how to bond RF fingers to steel chassis? or other methods to seal chassis  - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 01:33:52 am
content: Whats a good way to bond RF fingers (copper gasket) to a steel chassis? I welded/cut a steel chassis, and the top cover is going to mate on a prism made of angle irons, but I don't know how to attach the RF fingers to stuff.Would you use conductive epoxy? the thermal mass is ridiclous and trying to bond thin copper to heavy steel is going to be a major PITA. I thought about bonding nichrome wire to it so it can be carefully controlled to reach solder temperature maybe.How do they do this stuff (everything I took apart is usually in RF cans but we are talking big air cooled inductors for a ISN).its like going to be a cube thats 2.5 feet x 14 x 10 inches, with the top cover going over the whole thing.Does anyone sell really heavy duty fingers that can survive a gas torch for brazing? I have a feeling the flame would destroy their gasket properties/springynessMy idea was to use silver epoxy on the finger strip, and braze a copper strip over the angle iron and clean it good, so the fingers mate with the copper. I figure the finger strip should be on the angle iron since its stiffer then the cover. I have a feeling adding solder to these big steel planes is going to be a frustrating disaster.

author: profdc9
author_rank: Frequent Contributor
timestamp: October 07, 2019, 01:46:48 am
content: How about using pop rivets? I think some fingers even come with holes already in the strips.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 01:51:37 am
content: Quote from: profdc9 on October 07, 2019, 01:46:48 amWon't the chassis impedance be garbage? Not to mention be unstable over time. I mean maybe if it was some kinda nice stainless steel thing made in a factory, but this is literally made from garbage from the 50's. Panel beating, gas welding, grinding.. I doubt the chassis will be very stable mechanically.

author: profdc9
author_rank: Frequent Contributor
timestamp: October 07, 2019, 01:58:26 am
content: I think it depends on the frequencies you are trying to shield and how densely the rivets are placed. There probably should be something like 10 rivets per wavelength I would guess to have good uniformity and shielding properties. Perhaps you could use copper tape in between the rivets to close the gaps.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 07, 2019, 02:03:10 am
content: Quote from: coppercone2 on October 07, 2019, 01:51:37 amMost of the big RF stuff around uses either passivated mild steel or heavy aluminium, with strips of fingers attached with screws.As the covered surface area is fairly large, the shielding connection is usually OK.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 02:17:40 am
content: Do you just do a mask for paint and leave bare steel under the gasket if you intend to rivet it? My intention is to give it an 'automotive' paint job.Are there special coatings that can be used on mild steel? Does anyone see a problem with silver epoxy other then cost?what type of screw/rivet should be used to bond copper to mild steel electrically without corrosion issues?Maybe both rivets and epoxy to make it super durable?For passivation, is it possible to do on small areas of chassis with some kind of form (kind of like drilling ceramic tiles using the play-dough trick in your bathroom when you mount stuff to the floor if you ever do that). It's a big chassis and making tanks to do all this in, would be a bastard.

author: profdc9
author_rank: Frequent Contributor
timestamp: October 07, 2019, 02:31:54 am
content: The main problem I see with silver epoxy is that in my experience, it is rather brittle, and you may find the epoxy cracks and the gasket comes off. I am not sure if the silver in the epoxy wouldn't cause galvanic corrosion either.A possibility is that after you rivet or screw the strip, you put a varnish coat over the joint to seal out water. A rivet would have the advantage I think of making a crimp-like connection that you could then seal using a varnish spray. Of course the joint probably has to be pretty clean so that the varnish has good adhesion.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 02:33:01 am
content: Quote from: profdc9 on October 07, 2019, 02:31:54 amIt might be easier for me to make the inner skeleton structure really stiff ( I did not build the interior frame yet) so there is little flexing of the epoxy.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 02:34:11 am
content: How are finger gaskets tested anyway? Do you do the same test as safety where you get like a 30A constant current source and measure the voltage drop (for mains earth safety). Leave a few attachment points on the chassis that can get a modified bolt screwed into? And just record values and test it long time after to see how stable it is after you tighten it up?I was also thinking of using the deoxit brand greases (not the spray, they sell actual grease). maybe combine the test with some mallet tapping?I was going to braze stainless nuts to things to act as attachment points for bolts (since I can't do pems).

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 02:39:53 am
content: I will try plane soldering on some test plates with bottom heating to see if I can bond things without too much heat damage.. if both pieces are pre-covered in solder it might work, and it should still be electrically better then all the alternatives I think, even if its bootleg soldering. Maybe NASA can do better... (I wonder if they would just laser weld a special gasket on with lots of holes in it to increase weld area, since lasers don't really have penetration like solder).Or they might just use nanofoil?

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 02:44:31 am
content: can you put a small microwave antenna inside of a chassis operating at high frequencies (I can do 1W 18GHz) to test for 'hole' development? (just from a prospective of cavity resonators). When I did probes around here, I don't really have much energy in high microwave areas.Would need to fix the amplifier though.the idea would be to try to irradiate the interior of the chassis some how (maybe even hook it into a waveguide??) and measure the outside while its not plugged in with a horn.

author: profdc9
author_rank: Frequent Contributor
timestamp: October 07, 2019, 03:05:32 am
content: So for a research paper I wrotehttps://www.osapublishing.org/josaa/abstract.cfm?uri=josaa-34-4-472(Sorry for the paywall, but I have a picture of the structure below()I was able to get a really good seal by machining a "double walled" structure at 17-26 GHz. The cavity antenna is a machined aluminum box with a wide lip that has two edges on the inside and outside of the lip and a recess between them. The inside lip was about 0.5 mm lower than the outside lip. Into this recessed were periodically drilled and tapped holes. A PCB was placed over the holes and screwed down over the box to as the front surface. When the PCB flexed, it pushed against the inside lip, sealing the cavity shut. This structure was extremely effective at preventing RF leakage.To test it, we use a Keysight VNA that scans the radiation pattern of the antenna by moving an open ended waveguide in front of the antenna using a gantry and recording S21. This can be used to backpropagate and reconstruct the field at the plane of the antenna from which we can see where on the surface the antenna is radiating. There was pretty much no radiation from the edge, all of the detectable radiation was from the slots milled into the PCB.If you have some of field strength meter you might be able to do something similar, for example, based on Analog Devices / Hittite Microwave HMC948 (https://www.analog.com/en/products/hmc948.html) .But if you are really trying to achieve extremely low leakage, a double-walled structure might be needed.

author: coppercone2
author_rank: Super Contributor
timestamp: October 07, 2019, 04:23:06 am
content: damn that is too complicated for me. Right now my plan is to use high silver braze (55%) to cover the steel with a thin layer that is sanded flat, then cover it with more conventional metal solder, then put a solder covered gasket on top of that., and try to 'reflow' it. The harris brand 'steel' low temperature solder is actually pretty good, I managed to use a regular 65 watt iron to make a metal box with something like 3/32 inch steel!.. the flux was hell on the hakko though.. but it does work.. maybe it was even 1/8th I am not sure (its about 7 inches x 2 inches x 2 inches), 'hermetically' sealed I suppose, other then the RF connectors. Maybe if I silver plate it with braze I can use electrical solder...I figure maybe if I silver everything, and use the fingers, I can put a flexible rubber RF gasket ahead of the finger gasket... along with some greaseyour work requires skills/equipment I don't have.. I think I can make that, if its the size of a matchbox (or maybe altoids tin if I am feeling really ambitious)... If I am to machine things with a mill anywaysomething like thishttps://www.harrisproductsgroup.com/en/Products/Alloys/Soldering/Lead-Free-Solders/Stay-Brite-Kit.aspxalso the one I have smelled rather foul, if you are to solder this with a iron, use one of the nasty automotive ones and do it out doors. it smells funny and makes you feel sick/poisoned (at least mentally).Actually, solder would have been perfect for this project, but I am trying to work on welding skills and thought I might actually do some kind of magnetic field tests on this thing with helmholtz style coils.. beware though, steel is EASY. If you try to solder aluminum, even with their brand name aluminum solder, you will be fucking PISSED OFF. it smells even worse and barely works (though it will still work better then evil aluminum braze, its actually way better then aluminum braze, but I still find welding aluminum with the fucking oxy torch is easier then brazing or soldering it!!!!!!and it smells WAY better. ). Holy Jesus when I am working with brazing or soldering aluminum I am liable to grab the fucking work piece with out gloves and hurl it at the wall when its half molten. I can't stress enough how god damn infuriating it is... you will end up in the ward...........................the only thing nice about brazing aluminum is if you use thin flat bar that you can cut with bolt cutters and have a bench sander, you can put things together EXTREMELY fast on some firebricks. It cools fast, so long you cover both sides with braze and then stick it together by poking it with pliers on lap joints, you can make complicated things that are not too strong (very powerful if you have a vise and hammer nearby to make shapes out of flat bar).. I can see some quick stands being put together this way, but they will look horrid.. however its probably better (and faster) then lashing things together with wire etc.. ultra lazy man shit.. however do not expect to make nice boxes with this stuff easily.. small joints tend to crack during aggressive sanding etc.. working with this stuff makes me insane.. unless its a big lap joint its going to be brittle, I cut the flat bar with bolt cutters then even it on a belt sander and strip it on a sander (you hold it against the top of the sander where the belt goes over in a U shape, being careful not to fling stuff) to clean it up right before brazing.. don't worry its like working with hot glue (about as strong too!!!) you need a gentle hand and quick reflexes if you do this kind of unholy activity with belt sander btwalso its messy as hell to do this with a sander, you need goggles and you get aluminum in your hair etc

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 07, 2019, 06:23:52 am
content: Leave the steel bare. Or if you must plate, do it for corrosion resistance (tin, zinc, nickel?). Preferably use plated steel, though I guess that would be a bit annoying if it's welded (it doesn't need to be, but if that's convenient for fabrication, sure).There are some supposedly conductive paints for galvanizing or shielding, but test them for resistance (heck, on plastic even) before committing to their use.Fingers can be had with a slip-on or clip-on function, which may or may not need a specially shaped edge. Otherwise, there is strip that sticks on with double-stick tape (provided with the strip); the tape isn't conductive, but the tape, and the spring pressure, holds a ridge against the surface.tl;dr read the manufacturer's application notes for ideas.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2019, 05:04:23 am
content: this weld job makes me feel like im in a j.g. ballard novelanvil please