Heatsink Design
A practical Approach
Sridevi Iyengar
Global Application Engineer
Sapa Profiles
EFY Design Engineers Conference 2012
Agenda

Introduction

Heat sinks and Heat Transfer mechanisms
 Why use a heatsink
 Some facts you (N)ever wanted to know about heatsink

Thermal Interface materials

Liquid coolers

Friction Stir Welding
EFY Design Engineers Conference 2012
About Me – Sridevi ( Sri )

Joined Sapa in 2010

Have 10+ years of experience in electronics cooling and
thermal design. Worked mostly at telecom/networking
companies or consulted for projects in these areas.

Thermal Analysis, thermal testing – some of my key
strengths, area of expertise

Icepak, Flotherm, and currently Flow Simulation are the tools
I have used extensively for thermal simulations

Education

–
B.S – Chemical Engineering – NITK Suratkal ( Karnataka Regional Engg College)
–
M.S - Computational fluid Dynamics – University of California San diego
Passionate about South Indian Classical Music. I learn,
teach and perform regularly
EFY Design Engineers Conference 2012
What is a heatsink

Heatsinks are devices that enhance heat
dissipation from a component to a cooler ambient
– usually air, but sometimes to other fluids as well.

The primary purpose of a heatsink is to maintain
the temperature of the device being cooled within
acceptable limits as specified by the component
manufacturer.

Keeping the component temperature under the
specified limits ensures proper operation of the
device, and improves reliability and life of
component.
EFY Design Engineers Conference 2012
Factors to be considered
while designing heatsinks

Power that needs to be dissipated

Maximum allowable component temperature

Available space/volume for heatsink

Power density

Air Flow parameters

Pressure Drop

Bypass effects

Manufacturability

Costs
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Heat sinks for air cooling
Aluminium alloys are the
dominating materials for
air-cooled heat sinks
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Thermal conductivity of Alalloys
Copper (pure):
395 W/mK
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Principles of heat transfer

Heat transfer is “the science which seeks to
predict the energy transfer which may take
place between material bodies as a result of
temperature difference

The three modes:
 Conduction: Energy transfer within solids
 Convection: Transfer from a surface to a moving fluid
 Radiation: transfer by electromagnetic radiation
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Convection Cooling
•
Convection cooling achieved by two ways
• Forced Convection
 Air is forced over the components
with a fan or blower
 The velocity of air depends on the
fan and the local conditions
• Natural Convection or free
 The buoyancy effect forces hot air
to flow to the top and cold air to
come to the bottom.
 Typical velocity – 0.2 m/sec
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Conduction
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Convection
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Radiation
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Technical terms

Q = Total power that is dissipated by the device (s)
being cooled – (W)

Tj = Junction temperature of the device

Tc = Case temperature of the device

Ts = Heatsink temperature - Maximum
temperature of the heatsink at a location closest to
the device

Ta = Ambient temperature
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The basic equation
The governing equation which correlates the
total power, temperature difference and the
thermal resistance can be expressed as
The thermal resistance is analogous to the
electrical resistance used in Ohm’s law.
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=
Thermal Resistance
Rj-c is the Junction to case
thermal resistance. Usually a
parameter that is published by the
component manufacturer
Rc-s is the thermal resistance
across the thermal interface
material between the heatsink and
the component.
Rs-a is the thermal resistance of
the heatsink.
Junction to Ambient is the
sum of the resistances
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=
Heatsink Selection
Tj, Rjc and Q will be provided by the component manufacturer.
Rcs – Thermal resistance of the interface material
Ta – Ambient temperature
Ta and Rcs are parameters that we can control to
a certain extent
Rsa is the number that will help us identify a
heatsink that will meet our criteria.
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Heatsink Design parameters
 A heatsink can be optimised for performance by
varying the different dimensions shown.
 Of course, the optimised design should consider
manufacturability.
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Air-cooled heat sinks
forced convection - fan curve
High pressure-drop
Low pressure-drop
Optimal operating region
Fan law:
Air flow ∝ n (rpm)
Pressure drop ∝ n2
Noise ∝ n3
Characteristic
curve of the fan
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Fin efficiency
Apparent cooling area vs. effective cooling area
q = h·A ·(Ths-Tair)
T_fin => T_air
Low efficiency
forced air-cooling, medium speed
fin thickness t=0.7 mm
120
1
0,9
0,8
apparent cooling area
0,7
80
0,6
60
0,5
0,4
40
0,3
fin efficiency
fin area, m m2
100
effective cooling area
Fin efficiency
0,2
20
0,1
0
0
0
10
20
30
40
50
60
Fin height, mm
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Heat flow
Bypass Effects in Forced
Convection
When there is a significant gap
between the heatsink and the
top surface of the enclosure air
will bypass the heatsink. This
reduces the performance of the
heatsink. Bypass effect is more
pronounced in heatsinks with
closely packed fins.
Here the air is forced to go
through the heatsink and in
this case the performance of
the heatsink is optimised.
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Heatsink Fin
H
Heatsink Base
H
Conical fins vs. rectangular
fins
α
Conical fins seems have some
advantages when only heat flow is
considered
Die casting always
need a relief angle
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!
Heat source
Air flow in a conical
channel
When both air flow and heat
flow are considered,
rectangular fins are better
Temperaure increase vs. angles of conical fins
temperature increase, %
16%
14%
12%
10%
8%
6%
4%
2%
0%
0
1
2
3
4
angle of conical fins, degrees
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5
6
Cooling at Altitude
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Heat sink orientation
natural convection

gravity


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The buoyancy effects of air
forces hot air to move up and
cold air to come down.
Orient the heatsink keeping in
mind the direction of gravity
Fin thickness and fin pitch are
important factors to consider
while optimising the heatsink.
Comments on heat sinks used for
natural convection

Optimise the fin spacing according to
temperature and height.

Proper orientation of the heatsink with respect
to gravity is important.

Radiation heat transfer must be considered.

Proper surface treatment is often needed as
this increases the emissivity.
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Heatsink Orientation
Forced convection




Fluid is forced to flow over the surface by external help
(Fan)
Orient the heatsink in the direction of the Airflow.
Sometimes when the flow is erratic, can use pin fin
heatsinks.
In general, extruded plane fin heatsinks work better
and have lesser pressure drop across the Heatsink.
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Comments on Heatsinks used for forced
convection

Design must take the fan curve (and by-pass flow) into
account when appropriate.

Check the fin efficiency when the fin is fairly tall.

Avoid using conical fins.

Optimise the base thickness, fin thickness and fin
spacing based on the expected air velocity through the
channels.

Always remember that when you have more than one
heatsink in the system, the airflow to the downstream
heatsink will be affected by the upstream heatsinks
and components.
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Conduction, contact surface
Heat sink
Actual contact area
< 2% of apparent contact area
Heat source
 Perfect contact can never be ensured between the heatsink and the
package.
 This could lead to potential problems since trapped air acts as an insulator.
 The performance of the heatsink can be much lower than estimated leading
to high component temperatures.
 To combat this problem, it is necessary to use a thermal interface material.
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Thermal interface materials –
Different types

Double sided PSA
 Pressure sensitive adhesive is used to adhere the heatsink to the heat source
 Easy to assemble with protective liner tabs
 The component package type will determine the kind of tape to use – acrylic based
or silicone based
 The thermal conductivity of these tapes are moderate and depends on their thermal
performance depends on the contact area that can be achieved between the
bonding surfaces
 Typically 0.005 -0.10 “ thick
 Not recommended when the heatsink fins are oriented vertically – i.e along the
direction of gravity

Single sided PSA
 Provides adhesion only to the heatsink.
 Mechanical fastening of the heatsink to the component is needed.
 Typically 0.05 – 0.01” thick
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Thermal interface materials –
Different types

Phase Change Material
 Available as peel and stick pads at room temperature
 When heated the material reflows to fill all the interface voids
 Very good performance – high thermal conductivity
 Conforms to minimize thermal path thickness
 Mechanical fastening of heatsink is required
 Could be messy during re-work

Gap Filler
 Soft, thermally conductive silicone elastomers. Used in places where a large and
variant gap exists between the components and heatsink
 Typically used in places where a common heatsink is used for multiple components
 Mechanical fastening of heatsink required
 0.5mm – 5 mm thickness
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Thermal interface materials –
Different types

Epoxy
 Room temperature vulcanizing materials which function both as thermal pathway
and mechanical attachment
 Not favored by assemblers due to the possible prep work and inability to rework

Grease
 Excellent thermal conductivity and void filling capability
 Mechanical attachment of heatsink to component required
 Can be messy and not favored by assemblers
 Can be as thin as 0.01”
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What Next

At some point one reaches the limit of Air
cooling.

You may enhance the performance of the
heatsinks with different techniques like,
serrated fins, bonded fins, Skived fins.

Heatpipe heatsinks, Vapor chamber and
Liquid cooled heatsinks are the next
generation of thermal management products
when Air cooled heatsinks just will not do the
job for you.
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Heat pipe
Heat pipe
Vapour
flow
wick
Condense
returning
Heat in
(by capillary)
Heat out
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Heat pipes
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What is “liquid cooling”?

Conventional definition in
automotive analogy
 Circulating fluid driven by
pump
 Heat absorbed at source
by “cold plate! Or “water
block”
 Heat rejected to ambient
by “heat exchanger” or
“radiator”
 Multiple heat sources
possible in series or
parallel
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
May also include two
phase flow, evaporating
at heat source, e.g.
 Heat pipe
 Thermsyphon
Liquid cooling:
Channel design is important.
Heat source
30
Heat source
15
199
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Liquid cooling: temperature &
flow
“Star channel”
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Sapa’s channel
Disadvantages of liquid cooling:
System becomes more complex

Add significant complexity: more parts and more
units being involved

Pump reliability

Low heat flux parts still need cooling with
heatsinks/Fans

Investment required for testing and verifying system
performance

Still need to remove heat from liquid system to
ambient air (or other liquid)

In general, liquid cooling units will require more real
estate.
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Some comments on liquid
cooling

Channel design is important.

Contact thermal resistance between
component and heat sink may becomes
significant.

The choices of liquid (coolant) depends on
single phase or two phase.
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Friction stir welding

A rotating tool is plunged into the joint line and moved along the
joint. Neither flux nor filler material are used.

Friction Stir welding method of joining is based on the fact that the
metal is subjected to heavy plastic deformation at high
temperatures, but lower than the melting point.

When the rotating tool is plunged into the metal, friction heat is
generated. The tool produces severe plastic deformation under high
pressure, during which the weld interfaces are stirred together and a
homogenous structure is formed.

Process results in completely pore-free,tight joints with a high
strength

Minimum heat influence on the material

Good mechanical properties
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Friction Stir Welding
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Final Thoughts

Global market for Electronic Thermal management is
forecasted to reach $8.6 billion by 2015.

Miniaturization of products along with increase in
features is leading to higher power dissipations and
more importantly power density

Upfront, well thought out thermal design will eliminate
thermal related problems at later stages. At this time
there might be no recourse or if there is one, it might
be an expensive one.

Working closely with your thermal solutions provider
will ensure you have a solid thermal solution for your
electronic product.
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Sapa’s offer to you...
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Thank You

Feel free to contact me if you think I can be of
any help.

Sridevi.iyengar@sapagroup.com

91 – 99000 45726

Some websites that I visit for information on
thermal design
– www.coolingzone.com
– www.electronics-cooling.com
EFY Design Engineers Conference 2012