A comparative study on the growth behaviors of Sn whiskers and hillocks in a Sn-Al alloy coating under different environments

Abstract

In this study, the effects of factors such as temperature, solution and surface roughness on the growth of whiskers in a 100 μm thick Sn-Al alloy coating were comparatively investigated under different environments. The results indicated that whiskers did not grow after nucleation at 30 °C in oil. With the increase in aging temperature, whiskers started to grow after nucleation. The morphologies of whiskers showed the filament-type at 50 °C and the hillock-type at 100 °C. The residual compressive stress in the coating gradually decreased with the growth of whiskers and hillocks. Whisker nucleation was not found on the coating kept in water solution. The residual stress in the coating was rapidly released through the micro-cracks at the coating/substrate interface instead of growing whiskers. The oxide layer on the coatings with rough surfaces was easier to crack due to the higher thermal stress. The thermal stress on the oxide layer was in directly proportional to the depth of indentation and inversely proportional to the grain size. Whiskers preferentially grew on the (001)-oriented Sn grains in the coating. The finite element simulation results indicated that the grains with c-axis perpendicular to the coating surface had higher thermal strain and tended to grow whiskers. During creep deformation, the stress exponent increased from 7.0 to 11.5 with the increase in temperature. The mechanism for whisker growth was dislocation slip. Grain boundaries were observed in the hillocks. The power-law breakdown was the mechanism for the growth of hillocks.