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Flat heat pipe performance thresholds at ultra-thin form factors

Improved-efficiency heat spreaders must address ergonomic- and performance-driven thermal management demands of electronic devices of increasingly thin form factor. Heat pipes offer a potential high-conductance solution, but performance limitations unique to sub-millimeter thickness devices must be characterized. Using a reduced-order, one-dimensional resistance network and a two-dimensional numerical model, the thermal resistance of a flat heat pipe is benchmarked against a solid heat spreader as a function of geometry and power input. The reduced-order model enables a broad parametric study and analytical formulation of performance limitations, while the higher fidelity numerical approach is used to assess the accuracy of the thermal resistance network near these limits. The form factors and operating conditions for which a heat pipe is more effective than a solid heater spreader are identified. Two of the bounding performance limits have been commonly discussed in prior analyses - a capillary wicking limit and an increase in the heat pipe thermal resistance relative to the solid heat spreader at very large thicknesses. A third vapor-phase threshold is observed when the thickness is reduced below a critical limit. At this threshold, the vapor-phase thermal resistance imposed by the saturation pressure/temperature gradient in the heat pipe causes a crossover in the thermal resistance relative to a solid heat spreader. Devices are susceptible to this performance threshold at very low power inputs that would not otherwise induce a capillary limitation. Accurate prediction of this threshold is an important consideration in the selection and design of ultra-thin heat pipes.