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.