Effects of Cu on the interfacial reactions between Sn–8Zn–3Bi–xCu solders and Cu substrate

• The effect of Cu addition on the interfacial reaction between Sn–8Zn–3Bi solder and Cu substrate was investigated.
• Cu–Zn IMCs form in the solder matrix when Cu was added.
• Cu addition was beneficial in decreasing the thickness of IMC layers and increasing the activation energy.

The microstructure, thermal property, and interfacial reaction with Cu substrate of Sn–8Zn–3Bi–xCu (x = 0, 0.5, 1) lead-free solders were investigated in this work. Cu–Zn intermetallics formed in the solder matrix and the melting temperature increases slightly with Cu addition. After soldering at 250 °C for 90 s, a flat Cu5Zn8 layer and a scallop CuZn5 layer formed at the interfaces of all samples. The CuZn5 intermetallic compound (IMC) transformed to Cu5Zn8 IMC with longer reaction time due to the diffusion of Cu atoms from Cu substrate. The interfacial IMC layer grew thicker with the reaction time following a parabolic law which suggested the interfacial reactions were diffusion controlled. The calculation results show that the activation energy of IMC growth for Cu-containing solders is larger than that of Sn–8Zn–3Bi solder, which demonstrated that a small amount of Cu addition to the solder can effectively suppressed the growth of the interfacial IMC.