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aluminum heatsink anodizing

Discussion in 'Electronic Design' started by Jamie Morken, Dec 18, 2008.

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  1. Jamie Morken

    Jamie Morken Guest

    Hi,

    Does an anodized or painted aluminum heatsink perform better thermally
    than a bare aluminum heatsink for the situation of to-220 components
    attached to the heatsink with sil-pads?

    cheers,
    Jamie
     
  2. Eeyore

    Eeyore Guest

    No simple answer.

    I'd advise against paint since it forms a poor-thermally conductive layer.
    Black finish only aids *radiative* heat transfer and is often the lowest
    factor incolved.

    If the heatsink is fan blown, then the main heat transfer mechanism is
    conduction, not radiation or convection anyway.

    'Sil-Pads' vary from utter CRAP in their thermal conductivity ratings to
    utterly STUNNING.

    So how about some specifics. Your question is far too wildly generalised
    to make much sense.

    Graham
     
  3. Eeyore

    Eeyore Guest

    Try some of the Warth and Bergquist pads and take your words back ! NOT the
    grey ones. Look at the white, green and 'rust' coloured ones. They are
    STUNNING and conform *perfectly* to imperfect surface finishes, thus
    ensuring even better heat conductivitity over the likes of Thermapath Al2O3
    grease. Not sure about how they compare to the silver particle grease,
    never done a comparison.

    Read Motorola's AN1040.
    http://www.onsemi.com/pub_link/Collateral/AN1040-D.PDF

    Having said that I'd rather have a live heatsink with direct collector to
    heatsink contact any day. Even so, you have to fill the micro-gaps.
    Insulating a heatsink is easy.

    Graham
     
  4. Eeyore

    Eeyore Guest

    Hint: Understanding good heatsinking practice and methods is science in
    itself.

    Few people get beyond starter level which is why so many goods fail early.

    Graham
     
  5. NoSPAM

    NoSPAM Guest


    I beg to differ here, You have forced convection with a fan rather than
    natural convection without one, leading to improved heat transfer. If you
    really need to dissipate an enormous amount of heat from a small area, look
    at the Thompson CSF hypervapotron technology, a specialized form of
    nucleate boiling, used to cool extremely large transmitting tubes. They
    have achieved a heat transfer of 3000 watts per square centimeter surface
    using water as the boiling liquid.

    To go back to the original question, anodizing the surface of the heatsink
    will have little effect on the heat transfer while providing a scratch
    resistant, electrically insulating surface. Painting the heatsink will
    only lessen the heat transfer. At the temperatures normally found with
    semiconductor electronics, radiation heat transfer is usually negligible.
    For example a perfectly emitting (black body) object at 150 C radiating to
    a perfectly absorbing evacuated room at 20 C, the heat flux will be
    approximately 0.14 watts per square centimeter. If you have to insulate
    your TO-220 device from the heatsink, beryllium oxide provides an excellent
    electrical insulator which has a thermal conductivity nearly that of
    aluminum. Of course, BeO is extremely toxic if you should break one.

    73, Dr. Barry L. Ornitz WA4VZQ
     

  6. I would use a level 3 hard anodize for electrical resistance, and use
    silver filled IC chip bonding epoxy to nail the part to it. Less
    serviceable, but passes the thermals as best as can be done while
    maintaining no electrical contact.
     

  7. Use silver filled hard setting chip bonding epoxy for the absolute best
    result. That will fill all the gaps with a highly thermally conductive
    metallic media.
     
  8. Phil Allison

    Phil Allison Guest

    "Jamie Morken"

    ** When you need good heatsink performance with TO220 pack devices - avoid
    sil-pads entirely !!!!!!

    Use thin, mica insulators with a smear of white (usually zinc oxide loaded )
    thermal grease each side - not only does this conduct heat much better but
    mica will not *compress* over time like sil-pads do and result in your
    mounting bolts becoming so loose you can rotate them with your fingers.

    This loss of crucial pressure ruins the performance of the heatsinking.

    That said, a black anodised heatsink radiates IR energy much better than a
    silvery one - so it will generally be a few degrees cooler.



    ..... Phil
     
  9. Eeyore

    Eeyore Guest

    You're talking SHIT ! You're out of your depth. Where insulation is required,
    the high tech polymer pads outperform EVERYTHING.

    Graham
     
  10. Eeyore

    Eeyore Guest

    **** YOU !

    I designed audio 2 kW amps in 2u Rack units. I know WTF I'm talking about.

    Case temp rarely exceeded 80C for metal can TO-3.

    Now do the sums kiddie ! Heatsinking is an expert area that clearly no-one else
    here has much of a clue about. I've been studying it for over 30 years.

    Graham
     
  11. Eeyore

    Eeyore Guest

    Better hint. Don't use TO-220 for serious power and use pressure clips instead
    of screws. Screws used with devices with a single mounting hole lead to the
    awful scenario documented in AN1040 Fig 1 !
    http://www.onsemi.com/pub_link/Collateral/AN1040-D.PDF


    Graham
     

  12. It doesn't outperform hard anodized aluminum. On that, you do not even
    need a pad.
     
  13. Eeyore

    Eeyore Guest

    Hard anodised aluminium is hard to control and may crack. Any post anodising
    machining
    operations will render it void.

    You're better off with Al2O3 washers in fact.

    Graham
     
  14. Eeyore

    Eeyore Guest

    Who said mine need any machining ? It's a rubbish way to get a flat surface.

    Graham
     

  15. You're an idiot. My last large sink product was a 1500W supply for a
    CAT scanner for Philips. We used a custom extrusion and I outlined and
    trained the assemblers on the proper method of assembly to achieve
    perfect co-planarity across 25 installed components on a 16.5" wide sink.

    The best thermal interface in the world doesn't work worth a shit if
    every single device is not perfectly mated flat and evenly pressured.

    My methodologies are so good that we can use spring clips to hold down
    all the devices, and every unit was high reliability. They were almost
    all TO-247s and there were others as well, all perfectly aligned.

    Anybody with brains knows that a wave solder op is not possible for
    such assemblies for these reasons. So a good product will be hand
    assembled as it relates to the big guns and their heat sinks.

    I believe that there are more folks here that know a lot about sinking
    a heat source than you think, DonkTard.

    You should not be so full of yourself. You could end up getting
    flushed.
     
  16. NoSPAM

    NoSPAM Guest

    ----- Original Message -----
    From: "Eeyore" <>
    Newsgroups: sci.electronics.design
    Sent: Thursday, December 18, 2008 10:29 PM
    Subject: Re: aluminum heatsink anodizing


    No you don't. A fan blowing air across a heatsink provides for greater
    heat transfer from the sink to the aiir than does the natural convection
    moving the heated air away slowly. Obviously Eeyone has never blown air
    across a spoonful of hot soup to cool it faster than just letting the spoon
    sit in still air!

    You haven't learned much or else you are using incorrect terminology and
    trying to cover it up by bluster and profanity. Conduction provides most
    of the heat transfer from the part _to_the_heatsink_. The heatsink
    transfers its heat to the air by _convection_.

    As far as experience with heat transfer, I have been designing heat
    exchangers and such over 40 years. Heat transfer is a subject rarely ever
    taught to electrical engineers and what little there is taught is so
    simplified. Have you ever designed with heat pipes which use boiling
    liquids to carry away heat using the latent heat of vaporization? I have.
    And I have designed with the Thompson CSF Hypervapotron cooling technology
    for a 1.5 Megawatt rotating arc reactor for coal gasification. I'l like to
    see Eeyore try to get this much heat out of a box around a cubic foot in
    volume.

    Without looking it up via Google or other online search engine, try to
    explain Nusselt and Prantl numbers and how they relate to heat transfer.
    Extra credit: explain Rayleigh and Grashof numbers.

    By now, Graham, you should have figured out that at least one of my degrees
    is in chemical engineering, and that heat transfer is studied more by
    chemical engineers than any other engineers. Mechanical engineers do study
    quite a bit of heat transfer too, but they are usually only concerned with
    just air and water. For some strange reasons, mechanical engineering heat
    transfer texts often include dimensions to an equation based on
    dimensionless numbers.

    Go ahead and keep showing off you temper and ignorance. You are
    reinforcing the view of most readers here that your posts are usually not
    worth reading.

    Barry L. Ornitz, PhD
     
  17. Phil Allison

    Phil Allison Guest

    "NoSPAM"
    "Eeyore"
    ** You seem to have seriously MISREAD what he said.


    ** That is EXACTLY what his post says.

    " If the heatsink is fan blown, then the main heat transfer mechanism is
    conduction ...."

    This refers to the heat energy being conducted into the forced air stream
    and removed.


    ** Terminology often varies from one area of technology to another, one
    place to another plus it varies over time as well.

    In the world of *audio power amplifiers*, ones that have no fans are called
    " convection cooled " while those that have installed fans are often called
    " force air cooled " or just " fan cooled ".

    If a number of amplifiers * without fans* are fitted into a cabinet (
    called a " rack ") then normally several fans are fitted to the lower part
    of that rack and force air up and through the amplifiers and out the top .

    This is sometimes called " forced convection cooling " or more simply " a
    fan cooled rack" .

    Blowing an air stream over a hot surface might be called " assisted
    convection " in some quarters - but that is no more "correct" than
    saying the fast flowing air conducts the heat away.



    ...... Phil
     
  18. There are three levels of hard anodizing from a mil grade POV, and if
    the metal shop offers it, they are capable of supplying it, and any
    machining gets done prior to the operation.
    Pretty dumb to even consider machining a hard anodized Al surface.
    Absolutely not.
     
  19. Those are the same type of spring clip hold downs that we used.
     
  20. A point not often appreciated is the possible heat gain when a heatsink
    is operated in bright sunshine. A surface finish which is a poor
    radiator/absorber of radiant heat will work better in those
    circumstances.

    I have expeienced this problem with portable P.A. amplifiers used
    outdoors.
     
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