Dissolution and interfacial reaction of Nb in contact with the molten 52In-48Sn solder

The dissolution and interfacial reaction of Nb in contact with the molten 52In-48Sn solder has been investigated in the temperature range 192-260 oC for periods ranging up to 1 year. Intermetallic compounds (IMCs) grow at the interface between the Nb and the solder, after a latency time required for Nb to diffuse in the solder, and these have been identified by X-ray diffraction as NbSn2 hexagonal platelet crystals, with a highly preferred orientation. The crystal sizes have been found to follow a log-normal distribution and crystal growth begins first at the corners of the substrate, followed by the edges and then finally in the centre. The Nb/solder system is unusual in that the IMC growth is several orders of magnitude slower than observed in other solder-substrate systems, and the limiting size of the IMC layer actually decreases with increasing temperature. A model explaining these observations is presented where the IMC nucleation and growth are controlled by diffusion of Nb atoms in the liquid solder. Low meta-stable concentration of Nb at the interface, a critical supersaturated concentration of Nb very close to the meta-stable concentration required before the IMC can grow and negligible solid state diffusion in the IMC are identified as the key factors responsible for the observed kinetics of dissolution and interfacial reaction