Effects of minor alloying with Ge and In on the interfacial microstructure between Zn–Sn solder alloy and Cu substrate

The interfacial reactions of Zn30Sn (ZS), Zn30Sn-1Ge (ZS-1Ge), and Zn30Sn–1In (ZS-1In) on the Cu substrate were investigated. After reflowing at 450 °C for 5 min, the ε-CuZn₅ (scallop-type), γ-Cu₅Zn₈ (flat-type), and β^’-CuZn (flat-type) intermetallic compounds (IMCs) formed at the interface between the solders and Cu substrate. Phase transformation from ε-CuZn₅ and β^’-CuZn to γ-Cu₅Zn₈ was observed with an increasing aging time at 150 °C. The growth of γ-Cu₅Zn₈ layer was accompanied by the decrease of ε-CuZn₅ and β^’-CuZn layers. After aging for 360 h. γ-Cu₅Zn₈ layer became the main IMC layer at the interface. The addition of Ge and In decreased the diffusion rate of Cu and Zn in the intermetallic layer (IML). The first-principles calculations indicated that the diffusion activation energy per unit length of Cu and Zn atoms crossing a specific diffusion channel was similar. Based on first-principles calculations, the atomic concentration ratios of Cu to Zn in the three Cu–Zn IMC phases were calculated via a simple model. When the atomic concentration ratio of Cu to Zn exceeded 0.528, ε-CuZn₅ transformed into γ-Cu₅Zn₈, and when the ratio of Cu to Zn was less than 0.838, β^’-CuZn transformed into γ-Cu₅Zn₈. In addition, the precipitation of Sn–Ge and Sn–In solid phase at the IML suppressed the IMC growth. Accordingly, the diffusivity of Cu on the IMC layer of ZS, ZS-1Ge and ZS-1In solders was calculated to be 5.8 × 10⁻⁷, 5.7 × 10⁻⁷ and 4.6 × 10⁻⁷ m²/s, respectively.