Interfacial reaction and mechanical properties of Al/Sn-Zn-Ni/Cu solder joints

Sn-Zn-Ni lead-free solders with different Ni and Zn contents were used to join Al and Cu substrates in the present work. Coarsened Ni₃Zn₁₄ and needle-like Zn-rich phases dispersed in the β-Sn matrixes of the four Sn-Zn-Ni solders. The solders all showed good wettability on Al substrate and the 2# solder was the best. While the wettability on Cu substrate decreased with increasing Zn content. In the Al/Sn-Zn-Ni/Cu solder joints, Cu-Zn intermetallic compounds (IMCs) formed at the solder/Cu interface, while Al-Zn-Sn solid solutions formed at the Al/solder interface. Furthermore, Al₃Ni phases dispersed in the bulk solders. The shear tests of the solder joints showed that the failures occurred at the Al/solder interface. This is attributed to the relatively lower bonding strength between Al-Zn-Sn solid solution and Al substrate compared to that between Cu-Zn IMC and Cu substrate. The addition of Zn enhanced the dissolution of Al substrate into the molten solder, which made the Al interface more uneven, further improved the shear strength of Al/Sn-Zn-Ni/Cu solder joints. The addition of Ni improved the corrosion resistances of the Sn-Zn-Ni solders.