Microstructure and interfacial reaction of Sn–Zn–x(Al,Ag) near-eutectic solders on Al and Cu substrates

Sn–Zn–x(Al,Ag) near-eutectic solders, namely Sn–8.3Zn–0.73Ag, Sn–8.4Zn–0.44Al and Sn–7.4Zn–0.26Al–0.68Ag (in wt%) with melting points of 200.74, 198.00 and 197.32 °C, respectively, as well as the Sn–9Zn eutectic solder, were used to join Al and Cu substrates. The addition of Ag led to the formation of dendritic AgZn₃ phases, while the addition of Al obviously refined the microstructure of Sn–Zn eutectic, as well as the AgZn₃ phases. The Sn–Zn–Al solder possessed the best wettability on both Cu and Al substrates among the four solders. Al_4.2Cu_3.2Zn_0.7 intermetallic compound (IMC) layers formed at the Sn–Zn–x(Al,Ag)/Cu interfaces while Al-rich (Zn) solid solutions at the Sn–Zn–x(Al,Ag)/Al interfaces of all the as-soldered joints. The shear strength of the Al/Sn–Zn–Al/Cu solder joints was the highest among the four solder joints. The declining degree of the shear strength of the Sn–Zn–x(Al,Ag) solder joints in 3.5 % NaCl solution was in agreement with the corrosion-resistance order of the bulk solders. The Al/Sn–Zn–Ag/Cu joint thus owned the best corrosion resistance.