A nanostructure patterned heat spreader for on-chip thermal management of high-power LEDs

Experimental studies on micro/nanostructure patterned surfaces combining hydrophobic and hydrophilic zones for nucleation boiling were conducted. Two approaches for heat transfer enhancement were studied. One approach is to improve the heat transfer performance utilizing our previous that a well-designed network of hydrophobic and hydrophilic regions might promote nucleation, enhance the heat transfer coefficient (HTC), and increase the critical heat flux (CHF) by preventing dry out that may have great potential for high power LED cooling. Hydrophobic and hydrophilic regions will be manufactured on glass wafers and copper surfaces. Different surface hexagonal pattern size will be used. Indium Tin Oxide (ITO), Fluoroalkylsilanes (FAS) and Copper Oxide (CuO) will be used for glass and copper surfaces, respectively. Microsensors will be used to measure surface temperatures. It was found that a patterned surface can significantly improve the nucleation process which has great potential for enhancing heat transfer. Another approach is to develop a novel asymmetric vapor chamber in this study. In this vapor chamber, nanostructure is patterned on the inner top surface of condensing wall. And this condensing wall is made to be superhydrophobic to replace the conventional porous wick. This improvement not only results in drop-wise condensation which has a much higher heat transfer coefficient compared with film condensation, but also provides a shortcut for the condensed water to drop back directly to the center wick. Thus, smaller liquid flow resistance and high anti-dryout capability are achieved. The evaporator wick is made of sintered multi-layer copper powder. The dimension of the vapor chamber is 70×70×3mm³. The test module includes an aluminum block with recirculated cooling water going through it and a heater with an area of 1.5×1.5cm². The optimum working pressure is determined by testing the performance of vapor chambe- under different initial pressures. Heater temperature, horizontal resistance and vertical resistance are defined as key parameters to evaluate the performance of heat spreader. It is found that heater temperature increases with the increasing of heat flux. However, the vertical resistance shows the opposite tendency with the increasing of heat flux. The performance of our vapor chamber is compared with that of commercial vapor chamber and copper plate. The newly developed vapor chamber can greatly reduce the heater temperature. Furthermore, better temperature uniformity and lower vertical resistance can be found for the newly developed vapor chamber which is promising for the thermal management of high power LED.