Interfacial reactions between Sn–8Zn–3Bi–xNi lead-free solders and Cu substrate during isothermal aging

In this work, interfacial reactions of Sn–8Zn–3Bi–xNi (x = 0, 1) lead-free solders with Cu substrate and the growth of intermetallic compounds (IMCs) during isothermal aging were investigated. After soldering at 250 °C for 90 s, the Cu5Zn8 and CuZn5 phases formed at the Sn–8Zn–3Bi/Cu interface and only the Cu5Zn8 phase was found at the interface of the solder with addition of Ni. During aging treatment at 150 °C for 100, 400 and 900 h, the CuZn5 IMC at the Sn–8Zn–3Bi/Cu interface transformed to the Cu5Zn8 due to Cu atoms diffusing from Cu substrate. The Cu5Zn8 IMC layer at solder/Cu interfaces grew thicker with increasing the aging time and the growth was diffusion controlled. Moreover, the thickness of the IMC layer at the Sn–8Zn–3Bi/Cu interface was thicker than that at the Sn–8Zn–3Bi–1Ni/Cu interface. The reduction effect of Ni addition to the solder on the interfacial reaction might be attributed to the formation of the Ni5Zn21 IMC in the solder bulk, which effectively suppressed the diffusion of Zn atoms to the interface to react with Cu.