Inhibiting the growth of Cu3Sn and Kirkendall voids in the Cu/Sn-Ag-Cu system by minor Pd alloying

In this study, the metallurgical reaction between Cu substrates (electrolytic type) and a Sn3Ag0.5Cu-xPd alloy at 180°C was examined using a scanning electron microscope (SEM), electron probe microanalyzer (EPMA), focused ion beam (FIB) microscope, and transmission electron microscope (TEM). The results showed that the growth of Cu 3 Sn in the Cu/Sn-Ag-Cu solder joints was substantially suppressed by doping with a minor quantity of Pd (0.1–0.7 wt. %) in the solder alloy. The sluggish growth of Cu 3 Sn reduced the formation of Kirkendall voids at the Cu/Cu 3 Sn interface and significantly improved the mechanical reliability of the joint interface. It was argued that a minor addition of Pd into the solder stabilized the Cu 6 Sn 5 phase and enlarged the interdiffusion coefficient of Cu 6 Sn 5 but diminished that of the neighboring phase (Cu 3 Sn), thereby decreasing the Kirkendall effect in the Cu/Sn-Ag-Cu reactive system.