The thermal reliability of indium-doped low solver SAC/Cu joints and the corresponding alloys

Based on CALPHAD-guided alloy design, key experimental investigations, and first-principles calculations, this study explores the melting characteristics, microstructure, mechanical properties, and fracture mechanisms of Sn-0.5Ag-0.7Cu-3Bi-xIn (x = 4, 8, 12, 17, wt.%)/Cu solder joints. Research reveals that In doping lowers the solidus, liquidus, and peak temperatures, while increasing the melting range and undercooling. The matrix phases of 4In and 8In solders are β-Sn. When the In content exceeds 12 wt.%, the γ-InSn4 phase starts to appear in the matrix, the matrix is transforming from the original single-phase into a dual-phase matrix. Consistent with it, the interfacial compound phase also has In doped, forming Cu6(Sn, In)5. During isothermal aging at 170 °C for 0–750 h, In doped less than 12 wt.% inhibits the growth of Cu3(In,Sn) and Cu6(Sn,In)5, while the joints with just 12 wt.% In doping exhibit the strongest inhibition effect on the interface IMCs. After 750 h of aging, the 12In/Cu solder joint shows the best mechanical properties, with high shear strength and shear energy, minimal displacement damage, and consistent ductile fracture mode before and after aging. The nanoindentation results indicate that the hardness of the IMC layer remains nearly unchanged, and the modulus increases, along with an increase in Ei/H values and plasticity with increasing In content. These results are mainly due to the larger charge density difference between Cu and In atoms compared to those between Cu and Sn atoms, indicating a stronger Cu–In bond compared to the Cu–Sn bond. Because Young’s modulus is an intrinsic property, its magnitude may be related to the strength of bonding in the structure. This study, guided by CALPHAD for phase types and fractions, correlates nanoindentation with changes in mechanical properties and microstructure, and validates material performance through first-principles calculations. It has practical significance for controlling the growth of interface compounds, exploring the impact of the In element doping on solder joint performance, and stabilizing solder joint structures.