[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

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.