Relation of Sn whisker formation to intermetallic growth: Results from a novel Sn–Cu “bimetal ledge specimen”

Sn whisker formation was studied in a “bimetal ledge specimen” that consisted of a uniform layer of Sn that was covered by Cu on only half the sample. Whiskers are observed to grow in the Cu-free region of the sample, which is attributed to stress generated by the diffusion of Sn atoms along the network of columnar grain boundaries. The Sn diffusion is driven by the intermetallic growth, but the whisker density extends out much further than any measurable Cu concentration. Whiskers were also observed to grow in the Cu-coated region by punching through the overlayer, leaving a Cu-grain on the tip of the Sn whisker.