The interfacial reactions of Sn/Cu-xZn (x = 15 and 30 at.%) solder joints were investigated. Before aging, [Cu-6(Sn,Zn)(5)] and [Cu-6(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 A degrees C for 80 days, [Cu-6(Sn,Zn)(5)/Cu-3(Sn,Zn)/Cu(Zn,Sn)/CuZn] and [Cu-6(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 Cu-6(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.