Diffusion behavior and microstructural evolution of bonding interface between AuSn20 and tungsten-copper alloy

Purpose This paper aims to investigate the microstructural and diffusion behavior of the bonding interface between AuSn20 thermal interface material and tungsten-copper (WCu) alloy. Design/methodology/approach This research uses thermo-compression bonding, with a Ni/Au coated WCu alloy surface to examine the diffusion behavior of AuSn20 at the interface. Characterization techniques including scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy were used to analyze the interfacial microstructure and diffusion paths. Additionally, first-principles calculations were conducted to determine the diffusion coefficients of Cu and Sn in Ni, further elucidating the interaction mechanisms among elements during the soldering process. Findings The electroless nickel plating on WCu alloy surface promotes fast diffusion of AuSn solder into the grain boundary in Ni layer. It is found the solution of Cu in Ni layer also accelerates AuSn diffusion. As AuSn passed Ni layer and reached the WCu side, it tended to accumulate within the NiW rich region, forming an Au-Sn-Ni-W solid solution. Ni provides extra diffusion drive force for Au and Sn, on the other hand the moderate solubility of Au and Sn in NiW alloy improve solid solution stability via high-entropy effect. Under varying temperature and compositional conditions, the interfacial region displays phase transitions from a single-phase solid solution to multiphase structures, with elemental distribution inconsistencies. By combining first-principles calculations and energy-dispersive X-ray analysis, this study examined the temperature dependence of Ni-Cu and Ni-Sn interdiffusion coefficients, highlighting that larger difference in diffusion coefficients may lead to the Kirkendall effect, potentially affecting the bonding strength and interface stability. Originality/value This research offers both theoretical insights and experimental evidence for controlling AuSn and WCu alloy interfaces, but also sheds lights to the reliability of high-temperature soldering materials.