Thermal joint conductance for flexible graphite materials: analytical and experimental study

Increasingly, thermal interstitial materials (TIMs), such as metallic foils, solder, metallic coatings, polymeric matrices loaded with highly conducting filler particles (e.g., elastomers), greases, and phase-change materials are being employed to a greater extent in power generating systems. With greater use, follow an increased interest in the thermal transport and mechanical properties of these materials. These properties include thermal conductivity, thermal diffusivity, Young's modulus, Poisson's ratio, and the thermal resistance at the interface between the interstitial materials with the substrate material; however, these are just a few of the representative thermophysical properties that might be needed to model these thermal interstitial materials. In addition to these TIMs, a novel material has recently been fabricated, from highly pure graphite flakes, which has shown great promise in enhancing the joint thermal conductance without the limitations experienced by some TIMs (e.g., need for reflow temperatures for phase change materials to achieve good contact conductance). To provide information on the thermal joint conductance of an important interstitial material employed in microelectronic components, an experimental investigation has been undertaken for a class of TIMs called "Flexible Graphite". The experimental data were compared to an analytical model developed for elastic layers that was applicable to this class of thermal interface material. In addition, a comparison between the model, and a paraffin phase-change material deposited onto one surface of the elastic layer was conducted. The model assumes that complete wetting of the paraffin material takes place, therefore, only an additional gap conductance expression was incorporated. The model and data were found to be in good agreement over the pressure range within the investigation. The proposed model can be used to predict the lower bound on the joint conductance.