Influence of Alloying Elements on the Phase Structure, Stress–Strain Behavior, and Fracture Toughness of Ni3Sn: A First-Principles Study

Transient liquid-phase bonding (TLPB) enables the low-temperature fabrication of encapsulated solder joints with high-temperature resistance and electromigration resilience; yet, Ni-Sn TLPB joints suffer from brittle fracture due to intermetallic compounds (IMCs). This study investigates the Co, Cu, and Pt alloying effects on Ni₃Sn via formation energy, molecular dynamics, and first-principles calculations. Occupancy models of Ni_6-xM_xSn₂ (M = Co, Cu, and Pt) were established, with the lattice parameters, B/G ratios, fracture toughness (K_IC), and stress-strain behaviors analyzed. The results reveal that Co enhances fracture toughness and reduces Ni₃Sn anisotropy, mitigating microcrack risks, while Cu/Pt introduce antibonding interactions (Cu-Sn and Pt-Sn), weakening the bonding strength. The classical B/G brittleness criterion proves inapplicable in Ni-M-Sn systems due to mixed bonding (metallic/covalent) and the hexagonal structure's limited slip systems. The Ni_6-xCo_xSn₂ formation improves toughness with a low Co content, supported by an electronic structure analysis (density of states and Bader charges). The thermodynamic stability and reduced molar shrinkage (Ni + Sn → Ni₃Sn) confirm Co's efficacy in optimizing Ni-Sn solder joints.