Figure 5 shows the interfacial microstructure of the three kinds of solder joints. It is widely known that Cu6Sn5 is the main interfacial IMC in SnBi/Cu and SAC/Cu couples. As shown in Fig. 5a, Bi-rich phases densely distribute in the solder bulk of SnBi. The Bi-rich grains located on the interfacial IMC layer suppress the growth of Cu6Sn5. Therefore, the Cu6Sn5 IMC layer in SnBi/Cu is much thinner than that in the other two kinds of solder joints. The EDS results in Fig. 5d approve that the interfacial IMC in the SnBi/SAC/ Cu solder joint is also Cu6Sn5. Compared with SAC/Cu, the composite solder joint shows larger interfacial IMC grains. As introduced in the experimental part, the SAC bulk in the composite solder joint was soldered with the same soldering