Evolution of the Heatsink Technology
by Takeshi Hirasawa * , Kenya Kawabata * and Masaru Oomi *The environment around heatsinks is increasing in severity day by day. It is
known that the heat generated by electronic devices continues to increase, while
the casing is becoming more compact. Moreover, there is an urgent requirement for addressing
environmental issues including weight reduction and elimination of environmentally hazardous
substances. This paper presents the manufacturing technologies for heatsinks that have beendeveloped in response to such requirements together with the heatpipe products using suchtechnologies.

ABSTRACT1.

INTRODUCTION

Recently, the heat generated by the devices in electronicequipment unprecedentedly increases in density.
These electronic devices include: the integrated circuits
such as central processing unit (CPU) for the so-called
information-oriented home appliances like personal
computers together with graphic processor unit (GPU);and the semiconductor power devices represented
by insulated gate bipolar transistor (IGBT) for electricrailways 1). It is said that the heat generation density ofthese high heat-generating devices is reaching a level
corresponding to that of nuclear reactors, so that it isvirtually impossible to dissipate such a heat rate on an
individual component basis. Accordingly, there is a needfor heat dissipation using high-performance heatsinksthat can efficiently diffuse heat.
On the other hand, downsizing and weight reductionof the equipment that use these semiconductor devices
are being promoted, and, as the result, the vacant space
available to heat-dissipating devices such as heatsinks is
becoming much reduced. An urgent need for developing
high-performance heatsinks arises from this reason also.
Moreover, solder widely used for manufacturing heatsinks is required to be lead-free, because the RoHS
Directive (Restriction of the Use of Certain Hazardous
Substances in Electrical and Electronic Equipment)
requires that, as a general rule, electrical and electronicequipment marketed in Europe should be lead-free by
July 1, 2006.
In view of these requirements, Furukawa Electric has
been developing various high-performance heatsinks
based on heatpipe as well as various bonding methods
without using brazing metals such as solder, thereby
responding to the market needs.
* Thermal and Antenna Solutions Dept., Electronic
Components Div., Electronics and Automotive Systems Co.

2.

EVOLUTION PROCESS OF HEATPIPETECHNOLOGY

Here let us describe the trends in heatsinks focusing on
those for personal computers.
At first, the heat generation rate itself of CPUs was
small and hence the heat density was low necessitating
no heatsinks, and conventionally the heat was dissipatedinto the printed circuit board, taking advantage of the
CPU package made of plastics, through the socket. Then,as CPUs upgraded in integration scale and the design
rule for circuit patterns improved in preciseness together
with the increasing clock frequency, the heat generation
rate continued to upsurge. Consequently, a need for cooling using heatsinks arose, and heatsinks based on extrusions and die-castings were employed for desktop PCs.
Later on, as the heat generation rate of CPUs increased
higher, these blower-less air-cooling devices were foundto be insufficient, so that forced air-cooling using a motordriven fan was introduced, and this cooling method
continues to be used up until now experiencing many
improvements 2), 3).
On the other hand, with respect to notebook PCs, theheat was at first diffused over the entire casing using its
aluminum sheet metal to be naturally dissipated into theair. In the meantime, in 1993, heatpipe was combinedwith the casing to improve the heat-diffusing performance, and further, motor-driven fan was introduced thusachieving performance improvements year by year. As it
is now some CPUs generate a heat rate of more than 100
W, and those developed for notebook PCs in excess of
30 W. Moreover, the heat generation rate of other devices
such as GPU now exceeds 10 W, producing a tendency
toward increased temperature within the casing.
As described above, the heatsinks of today are used inmany crucial places where extremely high performance isrequired. Accordingly, it is essential for heatsinks to maximize the fin efficiency by adopting heatpipes that outper-

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Evolution of the Heatsink Technology

form diamond having the highest thermal conductivity
among solids.

3.

NEW REQUIREMENTS FOR HEATSINKS

Thus heatsinks are required to have high thermal performance as well as other advantageous characteristics.
For notebook PC application, for example, there is anincreasing need for lightweight heatsinks. The objective of weight reduction is portability enhancement ofcourse, but it should be noted here that recently there
is a tendency to install larger LCDs accompanied byincreased product weight. A significant weight reductionof heatsinks is therefore required to suppress the weightincrease in comparison to previous models and to compensate for the increase in weight due to other components.
There is also an intense need for profile reduction. As
mentioned earlier, the surface area of PC is growing insize due to the increased LCD size, so that it is desirable
to have a low-profiled casing so as to suppress volume
increase. Since heatsinks are also required to be lowprofiled, it is a normal practice to use a remote heatsink
system, whereby the heat is absorbed by a heatpipe,transferred to a fin array at the end of the casing, and thearray is cooled using a fan.
Moreover, there is an increasing need for dealing withenvironmental issues, including: elimination of lead incompliance with the RoHS Directive; improvement inrecyclability; and elimination of environmentally hazardous substances. It is essential to meet these requirementsthat are related with the disposal and recycle of products
as well as to improve the performance and user-friendliness of products. To this end, it is crucial for manufacturers to have advanced design competence in addition toprocessing capability.

4.

SOLUTIONS TO WEIGHT REDUCTION

Main measures for weight reduction include:(1) To use lightweight materials(2) To reduce the number of constituting members(3) To devise a suitable configuration
It is required that the material for heatsinks has superior thermal conductivity. Copper has an excellent thermal
conductivity of about 400 W/m·K, but its specific gravity
is 8.96 g/cm3 , so that, if copper is used in an entirety of aheatsink for desktop PC, it would weigh as much as 1 kg
because heatsinks have grown in weight in proportion to
the heat dissipation rate thus making them the heaviest
among the desktop PC components. Thus, although ametal plate is used for reinforcement, it is becoming difficult for the circuit board on which a heatsink is mounted
to maintain its configuration, resulting in a potential
trouble of poor thermal contact with the heat-generating
devices.
Accordingly, aluminum and its alloys with a specificgravity of 2.7 g/cm3 (i.e. one third of copper) are preferred, despite the fact that their thermal conductiv-

ity is 240 W/m·K, much lower than copper. These physicalparameters suggest that even if an aluminum member
twice as thick as a copper member is used to obtain an
equivalent heat transfer rate, the heatsink would be lighterby about 30 %, and further weight reduction can beobtained depending on design optimization.
Aluminum heatsinks have long before been adopted
for ease of fabrication using casting and extrusion, but
their adoption has been limited to such areas where highperformance operation is not necessary, because products with thin and narrow-pitched fins can not be realized
and because their thermal conductivity is poor. In orderto fabricate high-performance heatsinks of aluminum, it
is necessary to use thin sheets of pure aluminum for thefins so as to increase the surface area per volume. Whilethe heatsinks using a thin sheet are formed by means ofpress working, the thinner the sheet, the more skill andadvanced techniques the process needs. At present, ourprocess can manufacture heatsinks with 0.2-mm thickfins, thereby contributing to weight reduction and performance enhancement.
The use of heatpipes also significantly contributes toweight reduction, since they are provided with a thermal
conductivity of as high as about 10,000 W/m·K regardless of their extremely small apparent specific gravity, i.e.
approximately 2 g/cm3 for a heatpipe of 6-mm in diameter. Suitable configuration of such a heatpipe enables
much weight reduction while achieving considerable
performance improvements. For example, it has beenpossible to reduce the weight by about half as well as
to achieve a significant cost reduction while considerably improving the thermal performance by adopting a
structural design, such that a 4-mm thick copper sheet
has been replaced by a flattened heatpipe 2-mm highsandwiched between two 1-mm thick copper sheets. This
example demonstrates the overwhelming effects of heatpipes when they are optimally designed.

5.

SOLUTIONS TO PROFILE REDUCTION

As mentioned earlier, there is an intense need for profile
reduction in the field of notebook PCs especially. For thisreason, it is difficult to install cooling fins just above a heatgenerating device, so that normally the heat is transferred
to be dissipated to the end of the casing where fins are
installed with a fan, and it became essential to use aheatpipe in transferring the heat.
It has been a normal practice to bond a heatpipe on to
the end of a fin using solder and the like. With this bonding method, however, heat has to be transferred from one
end to the other of a fin, making inefficient use of the fin.
In contrast, when the heatpipe is bonded to the center ofthe fin, not only the entire circumference of the heatpipe
can be used to transfer the heat to the fin, but also the
distance to any end of the fin is shortened, and the fin efficiency is improved. Based on this idea, stacked fin heatsink has been developed to be put into practical application. See Figure 1.
What is more, the height of fins has been reduced as

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Evolution of the Heatsink Technologya) When bonded to fin ends,
temperature difference
occurs lowering the fin efficiency.
Figure 1b) When heat source is at fin
center, the fin efficiency
improves.

a sufficient mechanical strength. In particular, the stacked
fin heatsink with flattened heatpipe that has recently
been brought to practical application necessitated moreadvanced techniques, and considerable efforts weremade to establish the manufacturing technology realizing
a satisfactory degree of heat transfer. See Figure 3.

Effect of center bonding in stacked fin heatsinks.

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Figure 2

Effect of flattened heatpipe with stacked fins onpressure drop reduction.

the casing becomes low-profiled, and it was found thatthe influence of heatpipe itself on the airflow (i.e. pressuredrop) can not be neglected. Accordingly, the technology
of stacked fin heatsink using flattened heatpipe has beendeveloped. As shown in Figure 2, this technology has
made it possible to obtain higher airflow rates using an
identical fan, achieving an improvement in the overall performance of fins combined with a fan.
Moreover, another example of particularly high-performance heatsink has been demonstrated, in which a fan
casing is combined with a heatsink, and a heatpipe isinstalled there thus making those sections other than theheatsink share the cooling function.

6.

SOLUTIONS TO ENVIRONMENTFRIENDLINESS

While most heatsinks commercially available at present
use solder for bonding each part, there is an urgent needto eliminate the use of lead contained in the conventionalsolder, because the lead can leach due to acid rain after
the products are wasted, contaminating the soil andwater sources. Accordingly, Furukawa Electric always
uses a lead-free solder to replace conventional solders
used in manufacturing heatsinks.
Not only the company uses lead-free solder, but also
it has developed various bonding methods eliminating
the use of solder itself, such as the stacked fin technology mentioned above and the crimped fin manufacturing
technology.
Using the stacked fin technology, after fins are burreda heatpipe is press-fitted there to mechanically bond thefins. Thus, manufacturing capabilities of high-precision die
machining as well as process management are required
to bond the thin fins without damaging therely achieving

Figure 3

Photograph of stacked fin heatsink with flattened
heatpipe.

By means of the crimped fin technology, on the other
hand, after fins are inserted into the grooves cut in thebase plate, a blade is used to stamp the base plate in the
portion between the fins, and the fins are crimped by the
deformed portions from the both sides to be securely fastened. Because the fin crimping makes use of the plastic
deformation of metals in this way, it offers many advantages in that the thermal conductivity between the fin andthe base plate is superior since brazing materials suchas solder or adhesives do not exist in the bonding interface, and that it enables bonding of such combinations
as aluminum-to-aluminum where solder poorly works inaddition to copper-to-aluminum where any brazing filler
metals work insufficiently. Moreover, the caulking technique has been developed for bonding heatpipes on tometal blocks such as die-casts for heat-receiving blocks,
making it possible to manufacture heatsinks on a solderless basis.
These solder-less bonding techniques contribute toenhance environment-friendliness. First, since they arebased on plastic deformation of metals to fasten theparts, they increase the ease of disassembly, e.g. in thecase of crimped fins, the fins easily disengage by bending the base plate to form a fan shape. Second, since noadhesives are used, hence any generation of toxic gases
when the parts are remelted. Lastly, Ni-plating for soldering aluminum materials is not necessary, reducing environmental impacts.

7.

LATEST THERMAL SOLUTIONS

Let us introduce some examples.
Figure 4 shows a heatsink developed for a portable PC
often called a desk-notebook PC. The desk-notebook PC
has a high-performance, high-heat generating CPU nor-

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Evolution of the Heatsink Technology

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mally used for desktop PCs on board its compact casing
for notebook PCs, so that the PC has a strict limitation
on free space. Accordingly, in this product, four flattened
heatpipes 6 mm in diameter are used because it is known
that plural, flattened, small-diameter heatpipes are moreadvantageous than a large-diameter heatpipe of highheat transfer rate to suppress the pressure drop in a lowprofiled free space. The heatpipes are bonded to the center of a stacked fin array comprising lightweight aluminum
fins, and aluminum is also used for the heat-receiving
block thus reducing the weight.

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Figure 5

Thermal performance of aluminum and copper
heatsinks for desk-notebook PCs.

Figure 6

Photograph of Raman amplifier HPU-1001.

Heatsink for desk-notebook PC.

In this case, shortening the distance between the heatreceiving surface and the fin array would reduce the thermal resistance thereby improving the performance, but
it is also important to distribute the heat rate as evenly
as possible among the four heatpipes. Therefore, careful
designing is needed to obtain the optimum solution to theoverall configuration. Meanwhile the product uses caulking to fasten the heatpipe with heat-receiving block, without using solder or Ni-plating.
By adopting such a configuration, it was possible toreduce the weight by about 60 % in comparison to a heatsink that uses copper for the fin and heat-receiving block.
These two heatsinks were compared for the performanceusing a setup such that a sample heatsink was put in awind tunnel, a heat rate of 80 W was input to the heater
block provided with thermocouples, and the temperatureon the heatsink was measured as the airflow rate waschanged. It was confirmed from the experimental results
shown in Figure 5 that, since the design was optimized
in terms of the relative position of the fin and heatpipe aswell as the fin pitch, the aluminum heatsink has performance equivalent to that of the copper heatsink.
More specifically, it was demonstrated that the heatsink
has a thermal resistance of 0.25 K/W at an airflow rateof 0.3 m3/min, which indicates that under the conditionsof input heat rate of 120 W and ambient temperature of
40°C, the CPU temperature is suppressed to 70°C. Thisconfirms that the heatsink can sufficiently meet the performance requirements of high-performance CPU of thenext generation. It was further confirmed that the heatsink
does not change in the thermal resistance at a heat inputof 150 W, exhibiting a sufficient heat transfer rate.
Next an example is introduced where a high-performance heatsink was used to achieve a reduction of
power consumption.

Figure 6 shows a Raman amplifier for optical communications 4), 5). In the casing, eight high-power laser diodemodules (hereafter called LDM) each with a maximum
heat generation rate of 15 W are used to attain a totalheat generation rate of 120 W. Each LDM has a built-in
Peltier cooler, which, in order to hold the temperature of
the laser diode included in the LDM constant, cools thediode and in turn dissipate the heat into the heatsink. The
ability of the Peltier cooler is proportional to the electric
power supplied, and as the performance of the heatsink
deteriorates and as the LDM temperature rises, much
power is consumed accordingly to lower the LDM temperature to maintain a constant value, and this raises aneed for an efficient heatsink.
Meanwhile, the casing of this Raman amplifier has tobe low in profile to be accommodated in a rack, making
it impossible to install a heatsink with fins of sufficiently
large area just in the back of the LDM. It has been therefore decided to adopt a design such that the heat generated by the LDMs is individually transferred using a heatpipe to the casing side face, where eight units of stacked
fin arrays are installed to effect, notwithstanding the lowprofile casing, efficient heat dissipation. As a result, it
was possible to suppress the temperature increase of the
LDM much lower than using bulk heatsinks of solid aluminum extrusion that occupy an equivalent volume. This
reduced the load on the Peltier cooler built in the LDM
eventually resulting in a reduction of 30 % in the total
power consumption as shown in Figure 7.

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Evolution of the Heatsink Technology����������������� ��������
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Comparison of LDM power consumption between two
types of heatsinks.

THERMAL SOLUTIONS IN THE FUTURE

Thus it is thought that the heatsinks in the future would
be required to make efficient use of heatpipes besides
aluminum fins, thereby simultaneously achieving weight
reduction and performance improvement. In addition,ease of recycling should be taken into consideration indesigning so as to address environmental issues.
Furthermore, it would be necessary to actively adopt
high-performance heatsinks in consideration of the factthat the use of these efficient devices reduces the load onother active components such as Peltier coolers and fans,eventually resulting in reduction of power consumption.

REFERENCES
1) J.Niekawa et al.: Introduction to Micro-Heatpipe and Heat
Dissipation Technology, Nikkan Kogyo Shimbun Publishing. (inJapanese)2) M.Oomi et al.: Furukawa Review, 21 (2002) 69.
3) M.Shimada et al.: Furukawa Electric Review, 108 (2001) 23. (inJapanese)4) Emori et al.: OFC’99, PD195) Namiki: ECOC2001, We.M.2.1

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