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HOW TO SELECT A HEAT SINK
Seri Lee, Director
Advanced Thermal Engineering
Aavid Thermal Technologies, Inc.
Laconia, New Hampshire 03247
Tel: (603) 527-2339
Fmc (603) 528-1478
e-mail: lee@aavid. com
With the increase in heat dissipation from microelec-
tronic devices and the reduction in overall form factors,
thermal management
bmomes a more and more impor-
tant element of electronic product design. Both the per-
formance reliability and life expectancy of electronic equip-
ment are inversely related to the component temperature
of the equipment. The relationship between the reliability y
and the operating temperature of a typical silicon semi-
conductor device shows that a reduction in the tempera-
ture corresponds to an exponential increase in the reliabil-
ity and life expectancy of the device. Therefore, long life
and reliable performance of a component may be achieved
by effectively controlling the device operating temperature
within the limits set by the device design engineers.
Heat sinks are devices that enhance heat dissipation
from a hot surface, usually the case of a heat generating
component, to a cooler ambient, usually air. For the fol-
lowing discussions, air is assumed to be the cooling fluid. In
most situations, heat transfer across the interface bet ween
the solid surface and the coolant air is the lead efficient
within the system, and the solid-air interface represents
the greatest barrier for heat dissipation. A heat sink low-
ers this barrier mainly by increasing the surface area that
is in direct contact with the coolant. This allows more
heat to be dissipated and/or lowers the device operating
temperature.
The primary purpose of a heat sink is to
maintain the device temperature below the maximum al-
lowable temperature specified by the device manufacturer.
Thermal Circuit
Before discussing the heat sink selection process, it is
necessary to define common terms and establish the con-
cept of a thermal circuit. The objective is to provide basic
fundamentals of heat transfer for those readers who are not
familiar with the subject. Notations and definitions of the
terms are as follows:
Q:
T
j
:
total power or rate of heat dissipation in W,
represents the rate of heat dissipated by the
electronic component during operation. For the
purpose of selecting a heat sink, the maximum
operating power dissipation is used.
maximum junction temperature of the device
in
0
C. Allowable T
j
values range from 115°C
in typical microelectronic applications to aa
high as 180° C for some electronic control devices,
T
=
:
T,
:
Ta
:
Figure 1: Thermal Resistance Circuit
In” special and military applications, 65°C to 80°C
are not uncommon.
case temperature of the device in
0
C. Since the
case temperature of a device depends on the location
of measurement, it usually represents the maximum
local temperature of the case.
sink temperature in
0
C. Again, this represents
the maximum temperature of a heat sink at the
location closest to the device.
ambient air temperature in
0
C.
Using temperatures and the rate of heat dissipation, a
quantitative measure
locations of a thermal
of thermal resistance
of heat transfer efficiency across two
component can be expressed in terms
R, defined as
AT
R=—
Q
where AT is the temperature difference between the two
locations. The unit of thermal resistance is in
0
C/W, in-
dicating the temperature rise per unit rate of heat dissipa-
tion. This thermal resistance is analogous to the electrical
resistance
R
e
, given by Ohm’s law:
with AV being the voltage difference and
I the current.
Consider a simple case where a heat sink is mounted on
a device package
aa
shown in Fig. 1. Using the concept
of thermal resistance, a simplified thermal circuit of this
system can
be
drawn, as also shown in the figure. In this
simplified
modei, heat flows serially from the junction to
the case then across the interface into the heat sink, and
finally dissipated from the heat sink to the air stream.
The thermal resistance between the junction and the
case of a device is defined as
This resistance
though the
RjC
is specified by the device manufacturer. Al-
value of a given device depends on how and
where the cooling mechanism is employed over the package,
it is usuaily given as a constant value. It is aIso accepted
that
Rjc
is beyond the user’s ability to alter or control.
Similarly, case-to-sink and sink-to-ambient resistances
are defined as
Rc8=~=~
R
AT,a
T.
–
Ta
sa
=
—=—
QQ
respectively, Here, R
C$
represents the thermal resistance
across the interface between the caae and the heat sink and
is often called the interface resistance. This value can be
improved substantially depending on the quality of mating
surface finish and/or the choice of interface material.
R$a
is the heat-sink thermai resistance.
Obviously, the total junction-to-ambient resistance is the
sum of all three resistances:
Required Heat-Sink Thermal Resistance
To begin the heat sink selection, the first step is to de-
termine the heat-sink thermal resist ante required to satisfy
the thermal criteria of the component. By rearranging the
previous equation, the heat-sink resistance can be easily
obtained as
R
T
j
_
Ta
sa
=
Q
– R
jc
_
Rca
In this expression, T
j
, Q and
Rjc
are provided by the de-
vice manufacturer, and
Ta
and
Rcs
are the user defined
parameters.
The ambient air temperature
T
=
for cooling electronic
equipment depends on the operating environment in which
the component is expected to be used. Typically, it ranges
from 35 to 45”C, if the external air is used, and from 50 to
60” C, if the component is enclosed or is placed in a wake.
of another heat generating equipment.
The interface resistance
Rc$
depends on the surface fin-
ish, flatness, applied mounting pressure, contact area, and,
of course, the type of interface material and its thickness.
Precise value of this resistance, even for
a given type of
material and thickness, is difficult to obtain, since it may
vary widely with the mounting pressure and other case
dependent parameters.
However, more reliable data can
be obtained directly from material manufacturers or from
heat-sink manufacturers. Typical values for common in-
terface materials are tabulated in Table 1.
Table 1: Thermal Properties of Interface
Materialsl
Material
Ther-O-Link
Thermal Compound
High Performance
Thermal Compound
Kon-Dux
A-Dux
1070 Ther-A-Grip
1050 Ther-A-Grip
1080 Ther-A-Grip
1081 Ther-A-Grip
A-Pli 22o
@
20psi
1897
In-Sil-8
1898
In-Sil-8
Conductivity
W/in°C
0.010
0.030
0.030
0.008
0.014
0.009
0.010
0.019
0.074
0.010”
0.008
Thickness
inches
0.002
0.002
0.005
0.004
0.006
0.005
0.002
0.005
0.020
0.008
0.006
Resistance
in20C/W
0.19
0.07
0.17
0.48
0.43
0.57
0.21
0.26
0.27
0.81
0.78
With all the parameters on the right side of the above
expression identified,
R,a
becomes the
quid
mazirnmn
thermal resistance of a heat sink for the application. In
other words, the thermal resistance value of a chosen heat
sink for the application has to be equal to or less than
the above
R,d
value for the junction temperature to be
maintained at or below the specified
T
j
.
Heat-Sink Selection
In selecting an appropriate heat sink that meets the re-
quired thermal criteria, one needs to examine various pa-
rameters that affect not only the heat-sink performance
itself, but also the overall performance of the system. The
choice of a particular type of heat sink depends largely on
the thermal budget allowed for the heat sink and external
conditions surrounding the heat sink. It is to be empha-
sized that there can never be a single value of thermal
resistance assigned to a given heat-sink, since the thermal
resistance varies with external cooling conditions.
When selecting a heat sink, it is necessary to classify
the air flow as natural, low flow mixed, or high flow forced
convection. Natural convection occurs when there is no ex-
ternally induced flow and heat transfer relies solely on the
free buoyant flow of air surrounding the heat sink, Forced
convection occurs when the flow of air is induced by me-
chanical means, usually a fan or blower. There is no clear