Ex situ observations of fast intermetallic growth on the surface of interfacial region between eutectic SnBi solder and Cu substrate during solid-state aging process

We focused on the surface microstructure and morphology changes of Cu/eutectic SnBi/Cu joint during solid-state aging process through ex situ scanning electron microscopy (SEM) observations and energy dispersive X-ray spectroscopy (EDXS) analysis. Different intermetallic compound (IMC) growth behaviors on the surface from the bulk of solder joints were observed and investigated. The results indicated that at the initial stage of solid-state aging IMCs protruded from the interfacial region with two different microstructures, the small particles on the Cu surface and chunk-type IMC at the solder side. With the increment of solid-state aging time, although the total thickness of IMCs increased very slowly, Cu₃Sn phases grew fast by the consumption of Cu₆Sn₅ phase. Growth kinetic analyses for IMC on the surface and in the bulk of solder joints revealed that the IMC growth on the surface was faster than that in the bulk at the initial stage. For the long-term aging, although the total IMC thickness was still thicker than that in the bulk, the rate of the IMC growth on the surface was slower than that in the bulk. The growth of IMC on the surface of interfacial region was divided into two distinct stages, which corresponds to different IMC growth behaviors.