Formation of a Diffusion Barrier-Like Intermetallic Compound to Suppress the Formation of Micro-voids at the Sn-0.7Cu/Cu Interface by Optimal Ga Additions

Kirkendall micro-voids observed within ϵ-Cu3Sn or at the bulk Sn solder/Cu interface are undesirable imperfections in solder joints in electronic interconnections that significantly decrease the reliability of the joint. Recent studies have shown that micro-alloying could alter these interfacial reactions and improve the mechanical properties. In this study, we investigated the Cu-Ga-Sn phase equilibria at 200°C and the interfacial reactions between Cu substrate and Ga-doped Sn-0.7Cu solders with doping levels of 1.0, 2.0, and 3.0 wt.%. The assembled diffusion couples were isothermally annealed at 200°C for different time periods. The results showed that the thickness of the typical Cu-Sn IMCs (η-Cu6Sn5 and ϵ-Cu3Sn) progressively decreased along with the formation of the γ-Cu9Ga4 phase as the Ga-doping concentration was increased. The η-Cu6Sn5 and ϵ-Cu3Sn were completely suppressed and substituted by the γ-Cu9Ga4 phase when the Ga-doping concentration was 3 wt.%. More interestingly, the γ-Cu9Ga4 was the only IMC formed at the Sn-0.7Cu-3.0 Ga/Cu interface even when aging at 200°C for up to 1000 h, implying that the undesired defectiveness of Kirkendall micro-voids is possibly prevented by controlling the formation of the interfacial phase.