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Increasing computing power in military embedded electronics require innovative thermal solutions to meet the rugged environments at which they are expected to perform. One of the major challenges in cooling these devices is the conduction resistance associated with transporting the dissipated power from a PCB through aluminum and copper to the external cooling locations (liquid, forced convection, or natural convection). Vapor chambers offer conductivities of over ten times that of copper which in turn can drastically reduce the component case temperatures. Vapor chambers are flat two-phase devices that use the evaporation and condensation of a working fluid to produce a high conductivity thermal plane. A copper powder wick is sintered to the evaporator which acts as both a capillary liquid return path and a high surface area evaporation location. This copper wick has the ability to dissipate heat fluxes over 300 watts/cm2. Vapor chambers have demonstrated no degradation of performance through freeze-thaw cycles (-51Â°C to 76Â°C) and military vehicle shock and vibration requirements. In order to properly predict the affect that substituting a vapor chamber for an existing metallic spreader will have on the device temperature, models were created with Flotherm, a computational fluid dynamics program. These models represent four different approaches to conducting heat to a liquid cooled edge. An aluminum frame, a copper frame, a heat pipe embedded aluminum frame, and a vapor chamber were evaluated to determine the performance gains by using two-phase enhancement. To ensure the validity of these models, four frames were fabricated and thermally tested. The results of these tests and the comparison to the models are explored in this study as well as vapor chamber/heat pipe dependency on orientation with respect to gravity and maximum transportable power for the given geometry.
Date of Conference: 18-21 Oct. 2009