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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. k wallace

    k wallace Guest

    I have a design project that is nearing the build and test phase. I
    need some info re. thermal compounds for attaching a heatsink to an IC,
    and I'm wondering if this is a good NG to request that info on, or if
    someone would like to direct me to a more appropriate venue.
    thanks all,
    karinne
     
  2. Figaro

    Figaro Guest

    | I have a design project that is nearing the build and test phase. I
    | need some info re. thermal compounds for attaching a heatsink to an IC,
    | and I'm wondering if this is a good NG to request that info on, or if
    | someone would like to direct me to a more appropriate venue.
    | thanks all,
    | karinne

    this might help:
    http://www.heatsink-guide.com/content.php?content=compound.shtml
     
  3. Pooh Bear

    Pooh Bear Guest

    What's the IC ?

    Graham
     
  4. Chris

    Chris Guest

    Hi. This is an appropriate group.

    A good newbie starter is GC Electronics P/N 10-8109 Zinc Oxide and
    Silicone-based Heat Sink Compound. It's got a silicone and zinc oxide
    base, it's a highly visible white, and it's relatively inexpensive.
    This will allow you to do a visual inspection as you develop skill in
    applying the right amount of compound between the IC and the heat sink.

    Make sure both the surface of the IC and the heat sink are absolutely
    clean, and that the heat sink is free of burrs, warping, or other
    visible imperfections. I like to do an alcohol wipe with a lint-free
    cloth before application of heat sink compound to remove any dust or
    grit.

    Apply a very small dab of the white zinc oxide/silicone heat sink
    compound to your very clean finger (or ideally, with a fresh finger cot
    over your index finger) . Rub the compound over the mating surface of
    the IC, such that you can barely see through the heat sink compound to
    the surface. Then evenly place the heat sink on the IC, and evenly
    press down with the clip or fastener.

    Now that you've done the work, undo it. Carefully remove the clip or
    fastener so that the surface of the heat sink doesn't slide around on
    the IC. Then pull it straight off. Look at your work. You should see
    a kind of fine fish-scaling of the heat sink compound across the entire
    surface of the heat sink like lacework, with a tiny amount of the
    compound extruded out past the edges of the IC. If you don't see the
    fish-scaling, you (probably) put too much compound on the IC. It's
    always way too thick the first time. Start over and do it again, first
    wiping the IC and the heat sink clean with alcohol and the lint-free
    cloth (it's sometimes better to just scoop off most of it with a paper
    towel first, then use the alcohol and the lint-free cloth).

    Once you do this successfully a couple of times, you'll be able to do
    it well consistently in the future. It's like riding a bike -- you
    don't forget. Practice with each type of IC and transistor package.
    If you use a TO-3 or other heat sink that requires multiple screws, you
    should tighten evenly a little at a time on each side, ideally using a
    torque indicator to get the screws tightened down evenly. Just
    remember that way too much heat sink compound is actually worse than
    none. And by the way, make sure to clean off the heat sink compund
    carefully -- it's not good for you.

    It also might be a good idea, once you're in practice, to use another
    type of heat sink compound if your application is high reliability or
    you're dealing with a lot of heat. Plain zinc oxide-based heat sink
    compounds tend to dry out and lose their effectiveness over time
    (years). Others tend to extrude out over time with thousands of
    thermal expansion-contraction cycles. Once you're in practice, and if
    you need the reliability, there are a number of translucent
    silicone-based compounds that have good thermal conductivity if applied
    properly, will not dry out over time, and are viscous enough to stay
    put. You might want to look at

    http://www.aavidthermalloy.com/

    for a variety of good products, and some good technical papers and app
    notes.

    And of course, there's always the elusive search for the perfect heat
    sink compound. The last time I was walking along that path, I started
    seeing Monster Cables and other audiophile excesses lurking in the
    bushes, so I turned back to avoid brain cramp. But I've heard there
    are a number of very expensive silver-loaded heat sink compounds that
    have somewhat better thermal conductivity than silicone-based, and
    measurably better thermal conductivity than plain-Jane zinc
    oxide-silicone. For applications right on the bleeding edge (where the
    extra 0.2 degree C/W makes all the difference), you just might need
    something this good. But usually, you just put in a bigger fan or go
    to water cooling instead.

    You might get a more specific answer if you describe what you're doing,
    and some more about your project requirements.

    Chris
     
  5. k wallace

    k wallace Guest

    the integrated circuit. I'm working on a contract/extern basis for a
    well-known company that makes oscilloscopes and data probes. They are
    planning for the future with this R&D project- currently, their DAQ
    devices need to be able to dissipate about a Watt of heat. My assignment
    is to design a thermal management system that can dissipate 5W of heat
    in a quite small package (no larger than current data probes). I have a
    solution and design that will work, I need now to concentrate on test
    and build and retest; thermal compound is part of this thermal
    management equation and it's probably the part that I know least about,
    material-wise. I've done lots of reading...not only does this have to
    work but it has to be (reasonably) economical to manufacture. I've so
    far optimized the size and type of heatsink, figured out the rest of the
    management package, and need now to find a really good thermal compound
    that's not horribly expensive.
    regards,
    karinne
     
  6. k wallace

    k wallace Guest

    what else would you like to know? I can't discuss the exact solution
    we've come up with,(NDA stuff) but I can probably specify other things.
    For instance, this package is .013*.013*.114 m in size. junction temp no
    higher than 100C, case temp no higher than 60C. Need to dissipate 5W.

    Therefore, any extra cooling we can squeeze out of any part of this is
    essential. Your response is very helpful, and if you don't mind, I will
    print it to share with my other ME team member who has less hands on
    experience than I do with any electronic stuff, and I don't have a ton-
    limited to home PC builds and hobby RC and robotics projects.

    can you indicate why "too much is worse than too little"? We need to
    minimize contact resistance as much as possible; we have access to a
    totally kick-ass machine shop for the prototype of the heatsink. Water
    cooling is not an option, unfortunately, nor is thermoelectric cooling,
    but we've come up with something that should work; we now need to
    prototype and test.

    I've done this with home PC's, but nothing more demanding than that; I
    need something that performs really well for reasonable cost, that I can
    access an amount of to use in testing.

    and...aavid is a great site...as is another one I found,
    www.coolingzone.com - that has been quite helpful for tech papers and
    the like.
    regards,
    karinne
     
  7. k wallace

    k wallace Guest

    oops- I thought that said "Whats *AN* IC?"
    We don't know. We are designing a general thermal solution for future
    need. We can assume size of about 1 cm^2 to 1.2 cm^2 for the chip itself.
    regards,
    karinne
     
  8. Pooh Bear

    Pooh Bear Guest

    You can ? How so ? What package would that be ?

    It sounds like you're seriously in the dark about thermal management to me.

    What is it you're actually trying to acheive ?

    'General solutions' makes it sound like someone hasn't a clue what they're
    doing.

    You might care to check out a heatsink manufacturer's range. See Aavid for
    example

    Graham
     
  9. Pooh Bear

    Pooh Bear Guest

    The OP hasn't got an IC that you can screw / bolt to, so the above is
    irrelevant.

    Most likely they need some thermal epoxy.

    Since the OP doesn't even know the package dissipation there's very little
    help that can be offered.

    Either the Op gives some relevant info or shuts up.

    Graham
     
  10. Pooh Bear

    Pooh Bear Guest

    13mm x 13mm x 114mm ?

    What package is that ?
    Unusual spec. Normally it's 125C or 150C. It would help if you told us what
    the IC actually is.
    Irrelevant. The difference between case temp and junction temp is set by the
    junction-to-case thermal resistance. The junction temp is the one to target (
    case temp is not important ).
    You'll need to specify the max operating ambient temp too.

    Graham
     
  11. PeteS

    PeteS Guest

    I worked with an Infiniband switch from Agilent that had an absolute
    max junction temp of 110C (but it quit operating properly at 100C
    anyway). That was a fair sized device (about 28mm x 28mm square)
    flipchip, so junction to case/heatsink thermal resistance was almost 0.
    Had a big heatsink on it as it dissipated about 18W, 12 of them inside
    a 1U enclsure, so managing even the ambient temp was painful.
    Obviously, that system had fans.

    I agree that without knowing the device for junction to case thermal
    resistance and ambient, the OP will have an impossible time designing
    heat management for it.

    Cheers

    PeteS
     
  12. Chris

    Chris Guest

    Oh. Homework. Should've said so. Try sci.electronics.design.

    Good luck
    Chris
     
  13. Pooh Bear

    Pooh Bear Guest

    How can you do that without even knowing the ambient temperature ? Have you
    made one and measured its thermal resistance ?
    What's this 'management package' ?
    You simply want thermal transfer 'grease' ?

    Graham
     
  14. John  Larkin

    John Larkin Guest

    From that page:

    "Today, there are far more advanced thermal pads available, made by
    companies such as Power Devices, Bergquist or Chomerics, to name only
    a few. For links to the web sites of these companies, check out the
    links page. The performance of these pads can be roughly equal to
    standard thermal compound."

    That is, in nearly all situations, grossly untrue. The thermal pads,
    or the phase-change stuff, wind up being 2-10 mills thick, even if you
    manage to apply insane contact pressures. Regular thermal silicone
    grease or thermal epoxy will squish down below 100 microinches with
    mild pressure, so will have an interface thermal resistance of 1/20 or
    more likely 1/100 of the pads. At, maybe, 2% of the cost.

    And all the thermal pad vendors lie their asses off on thermal
    resistance specs. If you want to blow up power fets, use Bergquist
    pads.

    John
     
  15. k wallace

    k wallace Guest

    They've given a fairly large grant for this to be considered 'homework'.
    The fact that I work at a university rather than in industry does not
    automatically imply homework. I am simply the ME on the team. I don't
    know as much about some of this as the 3 EE's on the team, regarding the
    electronics. My work is in thermal-fluid sciences, so that's what
    they've got me for.

    I know more specifics on this device, but am constrained from discussing
    some of them. Therefore, any vagueness or confusion due to such is
    completely my fault.

    Thought I could get some general suggestions here for a thermal compound
    that needs to function in an enclosed space of prev. posted size at a
    T_amb, enclosure of 60C, T_amb, external of 20 C, T_j of 100C, etc. -
    that's all I was looking for. I did get a few good suggestions, but on
    the whole, it appears that this was not the right group to query. Will
    take your (generic) advice, shut up, and go elsewhere.

    regards

    kw
     
  16. John Fields

    John Fields Guest

    ---
    Check out GC Waldom at:

    http://www.gcwaldom.com/catalog.html

    Click on 'search' and enter "Catalog 145" to get to their heat sink
    compounds.

    They've also got spec's in there somewhere, but their site is a
    nightmare to navigate, so good luck... :)
     
  17. Pooh Bear

    Pooh Bear Guest

    That was the *space* and it's enclosed ?

    You don't have a hope in hell of dissipating 5W with a delta T of a few 10s
    of degrees in that space.

    You still haven't even mentioned if the 'thermal compound' has to provide
    mechanical adhesion ( as in a glued-on heatsink ) or is simply required as
    an interface filler where the mechanical aspect is provided by nuts and
    bolts or clips..

    If you simply *won't* provide any meaningful information *no-one* can help
    you.

    It's clear to anyone who knows about these things that the minimal info
    required is in no way going to create trouble over NDAs or whatever.

    You're being deliberately abstruse.

    Please don't repeat the same lame question in s.e.d. You'll get a similar
    respsonse.

    Graham
     
  18. John Fields

    John Fields Guest

    ---
    Not from me, he won't.

    All the guy's looking for is some information on what's available in
    heat sink compounds and you're ready to tear him a new asshole
    because he won't tell you everything you want to know about his
    widget?

    Get over yourself.
     
  19. Pooh Bear

    Pooh Bear Guest

    Hey, John......

    I realise I came over as being frustrated at the OP's lack of detail.

    I have my reasons though.

    The initial post suggested that it was just an interest in a thermal transfer
    compound but I dunno, I smelt something.... In any event it wasn't clear if it
    was a grease or an epoxy that was required ( hinted at ) and I don't like to
    give bad advice.

    Read the follow-up posts carefully and the horrible truth finally comes out. The
    transfer compound is the last of their problems and likely only was though about
    on account of anecdotal comment ( e.g CPU coolers ) . The following data I kind
    of extracted as best I could from the follow-up posts.

    The OP's data aquisition module is 13m x 13mm x 114? mm

    It contains an IC that's ~ 1cm^2 dissipating *5* Watts !

    The IC is inside the module which is unventilated.

    ( There is simply no hope ever of cooling this IC as required )

    In fact the determining characteristic thermally is the enclosure dimension
    since it's actually *the enclosure* that will need to dissipate the heat.

    This explains the posted 'package' temp of 60C. The OP didn't understand that I
    was originally referring to the unspecified *IC package* and gave the data probe
    package dimensions instead it seems...

    The reason for 60C is that IEC regs only allow a delta T of 40C for accessible
    parts and if the ambient temp is 20C then the temp of the probe package must not
    exceed 60C.

    In short it was like trying to get blood out of a stone to ge this far.

    The OP's DAQ seems destined for the big trash can in the sky since I know of no
    way ( I'm sure there is no way ) to get 5W out of such a small pacakge with a
    delta T of 40C.

    Typical idiot academics. The first thing they should have considered was the
    thermal constraint.

    I'll bet that IC has been designed around some power-hungry PLD or similar.
    They'll need to consider a new technology. In short, instead of ending, their
    project has merely begun.

    Graham
     
  20. Chris

    Chris Guest

    No good deed goes unpunished -- didn't really say that at all. First,
    I gave you quite a bit of _very_ specific advice for a newbie or
    hobbyist, which is the assumed audience for s.e.b., and could have been
    assumed given your lack of specificity in your original post.
    Something vague like your original post is charitably assumed to be the
    result of lack of knowledge rather than lack of effort. If you'd spent
    more than two minutes on your original post (from further posts, there
    obviously was quite a lot more you _could_ have said without bumping up
    against non-disclosure), or read the first couple of paragraphs of my
    first response, you'd see that and not get offended.

    Second, your problem extends a little beyond just heat sink compound (5
    watts dissipated by an 0.5" X 0.5" X 4.5" heat sink in free still air
    keeping t(j) below 100C and t(sink) below 60C). This seems to be a
    very ambitious engineering puzzle, for which there is no easy answer.
    Several engineers at s.e.d. have described things they've done with
    heat sinks that approach witchcraft, and getting their advice might be
    a good idea.

    Also, there happened to be some good advice on the heat sink compound
    to use in my first post. If you'll look at the website I suggested,
    you'd see the Ultrastick phase change product, which has thermal
    conductivity ratings better than the others, and looks like it might be
    a good starting point (and will probably be cost competitive with the
    higher-priced silver-loaded stuff). With something like this, it might
    be best to start with the best, then see what kind of margin you have
    to play with (unfortunately, I have the suspicion that your margin is
    negative already, and getting a really good heat sink compound will
    just eliminate that as a source of dither). The manufacturer also will
    provide samples, especially if they're talking to someone who is a
    potentially good customer.

    http://www.aavidthermalloy.com/products/options/greases.shtml

    Nobody told you to shut up, although I still feel you might get better
    advice elsewhere. Sorry you feel that way.

    (By the way, look closely at your mathematical modelling, check your
    assumptions, and make sure you can actually accomplish what you want
    first. I'm not too sure even a perfect heat sink of the size you
    specify could do the job you need in non-computer-modelled free still
    air.)

    Good luck
    Chris
     
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