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 degrees C for 5 s. The joints were subsequently aged at temperatures of 130-170 degrees 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(3)Sn and Cu(6)Sn(5). Cu(6)Sn(5) is formed during soldering. Cu(3)Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu(3)Sn 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(6)Sn(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. (C) 2011 Elsevier B.V. All rights reserved.