Non-conductive paint inside metal enclosure - Page 1
Forum section: electronics / beginners

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 05:18:17 am
content: HelloI want to put a circuit within a metal enclosure to minimise EMC issues. The thing is, both inside and out of the box are painted with a non-conductive paint.I actually thought this would be ok in order to contain any emissions. But now I read a book that said this is incorrect and that the inside of the enclosure must be conductive.Can someone confirm one way or otherwise (and explain why)? Thank you.

author: LabSpokane
author_rank: Super Contributor
timestamp: April 27, 2015, 06:28:52 am
content: All you need is a conductive patch on the enclosure to bond it to ground/common. Make sure that all pieces have the paint strategically removed so that all the parts have electrical contact. This is easy to verify by ohming it out.

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 06:46:04 am
content: Quote from: LabSpokane on April 27, 2015, 06:28:52 amOh, I see. So, if I can bond the enclosure to ground, that is enough in order to use the metal enclosure as a shield? Here's another question... is the shieldmoreeffective if the paint is not there, or is it the case that once it is bonded to ground at least somewhere, it will be fine?Thanks for this good news!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2015, 06:56:04 am
content: Do you have any cables or connectors entering or leaving the box?Tim

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 06:59:35 am
content: Quote from: T3sl4co1l on April 27, 2015, 06:56:04 amYes I do, USB only.

author: tggzzz
author_rank: Super Contributor
timestamp: April 27, 2015, 07:00:35 am
content: Quote from: LabSpokane on April 27, 2015, 06:28:52 amI would have thought that for EMC issues it was necessary to ensure that there were no electrically significantholes or slots. If you have a slot thenonetechnique is to cover the slot with copper tape, while ensuring the tape is electrically in contact all along its length. If the tape is only in contact at one point then it can act as an antenna and radiate at its resonant frequency. Seehttp://www.farnell.com/datasheets/1848360.pdfThe corollary is that a simply testing for an ohmic connection is not sufficient.It might be helpful for the OP to understand how microwave ovens ensure no leakage; some of the techniques are a little subtle. N.B. I am not an EMC/EMI expert.

author: Psi
author_rank: Super Contributor
timestamp: April 27, 2015, 07:09:23 am
content: The paint is relevant only from the standpoint of making an electrical connection to the case.You need to scrap the paint off at this point so it makes electrical contact with the screw,bolt or whatever your using to make the connection.

author: Ian.M
author_rank: Super Contributor
timestamp: April 27, 2015, 07:20:08 am
content: There will also be problems with the lid or access panel. If it doesn't make good electrical contact with the case all the way round, then the joint acts as a slot which as tggzzz pointed out, can be a significant problem. If the case wasn't designed with EMI shielding in mind, resolving this issue without ruining its appearance can be difficult.

author: LabSpokane
author_rank: Super Contributor
timestamp: April 27, 2015, 10:42:07 am
content: Quote from: pcblearner on April 27, 2015, 06:46:04 amThe paint is irrelevant RF absorption as long as the parts are all in good contact with one another. You might need to make sure the entire perimeter is scraped or sanded down to make full electrical contact as suggested. The reality is that the full perimeter of a lid won't be in full contact without some type of EMI gasket, so I wouldn't stress out over that too much. If you could post a picture of the box, the EMI experts can take a look. (Read: not me)Tim had a good point about cabling. That's where things get interesting.I read your question from the point of having a hobby project and wanting to knock down the EMI with a metal enclosure. If you're trying to submit for EMC compliance, that is a rabbit hole unto itself. And the reality is that most who submit designs for EMC compliance of any complexity don't pass on the first go. That's where you really need an expert to assist you at the design stage. There's just a lot of non-obvious stuff like small holes being better than narrow slots, interlocking features, etc.

author: Dago
author_rank: Frequent Contributor
timestamp: April 27, 2015, 11:06:29 am
content: If the case has multiple parts (like a lid or front/back panels) then the real problem is how to make good contact along the whole length of the lid. You can ground the lid from just one point but then it will be a very ineffective shield. It needs to have good contact along all the edges for good EMI protection.

author: pcblearner
author_rank: Contributor
timestamp: April 27, 2015, 11:32:51 am
content: Thank you for your help everyone. I definitely did not realise the importance of the join between top and bottom of the enclosure, so I will have t revisit that and possibly strip the paint at that point so that they bond nicely. Once it's all finished I will post photos!

author: VK5RC
author_rank: Unknown
timestamp: April 27, 2015, 12:37:44 pm
content: For the joint a good technique is "RF Finger Stock" looks like a strip of small springs that ensure good contact. For signals the typical technique is to use feed through capacitors, for USB it would have serious implications.I try to think of both radiated and conducted EMI.Disclaimer, NOT an EMC guru!!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2015, 03:10:08 pm
content: Quote from: pcblearner on April 27, 2015, 06:59:35 amAnd no other openings (e.g. LCD display?)? Then a solid enclosure, with contact to the USB connector shield, will do. As long as the circuit can put up with itself, anyway (there is a real "Dutch Oven" effect when you're dealing with high power switching, where the circuit interferes with its own operation, but I'm guessing it's not at that level here).You'll also want your circuit grounded to the USB shield. Other points probably won't matter, but it might still be useful for operational reasons to have multiple grounds. If you don't know where, it's probably not important.Paint doesn't matter, because what's important is that electric fields are positively terminated at the inside wall (the wall is conductive so it shunts any E-field going through air or paint -- check), and that magnetic fields are positively shunted by the wall (as long as it's conductive or permeable enough at the frequencies required, like steel or aluminum or copper, those will be blocked as well). As others have mentioned, you also need holes and slots kept to a minimum (a cover with a single-point ground acts as either a slot all the way around, or a 1/4 wave patch antenna at some frequency). If screws are providing contact, more is better than less; unless you've got microwaves inside the box, a screw in each corner is probably fine.Paint does matter around contact points, so obviously you need the screws or cover or shell able to form a complete ground. Occasionally you'll see an enclosure has been masked off, or the paint ground off, where contact is desired (i.e., under screws, or for a grounding lug, or..).There are also plastic enclosures lined with conductive paint or flame-spray metallization, which may be cheaper or lighter. The shielding is modest at lower frequencies, but if you're not trying to do induction heating inside, it's probably fine, too.Tim

author: pcblearner
author_rank: Contributor
timestamp: April 29, 2015, 01:07:19 pm
content: Quote from: T3sl4co1l on April 27, 2015, 03:10:08 pmExtremely helpful, thank you!