Experimental investigation of microheat pipes for high-power light-emitting diode modules
2013; Institution of Engineering and Technology; Volume: 8; Issue: 10 Linguagem: Inglês
10.1049/mnl.2013.0258
ISSN1750-0443
AutoresYi Luo, Gang Liu, Liang-liang Zou, Yang Yan-xia, Xiaodong Wang,
Tópico(s)Electronic Packaging and Soldering Technologies
ResumoMicro & Nano LettersVolume 8, Issue 10 p. 646-649 Special Section: Expanded Papers from NEMS 2013Free Access Experimental investigation of microheat pipes for high-power light-emitting diode modules Yi Luo, Yi Luo Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorGang Liu, Gang Liu Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorLiangliang Zou, Liangliang Zou Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorYanxia Yang, Yanxia Yang Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorXiaodong Wang, Corresponding Author Xiaodong Wang xdwang@dlut.edu.cn Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this author Yi Luo, Yi Luo Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorGang Liu, Gang Liu Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorLiangliang Zou, Liangliang Zou Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorYanxia Yang, Yanxia Yang Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this authorXiaodong Wang, Corresponding Author Xiaodong Wang xdwang@dlut.edu.cn Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian, 116023 People's Republic of ChinaSearch for more papers by this author First published: 01 October 2013 https://doi.org/10.1049/mnl.2013.0258Citations: 8AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract A light-emitting diode (LED) is a novel electronic light source that provides a direct conversion of electrical energy into light. A typical LED power package has a heat flux of 100 W/cm2, thus high power LEDs face severe thermal challenges because of their small size and general lack of a proper thermal path. The advantage of microheat pipes (MHPs) which are largely used in heat dissipation of microdevices is based on phase change. With the trend of using a silicon wafer as the substrate in LED manufacturing, a silicon-glass MHP was fabricated for the quick spread of heat from the LEDs. Flat plate grooved MHPs are introduced. The grooves were fabricated on the silicon wafer and followed by bonding with Pyrex 7740 glass. Water was selected as the working liquid and the charge rate was 40%. The temperature test experiments were carried out to test the feasibility of the MHPs in LED heat conduction, and the preliminary experimental results indicate that the heat conductivity of MHPs without a vapour chamber is better than that of MHPs with a vapour chamber, which is even better than that of the silicon wafer. 1 Introduction Generally, about 17% of the primary energy consumption in homes is attributed to lighting appliances, but a typical light-emitting diode (LED) power package has a heat flux of 100 W/cm2 and most common LED white bulbs used in homes employ a heatsink as a passive cooling device nowadays. Thus, high-power LEDs face severe thermal challenges because of their small size and generally the lack of a proper thermal path. In recent years, the ways to reduce high-power LED (HP LED) thermal resistance, junction temperature and hot spot influence have been studied by different research groups in passive and active cooling mechanisms. There are several aspects which have been studied in heat dissipation, such as package design [[1]], thermal interface material [[2]], low thermal resistance heatsink material [[3]-[5]] and cooling systems [[6]-[10]]. The best-known devices for effective heat transfer or heat spreading with lowest thermal resistance are the heat pipe and vapour chambers, which are two-phase heat transfer devices with excellent heat spreading and heat transfer characteristics [[11]]. Borrowing the idea of IC cooling, the heat pipe and heatsink have been studied for heat dissipation for LEDs. Kim et al. [[6]] reported using a heat pipe to control the junction temperature of an LED array. It was demonstrated that applying a heat pipe effectively decreases the total thermal resistance of an LED array, and proved to be a good solution for controlling the junction temperature of HP LED systems. Xiang et al. [[12]] proposed a novel phase change heatsink for high power LEDs with good heat transfer capability. Schneider et al. [[13]] reported a thermally optimised UV LED cluster with soldered LED chips using a room temperature liquid gallium alloy as thermal interface material between the substrate and heat pipe. All these above literature reported that HP LEDs with a heat pipe as the cooling system can efficiently lower the junction temperature. However, all these heat pipes were fabricated using the macroscale fabrication technique, thus the detailed structure of the heat pipe could not be well expressed, and the dimensions of the heat pipe were large. Using the MEMS technique to fabricate a microheat pipe has been an active research field for some decades. Youn and Kim [[14]] reported a silicon-based micropulsating heat spreader (MPHS). Ethanol was used as the working fluid, and ten parallel interconnected rectangular channels, which formed a closed loop, were engraved on a silicon wafer. The microheat pipes (MPHs) achieved the maximum effective thermal conductivity of 600 W/m K which was 3.5 times higher than that of silicon. The frequency and the amplitude were 0.67 Hz and 15 mm, respectively, at the maximum thermal performance. The literature on MHPs has shown that although the dimensions of the key components in the MHPs reached micronscale and the scale effect appeared, it is still a promising device for heat transfer with proper heatsink. However, since the heat source is not an LED, the temperature rising character and the power of the heat source are different, which will influence optimisation of the MHPs for the LED. In this Letter, the 10 W HP LED module was used as the heat source. A flat plate grooved MHP is introduced. The grooves were fabricated on the silicon wafer which was bonded with Pyrex 7740 glass (Corning Corporation). Silicon-Pyrex bonding technology has been utilised to allow flow visualisation. The grooves were fabricated on the silicon wafer using wet etching. Different dimensions of the grooves in the evaporator were studied by heat dissipating experiments. 2 Design and experiments 2.1 Design of MHPs Flat grooved MHPs not only had a good characteristic of normal heat pipes, but also had strong control of temperature in the hot zone of the discretely partial heat source. The flat MHPs could contact directly with the HP LED because their external surfaces were smooth. The flat MHPs created a whole isothermal surface, which could expand the area of heat dissipation effectively and improve heat dissipation efficiency. Flat grooved MHPs were fabricated on a solid substrate with a bunch of micro-grooved channels. Finite element analysis was carried out to determine the depth of the microchannels. FLUENT software was used, and the fluid flow in the channel was assumed as laminar. The volume of the fraction model was selected to track the gas–liquid interface, and element 200 which is a hexahedron element type was selected. The cross-section of the channels was non-parallel trapezoidal. The widths of the wide and narrow end of the channel were 230 and 180 μm, respectively. The cycle of the channel was 300 μm. The evaporator section had a 10 W heat source, whereas the condenser section had a constant temperature of 15°C. The simulation results showed that with the depth increased from 60 to 160 μm, the highest temperature decreased from overheat to 70°C. The possible reason is that with the increase of the channel depth, the capillary force increased too, which made the working fluid cycle more efficient. The dimensions of the LED modulus of 10 W were 8.5 × 8.5 mm, and the cycle of the channel was 300 or 400 μm, thus the number of channels were determined. Part of the designed MHPs had a vapour chamber which made the vapour channels interconnected, and it could decrease the interface friction produced by vapour migration. As a result, the vapour chamber highly improved the heat transfer ability of the heat pipes. The rest of them with no vapour chamber were the references. The photolithography masks are shown in Fig. 1. Figure 1Open in figure viewerPowerPoint Masks of MHP grooves a Parallel grooves b Non-parallel grooves c Vapour chamber The evaporator section provided the capillary force which could transfer heat to the gas–liquid interface; the adiabatic section provided capillary suction force which made the condensate liquid fly back to the evaporation part; whereas the condenser section transferred heat to the gas–liquid interface and turned the gas condensate into liquid [[15]]. To ensure the reflux of the condensed liquid, the wick should be capable of providing a high capillary pressure, which means a small effective capillary radius. On the other hand, to maintain a high flow rate of the liquid, the wick should have high permeability, which means a large pore size. Therefore MHPs with non-parallel grooved channels could transfer heat better. Based on the above thought, the design of the MHPs in this subject was that the number of channels in the evaporative part was larger than that of the condensate part, whereas the width was narrower. Specific design dimensions are shown in Table 1. Table 1. MHPs' design dimension Structure Number Shape of the channels Width of the channels, µm Space between channels, µm with vapour chamber 1 parallel 150 50 2 non-parallel part A∶300 part A∶100 part B∶260 part B∶140 3 non-parallel part A∶180 part A∶120 part B∶230 part B∶70 no vapour chamber 4 non-parallel part A∶150 part A∶50 part B∶100 part B∶100 5 parallel 230 70 2.2 Fabrication of microgrooves The microgrooves were fabricated on the n -type silicon (100) wafer (Tianjin Institute for Semiconductor Technology) using anisotropic wet etching. After cleaning with the RCA standard silicon wafer cleaning process, a 1 μm-thick SiO2 layer was grown and BP212 (Beijing Institute for Chemical Reagent) photoresist was spin-coated on the SiO2. KOH:IPA:H2O = 40 g:30 ml:100 ml was used as etchant at 73°C. When the depth reached 169 μm, we took the Si substrate out and rinsed it with DI water for 5 min, then removed the photoresist. The wafer was cut into small pieces with dimensions of 45 × 16 mm. For visualisation, the MHPs were bonded with Pyrex 7740 which had a thickness of 1 mm. We used ultrasonic vibration to drill two holes on the Pyrex 7740. The diameter of the holes was 1 mm. The fabrication processes of the MHPs and silicon-Pyrex MHPs are shown in Fig. 2. Figure 2Open in figure viewerPowerPoint Fabrication process of MHP and silicon-Pyrex MHP The temperature range of the melting point, boiling point and the critical point of DI water and alcohol ranges from −100 to 300°C, which can adapt to the working temperature area of the heat pipes. However, how to control the liquid filling ratio accurately and seal effectively are difficult in the fabrication of the MEMS heat pipes. 2.3 Working fluid charging and sealing According to the order of vacuum pumping and liquid filling, liquid filling methods can be divided into two styles. The first method is vacuum pumping before liquid filling. The other method is liquid filling before vacuum pumping. For sealing of the plate MHPs, the first method can change the volume of the vapour chamber, which not only can affect the MHPs, but also cannot control the filling liquid ratio accurately. The second method does not change the volume of the vapour chamber during sealing. Moreover, it can control the liquid filling ratio better. Therefore we chose the second method. We filled the MHPs with de-gassed DI water. We sealed one charging hole then inserted the other one into the container full of DI water and heated the evaporation part using a silica gel heater. The liquid in MHPs evaporated into gas and pushed the liquid level downwards with temperature rising. With the temperature of the silica gel heater rising, the vapour pressure of liquid increased, whereas the liquid filling ratio decreased. The device and the MHPs are shown in Fig. 3. By controlling the heat temperature in the sealing process, we could achieve an ideal filling ratio when the system reached equilibrium. Figure 3Open in figure viewerPowerPoint MHP charged with working fluid 2.4 Testing The test system for MHPs is shown in Fig. 4. It consisted of rapid response thermocouples (Chal-0005, Omega Company, USA), a data amplifier (AD524, Analog Devices Inc., USA), a multi-channel data acquisition board (PCL-841, Advantech, Taiwan) and an industrial control computer. Two thermocouples were fixed at A–B on the surface of the MHPs as shown in Fig. 4. The amplified signals from thermocouples were sampled by the data acquisition board with a sample rate of 50 kHz per channel. A data acquisition program was developed using LabVIEW to realise signal sampling and processing in the industrial control computer. Figure 4Open in figure viewerPowerPoint Schematic of the temperature experimental test system Figure 5Open in figure viewerPowerPoint Temperature curve of the MHPs a No. 1 MHP 90° b No. 1 MHP 45° c No. 2 MHP 45° d No. 3 MHP 90° e No. 4 MHP 45° f No. 5 MHP 90° g Silicon wafer 90° The 10 W HP LED module was mounted on the other surface of the MHPs by heat conductive silicone grease as the heating part of the MHPs, and its temperature could be tested by the thermocouple. We put the condensate part of the MHPs into water at a constant temperature of 15°C. The thermocouples were fixed at points A and B to test the temperature of the evaporator section and the adiabatic section, respectively. Figs. 5a–f show the experimental results of MHPs compared with experiments (as shown in Fig. 5g), a silicon wafer with the same dimensions of the MHP was used to replace the MHP. 3 Results and discussion Based on the data shown in Fig. 5, the effective thermal conductivity (K) of the MHPs can be calculated as follows: (1) where Qin is the heat input, T1 and T2 are the temperatures of the evaporation part and the condensation part, respectively, L is the length between the evaporation part and the condensation part and Ac is the cross-sectional area of the MHPs. The results are shown in Table 2. Table 2. Effective thermal conductivity of MHPs MHPs number MHPs mounted angle Heat input Qin, J T2, °C T1, °C Ac, mm2 K, W/m K 1 90° 800 15 80.268 24 3.689 1 45° 81.199 24 3.637 2 45° 88.431 24 3.278 3 90° 80.078 24 3.699 4 45° 85.161 16 5.146 5 90° 87.135 16 5.006 From Table 2, K represents the capacity of heat transmission of the MHPs: According to the comparison between the curve of the graph of Fig. 5g and the others, the capacity of heat transmission of the MHPs was better than that of the silicon wafer. According to the contrast between the graphs of Figs. 5a and b, for the parallel plate MHPs with a vapour chamber, the effective thermal conductivity of the MHPs which were placed at 90° was larger than those placed at 45°. The reason was that, as a result of the effect of gravity, the speed of gas flowing to the condensation part became faster. The K of MHPs 1–3 (having a vapour chamber) was lower than MHPs 4–5 (without a vapour chamber) significantly, which implies that MHPs without a vapour chamber have better capacity of heat transmission, the reason might be that under the testing situation, the capillary force in MHPs 4–5 was higher than that in MHPs 1–3. According to the contrast between the graphs Figs. 5c and d MHPs for non-parallel plate MHPs with a vapour chamber, MHPs with larger dimensions of microchannels were more favourable to heat conduction. 4 Conclusions Flat plate grooved MHPs were designed and fabricated in this Letter with dimensions of 45 mm (length) × 16 mm (width) × 1(1.5) mm (thickness). The grooves were fabricated on the silicon wafer, which was anodic bonded to Pyrex 7740. DI water was chosen as the working fluid and charged to the MHP at the ratio of 40%. Temperature test experiments were carried out with comparison between the silicon wafer and MHPs. 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