Tin whiskers prefer to grow from the [001] grains in a tin coating on aluminum substrate

Abstract

This work aims at understanding the features of the Sn grains from which whiskers preferentially grow. The growth behavior of Sn whiskers on a 50 μm thick hypereutectic Sn-Al alloy coating was observed in situ by mapping the grain orientations before and after aging using the electron backscatter diffraction (EBSD) technique. Sn whiskers were found to grow preferentially from the (001) or near-(001) grains surrounded by the grains having perpendicular orientations, such as (100), (110) and (210). The compressive stress in the coating was heterogeneous, and the (001) grains exhibited the higher compressive stress close to the grain boundaries. The orientation relationship between α-Al phase and β-Sn phase was confirmed as (2 0 0)α-Al ‖ (2 0 0)β-Sn, [0 11¯]α-Al ‖ [0 0 1]β-Sn. The plane matching resulted in approximately 0.7 % misfit strain in β-Sn, which had little impact on the growth of whiskers. Dislocations pile-ups were found in the (001) grains and repulsed by the Sn oxide layer, giving the probability of cracking the oxide. Grain boundaries were found between the whisker and underneath grain. The dominant diffusion mode for early whisker growth was grain boundary diffusion aided by pipe diffusion.