OT: Epoxy for RF/Microwave application?? Options

[Mueller]

I'm getting no real help from the local 3M rep (distributor)

Application is a microwave chamber that needs to be taken apart (soaked in a solvent), cleaned, and re-epoxied and tested for microwave leakage.

Still trying to get the conditions of operation of the unit from the customer, but no luck.

Thanks

[bondo]

PM jkeyzer, he *might* be able to help.

[TonyAKAVW]

I'm assuming you are looking for a conductive epoxy?

I've bought silver loaded epoxy from these guys before for microwave circuit construction...

http://www.2spi.com/catalog/spec_prep/cond...nd_adhes2.shtml

there's also these guys..

http://www.mgchemicals.com/products/8331.html


-Tony

[Mueller]

QUOTE (TonyAKAVW @ Dec 7 2005, 10:29 AM)

I'm assuming you are looking for a conductive epoxy? I've bought silver loaded epoxy from these guys before for microwave circuit construction... http://www.2spi.com/catalog/spec_prep/cond...nd_adhes2.shtml there's also these guys.. http://www.mgchemicals.com/products/8331.html -Tony

while the conductivity would help dissapate the heat, I believe the main objective of the epoxy is to contain the microwave inside the assembly...after a certain amount of cycles the epoxy breaks down needing to be replaced.

[TonyAKAVW]

Conductive epoxy would be the only kind of epoxy that could contain the microwave energy. It must form a faraday cage along with the rest of the cavity. However, conductive epoxy doesn't last forever, and it isn't perfectly reflective to microwave energy, so it sounds like its absorbing microwave energy, heating up, and getting temperature cycled fast, degrading the epoxy. I'm not sure what else you might use. Maybe evaporate metal onto the surface. Not easy to do I imagine...

Is this for filling in seams or what?

-Tony

edit: It may not be that the epoxy is absoring the microwave energy, but maybe some other source is heating it up, and causing it to degrade. Either way it sounds like you can't use conductive epoxy in this case..

This post has been edited by TonyAKAVW: Dec 7 2005, 11:57 AM

[Mueller]

QUOTE (TonyAKAVW @ Dec 7 2005, 10:55 AM)

Is this for filling in seams or what? -Tony

yep, it's an aluminum sheetmetal assembly....it cannot be welded shut since components inside must be r&r'd on a time schedual

[bondo]

What if you use high temp RTV to glue foil to it? As long as the glue layer was thin, it probably wouldn't leak.

Warning: High temp RTV isn't high temp until it cures.. I blackened a toaster oven that way.

[mightyohm]

Can you use copper tape instead of a glue?

3M makes copper tape in rolls. You can also get metallic tape from Home Depot, I think it is for duct work. It looks like aluminum or tin or something.

Silver loaded epoxies are dangerous, I have seen some that become very high resistance if there is no pressure pushing the "balls" of silver together, so it doesn't work well to bridge gaps or as caulking compound.

Edit - you can also buy RF gasket material. It is like metallic rope. You push it into the seams. They also have gasketing with fingers for things that open and close.

[TonyAKAVW]

QUOTE

What if you use high temp RTV to glue foil to it? As long as the glue layer was thin, it probably wouldn't leak. Warning: High temp RTV isn't high temp until it cures.. I blackened a toaster oven that way.

Foil could work... But it depends on the frequency of the microwave signal as well as the degree of 'containment' needed. It would work best to apply it to the whole surface of the cavity, not just the edges, etc. You still have the possibility for leakage where the foil is separated from the cavity walls by a layer of RTV. And the foil would have to make contact in many places to the walls...

-Tony

[mightyohm]

Something like this:

http://www.semsupplies.com/Copper%20Tape-CCT-3M.html

Note two important qualities - conductive adhesive and also high temp abilities.

Here's a link to 3M's EMI shielding tape product line. A few of these would work.

http://products3.3m.com/catalog/us/en001/e...ctronicsesm_3_0

[ppickerell]

Mike,
I can make beryllium copper finger gaskets that can form a EMI seal around the perimeter. I have also made fine diameter coil springs that sit in the channel and "crush" to form a seal when the cover is installed. We do a lot of RFI/EMI gasketing here.

[lapuwali]

QUOTE (ppickerell @ Dec 7 2005, 11:22 AM)

Mike, I can make beryllium copper finger gaskets that can form a EMI seal around the perimeter. I have also made fine diameter coil springs that sit in the channel and "crush" to form a seal when the cover is installed. We do a lot of RFI/EMI gasketing here.

Beryllium Copper? That's the unobtainium material the MPS diaphrams are made of...

[Mueller]

QUOTE (bondo @ Dec 7 2005, 11:02 AM)

What if you use high temp RTV to glue foil to it? As long as the glue layer was thin, it probably wouldn't leak. Warning: High temp RTV isn't high temp until it cures.. I blackened a toaster oven that way.

thanks for those alternative solution ideas, however, the repair/refurb must not deviate from the norm....

[Dave Bell]

Someone mentioned copper tape... We use it all the time on spacecraft electrical and RF boxes to seal RF from leaking out... MHz to GHz... your microwave is in the GHz regime.

Someone else mentioned mesh.. that will work too.... you don't need a solid surface.. You can use something conductive that has holes that are small compared to the wavelength of the RF wave...your typical microwave oven uses an RF wavelength measured in cm's... which is why the mesh screen on the front door of the microwave still works to reflect the waves back into your burrito... the holes in the mesh are mm's in diameter.

End of science lesson.

[bondo]

QUOTE (Mueller @ Dec 7 2005, 01:55 PM)

QUOTE (bondo @ Dec 7 2005, 11:02 AM) What if you use high temp RTV to glue foil to it? As long as the glue layer was thin, it probably wouldn't leak. Warning: High temp RTV isn't high temp until it cures.. I blackened a toaster oven that way. thanks for those alternative solution ideas, however, the repair/refurb must not deviate from the norm....

Epoxying on a panel that requires periodic removal is "the norm"? Sheesh. I vote copper tape if it can handle the heat. I have also seen shielding in the form of a copper braid about the size of a round shoelace. Wouldn't know where to get it though. If you could get some, perhaps you could use the epoxy to glue that in place, and put more epoxy on that.. then it would look just like "the norm". (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[jhadler]

What frequency RF? If it's not too high, then you shouldn't have any worries to begin with. A metal seam of an enclosure is not, in itself, a wide open port for RF. The gap of the seam must be wide enough, as does the length need to be long enough to transmit the signal.

If it's an enclosure that needs periodic access, then epoxy wouldn't fit the bill anyway. Why not use RF shielding gasket material. It's like a metalized foam gasket. And there's lots more options too, like the Cu-Be finger seals etc.

A quick google search will reveal a HOST of suppliers.

Here's just one....

-Josh2

[Katmanken]

I usta work on a SATCOM Microwave satellite communication system.

We ran a flexible 112 waveguide out to the Satellite antenna and had a rectangular slot containing a compressible wire mesh type gasket between the flexible waveguide and the antenna. Worked great whenever I checked for stray radiation. Except one day I found one without a gasket (IMG:http://www.914world.com/bbs2/html/emoticons/blink.gif)

That system ran enough power to almost melt the end of an 18 inch by 18 inch by 3 ft long aluminum dummy load.

Put a mesh gasket in there, you will be good.

Ken

[Mueller]

QUOTE (kwales @ Dec 7 2005, 06:06 PM)

Put a mesh gasket in there, you will be good. Ken

(IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)

no alternative materials or solutions allowed....the process engineers (customer) do not want to have to re-certify the new material and process....

got a line on a better 3M person that should be able to do an analysis for us...we'll see....

[ppickerell]

Mike,
I go to RS Hughes in Sunnyvale for adhesive issues. They are pretty good stocking distributor for loctite, 3m etc. Good luck.

[Mueller]

QUOTE (ppickerell @ Dec 7 2005, 07:03 PM)

Mike, I go to RS Hughes in Sunnyvale for adhesive issues. They are pretty good stocking distributor for loctite, 3m etc. Good luck.

that is who I meet with...the directions for cleaning and refurb given to me call out Epoxy* multiple times ...the RS Hughes rep kept saying it was RTV sil. on the unit.......of course I say it is still epoxy which has broken down due to at least the heat and conditions.

*just not what kind or we'd be able to give a qoute by now....