Microstructural Evolution and Mechanical Property Degradation of Sn0.5Ag0.7Cu5Bi Solder Joints with High Indium Alloying

This study systematically investigates the effects of indium (In) addition (4–17 wt%) on Sn0.5Ag0.7Cu5Bi solder joints through thermal analysis, microstructural characterization, and mechanical testing. Key findings reveal that 12 wt% In emerges as the optimal composition, offering superior mechanical performance with 87% ductile fracture and significantly suppressed intermetallic compound (IMC) growth. However, high‐In alloys (15–17 wt%) exhibit abnormal IMC thickening due to thermal activation near the aging temperature. Microstructural analysis suggests that In substitution at Sn sites causes the lattice to induce the aging local lattice contraction in η′‐Cu6Sn5, generating internal stresses that lead to cracking in low‐In alloys after prolonged aging. Shear strength shows a nonmonotonic dependence on In content, decreasing as In increases, attributed to InSn4 phase formation. This research identifies 12 wt% In as the optimal composition for Sn0.5Ag0.7Cu5Bi‐xIn solders, providing specific design rules for industrial applications: 1) enhanced drop resistance in mobile devices due to 87% ductile fracture; 2) cost reduction in automotive electronics by suppressing IMC growth (81% thickness reduction vs. 4In alloy); and 3) compatibility with flexible substrates via In‐induced lattice contraction, validated by density functional theory calculations. These guidelines enable reliable solder joints in high‐density IC packaging under thermal cycling.