High-Performance Micro Heat-Pipeby Koji Yamamoto *, Kenji Nakamizo *,Hideaki Kameoka *2 and Ken'ichi Namba *3

With greater electronic equipment speeds and higher performance, more heat is
generated posing acute problems of heat dissipation. Accordingly, Furukawa
Electric has been marketing heat sinks that incorporate micro heat-pipes since 1995. Whereas anumber of products with different configurations and components have been developed over theyears responding to ever increasing heat, the authors have recently improved thermal performance of micro heat-pipes per se in order to cope with further increases in heat generation
expected in future.

ABSTRACT1.

INTRODUCTION

Recent developments of electronic equipment in terms of
their speeds and performance are truly remarkable, posing at the same time the problem of increased heat generation that has to be dissipated. When in particular theequipment is required to be compact in size, as is thecase with notebook PCs, heat dissipation technology is
one of the key issues since this involves the stability andthroughput performance of such equipment. Against thistechnological background, Furukawa Electric has beenmarketing heat sinks that incorporate micro heat-pipes

Heat produced by MPU (W)30(hereafter denoted as µHP) since 1995. µHP generally
refers to small-sized heat-pipes with a diameter of 3 to 6
mm, which can be utilized to realize compact heat sinks ofhigh heat dissipation efficiency by incorporating such components as metal plate, heat radiation fin, fan, etc. Thus
as shown in Figure 1, new products with different configurations and components have been developed as the heatgenerated by MPUs continued to increase. In this work,
the authors have improved thermal performance of µHPs
per se in order to cope with further increases in heat generation expected in future.

Performance ultra upgrading
Performance upgrading

25

20

Ultra-high performanceheat-sinkHigh-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

1998
Pentium II

Third generation
1999

2000

Pentium III

2001

2002~

Pentium III Pentium 4(Tualatin) (Northwood)

Changes in the development of µHP-based heat sink.

* First Development Dept., Metal Research Center*2 Thermal Products Dept., Electronic Components Div.
*3 Parts and Mounting Technology Development Dept., Ecology
and Energy Lab.

1

Airflow

T2

T3

HeaterT1
Heater input power (Qin): 29~38 W
Aluminum block heater: 9 mm ✕ 11 mm ✕ 10 mm
Fan:
Maximum airflow is 6.0 CFM
Size is 18 mm ✕ 20 mm ✕ 45 mm
Fin:
18 mm ✕ 20 mm ✕ 0.3 mm, 45 fins, P=1.5 mm
φ 6, L=160 mm, 180 mm
Heat-pipe:

Figure 2

An example of heat sink using µHP.

Thermal Evaporation
insulator

2. HIGH-PERFORMANCE µHP
Figure 2 shows an example of heat sink that employsµHP. One end of the µHP is joined to an aluminum blockthat is in close contact with heat generating devices such
as MPU to absorb the generated heat, while the other end
is installed with aluminum fins to dissipate the heat using
fans, whereby efficient heat dissipation is effected thanks
to the high heat-transportation characteristics of µHP.
µHP per se consists of a copper pipe with a working fluidof water encapsulated therein, so that the working fluid
vaporizes at the heat-absorbing end and the vaporizedworking fluid condenses at the heat-dissipating end. The
condensed working fluid at the heat-dissipating end isreturned to the vaporizing end by means of capillary force
arising due to the wick structure formed on the inner surface of the copper pipe. Thus heat transportation is carried out by latent heat transfer, so that it is possible to dramatically increase the amount of heat transfer rate in comparison to ordinary heat conduction.
While wick structures of µHPs come in mesh, wire andsinter, most of Furukawa Electric's products employ an
inner groove structure where multiple fine grooves areformed on the inner surface of a copper pipe. Advantagesof the inner groove structure may be summarized as follows: the performance is relatively stabilized and the products can be manufactured at a moderate cost. The wick
structure on the inner surface is an important factor that
determines the performance of a µHP. We have developed in this study an inner groove structure of high-performance and compared it with the conventional structure.
The results will be described below.

3.

EXPERIMENTAL METHOD

Figure 3 is a schematic diagram of the measuring system.
One end of a µHP to operate as an evaporator section

2

Insulation

Condensation
Cooling block

Cooling
water

Heaterµ HP under test
Power source

Figure 3

Constant
temperaturewater tank

Schematic diagram of measuring system.

was wrapped with a heater wire at equal intervals, andvoltage was applied on the heater regulating the amountof heat input. The other end to operate as a condenser
section was inserted into a cooling block, and the amountof dissipation heat was regulated by the temperature ofcooling water circulating in the cooling block. The intermediate portion to operate as an insulation section was thermally insulated using insulation material. Assuming sufficient insulation, the amount of heat on the evaporator section was defined as a heat transfer rate.

4.

EXPERIMENTAL RESULTS

Thermal Characteristics of φ 4-mm HighPerformance µHP
4.1.1 Outer Surface Temperature Distribution
Figure 4 shows a typical temperature distribution on theouter surface of a φ 4-mm high-performance µHP in operation.
When the heat transfer rate is comparatively low --10 W
or smaller, the outer surface temperature distribution
along the entire length is almost even at the operation
temperature. As the heat transfer rate increases the outersurface temperature at the evaporator section gradually
4.1

Furukawa Review, No. 22

2002raises, while the outer surface temperature at the condenser section declines. Then the outer surface temperature at the end of the evaporator section begins to risewhen a certain level of heat transfer rate is arrived at. Thisis caused by "dryout" of the inner surface that manifests
itself as the amount of heat input reaches the maximum
heat transfer rate. On the contrary, the temperaturedeclines at the end of the condenser section. This isthought to be due to an accumulation of excessive working fluid on the condenser section end preventing heattransfer.
4.1.2 Thermal Resistance
Thermal resistance is one of the important characteristicsof µHPs. Thermal resistance can be calculated as follows:
Thermal resistance (°C/W) =(Average outer surface temperature of evaporation
section - Average outer surface temperature ofcondenser section) / Heat transfer rateThus, when a certain temperature difference exists
between the outer surface temperatures of the evaporator
and condenser sections of a µHP, the amount of heattransferable is determined by the thermal resistance ofthat µHP.
Figure 5 shows the thermal resistance of a φ 4-mm highperformance µHP in comparison to that of a conventional
µHP.

10 W
20 W
25 W
26.5 W

4.1.4 Effects of Inclination
It is known that, since the working fluid in a µHP is influenced by gravity, the maximum heat transfer rate is maximized when the µHP is installed with its evaporator section down, i.e., bottom-heat mode. To the contrary, whenthe evaporator section comes upside, i.e., top-heat mode,
the working fluid condensed in the condenser section has
to be transferred to the evaporator section defying gravity,
making performance degradation in the µHP hardly avoid-

50

40
0

50

100

150

Pipe length (mm)Outer surface temperature distribution of φ 4 µHP.

Figure 4

0.5

Thermal resistance (°C/W)

4.1.3 Maximum Heat Transfer Rate
As described above, µHP-based heat sinks deteriorate inthermal resistance when a heat load exceeding the maximum heat transfer rate is applied, rising drastically in thetemperature at the heat-absorbing end. Improving the
maximum heat transfer rate is therefore one of the important issues of µHPs.
Figure 6 shows the maximum heat transfer rate of a φ 4mm high-performance µHP in comparison to that of a conventional µHP.
It is seen that the high-performance type dramaticallyoutperforms the conventional type by 160~200 % at theoperation temperatures of 40~70°C where µHPs are generally used, demonstrating that the improvement in the
inner groove shape of the high-performance type has
been very effective in enhancing the maximum heat transfer rate.

High-performance type
Conventional type

0.4

0.3

0.2

0.1

0
0

10

20

30

Maximum heat transfer rate Qmax.Leff (W.m)

Outer surface temperature (°C)

60

It is said that a µHP with a wick structure of inner groove
has, because the wick is formed directly on its inner surface, a lower thermal resistance in comparison to othertypes such as the mesh or wire types. Accordingly, a conventional µHP of inner groove type is seen to have a low
thermal resistance of 0.1°C/W or less when the heattransfer rate is below 10 W, but the high-performanceµHP has a thermal resistance that is improved further.
What is more, whereas the conventional type reaches the
maximum heat transfer rate at a little over 15 W showing a
rapid increase in thermal resistance thereafter, the thermal resistance of the high-performance type remains low
until the maximum heat transfer rate is reached at 25 W orover.

Thermal resistance of φ 4 high-performance µHP incomparison to conventional µHP.

4

3

2

High-performance type
Conventional type

1

0
30

Heat transfer rate (W)Figure 5

5

40

50

60

70

80

Operation temperature (°C)

Figure 6

Maximum heat transfer rate of φ 4 high-performanceµHP in comparison to conventional µHP.

3

Maximum heat transfer rate Qmax.Leff (W.m)

0.5

Thermal resistance (°C/W)

5

4

3

2

1

High-performance type

Flatness ratio 0 %(round pipe)

0.4

Flatness ratio 50 %(profile height: 2 mm)
0.3

0.2

0.1

Conventional type
0

0
-40

-20

0

20

0

40

able because the transfer of the working fluid depends
entirely on the capillary force of the wick.
At this time, experiments were carried out within an
inclination range of ±30° with respect to horizontal installation of µHP, assuming the bottom-heat mode configuration to be positive inclination while the top-heat mode negative.
Figure 7 shows the influence of inclination on the maximum heat transfer rate.
Both the high-performance and conventional types
show improved performance in the bottom-heat mode. In
the top-heat mode, while the conventional type suffers asharp decline in the maximum heat transfer rate below an
inclination of -5°, the high-performance type shows amodest decline. Thus the difference in the maximum heattransfer rates of the conventional and high-performance
products is maximized at between -10~-20°. The fact thatthe high-performance type has been improved most significantly in the top-heat mode demonstrates that theenhancement in capillary force resulting from theimproved inner groove shape has been remarkably effective in enhancing the maximum heat transfer rate.
4.1.5 Characteristics of Flattened µHP
As notebook PCs are required to be downsized, µHPs are
frequently used with their partial or entire length flattened.
Hence we have measured the thermal characteristics of
φ 4-mm µHPs of high-performance type when they are flattened. Flatness ratio was calculated as follows:
Flatness ratio (%) =(Outer diameter of pipe before flattening - Profile
height of pipe after flattening) / Outer diameter ofpipe before flattening ✕ 100
Figures 8 and 9 show the thermal resistance and maximum heat transfer rate of flattened µHPs, respectively.
The thermal resistance increases as the µHP becomes
more flat, thereby tending to impede the thermal performance. The maximum heat transfer rate also tends todeteriorate as the flatness ratio raises. Even though thehigh-performance type has higher maximum heat transfer
rate than the conventional type when flattened, it has a

4

Figure 8

Maximum heat transfer rate Qmax.Leff (W.m)

Maximum heat transfer rate of φ 4 high-performanceand conventional µHPs vs. inclination.

20

30

Heat transfer rate (W)Inclination (°)

Figure 7

10

Thermal resistance of flattened φ 4 high-performanceµHPs.

3High-performance type
Conventional type
2

1

0
0

20

40

60

Flatness ratio (%)

Figure 9

Maximum heat transfer rate vs. flatness ratio of φ 4µHPs.

larger rate of performance degradation with respect to flatness ratio.
4.2 Effects of Pipe Diameter
As described above, it was seen that φ 4-mm µHPs ofhigh-performance type have dramatically improved in thermal characteristics including maximum heat transfer rate.
We then proceeded to fabricate φ 5-mm and φ 6-mm µHPs
of high-performance type with improved inner groove
structure to investigate their thermal characteristics.
Figure 10 shows the maximum heat transfer rates of φ 4mm through φ 6-mm µHPs of high-performance type.
The pipe diameter is seen to have substantial effects
such that the larger the pipe diameter, the greater the
maximum heat transfer rate improves. In particular, thehigh-performance type exhibits greater effects of improvements in the maximum heat transfer rate.
Figure 11 shows the maximum heat transfer rates of φ 4mm through φ 6-mm µHPs of high-performance type thatare flattened to a profile height of 2 mm. As without flattening, the maximum heat transfer rate dramatically
improved as the pipe diameter increased, but when flattened, the effects of the pipe diameter prior to flattening
tends to diminish. This is because the larger the pipe
diameter the greater the flatness ratio becomes when thepipe is flattened to a profile height of 2 mm, resulting in agreater decline of maximum heat transfer rate.

Furukawa Review, No. 22

2002Maximum heat transfer rate Qmax.Leff (W.m)

10

5.

8

Furukawa Electric has been developing, beginning since
1995, heat sinks that incorporate µHPs to cope with theever increasing heat brought about by greater electronicequipment speeds and higher performance. This paper
describes improvements in the thermal characteristics ofµHPs achieved by optimizing the inner groove structure.
Thus with respect to φ 4-mm through φ 6-mm µ HPs ofhigh-performance type, we have been successful in drastically improving the thermal characteristics including maximum heat transfer rate in comparison to conventional
types. We hope these improvements can be effectively
applied in heat sinks so as to deal with the growing heatgeneration of electronic equipment expected in future.

64High-performance type2

Conventional type
0
3

4

5

6

7

Pipe diameter (mm)Maximum heat transfer rate Qmax.Leff (W.m)

Figure 10 Maximum heat transfer rate of large-sized µHPs.

IN CONCLUSION

32

REFERENCES
1) Oomi and Fukumoto: Furukawa Review No.21, p.69 (2002)

1
High-performance type
Conventional type
0
3

4

5

6

7

Pipe diameter before flattening (mm)Figure 11 Maximum heat transfer rate of flattened µHPs with aprofile height of 2 mm.

5