Discussion in 'Electronic Design' started by D from BC, Mar 5, 2007.

1. ### John LarkinGuest

You can use a thermally conductive epoxy that has very small fill
particles, and add just a little dab of glass beads to enforce spacing
until the epoxy sets. The thermal properties of the beads doesn't
matter.

John

2. ### D from BCGuest

On Tue, 06 Mar 2007 16:09:05 -0800, John Larkin

[snip]
I think thermal epoxies have better thermal conduction than glass. I
might shoot for minimal glass beads.
Also, too much glass and the bond strength probably gets worse.
Too few glass beads and the part shorts out if it's glued on crooked.
I'll guess...maybe 10 parts glass beads to 1 part epoxy....
Somewhat of a probability problem.

Dabbing sounds better than math..
D from BC

3. ### D from BCGuest

oops ..Correction..
Not 10 parts glass beads to 1 part epoxy..
Should be
10 parts epoxy per glass bead...
D from BC

4. ### John LarkinGuest

No, 100 parts epoxy to 1 part beads.

John

5. ### D from BCGuest

Let's say.. the hot part has a 5mm x 5mm contact area with the heat
sink.
Assume the glass beads are 0.1mm in diameter.
There are 50 x 50 places for glass beads.
Using a 100:1 epoxy to glass bead ratio yields 25 glass beads over a
5mm x 5mm contact area.
Or 1 bead per sq mm..

Kinda like holding up a table with 25 legs...
100:1 looks good.
I just guessed at the 10:1 ratio..
Thanks
D from BC

6. ### GregSGuest

Diamonds.

I have used very small dust just for insurance. The micron sized
particles a a bit small, but clumps of the diamonds work for me.
Diamond is something like 20 times the conductance of copper.
Cheaper than you might think. I got three vials for \$50, probably at
least 20-25 teaspoons worth.15-25u 100 cts.
greg

7. ### GregSGuest

That was just one vial, I think I ordered 250 cts.

greg

8. ### John LarkinGuest

Diamond grit is not super expensive, but I doubt it's worth it as a
thermal filler. The thermal conductivity of a filled grease or epoxy
is dominated not by the filler but by the epoxy or grease itself.
There's not very good contact between the filler particles, so even if
they were thermal superconductors the net result wouldn't be much
better than some standard ceramic.

Diamond particles as epoxy spacers are really no better than glass
beads, since the density is low. The most important thing about spacer
beads is that the gap be accurately controlled, and most grits or
sandy things don't do that well. Cataphote does sell "spacer grade"
beads, sorted for diameter, but they're very expensive.

The key to good thermal conductivity is to have very flat, smooth
surfaces and, if you need insulation, the absolute minimal insulating
gap. A thin wafer of BeO or AlN, with silicone grease, under moderate
pressure, is about as good as you can reasonably do. No spacered
filled epoxy will be that good. Hard anodize and grease is very good,
too.

John

9. ### Rich GriseGuest

Just out of curiosity, where do you get these things, and how big
should they be? I googled for "glass beads" and got 30,000 pages
on how to make a necklace. ;-)

Thanks,
Rich

10. ### John LarkinGuest

Cataphote. Or your nearest beach, maybe. The Pacific stuff is kind of
coarse and non-uniform; I'd guess that Pensacola has the best domestic
sand.

The Cataphote stuff is sold mostly as retroreflectors, but they do

John

11. ### GregSGuest

They do sell flat polished diamond specifically for his purpose.
Some new semiconductors use diamond technology.

greg

greg

12. ### D from BCGuest

On Wed, 07 Mar 2007 13:48:37 GMT, (GregS) wrote:

[snip]
Probably ok for low breakdown voltage but the glass beads can set a
precise predictable bond thickness.
The more bond thickness, the higher the breakdown voltage.
D from BC

13. ### D from BCGuest

BeO or AlN wafers?? I haven't seen that yet.
Hopefully available at Digikey
I'll have to google that someday. ...
D from BC

15. ### gGuest

When I was epoxying LEDs, I set up a multimeter to measure ohms as I
to make sure there was no low ohm shorts. I don't know how the
Lumileds are spaced
when they are assembled to make stars, but it seems epoxy is the only
spacer and insulator.
greg

16. ### John LarkinGuest

That's tiny. If you need serious cooling, solder or thermal epoxy the
hot gadget to a copper heat spreader and insulate *that* from the
grounded heatsink. More footprint, less theta.

John