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Optimal Micro Heat Pipe Configuration for High Performance Heat Spreaders
Keywords: heat spreaders, micro heat pipes, high heat flux
Diamond heat spreaders have been shown to be effective for spreading heat from ultra-high heat flux sources, but their cost is prohibitive. In this work, radial micro heat pipe (MHP) arrays embedded in a copper substrate have been evaluated as a cheaper alternative. A square copper spreader with a centered 9 mm x 9 mm square heat source was modeled in MECHANICA 4.0, simulating heat generated by high-power LEDs. Seven radial array configurations, ranging from two MHPs to eight MHPs, were evaluated for a range of power inputs varying from 3 W to 21 W; maximum power that could be spread per heat pipe was determined using published equations relating to heat pipe limitations. Simulations were carried out for MHP effective thermal conductivities of 5,000 W/mC, 50,000 W/mC, and 100,000 W/mC, and the convective boundary condition was also varied. At a constant MHP effective thermal conductivity, the spreading resistance was independent of power input and the convection coefficient, as expected based on published work on spreading resistance. The spreading resistance decreased with increasing effective thermal conductivities until the conductivity reached about 50,000 W/mC, at which point the spreading resistance leveled off. The spreading resistance decreased with increasing numbers of MHPs in the array. Embedding eight MHPs in a heat spreader reduced the spreading resistance up to 70% compared to a heat spreader with no MHPs, and the MHP spreader had a lower spreading resistance than a sintered polycrystalline diamond spreader but a higher spreading resistance than a single crystal diamond spreader made from a high pressure CVD process. The second phase of this work, scheduled to begin in fall 2009, will involve a detailed analytical analysis of heat pipe limitations and effective thermal conductivity values.
Seema Singh,
San Jose State University
San Jose, CA

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