On 11/10/21 5:31 PM, Gerhard Hoffmann wrote:
A customer of mine uses Solitane, another one Mupsil.
I just wrote down the names in case I might need it.
Probably more for coating boards in space apps, no idea
if it fits.
Am 10.11.21 um 23:40 schrieb Richard (Rick) Karlquist:
I am looking for help choosing a potting compound that
has the following properties:
_
Yeah, the solithane (that's the name we use) is more used to repair
conformal coatings, stake fasteners, stick wires down to the board, glue
components to the board so it will survive vibe (think tall skinny
things, with the vibe in the plane of the board). Fairly fluid, cures
fairly quickly, low outgassing, and most important for space - someone
else used it and it worked without causing a disaster. There probably
is a potting version of it, and I'll ask one of the M&P folks at work
tomorrow what they think about Rick's need.
I've not heard of Mupsil, but we use a lot of Nusil - silicone
elastomers, often with alumina particles in it, as a thermal bonding
material. Say you've got a box with a fairly flat surface that you want
to clamp to another fairly flat surface. The problem is that tightening
the fasteners deforms both surfaces (unless you've got a zillion of
them) so the thermal contact area is just around the fastener, and there
is a perhaps a gap everywhere else. Spaceflight people hate "perhaps" so
they say, ok, put a thermal gasket in there (hey, many of us have used a
mica washer and silicone grease between part and heat sink, right?).
You can get elastomeric thermal gaskets from Chomerics and similar
companies, but they actually have the same problem with clamping force.
You tighten the fasteners, but to get the required clamping force over
the WHOLE gasket, you need a lot of fasteners, or a lot of force, and
you're back to the deformation problem.
So the answer is "thermally conductive glue" - you slather a thin layer
on, tighten the fasteners, which then causes the alumina particles to
poke into the surfaces on both sides, and hey - good thermal
conductivity. Of course, if you need to take it off, you need to get in
there with a wire saw and that's "not fun".
I will say the nifty-est thermal connection was a sort of velvet made of
carbon fibers. Carbon fibers have very high thermal conductivity. You
bond that furry velvet to both surfaces, and when you put it together,
the fibers slide along each other and make good contact along their
length, and there's millions of them. You aren't depending on clamping
force - it's the springyness of the very stiff fibers that provides the
contact force, and as you can imagine, it can tolerate a lot of
misalignment and gaps.
The actual stuff was developed originally to make a very optically
absorbing black coating over wide bandwidths - all those fibers bounce
the light around. And as a laser load (instead of the proverbial stack
of razor blades. It was then was used to coat mannequin forms, for
displaying lingerie for Victoria's Secret, of all places, because it was
very rugged and didn't shed lint. There's a whole exotic trade secret
about how they make the velvet - there's some sort of electrostatic
technique to making the fibers stand on end while they're bonded, and
some other exotic trick to getting them all the same length, and so
forth. I kept trying to use it in space (it is *so* much easier than
glue, gaskets, or zillions of fasteners), but it never took -> 1) nobody
else had used it before and 2) everyone was worried about little
conductive fibers shedding and floating around into places they
shouldn't be. Again, in the space world, no matter how tedious and
painful, if it worked before, we can do it again. thermally conductive
glue may be a pain, but it's "known to work".
For those of you doing bolted joints.. thermal conductances are around
0.1 to 1 W/K -
You want to google a chapter called "Mountings and Interfaces" by Gluck
and Baturkin - It's in Spacecraft Thermal Control Handbook Volume 1. but
there's tons of copies floating around the web, and it's a great
handbook reference for "just what is the thermal resistance with a 4-40
screw through that TO-220 tab onto an aluminum chassis"
It's one of those references which everyone cites.
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