Influence on Sn whiskering of controlled bismuth additions to sputtered Sn films

In 2006, the European Union restricted lead (Pb) to > 0.1% (by weight) in electronics. This caused a renewal of interest in Sn whiskers, as the absence of Pb in Sn films had been known for years to promote Sn whisker growth. The early work on Sn whiskers had suggested that elements other than Pb may suppress whiskers. Bi was chosen as an attractive candidate, as it has similar phase formation behavior to Pb, i.e., low solubility, eutectic formation, and no intermetallic compounds with Sn. In this work, the influence of Bi on Sn whiskering is investigated. Three custom Sn-Bi sputter targets with 0.5%, 1.0%, and 2.0% Bi (by weight) were used to generate ~2000Å sputtered Sn films on Si substrates. The samples were incubated for 77 days. Subsequently, three of the samples were placed in a thermal cycling chamber to accelerate whisker growth, while the others continued to incubate under lab conditions. An expected reduction in Sn whisker density was observed on all samples prior to thermal cycling, with the 0.5% Bi samples generating the highest whisker densities (980 cm-2). A dramatic explosion of whiskering occurred, however, in all the specimens after thermal cycling. The results clearly show that small amounts of Bi dramatically lowers the amount of Sn whiskering, but only under isothermal, room temperature conditions.