Thermal behavior of silicon-copper micro vapor chamber for high power LEDs

Abstract

Micro vapor chamber (MVC) for light emitting diodes (LEDs) can be designed and fabricated to enhance the heat dissipation efficiency and improve the reliability. In this paper, we used photoresist SU-8 and electroforming copper (Cu) to fabricate three kinds of wick structures, which are star, radiation and parallel ones, and the substrate is silicon with thickness of 0.5 mm. Electroforming Cu on silicon to make micro wick structure was a critical step, the ampere-hour factor was used, and accordingly the electroforming time was predicted. The composition of electroforming solution and parameters of electroforming were optimized too. After charging and packaging, thermal behavior tests were carried out to study the heat dissipation performance of MVCs. When the input power was 8 W, the parallel wick structure reached the equivalent temperature of 69.0 °C in 226 s, while the others were higher than that. The experimental results prove that the wick structures have significant influence on the heat transfer capability of MVCs.Vapor chamber (VC) is a solution to the poor heat dissipation capability of light emitting diodes (LEDs). Faghri et al [1] aimed to three kinds of Cu-water micro vapor chambers (MVCs) with different dimensions and structures, and the results demonstrate that the high-aspectratio channel heat pipe has better heat transfer performance. Peng [2] investigated the heat transfer performance of VCs with different cooling air-flow velocities, working fluid charge ratios and vacuum degrees. Naphon [3] conducted a series of experiments with different coolant types, flow directions of coolants and heat sink configurations. Recent years, many investigations about the incline angle [4] , new micro wick structures [5] and new types of heat pipes [6] have been also proposed to enhance the performance of VCs. Micro electroforming can be used to fabricate regular wick structure in MVC. Guan [7] analyzed the metallographic phases of electroforming deposits with different current densities. Nan [8] studied the composition of electroforming solution and summarized the optimal proportion of it. However, the microscopic structure nonuniformity is a problem of this technique. Therefore, how to improve the flatness of the microstructure is a main research orientation recently [9,10] . This paper is to fabricate micro copper (Cu) groove wick structures by electroforming to enhance the heat transfer capability of VC on account of the high heat conductivity coefficient of Cu. Moreover, the microstructure of electroforming was well organized and controllable, hence the capillary structure can be designed to be diverse. Three typical structures were designed and fabricated to investigate their influence on heat transfer capability.Three kinds of micro groove wick structures, namely star, radiation and parallel ones, were designed. The dimension of the Si substrate is 26 mm×20 mm×0.5 mm and the work area, also the electroforming area, is 20 mm×15 mm. The target electroforming height is 120 µm.The Si wafer was cleaned using ...