Effect of Ga additions as micro-alloying element to microstructure formation, thermal properties and mechanical properties of Sn-0.7Cu solder alloy

The present study investigates the effects of minor gallium (Ga) additions (0, 0.02, 0.05, 0.1, and 0.5 wt.%) on the microstructural, thermal, and mechanical properties of Sn-0.7Cu solder alloy. Cross-sectional microstructural analysis revealed that Ga is homogeneously distributed in the solder, forming a solid-solution structure and refining the bulk microstructure, with the most pronounced refinement observed at 0.1 wt.% Ga. This is evidenced by ∼16.9% reduction in the β-Sn area fraction region and the formation of finer, more uniformly dispersed Cu 6 Sn 5 intermetallic compounds. At the solder/Cu interface, Ga reduces Cu 6 Sn 5 scallop grooving and interfacial roughness, thereby limiting localized fast diffusion paths for Cu. The mechanical performance of Sn-0.7Cu-xGa solder joints results showed that Ga microalloying at 0.1 wt.% provided the best performance, increasing lap-shear strength by up to ∼43% and shifting the fracture surface toward dense, fine dimples, indicating enhanced plastic deformation and higher energy absorption. In contrast, excessive Ga addition (0.5 wt.%) reduced joint strength and promoted more brittle fracture features, likely due to the formation of Ga-rich phases that act as crack-initiation sites.