Phase Equilibria of the Cu-Si-Sn System at 700 and 500 °C

Intermetallic compounds formed in solder joints have a substantial effect on reliability. Because Sn-based alloys are alternatives to lead-containing solders, phase equilibria of the Cu-Si-Sn system were investigated for quenched samples annealed at 700 and 500 °C for 30 days. Nine three-phase equilibria were well established at 700 °C, and a previously unknown ternary τ phase with a possible homogeneity interval in the range Cu76Sn7.8Si16.2-Cu85Sn7.6Si7.4 was found for the first time. The τ phase has a hexagonal structure with a = 8.012 nm and c = 5.04 nm. Six three-phase regions were identified in the isothermal region at 500 °C. In contrast with the isothermal region at 700 °C, the new ternary τ phase was not observed at 500 °C. The solubility of Si in ε-Cu3Sn decreases from 12.8 to 1.4 at.%, and only small variations occur in the homogeneity ranges of η-Cu3Si and γ-Cu5Si.