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.