Interfacial reactions and joint strength of Sn-37Pb and Sn-3.5Ag solders with immersion Ag-plated Cu substrate during aging at 150 °C

Joint reliability of immersion Ag with two different solders, Sn-37Pb and Sn-3.5Ag, were evaluated. We first compared the interfacial reactions of the two solder joints and also successfully revealed a connection between the interfacial reaction behavior and mechanical reliability. The Sn-Pb solder produced a Pb-rich phase along the interface between the solder and the Cu substrate during aging. In contrast, the Sn-Ag solder exhibits an off-eutectic reaction to produce the eutectic phase and Ag₃Sn precipitate. The shear test results show that the Sn-Pb solder joint fractured along the interface showing brittle failure indications possibly due to the brittle Pb-rich layer. In contrast, the failure of Sn-Ag solder joint was only through the bulk solder, providing evidence that the interface is mechanically reliable. The results from this study confirm that the immersion Ag/Sn-Ag solder joint is mechanically robust, and thus the combination is a viable option for a Pb-free package system.