Effect of Aging on Tensile Properties and Microstructures of Eutectic Sn-Pb Solder With Small Amounts of Au and Pd for Aerospace Application

In the joint with eutectic Sn-Pb solder and a lead-free Ni/Pd/Au electrode, degradation of mechanical properties of the solder due to dissolution of Au and Pd into the solder would be anxious. In this study, the effect of aging was investigated on tensile properties and microstructures of the eutectic Sn-Pb solder with small amounts of Au and Pd added. In as-cast solders with both Au and Pd added, the tensile strength increases with increasing contents of Au and Pd. A similar tendency was observed after aging at 100°C for 1000 h. The effect of aging on elongation was relatively small and elongation degraded when brittle (Pd,Au)Sn₄ phases formed in the solder. In solders with Au ranging from 1 to 5 mass%, regardless of aging conditions investigated, the tensile strength is stable at approximately 50 MPa and 45 MPa before and after aging, respectively. The effect of aging on improvement of elongation was negligible and elongation degraded when rod shaped AuSn₄ formed in the solder. On the basis of the result of microstructural observation, it was clarified that the strengthening by dispersion of (Pd,Au)Sn₄ phases is superior to softening by microstructure coarsening upon aging when the contents of Au and Pd are 2 mass% and 1 mass% or above, respectively.