Minimizing Thermal Resistance with Direct Attach Heat Spreaders
Wednesday, February 01 2012
Page 2 of 3
Low CTE AlSic HiK Plate
Figure 3. Low CTE High Heat Flux Vapor Chamber
An embedded heat pipe plate (HiK
Plate), is a machined plate (typically aluminum)
that has heat pipes either soldered
or epoxied in place to create an
enhanced heat transfer path from the
source to the sink.
Aluminum silicon carbide (AlSiC) is a
metal matrix composite that combines
the highly conductive properties of aluminum
with the favorable CTE of silicon.
The CTE value can be matched to
that of the electronic device by adjusting
the composition of Al and SiC. The
isotropic controlled thermal expansion
values can range from 7-12 ppm/°C.
AlSiC typically has a thermal conductivity
of 200 W/m-K. While this can be effective
in many cases, high power or concentrated
heat loads will require greater
heat spreading capability. By embedding
heat pipes into the AlSiC, bulk thermal
conductivity values ranging from 500 to
1,000 W/m-K can be realized (dependent
on overall size of the heat spreader).
The increased conductivity is attributed
to the exceptionally high thermal
conductivity of heat pipes. Heat pipe
operation is illustrated in Figure 1.
Heat pipes operate by vaporizing the
working fluid at the heat source or evaporator
end, moving the vapor by internal
pressure difference and condensing
at the heat sink or condenser end. The
liquid is then returned to the evaporator
by capillary force provided by an
internal wick structure. For HiK plates
used in electronics cooling, the typical
heat pipe envelope material/working
fluid combination is copper-water.
Water heat pipes operate over the range
of 20°C - 150°C, making them an ideal
choice for electronics cooling. The
sealed heat pipe creates a passive closed
2loop system, which generates long life
and high reliability.
Figure 2 shows the temperature profile
of two heat spreaders of identical
dimensions comparing a HiK AlSiC
plate with a standard AlSiC plate. A heater was placed in the middle and the
edge temperature of each plate was controlled.
The hot spot temperature at the
source for the AlSiC HiK plate was less
than half that of standard AlSiC. This
decreased hot spot temperature can
translate into higher allowable electronics
power or increased thermal margin.
AlSiC HiK plates are particularly effective
in systems with multiple heat loads.
The heat pipe pattern can be optimized
to take advantage of the system geometry
and ambient conditions. When designing
a HiK plate it is important to consider
individual heat pipe limitations to
assure reliable operation for a specified
system. Whether the final design relies
on conduction through the plate to the
liquid cooled edge or spreading heat
over a larger area to enable an air-cooled
heat sink, the AlSiC HiK plate can provide
advantageous heat dispersion, lightweight
packaging, and direct component
attaching for lower interface resistance.
Figure 4. High Heat Flux Low CTE Vapor Chamber Test Results
A vapor chamber is a planar, twodimensional
heat pipe with exceptional
heat transfer capability and very low thermal
resistance. Like HiK plates, vapor
chambers are used for heat spreading
and heat transport applications.
Low CTE vapor chambers were
designed primarily for high power, high
heat flux electronic components. Often
individual components are directly
attached to the vapor chamber surface. Heat fluxes of 700 W/cm2 to over 1cm2
and total power of 2000W over 4cm2
have been demonstrated. Vapor chambers
operate similarly to heat pipes, utilizing
the benefits of two-phase heat
transfer and liquid return via an internal