Interfacial reactions in Sn-20In-2.8Ag/Cu couples

Interfacial reactions between Sn-20 wt.%In-2.8 wt.%Ag (Sn-20In-2.8Ag) Pb-free solder and Cu substrate at 250, 150, and 100 C were investigated. A scallop-type η-Cu₆Sn₅ phase layer and a planar ε-Cu₃Sn phase layer formed at the interface at 250 C. The indium content in the molten solder near the interface was increased with the formation of the η-Cu₆Sn₅ phase; and the η-Cu ₆Sn₅, Ag₂In, Cu₂In₃Sn, and γ-InSn₄ phases formed from the solidification of the remaining solder. At 100 and 150 C, only the η-Cu₆Sn₅ phase was found at the interface. However, unusual liquid/solid reaction-like interfacial morphologies, such as irregular elongated intermetallic layers and isolated intermetallic grains, were observed in the solid-state reactions. These η phase layers had less Sn content than the Sn-20In-2.8Ag alloy, resulting in an excess Sn-rich γ-InSn₄ phase accumulating at the interface and forming porous η layers on top of the initially formed dense η layers at 150 C. At 100 C, large elongated η grains were formed, whereas the interfacial layers remained almost unchanged after prolonged reaction. Based on the experimental evidence, the growth of the η phase was proposed to follow a diffusion-controlled mechanism at 250, 150 and 100 C, while that of the ε phase was probably controlled by the reaction.