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Ultra-thin Sheet-shaped Heatpipe “pera-flex ®”1.

INTRODUCTION

Recently, the development of mobile electronicequipment including notebook PC, digital camera, PDA
and mobile phone is truly remarkable. These electronicequipment are required to offer high performance as well
as slim body brought about by high-density mounting.
Thus the issue of heat dissipation in mobile electronicequipment, which was not viewed problematically, hascome up to the surface.
Whereas conventional heatpipes and graphite sheets
are known as an existing solution for heat dissipation,
application of these devices to mobile electronicequipment has been considered to be difficult due to theirmounting configurations.
Accordingly, applying the heatpipe technology thatthe company has long been studying, Furukawa Electric
has embarked on the research of ultra-thin sheet-shaped
heatpipe named “pera-flex ®” that can efficiently transportand dissipate heat within an extremely limited space,and has succeeded in developing the new product. See
Photo 1.

2.

STRUCTURE AND OPERATION OF
PERA-FLEXAs shown in Figure 1, pera-flex has such a structure
whereby a wick for capillary force generation and apressure withstanding spacer are encapsulated together
with a small amount of water into an envelope-shaped
container made of thin metal foil, and subsequently thePhoto 1

Appearance of pera-flex.

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Figure 1

Structure of pera-flex.

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Figure 2���� ������������������

Circulation of working fluid in pera-flex.

entire air is evacuated to seal the container hermetically.
The inner pressure, which equals to the saturated vapor
pressure of the working fluid at working temperature, islower than the ambient pressure at room temperature, sothat the wick and the pressure withstanding spacer arepressed together to become mechanically constrained
thereby leaving fluid paths among the wicks.
When heat is applied onto pera-flex, as illustrated in
Figure 2, the saturated vapor pressure of the working
fluid at the heat-absorbing section that is in contact
with the heat source increases, and the working fluid
vaporizes. Because the input heat is absorbed as latent
heat of the working fluid, the temperature rise at the heatabsorbing section is suppressed extremely low. The
vaporized working fluid diffuses thoroughly into the fluid
paths provided in the wick, and condenses at sectionsof relatively low temperatures, thereby dissipating thelatent heat. The condensed working fluid is absorbed
into the wick to reflow back to the heat-absorbing section
by gravity and capillary force. Because the working
fluid within pera-flex circulates taking advantage ofphase changes as described above, heat transportation
between sections with a very small temperature difference
becomes possible.

Furukawa Review, No. 25

200464

Ultra-thin Sheet-shaped Heatpipe “pera-flex®”������� ������������� ����������� ������������ ���� � ���Figure 3�� �� �� �� ��������� �������������

Setup for thermal performance measurement.

��� � ���Pera-flex in asymmetrical shape.

The mechanism of pera-flex at the time of flexure issuch that, since the wick and the pressure withstanding
spacer can move inside, the strain is absorbed so as
not to block the fluid paths in the wick due to buckling.
Thanks to this flexibility, pera-flex can be installed easily
without paying too much attention to the irregularities inheight between the housing and the heat source, and thisconstitutes a significant advantage in upgrading ease ofassembly.
Moreover pera-flex is provided with a high degree of
freedom in planar shape as shown in Photo 2, and thus
can deal with 2-dimensional heat transportation tasks
involving bends and indents, which was difficult using
conventional heatpipes.
What is more, the capillary force has beenstrengthened by the proprietary wick configuration andsurface treatment, so that it become possible to operatepera-flex in the top-heat mode, which was difficult for
conventional heatpipes, whereby the heat source islocated on the upside. In terms of materials, copper withhigh heat conductivity is usually used together with water
having high latent heat for the working fluid, resulting insuch advantages as good thermal performance, low cost,safety and low environmental impact.
Pera-flex can be applied to an extremely limited spacebecause of its exceptional small thickness in addition to
flexibility and freedom in the shape. Moreover, it can beused in all inclination angles.

3.

PERFORMANCE OF PERA-FLEX

Figure 3 shows an example of the measuring setup
for thermal performance. One end of the pera-flex isheated with a heater and the heat dissipating section
is provided with fins, and the remaining sections arethermally insulated. The results are plotted in Figure
4. It was shown that when heat amount of 4 W wasinput into a pera-flex 0.7 mm in thickness, under thecondition of the insulating section temperature T3 kept
at 50°C, the temperature difference ∆ T and the thermal
resistance could be suppressed within about 1°C and 1°C/W, respectively, in all inclination angles and over thelength from the heat insulating section to the end. The����������� �� ��

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Figure 4

Typical temperature profile of pera-flex
compared with copper sheet.

result of an experiment when the same amount of heat
was input into a copper sheet 1.0 mm thick is shownfor comparison, indicating that the ∆ T and the thermal
resistance reached 20°C and 7°C/W, respectively. In themeantime, the pera-flex 0.7 mm thick could control T3 to50°C at room temperatures higher than 30°C using naturalconvection cooling, while the copper sheet 1.0 mm
thick could not effect the same cooling unless at room
temperatures lower than 30°C using forced convection
cooling. Thus pera-flex was seen to have extremely
higher heat transportation efficiency in comparison
with copper sheet, being very effective in terms of heattransportation and heat dissipation.
Also like heatpipes, pera-flex has a limit in the amountof heat transfer rate. This phenomenon is called “dryout”,and Figure 5 shows the limiting value called “maximumheat transfer rate” usually represented by Qmax.
In the bottom-heat mode where the heat source islocated on the downside, the maximum heat transfer
rate reaches a relatively large value because gravity
acts additively on the reflow of the working fluid. On thecontrary, in the top-heat mode where the heat source islocated on the upside, gravity acts subtractively reducing
the maximum heat transfer rate to a relatively small value.
Thus limiting the heat transfer rate according to workingcondition designs makes it possible to operate pera-flex
at all inclination angles.

Furukawa Review, No. 25

200465

Ultra-thin Sheet-shaped Heatpipe “pera-flex®”��� � ��� ������ �������� � ���� ���������������

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Figure 5

4.

Relationship between maximum heat transfer
rate and inclination angle.

CONCLUSION

Because pera-flex has such features as extreme thinness,flexibility and freedom in shape and configuration, it iscapable of transporting and dissipating heat effectively,
making itself a powerful solution to thermal problems inmobile electronic equipment of today. We would like toinvite our customers in trouble with thermal problems tohave a look at pera-flex.

REFERENCES
1) J. Niekawa and Y. Kimura: “Introduction to Micro Heatpipeand Heat Dissipation Technology,” Nikkan Kogyo Shimbun. (inJapanese)
2) S. W. Chi, translated by Oshima et al.: “Theory and Application ofHeatpipe,” JaTech Publisher. (in Japanese)

For more information, please contact:
Engineering Sec., Thermal Management Products
Dept., Electronic Components Div.
TEL: +81-463-24-9784FAX: +81-463-24-9786

Furukawa Review, No. 25

200466