3. Results and discussion
The low-resolution scanning electron micrographs of the exfoliated graphite platelets are illustrated in Figure 1 for the production of the thermal interface material. Metal heat spring thermal interface materials can be made to preferentially release from a desired surface by adding a metal cladded aluminum foil, but require a high pressure in order to provide effective contact, minimize thermal resistance, and fill any interface gaps [45, 46]. Typically, some form of mechanical retention hardware is used to ensure adequate contact and optimal performance [47, 48]. Unfortunately, clamping forces of the mechanical retention hardware can damage the semiconductor component [49, 50]. The metal spring thermal interface materials also suffer from low elasticity, poor gap filling, and the potential for micro motion induced oxidation over time [51, 52]. Furthermore, the aluminum foil release layer can negatively affect thermal performance. Polymeric elastomer materials may be used as a thermal interface material [53, 54]. The thermal properties of polymeric elastomer materials can be modified with the addition of various forms of carbon, for example graphene layers, graphite flake, carbon fibers, or carbon nanotubes [55, 56]. Polymeric elastomer materials offer both high thermal performance and reasonable gap filling capability to enable good contact between a semiconductor component and a heat sink. However, like the metal heat spring thermal interface materials, polymeric elastomer materials also require high pressures in order to adequately fill interface gaps. Also like the metal heat spring thermal interface materials, excess pressure can cause damage to the semiconductor component, for example, cracking. Polymeric elastomer materials adhesively bond to surface of both the semiconductor component and the heat sink. If future removal of the heat sink is required, the risk of damaging the semiconductor component or the thermal interface material is high due to the adhesive properties, for example the inherent tackiness, of polymeric elastomer materials. Although conventional release layers, such as, for example, non-stick polymers or aluminum foil, can be applied to polymeric elastomer materials, they create an insulative barrier and degrade thermal performance. Expanded graphite materials may be used as a thermal interface material. Expanded graphite materials are available at relatively low cost, have good thermal performance and excellent dimensional compliance or gap filling capability, and release cleanly from device surfaces for easy reuse during semiconductor component repair or replacement. However, expanded graphite materials have limited elasticity and are not easily reusable for different surface topologies. Carbon nanotube materials may be used as a thermal interface material. Carbon nanotube materials include sheets of densely packed, generally vertically aligned and intertwined. Carbon nanotube materials offer extremely high thermal conductivity and thus excellent thermal performance. Carbon nanotube materials are durable and reusable, but have extremely limited elasticity and gap filling capabilities and require high mechanical loads to enable good thermal performance. Carbon nanotube materials are also relatively expensive to manufacture, and as such, are typically used as thermal beds on testers for lidded modules where high loads can be used without component damage and extremely high component test volumes are anticipated. Flexible graphite materials may also be used for their superior heat transfer properties. Flexible graphite materials generally have poor gap filling capabilities. Thermal grease or thermal gel may be used as a thermal interface material. Thermal grease or thermal gel is typically filled with various forms of thermally conductive media, such as, for example, carbon, metal particles, ceramic particles, or metal oxide particles. These materials have reasonable thermal performance and gap fill capability, but must be dispensed or printed onto surfaces. After being applied, thermal gel must be thermally cured as they rely on adhesive bonding at device and cooling hardware surfaces to ensure stable interface performance. Neither thermal grease nor thermal gel meets suitable reuse requirements. Phase change material may be used as a thermal interface material. Phase change material is a substance which can change from a solid to liquid at a certain temperature, for example, at room temperature. The phase change material absorbs heat when changing from a solid to a liquid, and releases heat when changing from a liquid to a solid. Advantages of phase change material include superior gap filling capability and good thermal performance.