This study reports on the processing and characterization of composite solders produced by liquid phase sintering, which comprise a high-melting phase such as Cu embedded in a matrix of a low-melting phase such as In. These solders combine higher electrical/thermal conductivities with high mechanical compliance, and are suitable for a range of next-generation thermal interface material and interconnect applications. After considering a range of compositions, a solder with 60 volume percent In was found to possess the requisite combination of compliance and conductivity. A thin interfacial Au layer was utilized for the dual purposes of (a) enhancing the wetting between Cu and In, and (b) reducing interfacial reaction between Cu and In to form coarse intermetallic compounds (IMC) scallops, which adversely affect both mechanical and electrical/thermal properties. The Au layer increased the thermal conductivity of the solder by a factor of similar to 2 while reducing the yield strength to make the solder more compliant. The effects of particle size, shape, and volume fraction are discussed, and a simple model is utilized to explain the trends in the mechanical and the thermal properties.