Effect of Bi on the microstructure evolution of Sn-3Ag-0.5Cu/Cu solder joint

The reliability of the solder joints is very important for lead-free solders to be applied extensively. The properties of joints have a close relationship with microstructure of the joints (W. Yang et al., J. Electr. Mater. vol. 24, no. 10, 1995). Therefore, this paper studied the microstructure evolution of Sn-3Ag-0.5Cu-XBi/Cu (where X=0-3%) solder joints after aging at varying temperatures. Sn-Ag-Cu-Bi solders have been selected for research because experiments revealed the eutectic Sn-Ag-Cu alloy to be a very promising solder in avionics and automotive applications where the solder joints are subject to numerous thermal cycles and mechanical vibrations. It has good wetting properties and forms good quality joints with copper, and also has good thermo-mechanical properties and soldering technology (C.M. Miller et al, ibid., vol. 23, no. 7, 1994), but compared with SnPb eutectic, it has higher melting temperature and high cost. It is well known that Bi has beneficial effect on mechanical properties of bulk solder when the addition of Bi is within 3%; however, there are only initial reports on the effect of Bi in solder joints, and a full understanding of Bi effects in solder applications is crucial.