Development of High-Performance Heatsink “Crimped fin®”by Hajime Noda * , Masami Ikeda * , Yuichi Kimura * and Kenya Kawabata * 2

As the heat generation of CPUs steadily increases with some bumps anddetours in recent years, there is an urgent requirement for the thermal
performance of heatsinks. Since the requirement includes many conditions with respect to thevolume, cost and environmental issues as well as noise reduction, it is often difficult to use highperformance heatsinks of remote type or Peltier devices, thus resulting in the use of a heatsinkof conventional type. This paper discusses the advantages and limitations of the so-called
Crimped fin® heatsink that belongs to the conventional type of heatsink, in which copper oraluminum heat-dissipating fins are mechanically bonded onto a base plate of either copper oraluminum. A brief description of the development and some application examples of Crimped
fin® heatsinks of new configuration will also be included.

ABSTRACT1.

INTRODUCTION

The heat generation rate of CPUs shows, while being
influenced by the surrounding circumstances such as
the market and the technology innovation of peripheral
equipment, the tendency toward a steady increase.
Accordingly, today it is required to dissipate some tens
watts —sometimes in excess of 100 W— of heat from ahigh-performance CPU to fully exploit its performance.
It is expected that the power consumption of CPUs
might decrease in future as the semiconductor design
rule becomes finer, but on the other hand, the density ofheat generation will surely increase. In either case, there
arises a need for high-performance heatsinks. In addition,these heatsinks are required not only to have satisfactory
thermal performance but also to meet many conditions
with respect to the volume, cost, environmental issues
and noise reduction. Due to the constraints on thevolume and cost especially, it is virtually impossible,
in most cases rather than in many cases, to usehigh technologies such as heatpipe-based remoteheatsinks or vapor chambers, but to resort to the use of
conventional heatsinks. It can be said that the heatsinkof Crimped Fin type to be discussed here would be theoptimum selection to reach a possible solution under theabove-mentioned conditions, because this type provides
the highest degree of freedom in designing among thoseheatsinks that consist of metals solely.
* Ecology and Energy Lab., R&D Div.
* 2 Thermal and Antenna Solutions Dept., Electronic
Components Div., Electronics and Automotive Systems Co.

2.

FEATURES OF CRIMPED FINHEATSINK

2.1 Positioning of Crimped Fin Heatsink
Crimped Fin heatsink is a heatsink in which copper oraluminum heat-dissipating fins are mechanically bonded
onto a base plate of copper or aluminum, thereby being
provided with high performance because the fins can bedensely arranged. The heatsink has been developed todeal with thermal solutions where integral heatsinks madeof aluminum extrusions conventionally in use are notapplicable.
The aluminum extrusion-based integral heatsink is
manufactured by extruding an aluminum extrusion to theshape of the heatsink itself and cutting it to a specified
length, thus featuring the lowest manufacturing cost. Dueto the geometrical constraints of the extrusion dies, however, the heatsink has a fin configuration limitation of 0.5
mm in fin thickness and 2.3 mm in fin pitch. To increase
the heat transfer area, we have no choice but to manufacture the base plate and the fin separately, then bonding
them by some means.
The bonding methods come in two ways, namely brazing including soldering and mechanical bonding like
crimping. The Crimped Fin heatsink is manufactured
by a mechanical method such that the base plate isgrooved, fins are inserted into the grooves and are fixedby mechanically deforming both sides of the grooves.
In other words, the Crimped Fin heatsink is one of
the alternatives to substitute the aluminum extrusionbased integral heatsink. Compared with this heatsink, theCrimped Fin heatsink is obviously superior in terms of

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Development of High-Performance Heatsink “Crimped fin®”

Crimped Fin heatsink with copper base plate andhybrid copper and aluminum fins.

thermal performance. Compared with soldered heatsinks,
on the other hand, it is almost comparable in thermal performance, but has the following advantages.
(1) Crimped Fin heatsink uses simple tooling and apressing machine to bond the fins so that it is low in
production costs, while the soldered heatsink is sure
to increase in cost by the amount corresponding to
the soldering material. With respect to the production
time, in addition, soldering is more time-consuming
than mechanical pressing thus building up costelements.
(2) Although they are equivalent with each other in termsof thermal performance, the individual difference inthe thermal performance of the Crimped Fin heatsink
is smaller than that of the soldered heatsink, because
the former is uniformly press worked mechanically.
Soldering process is more likely to fluctuate due tothe unevenness of solder and the fixing method at thetime of loading to the soldering furnace.
(3) Ordinary lead-tin solder is becoming difficult to use
due to the environmental regulations. Many kinds oflead-free solders are now available, but they are all
expensive. The Crimped Fin heatsink is completely
free from environmental impacts.
(4) Soldering of aluminum-to-aluminum and aluminumto-copper is technically difficult. Plating of aluminummakes the material as easy to be processed ascopper, but plating is expensive. In contrast to this,Crimped Fin heatsinks can use any combination ofcopper and aluminum. Not only different materials
for the base plate and the fin, but also a hybrid ofmaterials, e.g. copper fins and aluminum fins can beused like the one shown in Photo 1. Consequently,
design can be optimized in response to the required
performance.
For these reasons the Crimped Fin heatsink has
sufficient justification for existence at this time, whenheatsinks are required to address the new problems ofenvironmental regulations and noise as well as increasing heat generation by CPUs, and they are applicable tomany uses accordingly.

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Photo 1

2.2 Heat-Dissipation Performance
Figure 1 shows the performance curves of the Crimped
Fin heatsink. Basically the thermal performance of aheatsink is represented by thermal resistance Rca which
is defined as the difference between CPU temperature
T c and ambient temperature T amb divided by the heatgeneration rate of CPU Q. Following the same definition,
the Figure shows the airflow dependence of the thermal
resistance for Crimped Fin heatsink in comparison to
those of aluminum-extruded heatsink and solder-bonded
heatsink.
All the heatsinks have the same volume by dimensionsof 65 mm (W) x 90 mm (L) x 50 mm (H), and the fins are
all 0.4 mm in thickness with a pitch of 1.9 mm except for
the aluminum extruded heatsink, fins of which are —dueto the fact that such a small pitch can not be realized—
2.8 mm in pitch, 0.6 mm in thickness and 47 mm inheight.
In the Figure, the Crimped Fin heatsinks (plotted by ,and ) are obviously higher in performance than thealuminum extruded heatsink (plotted by ), still they areslightly superior to the solder-bonded heatsink (plotted by
). Specifically, the measurements of the solder-bonded
heatsink and the Crimped Fin heatsink (plotted by )
have been obtained using the models that are entirely
identical in the fin material (aluminum), base plate material (copper), fin pitch, fin thickness and base plate size. If it
is assumed that they are identical in the configuration andmaterial and that they have sound bonding, they shouldhave equivalent thermal performance because basicallythey can be considered as a metal block. The only difference to be noted is that, however, the solder-bonded
heatsink compared here uses fins of folded type, so that,even though the fins have the same surface area in thedesign drawings, they might be slightly curved causing
non-uniform airflows. That is to say, the reason why the
Crimped Fin heatsink slightly outperforms the solder-

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Figure 1

Thermal performance of Crimped Fin heatsink incomparison to other heatsinks.

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Development of High-Performance Heatsink “Crimped fin®”bonded heatsink can be attributed to the difference in thestraightness of the fins.
It is also possible for the Crimped Fin heatsink, as mentioned before, to select a design such that not aluminum
but copper is used for the base plate, and copper is used
for a part of the fin array adjusting the copper fin counts,
thereby making an attempt to render an achievement of
required performance compatible with cost reduction.
The hybrid-fin heatsink (plotted by ) in the Figure has
been designed based on such a concept.
2.3 Stability of Thermal Performance
Table 1 shows the results of confirmation of thermal
performance stability of the Crimped Fin heatsink,
whereby the thermal resistance of three types ofheatsinks, i.e. two Crimped Fin heatsinks and one solderbonded heatsink, were measured using eight samples
per heatsink and their individual differences werecompared. It can be seen from the standard deviation
over eight samples that the Crimped Fin heatsink isvery much smaller in the individual difference than thesolder-bonded heatsink. When it is claimed that industrial
products such as heatsinks should be evaluated withrespect not to mean value but to guaranteed value,comparisons should be made using the mean + 3σ .
In the previous Section it was described that if the heatsinks have the same configuration and material as well
as sound bonding, they should have the same performance independently of the bonding method. Referringto this description together with such a fluctuation factor
mentioned here, it may be concluded that the Crimped
Fin heatsink offers better performance than the solderbonded heatsink of the same configuration.

2.4 Comparison of Various Types of Heatsinks
Table 2 compares the Crimped Fin heatsink with othertypes of heatsinks. Whereas each heatsink has its own
advantages and disadvantages, the Crimped Fin heatsink
is seen to offer a wide range of optimum solutions.

3.

LIMITATIONS OF CRIMPED FINHEATSINK

3.1 Vertical Airflow and Horizontal Airflow
CPUs are generally cooled using a heatsink and a fan,and the cooling methods come in two types, i.e. theone in which the fan is directly mounted on the heatsink
effecting vertical airflow as shown in Figure 2, and theother in which the fan is arranged on the side of theheatsink effecting horizontal airflow as shown in Figure
3. The former is mainly adopted in desktop PCs and thelatter in servers.
The vertical airflow is adopted in cases where the heatsink has to be accommodated together with the fan in avacant space just above the CPU, but its cooling efficiency is inferior to that of the horizontal airflow. This is largely
because the fan is located near the heatsink fins with its

Figure 2
Table 1Base

CopperAluminum

CopperFin

Aluminum

Aluminum

Aluminum

Bonding method

Solder bonding

Crimping

Crimping

Number of sample

8

8

8

Mean

0.325

0.338

0.296

Standard
deviation σ

0.01050.00480.0042

Mean + 3σ

0.357

0.355

0.309

Material

ThermalresistanceRca (K/W)

Heatsink with vertical airflow.

Stability of thermal performance.

Table 2

Figure 3

Heatsink with horizontal airflow.

Comparison of various types of heatsinks.

Crimped Fin heatsink

Aluminum extruded heatsink

Solder-bonded heatsink

Fin pitch and height

No limitations

Manufacturing limitations are
posed on the height/width ratioNo limitations

Fin and base material

Either copper or aluminum canbe used in any configuration

Only aluminum can be used

Changing the material for a partof fin array is difficult

Manufacturing process

SimpleMost simple

Cost

Low

Low

Design flexibility

Complex, involving soldering in
furnace



High



Reliability

No problem

No problem

Zinc solder is weak against hightemperature and high-humidity

Environmental issues

No problem

No problem

Lead solder is environmentallyhazardous

Thermal performance

Thermal resistance is higher
than the other two

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Development of High-Performance Heatsink “Crimped fin®”Photo 2

Heatsink with a base plate smaller than the fin area.

shaft bearing facing the center portion of the heatsink,rendering this portion potentially windless. Another reason may be that the airflow bounces from the base plate
to go through horizontally causing accumulation and driftof airflows, so that a uniform airflow becomes difficult tobe realized.
Accordingly it can be said that, no matter how densely
the fins are bonded on the base plate, an appreciable
portion of the heat transfer area does not contribute toimprove the thermal performance. All heatsinks of verticalairflow type have such a drawback, whether they belong
to aluminum extruded heatsink, solder-bonded heatsink
or Crimped Fin heatsink. To solve this problem various
designs have been proposed to increase the downward
airflow rate that bypasses the base plate, e.g. reducing
the shaft bearing area of the fan, reducing the area of thebase plate, or removing a part of the base plate as shownin Photo 2, but none of them have achieved substantial
improvements.
3.2

Aluminum Extruded Heatsink with PressureInserted Copper BlockThus a new extruded heatsink based on a radical changeof idea has appeared in the market, in which, as shownin Photo 3, an aluminum cylinder extrusion provided withradial fins is extruded, and a copper cylinder is pressureinserted into the hollow core of the aluminum extrusion
to make the whole structure a heatsink. This is called
aluminum-extruded heatsink with pressure-inserted
copper block (hereinafter called AECIH). It may be wellto say that this is an optimum solution to the heatsinks
provided with a top-mounted fan, because this structure
can realize the following advantages that cannot be
achieved by the Crimped Fin heatsink.
(1) When viewed from the fan, the shaft bearing
corresponds to the copper cylinder as to the position,and the airflow faces toward the entire fin array.
(2) Since there is no base plate beneath the fin
array, the airflow goes straight downward without
generating accumulation and drift that constitute thedisadvantages of vertical airflow, resulting in airflows
comparable to horizontal airflow.
(3) It is possible to bend the fin to the same direction
as the airflow or to the opposite direction, allowing
fin configuration to be designed to adapt to the flow

Photo 3

Aluminum extruded heatsink with pressure-inserted
copper block (AECIH).

vector from the fan.
(4) Since the fin array is manufactured by extrusion ofaluminum, it is low in cost.
Owing to the advantages of the items (1) through (3)specifically, the AECIH is superior in terms of thermal
performance to the Crimped Fin heatsink, which ought tohave a larger heat transfer area due to its densely packed
fin array. Thus it was found out that among the heatsinks
with a top-mounted fan the AECIH is the most advantageous, and, accordingly, the use of the Crimped Fin heatsink was beginning to be limited to the horizontal airflow
heatsinks.
However, from another point of view it may be said that,
because the Crimped Fin heatsink is denser in the finarrangement per se, it would achieve better performance
if it is possible to use fin crimping to form a structure
equivalent to AECIH. It was against such a backgroundthat the authors embarked on the development of a
Crimped Fin heatsink of new configuration.

4.

DEVELOPMENT OF CRIMPED FINHEATSINK OF NEW CONFIGURATION

Basically, fin-crimping technique involves bonding of
two plane materials perpendicularly intersecting, so that
it is difficult to form a structure whereby fins are radiallyarranged around a cylinder. More specifically, in thistechnique rectangular fins are inserted into the parallel
grooves provided on the coplanar surface of a base plate,
and the gap area between each fin is pressed in the
direction perpendicular to the base plate, thereby fixing
the fins by mechanical deformation as shown in Figure
4. If the base plate is curved, therefore, crimping the fin
becomes virtually impossible.
The authors contemplated combining the two structural
images of a cylinder block with radiating fins and of perpendicularly intersecting planes by fin crimping, and finally arrived at an idea of adopting a polygon prism to forma structure with radially arranged fins using the crimping
technique. It was thought that not triangular or pentangular prisms having odd number of sides but regular polygon prism having even number of sides would allow fin

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Development of High-Performance Heatsink “Crimped fin®”

5.

Figure 4

Manufacturing method for Crimped Fin heatsink.

crimping as usual, based on mechanical pressing utilizing
two opposite sides; and further, that it was possible toform equally-spaced radial fins by bending the fins after
crimping.
To obtain radial fins approximating a circular shape,square prism is insufficient, so that polygon prisms higher
than hexagon in rank are required. Octagonal or decagonal prism is obviously better for its circularity, but either
would involve complicated mechanical processing. It wastherefore decided to use hexagonal prism.
As shown in Figure 5, the manufacturing process forthe hexagonal Crimped Fin heatsink includes three repetitions of both-sides crimping followed by bending of thecrimped fins.

Figure 5

Manufacturing method for hexagonal Crimped Finheatsink.

Table 3PERFORMANCE OF CRIMPED FINHEATSINK OF NEW CONFIGURATION

Table 3 shows the performance of four prototypes of
the hexagonal Crimped Fin heatsink with different fin
thickness, fin pitch, etc. All the prototypes have been
designed under the constraints of φ 90 mm x 35 mm (H) in
volume, and 350 gram in mass. The prototype heatsinks
completely utilize one of the advantages of the Crimped
Fin heatsink, i.e. suitable mixing of copper and aluminum
fins is permitted to adjust the weight.
Although the prototypes fulfill the specifications of the
new Crimped Fin heatsink such as the airflow as efficientas that of AECIH together with the densely arranged finsspecific to fin crimping, the first and second prototypes
were inferior to AECIH by 0.015 K/W with respect to thethermal performance. The reason for such results, despite
the fact that copper fins are employed in place of aluminum fins, is that the crimped fins are thinner than the
extruded fins unsatisfactorily counterbalancing the difference in the thermal conductivity between copper andaluminum, resulting in a temperature difference between
the fin base and the fin end; in other words, insufficient
fin efficiency. But even so, increasing fin thickness means
simulating the advantages of the AECIH, eventually
reducing the advantages of the Crimped Fin heatsink.
Accordingly we decided to improve the thermal performance by way of increasing the heat transfer area to sucha degree that fully compensates the poor fin efficiency,
i.e. densification of the fin array. It is a geometrical truththat radial fins tend to have coarse pitch at the outer circumference no matter how densely they are arranged at
the inner circumference. Probably in consideration of this,in the case of AECIH also each fin is made to branch atits end. The method we took was such that two fins wereinserted into each groove, and they were spread out after
crimping to form a V-shape, thereby increasing the densities of fins at the outer circumference. The prototype-3
manufactured in this way was the first to be comparablewith the AECIH in terms of thermal performance.

Structure and performance of hexagonal Crimped Fin heatsink prototypes, illustrating the development process.
AECIH

Fin pitch
Fin thickness

Prototype-1

Prototype-2

Prototype-3

Prototype-4

1.3 mm

1.1 mm

1.3 mm

1.3 mm

1.0 mm / 0.6 mm

0.4 mm

0.3 mm

0.2 mm

0.2 mm52 at base, 104 at end

Copper: 36
Aluminum: 30

Copper: 54
Aluminum: 24

Copper: 72
Aluminum: 60

Copper: 60
Aluminum: 132

Extended heattransfer area (cm2)

1740

1370

1620

2740

3150

Thermalresistance Rca
(K/W)

0.265

0.280

0.275

0.265

0.250

Fin count

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Development of High-Performance Heatsink “Crimped fin®”Photo 4������� ��������� ����������� ���������� ��� �����

An alternative way to increase the heat transfer area
involves increasing the surface area per fin. In the case of
AECIH fins are bent from the radial direction to the copper core center with an additional offset at the midpoint,
realizing an elongation of the fin length within a fixedcircle so as to increase the heat transfer area accordingly.
The Crimped Fin heatsink was in turn provided with a flexing point at an intermediate point of the fins, in addition
to the existing one, to have ultimately a greater bending
angle. The prototype-4 obtained in this way was identified
to have an increased heat transfer area, outperforming
the AECIH in terms of thermal performance.
Meanwhile, the data shown in Table 3 were gathered
under the conditions of: the fins are bent in the opposite
direction to fan rotation; the thermal resistance was measured using identical fans; and the revolution was set to anoise level of 45 dB.
The hexagonal Crimped Fin heatsink has been optimized with respect to the fin thickness and the heat transfer area, and its hexagonal core has been enlarged witha view to adapting it to various CPU areas, finally arriving
at a structure shown in Photo 4, and it was confirmed that
this structure exhibits the best thermal performance meeting the prerequisite conditions of volume and weight limitations.

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Figure 66.

Relationship between thermal resistance and heattransfer area of hexagonal Crimped Fin heatsink with
fin thickness as a parameter.

CONCLUSION

Whereas the Crimped Fin heatsink has superior
characteristics in terms of thermal performance, reliability
and environment-friendliness, it was found structurally
difficult for the heatsink to deal with those applications
whereby it was used with a top-mounted fan. In thispaper, it has been reported that the new Crimped Finheatsink has been realized by taking full advantageof techniques such as both-sides crimping and twofin insertion, and that the new heatsink can meet therequirements involved with such applications.

Hexagonal Crimped Fin heatsink with optimized
configuration.

The hexagonal Crimped Fin heatsink thus optimized
has a heat transfer area of 2850 cm2 using 132 fins of
0.3-mm thickness, where copper fin is not used for thesake of weight adjustment. Yet it has a thermal resistanceof 0.246 K/W, better than that of AECIH (i.e. 0.265 K/W) by
about 0.02 K/W.
Changes in the thermal resistance as a function ofheat transfer area of the five prototypes of the hexagonal Crimped Fin heatsink mentioned above is shown in
Figure 6, whereby fin thickness is taken as a parameter.
Although parameters such as the difference of material —i.e. copper or aluminum, the size of the center block, etc.
are not included, the Figure is thought to provide a guideline to the design of hexagonal Crimped Fin heatsinks incase of different conditions.

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