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An Investigation of Whisker Growth on Tin-Coated Wire and Braid
September 5, 2012 |Estimated reading time: 12 minutes
Editor's Note: This article originally appeared in the August 2012 issue of SMT Magazine.AbstractPure tin is a common finish for copper hook up wire, coaxial cable, ground braid and harness assemblies used on electronic assemblies. Historically there have been fewer reports of whisker growth on tin-coated copper wire than on other types of tin-coated parts. This paper presents data from humidity-temperature conditioning and electron microscopy inspection of tin-coated wire specimens of various size and age. After 4,000 hours of 85°C/85%RH conditioning, some of the wires showed tin whisker growth, but the growth was very sparse with near zero density distribution and whisker lengths were typically less than 10 microns. The data indicate very low risk for whisker growth on tin-coated copper wire, braid and cable.IntroductionElectrodeposited and dipped layers of tin can spontaneously generate hair-like conductive surface growths known as tin whiskers. Whiskers develop in any environment: Ground or vacuum/space, wet or dry, applied power versus no power. The time period before whiskers start to grow from a tin surface, known as the incubation period, can vary between minutes and decades.This paper continues the evaluation of whisker growth on tin-coated wire and cable. Thirteen samples of tin-coated wire and braid, manufactured between 1965 and 2009, from five different suppliers, were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) to evaluate the material composition, size, and density of whiskers. The samples were subjected to JESD201 testing in an attempt to accelerate whisker growth.Specifications and Literature DataPure tin is a common coating on bus wire, stranded wire, coaxial cable, and ground braid products. Several commercial and military specifications call out tin coating, including MIL-DTL-17, AA-59551 (superseding QQ-W-343), NEMA-WC27500 (superseding MIL-DTL-27500), and SAE-AS22759 (superseding MIL-W-22759).Literature data on whisker growth on tin-plated wire and cable were reviewed in Reference 3. Analysis of whisker growth on tin-coated wire and cable dates back to the 1950s; however, there is no mention of significant whisker growth on these specific types of tin-plated product. Several references suggest that there is little to no risk for substantial whisker growth on tin-coated wire and cable. In 1992, MIL-STD-1547 (“Electronic Parts, Materials and Processes for Space and Launch Vehicles rev B”) was revised to include an exemption for tin-plated drawn wire products, such as cables, shielding, and ground straps. All other types of tin-plated components were prohibited from use on space and launch vehicles. A recent update to this standard (Aerospace Corporation TOR-2006-8583-5236, dated 2006) also allowed for the use of tin-plated wire in high-reliability applications.ProceduresTest SpecimensA total of nine wires and four braided cables were selected for the investigation. The age, storage conditions, and supplier data varied widely. Aluminum specimen holders were fabricated for holding the test specimens to minimize the rigors of handling and to aid in tin whisker inspection/examinations. Figure 1 illustrates some of the test specimens mounted in the specimen holders. Table 1 and Table 2 list the wire and cable basic data.Figure 1: Examples of test specimens in aluminum specimen holders ready for inspection.
Table 1: Investigation wire data.
Table 2: Investigation braided cable data.
JESD201 Conditioning The JEDEC JESD201 specification is widely used by the component fabrication industry. The JESD201 specification defines the test conditioning parameters, inspection and tin whisker dimensional acceptability requirements. The JESD201 specification is not universally accepted across the Defense electronics community. Some users consider the results of the testing to only be appropriate for characterizing a specific plating bath or component lot evaluation for tin whisker susceptibility. However, many others in the industry feel that the specification requirements provide an industry-consistent indicator of component fabricator process capability and process consistency. Table 3 illustrates the JESD201 maximum tin whisker length criteria. The specification utilizes three conditioning parameter sets: (1) 4,000 hours of ambient (30°C/65RH) conditioning; (2) 1,500 thermal cycles (-55°C - +85°C thermal range); (3) 4,000 hours of 55°C/85RH high-temperature/high-humidity conditioning.Table 3: JESD201 maximum tin whisker length criteria [1].
Testing Parameters A modified JESD201 test parameter matrix was followed for the wire and cable investigation. The following test parameter matrix was used:
- No JESD201 process “preconditioning” conducted on component test specimens;
- Optical and SEM conducted for each sampling period;
- Optical lighting configuration used was in accordance with the NASA Goddard recommendations [2];
- Sampling period consisted of 500 hour intervals;
- Specific and random surface areas/regions of each wire/cable was inspected for tin whiskers for each sampling period; and
- Component conditioning parameters used:
- High temperature/humidity: 4.000 hours, 85°C /85% RH, non-condensing environment.
Figure 2 illustrates the test chambers used for the conditioning portion of the investigation.Figure 2: Conditioning chambers: High temperature/humidity.
Inspection Criteria Optical and SEM inspection was conducted for each sampling period. A sampling period consisted of 500-hour intervals. An optical lighting configuration was used in accordance with the NASA Goddard recommendations [2] and all surface areas of each test specimen were inspected for tin whiskers. Optical inspection was conducted using magnifications ranging from 10X to 200X and SEM was conducted using magnifications ranging from 100X to 5,000X magnification.
Test Results Summaries of each inspection interval are documented in the following sections. It should be noted that the test vehicles were not cleaned during the entire conditioning sequence duration to prevent the accidental removal of tin whiskers. Extraneous debris did accumulate over time making the inspection efforts more difficult.
Zero-Hour Inspection Interval Test specimen 2 was the only specimen that had tin whiskers observed at the zero-hour inspection interval. The tin whiskers on test specimen 2 were very small with lengths less than 10µm with no measureable area density. Figure 3 illustrates the typical tin whisker found on specimen 2. No tin whiskers were found on the other test specimens during the inspection interval. There was evidence of external debris, scratches, nicks and other handling type indications recorded. Figure 4 illustrates those observations for test specimen 5. Figure 3: Typical tin whisker observed on test specimen 2 during zero-hour inspection interval.
Figure 4: Typical no tin whisker observation (test specimen 5) during zero-hour inspection interval.
500-Hour Inspection Interval The 500-hour inspection interval yielded the documentation of tin whiskers on three other test specimens--test specimen’s 4, 14, and 15. The tin whiskers observed were very small (10µm or less in length) and were single isolated instances (see Figures 5, 6, and 7). The tin whiskers observed on test specimen 2 at the zero hour inspection interval did not appear to grow/change. A single instance of a tin whisker in the 10 to 20µm length was observed on test specimen 2 (Figure 8). Figure 5: Typical tin whisker observation (test specimen 4) during 500-hour inspection interval.
Figure 6: Typical tin whisker observation (test specimen 14) during 500-hour inspection interval.
Figure 7: Typical tin whisker observation (test specimen 15) during 500-hour inspection interval.
Figure 8: Typical tin whisker observation (test specimen 2) during 500-hour inspection interval.
1,000-Hour Inspection Interval
The 1,000-hour inspection interval observations were a mixed bag of information. The tin whiskers observed on testspecimens 4 and 15 had no changes in length or group density. The tin whiskers previously observed on test specimen 2did not appear to grow/change (Figure 9). Several instances of short, contorted tin whisker clusters were observed on testspecimen 14 (Figure 10). Tin whiskers were observed for the first time on test specimen 19. The tin whiskers observedwere very small (10µm or less in length) and were in isolated, contorted clusters (Figure 11). No tin whisker occurrenceswere observed on the other test specimens.
Figure 9: Typical tin whisker observation (test specimen 2) during 1,000-hour inspection interval.
Figure 10: Typical tin whisker observation (test specimen 14) during 1,000-hour inspection interval.
Figure 11: Typical tin whisker observation (test specimen 19) during 1,000-hour inspection interval.
1,500-Hour Inspection Interval
The 1,500-hour inspection interval observations again were a variety of changes. The tin whiskers observed on test specimens 15 and 19 had no changes in length or area density. The tin whisker clusters previously observed on test specimen 2, 4, and 14 at 1,000 hours had some density growth with individual cluster groups becoming more populated (Figured 12, 13, and 16). Tin whiskers were observed for the first time on test specimen 11 and 12. The tin whiskers observed on both of these test specimens were very small (10µm or less in length) and were isolated with little group density (Figures 14 and 15). No tin whisker occurrences were observed on the other test specimens.
Figure 12: Typical tin whisker cluster observation (test specimen 2) during 1,500-hour inspection interval.
Figure 13: Typical tin whisker cluster observation (test specimen 4) during 1,500-hour inspection interval.Figure 14: Typical tin whisker cluster observation (test specimen 11) during 1,500-hour inspection interval.
Figure 15: Typical tin whisker cluster observation (test specimen 12) during 1,500-hour inspection interval.
Figure 16: Typical tin whisker cluster observation (test specimen 14) during 1,500-hour inspection interval.
2,000-Hour Inspection Interval
The biggest trend of the 2,000-hour inspection interval was the lack of tin whisker growth activity. The tin whiskers observed previously on test specimens 2, 4, 11, 15, and 19 had no changes in length or group density. A single occurrence of a large, clumpy shaped tin whisker was documented on test specimen 12 (Figure 17). The most interesting observations of this inspection interval were the tin whiskers clusters observed on test specimen 14 were approximately 10µm increased lengths (Figure 18). The tin whiskers were measured in the 10-30 µm and were severely contorted. No tin whisker occurrences were observed on the other test specimens.
Figure 17: Large tin whisker observation (test specimen 12) during 2,000-hour inspection interval.
Figure 18: Large tin whisker observation (test specimen 14) during 2,000-hour inspection interval.
2,500-Hour Inspection Interval
The biggest trend of the 2,500-hour inspection interval was the complete stagnation of tin whisker growth activity. No changes in the tin whisker previously observed on test specimens or any occurrences were observed on the other test specimens that had yet to register a tin whisker observation.
3,000-Hour Inspection Interval
The 3,000-hour inspection interval observations documented only one test specimen change from the previous 2,500-inspection interval observations as tin whisker growth remained unchanged. The tin whiskers documented on specimen 11 at the 1,500-hour inspection interval began to form small cluster groups in a similar fashion as observed on test specimens 2 and 4 at earlier inspection intervals. Figure 19 illustrates the tin whisker clusters observed on test specimen 11 during the inspection.
Figure 19: Tin whisker cluster observation (test specimen 11) during 3,000-hour inspection interval.
3,500-Hour Inspection Interval
The 3,500-hour inspection interval was not conducted due to SEM equipment issues. The time duration between the completion of a 500-hour conditioning interval and the initiation of the subsequent 500 hour inspection interval had been maintained up to this point of the investigation. The amount of time required to complete an inspection interval was three to four weeks. It was decided that it would be better to skip the 3,500-hour inspection internal rather than introduce a new exposure variable especially with only one final inspection interval (4,000 hours) remaining in the test.
4,000-Hour Inspection Interval
The 4,000-hour inspection interval observations documented no changes in the test specimen tin whisker growth activity as they remained unchanged. Test specimen 14 was observed to have severe surface morphology changes occurring which was not observed on any other test specimens (Figure 20). The authors initially suspected that the surface morphology change might be an indication of tin pest, but in-depth SEM analysis of the test specimen 14 revealed that that plating was delaminating/flaking from the wire surface.
Figure 20: Severe surface morphology change observation (test specimen 14) during 4,000-hour inspection interval.
Discussion
A rigorous investigation of these wires and cables conducted in 2009 included SEM and metallographic analysis found no tin whisker instances on any of the samples despite a wide variety of factors: Ten suppliers, up to 44 years of age, and uncontrolled handling/storage procedures [3]. The current investigation was able to cause minor tin whisker initiation and growth using a high-temperature/high-humidity conditioning methodology. The tin whiskers produced during the 4,000 hours of conditioning can be characterized as follows:
- Morphology: single crystal, isolated, many instances of severely contorted;
- Length: small, typically less than 10µm in length, with some isolated instances of 10 to 30µm lengths being observed;
- Density: Some instances of small, isolated tin whisker converting to small isolated cluster groups; and
- Growth: Random, non-linear, erratic, in a large number of test specimens nonexistent.
Table 4 illustrates a tin whisker activity summary over the 4,000 hours of conditioning for the test specimens used in the test. Of the 13 test specimens included in the investigation, only test specimen 14 was documented to have some segment of continuous tin whisker activity.
Table 4: Investigation of tin whisker initiation/growth activity during testing.
No apparent correlation existed between tin whisker activity and the thickness of the tin-coating or intermetallic compound thickness. Three of the nine wire samples, and three of four braid samples, showed no tin whiskers after the inspection. Five out of five samples manufactured before 1982 showed whisker activity. Two out of eight samples manufactured after 1982 showed whisker activity. None of the tin whiskers observed exceeded 30µm in length which is below the JESD201 maximum tin whisker length 40µm value for Class 2 high-temperature/high-humidity conditioning. The typical tin whisker length measured for all samples when tin whiskers were observed was 10µm.
Conclusions
The tin whiskers observed and documented on a small sample size of wire and cable specimens, subjected to the high-temperature/high-humidity conditioning, were characterized as low risk potential for causing product issues.
Whisker suppression in tin electroplated wires is most likely due to a combination of relatively thin plating, geometry (no sharp edges) and post-plating processing operations that may relieve stresses in the tin layer. None of these three factors appear to be a cure-all for whisker growth, but the combination of factors appears to be effective in suppressing whisker growth. In hot tin dipped and electroplated wires with thin (type S) tin coating, whisker suppression is likely enhanced due to copper-tin intermetallic compound growth that consumes the tin layer a short time after manufacture.
It can be concluded that electrodeposited and redrawn tin-plated wire, braid and cable products can be used on high-reliability hardware with no further steps necessary to mitigate tin whisker growth.
Acknowledgements
The authors would like to thank Brian Smith, Rockwell Collins, for SEM fixture assistance.
References:
1. JEDEC Standard, JESD201, “Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes,” March, 2006.2. NASA tin whisker information.3. T. Lesniewski, “Assessment of Whisker Growth from Tin Coated Wire and Cable,” 2009 SMTAI Conference, pp. 619-627. Tom Lesniewski has worked for 14 years as a parts, materials, and processes engineer with TRW Space & Electronics Group and Northrop Grumman Corporation supporting, design and manufacture of high-reliability electronics for military avionics and spacecraft. He may be reached at tom.lesniewski@ngc.com. David D. Hillman is a metallurgical engineer in the Advanced Operations Engineering Department of Rockwell Collins Inc. in Cedar Rapids, Iowa. Hillman serves as a consultant to manufacturing on material and processing problems and served as a subject matter expert (SME) for the Lead-free Manhattan Project in 2009. He serves as the Chairman of the IPC JSTD-002 Solderability committee and as a member of the SMTA Journal and Soldering & Surface Mount Technology Journal Technical Paper Review committees. He is a member of ASM, TMS, SMTA, and IPC. He may be reached at ddhillma@rockwellcollins.com.