Evolution of the Sn-Bi Solder Microstructure vs. Temperature – an In- situ Scanning Electron Microscopy Study

Sn-Bi alloys are promising candidates as low temperature solders due to their low eutectic temperature of 139°C, non-toxicity, and low costs. However, the low eutectic temperature also implies these alloys are at high homologous temperatures during service and operation, and solid-state diffusion are faster. This, coupled with the large difference in the solubility of Bi in Sn vs. temperature, can result in substantial microstructural changes in the alloys during thermal cycling or multiple reflow processing. This study uses in-situ scanning electron microscopy to observe the microstructural changes of the TempSave B37 commercial solder as it is heated and cooled between room temperature and 150°C. The mechanisms of the dissolution and reprecipitation of Bi during heating and cooling are revealed. The understanding gained can contribute to better reflow process design to obtain favourable microstructures.