A study on the interfacial reaction of Sn58Bi/Cu soldered joints under various cooling and aging conditions

As one of the important factors affecting the quality of soldered joints, the interfacial intermetallic compound (IMC) formed at the solder/substrate interface is of increasing concern to researchers. Thus, the growth behavior of interfacial IMC in Sn58Bi/Cu solder joints was investigated as different cooling methods-quenched water, cooling in air, and cooling in a furnace during reflowing were used. In this work, the effect of cooling rate on interfacial IMC growth is studied in Sn58Bi/Cu system. Morphology and growth kinetics of the interfacial Cu–Sn IMC under various cooling rates were studied. It was found that only scallop-like Cu₆Sn₅ IMC layer was observed between solder and Cu substrate in cases of water cooling and air cooling, while bi-layer composed of scallop-like Cu₆Sn₅ and planar Cu₃Sn was detected under furnace cooling due to sufficient reaction time to form Cu₃Sn between Cu₆Sn₅ IMC and Cu substrate which resulted from slow cooling rate. Samples with different reflow cooling rate were further thermal aged at 373 and 393 K, respectively. The results showed that the morphology of interfacial Cu–Sn IMC layer would transform from scallop-like to planar-like with increasing aging time, and the thickness of IMC increased linearly with square root of aging time. The growth constants of interfacial IMC layer were obtained and compared for different cooling rates and aging temperatures, indicating that the IMC layer thickness increased faster in samples under slow cooling rate than the fast cooling ones under the same aging condition, and higher aging temperature could result in faster growth of IMC layer. The interfacial Cu₆Sn₅ grains gradually transformed to facet prismatic shape as the cooling rate decreased, and the grain size increased dramatically as the samples were cooled in furnace, comparing with that of samples cooled in water and air.