Kinetics of intermetallic phase formation at the interface of Sn–Ag–Cu–X (X = Bi, In) solders with Cu substrate

Abstract

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn–3.7Ag–0.7Cu; Sn–1.0Ag–0.5Cu–1.0Bi and Sn–1.5Ag–0.7Cu–9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130–170 °C for 2–16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface – Cu 3Sn and Cu 6Sn 5. Cu 6Sn 5 is formed during soldering. Cu 3Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu 3Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase – Cu 6(Sn,In) 5 instead of Cu 6Sn 5, with a higher rate constant. The mechanism of the Cu 6(Sn,In) 5 layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.