FABRICATING A HEAT PIPE STRUCTURE WITHIN A RADIATING PLATE FOR ELECTRONICS FAN-LESS COOLING

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1 Journal of Marine Science and Technology, Vol. 18, No. 3, pp (2010) 419 FABRICATING A HEAT PIPE STRUCTURE WITHIN A RADIATING PLATE FOR ELECTRONICS FAN-LESS COOLING Chun-Sheng Yu*, Shung-Wen Kang*, and Hong-Bin Chan** Key words: fan-less, radiating plate, heat pipe. ABSTRACT This study presents a method for fabricating a conductive metal with a heat pipe structure integrally formed within a radiating plate, and the heat transfer channels are arranged in an orthogonal matrix. The developed heat transfer device is monolithic and seamless, thus achieving low heat resistance and high heat transfer efficiency. The high conductivity radiating plate discussed in this study has significantly improved structural strength and a complete hot spot contact for efficient cooling. Heat generated from a heat source is transferring without passing through different interfaces of materials or any solder paste within the high conductivity plate. The proposed heat transfer device is suitable for mounting in various heat transfer systems or heat dissipating modules. Therefore, electronic system cooling using a fan-less design will have potential improved heat transfer efficiency of about 33% by natural convection. I. INTRODUCTION This study focuses on recent applications of heat plates in electronics industry. The operating principles, modeling, design, and testing of heat plates as applied to electronic applications with a comprehensive review of the specific applications are described. Using Computational Fluid Dynamics (CFD) analysis to calculate the thermal phenomena of electronic systems is increasing [1]. The benefits are undisputed when it comes to performing parametric studies in early design phases. The heat transfer behavior of electronic system is very complex. A common question is the numerical simulations accuracy compared to well-designed experiments. Many studies demonstrate excellent agreement when the design requirements Paper submitted 12/12/08; revised 01/21/09; accepted 07/03/09. Author for corresponding: Chun-Sheng Yu ( @s92.tku.edu,tw). *Department of Mechanical and Electro-Mechanical Engineering, Tamkang University, 151, Ying-Chuan Rd., Tamsui, 25137, Taipei, Taiwan. **ChrangRong Technology Co., LTD. were to maintain key temperatures in the Pentium processor case temperature. For operation and reliability; various ambient temperatures and conditions included 60 C, 55 C, 45 C, 35 C, and 25 C ambient. When electronics system adopted natural convection be the only thermal solution requirements and constrained chassis design at the same time, heat plate application plays an important role to enhance fin thermal performance. Electronic components thermal management must solve problems connected with the limitations on the maximum chip temperature with the requirements on the temperature uniformity level. One can use air and liquid coolers as well as coolers constructed on the phase change heat transfer principle in closed space to cool electronic components; i.e., immersion, thermosyphon, and heat plate coolers. Each of these methods has its merits and draw-backs. One must take into consideration the thermal parameters and system design and stability, durability, technology, price, application [10]. Heat plates, possessed of high thermal conductivity, provide an essential isothermal environment with very small temperature gradients between the individual components. The high heat transfer characteristics, ability to maintain constant evaporator temperatures under different heat flux levels, evaporator and condenser diversity and sizes variability make the heat plate an effective device for electronic components thermal control. II. FUNDAMENTAL THEOREM 1. Heat Plate Operation Heat plates are sealed by vacuum chambers that are partially filled with a working fluid, typically de-ionized water, as the heat transfer media. The heat plate envelopes are made of copper of different sizes, rectangular, or any other enclosed geometry. The heat plate embedded with extrusion fins is as shown on Fig. 1. The envelope wall is lined with a wick structure, which provides surface area for the evaporation and condensation cycle and capillary capability. Since the heat plate is evacuated and then charge with the working fluid prior to being sealed, the internal pressure is set by the working fluid vapor pressure [6].

2 420 Journal of Marine Science and Technology, Vol. 18, No. 3 (2010) 10 Radiating plate structure 160 mm Fig. 1. Mechanic dimension of Radiating Plate. 2. Performance Evaluation A heat plate works as an effective thermal conductivity (K eff ) device that can transfer a huge heat flux with a small temperature difference between vapor and condense zones. This shows high or low heat transfer efficiency from thermal conductivity of a heat plate [8]. Fourier s Law is used to investigate the thermal conductivity and compare with test data to understand the heat transfer efficiency of sintered wick structure in a heat plate. The equation is as follows: Q K A T measured in = eff eff (2) Leff Heat transfer between a solid surface and air flow The working fluid is vaporized as heat is applied to the surface of the heat plate. The vapor at the evaporator section is at a slightly higher temperature and pressure than other areas. This creates a pressure gradient that forces the vapor to flow into the cooler regions of the heat plate. For a heat plate to function properly, the net capillary pressure difference between the evaporator (heat sources) and condenser (heat sink) must be greater than the sum of all pressures losses occurring throughout the liquid and vapor flow path equation (1). This relationship, referred to as the capillary limitation can be expressed mathematically as follows [14]: P max > P 1 + P v + P g (1) P max is the maximum capillary pressure difference generated within the capillary wicking structure between the evaporator and condenser, P 1 and P v are the viscous pressure drops occurring in the liquid and vapor phases, respectively, and P g represents the hydrostatic pressure drop. When the maximum capillary pressure is equal to or greater than the sum of these pressure drops, the capillary structure can return an adequate amount of working fluid to prevent the evaporator wicking structure from drying out. When the sum of all pressure drops exceeds the maximum capillary pumping pressure, the working fluid is not supplied rapidly enough to the evaporator to compensate for the liquid loss through vaporization, and the wicking structure becomes starved of liquid and dries out to prevent the evaporator wicking structure from drying out [15]. When the sum of all pressure drops exceeds the maximum capillary pumping pressure, the working fluid is not supplied rapidly enough to the evaporator to compensate for the liquid loss through vaporization. The wicking structure then becomes insufficient of liquid and dries out. This condition, referred to as capillary limitation, varies according to the wicking structure, working fluid, evaporator heat flux, operating temperature, and body forces [16]. R T measured convection = = (3) Qin hc Aeff Equation (3) where: T = The driving temperature difference between average fluid and the average surface temperatures [ C] Q in = Power transferred [W] h c = Convective heat transfer coefficient [W/cm 2 - C] A eff = Surface area exposed to flow [cm 2 ] Heat transfer coefficient is a function of flow regime. From (3), the vapor and condense section temperature are the most important parameters for performance evaluation. The temperature differences ( T measured ) are obtained from thermocouple measurements, and calculate the heat loss by (3) from the interface to the heat plate wall that causes thermal resistance equation (4). Illustrated thermal resistance by network, then: T source R convection Q T base R fin Q 1 T fin Rconvection with fin efficiency = Rfin efficiency + Rconvection (4) 3. Heat Plate on Electronic Cooling Application The most versatile feature of using heat plates is the wide variety of geometries that can be constructed to take advantage of the available space around the electronics to be cooled. Proper heat source thermal isolated prevents heat loss during heating the heat plate vaporization area [7]. In many applications, the available heat sink size above the electronics is limited by the board-to-board spacing. In this situation, heat plates are used in a low profile system design that equalizes the heat to a large fin stack. In general, the smaller component cooling approaches transport 10 W to 30 W each. Currently industrial computers use a heat plate in the thermal management design. Most of these heat plate heat sinks use a 160 mm 90 mm as a platform to eliminate hot spot [2]. This type of passive design has been a very effective thermal management technique for CPUs which power ratings less then 30 W without cooling fan [9].

3 C.-S. Yu et al.: Fabricating a Heat Pipe Structure within a Radiating Plate for Electronics Fan-less Cooling 421 T 3 Pure Al heat sink test T 4 T 1 T2 T6 T 2 T 1 T 4 T 3 Pure Al heat sink test + Heat Spreader T 3 T 4 T1 T 2 T 1 T 2 Heater No. 1 Heater No. 2 Fig. 2. Experiment measured point description. Most thermal designs incorporate a heat sink or fins using a heat plate. This concept permits the computer system designer to locate the CPU independent of the heat sink. This novel methodology helps system designer to design the one of most effective heat exchangers and an optimal airflow path. The optimal design helps to reduce airflow requirements and acoustic noise. III. EXPERIMENTAL INVESTIGATION In situations which electronic is mounted on heat plate units inside electronic system. The structure strength of the heat transfer device in this study is significantly improved and thus complete contact between the electronic device to be cooled and the heat transfer device may be achieved. The proposed device is embedded in the plate and transports the heat to an air-cooled fin section. There are several different sized units like this being used in the field [11]. 1. Experimental Setup The HP data logger and thermocouple (T-type #36) are used to measure temperature at each point, as shown in Fig. 2. The measured temperature is transmitted to a personal computer via RS-232 from data acquisition system. The software observes and records the temperature variation on the heat plate. The HP data acquisition instrument main- Fig. 3. Heat pipe structure in a radiating plate. tains precision measurement at ±0.1 C. The T-type thermocouple is a precise temperature measurement sensor (test criteria is 0 C~100 C in this study). The measured temperature is calibrated by referring to the atmospheric boiling point at 100 C. In order to maintain the heat plate surface flatness and mean thickness of is the most important issue during testing, because thermal performance depends on the conductivity at each interface. The test system setup requires heat source thermal isolation to prevent data acquisition errors, which is always affected by ambient temperature. 2. Experimental Results Figure 3 shows an extrusion ( mm [L W H]). Thermal test is running by natural convection (ambient temperature is from 25 C to 60 C) with 2 major heat sources. The sizes are mm for each heater and total power consumption is 30 W. These tests were performed with the heat source in the center of the extrusion. Different fin configurations and measured temperatures are shown on Table 1. Standard fins are generally used for applications where high heat dissipation and airflow rate are low. For applications where heat dissipation is needed, embedded fins are employed to enhance the air convection. These fins must be optimized when designing the whole system to account for the pressure losses [13]. The temperature profiles at the surface of the extrusion with and without heat plates appear on Figs. 4, and 5, respectively [4]. The shape of the temperature profiles on Fig. 4 demonstrates that heat sink temperature. With heat plate assisted heat sink, thermal design needs to place the electrical components in convenient locations and is not limited by spreading resistance to the center of the heat sink. As shown in Fig. 5, the heat plates also significantly reduced the maximum surface temperature of the extrusion. By simply adding heat plates to the base of an existing heat sink, the overall sink to ambient temperature rises is reduced by 30 C. Figure 6 shows that a radiating heat plate presents better performance than original aluminum and copper plate design on electronics components cooling, and well proportioned temperature distribution on chassis skin. Figure 7 shows the lowest thermal resistance of this fan-less thermal solution

4 422 Journal of Marine Science and Technology, Vol. 18, No. 3 (2010) Table 1. Temperature differences on various fin direction and contact wall materials. Case Fin direction to Mounting T 1 T 2 place (NB) (CPU) T 3 T 4 T a Material C Rubber Rubber Air space Air space Wood Wood Al plate Al plate Spreader ARK1000 ARK1000 > Fig. 5. Temperature distribution without hot spot (Heat plate). 120 T T 1 T < 45 T 3 T a 20 g > Fig. 4. Temperature distribution with hot spot (Aluminum fin). is the best choice in natural convection in this study [12]. Faced with high spreading resistance, the thermal solution design must explore the feasibility of increasing the base thickness in an effort to reduce both the heat transfer by conduction and mass. The system design must decrease thermal resistance between any hot component and heat sink interface. In high power electronics system and server system, heat plates heat sink assemblies have been shown to offer both cost and performance advantages. With spreading acceptance, production volumes are increasing and costs are falling. This trend is, in turn, resulting in an expanding market with broader applica- < 45 Thermal resistance ( C/W) 0 Rubber Air space Wood Aluminum Heat plate Materials on mounting place Fig. 6. Temperature description of different fin performances Ta = 25 C Ta = 35 C Ta = 45 C Ta = 55 C Ta = 60 C Case study Fig. 7. Thermal resistance comparison of various ambient temperatures. tion boundaries. In the past few years, the use of heat pipes for the thermal management of industrial computers is a proven and widely applied technology. Furthermore, heat plate ap-

5 C.-S. Yu et al.: Fabricating a Heat Pipe Structure within a Radiating Plate for Electronics Fan-less Cooling 423 Table 2. Application field of fan less thermal solution. Item TDP (W) Fan-less thermal solution (Natural convection) Al extrusion heat sink Radiating heat plate AMD GX3 (1.1 W)* OK OK ULV-400 MHz (4 W)** OK OK Celeron-M 900 MHz (7 W) + CPU Intel 852 GM (3.2 W)** NA OK Pentium-M 1.1 GHz (12 W) + Intel 855 GME (4.3 W)** NA OK * [3] ** [5] plications have helped enhance the computer industry to the currently advanced level. Heat plate designs with optimal fin performance are easily handling the current requirements and allow additional industrial computer design flexibility. The next generation of industrial computers is limited by the increasing power that can be handled by optimized thermal management systems [2]. Heat plates are now being combined with other technology to help meeting the thermal requirements. In practice, these cooling units might be the best solutions for keeping pace with the increasing heat loads of high power semiconductors. After cooling the electronics which generate the most heat, there is the path to transfer heat from electronics components in the chassis. A sealed air-to-air heat exchanger is best for dealing with the residual heat inside the chassis. Heat plate reliability and flexibility has proven a valuable attribute that provides the system designer with increased layout possibilities and typically improves thermal performance. Heat plate assisted heat sinks can manage non-uniform heating and higher heat fluxes better than conventional heat sinks. Heat plates can be installed in the base of an extrusion to reduce the conduction spreading resistance. Spreading resistance occurs when a heat source s chip area is smaller than that of the heat sink s base. The electronic system can also be customized to fit the application as Table 2 listed. Heat plates offer an attractive approach to supplementing the conventional heat sink solution. The fin stack height, width, length, and fin spacing can be calculated using CFD numerical optimization, which allows the designer to extend the performance of more conventional heat sinks such as extrusions or castings. The heat plate and optimal fin design assembly permits the designer to achieve otherwise unreachable thermal resistance level via the efficient maximization of the convective surface area available for dissipation to the ambient air. Thermal simulation technology application means rapid computer performance increases. IV. CONCLUSION This study presented a progression of advance thermal design cooling approaches for the growing heat dissipation levels of power electronic devices. Heat plate assemblies (heat dissipation from 10 W to 30 W) allow increased heat sink performance within the volume available with little potential impact on the existing system design. Radiating heat plate applications can improve fin efficiency and eliminate hot spot for a fan-less system. This improvement, either in terms of permitting higher packaging densities or permitting higher power dissipation within a compact volume, a radiating heat plate heat sink, in this situation, extends the thermal solution to acquire additional surface area for heat dissipation by natural convection, thus eliminating the need for a fan. Heat plate applications become the accepted standard thermal design in industrial computers. Whereas volume constraints limit the use of a natural convection cooling solution, a heat plate to a miniature fan/sink can be more economical than a large system fan solution. REFERENCES 1. CFD Molding of a Therma-Base Heat Sink, Thermacore, Inc. (1998) Khrustalev, D. and Faghri, A., Thermal characteristics of conventional and flat miniature axially grooved heat pipes, ASME Journal of Heat Transfer, Vol. 117, No. 4, pp (1995). 7. 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