JESD22A121A

JEDEC STANDARD Test Method for Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes JESD22-A121A (Revision of JESD22-A121.01, December 2005) JULY 2008 JEDEC SOLID STATE TECHNOLOGY ASSOCIATION NOTICE JEDEC standards and publications contain material that has been prepared, reviewed, and approved through the JEDEC Board of Directors level and subsequently reviewed and approved by the JEDEC legal counsel. JEDEC standards and publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obtaining with minimum delay the proper product for use by those other than JEDEC members, whether the standard is to be used either domestically or internationally. JEDEC standards and publications are adopted without regard to whether or not their adoption may involve patents or articles, materials, or processes. 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For information, contact: JEDEC Solid State Technology Association 3103 North 10th Street Suite 240 South Arlington, Virginia 22201-2107 or call (703) 907-7559 JEDEC Standard No. 22-A121A -i- Test method A121A (Revision of A121.01) TEST METHOD A121 Test Method For Measuring Whisker Growth On Tin And Tin Alloy Surface Finishes Disclaimer This document is not a qualification standard. It contains a suite of recommended tin whisker growth tests. If these common tests are adopted, then the industry can collect common and comparable data that may improve the understanding of the fundamentals of whisker growth and allows comparisons between technologies. Tests in this document may be changed in the future as a better understanding of the mechanisms causing tin whisker growth is developed. Based on a variety of testing and data review from around the globe, three test conditions have been identified that appear to be suitable for monitoring tin whisker growth. The three test conditions include two isothermal conditions with controlled humidity and a thermal cycling condition. However, these test conditions have not been correlated with longer environmental exposures of components in service. Thus, there is at present no way to quantitatively predict whisker lengths over long time periods based on the lengths measured in the short-term tests described in this document. At the time of writing, the fundamental mechanisms of tin whisker growth are not fully understood and acceleration factors have not been established. However until such time that acceleration factors are determined all acceptance requirements for commercial tin finishes are provided in JEDEC standard JESD201. Certain applications, e.g., military or aerospace, may require additional and/or different tin whisker tests or evaluations. Introduction The predominant terminal finishes on electronic components have been Sn-Pb alloys. As the industry moves toward Pb-free components and assembly processes, the predominant terminal finish materials will be pure Sn and alloys of Sn, including Sn-Bi and Sn-Ag. Pure Sn and Sn-based alloy electrodeposits and solder-dipped finishes may grow tin whiskers, which could electrically short across component terminals or break off the component and degrade the performance of electrical or mechanical parts. JEDEC Standard No. 22-A121A Test method A121A -ii- (Revision of A121.01) JEDEC Standard No. 22-A121A Page 1 Test method A121A (Revision of A121.01) TEST METHOD A121 Test Method For Measuring Whisker Growth On Tin And Tin Alloy Surface Finishes (From JEDEC Board ballot JCB-08-42, formulated under the cognizance of the JC-14.1 Subcommittee on Reliability Test Methods for Packaged Devices.) 1 Scope The methodology presented in this document, see Annex A for process flow, is applicable for studying tin whisker growth from finishes containing a predominance of tin (Sn). This test method may not be sufficient for applications with special requirements, e.g., military or aerospace. Additional requirements may be specified in the appropriate requirements document. The purpose of this standard is to: • Provide an industry-standardized suite of tests for measurement and comparison of whisker propensity for different plating or finish chemistries and processes. • Provide a consistent inspection protocol for tin whisker examination. • Provide a standard reporting format. 2 Normative references JESD22-A104, Standard for Temperature Cycling IPC 7530, Guidelines for Temperature Profiling for Mass Soldering (Reflow & Wave) Processes JESD201, Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes JEDEC Standard No. 22-A121A Page 2 Test method A121A (Revision of A121.01) 3 Terms and definitions 3.1 surface corrosion: A localized corrosion of the silver-colored tin surface finish appearing in an optical microscope as non-reflective dark spots ranging in size from about 25 microns on the longest dimension to the entire terminal. 3.2 whisker: A spontaneous columnar or cylindrical filament, usually of monocrystalline metal, emanating from the surface of a finish. (See Annex C for example pictures of tin whiskers.) NOTE 1 For the purpose of this document, whiskers have the following characteristics: • An aspect ratio (length/width) greater than 2 • Can be kinked, bent, or twisted • Usually have a uniform cross-sectional shape • Typically consist of a single columnar filament that rarely branches • May have striations along the length of the column and/or rings around the circumference of the column • Length of 10 microns or more. Features less than 10 microns may be deemed important for research but are not considered significant for this test method. NOTE 2 Whiskers are not to be confused with dendrites: fern-like growths on the surface of a material which can be formed as a result of electromigration of an ionic species or produced during solidification. (See Annex D for a picture of a typical solidification dendrite.) 3.3 whisker density: The number of whiskers per unit area on a single lead or coupon area. 3.4 whisker growth: Measurable changes in whisker length and/or whisker density after exposure to a whisker test condition for a certain duration or number of cycles. 3.5 whisker length: The straight line distance from the point of emergence of the whisker to the most distant point on the whisker (i.e., the radius of a sphere containing the whisker with its center located at the point of emergence), see Figure 5 3.6 whisker test coupon: A piece of metal of specified size and shape that is plated or dipped with a tin finish for the purpose of measuring the propensity for whisker formation and growth. 4 Apparatus 4.1 Temperature cycling chambers Air to air temperature cycling chamber(s), capable of cycling from -55(+0/-10) °C to +85(+10/-0) °C or from -40(+0/-10) °C to +85(+10/-0) °C. The temperature cycling chamber(s) must be able to satisfy the cycle conditions defined in Table 4. JEDEC Standard No. 22-A121A Page 3 Test method A121A (Revision of A121.01) 4 Apparatus (cont’d) 4.2 Temperature humidity chambers Temperature–humidity (T&H) chambers capable of non-condensing 55 ±3 °C, 85 ±3% RH and 30 ±2 °C, 60 ±3% RH environment. NOTE 1 The elevated temperature–humidity condition of 55 ±3 °C, 85 ±3% RH is close to the condensation point. If water condenses on the tin finish during environmental exposure, the condensed moisture and resulting corrosion may affect the final test results. To prevent condensation in the T & H chamber, the chamber dry-bulb temperature must exceed the wet-bulb temperature at all times by not less than 2.4 °C (or equivalent for electronic sensors). Before opening of the chamber door for loading and unloading, the chamber temperature and humidity should be ramped down sufficiently close to room ambient (recommended within 10 °C and 10% RH) to prevent condensation on the test samples and chamber walls. NOTE 2 During operation, condensation is most likely to occur on the T & H test chamber walls and ceiling; therefore, it is recommended that the test samples be sufficiently shielded from any condensed water that may drip from the chamber ceiling and/or walls onto the samples. NOTE 3 When loading the test samples into the T & H test chamber, the sample temperature must be sufficiently higher than the chamber ambient temperature to avoid condensation on the test samples. It is recommended that the test samples and all sample trays or holders be preheated (to a temperature equal to the test temperature of the T & H test chamber) in a dry-bake oven prior to loading them into the T & H test chamber. NOTE 4 Frequent wet-bulb maintenance is required for proper control of this test condition. 4.3 Optical stereomicroscope (Optional) Optical stereomicroscope with adequate lighting capable of 50X to 150X magnification and capable of detecting whiskers with a minimum length of 10 microns, per Annex B. If tin whiskers are measured with an optical system, then the system must have a stage that is able to move in three dimensions and rotate, such that whiskers can be positioned perpendicular to the viewing direction for measurement. 4.4 Optical microscope (Optional) Optical microscope with adequate lighting capable of 100X to 300X magnification and capable of measuring whiskers with a minimum length of 10 microns, per Annex B. For tin whisker measurements, the optical system must have a stage that is able to move in three dimensions and rotate, such that whiskers can be positioned perpendicular to the viewing direction for measurement. 4.5 Scanning electron microscope Scanning electron microscope (SEM) capable of at least 250X magnification. An SEM fitted with an X-ray detector is recommended for elemental identification. 4.6 Convection reflow oven (Optional) A convection reflow system capable of achieving the reflow profiles of Table 3. JEDEC Standard No. 22-A121A Page 4 Test method A121A (Revision of A121.01) 5 Sample requirements and optional preconditioning For acceptance requirements of tin finishes for commercial use, the relevant test conditions, read points, and durations are given in JEDEC standard JESD201. For evaluating the whisker propensity of tin finishes for other purposes (e.g., scientific study, preliminary evaluations of various platings, etc.), it is recommended that all three conditions defined in Table 4 be used and that the durations given in JESD201, Table 4a for Class 1 and 2 products be used. In addition, each test condition is to be performed independently on separate samples. 5.1 Acceptance requirements: For acceptance requirements of tin finishes for commercial use: Use JEDEC standard JESD201 for the following: • Test conditions • Test samples • Read Points • Durations • Sample size • Test sample preconditioning • Reporting NOTE Coupons are not acceptable for acceptance requirements. 5.2 Scientific studies For evaluating the whisker propensity of tin finishes for other purposes (e.g., scientific study, preliminary evaluations of various platings, etc.), it is recommended that all conditions defined in JESD201 for Class 2 products are used. In addition, each test condition is to be performed independently on separate samples. Test coupons may be used for scientific studies. 5.2.1 Test coupons For comparison purposes, if using coupons, a total inspection area of at least 75 mm2 on at least 3 coupons is required for each test condition. For small coupons, it is recommended that there be sufficient coupons so that the total area inspected adds up to a minimum of 75 mm2, as described in Table 1. JEDEC Standard No. 22-A121A Page 5 Test method A121A (Revision of A121.01) 5.2 Scientific studies (cont’d) 5.2.1 Test coupons (Cont’d) Table 1 — Test Sample Size Requirements per Precondition Treatment for Coupons Sample Type Finished Area [1] Minimum Number of Samples Minimum Total Inspection Area for Screening Inspection Minimum Inspection Surface Area per Sample for Screening Inspection Minimum Total Number of Inspection Areas for Screening Inspection[2] Detailed Inspection (Number of areas per Readpoint)[2] Small Coupons < 25 mm 2 3 75 mm2 Top and two sides of coupon 75 mm2 ÷ (Plated area on top and 2 sides of coupon) 9 Large Coupons ≥ 25 mm 2 3 75 mm2 Top and two sides up to a total of 25 mm2 3 9 NOTE 1 See Figure 4 for detailed definition of the plated/finished area for inspection. NOTE 2 Each area for detailed inspection shall be a minimum of 1.7mm2. The same components or coupons evaluated for each test condition may be evaluated at all sequential readouts, including the final readout. Hence, to study a single finish, a minimum of 9 coupon samples are required to complete the three test conditions. Alternatively, the test may be started with a sufficient quantity of test samples equaling the minimum number of samples required per readout times the number of readouts. If a more accurate determination of growth kinetics is needed, it is recommended that the same test samples be used for each sequential readout instead of using re-populated samples. 5.3 Optional test sample preconditioning Table 2 lists optional test sample preconditioning treatments that are recommended prior to all subsequent Sn whisker growth tests. If the test method described in this standard is used as part of a tin finish acceptance evaluation, refer to JEDEC standard JESD201 for the required precondition treatments. JEDEC Standard No. 22-A121A Page 6 Test method A121A (Revision of A121.01) 5.3 Optional test sample preconditioning (cont’d) Table 2 — Optional Preconditioning Treatments for Tin Whisker Test Samples Condition Preconditioning Temperature Exposure Thermal Profile Exposure Use Guidelines A None Normal ambient exposure Intended to test for whisker growth under ambient temperature/humidity storage. B Room temperature storage for a minimum of 4 weeks after the finish is applied 15 -30 °C 30 – 80% RH Intended for samples without under-plating or post bake mitigation before exposure to high temperature/humidity storage, temperature cycling or preconditioning per conditions C or D. C Sn-Pb Temperature Preconditioning Sn-Pb profile per Table 3 Intended to test for whisker growth after thermal exposure to Sn-Pb SMT assembly temperatures (backward compatibility). D Pb-free Temperature Preconditioning Pb-free profile per Table 3 Intended to test for whisker growth after thermal exposure to Pb-free SMT assembly temperatures (Pb-free compatibility). 5.3.1 Optional test sample preconditioning profiles NOTE: J-STD-020 UTILIZES A PACKAGE TEMPERATURE REFERENCE; THE PROFILES HEREIN UTILIZE A LEAD OR COUPON TEMPERATURE REFERENCE. Test sample preconditioning profile information is shown in Table 3 and Figure 1. All profile criteria reference either the lead or solder joint temperature for components or the surface temperature for coupons. For the profile and the preconditioning process itself, it is recommended that non- metallized carriers or printed circuit boards be used to hold the samples during the reflow process. For components with leads, the orientation of the component shall be in the “live bug” configuration (i.e., leads down touching the carrier or board). JEDEC Standard No. 22-A121A Page 7 Test method A121A (Revision of A121.01) 5.3 Optional test sample preconditioning (cont’d) 5.3.1 Optional test sample preconditioning profiles (cont’d) Table 3 — Optional Preconditioning Reflow Profiles [1] Profile Feature Sn-Pb Profile Pb-Free Profile Average ramp-up rate (Tsmax to Tpeak) 3 °C/second max. 3 °C/second max. Preheat: - Temperature Min (Tsmin) - Temperature Max (Tsmax) - Time (Tsmin to Tsmax) (ts) 100 °C 150 °C 60–120 seconds 150 °C 200 °C 60–120 seconds Time maintained above: - Temperature (TL) - Time (t L) 183 °C 60–120 seconds 217 °C 60–120 seconds Lead or Solder Joint Temperature (Tpeak) 200–220 °C [2] 245–260 °C [3] Average ramp-down Rate (Tpeak to Tsmax) 6 °C/second max. 6 °C/second max. Time 25°C to Peak Temperature 6 minutes max. 8 minutes max. NOTE 1 All temperatures refer to lead or solder joint temperature for components or the surface temperature for coupons. NOTE 2 Maximum temperature of 220 °C ensures that the finish is not melted (i.e., melting point of pure Sn is 232 °C). NOTE 3 Minimum temperature of 245 °C ensures that the finish is melted. 25 Time Te m pe ra tu re Tpeak T L tL Time 25 oC to Peak tS Ts min Preheat AreaTs max Ramp Range Figure 1 — Optional Preconditioning Reflow Profile JEDEC Standard No. 22-A121A Page 8 Test method A121A (Revision of A121.01) 6 Whisker inspection, length measurement and test conditions The whisker inspection procedure includes three parts: (1) the initial pre-test inspection, (2) the screening inspection, and (3) the detailed inspection. The initial inspection should be performed once before the test samples are exposed to any test condition. The screening inspection should be performed at each readout. If whiskers are detected in the screening inspection, then the detailed inspection should be performed at that readout. The whisker inspections can be performed using either an SEM or a validated optical system meeting the conditions as outlined in Annex B. 6.1 Handling When handling test samples, care must be taken to avoid contact with the finish which may result in the detachment of whiskers. For SEM inspection, a conduct