High Heat Flux Heat Pipes Embedded in Metal Core Printed Circuit Boards

As LED applications continue to expand beyond lighting and sensors, the power levels and heat dissipation requirements will also continue to increase. Thermal management is becoming a major design issue for highpower LED systems. The size and weight of conventional bulk metal heat sinks cannot satisfy shrinking packaging constraints. Active cooling methods, such as forced air cooling or even pumped liquid, can provide acceptable performance but at the expense of increased energy consumption, reliability and most notably noise. Passive phase change (liquid to vapor) cooling devices, such as heat pipes, are well established in the electronics industry as a very effective and reliable way of removing excess waste heat at low thermal resistance. Successful application of heat pipes in general solid-state lighting (SSL) and other higher intensity lighting products will require adapting these heat pipe technologies to the form-factor, material and cost requirements unique to SSL products. This paper describes a recent development effort that integrates heat pipes with novel wick structures into metal core printed circuit boards (MCPCB) for high power LED devices. The novelty of the advanced wick structure lies in a low evaporative thermal resistance, which was engineered to address the high heat fluxes associated with LED devices. The embedded heat pipes use water as the working fluid, allowing the MCPCB to significantly improve heat spreading capability over conventional PCBs and MCPCBs. Experimental results show an average of 35 45% reduction in thermal resistance from typical MCPCB sizes and materials, which agrees with numerical modeling. The advanced wick structure was engineered to maximize the evaporative heat transfer coefficient near the heat input area (>8 W/cm-K) while maintaining high heat transport limits (>30 Watts per heat pipe). In this paper, the continuing study on heat transfer enhancement in a single-diode LED assembly is reported. Future development efforts will integrate the design in practical applications including arrays, address manufacturing issues and improving cost efficiency.