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The design requirement for electronics cooling is to maintain the hottest
location (hotspot) on the die (chip) below the specified temperature. Due to
the presence of multiple hotspots, the thermal resistance near the die is very
high. Total thermal resistance (Ψtot) can be written as:
click equation for larger view
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(1) |
where Rpackage,
Rsi,
RTIM1,
and Rspreader are the thermal impedances of
package, silicon, first-level Thermal Interface Material (TIM), and heat
spreader, respectively. ΨTIM2 and
Ψsink are the thermal resistances of
second-level TIM and the heat sink, respectively, and Density Factor (DF) is a
factor that accounts for the non-uniformity of heat generation. DF can possibly
be greater than 1 if highly non-uniform power distribution exists or if the die
size is very small. Since the thermal impedance near the die (Rpackage) gets
multiplied by the DF, any reduction in the package impedance results in a
larger reduction in Ψtot.
Because of this reason, the focus of next-generation
electronics cooling is on developing efficient cooling solutions near the
package. Futuristic cooling solutions may be based on micro and nano
technologies. These solutions might include a TIM made from micro and
nanoparticles, a microchannel heat exchanger, and a Thin Film Thermoelectric
Cooler (TFTEC) that is made of thin film superlattices, or nanocomposites,
placed directly above the hotspots to provide localized cooling. In this paper,
we focus on the technical merits of these technologies and discuss the
challenges that must be met to make these technologies a reality for
electronics cooling. The main challenges are: a) to reduce the boundary
resistance between the nanoparticles and the host medium for
nanoparticles-based TIMs and to increase the reliability performance of TIMs;
b) reduce the assembly-related parasitic effects seen in TFTEC (for example,
due to the very thin dimension of TFTEC, electrical contact resistance reduces
the effective ZT in a package making it much smaller than the intrinsic ZT);
and c) pumping requirements and pump reliability for microchannels. Water
cannot be used as a coolant because the freezing requirements for electronics
cooling is dictated by shipping and handling requirements and is much lower
than 0°C. Traditional antifreeze liquids have much lower thermal
conductivity and higher viscosity than water, forcing very severe pumping
requirements in order to get the same thermal performance as water.
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