Cu particle-containing In-matrix composites for thermal interface material (TIM) applications were prepared via liquid phase sintering, following chemical modification of the Cu-In interfaces. The optimized composite TIM possessed 1.5 times the thermal conductivity, and twice the yield strength, of pure In. Joints of the composite TIM between pairs of cylindrical Cu rods were used to measure shear behavior and thermal resistance as functions of three parameters: (i) joint thickness, (ii) thermal excursion history, and (iii) type of interfacial layers between Cu and In. The composite joints showed good shear compliance, with a shear yield strength of 2.7 MPa, as well as substantially lower joint thermal resistance (0.021 cm(2) K W-1) than pure In joints, which are commercially used in high-end TIM applications. The thermal resistance of the joints was found to be a sensitive function of the interfacial contact resistance between the Cu particles and In within the TIM, as well as between the TIM and the Cu substrates. The TIM-substrate interfaces, in particular, play an increasingly important role as the joint becomes thinner, limiting the joint thermal resistance. To reduce the interfacial contact resistance, a diffusion barrier of 1-2-nm-thick Al2O3 was applied by atomic layer deposition on both the Cu particles and the Cu substrates, followed by a 20-nm-thick Au layer, which served as a wetting enhancer. The engineered interfaces also improved the stability of the composite TIM joints under aging conditions.