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Thermal management challenges are becoming a major roadblock to the wide use of high-power LED lighting systems. Incremental improvements in conventional bulk metal heat sinks and thermal interface materials are projected to be insufficient to meet these challenges. Active cooling methods, such as forced air and pumped liquid cooling, may provide better performance but at the expense of higher cost and energy consumption. Passive phase change (liquid to vapor) cooling devices, such as heat pipes and thermosyphons, 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 and thermosyphons in solid-state lighting (SSL) products will require adapting the technologies to the form-factor, material and cost requirements unique to SSL products. This paper describes a recent development effort that integrates a planar thermosyphon into a printed circuit board (PCB) for LED devices. The planar thermosyphon/PCB uses a dielectric fluid as the heat pipe working fluid, achieving significantly improved heat spreading performances over conventional PCBs. Analytical modeling showed a more than 50% thermal resistance reduction from typical metal core PCBs. A low temperature electroplating technique was also investigated to fabricate wick structures onto PCB surfaces to enhance the boiling heat transfer performance of the dielectric fluids. Test results showed that a boiling heat transfer coefficient of 20,000W/m2-K can be achieved with the 3M Novec fluid. In this paper, the preliminary study on heat transfer enhancement by using the PCB planar thermosyphon in single LED assembly was reported. Future development efforts will verify the design in practical applications, address manufacturing issues and improve the cost efficiency.