Cooling Performance of Heat Sinks Used in Electronic Devices

Abstract

Existing passive cooling solutions limit the short-term thermal output of systems, thereby either limiting instantaneous performance or requiring active cooling solutions. As the temperature of the electronic devices increases, their failure rate increases. That's why electrical devices should be cooled. Conventional electronic cooling systems usually consist of a metal heat sink coupled to a fan. This paper compares the heat distribution on a heat sink relative to different heat fluxes produced by electronic chips. The benefit of adding a fan is also investigated when high levels of heat generation are expected.

Full text

Cooling Performance of Heat Sinks Used in
Electronic Devices
Ibrahim Mjallal, Hussein Farhat, Mohammad Hammoud*, Samer Ali, Ali AL Shaer and Ali Assi
Mechanical an Electrical Departments, School of Engineering, Lebanese International University
Beirut, Lebanon
*Correspondence: mohamad.hammoud@liu.edu.lb
Abstract
Existing passive cooling solutions limit the short-term
thermal output of systems, thereby either limiting
instantaneous performance or requiring active cooling
solutions. As the temperature of the electronic devices
increases, their failure rate increases. That’s why electrical
devices should be cooled. Conventional electronic cooling
systems usually consist of a metal heat sink coupled to a
fan. This paper compares the heat distribution on a heat
sink relative to different heat fluxes produced by electronic
chips. The benefit of adding a fan is also investigated when
high levels of heat generation are expected.
Keywords: Heat sink; Thermal management; Electronic
devices
I.
I
NTRODUCTION
Electronic equipment have made their way into practically
every aspect of modern life, from toys and appliances to high-
power computers. Recent increase in processing demands has
forced manufacturers to increase the performance and
functionality of integrated circuit chips in addition to
minimizing their size leading to high power dissipation
through smaller packages. This fact made thermal
management of integrated circuit chips a critical aspect of
successful processor system design, and finding new cooling
techniques much more valuable. Moreover, the failure rate of
electronic equipments increases exponentially with
temperature. Higher temperatures, increases power leakage
and degrades the chip reliability. Generally, the global
maximum allowable temperatures of various chips range
between 85℃ and 120 ℃ [1]. To avoid such problem,
packages of integrated circuits must be designed to remove
any predicted excessive heat.
Since high temperature reduces performance and degrades
chip reliability, the effective management of processor’s
temperature is an important issue that needs to be investigated.
In order to avoid excessive levels of heat and reduce
temperature, active and passive cooling techniques are used.
In many applications, standard cooling methods could be
insufficient for the heat load produced during continuous
operation over long periods due to some limitations such as
limited thermal conductivity of air for convection and copper
for conduction and small spaces decreasing the capabilities
performance.
In the present work, thermal management of the chip’s
temperature is performed using an aluminum heat sink that
can dissipate the heat generated by the chip and release it to
the surrounding.
II. C
LASSIC
C
OOLING
T
ECHNIQUES
For years, various active and passive cooling methods have
been applied to cool electronic chips. Active methods are
those who need power to work such as forced ventilation
using electric fans, liquid cooling through microchannels [2]
and heat pipes in fanless systems [3]. While in the Passive
cooling methods no power is needed to initiate the cooling
process as well as no mechanical moving parts are used. Heat
sinks is an attractive option for low power systems. However,
size constraints make the use of heat sinks difficult in
embedded or mobile systems.
A. Heat Sink
A heat sink is a passive heat exchanger that transfers the
heat generated by an electronic device (chip) to a lower
temperature fluid medium [4]. The main role of the heat sink
is to maximizes the surface area of contact between the
processor and the surrounding cooling medium, by the use of
fins. Heat generated by the chip is transferred to the heat sink
by conduction, Subsequently this heat is transferred to the
surrounding, which is usually air, by convection and radiation.
Most often, heat spreaders are used between the chip and the
heat sink due to the difference in size between the bottom of
the sink and the top of the chip, to ensure more heat
dissipation. A heat spreader is a heat exchanger that
moves heat between a heat source and another
secondary heat exchanger whose surface area and geometry
are more favorable than the source. Such a spreader is most
often simply a plate made of copper, which has a high thermal
conductivity.
Heat sinks as shown in Figure 1 are usually made of
metals such as aluminum or copper both having high thermal
conductivity to minimize the temperature difference between
the chip and the fin’s tip.
40ISBN: 978-1-5090-5173-1 ©2017 IEEE

The cooling package is modeled as a thermal resistance
circuit on basis of R (thermal resistance) or C (thermal
conductance) where the temperature across two locations can
be measured, leading to the calculation of heat dissipation rate
[5].
Fig.1. Heat Sink [6]
B. Other Cooling Methods
Various active and passive cooling methods have been
applied to cool electronic chips. Such as forced ventilation,
heat pipes, liquid cooled cold plates, immersion cooling, and
the use of Phase Change Materials [7].
III. E
XPEREMENTAL
S
ETUP
The experimental setup is shown schematically in Fig. 2.
The heat sink is placed on top of an electronic chip. The
dimensions of the sink are 20×20×6 mm. Both the chip and
the heat sink are drawn using SolidWorks in order to be used
in further simulation.
Fig. 2. SolidWorks drawing of the simulated domain.
A. Heat Transfer
The heat generated by the electronic chip is transferred
through the heat sink by conduction and then the heat is
carried out by the surrounding air by convection (natural
convection is considered in our study) and radiation. Usually,
a heat spreader is used to ensure the spreading of the heat all
over the whole base of the sink and to overcome the size
difference between the electronic chip and the heat sink.
B. Equations
1) Conduction: is the transfer of energy from the more
energetic, to less energetic particles of the same substance or
between two substances in contact due to interaction between
particles. [6]
TKq ∇−= ."
(1)
Where;
"q
is the heat transfer rate
)/(
2
mW
K is the thermal conductivity )./( KmW 








∂
∂
+
∂
∂
+
∂
∂
=∇ z
T
k
y
T
j
x
T
iT (2)
2) Convection: occurs between a fluid in motion and a
bounding surface when both are at different temperatures [8].
)("
∞
−−= TThq
s
(3)
Where;
h is the convective heat transfer coefficient )./(
2
KmW

s
T is the surface temperature (
o
K)
∞
Tis the fluid temperature (
o
K)
3)
Radiation: is the energy emitted by a matter that is at
non zero temperature. The energy emmitted by radiation field
is transported by electromagnetic waves [6].
GJq −=" (4)
Where;
J is the radiosity )/(
2
mW
G is the irradiation
)/(
2
mW
GEJ
ρ
+=
(5)
E is the emissive power )/(
2
mW
is the reflectivity
4)
Energy Balance:
The energy balance is given by the equation below:
)()()()( z
T
k
zy
T
k
yx
T
k
x
TC
t
p
∂
∂
∂
∂
+
∂
∂
∂
∂
+
∂
∂
∂
∂
=
∂
∂
ρ
(6)
C.
Fluent Modeling
This case is modeled using Ansys fluent simulation tool.
The solution is updated every 0.5 second with 40 iterations per
time step for a total flow of approximately 15 mins.
41ISBN: 978-1-5090-5173-1 ©2017 IEEE

In this study, a heat sink is placed in a fluid domain with
one inlet and one outlet. The heat sink will be studied under
three different heat fluxes 1250, 2500, and 5000 (
2
/mW ) at
26
℃ as an ambient temperature.
Because our study is a free convection study, pressure inlet
is specified as boundary condition for inlet with a pressure
gauge equal to zero. Due to symmetry, half of the domain can
be studied as shown in Figure 3.
Fig.3. The heat sink and the domain
The domain is divided into 2750000 hexahedral elements as
shown in Fig. 4.
Fig.4. Part of the mesh
IV.
R
ESULTS
As shown in figure 5, the temperature starts to increase
from the ambient temperature (300
o
K) to the steady state
temperature (326.5
o
K). At the beginning, the temperature
increases significantly, then its rate decreases with time, that’s
because, when the surface temperature of heat sink increases,
the temperature difference with ambient )(
∞
−TT
s
increases
so that the heat rejected by convection and radiation increases
with time until reaching steady state and becomes closer to
heat generated by the chip (1250
2
/mW ). Eventually, heat
generated is equal to heat rejected.
Fig.5. Variation of temperature of heat sink walls with function of time under
1250 (
2
/mW ).
The time needed for the heat sink to reach steady state
temperature is 9.5 min.
Fig.6. Variation of temperature of heat sink walls with function of time under
1250. 2500 and 5000 ( 2
/mW ).
As shown in figure 6, it is obvious that when the heat flux
increases, the steady state temperature increases, until it
reaches 348 and 383
o
K for 2500 and 5000
2
/mW ,
respectively.
Note that the temperature reached with 5000
2
/mW is
close to the harmful maximum allowable temperature (393
o
K), so the electronic ship fails to work properly, that’s why an
additional cooling technique is required.
42ISBN: 978-1-5090-5173-1 ©2017 IEEE

Fig.7. Variation of temperature of heat sink walls with function of time under
5000 ( 2
/mW
) and forced ventilated.
Figure 7 shows clearly that the forced ventilation decreases the
temperature of the heat sink to 353
o
K instead of 383
o
K which
is considered an acceptable temperature for a standard
operation of electronic chips.
F
ig.8. Temperature distribution through the heat sink
Figure 8 clearly shows that the temperature reaches its
maximum value at the center of the heat sink, and it decreases
gradually when moving away from the center and up to the top
of the fins.
From the simulation results shown above, one can conclude
that the heat sink might be used as good cooling technique of
most of electronic devices. However, traditional heat sinks
have some limitations. Based on the later observation, the
authors of this paper are currently testing a complementary
cooling technique which consists of adding Phase Change
Materials (PCMs). The PCM is to be integrated within the heat
sink [7].
V.
CONCLUSION
This paper aims to simulate the heat distribution over a
heat sink using Ansys fluent simulation tool to serve the goal
of investigating and studying the effect of integrating phase
change materials later on into the heat sink in order to improve
the cooling performance and consequently increase the
efficiency of electronic chips.
VI.
R
EFERENCES
[1]
Hill, M. (n.d.).
Cooling of Electronic Equipments"chapter 15".
Higher
education.
[2]
Coskun, A. K. 2010. "Energy-efficient variable-flow liquid coolingg in
3D stacjed architectures."
DATE,pp
111-116.
[3]
H. Xie, A. Ali. 2011. "The use of heat pipes in personal computers."
ITHERM
331-340.
[4]
Lasance, Clemens J.M. 2005. "Advanced in high-performance cooling
for electronics."
Electric cooling magazine
2-8.
[5]
Lee, S. (n.d.). How To Select a Heat Sink.
Advanced Thermal
Engineering
.
[6]
Kandasamy, Ravi. 2007. "Transient cooling of electronics using phase
change material (PCM)-based heat sinks."
Elsevier
, December 23: 11.
[7]
Ahmad hasan, Hassan Hijase, shaimaa Abdelbaqi, Ali Assi, Mohammad
Hamdan. "Comparative Effectiveness of Different Phase Change
Materials to Improve Cooling Performance of Heat sinks for Electronic
Devices."
Applied Sciences
, 2016: 2-3.
[8]
Frank P. Incropera David P. Dewitt , Theodore L. Bergman , Adrienne s.
Lavine Fundemantals of Heat and Mass Transfer [Book]. - [s.l.] : John
willey and sons, 2006.
43ISBN: 978-1-5090-5173-1 ©2017 IEEE