The influence of non-uniform copper oxide layer on tin whisker growth and tin whisker growth behavior in SnAg microbumps with small diameter

•A thin copper oxide layer formed on the surface of 10 μm-diameter Cu/SnAg microbump.•Copper surface diffusion is important in microbump with small diameter.•Tin whisker grows from the weak spots of the copper oxide layer.•Twin grain boundary existed between tin whisker and neighboring grain.•Tin atoms slip into tin whisker through dislocations at the twin grain boundary.