Microstructure Evolution of SnAgCuEr Lead-free Solders Under High Temperature Aging

In this work, we have systematically investigated the evolution of microstructure and of intermetallic compounds (IMCs), in particular, for lead-free SnAgCuEr solders during isothermal aging tests. The effect of trace amounts of the rare earth element Er on this process has also been studied. The results indicate that diffusion and reassembly occur in the solder matrix during the aging process, and the major influence of the rare earth element Er is concentrated on the nucleation sites. ErSn3 IMCs formed from the molten solder provide heterogeneous nucleation sites for the IMCs in the soldering and aging process. Subsequently, the Cu-Sn IMCs produced during soldering and Ag-Sn IMCs precipitated during the aging process have uniform size and evenly distribute in the solder matrix, and the refinement effect has been achieved in Er-containing solder joints. In addition, some cracks can be seen in Er-free solder joints, and the cracks may nucleate and propagate in the structure along the compound/solder boundaries after long aging times.