Figure 6. Transmission electron micrographs of the exfoliated graphite
platelets for the production of the thermal interface material.
The effect of pressure on the bond line thickness of the thermal
interface material for smooth surfaces is illustrated in Figure 7 for
graphite platelets and carbon black. The thermal interface material
structures include protruding surface features to reduce thermal
interface material migration. The thermal interface material structures
incorporate surface features onto a particular mating surface, such that
the features protrude into selected areas of an interface separating the
particular mating surface from another mating surface. In some cases,
the protruding features may be incorporated onto a mating surface of a
heat spreader that surrounds an electronic component and distributes
heat away from the electronic component [81, 82]. In other cases,
the protruding features may be incorporated onto a mating surface of a
heat sink that overlies the module lid and is separated from the module
lid by the interface [83, 84]. During thermal cycling, a coefficient
of thermal expansion mismatch between the module lid and the heat sink
may cause relative motion between the module lid and the heat sink
[85, 86]. By incorporating surface features that protrude from a
mating surface into selected areas of the interface, the potential
relative movement of the mating surfaces in the selected areas may be
limited [87, 88]. 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. A heat source dissipates heat using a
heat sink that is joined to the heat source by a thermal interface
material, such as a thermal grease or a thermal putty. Compressing the
thermal interface material between the heat source and the heat sink may
fill an interface gap between a mating surface of the heat source and a
mating surface of the heat sink in order to form an interface for
efficient removal of heat from the heat source via the heat sink. The
heat source and the heat sink may correspond to different materials that
have different coefficient values of thermal expansion. Due to the
coefficient of thermal expansion mismatch between the heat source and
the heat sink, thermal changes associated with thermal cycling cause
relative movement of the heat source and the heat sink. To illustrate,
during thermal cycling, the heat source may bow upward into a central
area of the interface, and the heat sink may bow downward into the
central area of the interface, resulting in a significant reduction of
interface thickness between the heat source and the heat sink in the
central area of the interface. The resulting strain may cause the
thermal interface material to migrate away from the central area of the
interface over time. Pump-out of the thermal interface material results
in increased thermal resistance due to loss of material from the
interface. By contrast, the thermal interface material structures
incorporate surface features onto a mating surface of a particular heat
transfer component of an electronic component cooling assembly that
includes two heat transfer components separated by an interface that
includes a thermal interface material. The heat spreader surrounds an
electronic component and distributes heat away from the electronic
component. In some cases, the surface features may be incorporated onto
a mating surface of the heat spreader. In other cases, the surface
features may be incorporated onto a mating surface of the heat sink.