EFY Design Engineers Conference 2012 Heatsink Design A practical Approach Sridevi Iyengar Global Application Engineer Sapa Profiles

EFY Design Engineers Conference 2012

Heatsink Design A practical ApproachSridevi Iyengar

Global Application Engineer

Sapa Profiles


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


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


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.


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


Heat sinks for air cooling

Aluminium alloys are the dominating materials for air-cooled heat sinks


Thermal conductivity of Al-alloys

Copper (pure): 395 W/mK


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


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


Conduction


Convection


Radiation


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


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.


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


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 extentRsa is the number that will help us identify a heatsink that will meet our criteria.


Heatsink Design parameters

A heatsink can be optimised for performance by varying the different dimensions shown.

Of course, the optimised design should consider manufacturability.


Air-cooled heat sinks forced convection - fan curve

Characteristic curve of the fan

Optimal operating region

High pressure-drop Low pressure-drop

Air flow ∝ n (rpm)Pressure drop n∝ 2

Noise n∝ 3

Fan law:


Fin efficiencyApparent cooling area vs. effective cooling area

forced air-cooling, medium speed fin thickness t=0.7 mm

0

20

40

60

80

100

120

0 10 20 30 40 50 60

Fin height, mm

fin

are

a, m

m2

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

fin

eff

icie

ncy

apparent cooling area

effective cooling area

Fin efficiency

Heat flow

Low efficiency

T_fin => T_airq = h·A ·(Ths-Tair)


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.

HHeatsink Base

HHeatsink Fin


α

Conical fins vs. rectangular fins

Heat source

Conical fins seems have some advantages when only heat flow is considered

Die casting always need a relief angle !


Air flow in a conical channel

Temperaure increase vs. angles of conical fins

0%

2%

4%

6%

8%

10%

12%

14%

16%

0 1 2 3 4 5 6

angle of conical fins, degrees

tem

per

atu

re i

ncr

ease

, %

When both air flow and heat flow are considered, rectangular fins are better


Cooling at Altitude


Heat sink orientationnatural convection

gravity 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.


Heatsink OrientationForced 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.


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.


Conduction, contact surface

Heat sink

Heat source

Actual contact area < 2% of apparent contact area

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.


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



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



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”


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.


Heat pipe

Heat pipe

Heat out

Condensereturning(by capillary)Heat in

wick

Vapourflow


Heat pipes


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

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


Liquid cooling: temperature & flow

Sapa’s channel“Star 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.


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.


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


Friction Stir Welding


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.


Sapa’s offer to you...Sapa’s offer to you...

Customizedheat sinks Design

R&D andHeat

calculation

NDA, GlobalPurchase

Agreement

Prototyping &serial

production

Complexmachiningexpertise

All-in-one

FSW Globalsales


Thank You

Feel free to contact me if you think I can be of any help.

[email protected]

91 – 99000 45726

Some websites that I visit for information on thermal design– www.coolingzone.com

– www.electronics-cooling.com

Documents

EFY Design Engineers Conference 2012 Heatsink Design A practical Approach Sridevi Iyengar Global Application Engineer Sapa Profiles