Investigation of Packaged Miniature Heat Pipe for Notebook ... · After performance test of a pressed and bent type MHP, the test with the same MHP packaged into the notebook PC is

© International Microelectronics And Packaging Society

The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number 4, Fourth Quarter, 2000 (ISSN 1063-1674)

Intl. Journal of Microcircuits and Electronic Packaging

488

Investigation of Packaged Miniature Heat Pipefor Notebook PC CoolingSeok Hwan Moon, Ho Gyeong Yun, Gunn Hwang, Tae Goo ChoyElectronics and Telecommunications Research InstituteKajong-Dong, Yusong-Gu,Taejon, Korea, 305-350Phone: +82-42-860-6087Fax: +82-42-860-5804e-mails: {shmoon, hgyun, hwangun}@etri.re.kr

Abstract

Miniature heat pipe (MHP) with woven-wired wick was used to cool the CPU of a notebook PC. The pipe with circular cross-sectionwas pressed and bent for packaging the MHP on the very limited space of a notebook PC. A cross-sectional area of the pipe isreduced about 30% when the MHP with 4mm diameter is pressed to 2mm thickness. In this study, a performance test has beenperformed in order to review the change of operating performance according to the pressed thickness variation and the heat dissipa-tion capacity of MHP cooling module that is packaged on a notebook PC. New wick type was considered for overcoming low heattransfer limit when MHP is pressed to thin-plate. The limitation of the pressed thickness is shown to be within the range of 2mm ~2.5mm through the performance test. When the wall thickness of pipe is reduced to 0.25mm from 0.4mm to minimize the conductivethermal resistance through the wall of heat pipe, limiting power and thermal resistance of MHP were improved about 10%. In themeantime, it is shown that the thermal resistance and heat transfer limit for the MHP with central wick type are higher than those ofMHP with existing wick types. As a result, the performance test of MHP cooling modules with woven-wired wick to cool a notebookPC shows the good stability as cooling system since Tj (Junction Temperature of Processor) satisfies a requirement condition of 0 ~100°C under CPU heat, 11.5W.

Key words:

Heat Pipe, Heat Sink, Maximum Heat Transfer Limit, Woven-wired Wick, and Electronic Packaging.

1. Introduction

Recently, the market of notebook PC has grown very rapidly.The importance of cooling problem in notebook PC is increasingsignificantly since the notebook PC has been made in the trendof thin-plate type packaging with high-density packaging throughtelecommunication and electronics industries.1 Especially, it isvery difficult to reserve enough space for cooling module since aspace in notebook PC is very small compared to other electronic

equipments. To design a notebook PC efficiently, not only theelectronic design but the thermal design should be considered.For adopting a heat pipe to the notebook PC, the most importantfactor is reserving a spare space for the heat pipe. Some compa-nies manufacturing heat pipes or notebook PCs remove heat fromthe CPU by connecting two heat pipes, i.e., the primary heat pipefrom the CPU to the notebook hinge, and the secondary heat pipefrom the hinge to the rear side of LCD panel by means of a ther-mal hinge. Other companies solve their thermal problem of note-book PC using keyboard or bottom side of notebook PC for cool-ing2,3 . If the final heat destination of notebook PC is contactedwith part of human, the user feels very uncomfortable and it willmake the market of the notebook PC unsuccessful.

In this study, to package a MHP into the very limited space ofa notebook PC, the MHP with circular cross-section was pressedand bent. In a circular type MHP, the woven-wired wick is tightlycontacted on the inner wall of MHP using elasticity of the metalwick and has enough space used for internal path of steamed air.However, the cross-sectional area for internal path of steamed air

Investigation of Packaged Miniature Heat Pipe for Notebook PC Cooling


© International Microelectronics And Packaging Society 489

2. Experimental Apparatus andProcedure

Prior to packaging a pressed and bent type MHP into the note-book PC, a pressed and bent type MHP is tested to compare itsperformance with a circular type MHP. The entire experimentalequipment is composed of the MHP, supporting unit, data acqui-sition system, and power supply unit. The thermal load is sup-plied by Ni-Cr thermic wires with the diameter of 0.36mm(10!/m) that is wound at an interval of 1.5mm on the wall of theevaporator. Wrapping thermic wires of the evaporator with aninsulation material, such as glass fiber, minimizes the heat lossto the air. Cooling pins (53mmx53mmx32mm), a fan, and ductsare attached at the condenser to achieve forced convection cool-ing. Thermal grease (0.74W/m°C) is charged between the outerwall of a heat pipe and the cooling pins of the condenser to mini-mize thermal contact resistance. The temperatures of the heatpipe wall are measured on the outer wall side of the heat pipeusing six K-type ("0.08mm) thermocouples attached on the wallby soldering. Two of them are located at the evaporator, one ofthem is at the adiabatic zone, and three of them are at the con-denser of the heat pipe. Distilled water is used as working fluidsince water has relatively large surface tension and latent heatcapacity under the operation temperature in the range of 30°C ~150°C. The optimal charging ratio is 31% of the inner totalvolume of the heat pipe for a 4mm MHP. The number of strandsof the woven-wick is selected 8 for optimum performance. Theinclination of MHP is changed within the range of –5o~ +5o.After performance test of a pressed and bent type MHP, the testwith the same MHP packaged into the notebook PC is also per-formed as shown in Figure 2. This unit composed of two MHPcools the CPU of notebook PC by dissipating the heat from CPUto condenser formed with heat sink and fan. The heat sink ofcondenser dissipates the transferred heat when the heat is under80°C. The fan installed at condenser operates and dissipates thetransferred heat when the heat is over 80°C, Reference5. Thethermal pad is attached to minimize a thermal contact resistancebetween the CPU and the aluminum block of evaporator. The K-type ("0.08mm) thermocouples are placed at the surface of CPU,evaporator, adiabatic zone, and condenser of the heat pipe to testa performance of the heat pipe installed on notebook PC. Thelocations of the thermocouples are shown at Figure 2b.

is reduced due to pressed and bent process. When a MHP with4mm diameter is pressed to the 2mm thickness, 30% of the origi-nal cross-sectional area of the MHP is decreased as shown inFigure 1 of Reference4. The cross-section of a MHP with woven-wired wick is also shown in Figure 1.

(a)

Pipe wall

Wick

(b)

Figure 1. Woven-wired wick. (a) SEM image of pressedheat pipe, (b) schematic diagram of the woven-wired wick.

The researchers investigated the performance of pressed andbent type MHP for cooling unit of notebook PC, especially incapability of heat transfer. A new type models of MHP are pro-posed and reviewed to overcome the heat transfer limit of MHP.These studies can be used to establish the optimal design of cool-ing by MHP in notebook PC.




490

(a)

?

? ?

‡ D

‡ @

‡ E

No Position 1 Center of CPU surface 2 Center of Al block,

evaporator 3 Adiabatic zone of MHP 4 Adiabatic zone of MHP 5 Surface of heat sink 6 Exit of fan

(b)

Figure 2. Cooling module with heat pipe. (a) Photograph ofcooling module by heat pipe, (b) Location of thermocouples.

The thermal load is supplied to the evaporator electrically from2W by 2W stepwise. The wall temperature of the MHP in eachstep is recorded when the temperature of the wall reached steadystate after it begins to be increased as the heat is supplied. Ittakes about 20 ~ 30 minutes to reach each steady state condition.The results of the present study may include the error rate in themeasurement, i.e., the error rate in the heat supply (±0.05V for

voltage, ±0.01 A for current) and that in the temperature mea-surement (±0.1°C).

3. Results and Discussion

An experimental study is performed to understand the cool-ing performance according to the pressed thickness of pipe. Press-ing process is essential in order to package a MHP to the limitedspace like a notebook PC. Figure 3 shows the performance testresults of the MHPs of which the pressed thickness are 3mm,2.5mm, and 2mm and these modules are made from the MHPswhose diameter is 4mm, pipe wall thickness is 0.4mm, the num-ber of wick strands is 8, and the amount of working fluid is 0.67cc.In Figure 1, the cross-sectional area for internal fluid flow pathin MHP is reduced as pressing the pipe but, nevertheless, thewick is tightly attached to the inner wall of pipe. That is due tothe property of elasticity of woven-wired wick. Though the cross-section of MHP with woven-wired wick is changed to be pack-aged into the limited cooling space, the role as a cooling modulecan be sufficiently performed. In Figure 3, the heat transfer rateof each MHP increased respectively as the inclination angle isincreased within the range of –5° ~ +5°. The maximum heattransfer limit is to be 6W ~ 22W with the 2mm ~ 4mm pressedthickness and show the difference of that of 66% ~ 120% as theinclination angle is changed from –5° to +5°. Heat transfer limitis somewhat largely dependent on the inclination angle, and thismeans that the cooling performance of this heat pipe under test isaffected by gravity force. That is undesirable result for the heatpipe but one should understand as that is due to the characteris-tics of woven-wired wick. Meanwhile, in Figure 3, the maxi-mum heat transfer limit is decreased when the pipe is graduallypressed from 4mm to 3mm or more. In the case of 2.5mm, thecooling performance is stable though the cooling performance isdecreased by 12% ~ 18%. The MHP shows the stable operatingcharacteristics up to pressed thickness of 2.5mm since the oneshows a small degree of the decreasing of 12% ~ 18% though thecooling performance may be decreased by the changes of pressedthickness. However, in the case of 2mm, the heat transfer limitis largely decreased by 40% ~ 50%. Therefore, one can concludethat 2mm as the minimum requirement for the MHP with wo-ven-wired wick.

To minimize the conductive thermal resistance through thewall among the total thermal resistance of thermal flow path, theminimum wall thickness6 of a MHP is 0.25mm. Figure 4 showsthe performance test results of the MHP having the pipe wallthickness of 0.4mm and 0.25mm with respect to the number ofwick strand, 8, the length, 300mm, and the pressed thickness,2mm. The maximum heat transfer limit of the MHP with 0.25mmwall thickness is about 10% ~ 19% higher than that of 0.4mmwith inclination angle of –5° ~ +5°. Therefore, it can be mini-mized for the conductive thermal resistance through the pipewall at the evaporator and condenser by reducing the pipe wallthickness. In the circular type heat pipe, the heat transfer limit is




decreased exponentially as the diameter is decreased based onthe identical structure3. When the heat pipe is pressed to thin-plate type, it is difficult to obtain high heat transfer limit sincethe reduction of cross-sectional area for fluid flow path.

Forced Convection Cooling(1m/s)

Inclination Angle (degree)

-5 0 5

Hea

t Tra

nspo

rt R

ate

(W)

5

10

15

20

t=4mmt=3mmt=2.5mmt=2mm

Figure 3. Effect of pressed thickness on the performance ofMHP.

Inclination Angle (degree)

-5 0 5

Max

imum

Hea

t Tra

nsfe

r Lim

it (W

)

5

6

7

8

9

10

11

12

13

Wall Thickness=0.4mmWall Thickness=0.25mm

Figure 4. Comparison of the maximum heat transfer limitin MHP with the 0.4mm or 0.25mm wall thickness.

The performance test is conducted to compare the heat trans-fer characteristics between the MHP whose wick is attached alongthe inner wall and the MHP whose wick is placed at the center ofthe MHP for flat plate type. Figure 5 shows the results. In Fig-ure 5, the thermal resistance of MHP is decreased as the thermal

load is increased. In the case of the MHP with existing wicktype, the thermal resistance is under the 1°C/W when the ther-mal load is larger than 6W, but, on the other hand, for the MHPwith central wick type, the thermal resistance is over the 2°C/Wfor the total thermal load range. When a heat pipe is pressed, theMHP with central wick type is able to protect the central partsinking of the pressed plate and minimize the reduction of fluidflow area. However, the test results show the abnormal state ofphase change heat transfer.

Input Power (W)0 2 4 6 8 10 12 14 16 18 20

Ther

mal

Res

ista

nce

(o C/W

)

0

1

2

3

4

5

6

7

8

Central Wick TypeExisting Wick Type

Horizontal Orientation,Natural Convection Cooling

Figure 5. Comparison of central wick type with existingwick type on thermal resistance of MHP.

Figures 6 and 7 show the results of performance test for theMHP with woven-wired wick packaged into a notebook PC. Inthe present notebook PC market, the CPU cooling module is madeof two heat pipes, a heat sink, a heat spreader, and a fan on heatsink. To minimize the thermal resistance between the heat pipeand the heat spreader, a thermal joint compound is used. On theaverage, it takes about 12 hours to test each heat pipe. The tem-peratures of each position are measured at each 2 seconds. Fig-ure 6 shows the temperature distributions of each position withthe operating conditions of a notebook PC under the continuousreading/writing thermal loads of HDD (Hard Disk Drive) andthe ones of continuous reading of CD-ROM Drive. The thermo-couple positions for measuring the temperature are shown to Fig-ure 2. In Figure 6, there is a significant gradient of the tempera-ture through the positions of 1, 2, 3, which means that the con-tact thermal resistance through the heat pipe, the heat spreaderand the surface of CPU (exactly, the surface of heat transfer plate)is relatively large. The temperature distributions obtained byperformance test satisfy the requirement of Tj (Junction Tem-perature of Processor) of 0°C ~ 100°C with respect to the operat-ing condition of CPU thermal load, 11.5W. Figure 7 shows theresults of performance test according to the thermal loads for acooling module of the heat pipe using a pseudo heat source




492

4. Conclusion

A test is performed in order to study the cooling characteris-tics of MHPs with woven-wired wick for application to a note-book PC. Conclusions are drawn from the results of the perfor-mance test as follows:

1. MHP with woven-wired wick shows the normal operatingperformance up to the pressed thickness of 2.5mm. However,cooling performance is significantly decreased when the pressedthickness is 2mm. Therefore, the limitation of pressed thicknessof MHP with woven-wired wick is between 2mm and 2.5mm.

2. Maximum heat transfer limit and thermal resistance canbe improved by about 10% as the wall thickness of MHP is re-duced from 0.4mm to 0.25mm.

3. Maximum heat transfer limit and thermal resistance ofMHP with central type wick is shown to be lower than that of theMHP whose wick is attached along the inner wall.

4. The results of performance test are satisfied the require-ment of Tj (Junction Temperature of Processor) of 0°C ~ 100°Cunder the operating condition, 11.5W of a CPU thermal load.

References

1. S. S. Nam, S. B. Choi, J. H. Kim, and H. Y. Kwak, “TransientCharacteristics of a Two-Phase Thermosyphon Loop forMultichip Module,” ETRI Journal, Vol. 20, No. 3, pp.284~299, September 1998.

2. K. Eguchi, M. Mochizuli, K. Mashiko, K. Goto, Y. Saito, Y.Nagaki, A. Takamiya, and T. Nguyen, “Cooling of CPU Us-ing Micro Heat Pipe,” Fujikura Co., Technical Note, Vol. 9,pp. 64~68, 1997.

3. H. Xie, M. Aghazadeh, and J. Toth, “The Use of Heat Pipes inthe Cooling of Portables with High Power Packages,”Thermacore Co., Technical Note, 1995.

4. B. R. Babin, G. P. Peterson, and D. Wu, “Steady-State Model-ing and Testing of a Micro Heat Pipe,” ASME Journal of HeatTransfer, Vol. 112, No. 3, pp. 595-601, August 1990.

5. M. Mochizuki, “Advanced Cooling System Using MiniatureHeat Pipes and High Aspect Ratio Fins for Electronics Cool-ing,” Second Heat Pipe Workshop, Taejon, Korea, pp. 11~20,1998.

6. D. Kurustalev and A. Faghri, “Thermal Analysis of a MicroHeat Pipes,” ASME J. of Heat Transfer, ASME HTD, Vol.236, pp. 19~30, 1993.

7. S. H. Moon, C. G. Choi, G. Hwang, and T. G. Choy, “Experi-mental Study on Performance of a Miniature Heat Pipe withWoven-Wired Wick,” 7th Intersociety Conference on Ther-mal and Thermomechanical Phenomena in Electronic Sys-tems, Las Vegas, Nevada, USA, Vol. 2, pp. 129~133, 2000.

(35mm×35mm×5mm) instead of CPU. In Figure 7, it is difficultto dissipate the heat by only conductive heat transfer through thealuminum plate without heat pipe since there is large tempera-ture difference between the evaporator and the condenser7. Asmentioned in Figure 3, the temperature difference between theevaporator and the condenser when the pressed thickness of apipe is 2mm is relatively higher than those of the pressed thick-ness of 2.5mm and 3mm. Therefore, one can conclude that thelimitation of pressed thickness of MHP is between 2mm and2.5mm.

Position of Thermocouple

0 1 2 3 4 5 6 7

Tem

pera

ture

Diff

eren

ce, ∆

T

10

20

30

40

50

60

Pressed Thickness=2mmPressed Thickness=2.5mm

Figure 6. Temperature difference according to position ofthermocouple on a notebook PC.

Input Power, W

8 12 16 20 24

Tem

pera

ture

Diff

eren

ce, o C

0

10

20

30

40

50No Heat PipePressed Thickness=2mmPressed Thickness=2.5mmPressed Thickness=3mm

Figure 7. Temperature difference between evaporator andcondenser according to input power.




About the authors

Seok Hwan Moon received a Ph.D.Degree in Mechanical Engineering fromthe SungKyunKwan University, Korea,in 1999. Currently, he is a Senior Mem-ber of Engineering Staff at the ElectronicPackaging Technology Team, for Elec-tronics and Telecommunications Re-search Institute (ETRI) in Korea. He hasbeen conducting research in the field ofheat pipes/thermosyphons and especiallymicro heat pipes for 8 years. His current

research interests are developing cooling technology for electronicequipment.

Ho Gyeong Yun is a member of Engi-neering Staff at the Electronic Packag-ing Technology Team for Electronics andTelecommunications Research Institute(ETRI) in Korea. He received his B.S.Degree in 1998, followed by his M.S.Degree in 2000, both in Materials Sci-ence and Engineering from the Univer-sity of Korea. His project deals with metalsintering process.

Gunn Hwang is a leader of the Elec-tronics Packaging Technology Team forElectronics; and TelecommunicationsResearch Institute (ETRI), where he hasbeen employed for sixteen years. He re-ceived his Ph.D. in 1999, his MastersDegree in 1985, both in Mechanical En-gineering from Korea Advanced Insti-tute of Science and Technology, and hisBachelors Degree from the University ofKorea in 1983. His current work centers

around packaging of electronics system, especially developingcooling techniques using heat pipe.

Tao Goo Choy is a Director of Com-ponent Technology Development Depart-ment for Electronics and Telecommuni-cations Research Institute (ETRI), wherehe has been employed for twenty threeyears. He received his B.S. Degree in1974, followed by his M.S. Degree in1976, both in Physics from the Univer-sity of Korea.

Documents

Investigation of Packaged Miniature Heat Pipe for Notebook ... · After performance test of a pressed and bent type MHP, the test with the same MHP packaged into the notebook PC is