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PS transf. to heat sink glue?

Discussion in 'Electronic Repair' started by SparkyGuy, May 27, 2007.

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  1. SparkyGuy

    SparkyGuy Guest

    Fixed a 26" Gateway LCD TV/monitor (bad solder joint in the SMPS).

    But exposing the component side of the power supply requires pulling off 2
    heat sinks that are glued to the tops of the 2 transformers. Some of the glue
    pulled off of the top of one transformer (about 3/16 inch, or 5mm thick) and
    a copper strap that winds around the laminates came loose.

    Is this really a heat conductor? Seems to be not possible, but then I'm no
    glue chemist.

    Should the missing glue be replaced? What should I use?

    Is it important to relocate the end of the copper strap?

    Regarding the other transformer (the glue is intact), should anything be
    placed between the glue and the heat sink (ie, heat sink compound), or just
    clamp down the heat sink?

    The "glue" is white and seems to be the same stuff used to attach some of the
    components to the PCB. It is a bit flexible but not very; it crumbles when
    scraped off or when a sliver of it is bent. Not epoxy, no way.

    Here's the top of the transformer:

    http://www.uploadwiz.com/WIZ2755487483

    Here's what's left of the material, stuck to the heat sink:

    http://www.uploadwiz.com/WIZ3413654658

    Thanks,
    Sparky
     
  2. N Cook

    N Cook Guest

  3. SparkyGuy

    SparkyGuy Guest

    how to download from that site ?
    Hmm... it works for me. I just tried it again. I'll post to another hosting
    site tomorrow (it's late...)

    Thanks,
    Sparky
     
  4. Palindrome

    Palindrome Guest

    I'm not sure exactly what that band is for. It could be there to reduce
    radiation of the transformer's eddy currents and thus satisfy emission
    regulations. It could be there as part of the heat transfer mechanism.

    However, they wouldn't have put it there without good reason and the
    mechanical design of the transformer is key to its electronic
    performance - so I would try to get it back in position and re-join it,
    idealy across the complete width, to form a complete electrical loop -
    if that was how it looked to be intended to be. It, or something else,
    could massively over-heat if the transformer's characteristics are altered.

    For "glue", I would use silicone adhesive/sealant eg maplin N71BZ. It
    may not be what they used originally, but it should do the trick nicely.

    For the interface between heatsink and copper - I would try to get as
    much mechanical metal to metal contact as possible. Then put a very,
    very thin coating of heat sink compound on. But the aim would be to
    ensure that the heatsink remains in close contact with the heat source -
    even if that means adding additional clamping.

    If the copper pad looked to be providing electrical, as well as
    mechanical contact, I'd want to make sure that was re-established too.
     
  5. N Cook

    N Cook Guest

    My error, I have to copy and paste into browser from newsreader and not full
    URL copied across.
    I've often seen that gel type stuff, I assumed it was a silicone like RTV
    and assumed it was for acoustic damping of any proneness to sing.
     
  6. Guest

    Farnell stocks two thermally conductive adhesives. The Electroline
    ETCR75S (order code 130-485) is probably just rubber loaded with
    finely divided zinc oxide. I used something very similar twenty years
    ago, and it worked well.

    The Loctite 1705052 (order code 537-020) is a "self-shimming"
    adhensive, where a methacrylate based adhesive has been loaded with
    0.15mm diameter balls, and is intended for applications whee there is
    a relatively high potential difference across the ahdhesive layer.

    Both adhesives are quite expensive.

    <snip>
     
  7. Eeyore

    Eeyore Guest

    That's exactly what it's there for. Actually it uses eddy currents in the copper
    band to reduce the transformer's stray magnetic field.

    Or improve subjective performance in some way (far more likely imho).

    I agree.

    Graham
     
  8. Guest

    | |> Fixed a 26" Gateway LCD TV/monitor (bad solder joint in the SMPS).
    |>
    |
    |>
    |> http://www.uploadwiz.com/WIZ2755487483
    |>
    |> Here's what's left of the material, stuck to the heat sink:
    |>
    |> http://www.uploadwiz.com/WIZ3413654658
    |>
    |> Thanks,
    |> Sparky
    |>
    |
    | how to download from that site ?
    | I just get auto-redirected off site

    The site works fine for me (Firefox browser under Slackware/Linux). Maybe
    it is doing something because of teh referrer URL it sees from how you
    click on the URL. Try copy and paste the URL as if you had typed it in.
    I always copy and paste URLs I get from newsgroups and email, and these
    did work for me that way.
     
  9. JackShephard

    JackShephard Guest

  10. JackShephard

    JackShephard Guest

    You probably don't know enough to make that assertion.

    It may well be RTV with a filler to increase its thermal properties.
    They typically make the media more "crumbly".

    http://rshughes.com/products/rtv11_white_012.html
     
  11. SparkyGuy

    SparkyGuy Guest

  12. SparkyGuy

    SparkyGuy Guest

    For "glue", I would use silicone adhesive/sealant eg maplin N71BZ. It
    So you don't think there is any thermal conductive property to the original
    stuff?
     
  13. SparkyGuy

    SparkyGuy Guest

    I'm not sure exactly what that band is for. It could be there to reduce
    The band doesn't look like it was soldered or a continuous strip that I
    ripped apart (it's not *that* fragile, nor did I "gorilla" it *that* much
    when separating the heat sink). I'll lay it down to the lams when I replace
    the adhesive.
    So you don't think the glue is thermally conductive at all? Silicon would,
    surely, insulate.
    Other components won't allow such close contact between transformer and heat
    sink. Some heat sinks soldered to the PCB make contact with this plate above
    the PCB which leaves ~5mm gap between lams and the plate. I'd like to make
    contact between the 2, but this isn't easily do-able.
    As best I can tell, it isn't an electrical issue; there isn't a soldered
    connection nor a solid physical one. The two ends of the band just seem to
    meet at the corner without any assurance that they'd be connected. (Upon
    closer examination there's what remains of a drop of... cyanoacrylate? ...
    under the end of the copper band. I'll re-do that so it's laying back down.)

    Still not sure about the glue and whether it may have been thermally
    conductive...
     
  14. John Larkin

    John Larkin Guest

    The amount of heat flow into that sliver of metal will be tiny, so any
    adhesive will work the same. The thermal resistance of the metal to
    the air will dwarf the thermal resistance of any adhesive. I suspect
    the function of the metal may not be thermal anyhow.

    Besides, filled adhesives aren't all that much better than unfilled
    ones, even if the filler is diamond.


    Run it for 30 minutes, as-is, and see if the transformer gets hot.

    John
     
  15. SparkyGuy

    SparkyGuy Guest

    The amount of heat flow into that sliver of metal will be tiny, so any
    That bad?
    I was thinking to put just a thin layer of silicon heat sink compound on the
    top of the transformer (on top of the copper strip) and refit the chunk that
    is stuck to the heat sink. That way, if the original adhesive *is* thermally
    conductive, I'd restore most of its original function.

    Thanks.
     
  16. Gibbo

    Gibbo Guest

    I made some measurements on Araldite filled with copper filings to quite
    a high density, basically as much copper as I could get in there and
    keep it workable. The heat transfer through it was amazing. I was quite
    impressed.

    Then my techie did the same tests with plain Araldite and the results
    were a few percent worse. It actually made hardly any difference. So
    you're bang on.
     
  17. Jim Yanik

    Jim Yanik Guest

    3M makes a thermally conductive glue for gluing heat sinks to
    semiconductors. We used to use it at TEK.
    It's a 2 part system,a glue and catalyst,like a cyanoacrylate and
    accellerator.
     
  18. John Larkin

    John Larkin Guest

    Yeah. Apparently random particles packed into an adhesive don't
    actually contact very well, so the base material still dominates.

    The key to good heat sinking is to keep the gap small, which means
    very flat surfaces, and using a material that squeezes out as thin as
    possible, with appropriate pressure. In that situation, sil-pads,
    phase-change materials, and some filled compounds can be much worse
    than plain grease, which can squeeze down into the microinches. Your
    copper filings probably increased the gap more than they helped.

    John
     
  19. R!

    R! Guest

    I don't think it was heatsinking at all just glue. I would just glue the
    top part of the transformer back on with "super glue" let it cure for a
    day then use rtv102 "silicone rubber" to put the heatsink back together
    and keep it quiet.

    R!!
     
  20. Eeyore

    Eeyore Guest

    If it's not soldered into a continuous loop it won't do very much !

    Graham
     
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