Thermal adhesive how thick or thin

[DocD]

Hi i had a hic-up with a P7 led turning blue it's not down to voltage but i do think it is down to a very fast build up of heat

So how thick or thin should thermal adhesive be ?

cheers many thanks DocD

 

[greenLED]

as thin as possible

 

[EngrPaul]

as thin as possible

And then get it a little thinner than that.

If there's no electrical reason that the metal can't touch, metal should be touching (theoretically in three places) and the rest takes up the space where a minute gap exists.

 

[DocD]

Hi,
thanks for the info, so were talking about a 10th of a mm or just as thin as possable ? and how do you guys get it as thin as possable?

 

[jtr1962]

Hi,
thanks for the info, so were talking about a 10th of a mm or just as thin as possable ? and how do you guys get it as thin as possable?

Spread it around fairly evenly and then apply as much pressure as you can (not to the LED dome of course) to squeeze out the excess. Ideally, you should end up with a layer a few thousanths of inch thick (i.e. well under 1/10 of a mm). The roughness of both surfaces affects how thick a layer you'll ultimately end up with of course.

 

[PhotonFanatic]

While the goal is to have as thin a layer of adhesive as possible, the goals of using an thermal adhesive are two-fold: 1) to isolate the slug electrically from the heatsink, and 2) to provide thermal transfer.

Many users of thermal adhesive fail to realize that to obtain the first goal, i.e., isolation, that you can not press so hard that the slug makes contact with the heatsink, and allow an electrical connection to occur.

You really need a very rigid set-up, like a lathe, or other jig, to ensure that the slug is perfectly perpendicular to the heatsink while applying pressure. Or, you need to be very experienced. :devil:

There is another approach that one can take: Apply a very thin layer of thermal adhesive to the slug, then press the LED onto a sheet of waxed paper as hard as possible, to make the layer extremely thin. Then let it cure for hours, yes, hours, so that the adhesive becomes rock hard. Then remove the waxed paper--it just peels away.

Then one can apply the LED to the heatsink without worry because the previously applied layer will now physically isolate the slug from the heatsink--just apply a little thermal adhesive and press away. The second layer will fill in any voids from the first application and bond the LED in place.

You will get good thermal transfer and good isolation using this methodology. At least it works for me.

 

[EngrPaul]

There is another approach that one can take: Apply a very thin layer of thermal adhesive to the slug, then press the LED onto a sheet of waxed paper as hard as possible, to make the layer extremely thin. Then let it cure for hours, yes, hours, so that the adhesive becomes rock hard. Then remove the waxed paper--it just peels away.

Then one can apply the LED to the heatsink without worry because the previously applied layer will now physically isolate the slug from the heatsink--just apply a little thermal adhesive and press away. The second layer will fill in any voids from the first application and bond the LED in place.

You will get good thermal transfer and good isolation using this methodology. At least it works for me.

I posted this method a long time ago, I'd like to think *I* invented it. :devil:

Teflon tape can also be used, and then press it against something known to be very flat. This allows you to turn over the part and inspect how thin the layer is before it cures. It also peels away nicely.

 

[PhotonFanatic]

I can assure you that it wasn't me who came up with the idea--so kudos to you. :bow::bow:

There's a lot to be learned here on CPF, that's for sure and I appreciate all who post their techniques and tips. It is just that I will never remember who posted the original article, not with my feeble memory. :devil:

 

[likeguymontag]

something known to be very flat.

For those of us who aren't machinists with slabs of granite or whatever it is that they use, panes of glass are a pretty good substitute for a perfectly flat surface. I don't have any quantitative info to back up this statement; it's just what I've heard.

You can also sand the surface of your heatsink and LED star flatter than they probably came from the factory if you're careful.

 

[LukeA]

For those of us who aren't machinists with slabs of granite or whatever it is that they use, panes of glass are a pretty good substitute for a perfectly flat surface. I don't have any quantitative info to back up this statement; it's just what I've heard.

You can also sand the surface of your heatsink and LED star flatter than they probably came from the factory if you're careful.

As long as it's float glass.

I suppose I should say that float glass is the flattest. However, it's not especially difficult to find something flat enough for LED mounting.

 

[likeguymontag]

As long as it's float glass.

I suppose I should say that float glass is the flattest.

I thought that was basically the only kind of flat glass available these days? Anyway, it's cheap and readily available. You can get an 8x10 piece at the Borg for a couple of bucks.

 

[Anglepoise]

I agree with everyone. Thin as possible, both surfaces flat as possible and clean with alcohol swab just before setting.

If I have a concern about electrical isolation, I mix a little diamond dust in the
Arctic Alumina. Dust is readily available from any Lapidary shop and is an electrical insulator but great heat conductor.

 

[EngrPaul]

If I have a concern about electrical isolation, I mix a little diamond dust in the
Arctic Alumina. Dust is readily available from any Lapidary shop and is an electrical insulator but great heat conductor.

Great idea! Plus, you can justify giving it to your spouse for V-day, since you're giving her diamonds. :nana:

 

[Black Rose]

There is another approach that one can take: Apply a very thin layer of thermal adhesive to the slug, then press the LED onto a sheet of waxed paper as hard as possible, to make the layer extremely thin. Then let it cure for hours, yes, hours, so that the adhesive becomes rock hard. Then remove the waxed paper--it just peels away.

That is perfect; I wish I had come across something like this when I was replacing a Lux I with a SSC P4. I made an awful mess of the base of the SSC LED trying to isolate it.

I'll have to remember that for the next time I do a transplant.