Effect of reflow and thermal aging on the microstructure and microhardness of Sn-3.7Ag-xBi solder alloys

This work investigates the effect of reflow and the thermal aging process on the microstructural evolution and microhardness of five types of Sn-Ag based lead-free solder alloys: Sn-3.7Ag, Sn-3.7Ag-1Bi, Sn-3.7Ag-2Bi, Sn-3.7Ag-3Bi, and Sn-3.7Ag-4Bi. The microhardness and microstructure of the solders for different cooling rates after reflow at 250°C and different thermal aging durations at 150°C for air-cooled samples have been studied. The effect of Bi is discussed based on the experimental results. It was found that the microhardness increases with increasing Bi addition to Sn-3.7Ag solder regardless of reflow or thermal aging process. Scanning electron microscopy images show the formation of Ag₃Sn particles, Sn-rich phases, and precipitation of Bi-rich phases in different solders. The increase of microhardness with Bi addition is due to the solution strengthening and precipitation strengthening provided by Bi in the solder. The trend of decrease in microhardness with increasing duration of thermal aging was observed.