Effective suppression of interfacial intermetallic compound growth between Sn-58Â wt.% Bi solders and Cu substrates by minor Ga addition

Overgrowth of the intermetallic compound (IMC) layer between Sn-58 wt.% Bi (Sn-58Bi) solders and Cu substrates significantly degrades the reliability of electronic products. Doping active elements into Sn-58Bi solders is a common approach for improving joint properties. In this study, minor amounts of Ga, ranging from 0.25 to 3.0 wt.%, were doped into Sn-58Bi solders. The interfacial reactions between the Ga-doped Sn-58Bi solders and Cu substrates at 200 °C were investigated using electron probe microanalysis (EPMA) and CALPHAD thermodynamic modeling. Four IMCs, namely θ-CuGa2, γ1-Cu9Ga4, η-Cu6Sn5, and ε-Cu3Sn phases, were observed. For longer reaction time or lower doping levels of Ga in the solders, the diffusion paths across the Ga-doped Sn-58Bi/Cu couples changed from L/θ/γ1/Cu to L/γ1/ε/Cu and L/γ1/η/ε/Cu. The dense γ1-Cu9Ga4 layer acted as a native diffusion barrier. The growth of the IMC layer was effectively suppressed with minor Ga addition.