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.