SMART GROUP STANDARD. Control of Solder Paste used in Electronic Assembly Process. SMART Group. 2 Normative References

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1 2 Normative References The Test Methods employed are adapted from IPC-TM-650 comprising: SMART GROUP STANDARD Control of Solder Paste used in Electronic Assembly Process Number: SG PCT 01 Control of Solder Paste used in Electronic Assembly Process Date: 10/09 FOREWORD The SMART Group is a British trade association. The object of this specification is to aid and assist our members and affiliates in the electronics industry and to enhance the design, performance and reliability of electronic circuit assemblies. These specifications are prepared by the Technical Committee of the SMART Group. The SMART Group provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with one of its specifications. Attention is drawn to the possibility that some of the elements of this specification may be the subject of patent rights. The SMART Group shall not be held responsible for identifying any or all such patent rights. 1. Scope: This Standard is intended to determine the suitability of solder paste prior to its employment on the production line; reduce process defects and minimise rework. The tests are, by necessity, fast and simple to perform; with commercially available equipment and may be completed in less than 1 hour. A list of defects, test methods and the time/cost implications are shown in Table 1 Slump Test (Modified 1 ) Solder Ball Test (Modified 1 ) Tack Test (Modified 1 ) Wetting Test (Modified 1 ) Spread Test (Modified 1 ) Other Standards include: IEC JIS Z 3284 Open Time/Stencil Life Test 1 NOTE: By courtesy of the IPC, these Test Methods have been modified primarily to the inclusion of lead-free alloys. NB Copies of the IPC-TM-650 Test Methods can be downloaded free of charge from the IPC website: Methods 3 General Outline of the Methods and Purpose: 3.1 Tack Testing This test is to determine the ability of a printed pattern of solder paste to retain a probe placed in the solder paste by measuring the force required to separate the probe from the paste. Times between printing and probe placement are progressively increased to simulate variables in a manufacturing process. The tackiness of the solder paste is affected by its environment in terms of both temperature and humidity. Incorrect tackiness can give rise to process defects such as tombstoning, missing component, misalignment etc. High speed component placement puts high demand on correct tackiness to better hold the components. Incorrect tackiness can lead to printing problems during the application process. Examples of missing component due to poor paste tack strength during high speed placement are shown in Figure 1. Examples of component movement during placement cycle due to poor tack strength are shown in Figure 2. Figure 3 illustrates typical results. 1 SMART Group

2 Figure 1 - missing component 2 Figure 2 - component misalignment 2 Figure 3 - tack testing comparisons 1 Table 1: List of defects, test methods, and the time/cost implications to the assembly process. 2 SMART Group

3 3.2 Solder Balling Test This test is carried out to determine the reflow properties of the solder paste. The ability of the pre-alloyed solder particles in the paste to reflow into a sphere on a non-wettable substrate is determined under defined test conditions. Solder-balling is a common occurrence that can lead to circuit failure in the field. Figures 4 and 5 show examples of this problem. (More information on this topic can be found in IPC-A610) Figure 6 Solder balling test on ceramic tile 1 A solder paste that is poor condition can result in solder balls, often microscopic in size. These present a potential hazard to circuit reliability as they may dislodge and bridge to adjacent conductors. Cleaning after processing can largely offset such a problem, however most circuit assembly processes are conducted without cleaning. Figure 6 shows the test being performed. Figure 7 shows a method employed to compare the results using a computerised camera system. Figure 7 - Solder paste testing for solder balling Slump This test determines both the horizontal and vertical slump characteristics of the solder paste. Solder paste that slumps prior to reflow can lead to unwanted bridging between adjacent component pads. Such influences are difficult to rework on ultra fine pitch components. Figure 4 Solder balling on resistor network 2 Examples of bad slump can be seen in Figures 8 & 9. Figure 10 shows a method employed to compare the results using a computerised camera system Figure 5 Solder balling seen on BGA during x-ray inspection 2 Figure 8 Cooper plate after testing 2 3 SMART Group

4 Figure 9 Slump of paste on a board assembly prior to final reflow 2 Figure 11 - poor wetting on solder pad 2 Figure 10 - comparing slump tests Wetting or Spreading This test method will give an indication of activity of wave solder fluxes, core solder fluxes, and solder paste. Poor wetting or spreading leads to unstable operations on the assemblies and increases defect levels. This increases rework and can adversely affect circuit reliability. Examples of poor wetting or spreading are shown in Figures 11 & 12. Figure 13 shows the test being performed. 4 Figure 12 Insufficient solder paste wetting on component termination 2 SMART Group

5 Malcom Instruments Corp Industrial Blvd. Hayward, CA Phone: Sharemate Technologies 2F, No52 Joutz Street Taipei, Taiwan R.O.C. Phone: +886 (0) Figure 13 - spread testing 1 4 Recommended Test Procedures This comprises of 5 individual tests: Test 1 Slump Test Test 2 Solder Ball Test Test 3 Tack Test Test 4 Wetting Test Test 5 Spread Test Images are courtesy of Gen3 Systems Limited 2 Images are courtesy of ASKbobwillis.com 5 Test Equipment Sources The equipment sources described below represent those currently known to the industry. Users of this document are urged to submit additional source names as they become available, so that this list can be kept as current as possible. This list is in alphabetical order: AMETEK 8600 Somerset Drive Largo, FL USA Phone: (800) chatillon.fl-lar@ametek.com Ascentech LLC Chester, Connecticut USA Phone: (860) rallinson@snet.net Gen3 Systems Limited B2 Armstrong Mall Southwood Business Park Farnborough GU14 0NR UK Phone: sales@gen3systems.com 5 SMART Group

6 Standard Improvement Form SG PCT 01 The purpose of this form to to provide the Technical Committee of SMART with input from the industry regarding usage of the subject standard. Individuals or companies are invited to submit comments to SMART. All comments will be collected and dispearsed to the approproate committee(s). If you can provide input then please complete this form and return to: SMART Group 94 Easton Street High Wycombe Bucks United Kingdom HP11 1LT Fax 44 (0) I recommend changes to the following: Requirement, paragraph number... Test Method number, paragraph number.. The referred paragraph number has proven to be: Unclear Too Rigid In Error Other.. 2. Recommendations for correction: Other Suggestions for document improved: Submitted by: Name Telephone Company Address City Date 6 SMART Group

7 4.2.1 SMART GROUP STANDARD Control of Solder Paste used in Electronic Assembly Process Slump Test Date: 07/09 Test 1 Slump Test This procedure determines vertical and horizontal slump for solder pastes. 1 Test Specimen A standard specimen shall be an alumina substrate measuring 76mm x 25mm, minimum 0.8 mm thick. 2 Equipment/Apparatus Stencils - IPC-A-21, IPC-A-20 Steel Squeegee Hot plate or Oven capable of controlled heating to C Microscope 10x magnification 3 Procedure 3.1 Paste Conditioning Before conducting any of these tests, it is important to prepare the solder paste properly The solder paste must be allowed to achieve ambient conditions that shall be nominally 25 0 C ± 5 0 C and 50% RH ± 10% for 8 hours or in accordance with the manufacturer s data sheet Open the supply container(s); remove any internal cover, scrape off paste adhering to the lid(s), internal covers, and the container walls; and add this material to the paste in the supply container(s) Using a spatula, stir the paste gently for 1 to 2 minutes to homogenize it; taking care to avoid the introduction of air If necessary, gently transfer the paste to a test container of sufficient volume, taking care to avoid the introduction of air. 3.2 Preparation Specimen preparation using appropriate stencil pattern IPC-A-21 or IPC-A-20. (Figures 1 & 1A & 2 & 2A) Deposit solder paste patterns on 2 substrates for each stencil pattern. The printed pattern shall be uniform in thickness with no solder particles separated from the pads. One test specimen shall be marked as specimen #1 and one specimen as #2. 4 Test 4.1 Tin Lead Alloy The 2 specimens shall be stored for 10 to 20 minutes at room temperature conditions (25 C ±5 C and 50% relative humidity ±10%) and specimen #1 examined for slump. Specimen #2 shall be heated using the hot plate or oven to 150 C ±10 C for 10 to 15 minutes, cooled to ambient and examined for slump. 4.2 Lead-Free Alloy The 2 specimens shall be stored for 10 to 20 minutes at room temperature conditions (25 C ±5 C and 50% relative humidity ±10%) and specimen #1 examined for slump. Specimen #2 shall be heated using the hot plate or oven to 180 C ±10 C for 10 to 15 minutes, cooled to ambient and examined for slump. 5 Examination The examination of each of the specimens shall be conducted using a 10x magnification microscope. 6 Evaluation Enter data in Table 1 and/or Table 2 by entering spacings which have bridged with a suitable check mark. 7 Accept / Reject Criteria AABUS (As Agreed Between User and Supplier) Notes: This test should be carried out on each new batch of solder paste as part of incoming inspection. The test should then be repeated on a sample prior to the paste being put into production. It will be useful to compare test results over a number of batches and over your maximum and minimum storage time. This will permit the user to establish the maximum or minimum variation permitted.

8 4.2.1 Slump Test Date: 07/09 (Tables and Figures are by courtesy of IPC) Figure 1A Figure 2A

9 4.2.1 Slump Test Date: 07/09

10 4.2.2 SMART GROUP STANDARD Control of Solder Paste used in Electronic Assembly Process Solder Ball Test Date: 07/09 Test 2 Solder Ball Test This test is carried out to determine the reflow properties of the solder paste and the ability of the pre-alloyed solder particles in the paste to reflow into a sphere on a non-wettable substrate. 1 Test Specimen A standard specimen shall be an alumina substrate measuring 76mm x 25mm, minimum 0.8 mm thick. 2 Equipment/Apparatus Stencil for Type 1-4 provided with at least 3 round holes of 6.5mm diameter apertures with a minimum distance between centers of 10mm. (Figure 2 and Table 2A) Stencil for Type 5-6 provided with at least 3 round holes of 1.5mm diameter apertures with a minimum distance between centers of 10mm. (Figure 3 and Table 2B) Spatula Flat hot plate capable of controlled heating to C Magnifying glass with a 10x magnification. 3 Procedure 3.1 Paste Conditioning Before conducting any of these tests, it is important to prepare the solder paste properly: The solder paste must be allowed to achieve ambient conditions that shall be nominally 25 0 C ± 5 0 C and 50% RH ± 10% for 8 hours or in accordance with the manufacturer s data sheet Open the supply container(s); remove any internal cover, scrape off paste adhering to the lid(s), internal covers, and the container walls; and add this material to the paste in the supply container(s) Using a spatula, stir the paste gently for 1 to 2 minutes to homogenize it; taking care to avoid the introduction of air If necessary, gently transfer the paste to a test container of sufficient volume, taking care to avoid the introduction of air. 3.2 Preparation Prepare two test specimens with either/or both stencils listed above Deposit solder paste onto a specimen using the appropriate stencil pattern The printed pattern shall comprise at least 3 circular deposits that shall be 6.5mm in diameter or 1.5 mm in diameter and ensuring that the printed pattern is uniform in thickness Mark the test specimen in a suitable manner to identify the sample Set the temperature of the hot plate to 25 0 C ±3 0 C above the liquidus / melting point of the solder alloy. 4 Test 4.1 Test one specimen within 15 ±5 minutes after placement of solder paste on test specimen. 4.2 Test the second specimen 4 hours ±15 minutes after placement of solder paste on test coupon. (Storage for 4 hours shall be at room temperature conditions that is 25 C ±3 C and 50 ±10% RH.) 4.3 Place the substrate on the hot plate. As soon as the solder has melted, withdraw the substrate from the hot plate maintaining a horizontal position. The reflow shall occur within 20 seconds after the specimen is placed in contact with the hot plate. 4.4 For lead free solder paste, reflow shall occur within 30 seconds after the specimen is placed in contact with the hot plate. 5 Evaluation Examine the reflowed specimens under 10x magnification. Solder ball size and number should be compared with Figure 1. Record the degree of reflow in comparison with Figure 1 for the 6.5 cm and 1.5 cm acceptance/reject conditions, respectively.

11 4.2.2 Solder Ball Test Date: 07/09

12 4.2.2 Solder Ball Test Date: 07/09 Figure 2 Solder Ball Stencil 6.5 Figure 3 Solder Ball Stencil 1.5 Type None Larger Than Less Than 1% Larger Than 150 µm 80% Minimum Between 10% Maximum Less Than µm µm 2 80 µm 75 µm µm 3 50 µm 45 µm µm 4 40 µm 38 µm µm Table 2A % of Sample by Weight Nominal Size Type None Larger Than Less Than 1% Larger Than 80% Minimum Between 20 µm 20 µm 20 µm 20 µm 10% Maximum Less Than 5 30 µm 25 µm µm 15 µm 6 20 µm 15 µm 15-5 µm 5 µm Table 2B % of Sample by Weight Nominal Size

13 4.2.3 SMART GROUP STANDARD Control of Solder Paste used in Electronic Assembly Process Tack Test Date: 07/09 Test 3 Tack Test This test is to determine the ability of a printed pattern of solder paste to retain a probe placed in the solder paste by measuring the force required to separate the probe from the paste. Times between printing and probe placement are progressively increased to simulate variables in a manufacturing process. 1 Test Specimen A standard specimen shall be an alumina substrate measuring 76m x 25mm, minimum 0.8 mm thick. 2 Equipment/Apparatus A Force Gauge shall be used that is capable of applying a force up to 500g and provided with a reversible speed control from 1mm/min to 5mm/min at an accuracy of ±0.5mm/min. The equipment shall have a stainless steel test probe with a nominal 5mm diameter bottom surface, which is smooth, flat, and aligned parallel to the plane of the subject test specimen. A graphical means of recording the test results. Stencil Steel Squeegee 3 Procedure 3.1 Paste Conditioning Before conducting this test, it is important to prepare the solder paste properly: The solder paste must be allowed to achieve ambient conditions that shall be nominally 25 0 C ± 5 0 C and 50% RH ± 10% for 8 hours or in accordance with the manufacturer s data sheet Open the supply container(s); remove any internal cover, scrape off paste adhering to the lid(s), internal covers, and the container walls; and add this material to the paste in the supply container(s) Using a spatula, stir the paste gently for 1 to 2 minutes to homogenize it; taking care to avoid the introduction of air If necessary, gently transfer the paste to a test container of sufficient volume, taking care to avoid the introduction of air. 3.2 Preparation Deposit solder paste onto a specimen using the appropriate stencil pattern. (Figure 1) The printed pattern shall comprise 6 circular deposits that shall be 6.5mm in diameter and ensuring that the printed pattern is uniform in thickness. Mark the test specimen in a suitable manner to identify the sample and the time after printing when tackiness is to be measured. The prepared samples shall be stored at normal room temperature conditions (25 C ± 2 C and 50 C ± 10% relative humidity (RH)) until evaluated. Note: The samples shall not be stored in an enclosed cabinet or container, which allows the solder paste solvent vapors to saturate the environment surrounding the printed paste, thus preventing natural drying of the material.) 4 Test The prepared specimen shall be securely placed centrally beneath the force gauge probe such that each individual solder paste sample (circular pattern) may be tested and sequentially. Bring the test probe in contact with the printed paste specimen at a rate of 2.5 mm/min. ± 0.5 mm/min. and apply a force of 300 g ± 30 g to the specimen. Within five seconds following application of this force, withdraw the probe from the specimen at a rate of 2.5 mm/min. ± 0.5 mm/min. and record the peak force required to break the contact. Take at least five additional measurements under the same test conditions and average all the readings. Record both the tack force and time following paste printing.

14 4.2.4 SMART GROUP STANDARD Control of Solder Paste used in Electronic Assembly Process Wetting Test Date: 07/09 Test 4 Wetting Test This test is used to determine the ability of a solder paste to wet an oxidized copper surface and to qualitatively examine the amount of spatter of the solder paste during reflow. 1 Test Specimen A standard specimen shall be an epoxy glass (FR4) substrate measuring 76mm x 25mm, minimum 0.8 mm thick and printed with 3 square shaped copper pads centrally located measuring 10mm x 10mm. 2 Equipment/Materials/Apparatus Flat hot plate capable of controlled heating to C Magnifying glass with 10x magnification Specimen tongs Isopropyl alcohol Solvent for residual flux removal Stencil 76mm x 25mm x 0.2mm provided with at least 3 round holes (apertures) 6.5mm diameter with a center to centre separation of at least 10mm (Figure 1) 3 Procedure 3.1 Paste Conditioning Before conducting any of these tests, it is important to prepare the solder paste properly: The solder paste must be allowed to achieve ambient conditions that shall be nominally 25 0 C ± 5 0 C and 50% RH ± 10% for 8 hours or in accordance with the manufacturers data sheet Open the supply container(s); remove any internal cover, scrape off paste adhering to the lid(s), internal covers, and the container walls; and add this material to the paste in the supply container(s) Using a spatula, stir the paste gently for 1 to 2 minutes to homogenize it; taking care to avoid the introduction of air If necessary, gently transfer the paste to a test container of sufficient volume, taking care to avoid the introduction of air. 3.2 Deposit solder paste patterns onto the specimens ensuring that the printed pattern is uniform in thickness. 3.3 Set the temperature of the hot plate to 25 0 C ± 3 0 C above the liquidus/melting point of the solder alloy. 4 Test 4.1 Place the substrate onto the hot plate and heat to peak reflow temperature according to the alloy type. As soon as the solder paste has melted, withdraw the substrate from the hot plate maintaining a horizontal position. The reflow shall occur within 20 seconds after the specimen is placed in contact with the hot plate. After reflow, the residual flux shall be removed with a suitable solvent for residual flux removal. 4.2 For lead free solder paste reflow shall occur within 30 seconds after the specimen is placed in contact with the hot plate. 5 Evaluation When examined visually at 10x, the solder shall have uniformly wet the copper and there should be no evidence of dewetting or non-wetting of the copper and there shall be no solder spatter around the printed dots. Figure 1 Stencil

15 4.2.5 SMART GROUP STANDARD Spread Test Date: 07/09 Control of Solder Paste used in Electronic Assembly Process Process Control Testing of Solder Paste Test 5 Spread Test This test is used to determine the ability of a solder paste to wet a copper surface and to quantitatively examine the solder activation by the ratio of spreading of the solder paste during reflow. 1 Test Specimen 1.2 The standard specimen shall be an epoxy glass(fr4) substrate measuring 76mm x 25mm, minimum 0.8 mm thick and printed with 3 square shaped gold pads central located measuring 10mm x 10mm. 2 Equipment/Materials/Apparatus 2.1 Flat hot plate capable of controlled heating to C 2.2 Magnifier with minimum of 10x magnification 2.3 Stencil 76mm x 25mm x 0.2mm provided with at least 3 round holes (apertures) 6.5mm diameter with a center to centre separation of at least 10 mm (Figure 1) 2.4 Camera 3 Procedure 3.1 Prepare a minimum of 2 specimens. 3.2 Using the stencil described in 2.3 print the solder paste patterns onto the 2 specimens ensuring that the printed pattern is uniform in thickness. 3.3 Number the specimens 1 & Set the temperature of the hot plate to 25 0 C ±3 0 C above the liquidus / melting point of the solder alloy. 3.5 Set up the camera horizontal position. The reflow shall occur within 20 seconds after the specimen is placed in contact with the hot plate. 4.4 For Lead Free solder paste, reflow shall occur within 30 seconds after the specimen is placed in contact with the hot plate. 4.5 Take a second photograph of specimen 2. 5 Evaluation 5.1 When examined visually at 10x, the solder shall have uniformly wet the substrate and there should be no evidence of de-wetting or non-wetting of the copper pads and there shall be no solder spatter around the printed dots. 5.2 Compare Specimen 1 with Specimen 2 noting the different amount of spread between them. The spread difference between each specimen should be >10% 5.3 Compare the 2 photographs that will show the area covered before and after reflow. The difference should be >10% 6 Accept/Reject Criteria Spread ratio to be minimum 10% or AABUS (As Agreed Between User and Supplier) NOTES: Commercial equipment is available that has software that permits the Test & Evaluation to be done automatically. See figures 1 and 2. 4 Test 4.1 Photograph the printed specimens using the camera 4.2 Retain Specimen 1 unprocessed. 4.3 Place the second specimen onto the hot plate and heat to peak reflow temperature according to the alloy type. As soon as the solder paste has melted, withdraw the substrate from the hot plate maintaining a

16 4.2.5 Spread Test Date: 07/09 Figure 1 Figure 2