Study of the microstructure and the mechanical properties of Pb-2.5Ag-2Sn solder joint

In this study the microstructure and the mechanical properties of Pb-2.5Ag-2Sn solder, the most used die attach material for power devices, were investigated. Simple lap joints with different microstructure in the solder joint layer were fabricated by changing the temperature profile of soldering process. The microstructure was significantly changed by the release of tin from inside the solder joint layer during the melting process of the solder. As the tin release continued, the microstructure tended to become finer and when the tin was depleted, the microstructure changed to coarser. Among mechanical properties of the solder joint, the fatigue life was much sensitive to changes in its microstructure; long fatigue life was obtained for the solder joint layer with Ag-Sn finely dispersed structure. These results indicate that to obtain a reliable Pb-2.5Ag-2Sn solder joint layer, it is necessary to understand growth mechanisms and design an appropriate microstructure.