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Proper group for heatsink question?

Discussion in 'Electronic Basics' started by k wallace, Dec 11, 2005.

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  1. Pooh Bear

    Pooh Bear Guest

    To be fair, I indicated that the OP should post more detailed info ( which
    certainly needn't have encroached on an NDA ) or 'shut up'.
    It seems that you too have come to the same conclusion that the enclosure in
    which this IC is to be installed is the limiting factor.

    I despair sometimes at the poor level of practical skills in engineering
    exhibited today - and especially by 'academics'. A basic understanding of heat
    flow would have fixed this.

    Something this simple should have been considered at the beginning. Leaving '
    thermal manageent ' to the end is begging for failure. And sadly it looks like
    failure is the conclusion here.

    Graham
     
  2. Bob Masta

    Bob Masta Guest

    Just a note of caution: You may run across BeO (beryllium
    oxide) touted as a good heat sink material. No doubt, but
    be aware that the stuff is incredibly toxic if you get it into a
    cut, breathe dust, etc. The manufactured ceramic pieces are
    probably not a serious danger (compared to the raw metal or
    powdered oxide), but not something to mess around with
    as a prototype. No filing, grinding, etc, and handle it with
    care. Personally, I'd avoid it if at all possible.

    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  3. Tim Williams

    Tim Williams Guest

    On a related question, what's your opinion of thermal paste vs. silicone
    pads vs. mica vs. aluminum nitride or any other conductive ceramic, with or
    without paste?

    I'm interested in putting a half bridge of 600Vceo IGBTs on the same
    heatsink (which will be copper, if I can cast and machine a slab suitably
    :).

    Tim
     
  4. John  Larkin

    John Larkin Guest

    AlN is almost as good as BeO. Both should be used with filled silicone
    paste. The problem with any insulator is its thickness... thermal
    resistance is proportional to thickness, and it's hard to beat the 100
    micro-inches of just silicone grease.

    I like to mount an electrically hot semoconductor directly to a copper
    heat spreader, and insulate *that* from the main heatsink. You can
    increase the effective thermal footprint of the part many-fold.

    Lateral heat spreading on a heatsink is a much-ignored issue, and a
    spreader helps here, too.

    John
     
  5. Tim Williams

    Tim Williams Guest

    Ah yes, good point! But where do you get insulators for the nonstandard
    footprint? Machinable AlN? Somehow I doubt that. :(

    Tim
     
  6. Fred Abse

    Fred Abse Guest

    Does "attaching" mean you need an adhesive, as well as thermally
    conductive compound?

    Look at:

    http://www.dowcorning.com/applications/product_Finder/pf_main.asp?selIndustry=009&pg=00001122
     
  7. John  Larkin

    John Larkin Guest

    One way is to hard-anodize the main heatsink, and the spreader as well
    if it's aluminum. Then just use grease or epoxy. 0.001" of hard
    anodize, chemically Sapphire, is a reliable insulator up to, say, 200
    volts, and has a low thermal resistance.

    You can buy AlN in slabs, cut to order, if you have modest quantities.
    I haven't tried to machine it myself, but I'd bet you could work a
    thin slab with a Dremel.

    Or, bond the spreader to the main sink with thermal epoxy, using tiny
    bits of mylar or monofilament as spacing shims, or load the epoxy with
    just a dash of Cataphote glass beads to enforce spacing before it
    cures.

    Even better, don't insulate the semiconductor, insulate the entire
    heatsink!

    John
     
  8. Bob Masta

    Bob Masta Guest

    Of course, the simplest approach for standard items like
    IGBTs is to buy parts that are already electrically insulated, then
    just use silicone grease. I haven't checked out if the insulated parts
    have higher thermal resistance than their hot counterparts, but they
    sure are a snap to use!

    Best regards,



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  9. Tim Williams

    Tim Williams Guest

    I would love to have a 600V, 100A, high speed, half bridge IGBT module, but
    a quad of 600V, 50A, TO-247's was cheaper ($44 from Allied, saved 10% vs.
    Digikey!).

    Tim
     
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