EMI spray - experience?

[tkamiya]

I have been working on cases for my DIY projects.  Then thought occurred to me.

Many of commercial products, even mid to high-end ones, are using plastic.  Then they spray EMI stuff on the inside.  Then I looked at what they cost.  $25 a can?  Pretty pricey I thought.  Assuming heat conductivity isn't an issue, how good are they?  Has anyone used these for DIY projects?  Apparently, there are different composition for different purposes.  Which one did you use and how good was the end product/property?

[helius]

You may want to take a look at https://www.nordson.com/en/divisions/asymtek/your-process/applications-corner/electromagnetic-interference-shielding
for information about how spray coated EMI shielding is used industrially. They use robot spray and fluid application systems. With this kind of fine control it's possible to shield one section of a MCM while leaving others exposed for integrated antennas.

[DaJMasta]

TheSignalPath just put out a video where they use it to coat 3d printed parts to make custom designed waveguide bits, seemed to work alright.  Pricey, but at least the stuff he used contained silver for its conductive component, and getting metal flakes out of a spray nozzle takes a bit of doing - a can of good brand regular spray paint is probably 1/3 or 1/2 of that price anyways.

[Gyro]

I've used conductive spray on a consumer product before - really not my choice, the customer wanted in their 'house' smoked plastic case. It was a Copper- Nickel one.

Screening efficiency depends almost entirely on the electrical continuity between the case halves, panel flanges etc. It's very easy to create a 'slot antenna' if it's not a perfect fit.

Not that particular product (and mentioning no names  ) but it looks as if they still use it - http://www.hauppauge.com/site/press/presspictures/PVRUSB2_black_angle.jpg

[T3sl4co1l]

I haven't tried applying any paint -- but I have used boxes so prepared.  (The boxes are awfully expensive, too...)  They were... uh, hmm, this kind https://www.budind.com/pdf/hb1323.pdf but I don't see the EMI/RFI/shielded version available (anymore?), so that doesn't help my point very much...  Anyway, the resistance of the coating was quite reasonable, a couple ohms between any two points (DMM probes), including the gasket, which was a bit of a surprise (it has a chalky texture and powdered-brassy color, but it does indeed seal both RF and water!).  So, I would expect better than 40dB shielding for most purposes, which is quite reasonable.

So, if nothing else, you should be able to test your own coatings -- the resistance between points should be low, but even if it's not measuring well, that may just be a sign of having bad contact; try making contact with EMI springs, tape, or clamps?

Tim

[TheSteve]

I've made antenna's with it, so it certainly does work. I am sure you can use the lowest cost version without lots of silver/copper in it - none of it is cheap though.

[MagicSmoker]

As already mentioned, the key to success with this sort of thing is not creating inadvertent slot antennae from gaps in coverage or the interface between case halves.

You might want to try "cold galvanizing" spray paint on the enclosure first, both to get the process down and make some before/after EMI measurements. See, for example, this product which is about $6 a can in the US: https://www.homedepot.com/p/Rust-Oleum-Professional-20-oz-Flat-Gray-Cold-Galvanizing-Compound-Spray-7585838/100142963

Yes, zinc has a much higher bulk resistivity than copper or silver, but this hardly matters unless you are making a waveguide.