First-principles calculation of the effects of In, Ga, and Al on the stability and mechanical properties of AgCu alloy

Silver–copper solder is extensively utilized in electronic packaging; however, its application is constrained by a relatively high melting point and insufficient mechanical strength. Although experimental studies have demonstrated that microalloying with In, Ga, and Al can effectively lower the melting point and improve the mechanical properties of AgCu alloys, the underlying microscopic mechanisms—particularly how the alloying elements selectively occupy lattice sites and how electronic structure modulation contributes to strengthening and toughening—remain insufficiently understood. This gap in systematic knowledge poses a significant barrier to the rational design of high-performance AgCu-based brazing materials. This paper, for the first time, adopts the first-principles method based on density functional theory to reveal the site preference (preference for Ag3 sites) of In/Ga/Al in AgCu alloys and their differentiated impacts on mechanical stability: Ga and Al enhance the thermodynamic stability (binding energy: Al > Ga > In), while In has the least effect. In terms of mechanical properties, Al yields the highest bulk modulus (115.568 GPa), shear modulus (36.332 GPa), and Young's modulus (98.659 GPa), while Ga doping results in the highest charge transfer. Ga and Al reduce the density of states at the Fermi level (10.3 and 10.2, respectively, compared to 10.5 states/eV for AgCu) and enhance covalent bonding (for Ga, the Ag–Cu occupancy increases from 0.17 to 0.21). These findings provide microscopic insights for optimizing AgCu-based solders. They are of substantial guiding significance for the development of low-melting-point, high-strength, and high-toughness electronic packaging materials.