A Microjet Array Cooling System for Thermal Management of Active ...

A Microjet Array Cooling System For Thermal Management of Active Radars and High-Brightness LEDs Sheng Liu1, 3 , Tim Lin 4, Xiaobing Luo2, Mingxiang Chen1 , Xiaoping Jiang1 1

Institute of Microsystems and Wuhan National Lab of Optoelectronics, Huazhong University of Science & Technology, Wuhan, 430074, China 2 School of Energy and Power Engineering, Huazhong University of Science & Technology,, Wuhan, 430074, China 3 Department of Mechanical Engineering, Wayne State University, Detroit, Michigan 48202, USA 4 Department of Mechanical Engineering, Florida Atlantic University, Boca Raton, FL 33431. USA Phone: 313-577-3875, Email: [email protected]

Abstract Advancement in high heat thermal management technology and its successful integration into emerging GaNbased amplifiers is imperative to meet the long-term requirement of future X-band radar systems. It is also known that the efficiency and reliability of light emitting diode (LED) strongly rely on successful thermal management due to its inherit low junction temperature in the LED chip. In this paper, a new cooling solution for thermal management of GaN based power amplifiers for X-Band radars (XBR) and high-brightness LEDs, by a closed microjet array cooling system, is proposed and investigated by experiments. For the active radar part, we developed a circuit consisting of Si resistor chips to generate heat. The resister chip has a resistance of 10 Ohm. It has a chip dimension of 2.4 mm x 2.4 mm x 0.50 mm, with a top side 4 um Al and bottom side 1 um Al/Ti/Ni/Ag. The testing results are shown to achieve a thermal resistance of 0.192oC/W under a flow pressure of 32psi. For LED packaging, the tests show that it can achieve good cooling effect, for a 16.4W input power, the surface temperature of 2x2 LED array is just 44.2 ℃ after 10 minutes’ operation, much lower than 112.2 ℃ , which is measured without any active cooling techniques at the same input power. To compare its performance with conventional thermal management means, three cooling methods for LED array such as natural convection, heat sink and heat pipe, are also experimentally studied. The comparison results demonstrate that present microjet cooling system has the best cooling performance. To fully understand present cooling system for real world applications, the cost, reliability and volume comparison and analyses among different cooling means are also briefly conducted in the final section of this paper. Introduction Advancement in high heat thermal management technology and its successful integration into emerging GaNbased amplifiers is imperative to meet the long-term requirement of future X-band radar systems. In the sense of thermal management, light emitting diode (LED) has received outstanding attention in recent years because of its advantages compared with common light sources. Currently, the common light sources are incandescent filament lamp and fluorescent lamp, both of which are associated with large energy losses that are essentially inherent because of high temperatures and large stokes shifts involved [1]. LED depends on semiconductor material to emit the light; it has many

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distinctive advantages such as high efficiency, good reliability, long life and low power consumption. Now, LED begins to play an important role in many applications [2]. Typical applications include back lighting for cell phones and other LCD displays, interior and exterior automotive lighting including headlamps, large signs and displays, signals and illumination. For present LEDs, especially for high-brightness LEDs, both optical extraction and thermal management are critical factors for the high performance of LED packaging. Without an efficient mechanism to remove the heat produced in the LED chip and the luminous efficiency is still low. For example, the efficiency of green LED chip is just 30%~40%, it means that 60%~70% of electronic power will convert into heat. In general, the produced heat will greatly reduce device luminosity. In addition, the high junction temperature will shift the peak wavelength of LED, which will change the light color of LED. Resultantly, lower operation temperature LED has, higher efficiency it will become. Therefore, low operation temperature is strongly needed for LEDs. Since the market requires that LEDs have high power and packaging density, it poses a contradiction between power density and operation temperature, especially when applications require LEDs to operate at full power to obtain desired brightness. The problem leads to the emerging of major advances in thermal management of LED illumination. To solve the thermal problem of LEDs, Arik et al. [3] carried out numerical study to understand the chip temperature profile due to the bump defects. Finite element techniques were utilized to evaluate the effects of localized hot spots at the chip active layer. Sano et al. [4] reported an ultra-bright LED module with excellent heat dissipation characteristics. The module consists of an aluminum substrate circuit board having outstanding thermal conductivity and workability, the mount for LED chips being formed into fine cavities with high reflectance to improve light recovery efficiency. Furthermore, the condenser of this LED module is filled with high-refraction resin on the basis of optical simulation. An improvement in luminance of 25% or more was observed by taking the aforementioned measures in their paper. Petroski et al. [5] developed a LED-based spot module heat sink in a free convective cooling system. Cylindrical tube, longitudinal fin (CTLF) heat sink is used to solve the orientation problem of LED. Chen et al. [6] presented a silicon-based thermoelectric (TE) for cooling of high power LED. The test results show that their TE device can effectively reduce the thermal resistance of the high power

1634 2006 Electronic Components and Technology Conference

LED. Hsu et al. [7] reported a metallic bonding method for LED packaging to provide good thermal dissipation and ohmic contact. Zhang et al. [8] used multi-walled carbon nanotube and carbon black to improve the thermal performance of thermal interface materials (TIM) in highbrightness LED packaging. Tests show that such a TIM can efficaciously decrease the thermal resistance. Acikalin et al. [9] used piezoelectric fans to cool LEDs. Their results show that the fans can reduce the heat source temperature by as much as 37.4℃. The piezoelectric fans have been shown to be a viable solution for the thermal management of electronic component and LED. In this paper, a closed microjet array cooling system is proposed to realize the thermal management for both the active radar systems and high power density LEDs, in particular for LED array. To compare its performance with existed conventional technologies such as heat sink and heat pipe, the comparison study is experimentally conducted. Four cases that adopt different cooling methods are investigated. The four cooling methods are natural convection, heat sink, heat pipe and present microjet array respectively. By detecting temperature dependence of LED on time, the effect of every cooling method is observed. Microjet array cooling system Fig.1 demonstrates present cooling system. It composes of three parts: microjet array device, micropump and mini water container with heat sink. It is obvious that such a system is a closed one. When radar amplifiers or LEDs need to be cooled, present system starts to work. Water in closed system is driven into microjet array device through inlet by micropump. Many micro jets will form inside the jet device, which are directly impinged onto the bottom plate of radar amplifiers or LED array. Since impinging jet has very large heat transfer coefficient, the heat created by radar amplifiers or LEDs is easily taken out by the recycling water in the system. The water is heated and its temperature increases after flowing out the jet device, then the heated water enters into the mini water container. The heat sink with fan on the water container will cool the water and the heat will dissipate into environment. The cooled water will be delivered into the jet device to cool the radar amplifiers or LED array again by the force of micropump in system. The above processes constitute one operation cycle of the total system. It should be noted here that the real size of present system can be designed as one small package according to application requirements.

Bottom plate of LEDs or radar power amplifer A A

Chip layer Top plate of jet

Single LED chip Jet shape

Impinging jet cavity Microjet array layer

Jet array outlet

Jet array inlet Bottom cavity

Microjet A-A cross section

Fig. 2: Microjet array cooling device Impinging jet cavity

Fan

Bottomplate of LED array or radar amplifiers

Heat sink Outlet

Micro jet array

Fig.2 shows the structure of jet device in detail in Fig.1. It consists of several layers from the top to the bottom, which are chip array layer, top plate of jet cavity, impinging jet cavity, microjet array layer and bottom cavity respectively. Cooled water enters into the device through the inlet opened at one side of bottom cavity layer, which will flow through the microjet array and forms many micro jets, as shown in Fig.2. With enough driving force, the jets will impinge onto the top plate of jet cavity which is bonded with the chip substrate of radar amplifiers or LEDs. The heat conducted into the top plate of jet cavity through the radar amplifiers or LED chips will dissipate into the cooled water quickly due to the high heat transfer efficiency of impinging jet. The water temperature increases and the heated water flows out from the jet array outlet which is opened at one side of top jet cavity layer. Through such a process, the heat from radar amplifiers or LED chips will be transferred into the water efficiently. To satisfy different applications of radar amplifiers or LEDs, the present microjet array structure shown in Fig.2 can be changed into several types, the impinging jet cavity can be polyhedron or cylinder shape, the microjet arrays can be distributed at several faces or all faces of polyhedron or cylinder, correspondingly, the radar amplifiers or LED chips can be located at several or all directions for cooling. Fig.3 shows such a design. The micro jets are formed on both the top and bottom sides, thus radar amplifiers or LED chips can be collocated on both sides for cooling. Such a design greatly facilitates radar amplifiers or LED thermal management in many applications that require multi-directions distribution of radar amplifiers or LEDs.

Mini water container

Microjet array layer Jet array inlet

Jet device inlet

Jet array outlet Bottomcavity of jet device Micropump

Central flow cavity

Impinging jet cavity

Fig.3. Microjet array in multi-faces Fig.1 Present closed-loop microjet array cooling system

Present system is based on the impinging jet to transfer heat, which has been demonstrated as an effective cooling solution in many applications [10-11]. Therefore, the most


distinguishing advantage of the present concept is its high heat transfer efficiency. Secondly, it provides nice temperature uniformity. In addition, the present system is compatible with and easily incorporated into radar amplifiers or LED packaging structures, it is therefore possible to simplify the package structure and reduce the thermal path, as the impingement is performed onto the package substrate. Cooling performance of present system

Fig. 4 is a prototype of the bottom-side microjet array cooling heatsink. To test the thermal performance of our thermal management/package, we have designed a circuit consisting of Si resistor chips to generate heat. The resister chip has a resistance of 10 Ohm. It has a chip dimension of 2.4 mm x 2.4 mm x 0.50 mm, with a top side 4 um Al and bottom side 1 um Al/Ti/Ni/Ag. Under a flow pressure of 32psi, we can achieve a thermal resistance of 0.192oC/W. An even lower thermal resistance can be achieved if using higher flow resistance. Fig. 5 shows a functional package module. It should be noted that this performance is only based on the first prototype and more optimization can be made on the hole sizes, shapes, density, etc.

26.4℃. It is noted that no intent is made to optimize the dimensions, materials, and structures. 2*2 LED chips packaged with present microjet devices

Power input for LEDs Raytek non-contact thermometers

Inlet tube connected with micropump

Outlet tube connected with small water container

Fig.6. Experiment measurement on cooling effect of present system Fig.7 demonstrates the LED distribution structure in our present experiment. The four LED chips are distributed on a substrate. Every two chips are connected in series. Then they have parallel connections with each other. The size of each LED chip is 1mm by 1mm. The substrate size is 15mm by 15mm. LED chips LED chips

Sustrate

Electronic pads

Fig. 7: Photo of LED chip layout in present experiment

Fig.4. A microjet heatsink

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Temperature ( C)

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0.3A, 6.3V, no cooling 0.6A, 7.1V, with cooling 1A,7.4V, with cooling 1.5A, 7.9V, with cooling 2.0A, 8.2V, with cooling

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Fig.5. A functional package module Fig.6 shows the experiment measurement on cooling effect of present system. Four LED chips are packaged with present microjet cooling device. Raytek infrared thermometer is used to measure the LED surface temperature. The micropump in experiment is based on electromagnetic principle, which is custom designed and made locally in China. The small heat sink with fan, bought from commercial electronics market, is bonded on the top surface of our designed small water container. The working media of the system is water, the flow rate of micropump is 10mL/s, the diameter of micro jets is 0.5mm and the room temperature is

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Fig. 8: LED surface temperature with time at different input power Fig.8 reveals the surface temperature dependence of LED chip on time at different input power levels without and with the cooling of present microjet system. In each experiment case, the input electric current are 0.3A, 0.6A, 1A, 1.5A, 2A, corresponding voltages are 6.3V, 7.1V, 7.4V, 7.9V, 8.2V


Heat sink with fan

LED chips

convection, heat sink with fan and heat pipe with condenser. Fig.9 demonstrates the LED chips cooled by heat sink and fan. Fig.10 shows the LED chips cooled by heat pipe with condenser. Here, the condenser consists of conventional heat sink and fan. In both cases, the heat sink with fan and heat pipe are purchased from commercial electronic market. Usually they are used for CPU cooling in personal computer. 2*2 LED array

140

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Heat sink at 4.2W Heat pipe at 4.2W Present cooling system at 4.2W Natural convection at 4.2W

100

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Temperature ( C)

respectively. It is obvious from Fig.8 that without present cooling system, the surface temperature of LEDs increases very quickly. For 0.3A and 1.89W power, the temperature increases from 27℃ to 112.2℃ in ten minutes,. However, with 0.6A and 4.26W, after packaged with present cooling system, the temperature just changes from 27℃ to 32.4℃. The comparison clearly demonstrates the cooling performance of the present system. Fig. 8 also shows that by using present cooling system, even the electric current increases to 2A, voltage is 8.2V, the surface temperature of LED chips only varies from 27℃ to 44.2℃ in ten minutes. These results demonstrate that with our present cooling system, it is promising that LED chips can work at large electric current and low operation temperature, which can achieve high lightoutput efficiency, good light quality and longer life.

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Fig. 11: Surface temperature dependence on time after cooling by different methods at 4.2W input power

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Heat conduction glue

Fig. 9: LED chips cooled by a heat sink with fan Heat pipe with condenser

LED chips

Heat sink at 1.89W Heat pipe at 1.89W Present cooling system at 1.89W Natural convection at 1.89W

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Fig.12: Surface temperature dependence on time after cooling by different methods at 1.89W input power

Fig. 10: LED chips cooled by heat pipe Comparison study To compare the cooling performance of the present system with other conventional cooling methods, comparison studies by experiments are also conducted. The common cooling solutions used in our present experiments are natural

Fig.11 and Fig. 12 exhibit the temperature dependence of LED chip on time after cooling by different means at 4.2 W and 1.89 W input power respectively. From Fig.11, it is clear that if we do not use any other active or passive means, just by natural convection, the LED temperature increases very quickly. In three minutes, the temperature changes from room temperature to 145℃ at 4.2W. It is extremely harmful to LED operation efficiency and life. After using heat sink and fan, in ten minutes, the LED temperature just increases to 37.6℃ from room temperature at 4.2W input power. For the heat pipe cooling, the LED temperature varies from room temperature to 31.6℃ at 4.2W. However, during the same operation period, when present microjet cooling system cools the LED chips, the LED temperature just shifts from room temperature to 28.2℃ at 4.2W input power. The comparison


clearly proves that present cooling system can achieve better performance. Same as Fig.11, Fig.12 also obviously demonstrates that present cooling system can obtain better cooling performance compared with other three means at 1.89W input power. It should be noted that the above comparison is based on cooling performance. In real world applications, when choosing thermal management solution, many other factors such as volume, cost, reliability should also be considered together besides the performance,. Fig. 9 and Fig.10 show that present cooling system has the best performance, heat pipe is better than heat sink and fan in cooling effect. However, from the angle of cost, heat sink with fan is the cheapest solution in three cooling means. As for the present cooling system, since it adopts micro pump to drive, its cost is slightly higher than other two methods. However, it is known that LED life strongly depends on LED temperature, there is exponential relation between them as demonstrated in reference [3], longer life means cost increase. For present cooling system, it will cool the LEDs to a lower temperature than that by other means, therefore, present cooling system can make the LEDs work longer, from this viewpoint, present cooling system will increase the LEDs cost, although the initial cost of its system devices is a little higher than other means. In terms of volume, if just simply adding the volumes of different components in every method, it is nearly the same for three methods. However, compared with the other two solutions, the present microjet cooling system can be easily packaged with LED chips that have different scales and shapes due to its flexible design,. Secondly, from the viewpoint of heat transfer, our present microjet cooling system composes of two processes. Firstly it removes the heat from LED chips into water by a small microjet array device, and then the heat in water dissipates into environment by the heat sink with fan. The above two processes are separated and can be conducted at different places according to requirements. Such a characteristic means that the free or dead volume in system can be used, especially for the latter heat transfer process, which provides the high possibility to realize the flexible volume distribution and satisfy the large power density requirement. Comparatively, the other two methods do not have the above characteristics; the heat transfer distance cannot be far. Thirdly the LED package cannot be made very small due to the fabrication limit of the heat pipe and heat sink, especially for the heat sink,. Reliability for present cooling system is strongly affected by the life and reliability of micropump. Fortunately, present micropumps have been made with reliability of 50,000 hours MTBF. Long term reliability in the system level is in progress. In addition, since the present cooling system is a closed one, leakage should not be problem if having good connections among different components. For other two methods, the reliability should be good, especially for the heat sink. Conclusion In this paper, a closed microjet cooling system for thermal management of active radar systems and LEDs is presented.

Its performance experiments demonstrate that it can obtain very good cooling effect, which is very suitable for high power active radar systems and LEDs. The performance comparisons of present cooling system with natural convection, heat sink and heat pipe are also experimentally conducted. For 1.89 W and 4.2W input power, both cases demonstrate that the present cooling solution has the best efficiency for thermal management of LEDs. The cost, volume and reliability analyses show that there should have a tradeoff consideration among performance, cost, volume and reliability in real applications. For our present thermal management solution, it is very suitable for LEDs applications that require high heat flux cooling, flexible design and volume distribution. If the applications need very low cost design and the heat flux is not high, it is more appropriate to choose other thermal management methods. References 1. Zukauskas, A., Shur, M.S., Gaska, R., Introduction to solid-state lighting, John Wiley & Sons, Inc., New York, 2002 2. Alan, M., Solid state lighting- a world of expanding opportunities at LED 2002, III-Vs Review v16 (1) : 30-33, 2003 3. Arik, M., Weaver, S., Chip scale thermal management of high brightness LED packages, Fourth International Conference on Solid State Lighting: 214-223, 2004 4. Sano, S.; Murata, H.; Hattori, K., Development of flat panel LED module with heat sink, Mitsubishi Cable Ind. Rev., Vol 86: 112-118, 1993 (in Japanese) 5. Petroski, J., Understanding longitudinal fin heat sink orientation sensitivity for Light Emitting Diode (LED) lighting applications, International Electronic Packaging Technical Conference and Exhibition: 111-117, 2003 6. Chen, J.H., Liu, C.K., Chao, Y.L., Tain, R.M., Cooling performance of silicon-based thermoelectric device on high power LED, 24th International Conference on thermoelectrics:53- 56, 2005 7. Hsu, C.C.; Wang, S.J.; Liu, C.Y., Metallic wafer and chip bonding for LED packaging, The 5th Pacific Rim Conference on Lasers and Electro-Optics, Vol 1: 26, 2003 8. Zhang, K., Xiao, G.W., Wong, C.K.Y., Gu, et al., Study on Thermal Interface Material with Carbon Nanotubes and Carbon Black in High-Brightness LED Packaging with Flip-Chip, Proceedings of Electronic Components and Technology: 60-65, 2005 9. Acikalin, T., Garimella, S.V., Petroski, J., Arvind Raman, Optimal design of miniature piezoelectric fans for cooling light emitting diodes, The Ninth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems: 663-671, 2004 10. Wu, S., Mai, J., Tai, Y.C., Ho, C.M., Micro heat exchanger by using MEMS impinging jets, Twelfth IEEE International Conference on Micro Electro Mechanical Systems :171 – 176, 1999 11. Fleischer, A.S., Nejad, S.R., Jet impingement cooling of a discretely heated portion of a protruding pedestal with a single round air jet, Experimental Thermal and Fluid Science Vol 28: 893-901, 2004
