Figure 2. Moderate-resolution scanning electron micrographs of the exfoliated graphite platelets for the production of the thermal interface material.
The effect of thermal interface material thickness on the thermal resistivity of the thermal contact is illustrated in Figure 3 for graphite platelets and carbon black. The heat dissipating components facilitate heat dissipation from the surface of a heat source, such as a heat-generating electronic device, to a cooler environment, usually air [65, 66]. In many typical situations, heat transfer between the solid surface of the electronic device and the air is the least efficient within the system, and the solid-air interface consequently represents the greatest barrier for heat dissipation [67, 68]. The heat dissipating components seek to increase the heat transfer efficiency between the electronic device and the ambient air primarily by increasing the surface area that is in direct contact with the air or other heat transfer media. This allows more heat to be dissipated and consequently lowers the electronic device operating temperature. The primary purpose of a heat dissipating component is to help maintain the device temperature below the maximum allowable temperature specified by its designer or manufacturer [69, 70]. Typically, the heat dissipating components are formed of a metal, especially copper or aluminum, due to the ability of metals like copper to readily absorb heat and transfer it about its entire structure. In the case of heat sinks, copper heat sinks are often formed with fins or other structures to increase the surface area of the heat sink, with air being forced across or through the fins to effect heat dissipation from the electronic component, through the copper heat sink and then to the air [71, 72]. Limitations exist, however, with the use of metallic heat dissipating components. One limitation relates to the relative isotropy of a metal that is, the tendency of a metallic structure to distribute heat relatively evenly about the structure. The isotropy of a metal means that heat transmitted to a metallic heat dissipating component becomes distributed about the structure rather than being preferentially directed to a desired location. In addition, the use of copper or aluminum heat dissipating elements can present a problem because of the weight of the metal, particularly when the heat transmitting area of the heat dissipating component is significantly larger than that of the electronic device. For example, several heat sinks need to be arrayed on a circuit board to dissipate heat from a variety of components on the board. If metallic heat sinks are employed, the sheer weight of the metal on the board can increase the chances of the board cracking or of other equally undesirable effects, and increases the weight of the component itself. In the case of larger heat dissipating components such as for example that class of components known as heat spreaders, the weight of a pure copper heat spreader requires special mechanical features and designs to hold the heat spreader. What is desired, therefore, is a heat dissipating component effective for dissipating heat from a heat source such as an electronic device. The heat dissipating component should advantageously be relatively anisotropic, as compared to a metal like copper or aluminum and exhibit a relatively high ratio of thermal conductivity to weight. One group of materials suitable for use in heat sinks are those materials generally known as graphite, but in particular anisotropic graphite such as those based on natural graphite and flexible graphite.