Interfacial microstructure evolution and tensile behavior of Sn-xAg micro solder joints during aging

The microstructure evolution and tensile behavior of Sn- x Ag/Cu micro solder joints during isothermal aging were investigated. The Ag content not only affected the initial orientation and morphology of interfacial Cu 6 Sn 5 grains, but also precipitated as Ag 3 Sn particles on the surface of the Cu 6 Sn 5 gains to exhibit a shielding effect. As a result, the coupling effect dominated the growth behavior of both Cu 6 Sn 5 and Cu 3 Sn interfacial IMCs. The IMC growth was controlled by grain boundary diffusion for the joints with x < 2.0 wt%; when x ≥ 2.0 wt%, it was co-controlled by both bulk diffusion and grain boundary diffusion. The final total thickness of the IMCs followed the trend: Sn-0.5Ag < Sn < Sn-5.0Ag < Sn-2.0Ag < Sn-3.5Ag. Additionally, the effect of Ag content on the tensile strength of the as-soldered and aged solder joints was discussed in view of fracture morphology. Mixed ductile-brittle fracture mode was observed in the as-soldered joints, while ductile fracture mode was observed in the aged joints. The Sn-3.5Ag/Cu showed the thickest IMCs with preferred orientated Cu 6 Sn 5 and the lowest proportion of Cu 3 Sn; while the Sn-0.5Ag/Cu showed little effect on Cu 6 Sn 5 grain orientation similar to Sn/Cu, but presented the lowest decrease in tensile strength after aging. The results could provide theoretical support for optimizing the composition of Ag-bearing solders.