Heat transfer is a discipline of thermal engineering that concern the generation, use, conversion and exchange of thermal energy, heat between physical systems. Heat transfer is classified in to various mechanisms such as heat conduction, convection, thermal radiation & transfer of energy by phase change. Most of the electronic equipment are low power and produce negligible amount of heat in their operation. Some devices, such as power transistors, CPU's, & power diodes produce a significant amount of heat. so sufficient measures are need to be taken so as to prolong their working life and reliability.

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 6, Issue 11, Nov 2015, pp. 145-153, Article ID: IJMET_06_11_017
Available online at
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=6&IType=11
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
THERMALANALYSIS OF A HEAT SINK
FOR ELECTRONICS COOLING
M. Chandra Sekhar Reddy
Associate Professor,
Department of Mechanical Engineering,
UCE (A), Osmania University, Hyderabad – 500 007, India
ABSTRACT
Heat transfer is a discipline of thermal engineering that concern the
generation, use, conversion and exchange of thermal energy, heat between
physical systems. Heat transfer is classified in to various mechanisms such as
heat conduction, convection, thermal radiation & transfer of energy by phase
change. Most of the electronic equipment are low power and produce
negligible amount of heat in their operation. Some devices, such as power
transistors, CPU's, & power diodes produce a significant amount of heat. so
sufficient measures are need to be taken so as to prolong their working life
and reliability. Here, we deal with the design of a heat sink of Aluminum alloy
for cooling of a PCB of dimension 233.3×160 with FPGA, fine pitch BGA
package, which dissipates 19.5 watts of heat energy. The whole mode of heat
transfer is carried out through forced convection with help of a cooling fan of
specific velocity. Heat sinks are passive components that cool a device by
dissipating heat into surrounding air. We need to introduce discontinuities in
the fin surface to break up the boundary layer. This can be accomplished by
cross cutting an extruded 'Al' heat sink to create a segmented fin. Heat sinks
have a wide range of applications mainly in micro processors, BGA's, PCB's,
Airplanes, Satellites, Space vehicles & missiles.
Cite this Article: M. Chandra Sekhar Reddy. Thermal Analysis of A Heat
Sink For Electronics Cooling, International Journal of Mechanical
Engineering and Technology, 6(11), 2015, pp. 145-153.
http://www.iaeme.com/currentissue.asp?JType=IJMET&VType=6&IType=11
1. INTRODUCTION
Electronic devices are strategic devices such as radars, communication sets, and
amplifiers, twt’s controls which are used in aircraft, space vehicles, missiles and
ground stations. The use of these devices in strategic and military application is
rapidly increasing and these equipments are becoming more and more sophisticated.
They demand a great deal of mechanical design skill for achieving sound and reliable
products. The equipments are liable at higher temperatures. Thermal design has
become a crucial contribution in increasing the life of the equipment. There are many

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sources of heat in an electronic circuit, all of which must be considered. It is common
to consider only heat created by the operation of active components, but such
components are resistors, conductors and even small wire bonds can generate
significant heat. The location of heat is just important as the amount. Heat generated
component in close proximity can cause a temperature raise greater than the heat
generated by the component itself. The general heat must be carried away from the
devices and dissipate safely this requires the design and construction of an efficient
heat path from devices to the mounting surface and to the outside ambient. This is
achieved by a heat sink.
Mohan and Govindarajan [1] described about pin fin and slot parallel plate heat
sinks with copper and carbon carbon composite (CCC) base plate material mounted
on CPU’s. The parameters such as fin geometry, base plate material, base plate
thickness, number of fins, fin thickness are considered. The objective of “Modelling
and Analysis of Heat Sink with Rectangular Fins Having through Holes” is to present
a best possible Heat Sink for efficient cooling of electronic devices. The choice of an
optimal heat sink depends on a number of geometric parameters such as fin height, fin
length, fin thickness, number of fins, base plate thickness, space between fins, fin
shape or profile, material [2] etc.. Stadler [3] initial attempts at developing a
systematic method to determine an optimal layout for a heat sink have proven
unsuccessful. Ritzer and Lau [4] describes the analysis and derivation of an optimum
heat sink design for maximizing the thermoelectric cooling performance of a
laboratory liquid chiller. Cengel [5] presented the basic principles of heat transfer and
a broad range of applications in a flexible format, provides the perfect blend of
fundamentals and applications. Bailey [6] has discussed that extracting heat from
electronic systems has always been a challenging task for electronic package
designers. Ever increasing miniaturisation and functionality of Microsystems
packaging technologies is resulting in increased power densities that current thermal
management techniques are finding difficult to manage. Tien [7] presented Failure
Criteria, Accelerated Life Testing, Modeling and Qualification-Based on field return
and test data, the major failure mechanisms and failure modes of cooling fan system.
Heat from electronic devices is an integral part of information processing [8], not a
nuisance that can someday be eliminated. This is a physical principle that is
independent of the device of information processing. Optimum Design of Rectangular
Plate Heat Sink, the trends in electronics are toward decreasing size and cost, while
increasing speed. Performance and reliability is resulted by increasing heat dissipated
from the operating devices [9].
2. DESIGN METHODOLOGY
A PCB consumes electrical power for its electrical functionality. Rest of the power is
dissipated as heat energy due to its very low efficiency.
PCB consisting of various active components which generates different heat
loads. Varying from 0.1W to 5.27W.This PCB temperature is to be maintained below
85o
c for safe and reliable operation of component in the atmospheric condition of
35o
C temperature.
Forced air convection cooling mechanism is inadequate for high power
applications even though it is simple and mostly preferred thermal management
option for electronic applications. An heat sink with dimensions is shown in Fig.1.

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Figure 1 Heat sink
Given
Heat load/dissipation 19.5
Base size
Assumptions
The ambient temperature of air is TA = 35°C
Surface Temperature TF = 75°C
Thickness of fin =1.5
Heat transfer coefficient =4
Average temperature (or) film temperature TF (Ts+TA)/2
(75+35)/2=55°C
The properties of dry air at 35°C from “heat and mass transfer data book” by
“C.P.KOTHANDARAMAM” are,
Density ( ) 1.0755
Thermal conductivity (k)
Coefficient of viscosity ( )
Prandtl number (Pr) 0.7215
β = 2/ (Ts+TA) 0.0030

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DESIGN CONSTRAINT
The surface temperature of components should not exceed 85°C
3. NATURAL CONVECTION CALCULATION
FIN SPACING: To find optimum fin spacing, it is first necessary to find out
Rayleigh number ( ).
It can be calculated using the formula:
Where,
g= Acceleration due to gravity = 9.81
L=Base length = 233.3 = 0.2333
Therefore,
=1.0197 107
Optimum Fin Spacing =
7.68
NUMBER OF FINS (n)
The number of fins is given by
Therefore the number of fins is taken to be 15.
FIN HEIGHT
The heat dissipation through Natural convection is given as
Where, =Heat dissipated = 19.5w
= Natural convention heat transfer coefficient = 4
=Total area
Therefore,
But A

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Fin Height = H =22
But for the given Base area of 160 , the height of the fin obtained
through natural convection design is very high. It is also not possible to fabricate a fin
of such height by conventional methods. Hence, design of this heat sink is not feasible
with natural convention cooling systems and so we resort to Forced convention
cooling system.
4. FORCED CONENTION CALCUATION
ASSUMPTIONS
Thickness of fin= 1.5
Fin spacing= 5
Velocity of air = 3
HEATFLUX
Where,
= heat dissipation in
= base area in
Therefore,
Therefore the heat sink is suitable for forced convention cooling.
NUMBER OF FINS (n)
The number of fins is given by
Therefore the number of fins is taken to be 21.
FIN HEIGHT
For calculating the fin height, first the total fin area must be calculated which is
obtained from the total area.
The total are can be obtained using the relation:

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0
20
40
60
80
100
120
140
160
180
0 2 4 6 8 10 12
Pressuredifference[Pa]
Volume flow rate[ft^3/min]
Total fin area Total area – Base area
0.0793-0.016135
0.0.6324
Now, fin height can be obtained used by using the relation:
Where
= base length in
= fin height in
= no of fins
The fin height obtained is approximately 9. 41 .
SELECTION OF FAN
Volume flow rate = no of slots X height of fin X fin spacing X velocity of air.
= .
1 = 2118.88 .
Volume flow rate = 5.9815 .
The radial fan should be selected whose CFM value should be 5.9815 cubic
feet/min or slightly more than that. Pressure difference versus volume flow rate is
shown in Fig.2. It is observed that as flow rate is increasing pressure difference is
decreasing. Fig.3. shows the isometric view of PCB-Enclosure with Fan and Heat
sink.
Figure 2 Pressure difference versus flow rate

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Figure 3 PCB-Enclosure with Fan and Heat sink (Isometric view)
5. RESULTS AND DISCUSSION
Based on designed parameters, CFD analysis is carried out using Solidworks-FloEFD
Pre processing Stage
1. The first step followed in this is the creation of the domain and dimensions of the
domain and the temperature of the domain is at Tamb=35o
c are given and this is the
first step in pre processor.
2. In the second step boundary conditions are applied and inlet and outlet blocks are
created for the flow.
3. A fan is one of the types of boundary condition that defines flow. Properties of fan
are assigned to the fan in the PCB-Enclosure assembly.
4. In the next step heat loads are defined as surface loads.
5. In this stage the material properties are assigned to PCB and Heat-sink and
6. Finally the engineering goals are specified to know the temperature and velocity of
the air.
Meshing Stage: [without Heat sink and Fan]
1. In the solution stage meshing of the component is done and in Solidworks-FloEFD
default meshing is done.
2. The Max Mesh level is 4
3. The mesh type is a Hexahedral in shape
4. General Info
Iterations: 129
CPU time: 2281 s
5. Calculation Mesh
Basic Mesh Dimensions
Number of cells in X 24
Number of cells in Y 4
Number of cells in Z 36

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Post Processor Stage
After the iterations are done now the final solution is visible and accordingly the
temperature of the component or the surface is shown in figure 4.
Figure 4 Temperature distribution in PCB-Enclosure without fan
Figure 5 Air flow Direction in PCB-Enclosure with Fan and Heat sink.
Designed Heat sink assembly is modelled in the Solidworks-2013 and it is
observed from the FloEFD simulation analysis, the maximum base temperature
produced by the PCB is 88o
C as shown in figure 4. Fig.5 shows the air flow Direction
in PCB-Enclosure with Fan and Heat sink. By forced convection with the help of fan
and heat sink assembly the temperature of the PCB is reduced to 50.98o
C.
6. CONCLUSION
The results of the simulation of natural convection at ambient conduction is 88ᴼC.
Then by forced convection using a heat sink and fan of 7cfm the temperature is
reduced to 50ᴼC which is under allowable limits. The temperature is reduced by 43%.
Therefore the PCB is working under a given temperature range of 85o
C. From this we
can say that the component is desirable in working for a given Heat sink design.

9. M. Chandra Sekhar Reddy
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