Transient liquid phase soldering for lead-free joining of power electronic modules in high temperature applications

The study focuses on a new variant of transient liquid phase soldering (TLPS) using tin based solder with copper powder. This technology may act as an alternative for lead free joining of semiconductor dies in power electronic applications at high operating temperature. Lead-free joining technologies currently used like gold-rich solders and silver sintering are well suited for high temperature applications. However, due to the high metal price they have a limited acceptance. Using a special soldering process it is feasible to produce an almost void-less solder joint, using a paste of tin-based solder powder (e.g. SAC305), copper powder and a solvent which is hardly activated. The resulting interconnection is characterized by an almost complete transformation into intermetallic phases of Cu6Sn5 and Cu3Sn. Thus the melting point of the transformed interconnect can be increased up to the decomposition temperature of the Cu6Sn5 intermetallic phase which is 415 °C. A two-st ep soldering process allows to eliminating the typical skeleton structure that forms as a result of the immediate reaction of the liquid tin-based solder with the higher melted copper powder to form the Cu6Sn5 and Cu3Sn intermetallic phases. An alternative way compared to the two-step-process is also explained in this study: Capillary forces let the solder flow into the gap filled with Cu spheres.