Thermal Paste

Ozythemandias

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Thanks Ozy[emoji4]

I wonder if there's another method? I mean I don't want the star to move at all. nothing hold the star but thermal paste and I don't like that.

That's a less than ideal setup. Is there a reflector or optic applying pressure to keep the star down?
 

iamlucky13

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I assume you already considered using small screws to hold it in place and don't have space?

Search for thermal adhesive. Arctic silver makes one, and there are other options. Often if you look inside a desktop computer, you will see small heatsinks on the motherboard with no screws or clamps holding them in place. They're simply glued straight to the chip they are cooling with thermal adhesive.

I've never used any myself, so I can't say exactly how tightly it sticks, but expect to have difficulty removing it once fully cured.
 

Ozythemandias

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Optic, but still I want it to be rock solid.

The optic will hold it in place, the legs sit in corresponding holes on the mcpcb so it can't shift laterally.

You can use a thermal epoxy or adhesive but their usually less efficient the regular thermal paste. Honesty the thermal paste is overrated and you're more likely to do lose efficiency if you over apply. Just smooth out both surfaces as much as possible and put a very small amount of paste.
 

TexasLumens

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I use the Arctic Silver thermal adhesive....but remember, it is stuck for life. I sometimes use the thermal paste but a week or so back, I had it fog up a reflector and lens on a C8 we built with an HXP 50.2. This was the ONLY time that ever happened but I was not happy!! TL
 

DrafterDan

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I have used thermal adhesive (pretty sure I purchased it from Mtn Elec) on almost every mpcb project. It does stick very well, but I suppose you could use a chisel or something to pry it up. My thinking is that if I wanted to do something so drastic, I'd just make up a new pill/ drop-in
 

Julian Holtz

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The optic will hold it in place, the legs sit in corresponding holes on the mcpcb so it can't shift laterally.

You can use a thermal epoxy or adhesive but their usually less efficient the regular thermal paste. Honesty the thermal paste is overrated and you're more likely to do lose efficiency if you over apply. Just smooth out both surfaces as much as possible and put a very small amount of paste.

I wonder if it is actually possible to over-apply thermal paste. I once made an experiment: I put a drop between two acrylic glass parts from two CD cases, and gently squeezed them together. After a short while, it spread out so thin that it was practically transparent. From then on, I believe that excess thermal paste simply squeezes out at the edges, where it does not do much harm if one has the foresight not to use an electrically conductive type.
I also never try to apply it on the complete surface. I believe this is an easy way to trap bubbles. If I use it on a round surface, I put a drop in the middle and let it spread out on it's own when assembling the part with a little pressure. When I use it on a square surface, I apply it in an "X" shape, from where it spreads out, and so on. I always try to think about the way the thermal past would spread out on it's own without trapping air bubbles while covering the area completely, and apply a pattern accordingly.
 

DIWdiver

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I wonder if it is actually possible to over-apply thermal paste. I once made an experiment: I put a drop between two acrylic glass parts from two CD cases, and gently squeezed them together. After a short while, it spread out so thin that it was practically transparent. From then on, I believe that excess thermal paste simply squeezes out at the edges, where it does not do much harm if one has the foresight not to use an electrically conductive type.
I also never try to apply it on the complete surface. I believe this is an easy way to trap bubbles. If I use it on a round surface, I put a drop in the middle and let it spread out on it's own when assembling the part with a little pressure. When I use it on a square surface, I apply it in an "X" shape, from where it spreads out, and so on. I always try to think about the way the thermal past would spread out on it's own without trapping air bubbles while covering the area completely, and apply a pattern accordingly.

That's an interesting idea. I've always tried to put on as uniform a layer as possible.

Success also depends on the paste and how you apply pressure. If you apply a glob of paste, tighten the screw, and move on, then when the paste oozes out over time it will reduce the screw tension. This is not optimal. You need something with some springiness, or you have to come back and tighten the screws again.

At room temperature and with the paste I use, I find a minute or two is usually enough, but with some of the thermal pads, they melt when they get hot, and the screws get quite loose. Those really should be done with springs.
 

Agpp

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May 12, 2017
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With CPU cooling it was shown that the way you apply it has measurable effect on performance. Making a cross works best because it allows the paste to spread without making air bubbles. It's better than spreading it manually, making a dot or a smiling face. It's not the same with flashlights as you have a hole in the middle of the shelf / pill where paste (and air) can escape.
But does it mean any way of spreading is equally good? I don't think so. A thin spread will form bubbles just like with CPUs. A circle? Hugely wasteful. Maybe a couple of small blobs?
Or maybe air bubbles don't matter as much? After all, lights tend to be less sensitive to thermal paste selection than CPUs are, they may be less sensitive to bubbles too.
But it may depend on particular LED use too. Recently djozz found that with Oslon Black Flat thermal transfer from LED to MCPCB is way more critical than it is with XM-L2. Ultra thin dielectic layer with thermal resistance being equivalent to a bit thicker solder caused drop of lumen from (speaking from memory) over 700 to 500.
The farther from die you are the less important the thermal transfer is. Thermal paste is one layer lower than solder. Is it worthwhile to re-analyse our techniques with LEDs that have higher thermal density?
I don't know.
 
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