Currently the International Thermonuclear Experimental Reactor (ITER) is constructed in France. ITER aims to generate 500 MW power by nuclear fusion in a D-T plasma. The plasma facing components (PFC) in ITER are designed to absorb a steady state heat flux load 10-times higher compared to the re-entry phase of a space shuttle. Such PFC require the joining of Cu to carbon fibre reinforced carbon (C-C) composites. For that purpose PLANSEE SE drills holes by means of a laser into the C-C surface prior to a Cu casting process in the presence of activating elements. Aiming at an increased reliability in operation an optimized joint is being developed and characterized in this work. It features new interface metallurgy. This results in increased fracture toughness as well as shear and tensile strength, respectively. To explain the improved behaviour the interface region of differently processed joints are investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). In addition, the crack path is analyzed by stereomicroscopy. The improved properties can be explained by a fracture path that changes depending on the local structure in the C-C composite. Differences in the performance of the investigated joints can be correlated to interface morphologies and metallurgical processing parameters. A simple testing method and key features of the joint have been identified, which allow further optimizing the thermal fatigue behaviour of Cu to C-C joints in PFC.