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

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 – Cu3Sn and Cu6Sn5. Cu6Sn5 is formed during soldering. Cu3Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu3Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase – Cu6(Sn,In)5 instead of Cu6Sn5, with a higher rate constant. The mechanism of the Cu6(Sn,In)5 layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.

► In substitutes Sn in intermetallic compounds formed at the Cu–solder interface.
► Bi and In decrease the parabolic rate constant of Cu3Sn layer growth.
► In increases the parabolic rate constant of Cu6Sn5 layer growth.
► High In concentrations should be avoided since they may lead to a pre-mature solder joint degradation.