Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
Table 1. Detailed results of uniform vs. tailored heat-transfer cold plates.
TIM-attach: case 1; direct-attach: cases 2 and 3.
Fig. 4. Simplified processor power map with hotspot area (red, orange).
The heat transfer surface is discretized with a granularity of
1 mm 2 according to the fabrication limit of one unit cell.
The heat-transfer coefficients of these unit cells can be
changed independently (Fig. 5). The heat-conduction problem
is solved with finite-element analysis using the commercial
solver ANSYS. As initial value a uniform heattransfer
coefficient was applied calculated from the package
thermal impedance, total chip power and available thermal
budget. The spatial distribution of the heat transfer coefficient
after each iteration is adjusted in proportion to the heat
flux map at the cooler base obtained in the preceding
iteration. This factor is adjusted in the course of sub-sequent
iterations to reach a convergence stop condition given by a
predetermined threshold value for ΔT jmax = T jmax N+1 - T jmax N .
A less stringent thermal budget was chosen for TIM-attach
owing to the additional TIM thermal resistance of 12 K
mm 2 /W. In both cases, fluid outlet temperatures ≥ 60°C are
achieved while maintaining a maximal junction temperature
of 80°C.
The cost ratios for flow rate, pressure drop, and pumping
power of the cold plate and the server-rack cooling loop is
shown in Fig. 6. The ratio is defined as performance of the
custom-tailored divided by the uniform heat-transfer cold
plate with the same package configuration. As expected
non-uniform heat transfer results in a reduced flow rate and
an increased fluid outlet to inlet temperature. This is
especially pronounced for heat removal close to the heat
source, such as case 3, corresponding to a quasi-onedimensional
heat flux. For the realistic case 1, the reduction
is 28%. Heat-transfer coefficient tailoring results in an
increased cold-plate pressure drop because of the need for
higher peak heat-transfer coefficients. Moreover, also an
increased cold-plate pumping power is needed in cases 1
and 2. Nevertheless, the system pumping power is reduced
by 43% for case 1 and by up to 97% for case 3 because of
the total flow rate reduction.
Fig. 5. Definition of boundary condition for the finite- element analysis.
Spatially resolved heat flux and heat transfer coefficients are applied on the
chip (orange) bottom side and cooler base (blue) back-side, respectively.
The resulting heat flux map at the cooler is the base for heat transfer
scaling for the next iteration.
The resulting flow rates and pressure drops for uniform and
tailored heat transfer for the TIM-attach (case 1) and directattach
(cases 2, 3) mode with a temperature gradient budget
of 20 and 10 K, respectively, is presented in Table 1.
Fig. 6. Cost ratios (parameter for tailored cold plate divided by that for
uniform cold plate) of cold-plate volumetric flow rate and pressure drop
(dp cp), as well as pumping power for the individual cold plate (P cp) and the
complete system (P system). TIM-attach: case 1, direct-attach: cases 2 and 3.
IV. CROSS-FLOW COLD-PLATE STUDY
The cross-flow cold plate is built of several 0.4-mm-thick
copper sheets (referred to as mesh layers) patterned using
©EDA Publishing/THERMINIC 2009 152
ISBN: 978-2-35500-010-2