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Thermal Design for NOTEBOOK PC by using thermal analysis Man-In Baek*, Jung-Mi Lee LG Electronics Inc., LG Production Research center [email protected] Key word : Thermal design, RHE(Remote Heat Exchanger), Thermal analysis, PCB(Printed circuit board), Abstract The portable personal computer is more popular than ever. On the other hand, the power dissipation of notebook PC is higher. So many portable personal computer makers have a problem in thermal management. In this paper, we will review the various thermal enhancements from the component, board, and system level for the use of the Pentium processor in the ball grid array(BGA) in notebook PC. CFD and heat transfer simulations are applied to the layout design of the notebook PC. The CPU location is decided by the result of optimization of the fan and inlet air ventilation. To escape hot spots in the outcase we suggested Magnesium. And to maintain the device temperature and the surface temperature of the enclosure within limits, the thermal models have been developed for compact simulation and equivalent accuracy with experiments. 1. Introduction Electronic portable devices, especially portable computers, have become pervasive and popular nowadays. As the technology progresses and the market demands, more functions have become integrated and packed into these devices in conjunction with higher performance semiconductor chips, while the size of the devices is decreasing. The CPU in a personal computer has the largest power consumption of all devices and its consuming trend towards increasing. In addition, Chipset, audio, graphic processor, memory, and etc. are also beginning to generate more heat due to higher performance priority. On the other hand, there is a trend toward portable computer becoming thinner and lighter. They offer users the advantage of full personal computer capabilities in a portable system. And the disparity between notebook and desktop performance continues to lessen. The consumer’s need to get hold of portable property continues to escalate. Sales of portable units will grow twice as fast as those of desktops and account for a third of PC market in past ten years. Needless to say, this trend has not been lost on the industry’s major players. High performance processor and new features in notebooks, such as L2 cache, more DRAM, larger hard disk drivers, PCMCIA, etc., lead to higher power dissipation inside the notebook. This creates a greater need for high performance thermal solutions without compromising the notebook’s size and weight. While advancing processor technology is reducing the power consumption of components, notebook computer manufacturers are pushing the performance envelope to produce even higher performance notebook PC’s. The system designer is caught in the dilemma of having less time to design thermal management solutions for more complex, more compact, and higher powered notebook systems. The peripheral products for notebook computers are progressing very fast, such as larger TFT LCD, larger capability hard disk and added many additional devices. In this way the competition between vendors are so intense that make the price of notebook computers motivates the market to expand very fast. There are some problems when notebook computer design. Space allocation, definition and area of printed circuit boards, heat handling, noise interruption are the topics which determine whether notebook computers are successful or not. Among them, the heat problem plays a more important role than before. In the past, there was no overheat consideration, for the performance of CPU is not as high as it is now. However, the average power consumed by CPU reaches 14-16watts. It is an inevitable trend that CPU must include more and more power inside for stronger ability and more function. More power input means more heat generated. The reliability, functionality and even consumer satisfaction are concerned with thermal management. Thus, many kinds of heat control technology, theory and device are developed. In order to accomplish the principle of “TIME TO PROFIT”, designers take the double pressure of low cost and high performance on their shoulders. Usually, the failure of final layout in notebook computers is due to bad thermal considerations. This is because thermal aspect is always put in the last part of design. 2. Model Concept Heat Dissipation of CPU Figure 1 shows the power dissipation of Intel microprocessors as a function of time. The scatter 10 in data clearly suggests technology advancements where (T1-T2)and h are temperature difference and heat transfer coefficient between the two walls, g,ß,v and Pr are gravitational acceleration, volumetric thermal expansion coefficient, kinematic viscosity, and Prandtl number, respectively. to reduce power. But the overall trend is the exponential increase in power dissipation with Conductio time. This trend is not particular to one company’s technology. Nu Convectio Rac Nu =1 Fig.4 Heat transfer through horizontal fluid layer 1 Fig. 1 power dissipation of microprocessors Model of heat transfer Figure 3 shows the property of heat transfer between two horizontal surfaces which confront each other as illustrated in Figure 2 X-axis represents the Rayleigh number Ra and Y-axis represents the Nusselt number Nu. The characteristic length for these numbers is the gap of the two surfaces L. 6 2 3 where Rayleigh 4 5 number 10 10 In the region 10 10 10 is smaller than its critical value Ra Racr, the Nusselt number is 1.0. This indicates that for small Ra. Natural convection does not occur. Fluid between two walls does not move and heat transfer takes place only by conduction and radiation. Notebook PCs are designed so as to minimize dead space. Hence, in many cases, the Rayleigh number inside the PC is lower than its critical value and the thermal analysis for this type of PCs can be performed on the basis of heat conduction analysis. Heat conduction analysis does not need much computational power compared with that for fluid analysis. Even for the case that the Rayleigh number exceeds its critical value, if the corresponding part is independent or the domain of natural convection is small, it is not difficult to simulate the heat transfer by stuffing material, of which thermal conductivity k is represented by equation(3), instead of air in the corresponding portion of the numerical model. k = k air Nu Ai L Q Fig.2 Confronting surfaces Ra = Gr Pr Gr = gβ (T1 − T2 ) L3 / v 2 T1 Assuming that the view factor is one or gaps between PCBs or between PCB and other parts are small enough. Heat transfer Q due to thermal conduction and radiation between parallel surfaces is k Q = A( (T1 − T2 ) + σf 12 (T14 − T24 )) L 1 f 12 = 1/ ε 1 + 1 / ε 2 − 1 where ε i is emissivity of each surface and s is Stefan-Boltsmann constant. To simplify the calculation, heat transfer is converted to a form of thermal conductions as follows. RHE thermal solutions The mechanisms and functions of heat pipes are described in various publications. A brief review is introduced below on the basic structures and key properties of heat pipe. Heat pipes are sealed and evacuated vacuum tight vessels which are partially backfilled with a fluid. The fluid vessel are lined with aporous media(the wick) which acts as a passive pump, via capillary action, to circulate the condensate within the heat pipe. Earlier heat pipe thermal solutions have tried to avoid the use of miniature fans, due to the cost, reliability, power consumption of the fans. The past few years have seen a lot of improvements in miniature fans. Cost has been continually decreasing, making it quite affordable in the notebook PCs environment. Although miniature fan’s reliability is still not as good as the bigger fans used in desktop PCs, the use of ball bearing and seal of the bearing have improved miniature fans’s reliability of operation. Power consumption is also dropping, with some fans consuming only 0.2watts of power for typical operation. The RHE thermal solution was then developed to take advantage of the emergence of miniature fans in notebook PCs cooling. Figure 4 shows a setup of an RHE thermal solution of mechanical design that has been adopt in our new notebook PC. finned heat sink and a miniature fan to provide air flow for the heat sink. A special vent is created by the bottom of the notebook to provide inlet air flow to the heat sink. Inlet air flows through the heat sink, carries away the heat from the heat sink fins and finally exits through an exhaust fan dissipating heat to the ambient. The material of the case. As the technology progresses and the market demands, more functions have become integrated and packed into these devices in conjunction with higher performance semiconductor chips, while the size of the devices is decreasing. For instance, notebook PCs are getting faster and thinner. However most customers demand more reliability. In this notebook PC, we adopt a Mg case that is more than 100 times more conductive than a plastic case. 3. Numerical analysis Simplified Keyboard The keyboard has a plate which consists of a PCB and a aluminum plate. On the base plate, there are many switches with key-caps. Components beneath the keyboard heat up the base plate. From the base plate and key-caps, heat dissipates under natural convection. The phenomenon involved in the effect of key rows on the heat transfer is complicated. Fig.5 is the real mechanical design and the simulation model of keyboard. Since the contribution of key array to horizontal thermal conduction is small, the keyboard is simplified to a base plate and modified heat transfer coefficient which includes the influence of the key array is applied on the base plate. The keyboard is assumed to be a plain plate on which equivalent heat transfer coefficient hk is calculated as follows. hk = Figure 4: RHE thermal solution of mechanical design As shown in the figure, the heat generated from the CPU is conducted to a metal block directly in contact with the CPU through thermal interface material. The heat is then transferred to two paths for heat dissipation. One is a passive solution represented by the heat pipe to keyboard thermal solution on the top side of the block. The other is an active solution represented by the heat exchanger/fan assembly which consists of a Q where LW (Tk − Ta ) - Q,L,W are heat dissipation from key side of the keyboard, keyboard length and width respectively. - Tk and Ta are temperature of base plate and ambient temperature. Since the contribution of the keys to horizontal thermal conduction is small, it is appropriate to use the base plate as the simplified keyboard. Battery Card Bus PCI 440Zx Grph. Pix HDD Equivalent heat Figure Plain 5 : Keyboard plate design Equivalent PCB Model Printed Circuit boards(PCBs) consist of glassepoxy layers and copper layers. In the present analysis, the PCBs are simplified as simple plates with anisotropic thermal conductivity for the whole domain analysis model. The equivalent horizontal thermal conductivity and normal thermal conductivity are calculate by use of the Flotherm web site. Numerical and Experimental result Fig.6 illustrates the numerical model for the whole domain of the notebook PC. This numerical model was created using of Flotherm. The boundary conditions are turbulent flow and radiation effects attached to each component. Table 1 shows heat dissipation of each component and was obtained from Intel corporation and other component manufactures. Total heat dissipation from the notebook PC model was 19.35watts. CFD-base thermal simulation were carried out using the software Flotherm. Components Heat dissipation(watts) CPU 11.2 Graphic chip 2.01 Audio chip 0.38 Video chip 2.23 Card bus 0.35 Dimm socket 0.33 Memory chips 0.2 Modem card 1.2 PC card 1.35 HDD 0.5 Table 1 : Heat dissipation of each components CPU Fan Figure 6 : Simulation model of notebook PC Two simulation cases were run for each of the without docking station and with docking station, slice Figure7 and Figure 8 are simulation result of each case. In each components temperatures were recorded for the two cases in table 2. Components Sim. Result Exp. Result CPU 97.5 C 95.0 C Graphic chip 83.0 C 80.2 C Audio chip 82.5 C 81.0 C Bottom case1 45.3 C 42.9 C Bottom case2 46.8 C 43.2 C HDD 46.4 C 44.6 C Keycap 38.1 C 34.0 C Palm rest 35.8 C 31.5 C Ambient 25.0 C 25.3 C Table2: Comparison of experimental and simulation results Conclusion With the continuing increase of CPU and other system power dissipation in notebook PC. Metal outcase and RHE (remote heat exchanger) use in notebook PC has been quite widely within the introduction of the high power microprocessor. Thermal analysis and experimental for the notebook PC were carried out. The main purpose is to decrease the CPU temperature and case temperature. When we used metal case in notebook PC, the skin temperature and main heat dissipate components temperatures are decreased. The design of RHE (remote heat exchanger) is carried by result of simulation and experimental result. As a result of applying this method, the CPU with fan and other components were located on a unique position. Reference 1.Avram Bar-Cohen,Allan D. Kraus,”Advances in Thermal modeling of electronic components and systems Volume2” ASME press series 2. Kim sung jin and Lee Sang Woo,1996,”Air Cooling Technology for Electronic Equipment”,CRC Press,Boca Raton, New York. 3. Nakayama ,W.,1988,”Thermal Management of Electronic Equipment: A Review of Technology and Research Topics,” Chapter 1 in advances in thermal Modeling of Electronic Component and System – Vol.1, eds., A. BarCohen and A.D. Kraus, Hemisphere Publishing, New York. 4. Steinberg, D.S.,1991, “Cooling Techniques for Electronic Equipment”, John Wiely & Sons, Inc, 2nd Edition, pp.129. 5. Martin Wills, 1983, “Thermal analysis of Air Cooled PCBs ”, Electronic Production May. 6.A.Bar-Cohen, and Krueger, W.B, 1997, “ Thermal Characterization of Chip PackagesEvolutionary Development of Compact models ”, IEEE CPMT, Vol.20 No.4,pp 399-409 7. Hendrik Decruyenaere, 1999,”Advanced Thermal Resistance Characterization Technique for IC Packages”, ASME INTERPAK ’99, No. 252, Vol.1 pp. 987-992 8. 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