TEM analysis of whiskers formation over tin-plated films

Whiskers grown from tin films plated on brass plates were examined by transmission electron microscopy. After plating, the tin film, which was held at 323 K for five days, was composed of an amorphous phase and spherical β-Sn particles. The amorphous phase formed immediately upon plating, while after being maintained at this temperature, the amorphous phase crystallized to form β-Sn particles. After holding for 14 days, many rod-like whiskers with diameters of 0.1–0.2 μm and lengths of 1 μm were observed. The whiskers comprised β-Sn single crystals covered with SnO2 crystals several nanometers in size. It appeared that the growth of the oxide did not inhibit whisker growth. The reason is that the small oxide particles slide and rotate relative to each other, and the oxide covering β-Sn deforms during whisker growth. If the surface layer cannot be deformed, the whisker will continue to grow, maintaining the core shell structure through destruction and regeneration of the oxide coating.