Growth mechanisms of interfacial intermetallic compounds in Sn/Cu–Zn solder joints during aging

The interfacial reactions of Sn/Cu–xZn (x = 15 and 30 at.%) solder joints were investigated. Before aging, [Cu₆(Sn,Zn)₅] and [Cu₆(Sn,Zn)₅/Cu–Zn–Sn] intermetallic compounds (IMCs) formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces, respectively. After thermal aging at 150 °C for 80 days, [Cu₆(Sn,Zn)₅/Cu₃(Sn,Zn)/Cu(Zn,Sn)/CuZn] and [Cu₆(Sn,Zn)₅/Cu(Zn,Sn)/CuZn] IMCs, respectively, formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces. Increasing the amount of Zn in the Cu–Zn substrates evidently suppresses the growth of Cu₃Sn and Kirkendall voids at the solder joint interfaces. Transmission electron microscopy images show the different microstructure of CuZn and Cu–Zn–Sn phases in Sn/Cu–Zn joints. These Cu–Zn phases act to inhibit the growth of Cu₆Sn₅ and Cu₃Sn IMCs. As the content of Zn increased in Cu–Zn substrates, both CuZn and Cu(Zn,Sn) grew significantly. In addition, the growth of the Cu₆(Sn,Zn)₅/Cu₃Sn IMCs approached a reaction-controlled process. The formation mechanisms of the CuZn and Cu(Zn,Sn) phases were probed and proposed with regard to the interfacial microstructure, elemental distribution, and the compositional variation at Sn/Cu–xZn interfaces.