PS transf. to heat sink glue?

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
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how to download from that site ? I just get auto-redirected off site
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net /
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Hmm... it works for me. I just tried it again. I'll post to another hosting site tomorrow (it's late...)
Thanks, Sparky
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| |> 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.
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SparkyGuy wrote:

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.
--
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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.
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Palindrome wrote:

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).

No.
I agree.
Graham
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So you don't think there is any thermal conductive property to the original stuff?
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wrote:

Get some silica powder or fiberglass shards to add to it to increase the thermal properties.
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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...
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SparkyGuy wrote:

If it's not soldered into a continuous loop it won't do very much !
Graham
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glue
and
just
the
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.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net /
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wrote:

There is a version of "Stycast" that has thermal properties, and you can add silica powder to nearly any epoxy and increase its capacity to pass heat a huge amount.
http://www.ellsworth.com/display/productdetail.html?productidc4&Tab=Vendors
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wrote:

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
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Looks like it might do the job. Expensive! And doesn't seem to be available in quantities less than 1 pint:
< http://www.alliedelec.com/Search/ProductDetail.asp?SKUf1-0117&desc=RTV11 - 1P&Rf1%2D0117&sidF58CA0017B3E17F>
A bit much, when all I need is a few grams...
Hmm... anybody have a small amount they want to give / sell?
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wrote:

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
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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.
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John Larkin wrote:

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.
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wrote:

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
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I would say that those "tests" were flawed then, as were the results, and likely, the methods.
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