EMI shielding a plastic DIY electronics box (Al foil)

[faekjarz]

Hey there, Enginerds and advanced Wannabees!  *waves*

I'm thinking about shielding this plastic project box (see photo below) against EMI (Electro Magnetic Interference) (in-?)egress.



It's supposed to become some kind of DIY router / pentesting & networking appliance / spare x86 machine, based on an Advantech MIO-5270D (AMD dual core T56N) + MIOe-220 triple Intel GbE + dual band Atheros wifi.

I discovered that the plastic case of my old laptop is coated with some kind of (coppery?) conductive layer (see photo below) …i guess that's for EMI shielding. (Is it?)



My idea (Plan A) involves glue sticks and Alumin(i)um foil …yes, the kitchen type.

Here are some Questions …or contribution opportunities 

1) I'm a n00b / layman / maker …so, is "EMI shielding" the correct term?
2) What's EMI shielding actually meant for; suppress in- or egress? (I'm sure some super fancy RF things [or quantum computers] need to be shielded against ingress …but isn't it about keeping those pesky EMIs in the box …to not affect other devices?)
3) Should that EMI shielding be grounded …physically connected to the GND of the enclosed electronics?

[Jeroen3]

1. Yes.
2. To prevent electromagnetic interference from leaving or entering the box.
3. Yes, thats what the conductive sponges are for.

I would not use aluminium foil. But I'd buy the aluminium version of the box. Or perhaps EMI paint. (copper paint)

[faekjarz]

…Yes…

Ohai, thank you very much! Are you the Jeroen (Domburg) from
?

Why wouldn't you use AL kitchen foil?

I mean, it's conductive and can be formed into any shape, even multiple layers, if you think that would be necessary. Please keep in mind, that i'm on a very tight - better than nothing type - budget.

…would AL kitchen foil be sufficient? …how many layers?

[Jeroen3]

No, that's not me.

Glued foil is not an elegant solution. I prefer elegant solutions. But foil will work if properly sealed on the edges.

Perhaps you can use a water balloon to force the foil into all the corners of the box?

[Benta]

Another option would be conductive graphite spray paint.

[hagster]

http://www.mgchemicals.com/products/emi-and-rfi-shielding/acrylic-conductive-coatings-original-series/super-shield-nickel-841

This stuff is the business for EM shielding of plastic cases.

[faekjarz]

…Glued foil is not an elegant solution… …Perhaps you can use a water balloon to force the foil into all the corners of the box?

I know it's not an elegant solution …oh wow, using a water balloon seems rather overkill …i could just "massage" it real good into all the corners …so, i see no real "dealbreaker" with AL foil, as long as i ground it properly. Thanks!

Another option would be conductive graphite spray paint.

I agree, but that would nuke my pensionist - better than nothing - budget.

http://www.mgchemicals.com/products/emi-and-rfi-shielding/acrylic-conductive-coatings-original-series/super-shield-nickel-841 This stuff is the business for EM shielding of plastic cases.

Aye, seems rather fancy for a tight pensionist budget …but thank you for mentioning it.


…any other comments or suggestions about reasons why (a single layer of) AL kitchen foil would totally fail? (How about dual or multiple layers?)

[Gregg]

Google copper tape with conductive adhesive for an easier to install solution.  Most of these tapes have a peel off cover on the adhesive side that makes it easier to cut and fit prior to actually sticking it in place.  I don't know how conductive the adhesive may be, but with sufficient overlap of joints it should provide the required EMI shielding.  The other advantage is that you can solder to it directly.

[bjcuizon]

How about double sided tape on the back of the al foil. It should stick pretty well.

[2N3055]

Thin steel (like one for cans of canned goods type) is solderable and will shield a bit of magnetics too...  It will be too thin for low frequencies, but should attenuate high frequency PC board EMI quite well...Also easy to cut with plain strong scissors... You should be able to get it very cheaply... For only electrostatic shielding Kontakt Chemie has graphite spray...
But for a PC (router) you don't need shielding at all for it, you want to shield rest of the world from it, and magnetic shielding is most important.. But you must make sure whole shield has tight connections, no openings, except for connections, every discontinuity might become aperture antenna... Also it depends how noisy is PC motherboard by itself...

[cdev]

A fire would be even worse for your budget!

Watch out for aluminum foil.. it will short out your equipment.

Copper tape - is a different story. With a rigid plastic box it will definitely be doable. Who cares what it looks like on the inside, people wont see the inside.  But.. okay, suppose you are entering the "pretty electronics contest".

You might be able to get the tape into the corner fairly closely by folding it to exactly the right size before removing the coating over the adhesive and being careful not to trap air behind it pushing it in there with something thats the appropriate shape..(I have a approximately 25 mm wise plastic drywall tool I use). You'll only be able to push it in in a manner to avoid sticking it to the perpendicular side wall by leaving half of the adhesive on until the first side your'e covering is properly stuck on. Then remove the rest of the backing and with a smooth motion as if you are brushing butter onto toast with a strange square knife stick the other side down.  Copper tape is less forgiving than many other kinds of tape. It will be hard to remove it without tearing it once it sticks to something.

Also, dont use aluminum foil or aluminum foil tape, the cheap aluminum foil tape isnt just bad for shielding, it disintegrates into something which could only be described as aluminum foil DUST. Which is around the worst thing you would ever want near your electronics.

OTOH, copper EMI shielding tape is great stuff. Its also good for repairing PCB traces.

Its cheap, if you shop around.  So cheap I wonder how they could possibly be making any money on it. My last roll was wide and I got a fairly big roll of it for under $5 including shipping.  The prices vary a real lot, so plug in the length and width you want and search on that and copper.

Quote from: faekjarz on Today at 11:03:50>Quote from: Jeroen3 on Today at 09:06:35

…Glued foil is not an elegant solution… …Perhaps you can use a water balloon to force the foil into all the corners of the box?

I know it's not an elegant solution …oh wow, using a water balloon seems rather overkill …i could just "massage" it real good into all the corners …so, i see no real "dealbreaker" with AL foil, as long as i ground it properly. Thanks!
Quote from: Benta on Today at 09:07:43

Another option would be conductive graphite spray paint.

I agree, but that would nuke my pensionist - better than nothing - budget.
Quote from: hagster on Today at 09:16:44>http://www.mgchemicals.com/products/emi-and-rfi-shielding/acrylic-conductive-coatings-original-series/super-shield-nickel-841 This stuff is the business for EM shielding of plastic cases.
Aye, seems rather fancy for a tight pensionist budget …but thank you for mentioning it.


…any other comments or suggestions about reasons why (a single layer of) AL kitchen foil would totally fail? (How about dual or multiple layers?)

[faekjarz]

Google copper tape with conductive adhesive for an easier to install solution.

I'd rather "duck" (ddg.gg) those key words
…but yeah, your're right, i've thought of Cu.
But i'd like to work with what i already have. Which is Ally kitchen foil and that good ole glue stick.

How about double sided tape on the back of the al foil. It should stick pretty well.

Aye, good idea, but i've got to work with what i already have …which is glue stick and Ally foil.

I use copper foil tape for that now. Its not that expensive and its works well. With cardboard boxes/copper tape you can even solder to it (usually works best if the soldered to area is not large- also, be quick). Ive never used copper tape on a plastic box but it likely works just as well. Just if you solder to that tape, you likely need to be even faster than with cardboard or it will melt. Cardboard can be used to make a surprisingly nice case.

Haha yeah! Actually that SBC has been used in a "Cardbox Router", for months without issues, without any EMI shielding, plain cardboard, worked well too. I just thought, being nice, and having EMI shielding, would be ...well... NICE.

…soldering(very quickly) in a few places to anchor the copper sides together. This would be easy to screw up if you apply heat for too long… …Burnt plastic will make your device smell. You really don't want that.

Yup, It's not only the smell of burnt plastic. But those melted seams could easily be sanded down and painted over, on the outside. I'm not worried about melting plastic.


…concluding, for now: no "real"(?) arguments against using AL kitchen foil, as long as it's properly connected / grounded to the enclosed board(s). No comments about required amount of AL kitchen foil layers for "proper" shielding ...do i hear any comments? ...vetos? ...3, 2, 1, LOL, ?

[SeanB]

Use a thick aluminium foil, just because it will hold shape better and not be as easy to tear. You can solder aluminium, just need to place a square of FR4 board behind the spot you are using, to keep the plastic from melting too fast, and then use some gel flux on the top side, and scrape with a soldering iron tip through the flux till you tin a section of the exposed bare aluminium metal, then put the wire on the solder blob and solder it.

[cdev]

as somebody who has tried all that I can tell you it wont work well. Sure it conducts electricity, but it also comes apart unpredictably. The only way aluminum foil works in shielding is in things that arent going to become stressed in any manner or short term.

for quick testing to see if shielding helps and then only on the outside of the plastic case.
take an LNA thats inline on a piece of coax, would shielding help?

 twisting of a square of aluminum foil around some device still in its plastic case with strong clothes pins to anchor the foil over the RF connectors. Great, it works, lets shield the thing. Then you throw your crumpled up foil away. Another RF application its great for is waveguide atennas. You can turn a cardboard appliance box into a horn to receive RF from space - a great science fair project that mimics the first radio telescopes. Aluminum foil works great in that setting. And many others.
But dont use it inside electronics. because little pieces come off.

The absolute worst thing you could use is cheap aluminum tape which literally disintegrates over a fairly short time frame.

You wont be saving anything either as EMI shielding tape from Chinese ebay sellers is really cheap, and actually works. The adhesive sticks, it works as advertised..

Especially think about the likelihood of causing shorts. little pieces of disintegrating aluminum and electronics don't mix.

[T3sl4co1l]

http://www.mgchemicals.com/products/emi-and-rfi-shielding/acrylic-conductive-coatings-original-series/super-shield-nickel-841

This stuff is the business for EM shielding of plastic cases.

That, or you can get boxes already coated with it.  I've used a Hoffman QLine (IP67) box like this.  The coating looks like metal flake paint, and the resistance between most any points inside the enclosure is just a couple ohms.  So the attenuation looking in from the outside, through a metal housing connector bolted into the box, is quite good (>60dB?).[1]

Keep in mind that exactly this is necessary.  The metallic connectors, that is.  If you aren't shunting outside noise, conducted along cables (similarly shielded), into and over the shielding, then you aren't doing much of anything with the shielded box.  Or if your signals are unshielded but low bandwidth, so you can filter away the noise, then the EMI filter needs to be referenced to the shield, not necessarily circuit ground as such.

If you have good contact to the shield-ground, on board (such as by having many screw bosses, or using EMI spring tabs), then you can use that for the EMI filter or shield-to-ground connection, without too much lost performance.  (Maybe you'll get 40 instead of 60dB attenuation, but maybe that's plenty for the application.)  As a bonus, you then have tons of low impedance grounds around your board, which means you don't need to play any ground-splitting tricks whatsoever -- a single ground plane will do nicely.

If you still need separation between board-ground and shield-ground, then you have the perfect opportunity to add ferrite beads on the signals, improving CMRR even further.  Do this cautiously, of course.  Don't make CM-to-diff (mode) conversion happen.  (Example: you have shielded cables, coming in on metallic connectors, bolted into the box.  That's your main shield scheme.  The connectors, inside, have wiring harnesses that carry the signals from the bulkhead up to the board.  These cables may exhibit mode conversion, because they aren't shielded, or at least the signals aren't paired, symmetrically, with ground, and enough ground lines, for that matter.  In such a case, ferrite beads help, by making the ground-loop voltage drop more even across all wires in the enclosed cable.  Even better, is to continue the shielded cable on the inside as well, so that the shield is grounded in two places: the bulkhead and the PCB.  A ferrite bead around that will have little effect on mode conversion -- because the shield is already quite good on CMRR -- but can further increase CMRR or immunity, especially against endogenous noise signals: that is, unbalanced emission within the enclosure itself, that can interfere with itself.)

And yes, foil is perfectly acceptable.  Note that foil tape needs to overlap properly; not all foil tapes have conductive adhesive.  It can help to curl the edge over, so it makes contact at least at a few points.

[1] Did you notice it's an IP67 (water tight) box?  How does that work?  Surely the cover won't connect.  Aha, they thought of that: the o-ring is also coated, or made of some sort of conductive material anyway.  I'm not entirely sure what material it is, but it seems to do the business!

Also, I'm not recommending QLine boxes, specifically -- they work, but they're rather expensive, and it looks like there are cheaper equivalent options out there.

Tim

[SeanB]

Oring is made of carbon loaded nitrile rubber, the same as used in Zebra stripe connectors as the conductor, or on others it is regular nitrile rubber with added silver flakes, to make it conductive. Another method is to extrude the oring around a nickel coated copper or brass spring, so that the spring coils are just flush with the rubber, making multiple contact points that will deform when closing the box providing the contact pressure needed. Others use a woven tinned copper mesh pulled tight on the rubber elastomer, for larger joints that need a good RF seal, but less issues with being IP68 rating, just a lot easier IP44 or IP55 rating.

[faekjarz]

Use a thick aluminium foil, just because it will hold shape better and not be as easy to tear. You can solder aluminium, just need to place a square of FR4 board behind the spot you are using, to keep the plastic from melting too fast, and then use some gel flux on the top side, and scrape with a soldering iron tip through the flux till you tin a section of the exposed bare aluminium metal, then put the wire on the solder blob and solder it.

Why would you solder the AL foil?
Considering, it's basically foldable like a sheet of paper. That SBC is of the EBX(if i recall correctly) form factor. However the box is 12x20x7.5cm, that should easily be covered by only one layer of kitchen type Ally foil - without tearing or seams - like a monolithic (bath) tub.

as somebody who has tried all that I can tell you it wont work well. The only way aluminum foil works a shielding is for quick testing to see if shielding helps and then only on the outside of the plastic case. Think "Big Bambu" type…

I'm not familiar with "Big Bambu (type)" a ddg.gg search seems to only result in some kind of musical / artsy type links …sorry i can't do that art shizzle (and no, i'm not sorry about calling art SHIT …it's just too frikkin much meta-meta-funky-meta-frikkin-meta *burps* …no offence (seriously), i just can't cope with that …not my culture)

http://www.mgchemicals.com/products/emi-and-rfi-shielding/acrylic-conductive-coatings-original-series/super-shield-nickel-841

This stuff is the business for EM shielding of plastic cases.

That, or you can get boxes already coated with it.  I've used a Hoffman QLine (IP67) box like this.  The coating looks like metal flake paint, and the resistance between most any points inside the enclosure is just a couple ohms.  So the attenuation looking in from the outside, through a metal housing connector bolted into the box, is quite good (>60dB?).[1]

Keep in mind that exactly this is necessary.  The metallic connectors, that is.  If you aren't shunting outside noise, conducted along cables (similarly shielded), into and over the shielding, then you aren't doing much of anything with the shielded box.  Or if your signals are unshielded but low bandwidth, so you can filter away the noise, then the EMI filter needs to be referenced to the shield, not necessarily circuit ground as such.

If you have good contact to the shield-ground, on board (such as by having many screw bosses, or using EMI spring tabs), then you can use that for the EMI filter or shield-to-ground connection, without too much lost performance.  (Maybe you'll get 40 instead of 60dB attenuation, but maybe that's plenty for the application.)  As a bonus, you then have tons of low impedance grounds around your board, which means you don't need to play any ground-splitting tricks whatsoever -- a single ground plane will do nicely.

If you still need separation between board-ground and shield-ground, then you have the perfect opportunity to add ferrite beads on the signals, improving CMRR even further.  Do this cautiously, of course.  Don't make CM-to-diff (mode) conversion happen.  (Example: you have shielded cables, coming in on metallic connectors, bolted into the box.  That's your main shield scheme.  The connectors, inside, have wiring harnesses that carry the signals from the bulkhead up to the board.  These cables may exhibit mode conversion, because they aren't shielded, or at least the signals aren't paired, symmetrically, with ground, and enough ground lines, for that matter.  In such a case, ferrite beads help, by making the ground-loop voltage drop more even across all wires in the enclosed cable.  Even better, is to continue the shielded cable on the inside as well, so that the shield is grounded in two places: the bulkhead and the PCB.  A ferrite bead around that will have little effect on mode conversion -- because the shield is already quite good on CMRR -- but can further increase CMRR or immunity, especially against endogenous noise signals: that is, unbalanced emission within the enclosure itself, that can interfere with itself.)

And yes, foil is perfectly acceptable.  Note that foil tape needs to overlap properly; not all foil tapes have conductive adhesive.  It can help to curl the edge over, so it makes contact at least at a few points.

[1] Did you notice it's an IP67 (water tight) box?  How does that work?  Surely the cover won't connect.  Aha, they thought of that: the o-ring is also coated, or made of some sort of conductive material anyway.  I'm not entirely sure what material it is, but it seems to do the business!

Also, I'm not recommending QLine boxes, specifically -- they work, but they're rather expensive, and it looks like there are cheaper equivalent options out there.

Tim

Hello there! Here we have a comment, which looks like composed by someone who knows his stuff. Yes and no; I'm talking about foil, yes, but i'm talking about KITCHEN TYPE AL FOIL ...


...all right, i just measured it, it's 30cm wide, and almost as long as one can possibly imagine.... THAT should be perfectly fine to cover the whole bottom half of the box, without seams… …the lid will be screw-clamped down onto the body. Thank you very much. (no sarcasm, just booze) Please keep in mind, that i intend to achieve a "good-enough" solution, a.k.a. "better-than-nothing". 

…here, i attached some hopefully self-explanatory photos, measuring resistance of a sheet of kitchen type AL foil, layers of kitchen AL foil (gravity only), and connector grounds to PCB GND (that mounting post thing), Rprobe included, almost the same ...she'll be all right, right? 

…wow, you're really going to town! So much technical details and non-hobbyist production insights! Thank you! Keep those juices flowing!

[T3sl4co1l]

…here, i attached some hopefully self-explanatory photos, measuring resistance of a sheet of kitchen type AL foil, layers of kitchen AL foil (gravity only), and connector grounds to PCB GND (that mounting post thing), Rprobe included, almost the same ...she'll be all right, right? 

Entirely lead and contact resistance.  You're down in the milliohms, at best, there!

Tim

[eugenenine]

How about double sided tape on the back of the al foil. It should stick pretty well.

You can buy aluminum tape
https://www.lowes.com/pd/3M-1-88-in-W-x-150-ft-L-HVAC-Tape/4009603

its in a roll like the typical fabric duct tape but thick solid like aluminum foil.

[cdev]

Could you describe the problematic interference (I'm assuming thats the problem?) that you're currently getting that you want to address?

[faekjarz]

Could you describe the problematic interference (I'm assuming thats the problem?) that you're currently getting that you want to address?

I don't want to address any particular / current problem. I had this board running for months in a cardboard box. I'm not trying to solve any problems. Afaik, there are / were no EMI issues. All i want is to be an at least somewhat nice participant in the EM spectrum. …just trying to be nice …and maybe learn a few things about EMI along the way.