Micro Heat-pipe with 1-mm Thickness

1.

INTRODUCTION

As computers improve in performance and speed, theheat generated by the micro processing unit (MPU)
increases rapidly, making heat dissipating measures indispensable. In particular, notebook PCs where heat dissipation poses a serious problem due to the small packaging
volume, heat dissipation technology has become one of
the key technologies. Micro heat-pipe (hereafter denotedas µHP) is capable of coping with this problem, and has
been in practical use since 1995.
In the MPU industry, while MPUs with extreme heatgeneration are marketed for high-end applications, powersaving type MPUs with small heat generation have also
been developed.
µHP heat-sinks used in notebook PCs equipped withsuch power-saving MPUs are generally required to be low
in profile in order to improve portability. Thus, a µHP having a thickness as small as 1 mm --the thinnest in theworld among practical µHPs-- has been developed to realize such heat-sinks. Below will be introduced the structure
and performance of 1-mm thick µHP.

Passage for vaporized working fluid

Type A

Type BWickType CSpecifications■Container: Pure copper■Wick: Pure copper mesh
■Working fluid: Deionized water■Standard dimensions: 1.0 (1.5) mm in thickness
by 9.0 mm in width

Figure 1

Structure of µHP with 1-mm thickness.

20

STRUCTURE

A µHP for room temperature use is normally comprised ofa copper container and a working fluid of water. The working fluid of water has a large amount of latent heat providing the µHP with a great deal of heat transfer rate. Heattransfer is effected by the working fluid, in which the watervapor used in heat transfer is cooled down to liquid waterat the heat radiation portion of the µHP, making it necessary to return the liquid to the heat receiving portion by thehelp of capillary force arising from the wick. The wick
structure may include grooved copper tubes, insertion ofmesh or fiber within the container and sintering of metals
resulting in porous materials.
Figure 1 shows the structure of this µHP with a thickness of 1 mm. A mesh constituting the wick is configuredin the center or sides of a flattened copper container, featuring the isolation of the passage for vaporized workingfluid from that for returning the liquefied working fluid.
Such a structure ensures a passage for the vaporizedworking fluid, permitting sufficient circulation of water andvapor even when the container is flattened down to a60°C

19Maximum heat transfer rate Qmax (W)

2.

50°C
40°C

18
1716151413121110-30-15

01530

Inclination angle θ (°)
Length of µHP: 200 mm
Length of heated portion: 50 mm
Length of cooled portion: 100 mm

Figure 2ng

oliCong

atiHePµHAngle

Maximum heat transfer rate of µHP with 1-mm thickness.

1thickness of 1 mm, thereby carrying out an effective function as a heat pipe.

160°C

0.950°C
Thermal resistance R (K/W)

0.840°C

3.

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0-30-1501530

Inclination angle θ (°)
Length of µ HP: 200 mm
Length of heated portion: 50 mm
Length of cooled portion: 100 mm

Figure 3ng

oliCong

atiHePµHAngleThermal resistance of µHP with 1-mm thickness.

PERFORMANCE

Figure 2 shows the characteristic curve for maximum heattransfer rate of this 1-mm thick µHP, which increases as
the working temperature increases. The working temperature in a notebook PC equipped with a heat-pipe normallyrises to 60~80°C. At this working temperature, the maximum heat transfer rate of this 1-mm thick µHP is known to
be 18 W or more, so that it is estimated that a heatamount of as much as about 35 W generated by an MPU
can be satisfactorily dissipated by employing a heat-sinkprovided with a fan, a heat-radiating plate or a die-castradiator.
Figure 3 shows the characteristic curve for thermal
resistance of this 1-mm thick µHP. When installed on anotebook PC, a heat-pipe is required to be lower in thermal resistance the higher the working temperature. This 1mm thick µHP is shown to have lower thermal resistances
at higher working temperatures, thus sufficiently guaranteeing practical applications.
Figure 4 shows a heat-sink, i.e., an application exampleof this 1-mm thick µHP, which successfully eliminates a
fan to dissipate the heat generated by a low heat generation MPU of about 8 W.
For more information, please contact:
Engineering Section, Thermal Products Department,Electronics Components Division
TEL: +81-463-24-9784
FAX: +81-463-24-9786

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

Figure 42An application example of µHP with 1-mm thickness.

Furukawa Review, No. 222002