Improving materials for thermal interface and electrical conduction by using carbon
Leong, Chia Ken
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Thermally and electrically conductive materials are needed in electronic packaging for heat dissipation and electrical interconnection respectively. Carbon is both thermally and electrically conductive, though it is less conductive than metals. This dissertation shows that carbon in the form of carbon black is highly effective for formulating thermal interface materials and electrically conductive paints/pastes, due to its being in the form of porous agglomerates of nanoparticles. This structure promotes conformability. The thermal interface materials (TIMs) are in the form of pastes, which are optionally in combination with a solid sheet carrier, such as an aluminum foil. They serve to enhance thermal contacts. The vehicle (e.g., polyol esters, polyethylene glycol, etc.) of the thermal paste is chosen to promote conformability and spreadability. Synergistic use of carbon black and silver pastes is effective and is particularly valuable for thermal contacts that involve rough (15 μm) proximate surfaces. For smooth (0.009 μm) surfaces, the carbon black paste is more effective than a widely used commercial silver paste. Comparative evaluation of the performance of the TIMs developed in this work is provided through thermal contact conductance measurement (heat flux measurement across copper surfaces) and temperature measurement of a computer in operation. The performance of TIMs is understood through finite element modeling of both carbon black paste and silver paste sandwiched by copper surfaces. These pastes differ in the elastic modulus, thermal conductivity and spreadability. The performance of the carbon black paste hinges on its low modulus; the performance of the silver paste hinges on its high thermal conductivity. The modeling and experimental results agree reasonably well. An electrically conductive paste for providing pressure contacts of low contact electrical resistivity has been developed by using carbon black as the sole conductive component. The vehicle involves either polyethylene glycol or butyl ether. An electrically conductive paint that provides low volume electrical resistivity has been developed by using carbon black as a minor conductive component that partially replaces silver in the paint. The vehicle involves silane. The partial replacement enhances both the electrical conductivity and the scratch resistance, in addition to decreasing the cost.