Mechanisms for interfacial reactions between liquid Sn-3.5Ag solders and Cu substrates

Intermetallic compounds formed during the soldering reactions between Sn-3.5Ag and Cu at temperatures ranging from 250degreesC to 375degreesC are investigated. The results indicate that scallop-shaped eta-Cu-6(Sn-0.933 Ag-0.007)(5) intermetallics grow from the Sn-3.5Ag/Cu interface toward the solder matrix accompanied by Cu dissolution. Following prolonged or higher temperature reactions, epsilon-Cu-3 (Sn-0.996 Ag-0.004) intermetallic layers appear behind the Cu-6(Sn-0.933 Ag-0.007)(5) scallops. The growth of these interfacial intermetallics is governed by a kinetic relation: DeltaX = t(n), where the n values for eta and epsilon intermetallics are 0.75 and 0.96, respectively. The mechanisms for such nonparabolic growth of interfacial intermetallics during the liquid/solid reactions between Sn-3.5Ag solders and Cu substrates are probed.