What Can No Longer Be Ignored In Today s Electronic Designs. Presented By: Dale Lee

Transcription

1 What Can No Longer Be Ignored In Today s Electronic Designs Presented By: Dale Lee Dale.Lee@Plexus.Com 24 January 2008

2 Introduction Component packaging technology continues to decrease in size (length, width, thickness), interconnection density per unit area is increasing (thinner PCB s, smaller lines & spaces), functional performance is increasing (thermal, mechanical, electrical) at the same time as assembly processes are changing to leadfree assembly. Because of this migration, many of the elements that use to be ignored, can no longer be ignored.

3 Elements of Design Process Cultural Differences Religious Race Political Geographical Differences Language Environmental (Local/Regional) Unit of Measure (Inch vs Metric) Time Line Management Multiple Idea Selection Quickest To Success Quickest To Failure Process Development Nominal Vs At Limits

4 Electronic Designs Elements That Were Previously Insignificant, May Be A Significant Source of Assembly Defects Part Tolerance Trace Connections Via Hole Type Via Hole Location Via Pad Size Uniformity Tooling Tolerance Process Tolerance Facility Environment

5 Component Issues - Warpage Higher Lead Free Solder Solidification and Process Temperatures, Increases The Amount Of Thermal Expansion Mismatch Of Components Which Can Increase Amount Of Component Warping During Assembly Process May Require Redesign Of Package (Material Selection) For Thermal Mass And Expansion Balance.

6 Component Issues Warpage & Thermal Shock May Require Change In Production Process Tooling To Bridge Warpage Gap. (Increased Solder Paste Volume Application, etc.) Reflow Profile To Bridge Warpage Gap. (Decreased Thermal Change Rate Reduce Surface To Cooler Location Temperature Delta - TCE Induced Warpage) Reflow Profile To Reduce Micro Cracks In Multilayer Ceramic Capacitor Chips & Arrays. (Decreased Thermal Change Rate Reduce Surface To Cooler Location Temperature Delta For Chip > 1210)

7 Component/PCB Warpage Impacts Split Planes/Unused Pad Removal: Localize Changes In Thickness/Coplanarity Of PCB Potential Opens From Tilted Components (Teeter-Totter Effect) Potential Opens From Dropped Solder Connection Potential Reduced Reliability From Stretched Solder Joints

8 Trace Routing Impacts Solder Joint % PAD SIZE INCREASE EFFECTIVE PAD SIZE ANALYSIS WIDTH OF TRACE 10 Mil Nom 15 Mil Nom 20 Mil Nom 10 Mil Max 15 Mil Max 20 Mil Max Increased Mounting Pad Size Affected By: Number Of Trace Connections To Each Pad Width Of Trace Connections To Each Pad Size of Pad Small Pads Have Less Margin Uniformity Of Trace Egress Direction Some Package Types Are More Sensitive Than Others Uniformity Of Trace Sizes

9 Trace Routing Impacts Solder Joint Gradient Of Different Trace Sizes Localized Concentrated Large Trace Connections Increase Defect Potential Concentrations Of Design Variability Can Create: Solder Bridge, Open Connection, Insufficient Solder, Tilted Components

10 Micro-Via in Pad Micro-Via Locations Should Be Consistent Area Array Components - Center of Pad Leadless (Chips, Chip Array, LCC, QFN, etc) and Leaded (QFP, SOIC, SOT, etc.) Components Near Toe Fillet X-ray Image

11 Micro-Via in Pad Voids in Solder Joints Unfilled Via in Pad Provide Flat Pad With Filled/Plated Closed Via Solder Joint Formation Thermal Connection Plane Connection Multiple Connections Stacked Via Solder Volume Via Location - Edge Unfilled Via Plated Closed Via Filled Via

12 High Design Density Spacing Limitations Are Not A Driven By The Manufacturing/Assembly Process SMT Pads Minimum Or No Visible Solder (Toe) Fillets Potential AOI/Inspection Issue Component Spacing Minimum Component to Component Spacing Limited/No Rework Limited AOI/Test Point Access

13 Component Pad - Thermal Imbalance Multiple Trace Connections Number Of Trace Connections Per Pad Uniformity Across All Pads On Single Component Solder Mask Defined Pad Increased Soldering Defects Delayed Reflow Across SMT Components Tombstone Components Ball in Socket Area Array Component

14 Solder Paste Printing Volume Leadless Device Usage Increase (DFN, QFN, LCC LGA) & Ultra-fine Pitch Components Impacts: Tighter Tolerance On Solder Paste Volume Thinner Stencil Increased Uniformity Of Paste Volume Across Component (Pad to Pad) Paste Volume/Pad Trace Egress Direction Impact Some Package Types Are More Sensitive Than Others Open/Unwetted LGA Solder Connection

15 Interconnection Failure Mode Shift Courtesy of Universal Instruments

16 PCB Performance Consideration Performance Requirements May Dictate Material Selection Mission Critical Applications; Military, Aerospace, Medical Decisions Should Be Conservative To Allow For A Safety Zone Non Fault Tolerant Systems Where Additional Reliability Requirements Are Specified Mainframes, Servers, Communications Equipment, Automotive Life Expectancy Of The Product Consumer Electronics Vs. System Electronics System Environment Additional Performance Criteria Such As CAF Resistance May Make Certain Materials More Appropriate Photos Courtesy of Universal Instruments

17 PCB Lead Free Laminate Traditional DICY Cured FR4 Laminates Cannot Survive Multiple Elevated Pb Free Reflow Profiles Or Sustain Impact To Reliability Tg Alone Is Not An Indicator Of Lead Free Capability T260 And T288 Values Alone Do Not Assure Lead Free Capability Of A Laminate System Base Resin Decomposition Temperature (Td) And CTE Values Are Good Indicators Of Lead Free Survivability Reliability Impacts Following Assembly Simulation Critical Reflow Conditions Will Vary Depending Upon Board Design Board Design Contributes To The Lead Free Equation Electrical Properties Need To Be Accounted For In Design Resin Content Changes Electrical Performance Halide Free Change Electrical Performance and Moisture Absorption Photos Courtesy of Cookson and Universal Instruments

18 Internal PCB Impacts 1 Oz 2 Oz Copper Plane Uniform Thickness Balanced Thickness To PCB Centerline Thicker Copper Plane Layers Increase Thermal Mass Thinner Is Better Increase Thermal Pad Isolation Issues Include: PCB Warping PTH Hole Fill

19 Internal PCB Impacts Number Of Layer Connections to Plated Through Hole Increased Number Of Layer Connections Increases Thermal Mass Of Plated Through Hole Increased Number Of Plane Layer Connections Greatly Increases Thermal Mass Of Plated Through Hole Increase Thermal Pad Isolation To Improve Solder Flow To Topside Issues Include: PTH Hole Fill

20 What To Do? Increase Solder Temperature? Lead Free Solder Issue Higher Solder Temperatures Or Increased Solder Dwell Times Create Problems With Pads On Solder Side * Dr. S. Zweiger, Solectron GMBH, Productronica Green Day, November 2005

21 Wave Solder & Rework Issues Limit Effects Of Copper Dissolution Use Lower Dissolution Rate Solder Alloy Modified SAC Alloy (Sb, Ni, Zn, Ge, In, Etc) Non-SAC Alloy (Sn/Cu/Ni, Etc) Pad Trace Connection Tear Drop Snow Man Connection Wide Trace Greater Than Trace Dissolution in 30 Seconds PCB Photo Courtesy of Cookson Alpha Metals Byle, et al, Copper Dissolution Rate in Pb-Free Soldering Fountain Systems, SMTA-I 2006

22 Lead Free Process Impacts On Solder Mask Higher Reflow Temperature - Additional Stress On Solder Mask Bond Line - Potential Delamination - Discoloration - Solder Wicking Under Mask (Improperly Cured) Properly Cured Improperly Cured Board Challenges (and Solutions) For Next Generation Designs, J. Fix, Taiyo America, Webinar, May, 2006

23 Solder Mask Selection Impact A Low Surface Energy Glossy Finish Can Generate Solder Defects Similar In Appearance Of No Flux Or Too Little Flux On The Board. Resulting in Whiskers, Strings, Peaks, Bridges, Icicles, Solder Balls, Solder Web, Solder Flags

24 Lead Free Solder Advantages Material Costs NO More Expensive Reflow Temperature NO Higher Temperatures For Reflow & Rework Increased Component & PCB TCE Mismatch (Warp, Crack) Through Hole Soldering NO Higher Hand and Rework Solder Temperature Greater Lead to Hole Clearance Increased Copper Dissolution Correct Placement Location and Orientation Analysis Assembly Process Physics Potential Tighter SMT Lead Pitch Packages Increase Secondary Side SMT Package Mass

25 Physical Property Comparison Alloy Sn62 SnAg3.8Cu0.7 Melting point 179C 217C Density (g/mm2) Surface ~35 C ~460 dyne/cm* ~510 dyne/cm** above melting point in air Wetting angle on Cu (deg) * Values for Sn63Pb37 ** Values for SAC305 Increase Surface Tension = Higher Mass Components on Secondary Side of Double Sided SMT Assemblies ( 10-12%) Physical Values From: Henkel Technologies SMTA Chapter Presentation, Orange County, Oct, 2005 Surface Tension From Indium Corporation

26 Coalescent Performance Comparison Lead-free has significantly higher coalescent force (Decrease Solder Spread) Indium Corp. Pb-Free Workshop 2003

27 Lead Free Solder Spread Stencil Alignment of Solder Paste To Pad Tolerance May Be Critical To Good Manufacturing Yields (Dependant Upon PCB Surface Finish) Tin-Lead Paste Example OSP Finish Lead Free Paste Alpha Metals, SMT Mag Webcast, Jan 2006

28 PCB Finish Vs Solder Spread OSP Immersion Silver Immersion Tin ENIG Amount Of Lead Free Solder Wicking Is Dependant Upon Finish

29 Tin-Lead Vs Lead Free Wicking Depending Upon The Pairing Of PCB Surface Finish And Component Lead Finish, The Amount Of Solder Wicking / Spread Can Induce or Reduce Solder Defect Formation. Lead Free Tin-Lead

30 Component Placement Tolerance Fallacy No problem We place 0.5-mm (20 mil) pitch QFPs every day Pad Width 0.3-mm Foot Length 0.76-mm Lead Width 0.2-mm QFP Misplaced 0.15-mm in x & y 0.3-mm ball 0.25-mm pad 0.5-mm CSP Misplaced 0.15-mm in x & y Top View Courtesy of Xetel Corporation

31 Component Placement Tolerance Fallacy Top View Profile View 0.3-mm ball Solder Paste Shift 0.25-mm pad Courtesy of Xetel Corporation

32 Match Tooling Design To Parts Normal Manufacturing Process Variability May Exceed Allowable Assembly Process Tolerance For High Yield, Reliable Assembly Matched Tooling (Stencils) To Materials (PCB) May Be Required PCB to Stencil Pad Positional Deviation Measurements No Adjustments PCB to Stencil Pad Positional Deviation Measurements With Scaling Adjustments

33 Lead To Hole Clearance Lead Angle In Hole Combined With Lead Clearance Can Impact Void Creation In Plated Through Hole. Lead Type Can Impact Void Creation Round Versus Flat Ribbon Lead / Hole Metallization Can Impact Void Creation Faster Wetting Rate = Greater Risk Of Void Creation * K. Seelig, et al, Lead-Free Solder Assembly For Mixed Technology Circuit Boards, SMTA-I 2006

34 Lead To Hole Clearance Lead Free Soldering Lead Clearance Minimum May Increase Increasing Board Thickness May Further Increase Lead To Hole Clearance (Aspect Ratio) Larger Holes Create Less Voids Smaller Hole To Lead Clearance Decreases Shrinkage Holes / Hot Tear Joints IPC-A-610D, Fig. 5.67

35 Through Hole Pad Design Square Pads Should Not Be Used On Solder Side Increased Pad Lifting* Increased Solder Defect Bridge/Flag/Web Decrease Component / Top Side Pad Size** Reduced Fillet Lifting * Dr. S. Zweiger, Solectron GMBH, Productronica Green Day, November 2005 ** K Puttlitz, K Stalter, Handbook of Lead-Free Solder Technology For Microelectronic Assembly, pp 628, Fig 48

36 Through-Hole Component Mounting Leads Should Extend Below Bottom of Board Lead Protrusion: Shorter is Better Decreased Solder Bridging Decreased Component Tilting Lead Free Solder: Lead Length Should Be Even Shorter Than Tin-Lead Solder.

37 SMT Component Placement Secondary Side No SMT Parts Under Solder Tail Parts No Passive Packages Smaller Than 0603 No Passives Packages Greater Than 1210 Lead Free Wave Soldering Maximum Component Size Is Smaller Than Tin/Lead No SMT QFP Packages No Large/Tall SMT SOIC Packages on Wave Side May Require Increased Pad Length Extension From Component Body May Require Increased Component to Component Spacing (Requires Very Long Pad Length or Spacing) Tighter Component Spacing Require Use Of Nitrogen

38 SMT Thieving Pads Purpose Is: To Wick Excess Solder Away From Pins Provide Solder Wave Surface Tension Breaking Points Direct Flow of Solder to Pins and Around Features Photos Courtesy of Cookson Alpha Metals

39 Cleaning Remember it is not the cleanliness of the entire board that causes the electrical failure but the amount of residue between two pads on a critical circuit the define the cleanliness of the assembly. Terry Munson, President/Senior Technical Consultant, Foresite Ask the Experts, Circuitnet.com, 7 Jan. 2008

40 Cleaning Chemistry Compatibility Cleaning Chemistries Must Work With Tight Component Spacing Component To Component Spacing Component Stand-off Height Compatible With Greater Diversity Of Materials PCB s Substrate Materials Solder Mask Silk Screen Ink Pad Finish Component Materials Package Seals Marking Inks Solder

41 Source Of Contamination What is Source of Whiskers in Removed BGA Component Package Solder Joints at PCB Interface Surface? 1. Solder Paste 2. Stencil Printer 3. Reflow 4. PCB Finish 5. Thermal Aging

42 Closing Thought We can t solve problems by using the same kind of thinking we used when we created them. Albert Einstein Don t Forget About Reflow Process Induced Warpage/Coplanarity Issues.

43 Thank You! Questions