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.