Effect of Bi addition on the activation energy for the growth of Cu5Zn8 intermetallic in the Sn–Zn lead-free solder

The growth kinetics of Cu5Zn8 phase has been investigated under long-term thermal exposure conditions using single shear lap joints. The Cu5Zn8 phase was formed by reacting Sn–Zn and Sn–Zn–Bi lead-free solders with Cu substrate. A scanning electron microscope (SEM) was used to observe the morphology of the phases and energy dispersive X-ray (EDX) was used to estimate the elemental compositions of the phases. The morphology of the Cu5Zn8 phase is rather flat for all the aging temperatures. The Cu6Sn5 isolated phase was observed after long-term aging. Intermetallic thickness measurements show that the thickness of the Cu5Zn8 phase in the Sn–8Zn–3Bi/Cu system is lower than in the Sn–9Zn/Cu system. Subsequently the activation energy for the growth of Cu5Zn8 intermetallic is higher in the presence of Bi. Therefore, the presence of Bi in the Sn-matrix lowers the Zn diffusion and hampers the higher-growth of intermetallics in the Sn–8Zn–3Bi/Cu system compare to the Sn–9Zn/Cu solder interface.