The development of a natural graphite heat-spreader

Thermal management systems consist of external cooling mechanisms, heat dissipaters, and thermal interfaces. The primary function of heat dissipaters, e.g. heat sinks, is to create the maximum effective surface area where heat is transferred into and removed by the external cooling medium. Heat dissipater performance is characterized by its intrinsic thermal conductivity, physical surface area, and pressure drop (or drag) coefficient (Kraus and Bar-Cohen, 1995). Another variable, the heat spreading coefficient, introduced by Tzeng et al (PCIM, 2000), must be considered when the heat dissipater is a thermally anisotropic material. A high degree of thermal anisotropy reduces the temperature gradient in the component plane and increases effective heat transfer area, characteristics that are most desirable for electronics with high heat-intensity components. The ability to direct heat in a preferred direction is a further advantage of anisotropic heat-spreader materials. Carbon and graphite-based materials are attracting interest as anisotropic heat-spreaders, with another advantage being their low density. Most carbon and graphite-based materials used to date are based around carbon fibers. These are high cost due to the need for high temperature graphitization processes to develop the required fiber thermal properties. A new form of graphite heat-spreader material is described in this paper, based around naturally occurring graphite. Since this material has been graphitized by nature, anisotropic heat-spreaders with high thermal conductivity can be manufactured without carbon fiber-based additives.