Effects of the melt state on the microstructure of a Sn–3.5%Ag solder at different cooling rates

The effects of the melt state on the microstructure of Sn–3.5%Ag solder were investigated at two different cooling rates. The results show the solid–liquid interface energy is found to increase when the alloy is melted at above liquid–liquid structure transition (L-LST) temperature. Consequently, the melts require a larger undercooling to nucleate and a slower rate to grow. Finally, the solidification microstructures of the Sn–3.5%Ag alloy are refined. During L-LST, the eutectic growth pattern changes from a metastable cellular eutectic growth into a rod-like regular eutectic in some grains. L-LST refines the microstructure, inevitably leading to increased microhardness. At different cooling rates, the effects of the melt state on the microstructure and microhardness of Sn–3.5%Ag solder are similar.