Phase identification and growth kinetics of the intermetallic compounds formed during in-49Sn/Cu soldering reactions

For the application of In-49Sn solder in bonding recycled-sputtering targets to Cu back plates, the intermetallic compounds formed at the In-49Sn/Cu interface are investigated. Scanning electron microscopy (SEM) observations show that the interfacial intermetallics consist of a planar layer preceded by an elongated scalloped structure. Electron-probe microanalyzer analyses indicate that the chemical compositions of the planar layer and the scalloped structure are Cu_74.8In_12.2Sn_13.0 and Cu_56.2In_20.1Sn_23.7, respectively, which correspond to the ε-Cu₃(In,Sn) and η-Cu₆(In,Sn)₅ phases. Kinetics analyses show that the growth of both intermetallic compounds is diffusion controlled. The activation energies for the growth of η- and ε-intermetallics are calculated to be 28.9 kJ/mol and 186.1 kJ/mol. Furthermore, the formation mechanism of intermetallic compounds during the In-49Sn/Cu soldering reaction is clarified by marking the original interface with a Ta-thin film. Wetting tests are also performed, which reveal that the contact angles of liquid In-49Sn drops on Cu substrates decline to an equilibrium value of 25°C.