Sn whiskers mitigation by refining grains of Cu substrate during the room temperature exposure

The effect of Cu grain size on the growth of Sn whiskers was studied. A fine grained Cu film was obtained by vacuum sputter (VS) method, the average grain size is around 350 nm; while the coarse grained Cu foil (high purity-HP Cu) has an average grain size of about 25 µm. After electroplating, the Sn deposited on Cu samples were then subjected to room temperature storage. It is found that thin filaments like Sn whiskers were prone to be formed on the surface of Sn/HP Cu sample, but this tendency was significantly reduced after refining grains of Sn/VS Cu sample. The analysis result shows that the fine grained VS Cu film could slow the growth of Cu6Sn5 intermetallic compound (IMC) layer, for that the VS Cu film has more intertwined grain boundaries (GBs) and thus longer diffusion paths than those of HP Cu foil. The thinner IMC layer could reduce the compressive stress of Sn coating, thus mitigating formation and growth of Sn whiskers.