1. Introduction
In an electronic device, a thermal interface material is a material that is disposed between a heat generating component of an electronic device and a heat dissipating component in order to facilitate efficient heat transfer between the heat generating component and the heat dissipating component [1, 2]. The powering up or powering down of the electronic device may cause temperature changes which may cause a relative motion between the heat generating component and the heat dissipating component, including in-plane motion and out-of-plane motion due to coefficient of thermal expansion mismatch [3, 4]. With the development of more sophisticated electronic components, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high-power optical devices, relatively extreme temperatures can be generated [5, 6]. However, microprocessors, integrated circuits, and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure [7, 8]. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates these negative effects.
With the increased need for heat dissipation from microelectronic devices caused by these conditions, thermal management becomes an increasingly important element of the design of electronic products [9, 10]. As noted, both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment [11, 12]. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an exponential increase in the reliability and life expectancy of the device [13, 14]. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance. A thermal management system is designed to assist with this objective. One element of a thermal management system is a thermal interface material [15, 16]. A typical use for a thermal interface material is to thermally connect a computer chip to a cooling module to overcome contact resistance and lack of surface conformity between the heat sink, or the cooling module and the chip or other heat source. Typically, thermal interfaces consist of thermal greases, phase change materials, and tapes [17, 18]. Flexible graphite is readily applicable to such applications because of its low thermal resistance and its ability to conform to the surfaces to be interfaced, especially when either or both surfaces are not completely flat [19, 20]. Such characteristics are important in a thermal management system because reducing the thermal resistance as much as possible is of paramount importance.
The process for manufacturing the flexible graphite used in the thermal interface is well-known [21, 22]. In general, flakes of natural graphite are intercalated in an acid solution. After the flakes are intercalated, they are washed and dried and then exfoliated by exposure to a high temperature for a short period of time. This causes the flakes to expand or exfoliate in a direction perpendicular to the crystalline planes of the graphite [23, 24]. The exfoliated graphite flakes are vermiform in appearance and are therefore commonly referred to as worms. The worms may be compressed into sheets or foils with a density approaching theoretical density although a density of about 1.1 grams per cubic centimeter is considered typical for most applications. The sheets of flexible graphite can be cut into any desired configuration to suit a particular application. Graphite is made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another [25, 26]. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphite consists of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well-ordered carbon layers. In other words, highly ordered graphite has a high degree of preferred crystallite orientation [27, 28]. It should be noted that graphite possesses anisotropic structures and consequently exhibit or possess many properties that are highly directional such as thermal and electrical conductivity.
The present study aims to provide a thermal interface material with aligned graphite nanofibers in the thermal interface material to enhance the thermal interface material performance. The method includes preparing the graphite nanofibers in a herringbone configuration, and dispersing the graphite nanofibers in the herringbone configuration into the thermal interface material. The method further includes applying a magnetic field of sufficient intensity to align the graphite nanofibers in the thermal interface material. Thermal materials are used in packaging as interfaces between devices to dissipate heat from these devices. One typical thermal interface material typically includes a polymer matrix and a thermally conductive filler. As electronic components have become smaller and more densely packed on integrated boards and chips, designers and manufacturers now are faced with the challenge of how to dissipate the heat which is generated by these components. The thermal interface material technologies used for electronic packages encompass several classes of materials, such as phase change materials, epoxies, greases, and gels. However, there is still a need for thermal interface materials and methods for making thermal interface materials having improved thermal conductivity property by maximizing the anisotropic benefit of exfoliated graphite platelets to the fullest extent. The effect of filler volume fraction on the thermal resistivity of the thermal contact and the thermal conductivity of the thermal interface material is investigated for graphite platelets and carbon black. The effect of pressure on the bond line thickness of the thermal interface material is evaluated for smooth and rough surfaces. Particular emphasis is placed upon the heat conduction properties of thermally conductive interface materials with exfoliated graphite platelets.