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Undercooling and microstructure analysis for the design of low melting point solder

The undercooling in solidification of eutectic Sn37Pb, Sn3.5Ag, Sn58Bi and Sn52In solders was inspected. Sn37Pb and Sn58Bi have a similar undercooling, around 16°C, and Sn52In has the lowest undercooling of around 3°C. The larger the undercooling, the higher the barrier to nucleation. A small undercooling is favorable for solder joint array to solidify at the same time in chip stacking on a packaging substrate.