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					    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
EFY Design Engineers Conference 2012
   Heat sinks for air cooling

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




EFY Design Engineers Conference 2012
Thermal conductivity of Al-
alloys




                                       Copper (pure):
                                       395 W/mK




EFY Design Engineers Conference 2012
   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




EFY Design Engineers Conference 2012
       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




    EFY Design Engineers Conference 2012
   Conduction




EFY Design Engineers Conference 2012
   Convection




EFY Design Engineers Conference 2012
   Radiation




EFY Design Engineers Conference 2012
   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


EFY Design Engineers Conference 2012
   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.




EFY Design Engineers Conference 2012
=      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                                     =
    EFY Design Engineers Conference 2012
   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.
EFY Design Engineers Conference 2012
   Heatsink Design parameters

    A heatsink can be optimised for performance by
     varying the different dimensions shown.
    Of course, the optimised design should consider
     manufacturability.




EFY Design Engineers Conference 2012
   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
EFY Design Engineers Conference 2012
Fin efficiency
Apparent cooling area vs. effective cooling area

                         q = h·A ·(Ths-Tair)                                                                   T_fin => T_air



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

                   120                                         1
                                                               0,9
                   100
                                                               0,8
                                                                                      apparent cooling area
                                                               0,7
                   80                                                                 effective cooling area
   fin area, mm2




                                                                     fin efficiency
                                                               0,6                    Fin efficiency
                   60                                          0,5
                                                               0,4
                   40
                                                               0,3
                                                               0,2
                   20
                                                               0,1
                    0                                          0
                         0   10   20     30     40   50   60
                                   Fin height, mm




EFY Design Engineers Conference 2012                                                                                     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               Heatsink Fin
                                           H




closely packed fins.                   Heatsink Base
                                       H




 Here the air is forced to go
 through the heatsink and in
 this case the performance of
 the heatsink is optimised.

EFY Design Engineers Conference 2012
  Conical fins vs. rectangular
  fins
                                      Conical fins seems have some
                         α            advantages when only heat flow is
                                      considered




                      Die casting always
                      need a relief angle
                                                  Heat source
                      !
EFY Design Engineers Conference 2012
   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


                                                                 16%




                                       temperature increase, %
                                                                 14%
                                                                 12%
                                                                 10%
                                                                 8%
                                                                 6%
                                                                 4%
                                                                 2%
                                                                 0%
                                                                       0       1        2        3         4        5   6
                                                                                   angle of conical fins, degrees




EFY Design Engineers Conference 2012
   Cooling at Altitude




EFY Design Engineers Conference 2012
   Heat sink orientation
   natural convection
                                             The buoyancy effects of air
                                gravity       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.




EFY Design Engineers Conference 2012
   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.




EFY Design Engineers Conference 2012
   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.




EFY Design Engineers Conference 2012
   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.
EFY Design Engineers Conference 2012
   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.
EFY Design Engineers Conference 2012
   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



EFY Design Engineers Conference 2012
   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




EFY Design Engineers Conference 2012
   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”




EFY Design Engineers Conference 2012
   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.

EFY Design Engineers Conference 2012
   Heat pipe

                                        Heat pipe

                                       Vapour
                                       flow




                                                                       wick



                                           Condense
                                           returning
                             Heat in       (by capillary)


                                                            Heat out



EFY Design Engineers Conference 2012
   Heat pipes




EFY Design Engineers Conference 2012
   What is “liquid cooling”?

     Conventional definition in          May also include two
      automotive analogy                   phase flow, evaporating
       Circulating fluid driven by        at heat source, e.g.
        pump                                Heat pipe
       Heat absorbed at source             Thermsyphon
        by “cold plate! Or “water
        block”
       Heat rejected to ambient
        by “heat exchanger” or
        “radiator”
       Multiple heat sources
        possible in series or
        parallel

EFY Design Engineers Conference 2012
   Liquid cooling:
   Channel design is important.

                                       Heat source

    30




                                       Heat source
    15



                                             199



EFY Design Engineers Conference 2012
   Liquid cooling: temperature &
   flow
               “Star channel”          Sapa’s channel




EFY Design Engineers Conference 2012
   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.
EFY Design Engineers Conference 2012
   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.




EFY Design Engineers Conference 2012
   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

EFY Design Engineers Conference 2012
   Friction Stir Welding




EFY Design Engineers Conference 2012
   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.
EFY Design Engineers Conference 2012
   Sapa’s offer to you...




EFY Design Engineers Conference 2012
   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

				
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