Microstructure and mechanical properties of Sn-xGa alloys and solder joints

Though Ga is a promising addition used in third-generation lead-free solders for harsh-environment applications, its influences on βSn structure and mechanical properties of solder alloys/joints are still unknown. This study demonstrated that Ga additions in Sn-XGa (X = 0.5, 1.0, 2.0 in wt%) can moderately refine βSn grains but render the alloys more susceptible to recrystallization during cross-section and polishing. Sn-1.0Ga shows substantially elevated yield/tensile strengths (45.1 MPa/64.9 MPa) compared with pure Sn, attributed to the strong solid-solution strengthening effect of Ga, and it also possesses good tensile ductility of >25%. Compared with pure Sn and Sn-3.0Ag-0.5Cu (SAC305) solder joints on Cu, Sn-XGa/Cu solder joints show significantly reduced dissolution of Cu substrate that benefits in suppressing the formation of primary Cu₆Sn₅. Among Sn-XGa/Cu, Sn-2.0Ga/Cu joint exhibits high shear strength (69.4 MPa) comparable to that of SAC305/Cu joint (70.5 MPa), owing to the Ga solid-solution strengthening and the extra second-phase strengthening from Cu₉Ga₄.