The interfacial intermetallic formation at 150 °C between Cu and various solders, including Sn–9Zn, Sn–8.55Zn–1Ag, and Sn–8.55Zn–1Ag–XAl was investigated. The Al contents X of the quaternary solder alloys investigated were 0.01–0.45 wt.%. The compositions and the growth kinetics of intermetallic compounds (IMCs) were investigated. The IMC consisted of three layers for Sn–9Zn/Cu, Sn–Zn–Ag/Cu, and Sn–Zn–Ag–XAl/Cu specimens after aging for 100–600 h. These three layers included the Cu3(Zn, Sn) phase adjacent to the solder, the Cu6(Sn, Zn)5 phase in the middle, and the Cu–rich phase near to Cu. For long–term aging time over 1000 h, the Cu6(Sn, Zn)5 phase grew, while the Cu3(Zn, Sn) phase diminished. Al segregation formed in the IMC for all of the Sn–Zn–Ag–XAl/Cu specimens after aging.Cracks formed, when aged for 1000 h, at the solder/IMC interface or within the IMC layer for the following solders: Sn–9Zn, Sn–8.55Zn–1Ag, Sn–8.55Zn–1Ag–0.1Al, Sn–8.55Zn–1Ag–0.25Al, and Sn–8.55Zn–1Ag–0.45Al. The crack was not detected up to 3000 h for the Sn–8.55Zn–1Ag–0.01Al/Cu couple, of which the IMC growth rate was the slowest among all solders.