State-of-the-art Technologies of Micro Heat-Pipe Heat-Sinks for
Notebook PCs
by Masaru Oomi *, Toshiro Fukumoto * , Takao Kobayashi *Masanobu Sugiura * , Katsuo Nakayama * and Ken'ichi Namba *2

In recent years, the heat produced in notebook PCs has increased rapidly alongwith the upgraded speed of the MPU, making heat dissipation measures thereof
indispensable. Micro heat-pipe heat-sinks which are capable of dealing with this problem havebeen in practical use since 1995. Conventional heat-sinks, however, can no longer provide sufficient cooling for the latest version of MPUs in notebook PCs where increased heat production isseen caused by high-performance peripheral components. This paper reports on the technological background of micro heat-pipe heat-sinks for notebook PCs together with fin-composite micro
heat-pipe heat-sinks that are expected to dominate the field of heat-sinks of the next generation.

ABSTRACT1.

INTRODUCTION

As computers improve in speed and performance, theheat produced by the micro processing unit (MPU)
increases rapidly, making heat dissipation measuresurgently needed. With notebook PCs where heat dissipation poses a problem due to their small packaging volume,in particular, heat dissipation technology has become oneof the key technologies. Micro heat-pipe (hereafter denoted as µHP) heat-sinks which are capable of dealing withthis problem have been in practical use since 1995.
µHP generally refers to small heat pipes with a diameter of 3 to 6 mm, which permit easy bending and flattening, and is applicable to thermal wiring for heat dissipation. In practical applications, µHPs are combined withthin metal plates, die casts, fans and rubber to comprise avariety of heat-sinks of compact size. In recent years,
appreciated for their advantages, µHPs are well on their
way to practical applications in the fields of information oriented home appliances and multi-media terminal equipment, in which small space for installation is essential.
At the time when µHPs were first employed for notebook PCs, the heat produced by the MPU was 5 to 6 W at
most, so that µHP heat-sink products having a simple
structure consisting of a µHP and a metal plate accounted
for the main stream. MPU continued to improve in speed
and performance after that, leading to an increase of heatproduction that is reaching 20 to 25 W with the latest models. Moreover, the MPUs now under development are estimated to produce an amount of heat of 30 W or more,* Thermal Products Dept., Electronic Components Div.
*2 Components and Mounting Technology Development Dept.,
Ecology and Energy Lab.

raising the entire heat including that from other heat-producing components as high as 35 W approximately. Thus,
it is anticipated that notebook PCs in the near future will
face a difficulty in cooling the total system including
MPUs, unless they employ advanced heat-sinks that combine µHPs and fans.
The present paper reports on the technological background of µHP-based heat-sinks, describes the structuralchanges in µHP-based heat-sink products until now, andintroduces state-of-the-art technology together with innovative products that are expected to dominate the field ofheat dissipation, including: heat-sinks using heat-pipes as
small as 1 mm in thickness; heat-sinks that utilize heatpipe hinges; and heat-sinks that use fin-composite µHPs
having a multitude of fins mounted by a unique process.

2.

CHANGES IN THE µHP HEAT-SINK
PRODUCTS FOR NOTEBOOK PCS

Performance of heat-sinks has to be improved along with
the increase in the heat produced in the MPU of notebook
PCs. Figure 1 shows the changes in the µHP heat-sinkproducts for notebook PCs, and the features of each generation are described below.
2.1 Heat-sinks of the First Generation
Heat-sinks of the first generation has a structure such thatthe heat produced in the MPU is transferred to the µHP
through heat-receiving block, is diffused to the entire
metal parts such as shielding case, and is dissipated fromthere by natural convection. The bottom of the housing or
the rear side of a keyboard is often used as an area forheat dissipation. Heat-sinks of this generation, while being
simple in structure, ensure sufficient cooling as long as an

69

Heat produced by MPU (W)30

Ultra-high performanceheat-sink

Performance ultra upgrading
Performance upgrading

25

20

High-performanceheat-sink

Fan-composite heat-sink
Profile reduction

15

Performance
upgrading

1 mm-thick heat pipe

10Standard heat-sink

Noise reductionµHP thermal hinge

Low cost

5
Second generation

First generation
1995
MPUs

1996

1997

Pentium, Pentium Pro

Figure 1

Structure
Type A

1998
Pentium II

Pentium III

2001

2002~

Pentium III Pentium 4(Tualatin) (Northwood)

PC when the housing main body only is insufficient in heatdissipation. These two heat-sinks are described in detail in
Chapter 3, "STATE-OF-THE-ART TECHNOLOGIES OFµHP HEAT-SINKS".

Wick

Specifications• Container:
Pure copper• Wick:Pure copper mesh
• Working fluid:
Deionized water• Standard dimensions: Thickness: 1.0 (1.5) mm
Width:
9.0 mm

Structure of 1mm-thick µHP

area of adequate thermal margin is provided, so that they
are still in use for various notebook PCs that employ
MPUs of low heat production.
2.2 Heat-sinks of the Second Generation
As the heat produced by the peripheral devices andequipment besides MPU increases in notebook PCs
reducing areas of adequate thermal margin, conventional
heat-sinks --first-generation heat-sinks and those basedon fans-- encountered difficulties in coping with heat dissipation. Accordingly, heat-sinks have begun to employ a
new structure such that the heat produced by MPUs istransferred using µHPs to thermal fins located at the side
portion of the housing securing a space, and is discharged
outside thus effecting forced cooling. Meanwhile, sincenotebook PCs are increasingly required to be lower in profile and better in portability, new µHP-based heat-sinks
are appearing to meet such requirements. They are: heatsinks that use µHPs 1 mm in thickness enabling thermal
wiring in limited places where conventional 2 mm-thick flatµHP is unsuitable; heat-sinks based on thermal hinges
permitting efficient heat conduction to the display panel of

70

2000

Changes in the µHP heat-sink products.

Passage for vaporizedworking fluid

Figure 2

Third generation
1999

2.3 Heat-sinks of the Third Generation
It is anticipated that even the heat-sinks of the second
generation are going to be insufficient in coping with theever increasing heat produced in notebook PCs in thenear future. Only heat-sinks of the third generation arethought to be capable of managing such a situation.
These heat-sinks are provided with high-performance finsof lightweight and compactness having a large working
area per unit volume, which are installed at the outlet of a
fan to achieve extensive improvement than ever in performance.
Thus, heat-dissipation technology represented by µHP
heat-sinks has made a remarkable progress along with
the improvement of MPUs. Chapter 3 describes some of
the new products of the second and third generations representing the latest technologies for heat dissipation.

3.

STATE-OF-THE-ART TECHNOLOGIESOF µHP HEAT-SINKS

3.1 Heat-sinks using 1mm-thick µHPs
µHP heat-sinks used in notebook PCs are required to be
low-profiled in order to improve portability. A µHP having athickness of as small as 1 mm --the thinnest in the worldamong practical µHPs-- has been developed to realize
such heat-sinks.
Figure 2 shows the structure of a 1-mm thick µ HP,
which is characterized by providing a return passage forworking fluid at the center and two passages for vaporizedworking fluid at the both sides. Figure 3 shows the maximum heat transfer rate of a 1 mm-thick µHP. The operating temperature within a typical notebook PC using such a

Furukawa Review, No. 21

2002

20

60°C

19

50°C
40°C

Maximum heat transfer rate (W)

18
17
16
15
14

Features
Small thickness of 1 mm
Applicable to bending process
Large heat transfer rate
Direct mounting to heat-producing devices permitted

13
12
11

Figure 5

Heat-sink using 1 mm-thick µHP.

10
-30

-15

0

15

30

Angle of inclination (deg)

ng

oli

Co

Total length of heat-pipe: 200 mm

ng
Length of heated portion: 50 mm
ati
Hee
Length of cooling portion: 100 mm
pip
atHe

Figure 3

Angle ofinclination

SpringµHP
Heat transfer
Rotatable

Maximum heat transfer rate of 1 mm-thick µHP.

Heat producing device

1

Base plate

60°C

0.9

50°C
40°C

0.8

Thermal resistance (K/W)

Figure 6

Structure of µHP hinge.

0.7

1 mm-thick µHPs.
Figure 5 shows a heat-sink using this 1 mm-thick µHP,
in which fan was removed by employing the heat-sink todissipate the heat of 8 W produced by the low-power consumption MPU newly developed.

0.6
0.5
0.4
0.3
0.2
0.1
0
-30

-15

0

15

Angle of inclination (deg)30

Total length of heat-pipe: 200 mm
ng
ati
Length of heated portion: 50 mm Hee
Length of cooling portion: 100 mm
pip
atHe

Figure 4

ng

oli

Co

Angle ofinclination

Thermal resistance of 1 mm-thick µHP.

1 mm-thick µHP is estimated to rise to 60~80°C. It is seenfrom the Figure that the maximum heat transfer rate of theµHP at this temperature is more than 18 W, so that aheat-sink with a heat-transfer capacity of around 35 W
sufficiently capable of coping with the MPU's heat can berealized when metal plate or die cast is combined.
Figure 4 shows thermal resistance characteristics of a 1
mm-thick µHP. The higher the operating temperature, thelower should be the thermal resistance of 1 mm-thickµHPs for notebook PCs. The Figure shows such thermal
characteristics, demonstrating the practical applicability of

3.2 Heat-sinks Using µHP Hinges
Conventional design of notebook PCs has usually beensuch that heat dissipation is effected on the main body
(keyboard) side only. However, the heat produced by themain body has increased so much that sufficient heat dissipation through the limited area has become difficult,making it effective to dissipate the heat by conduction
from the main body to the display panel where remains
ample thermal margin.
Figure 6 shows the structure of a µHP hinge developed
for such a purpose. The heat produced by the MPU isreceived by the µHP hinge assembly connecting the main
body to the display panel, is transferred to the heat dissipating metal plate on the display panel where dissipation
is effected. Thermal hinge structure of small heat resistance is therefore needed for practical application of µHP
hinges.
Figure 7 shows the results of thermal experiments on aheat-sink having a structure of µHP hinge. It can be seenthat the heat produced by the MPU on the main body isefficiently transferred under a low thermal resistance,
which has been achieved by fastening the hinge assemblyto the µ HP using a specially designed spring thereby

71

90

MPU

Temperature (°C)

80
70
60
50
40
30
T1

T6

T7

Tair

{

T5

{

T4

{

T3

{

T2
DeviceBaseplate

µHP

Panel

ConditionsµHP: φ4 HP x 2
Fan:
45 mm x 45 mm x 10 mm
Frame: ADC12, 3 mm thickFin:
15 fins, 1.5 mm thick at 2.5 mm pitch

Spot of measurement

Figure 7

Results of thermal experiments on a heat-sink with
µHP hinge.
Figure 9

Figure 8

Appearance of a notebook PC using µHP hinge.

reducing the thermal contact resistance of the hinge.
Moreover, no degradation in performance was observed
at open-close tests conducted 30000 times prior to practical application.
Figure 8 shows the appearance of a notebook PC thatutilizes a µHP hinge assembly.
3.3 Heat-sinks Using Fin-composite µHPs
As mentioned earlier, the heat produced by MPU is anticipated to exceed 30 W in the near future. In the meantime,
notebook PCs are required to be low in profile and compact in size, reducing the space for heat dissipation. Diecast fin-composite fans conventionally used for cooling
have limitations in terms of the thickness, height, and pitchof the heat dissipating fins since the fins are integrally
structured by die casting, so that the fins tend to grow insize to provide sufficient cooling area thus resulting inlarge sized fans.
Heat-sinks using fin-composite µHP have been devised
to overcome such a difficulty. These heat-sinks have astructure such that the heat produced by MPU is conducted to the fin-composite µ HP via heat-receiving block,transferred by the µHP to the fins located in front of thedraft exit of the housing, and is dissipated externally by
the fan thereby effecting forced cooling. Fin-compositeµHPs can be structured compact and lightweight since thefin thickness as well as the fin mounting pitch can be

72

Result of numerical analysis of die cast-composite
fan heat-sink.

ConditionsµHP: φ4 HP x 2
Fan:
45 mm x 45 mm x 10 mm
Block: A6063, 5 mm thickFin:
A1050, 32 fins, 0.3 mm thick at 1.2 mm pitch

Figure 10 Result of numerical analysis of fin-composite µHP
heat-sink.

reduced. Specifically, the fin pitch of fin-composite µHPs
can be made as small as 1.2 mm that is less than half of
die-cast fin pitch of 2.5 mm, enlarging the heat dissipation
area by more than two times. Thus remarkable improvement in performance is anticipated.
Accordingly, in order to study practical application of fincomposite µHPs, a performance comparison between theheat-sinks using die cast-composite fans and those using
fin-composite µHPs was carried out by numerical analysis, assuming the same heat dissipating space. Figures 9
and 10 show the results of the numerical analysis, respectively. It is seen that fin-composite µHP can lower the temperature of MPU by more than 30°C at the heat production of 20 W. Then we proceeded to thermal experimentusing a prototype having the same geometry as the modelthat was subjected to numerical analysis.
Figure 11 shows the appearance of a fin-compositeµHP heat-sink with 20 W rating and the schematic of thethermal experiment, and Table 1 summarizes the results.
The results of measurement are seen to be in fairly good
agreement with those of numerical analysis. The fin-com-

Furukawa Review, No. 21

2002DraftHeaterHeater input power (Q): 20 W
Aluminum block heater: 9 mm x 11 mm x 10 mm
Fan: Maximum airflow: 2.5 CFM
Noise level:
24.5 dBA
Revolution:
5000 rpm
Size:
45 mm x 45 mm x 10 mm
Fin:
10 mm x 20 mm x 0.3 mm, 32 fins, P=1.5 mm
Heat pipe
φ4, L=134 mm, 136 mm

Figure 11 Fin-composite µHP heat-sink with 20 W rating and schematic of thermal experiment.

Draft

HeaterHeater input power (Q): 29~38 W
Aluminum block heater: 9 mm x 11 mm x 10 mm
Fan: Maximum airflow: 6.0 CFM
Size:
60 mm x 60 mm x 8 mm
Fin:
18 mm x 20 mm x 0.3 mm, 45 fins, P=1.5 mm
Heat pipe
φ6, L=160 mm, 180 mm

Figure 12 Fin-composite µHP heat-sink --one armed-- with 30 W rating and schematic of thermal experiment.

Table 1Results of experiment for fin-composite µHP heatsink with 20 W rating.
Heat

HeaterFan input

T1

T2

T3

Tair

input

voltage

(°C)

(°C)

(°C)

(°C) resistance

Measured value 77.8

66.6

65.0

35.0

2.14

31.6

30.0
35.0

1.94

(°C/W)

power (Q)
20 W

3V

∆T
20 W

4V

Measured value 73.9

62.2

60.4

38.9

27.2

25.4

Measured value 73.0

60.0

58.2

25.0

23.2

∆T
20 W

5V

42.8

∆T

38.0

Table 2

Results of experiment for fin-composite µHP heatsink --one armed-- with 30 W rating.
Fan input

T1

T2

T3

Tair

input

voltage

(°C)

(°C)

(°C)

(°C) resistance(°C/W)

power (Q)29 W

3V

Measured value 64.5 53.6 52.7 35.0
∆T

33 W

4V

1.90

Notes: 1) Heat resistance = (T1-Tair) / Q
2) ∆T = Ti-Tairposite µHP heat-sink is one of the major candidates fornotebook PC heat-sinks of the next generation, and has
already been employed in selected notebook PCs.
We are currently developing new fin-composite µHP
heat-sinks, one-armed and two-armed, for 30~40 W tocope with increased heat production of MPUs assumablein future. Figure 12 shows the appearance of a one-

38 W

5V

1.02

33.7 20.9 19.7

Measured value 73.6 58.9 57.3 35.0
∆T

1.02

29.5 18.6 17.7

Measured value 68.7 55.9 54.7 35.0
∆T

35.0

Heat

Heater

1.02

38.6 23.9 22.3

Notes: 1) Heat resistance = (T1-Tair) / Q
2) ∆T = Ti-Tairarmed, fin-composite µHP heat-sink with 30 W rating andthe schematic of the thermal experiment, and Table 2
summarizes the results. The one-armed heat-sink proveda heat resistance of around 1.0°C/W, demonstrating that
MPUs producing 40 W of heat can be dealt with using
these heat-sinks.

73

4.

IN CONCLUSION

We have reported on heat-sinks for notebook PCs, focusing on µHP heat-sinks regarded to dominate the field.
These µ HP heat-sinks have configurations specific toeach PC, manifesting the incorporation of design engineers' inventive ideas. This means that the products havebeen realized by the zeal and efforts of those engineers of
PC manufacturers, to whom we wish to express our gratitude.
Hereafter, heat dissipation measures for notebook PCs
are expected to grow in importance, so that we intend to
standardize the heat-sinks of the third generation introduced in this report, and at the same time, to make efforts
to develop new heat-sinks of the next generation.

REFERENCES
1) Kitanoya et al.; Furukawa Electric Review, No.97, 1995. (inJapanese)2) Hashimoto et al.; Furukawa Electric Review, No.101, 1998. (inJapanese)

Manuscript received on July 23, 2001.

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Furukawa Review, No. 21

2002