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meshes with highest heat removal rate are deployed on the
hot spots. Three layer meshes on low heat flux areas
minimize the pressure drop.
Fig. 11. Mesh design of test cold plates. Colors represent number of mesh
layers (pink 3, red 2, blue 1, turquoise no mesh). Dark blue squares are
inlets and outlets. In the lower left panel, the unit-cell alignment is shown.
With a silicon die size of 720μm and a thermal interface
resistance of 15 K*mm 2 /W the temperature gradient from
highest junction to fluid inlet temperature is not
significantly changed for all test cases (Fig. 12). Pressure
drop wise the uniform heat transfer utilizing all chip area for
heat transfer is performing best followed by the stream wise
cavity modulation experiment E). Non-uniform heat
removal only improves the temperature uniformity (ΔT j ) on
the die as indicated with the dashed lines. Case B)
outperforms uniform case A) by 5K.
7-9 October 2009, Leuven, Belgium
V. CONCLUSION
Tailored, steady-state normal-flow cold plate results clearly
demonstrate the potential of hotspot cooling in case of quasi
one-dimensional heat flux. Modeling suggests that the flow
rate can be reduced by 81% in case of 1μm die thickness
and direct-attach heat transfer mode. Both system pumping
power and fluid outlet temperature relevant for heat re-use
benefit from such low flow rate cold plates. However for a
realistic chip thickness of 720μm and thermal interface
(R TIM =12 K mm 2 /W) the benefit is shortened to a flow rate
reduction of 28% and system pumping power saving of
43%.
This pumping power and volumetric flow rate reduction
results in a improved power usage effectiveness in the
datacenter. Furthermore the increased fluid outlet
temperature increases the monetary value of the heat
potentially sold to a neighborhood-heating network.
In case of the presented cross-flow heat exchange
architecture the benefits for a realistic package including a
thermal interface are vanishing. The results show clearly,
that uniform heat transfer on the total chip backside is
resulting in equal peak junction temperature but reduced
pressure drop. The additional constraint of stream wise
constant flow rate in a certain fluid zone in combination
with the low change in heat transfer coefficient for varying
mesh cavity heights are responsible for poor heat transfer
contrast at the cold plate base. The only benefit of tailored
cold plates is the reduction in thermal gradient on the
processor die with the benefit of increased reliability.
We presented the benefits but also limits of hotspot cold
plates as a single component and applied in a server rack
with multiple processors. In standard TIM-attach packages
only cold plates with high heat transfer contrast in x and y
direction showed improved performance.
Further work on the normal-flow cold plate should
concentrate on eliminating the throttles by considering heat
transfer geometry modulation. For cross-flow heat
exchangers means to increase stream wise heat transfer
contrast have to be studied. We propose to change hydraulic
diameters instead of mesh cavity height.
Fig. 12. Maximal junction to fluid inlet temperature gradient (T jmax-
T fin)(full lines), chip temperature uniformity (ΔTj)(dashed lines with full
bullets), and pressure drop (full lines, empty bullets) of all cross-flow
heat exchange cases.
©EDA Publishing/THERMINIC 2009 154
ISBN: 978-2-35500-010-2