Interfacial reactions between molten Sn–Bi–X solders and Cu substrates for liquid solder interconnects

This paper reports on the interfacial reaction kinetics between molten Sn–58Bi solder and Cu substrates for extended temporal scales. Also studied are the effects of a series of elemental additions of 1–2 wt.% Al, Cr, Cu, Si, Zn, Ag, Au, Pt and Nb into the basic Sn–58Bi solder, in an attempt to produce a barrier layer that slows down intermetallic compound (IMC) growth. At temperatures between 200 and 240 °C, grain boundary-/molten channel-controlled growth of η-phase (Cu 6Sn 5), followed by diffusion-controlled simultaneous growth of the ε-phase (Cu 3Sn) and η-phase (Cu 6Sn 5), between the molten Sn–58Bi solder and the Cu substrate was observed, and a layer of 6 μm Cu could survive for approximately 24–48 h. Addition of 1 wt.% Zn resulted in a layer of γ-Cu 5Zn 8, instead of Cu 3Sn and Cu 6Sn 5, forming at the interface, leading to an increase in the lifetime of the Cu substrate to greater than 120 h at 200 °C, while the other additions failed to slow down IMC formation.