Composite Soldering Materials Based on BiSnAg for High-Temperature Stable Solder Joints

The thermal load on electronic assemblies is constantly increasing. The reasons for this increase are, on the one hand, the integration of power electronic components in an ever smaller space and, thus, an increasing power density and, on the other hand, the increasingly harsh environmental conditions with high temperature load. In addition to electronic components and substrate materials, the soldered connections are also exposed to this stress and must withstand it. The thermal stability is primarily determined by the melting temperature of the solder material or by the remelting temperature of the final solder interconnection. The remelting temperature can be purposefully increased through diffusion soldering. The advantage of diffusion soldering is that the operating temperature of the final solder joint can exceed the joining process temperatures. By using the composite soldering materials and diffusion soldering process, it is possible to produce the solder interconnections that can withstand the high thermal and thermomechanical stress. In this work, the composite solder material, consisting of the base solder alloy BiSnAg in eutectic composition with a melting point of 139°C and added copper particles, was examined. The added copper particles have a direct influence on the dynamics of the diffusion process. Diffusion can also be influenced by adjusting the soldering process parameters, such as maximum temperature and time above liquidus of the base solder alloy, with the aim of achieving isothermal solidification. The solidification can take place through the parallel reactions: the reaction between tin and copper with the formation of high-melting intermetallic phases Cu3Sn and Cu6Sn5 and the growth of bismuth (Bi) crystals through coarsening of the structure and tin depletion in the original eutectic solder alloy. © 2022 International Microelectronics Assembly and Packaging Society.