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.