The use of electroplated tin on component leads introduces a finite, long-term reliability risk through its propensity to grow tin whiskers – single crystal, needle-like protrusions which grow from plated surfaces. These metallic, highly conductive needles are only a few microns in diameter, but they can grow to lengths of several millimeters and cause electrical shorts between adjacent, fine-pitch conductor leads.

Component manufacturers, as well as the technical community, recognize the risks posed by tin whiskers and have initiated experimental studies to investigate the tin whisker growth mechanism and potential mitigation strategies. In spite of intense technical activity over the past few years, the exact growth mechanism is still not fully understood, so the acceleration factors remain undefined.

With the approaching deadline for RoHS conversion, component manufacturers have begun phasing in tin plating without fully addressing the tin whisker risk. Since tin whiskers do not grow immediately but over a number of years, each segment of the electronics industry views the risk differently. The consumer electronics industry, with its shorter product life cycles, does not view tin whiskers as a major reliability risk. But component users in the defense and aerospace electronics community, whose electronics are mission critical, view tin whiskers as a serious risk.

Current data from numerous studies point to different factors and causes, but there is no scientific consensus on tin whisker formation and growth fundamentals.2 It is known that room temperature conditions are most conducive to tin whisker growth, and the most commonly cited driving force for whisker formation is a buildup of compressive stresses in the plated tin layer. These compressive stresses originate from intermetallic growth, grain size, surface damage, and environmental stresses. Based on this understanding, an assortment of mitigation strategies are being implemented: nickel underplating, low stress tin finishes, solder dipping, and annealing. Although these mitigation strategies are fundamentally sound, there is little experimental proof of their long-term effectiveness. In the absence of a fundamental growth mechanism for tin whiskers, there are no verified acceleration factors with which to accelerate the phenomenon for testing purposes. The International Electronics Manufacturing Initiative (iNemi), an industry-led consortium, has proposed a test to promote and accelerate whisker growth, but even this test’s validity remains unproven. For now, to verify the effectiveness of any strategy on a component’s 25-year life, researchers will have to wait 25 years.

To mitigate whisker-related risks, iNEMI, the Aerospace Industries Association (AIA), and Government Electronics and Information Technology Association (GEIA) have proposed draft guidelines, recommending restrictions on the use of tin plated components for Level IV and Level V electronic assemblies. Continued dependence upon commercial off-the-shelf (COTS) components and the future unavailability of tin-lead finished parts have forced component users to face the laborious task of identifying, screening out, or refinishing tin plated parts to meet military and aerospace reliability standards.

In the absence of a technological “silver bullet” solution to the tin whisker problem, alternative options must be evaluated, which could allow past reliability standards to be met while achieving environmentally significant reductions in electronics lead content. As a last resort alternative, the use of a low-Pb content SnPb finish should be considered. There is published evidence that a tin alloy containing greater than 3 to 4 percent Pb has a greatly reduced tendency to form tin whiskers, and those formed are much smaller. If this type of plating is accepted universally, it would reduce the amount of lead by nearly 85% compared to the current 63/37 tin-lead finish. This low-Pb tin finish could be easily implemented and would significantly lower the tin whisker risk without the need for extensive verification.

Nevertheless, the low-Pb tin finish raises concerns of Pb contamination of SnAgCu (tin silver copper) solder joints. As the Pb from the finish mixes with the SnAgCu solder, it forms an alloy phase with a lower melting point and lower strength. Macro-segregation of this phase within a solder joint is believed to be the cause of the reduced reliability. When the low-Pb (3 to 4 percent) tin finish is used in conjunction with the SnAgCu solder, the resulting alloy will have less than 1 wt% Pb. The EMPF is actively evaluating the lead contamination of SnAgCu solder and its impact on solder joint reliability. It appears that below 2% Pb concentration, the mixture does not form the lower melting phase and should not affect reliability.

The reintroduction of low-Pb tin plating may seem like taking a step backward, but it is a step in a safer direction, which also allows a significant reduction in lead levels. This approach would need to be pursued on both legislative and technological fronts. It is not unusual for the EU Commission to grant an exemption for technological problems which do not have a solution. A case can be made for the reliability issues surrounding the use of tin whisker-prone pure tin plating as compared to the mitigation which a low-lead tin alloy plated finish would offer for long-term reliability.

Evidence showing the lack of a reliable and proven replacement finish and the technological difficulties associated with testing finishes could be presented to the EU commission. A technical argument can be made to justify a temporary exemption for using reduced-Pb tin finishes until 2010. In the next 5 years, a lot of necessary data regarding the validity of current acceleration tests and the viability and effectiveness of many mitigation techniques will become available. This will allow selection of an optimal mitigation technique. In the same time frame, experimental work to determine the lowest Pb content necessary to minimize tin whiskers can be performed. At the end of the 5-year extension, if the tin whisker issue remains unresolved, the use of low-Pb Sn alloy could continue, providing a technologically reliable and environmentally-sound “win-win” solution.

References
1. NASA Goddard Space Flight Center Whisker Failures, http://nepp.nasa.gov/whisker/failures
2. “Interim Recommendations on Lead-Free Finishes for Components Used in High Reliability Products,” NEMI Tin Whisker User Group, March 2004

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