Evolution of tin whiskers and subsiding grains in thermal cycling

The evolution of surface morphology, including whisker formation, grain boundary cracking, and subsiding grains, was studied in Sn thin films on Si substrates with a Cu interlayer during thermal cycling from −40 to 85 °C in air for up to 250 cycles and was compared with surface morphologies resulting from room temperature aging. Multiple areas were tracked, and the areal density of whiskers and the grain morphologies within these areas were monitored over time for room temperature aging and with increasing number of thermal cycles. During room temperature aging, the whisker density increased with time until saturation ~3 weeks after plating. As for thermal cycling, the whisker density was observed first to increase but then to decrease as a result of a whisker pinch-off phenomenon. The characteristic features of whiskers formed during thermal cycling included the formation of deep grooves along the in-plane grain boundaries of whiskers (“whisker root”), a decrease in whisker radii as they grew, striation rings on whiskers perpendicular to the whisker growth direction, corresponding striations along grooved surfaces in the film, albeit at different spatial periodicities than those on their corresponding whiskers, and whisker pinch-off as whiskers became prone to fracture as their radii decreased. Whiskers formed during room temperature aging did not display such grooving or pinch-off. A whisker pinch-off model was proposed to explain the observed morphological changes and the resulting decrease in whisker density during thermal cycling, with a calculated whisker growth rate that agrees with the experimental observation.