Gallium Microalloying in Bi–Sn Solders: Interfacial Phase Formation, Wettability, and Long‐Term Shear Reliability Under Isothermal Aging

Low‐temperature Bi–Sn solders are promising candidates for next‐generation electronic packaging. However, the influence of gallium microalloying on interfacial reactions and mechanical performance, together with the combined effect of gallium and flux chemistry on wettability, remains insufficiently understood. This study systematically investigates 58Bi–42Sn, 57.5Bi–42Sn–0.5Ga, and 57Bi–42Sn–1Ga alloys in terms of melting behavior, wettability using three distinct fluxes at two solder bath temperatures, and intermetallic layer evolution and mechanical performance during annealing at 100 and 125 °C for up to 2000 h. Ga addition slightly reduced the melting temperature (∼133 °C) and enhanced the maximum wetting force under optimized flux conditions. Microstructurally, Ga redirected the interfacial reaction pathway by promoting formation of Cu 9 Ga 4 instead of the conventional Cu 6 Sn 5 /Cu 3 Sn bilayer. Although the Cu–Ga phase exhibited approximately 30% lower activation energy, intermetallic layer growth remained markedly suppressed because of lower diffusion coefficients and the limited Ga content available for layer thickening. Consequently, Ga‐containing joints developed thinner interfacial layers and avoided pronounced Sn depletion and Bi enrichment near the interface. These results demonstrate that Ga microalloying modifies both interfacial chemistry and processing behavior, making Ga‐containing Bi–Sn solders attractive for low‐temperature assembly.