Potting material recommendations?

speff wrote in news: snipped-for-privacy@googlegroups.com:

I know about the only NASA approve potting for HV space circuits, but it is pretty rigid, not foam. Called "CONAP" it is a polyurethane. Perhaps it could be made foamlike. Absolutely non-serviceable. Once attached and cured, good luck. But there are several formulations too.

This stuff stops a sharp knife thrust at less than 7mm penetration. I think it could stop a bullet. I know it is not what you are after though, just wanted to mention it.

Don't know if a foam solution would work against high G forces. You might actually want a more rigid media.

Getting shot into space? I wonder if there are any good NASA technical documents?

What's the temperature range?

George H.

Any lack of rigidity would allow the various connections to flex, which is seldom good on a pcb, espeically one encountering shock forces. Rigid potting then surrounded by something that can absorb impact forces would be a lot better.

NT

The standard for high temperature downhole use in extreme vibration seems to be RTV silicone, degassed under vacuum.

'Heavy' things like power transistors are wired to the PCB with stranded PTFE wire wrapped, soldered and sleeved onto the legs and then through strain relief holes on the board, looping back up to solder through a pad.

Pad size is bigger than normal too and more solder. Lots of mounting holes for the PCB onto a rigid frame which can also form the 'potting box'. Component orientation can be optimised too if there's an obvious 'preferred' flex direction.

I'm not an encapsulation expert, but I do work with this stuff.

Some of my products were cast in filled epoxy. We sent out to encapsulating specialists who made customized steel molds. They did all the mixing, vacuum and oven cureing etc.

The TCE of the rigid stuff was such that over wide temperature swings glass diodes (1N4148 etc) would crack so initially we sleeved the diodes and later brush precoated them either RTV or unfilled, more flexible, epoxy.

Can't recall the details but the resin was made by Ciba-Gigy and the filler was mineral dust, possibly silica? Colorant dye was added to customize product lineup. Environmental performance was superb, cannot speak for vibration but we'd drop them from height onto stone and they'd survive the shock.

piglet

Silicon (google 2 component silicone )

Hi, thanks!

Rigidity often causes issues with temperature changes, even over automotive range. Lots of failures until they started using more flexible compounds. I'd prefer to avoid having to do a lot of testing on that.

To minimize the deflection I think a small relatively thick board will suffice. Shock from things like dropping a consumer product on a hard surface can be quite high.

Best regards, Spehro Pefhany

Hi, George:

Launching isn't all that bad if you don't hit a resonance- not too many g's .. some bad vibration for a short while, even in the cheap seats. After all, meat bags and flimsy rockets can survive it.

Probably -55 to 125 or 135

Best regards, Spehro Pefhany

Hi,

Thanks, I'll look into that one. Shore D is indeed pretty hard, but maybe there's a filled version or one using a different hardener.

Best regards, Spehro Pefhany

and helicopters fly, I've heard them described as a machine that vibrate so violently that earth repels it

They beat the air into submission.

Gliders, OTOH, seduce the air into letting it do what the pilot wants (on a good day :) )

Hardness isn't so important for acceleration tolerance, but buoyancy might be. You want the density of the support medium to be similar to the components and PCB. Even a liquid shape will remain intact through shock, if it's in a medium that matches it acoustically. I suppose this is a vote for a soft epoxy...

Clive Arthur wrote in news:r11pif$7qv$1 @dont-email.me:

Most of the RTV silicone potting media I have worked with sheared pretty easily. Not approved for space so I would not trust it for high vibe settings.

But hey if that is what you guys used...

If it does not get attacked by the petrols, etc. I would use the CONAP polyurethane.

I could stab a slab of that stuff with a sharp, stout knife and barely penetrate. So strong it seems like it would even stop a bullet.

One shot though. Zero serviceability compared to RTV.

piglet wrote in news:r11r85$ii7$ snipped-for-privacy@dont-email.me:

Conformal coat with the UV cure stuff. It is thick and soft, and then pot over that. It looks clear-ish but lights up under UV for inspection of coating completion. That is what the mil boys use. Weather seal! Double protection! Stops those danged tin whiskers too (a little bit)(on RoHs crap).

Hey, I know! Make a thin potting case for the softer stuff (RTV) and another case that goes into for a cladding of the tougher stuff. You could even used harder epoxies for the tougher layer. That would even make the interior 'package' removeable and serviceable, whereas epoxy or urethane potting usually makes servicing very difficult. Your final enclosure would be the potting shell for the outer layer.

Lasse Langwadt Christensen wrote in news: snipped-for-privacy@googlegroups.com:

I saw that video.

The early whirlys

whit3rd wrote in news: snipped-for-privacy@googlegroups.com:

Soft durometer epoxies with good surface adhesion sounds great. The RTVs suggested usually shear quite easily.

I suggested a polyurethane and said the word 'rigid', but I meant as rubbery things go, not like solid, firm epoxies or such.

Rigid compounds can shear AND components off the board when it cures

We use flexible compounds for that reason

Cheers

Klaus

Typo

AND = SMD

snipped-for-privacy@gmail.com wrote in news:34dda629-318e-40ec-8404- snipped-for-privacy@googlegroups.com:

cures

When I said 'rigid' about CONAP polyurethane, I maent compared to RTV. A more rigid RUBBERY compound.

Not like HARD (better term than rigid for what you want to convey) epoxies such as StyCast, etc.

It is rigid inasmuch as it is a dense 'rubbery' like medium. It gives under thermal cycling and stresses nothing when curing or after cure. StyCast, a hard curing epoxy, would break SMD diodes and though hole too.

They had a thermal version that was blue, and we gave it better thermal performance by adding small silica dust to it at mix time. It was not as hard, but was still able to shear parts from their mounts.

So referring to the polymer types, the term 'rigid' usually means 'more firm', but NOT 'rigid' like when describing the cured feel of an epoxy.