With the increased deployment of power modules in demanding conditions, sintering materials, especially composite sintering materials, have raised growing interest due to their cost-effectiveness and suitability. Therefore, this study explores the viability of Cu–Ag composite sintering material, focusing on solvent influence through microstructure and mechanical behavior analysis. Micron-sized particle-based Cu–Ag composite pastes were designed and compared using eight solvents (four epoxy-free and four epoxy-added) based on fluidity and thermal stability. The sintered joints' performance, assessed through shear strength analysis, showed comparable values to pure silver sintering for both epoxy-free and epoxy-added samples. Optimized samples from each solvent system underwent reliability analysis, demonstrating that Cu–Ag joints with epoxy resin exhibited significantly higher shear strength after high-temperature storage and thermal cycling tests. Micromorphology and elemental composition analysis revealed differences in aging mechanisms, primarily attributed to variations in porosity due to oxide formation and pore filling by epoxy resin under different solvent systems. Further nanoindentation characterization of micromechanical properties, including hardness, modulus, and creep properties, during high-temperature aging, established constitutive models for insights into reliability evolution. In conclusion, the optimized epoxy-added Cu–Ag sintered joints proposed in this study demonstrated exceptional reliability and acceptable micromechanical properties, presenting a promising option for high-temperature power packaging.