Figure 4. Effect of filler volume fraction on the thermal conductivity
of the thermal interface material for graphite platelets and carbon
black.
The high-resolution scanning electron micrographs of the exfoliated
graphite platelets are illustrated in Figure 5 for the production of the
thermal interface material. Conventional polymeric thermal interface
materials may be used as the thermal interface material. However,
currently used polymeric thermal interface materials are typically cure
in place silicone gel materials that are required to be shipped and
stored frozen. They also have short pot lives upon opening, short shelf
lives, and require special dispensing equipment to apply. Therefore, it
is necessary to eliminate, avoid, or at least reduce these
aforementioned drawbacks associated with conventional polymeric thermal
interface materials. A thermal interface material in the form of a pad
of self-healing, thermoplastic material may or may not be naturally
tacky. The thermal interface material may have a softening or melting
temperature higher than a normal operating temperature of a central
processing unit, for example, normal operating temperature from about 60
°C to about 100 °C or from about 30 °C to about 40 °C. Consequently, the
pad of thermoplastic material will soften or melt once, for example,
during an adhesive curing stage or during an initial operation of the
central processing unit, and then solidify. Thereafter, the pad of
thermoplastic material may be used below its softening or melting
temperature and remain solidified. The thermal interface material
comprises a thermoplastic phase change material having a softening or
melting point temperature that falls within a range from about 80 °C to
about 200 °C or about 100 °C to about 180 °C. Or, for example, the
thermal interface material may have a softening or melting temperature
of about 40 °C, 60 °C, or 80 °C. The thermal interface material may have
a thermal conductivity of about 0.8 Watts per meter per Kelvin or more,
or 2.0 Watts per meter per Kelvin or more, which thermal conductivity
may be enhanced by incorporating thermally-conductive filler into the
thermoplastic material. The thermal interface material may comprise a
low melting alloy having a melting temperature of about 200 °C or less.
The surface features protrude from the mating surface of the particular
heat transfer component into selected areas of the interface. The
protruding features may limit the relative movement of the mating
surfaces in the selected areas of the interface during thermal cycling
due to coefficient of thermal expansion mismatch. Limiting the relative
movement may reduce strain on the thermal interface material in areas of
the interface proximate to the protruding features. Reducing the strain
on the thermal interface material may reduce the potential for thermal
interface material pump-out and the associated increase in thermal
resistance due to loss of material from the interface. The protruding
features are incorporated onto the heat sink and protrude out from the
heat sink mating surface into selected areas of the interface between
the heat sink mating surface and the module lid mating surface. The
protruding features may be incorporated onto the heat sink through a
forming process during manufacturing of the heat sink. Channels may be
machined or otherwise incorporated into the heat sink, and the channels
may be filled with a material that is appropriate for the heat sink in
order to form the protruding features. In some cases, the heat sink may
be formed from a heat sink material, such as an aluminum-based material
or a copper-based material, among other alternative materials. When the
heat sink is formed from an aluminum-based material, the channels may be
filled with an aluminum-based material that is the same as the
aluminum-based material of the heat sink or that is substantially
similar to the aluminum-based material of the heat sink. Alternatively,
with respect to the example in which the heat sink is formed from an
aluminum-based material, the channels may be filled with other materials
compatible with the aluminum-based material for efficient transfer of
heat to the heat sink.