Effect of Bi addition on the activation energy for the growth of Cu 5Zn 8 intermetallic in the Sn–Zn lead-free solder

Abstract

The growth kinetics of Cu 5Zn 8 phase has been investigated under long-term thermal exposure conditions using single shear lap joints. The Cu 5Zn 8 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 Cu 5Zn 8 phase is rather flat for all the aging temperatures. The Cu 6Sn 5 isolated phase was observed after long-term aging. Intermetallic thickness measurements show that the thickness of the Cu 5Zn 8 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 Cu 5Zn 8 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.