Thermal Design for NOTEBOOK PC by using thermal analysis

Man-In Baek*, Jung-Mi LeeLG Electronics Inc., LG Production Research center

mandol@lge.com

Key word : Thermal design, RHE(Remote Heat Exchanger), Thermal analysis, PCB(Printed circuitboard),

AbstractThe portable personal computer is more popular than ever. On the other hand, the power dissipation ofnotebook PC is higher. So many portable personal computer makers have a problem in thermalmanagement. 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. CFDand heat transfer simulations are applied to the layout design of the notebook PC. The CPU location isdecided by the result of optimization of the fan and inlet air ventilation. To escape hot spots in the outcasewe suggested Magnesium. And to maintain the device temperature and the surface temperature of theenclosure within limits, the thermal models have been developed for compact simulation and equivalentaccuracy with experiments.

1. IntroductionElectronic portable devices, especially portablecomputers, have become pervasive and popularnowadays. As the technology progresses and themarket demands, more functions have becomeintegrated and packed into these devices inconjunction with higher performancesemiconductor chips, while the size of the devicesis decreasing. The CPU in a personal computerhas the largest power consumption of all devicesand its consuming trend towards increasing. Inaddition, Chipset, audio, graphic processor,memory, and etc. are also beginning to generatemore heat due to higher performance priority. Onthe other hand, there is a trend toward portablecomputer becoming thinner and lighter.

They offer users the advantage of full personalcomputer capabilities in a portable system. Andthe disparity between notebook and desktopperformance continues to lessen. The consumer’sneed to get hold of portable property continues toescalate. Sales of portable units will grow twice asfast as those of desktops and account for a third ofPC market in past ten years. Needless to say, thistrend has not been lost on the industry’s majorplayers.

High performance processor and new features innotebooks, such as L2 cache, more DRAM, largerhard disk drivers, PCMCIA, etc., lead to higherpower dissipation inside the notebook. Thiscreates a greater need for high performancethermal solutions without compromising thenotebook’s size and weight. While advancingprocessor technology is reducing the power

consumption of components, notebook computermanufacturers are pushing the performanceenvelope to produce even higher performancenotebook PC’s. The system designer is caught inthe dilemma of having less time to design thermalmanagement solutions for more complex, morecompact, and higher powered notebook systems.

The peripheral products for notebook computersare progressing very fast, such as larger TFTLCD, larger capability hard disk and added manyadditional devices. In this way the competitionbetween vendors are so intense that make theprice of notebook computers motivates the marketto expand very fast. There are some problemswhen notebook computer design. Spaceallocation, definition and area of printed circuitboards, heat handling, noise interruption are thetopics which determine whether notebookcomputers are successful or not. Among them,the heat problem plays a more important role thanbefore. In the past, there was no overheatconsideration, for the performance of CPU is notas high as it is now. However, the average powerconsumed by CPU reaches 14-16watts. It is aninevitable trend that CPU must include more andmore power inside for stronger ability and morefunction. More power input means more heatgenerated. The reliability, functionality and evenconsumer satisfaction are concerned with thermalmanagement. Thus, many kinds of heat controltechnology, theory and device are developed.

In order to accomplish the principle of “TIME TOPROFIT”, designers take the double pressure oflow 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 thelast part of design.

2. Model Concept

Heat Dissipation of CPUFigure 1 shows the power dissipation of Intelmicroprocessors as a function of time. The scatterin data clearly suggests technology advancementsto reduce power. But the overall trend is theexponential increase in power dissipation withtime. This trend is not particular to onecompany’s technology.

Fig. 1 power dissipation of microprocessors

Model of heat transfer

Figure 3 shows the property of heat transferbetween two horizontal surfaces which confronteach other as illustrated in Figure 2 X-axisrepresents the Rayleigh number Ra and Y-axisrepresents the Nusselt number Nu. Thecharacteristic length for these numbers is the gapof the two surfaces L.

Fig.2 Confronting surfaces

PrGrRa =

2321 /)( vLTTgGr −= β

where (T1-T2)and h are temperature differenceand heat transfer coefficient between the twowalls, g,ß,v and Pr are gravitational acceleration,volumetric thermal expansion coefficient,kinematic viscosity, and Prandtl number,respectively.

Fig.4 Heat transfer through horizontal fluid layer

In the region where Rayleigh number is smallerthan its critical value Racr, the Nusselt number is1.0. This indicates that for small Ra. Naturalconvection does not occur. Fluid between twowalls does not move and heat transfer takes placeonly by conduction and radiation.

Notebook PCs are designed so as to minimizedead space. Hence, in many cases, the Rayleighnumber inside the PC is lower than its criticalvalue and the thermal analysis for this type of PCscan be performed on the basis of heat conductionanalysis. Heat conduction analysis does not needmuch computational power compared with thatfor fluid analysis.

Even for the case that the Rayleigh numberexceeds its critical value, if the corresponding partis independent or the domain of naturalconvection is small, it is not difficult to simulatethe heat transfer by stuffing material, of whichthermal conductivity k is represented byequation(3), instead of air in the correspondingportion of the numerical model.

Nukk air=

Assuming that the view factor is one or gapsbetween PCBs or between PCB and other partsare small enough. Heat transfer Q due to thermalconduction and radiation between parallelsurfaces is

))()(( 42

411221 TTfTT

Lk

AQ −+−= σ

1/1/11

2112 −+

=εε

f

Ai LQ T1

Nu

Ra102 103 104 105 106

Nu =1

Conductio Convectio

1

10

Rac

where iε is emissivity of each surface and s is

Stefan-Boltsmann constant. To simplify the calculation, heat transfer isconverted to a form of thermal conductions asfollows.

RHE thermal solutions The mechanisms and functions of heat pipes aredescribed in various publications. A brief reviewis introduced below on the basic structures andkey properties of heat pipe. Heat pipes are sealed and evacuated vacuum tightvessels which are partially backfilled with a fluid.The fluid vessel are lined with aporous media(thewick) which acts as a passive pump, via capillaryaction, to circulate the condensate within the heatpipe.

Earlier heat pipe thermal solutions have tried toavoid the use of miniature fans, due to the cost,reliability, power consumption of the fans. Thepast few years have seen a lot of improvements inminiature fans. Cost has been continuallydecreasing, making it quite affordable in thenotebook PCs environment. Although miniaturefan’s reliability is still not as good as the biggerfans used in desktop PCs, the use of ball bearingand seal of the bearing have improved miniaturefans’s reliability of operation. Power consumptionis also dropping, with some fans consuming only0.2watts of power for typical operation. The RHE thermal solution was then developed totake advantage of the emergence of miniature fansin notebook PCs cooling. Figure 4 shows a setupof an RHE thermal solution of mechanical designthat has been adopt in our new notebook PC.

Figure 4: RHE thermal solution of mechanicaldesign

As shown in the figure, the heat generated fromthe CPU is conducted to a metal block directly incontact with the CPU through thermal interfacematerial. The heat is then transferred to two pathsfor heat dissipation. One is a passive solutionrepresented by the heat pipe to keyboard thermalsolution on the top side of the block. The other is an active solution represented by theheat exchanger/fan assembly which consists of a

finned heat sink and a miniature fan to provide airflow for the heat sink. A special vent is createdby the bottom of the notebook to provide inlet airflow to the heat sink. Inlet air flows through theheat sink, carries away the heat from the heat sinkfins and finally exits through an exhaust fandissipating heat to the ambient.

The material of the case. As the technology progresses and the marketdemands, more functions have become integratedand packed into these devices in conjunction withhigher performance semiconductor chips, whilethe size of the devices is decreasing. For instance,notebook PCs are getting faster and thinner. However most customers demand morereliability. In this notebook PC, we adopt a Mg case that ismore than 100 times more conductive than aplastic case.

3. Numerical analysis

Simplified Keyboard The keyboard has a plate which consists of aPCB and a aluminum plate. On the base plate,there are many switches with key-caps.Components beneath the keyboard heat up thebase plate. From the base plate and key-caps, heatdissipates under natural convection. Thephenomenon involved in the effect of key rows onthe heat transfer is complicated.Fig.5 is the real mechanical design and thesimulation model of keyboard. Since thecontribution of key array to horizontal thermalconduction is small, the keyboard is simplified toa base plate and modified heat transfer coefficientwhich includes the influence of the key array isapplied on the base plate. The keyboard is assumed to be a plain plate onwhich equivalent heat transfer coefficient kh is

calculated as follows.

)( akk TTLW

Qh

−= where

- Q,L,W are heat dissipation from key side of thekeyboard, keyboard length and widthrespectively.

- kT and aT are temperature of base plate and

ambient temperature. Since the contribution of the keys to horizontalthermal conduction is small, it is appropriate touse the base plate as the simplified keyboard.

Figure 5 : Keyboard design

Equivalent PCB Model Printed Circuit boards(PCBs) consist of glass-epoxy layers and copper layers. In the presentanalysis, the PCBs are simplified as simple plateswith anisotropic thermal conductivity for thewhole domain analysis model. The equivalent horizontal thermal conductivityand normal thermal conductivity are calculate byuse of the Flotherm web site.

Numerical and Experimental result Fig.6 illustrates the numerical model for thewhole domain of the notebook PC. Thisnumerical model was created using of Flotherm. The boundary conditions are turbulent flow andradiation effects attached to each component. Table 1 shows heat dissipation of eachcomponent and was obtained from Intelcorporation and other component manufactures.Total heat dissipation from the notebook PCmodel was 19.35watts. CFD-base thermalsimulation were carried out using the softwareFlotherm.

Components Heat dissipation(watts)CPU 11.2Graphic chip 2.01Audio chip 0.38Video chip 2.23Card bus 0.35Dimm socket 0.33Memory chips 0.2Modem card 1.2PC card 1.35HDD 0.5

Table 1 : Heat dissipation of each components

Figure 6 : Simulation model of notebook PC

Two simulation cases were run for each of thewithout docking station and with docking station,sliceFigure7 and Figure 8 are simulation result of eachcase. In each components temperatures wererecorded for the two cases in table 2.

Components Sim. Result Exp. ResultCPU 97.5 C 95.0 CGraphic chip 83.0 C 80.2 CAudio chip 82.5 C 81.0 CBottom case1 45.3 C 42.9 CBottom case2 46.8 C 43.2 CHDD 46.4 C 44.6 CKeycap 38.1 C 34.0 CPalm rest 35.8 C 31.5 CAmbient 25.0 C 25.3 C

Table2: Comparison of experimental andsimulation results

Conclusion With the continuing increase of CPU and othersystem power dissipation in notebook PC. Metaloutcase and RHE (remote heat exchanger) use innotebook PC has been quite widely within theintroduction of the high power microprocessor.Thermal analysis and experimental for thenotebook PC were carried out. The main purposeis to decrease the CPU temperature and casetemperature. When we used metal case in notebook PC, theskin temperature and main heat dissipatecomponents temperatures are decreased. Thedesign of RHE (remote heat exchanger) is carriedby result of simulation and experimental result. As a result of applying this method, the CPU withfan and other components were located on aunique position.

Plain plate

Equivalent heat

CPU

Fan

PixGrph.440Zx

HDD

Battery Card Bus

PCI

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