Soft foam damping between Elec. Boards under shock

[kelroy]

I am looking for a good (cheap/dirty) way to dampen electronics boards in a very space limited enclosure under high shock. I was hoping to find a COTS foam tape (~.15 and ~.25 thk) that I could stick between the boards in certain places to prevent high frequency vibration.

Anyone know of a foam/material I can stuff between the boards? Oh, and it can't particulate....

Any other thoughts are welcome also.

 

[waross]

You probably know that it should be a closed cell type of foam. I have had some problems with some equipment that used a piece of foam across the top of the capacitors. It was porous. In high humidity it got damp and shorted out the capacitors. Two similar pieces of equipment at two different sites had the same failure.

 

[jraef]

We use silicone gel, i.e. RTV. It provides shock absorbtion, is insultating, non-hygroscopic and doesn't decompose. Don't use the bathtub/sink sealing type from a hardware store though, they are not all silicone, and even some that are have other materials in them for anti-fungal treatment, paintability etc.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[sreid]

3M VHB tape might work. Up to 0.120 thick, adhesive on both sides. Incredible bond strength.

http://products3.3m.com/catalog/us/...vetape_3_0/command_AbcPageHandler/output_html

 

[sreid]

Also, search McMaster-Carr for "Polyethylene Foam Strips."

http://www.mcmaster.com

 

[MikeHalloran]

What is this soft foam tape expected to do, exactly, when the shock comes along?



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

What jraef meant to say "I believe" is don't use standard silicon because it generates ascetic acid while curing. That's the vinegary smell.

Use an electronics rated silicon which does not generate ascetic acid while self vulcanizing.

I think that is a good idea, by the way, as it can be separated but won't normally, and will fit any shape, and won't degrade, and can be right on the parts, and..

 

[kelroy]

Thanks for all the tips. We have some standard space grade silicones around here at my disposal. I'm worried that might be too stiff though. I don't want to impart a large pre-load on the boards either when I stick this thing in between them. Also, I'd have to go with a flouro-polymer likely due to the low temp extreme.

MikeH -- my thought is simply to provide some damping on some unsupported regions of the boards. The G levels are quite high at high frequencies (5-10kHz), and if I can attenuate those (keep the board from vibrating at that natural frequency, in those regions), I can handle the levels at the lower frequencies. I can't use more standoffs, because the EE guys insist they're at their limit for space and can't deal with another standoff. The assembly has to be servicble (can't use anything permanent).

 

[MikeHalloran]

The acceleration at frequency stuff is pretty much a mathematical artifice, to make comparative analysis easier. An actual shock test involves an enormous hammer or a big explosive charge. I.e., the forcing function only lasts for half of a very fast cycle. Everything after that is ringing.

Elastomeric dampers are too slow developing reaction force to help when the dynamite goes boom. The compression waves traveling through them may add some interesting dynamics of their own.

So, don't add standoffs. Substitute tension members, and put the components between the boards into compression. Use the boards to reinforce each other.

Or pot the box. Or pack it with sand.




Mike Halloran
Pembroke Pines, FL, USA

 

[btrueblood]

Or put isolators between the structure and your boards. As Mike says, the loose-filled isolators work best against shocks, silicone rubber works best damping continous (e.g. random) vibrations.

 

[kelroy]

Mike H. Thanks. Yes, we have a shock machine that emulates the SRS that the parts are exposed to (we have random vibe, standard shock, and a repetition of specific pyrotechnic events (not quite dynamite -- one boom and you're done). Boards are compressed via screws into standoffs (a stack of boards seperated by standoffs), there's just too much unsupported board in my opinion in certain regions. There is no "structure" external to the board edges -- I agree. Let the boards "support themselves." Lol -- sorry, sand might blow my weight budget.

Again, thanks to everyone for the responses.

 

[analogkid2digitalman]

Sand may also abrade your PWB traces over time. Think "sand" paper:/

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[felixc]

I remember that Kontron was touting a sort of foam in their systems, that was used to dampen vibrations, and that also was heat conductive. I don't remember the name they were giving to it. Maybe they still use it. If they don't carry it anymore, well, it says it all about how good it was.