Heatsink Design A practical Approach

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Heatsink Design A practical Approach

• Sridevi Iyengar
• Global Application Engineer
• Sapa Profiles

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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

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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

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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.

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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 Al-alloys
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 Ohms 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
Fan law
Air flow ? n (rpm) Pressure drop ? n2 Noise ? n3
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Fin efficiencyApparent cooling area vs.
effective cooling area
q hA (Ths-Tair)
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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.
HHeatsink Fin
HHeatsink Base
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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Conical fins vs. rectangular fins
Conical fins seems have some advantages when only
heat flow is considered
Die casting always need a relief angle !
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Air flow in a conical channel
When both air flow and heat flow are considered,
rectangular fins are better
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Cooling at Altitude
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Heat sink orientationnatural convection

• 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.

gravity
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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 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.

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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
Actual contact area lt 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.

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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 (by capillary)
Heat in
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

• May also include two phase flow, evaporating at
  heat source, e.g.
• Heat pipe
• Thermsyphon

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Liquid cooling Channel design is important.
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199
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Liquid cooling temperature flow
Sapas channel
Star channel
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Disadvantages of liquid coolingSystem 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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Sapas 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_at_sapagroup.com
• 91 99000 45726
• Some websites that I visit for information on
  thermal design
• www.coolingzone.com
• www.electronics-cooling.com