What put Sn0.6Ag0.6Cu3Bi at the opposite end of the whiskering spectrum? A balance of the right elements. This tallest projection, at 29µm tall, dwarfs the alloy’s typical whiskers seen surrounding it, which are usually less than 10µm.įIGURE 1. The largest whisker grown by the top performing alloy, Sn0.6Ag0.6Cu3Bi. Its largest whisker, recorded after 5000hr of high heat and humidity storage, is shown in FIGURE 1. The low silver SAC alloy with bismuth didn't grow any whiskers for the first 3000hr, and when it did produce them at 37hr, they were smaller and/or fewer than the other alloys. We documented whisker growth at 2500, 3000, 37hr. In this final round of tin whisker growth tests we aged the samples at 85%RH and 60☌ for 5000hr, or roughly seven months. It also performed well in the other electronic solder suitability tests such as wetting, oxidation resistance, melting temperature and mechanical properties.
Sn0.6Ag0.6Cu3Bi, labeled “Alloy #69-2” in our test matrix, a tweak on one of the original nine alloys, produced the least and smallest whiskers. In our year-long study of the relationship between alloy composition on tin whisker formation, SAC 305 notoriously produced more and larger whiskers than any of the other alloys tested. Alloy composition has a strong influence on tin whisker production and has proven to either mitigate or exacerbate the propensity for a solder to whisker. Silver is a known whisker promoter but small amounts of bismuth mitigate the problem.
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