Thickness, Sheet Resistivitv, Edge Raggedness, Width Gain, % Ink Film Thickness, Sheet Resistivitv, Edge Raggedness, Width Gain, % Ink Film Thickness, Sheet Resistivitv, Edge Raggedness, Anilox (18.( ium) 80% % % % Anilox (23.: lum) 80% % %

115 6.2.1 Effect of Different Factors on Sheet Resistance ANOVA results for sheet resistance are given in Table 29 with a coefficient of determination R 2 (81.73%) gives the variability in the data set accounted for by the model. Individual factors substrate, ink, cell volume and tone step have a significant effect on sheet resistance. Main effects and interaction effects of various factors on sheet resistance are shown in Figures 28 and 29. Lowest sheet resistance was obtained by printing with WB ink and 18.6um anilox cell volume. The difference in sheet resistivities of WB and SB inks might be due to the different solvents and particle size of the silver flakes in the two inks. For the WB ink, the silver particle size is less than 3um compared to 7um for the SB ink. The WB ink was higher in solids, 85% versus 64% solids for the SB ink. Due to its higher solid content, the WB silver particles might have better contact, giving rise to less sheet resistance. Sheet resistance obtained for traces printed with 23.3 um cell volume was higher than for 18.6 um and was significant as shown in Table 34. Higher cell volume of the anilox band would contain more ink in its cells. Though there is higher ink volume in the anilox cells, ink released from the anilox cells is dependent on the interaction between the plate and anilox cells and between plate and substrate. Interaction between factors such as cell 99

116 geometry, pressure applied at the nip is detrimental in the transfer characteristics of ink 122. Sheet resistance measured for traces on substrate P2 were the highest among the substrates printed with both WB and SB ink at 80% tone step for 18.6 (am and 23.3 um cell volumes. Lines printed on P2 with WB ink, shown in Figure 30, has discontinuities in the ink film which affects the conductivity of the trace. Moreover the porosity and calculated permeability coefficient of P2 was the least and the surface energy was the highest among the paper substrates. There is a possibility that the adhesive forces between the ink and the substrate was higher than the cohesive forces in the ink due to which a non- continuous film might have been formed leading to high sheet resistance. Ink film thickness on P2 had a variance of 50%. Due to the large variance in ink film thickness on substrate P2 the effective cross sectional area available for conductivity varies. There is no significant difference in sheet resistance printed traces on PI or P3 and tone step 100% or 90% as shown in Table 30. Source Substrate Ink Cell Volume Tone Step Substrate*Ink Substrate*Cell Volume Substrate*Tone Step Ink*Cell Volume Ink*Tone Step Cell Volume*Tone Step Error Total Table 29: Analysis of Variance for Sheet Resistance DF Seq SS Adj SS Adj MS F P S = R-Sq = 31.73% R-Sq(adj) = 79.64% 100

117 «30 4 o cn <D "4-1 O 20- S 15- Substrate Pl P2 P3 P4 SB WB Ink i r ' <u Cell Volume Tone Step , S / \ 1 I I I I 12BCM 15BCM 80% 90% 100% Figure 28: Main Effects Plot for Sheet Resistance Table 30: Tukey's Pair Wise Comparison Test-Sheet Resistance Factor Level Difference of Means P- Value P2-P P4-P Substrate P2-P P2-P P4-P Ink SB-WB Cell Volume 23.3 urn Tone Step 80%-90% %-100%

118 SB WB 12 BCM 15 BCM I 80% 90% 100% I I I tc / / \ Ni- 30 Substrate PI P2 P3 P4 Ink SB WB Cell Volume 12 BCM - 15 BCM Cell Volume Tone Step Figure 29: Interaction Effects Plot for Sheet Resistance! Figure 30: WB Traces on P2,15BCM Anilox, 80%, 90% and 100% Tone Step Effect of Different Factors on Line Raggedness ANOVA results for line raggedness are given in Table 31. The coefficient of determination R 2 (36.15%) gives the variability in the data set accounted for by 102

119 the model. Individual factor substrate and interaction effect between substrate and cell volume have a significant effect on line raggedness. Main effects and interaction effects plots for line raggedness are shown in Figure 31. From the interaction effects plot between substrate and cell volume similar trends are shown by substrates PI and P4 with higher raggedness for 23.3 urn cell volume than 18.6 um cell volume. At 23.3 urn anilox cell volume the printability of SB was better than WB ink. The best printed traces were obtained by printing with a 18.6 um anilox cell on PI and with a 23.3 um anilox cell on P3. Line raggedness gives an indication of the printability of the inks on the substrates. To prevent contact between closely printed lines, the raggedness of the lines has to be as close as possible to zero, which indicates a perfect straight line. Table 31: Analysis of Variance for Line Raggedness Source DF Seq SS Adj SS Adj MS F P Substrate Ink Cell Volume Substrate*Ink Substrate*Cell Volume Ink*Cell Volume Error Total S = R-Sq = % R-Sq(adj) = 31 87% 103

120 o.oim « C "g bo o c BCM 15BCM Cell Volume 12BCM J 15BCM l_ Substrate h h Cell Volume Figure 31: Main Effect and Interaction Effect Plot for Line Raggedness 104

121 6.2.3 Effect of Different Factors on Line Width ANOVA results for line width are given in Table 32. The coefficient of determination R 2 (78.94%) gives the variability in the data set accounted for by the model. Individual factors substrate, ink and cell volume have a significant effect on line width as well as interaction effect between substrate and ink, and ink and cell volume. Main effects and interaction effects plots for line width are shown in Figures 32 and 33, respectively. Image size in flexography either increases or decreases due to the flexible plate that compresses from pressure during printing. Line width gain in flexography printing is due to deformation of the flexible printing plate and displacement of the ink during printing 123. An increase in line width limits the reproduction of fine detailed features necessary for printed electronics 124. Traces printed with both WB and SB inks experienced width gain. Line width gain measured for traces printed with SB ink ranged from 14%-23% and 24%- 40% for WB ink. From the interaction effects plot between substrate and ink, lowest line width gain is obtained with PI and SB ink. Line width gain was higher for traces printed with 23.3 am anilox cell volume. Due to the higher cell volume, a greater amount of ink is available in the anilox cells and this might have led to more extensive ink spreading on the substrate before it 105

122 completely immobilized and dried. The lowest line width gain is obtained by printing on PI with SB ink and 18.6 jam cell volume of anilox and is significant as shown in Table 33 and tone step has no significant effect. Table 32: Analysis of Variance for Line Width Source DF Seq SS Adj SS Adj MS F P Substrate Ink Cell Volume Substrate*Ink Ink*Cell Volume Error Total S = R-Sq = 78.94% R-Sq(adj) = 78.25% i g J g PI P2 Substrate P3 P4 1.26' f g 1.25 s S / << 12BCM / Cell Volume / 15BCM g g ' SB Ink WB Figure 32: Main Effects Plot for Line Width 106

123 12BCM Cell Volume 15BCM Figure 33: Interaction Effects Plot for Line Width Table 33: Tukey's Pair Wise Comparison Test-Line Width Factor Level Difference of Means P-Value P2-P Substrate P3-P1 P4-P P4-P Ink WB-SB Cell Volume 15BCM-12BCM

124 6.3 Analysis of Printed Traces on Paperboard Substrates The following analysis is for board substrates printed using flexography. Factors and levels used for ANOVA analysis is shown in Table 34. The response variables sheet resistance, line width and line raggedness. Table 34: Factors for ANOVA Analysis Factors Substrates Inks Cell Volumes Tone Steps Level Values Bl, B2, B3, B4 SB,WB 18.6um, 23.3um 80%, 90%, 100% Effect of Different Factors on Sheet Resistance ANOVA results for sheet resistance are given in Table 35. The coefficient of determination R 2 (88.94%) gives the variability in the data set accounted for by the model. The individual factors of substrate, ink, cell volume and tone step have a significant effect on sheet resistance, as well as the interaction effects between them. Main effects and interaction effects of various factors on sheet resistance are shown in Figures 34 and 35 and the pair-wise comparison of different levels each factor is shown in Table

125 Table 35: Analysis of Variance for Sheet Resistance Source DF Seq SS Adj SS Adj MS F P Substrate Ink Cell Volume Tone Step Substrate*Ink Substrate*Cell Volume Substrate*Tone Step Ink*Cell Volume Cell Volume*Tone Step Error Total S = K -Sq = 8E.94% R-Sq(adj) = Sheet resistance is the lowest when printed with SB ink and 18.6um anilox cell band. There is an interaction between the two factors substrate and ink on sheet resistance. Substrates Bl, B2 and B3 have lower sheet resistances printed with SB ink where as B4 has with WB ink and is the lowest of all the substrates. Variation in sheet resistance arises due to the arrangement of silver particles on the substrate. If the thickness of ink film is low then there is a possibility that the silver particles are not in proper contact to impart desired conductivity. Hence, Bl with lower ink film thickness has higher sheet resistance and B4 has the lowest value. Since the aspect ratio of the silver particles are not known it can be speculated that the larger silver particles in SB might have led to better contact among the particles than those in WB ink and hence lower sheet resistance. 109

126 Substrate Ink 24 H OS 21H H <u '55 19 Pi ^6 18H c ca * x Bl B2 B3 B4 SB WB Cell Volume Tone Step / I I % 90% 100% Figure 34: Main Effects Plot for Sheet Resistance Table 36: Tukey's Pair Wise Comparison Test- Sheet Resistance Factor Level Difference of Means P-Value B1-B Substrate B1-B3 B1-B B4-B Ink WB-SB Cell Volume 15BCM-12BCM %-90% Tone Step 80%-100% %-100%

127 SB WB i i 80% 90% 100% * \ Substrate fc^rr.. S»-- ll ^fcasj Substrate -30 B2 - * B3 A- B4 Ink _^ -* -20 Ink SB m WB Cell Volume Cell Volume Tone Step Figure 35: Interaction Plot for Sheet Resistance Sheet resistance depends on the type of substrate, ink, cell volume and tone step used for printing the traces. Many factors such as ink release characteristics of the ink from the anilox roll, cell geometry, pressure applied, geometry of dots on the plate play an important role on the transfer characteristics of the ink onto the substrate affecting the sheet resistance 125. Ill

128 6.3.2 Effect of Different Factors on Line Width ANOVA results for line width are given in Table 37. The coefficient of determination R 2 (83.42%) gives the variability in the data set accounted for by the model. The individual factors that have a significant effect on line width gain are substrate, ink and cell volume as well as interactive effects between substrate and ink, substrate and cell volume and ink and cell volume. The main and interaction effect plots for line width are shown in Figures 36 and 37 and the pair-wise comparison is shown in Table 38. The flexible plate used in flexography that carries the image compresses due to pressure during image transfer that causes the deformation of plate and displacement of ink during printing. An increase in line width limits the reproduction of fine detailed features necessary for printed electronics. SB ink printed lines had an average line width gain of 37% and WB ink lines had 30% gain. The smoothest substrate Bl had the least line width gain while the roughest substrate B4 showed the greatest increase in line width. Different factors such as surface energy, roughness of substrates, and evaporation of solvent in the inks play an important role in determining the interaction at the ink paper interface. To compensate for the increase in line width gain the printing image on the plate has to be adjusted for gain during printing in 112

129 flexography. Lines with the least line width gain were printed on Bl with WB ink and 18.6 am anilox volume. Table 37: Analysis of Variance for Line Width Source DF Seq SS Adj SS Adj MS F P Substrate Ink Cell Volume Substrate*Ink Substrate*Cell Volume Ink*Cell Volume Error Total S = R-Sq = 83.42% R-Sq(adj) = 82.69% Substrate Ink ^ ^ ^ *d l-h cu s C / Bl B2 B3 B4 Cell Volume SB ^ WB S BCM 15 BCM Figure 36: Main Effects Plot for Line Width 113

130 SB WB 12BCM 15BCM 1 1 i l Substrate Substrate - Bl -m- B2 4> B3 -A- B4 Ink SB WB ^_ \ Ink Cell Volume Figure 37: Interaction Effects Plot for Line Width Table 38: Tukey's Pair Wise Comparison Test-Line Width Factor Level Difference of Means P-Value B2-B B3-B Substrate B4-B1 B3-B B4-B B4-B Ink SB-WB Cell Volume 23.3 pm

131 6.3.3 Effect of Different Factors on Line Raggedness ANOVA results for line raggedness are given in Table 39. The coefficient of determination R 2 (62.41%) gives the variability in the data set accounted for by the model. Table 39: Analysis of Variance for Line Raggedness Source DF Seq SS Adj SS Adj MS F P Substrate Ink Tone Step Substrate *Ink Ink*Tone. Step Error Total S = R-Sq = 62.41% R-Sq(adj) = 60.12% Substrate o.oioo- -*- x Substrate Bl B3 B4 -A *"""".» " " Vs x ""v 'il Figure 38: Main Effects and Interaction Effects Plot for Line Raggedness 115

132 The individual factors substrate, ink and tone step have a significant effect on line raggedness as well as the interactive effect between substrate and ink. The main effects and interaction plots for line raggedness are shown in Figure 38. Lower line raggedness is obtained by printing on substrate B2 with WB ink at 100% tone step. 6.4 Ink Film Thickness Analysis The ink film thickness of printed traces for each substrate was examined at 10 different locations. The standard deviations for the measurements are very high for most of the measured values as can be seen in Figure 39. ANOVA analysis performed on ink film thickness gave a very low R 2 value indicating a lot of variability. Few substrates have a standard deviation as high as 50% of the mean value. The ink film thickness of the paper substrates varied from 3um-8um and from 3um-13um for the board substrates. For the board substrates, the SB ink had the lowest ink film traces where as the WB ink had the highest ink film traces printed. The ink film thickness variation for each substrate among different inks followed the same trend with WB having the highest and SB traces having the lowest values. Traces printed with the 18.6 um anilox cell volume had a higher ink film thickness than 23.3 urn. Though there is higher ink volume in the 23.3 um anilox cells, ink released from the 116

133 anilox cells is dependent on the interaction between the plate and anilox cells. Interaction between factors such as cell geometry, pressure applied and geometry of the dots on the plate is detrimental in the transfer characteristics of ink 123. Ink in the 23.3 um anilox band might have dried at the bottom of the cells and hence the amount of ink available for transfer was less. Surface roughness of the substrate plays an important role in ink transfer 126 and can be evidenced by the higher ink film thickness on P4 and B4 with WB ink Mill BUV ti P1 P2 P3 P4 B1 B3 B4 Substrates SB WB Figure 39: Ink Film Thickness at 90% Tone Step with 18.6 am Anilox Band 6.5 Printed Antenna Radio Frequency Testing Trial -1 Antenna testing provides insight into the ability and usefulness of various combinations of substrates and inks for printed antennas. The initial RF 117

134 testing was performed using a laboratory antenna measurement range. Antenna performance measurements were taken for 27 different Alien antennas printed using different ink systems (SB solvent based, WB water based, and UV ultra-violet based) and substrates. The magnitude limit for acceptable antenna was set at 46.4dB indicated by the dotted line in Figure 40. For SB ink systems, acceptable substrates were PI, P3, and B3 and the other substrates had a slightly higher performance taking into consideration standard deviation. For UV based ink systems only PI had an acceptable performance indicating that the UV system is unreliable or undesirable based on the process used for printed RFID antennas. For WB based ink systems, all substrates except B3 provided acceptable results. Based on these results UV based ink system was found to be clearly inferior. Substrates PI, P3 and B3 appear to be superior. Printed antennas were tested for read range measurements. Antenna testing was performed for readability at increasing distances from reader's antenna, ability to reassign an ID and measure the reads per second at a distance of 1 foot from the reader's antenna. 118

135 Figure 40: Alien Antenna Performance Data Set- Trial 1 When a tag is to be read, the ALR-9780 Alien Technology RFID reader generates a radio signal where a portion of it is absorbed from the reader and returns the signal with the tags information from the memory. Alien technology RFID Gateway software was used to identify and reassign an ID. The test performed on Alien antennas printed with UV, WB and SB inks and the read ranges are reported in Table 40. Sample Table 40: Alien Printed Antenna Performance with IC Readability Reassign ID Reads/Second B3 UV Ink Up to 6" Fail Fail B3 SB Ink Up to 1" Fail Fail B3 WB Ink Up to 5" Pass

136 6.5.2 Trial-2 Antenna pattern measurements were taken the Alien antenna printed using the three ink systems (SB, UV and WB) on all the substrates. The magnitude limit for acceptable antenna was set at 46.4 db. The entire antenna set with a magnitude value greater than db had passed the performance test. The acceptable performance for UV ink system was only 29% of the entire set. For the um cell volume there was no printed antenna that could meet the acceptance criteria. Antennas printed with a cell volume of 23.3 um showed the highest number of acceptable antennas and the best performing antenna was obtained by printing on PI. Antennas printed with solvent based ink system showed a greater percentage of acceptable performance at 87.5%. Of all the samples tested only 3 failed to perform and these were printed using the um anilox band. Antennas printed using the 18.6 um anilox band gave the highest number of acceptable performance though the best performing substrate was P3 printed with 23.3 um anilox band. Antennas printed with the WB ink system performed higher than the other ink systems with 92% of the printed substrates showing an acceptable performance. Substrate P2 failed to produce acceptable antennas at um 120

137 and 23.3 um cell volumes which were due to drying issues on the press during printing. For the WB ink system the best performing antennas were produced using 18.6 um anilox band and the best substrate was PI. The antenna performance of all the ink systems using 18.6 um anilox cell volume is compared in Figure 41. WB and SB ink systems have antenna performance around -40dB where as UV ink system is less than -45dB for most of the printed antenna set. The acceptable performance for UV ink system was only 29% of the entire set. For the um cell volume there was no printed antenna that could meet the acceptance criteria. Antennas printed with a cell volume of 23.3 um showed the highest number of acceptable antennas and the best performing antenna was obtained by printing on sub 1. Figure 41: Antenna Performance at 18.6 [im Anilox Cell Volume 121

138 Antennas printed with solvent based ink system showed a greater percentage of acceptable performance at 87.5%. Of all the samples tested only 3 failed to perform and these were printed using the um anilox band. Antennas printed using the 18.6 um anilox band gave the highest number of acceptable performance though the best performing substrate was sub 3 printed with 23.3 um anilox band. Antennas printed with WB ink system performed more than the other ink systems with 92% of the printed substrates showing an acceptable performance. Substrate 2 failed to produce acceptable antennas at um and 23.3 um cell volumes which were due to drying issues on the press during printing. For the WB ink system the best performing antennas were produced using 18.6 um anilox band and PI was the best as shown in Figure 42. Overall the paper substrates performed better than the board substrates with PI showing the best performance. Among the tested anilox bands, 18.6 um printed antennae performed the best. Antenna performance for SB ink printed with 18.6 um and 23.3 um was comparable. However for the WB ink, the 18.6 um anilox band performed batter than 23.3 um. This can be the result of differences in ink release characteristics. 122

139 Figure 42: Antenna Performance for WB Ink 6.6 Simulation Results Monte Carlo simulations were run for WB and SB ink systems. Variations in width and ink film thickness were included in the models. Material resistivity values of the silver inks specified by the manufacturer were used to calculate the theoretical and statistical resistance. For the simulation, 10,000 trials for resistors having 1024 discrete regions with width and thickness variance as uniform random variables were performed. The conditions for WB ink considered for the simulation are as follows: Length urn Average width- loooum Variance in width- 30% of average width 123

140 Average thickness- 8 urn Variance in thickness- 75% of average thickness p- 500 Q-nm The simulation results for 10,000 trials run are shown in Figure 43. ' } Figure 1 File Edit View Insert Tools Desktop Window Help i-'ckx 1 Resistance Variance Monte Carlo Trial 35 J i i i i i i_ Figure 43: Simulation Resistance Results for WB Ink The theoretical resistance of the samples was found to be Q. The statistical average of variance resistor across trials was found to be Q or % greater. 124

141 Comparing this model with the measured values for the samples, both the statistical average and the theoretical value of resistances were less than the measured value of 15 Q. The simulations were then run to calculate the resistivity of the ink considering an average resistance value of 15Q with the other parameters constant. The simulation results are shown in Figure 44.»! Figure 1 File Edit View Insert Tools Desktop Window Help DcSHS It ^ ^ 0 8 V. OES USSSJ flan Resistivity Variance Monte Carlo Trial 2550 r Figure 44: Simulation Resistivity Result for WB Ink 125