Figure 3. Effect of thermal interface material thickness on the thermal resistivity of the thermal contact for graphite platelets and carbon black.
The effect of filler volume fraction on the thermal conductivity of the thermal interface material is illustrated in Figure 4 for graphite platelets and carbon black. In most polymer composite applications, the resin system is mixed with expanded graphite, chopped fibers or other additives for processing and durability requirements and sometimes with fillers for further cost reduction. These additives are incorporated into the polymer in specific amounts in order customize properties of the resulting composite. Additionally, thermal, electrical, and mechanical properties are all affected by the form and matter of the particulate in polymers. Desired properties can be obtained by varying filler content and process techniques. However, the orientation and accommodation of the particulate in the component may cause weak areas, susceptible to crack initiation and propagation at sharp bends. Thermal interface materials are used between heat-generating components and heat sinks to establish heat-conduction paths therebetween. However, thermal interface materials provide a thermally conducting heat path that is substantially contained between the heat generating components and the heat sink, which results in a relative narrow heat conduction path that causes heat to be localized around the electronic component. That is, a substantial portion of heat generated by the electronic component is conducted via the path of least impedance through the thermal interface material that lies directly between the electronic component and the heat sink. This results in limited heat spreading throughout the thermal interface material and the heat sink. Thermal interface materials provide a limited heat-conduction path and may include flexible heat-spreading materials and one or more layers of soft thermal interface material. Flexible heat-spreading materials may generally refer to and include a wide range of materials having flexibility equal to or greater than a sheet of stamped aluminum and flexibility equal to or greater than a sheet of stamped copper. Within the flexible heat-spreading material, heat laterally spreads out such that there will be more surface area from which heat may be transferred from the flexible heat-spreading material. The greater surface area due to the laterally spreading of the heat may increase and improve the heat transfer efficiency associated with the flexible heat-spreading material and the overall thermally-conductive interface assembly. Heat may be transferred from the flexible heat-spreading material via conduction in the normal direction to an outer layer of thermal interface material, in which flexible heat-spreading material is sandwiched between, bonded to, or encapsulated within layers of thermal interface material. Heat may be transferred from the flexible heat-spreading material via convection to air or other ambient environment, in which a heat-spreading material includes thermal interface material on only one side such that other side of the heat-spreading material is exposed to air or another ambient environment. In cases in which thermal interface material is on or along only one side of a heat-spreading material, and the thickness of the thermal interface material may be greater than the thickness of the flexible heat-spreading material. Alternatively, the thickness of the thermal interface material may be about equal to or less than the thickness of the flexible heat-spreading material in other cases. In cases in which flexible heat-spreading material is sandwiched between, bonded to, or encapsulated within layers of thermal interface material, the layers of thermal interface material along one side of the flexible heat-spreading material may be thicker, thinner, or about equal to the layers of thermal interface material along the other or opposite side of the flexible heat-spreading material. For example, a flexible heat-spreading material may have inner and outer layers of thermal interface material, where the inner layer, which is intended to contact one or more electronic components, is thicker than the outer layer. Thermally-conductive interface assemblies include one or more outer layers of soft thermal interface materials that are relatively flexible, soft, and thin, for example, for good conformance with a mating surface. This, in turn, may help lower thermal impendence as thermal impedance depends upon the degree of effective surface area contact therebetween. The ability to conform to a mating surface tends to be important as the surfaces of a heat sink and a heat-generating component are typically not perfectly flat and smooth, such that air gaps or spaces tend to appear between the irregular mating surfaces.