Anomalous growth of whisker-like bismuth–tin extrusions from tin-enriched tin–Bi deposits

This article shows the first finding that the anomalous growth of Bi–Sn extrusions from tin-enriched alloys (Sn–xBi with x between 20 and 10 wt.%) can be induced by post-plating annealing in N2 between 145 and 260 °C for 10 min although metal whiskers were commonly formed on the surface of pure metals or alloys of the enriched component. From SEM observations, very similar to Sn whiskers, Bi–Sn extrusions vary in size, shape, length, and diameter with changing the annealing temperature, which are highly important in regarding the potential for failure of electronic products. Annealing resulting in thermal expansion of Sn grains is believed to squeeze the Bi–Sn alloys with relatively low melting points to form whisker-like extrusions although the exact mechanism is unclear.