Metallic Ink Spray for Shielding! 3

[var10]

Hi,

I am trying to stop emi losses by upgrading the current design of our product. And I came across metallic ink that can be sprayed or just painted. Just wondering if this is better than conventional ways meaning using finger stock gaskets, conductive seals, conductive tapes etc? Curious to know what they are like if someone has used them? Are they very expensive?

 

[IRstuff]

A paint or ink is not going to completely seal a joint, so there's some risk of leakage

TTFN
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[var10]

EMI through lcd screens? Any one faced this kind of an issue before?

Sorry for giving a very bland question. I am still researching and talking to our electrical engineers.

 

[IRstuff]

Your EEs should already know about this. Indium Tin Oxide is one option. Vapor deposited gold mesh is another.

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[MikeHalloran]

I have been associated with several generations of medical electronics where various nontechnical leader types forced the use of plastics as electronic enclosures to save money. Every one of them was convinced that 'shielding paint' would take care of the electromagnetic radiation/ susceptibility problem all by itself. (This in products where the normal preamp output pulse range goes from 6 volts to 6 microvolts.)

Unfortunately, 'we' found out the hard way, several times in succession, that you can't effectively and durably ground paint, no matter how conductive the paint itself may be, and no matter how many thousands of dollars a gallon of paint might cost.

We always eventually more or less followed Apple's example, concealing nickel-plated steel inside the plastic, and grounding the steel. Apple was smart enough to design in the steel in the first iteration of the plastic molds; I could not sell even that small bit of prevention.





Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

The only other, viable alternative to steel, is copper foil, which we've used in several of our systems. Our fiberglass composite shells are internally taped completely with copper foil, and the sensor windows are covered with a gold mesh.

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[hendersdc]

If you are dead set on a plastic enclosure it may be easiest to shield emissions at the source, especially in the case of a large enclosure. Can you add board level shielding? Can you enclose the boards/power supplies/whatever in a small internal sheet metal enclosure?

Also, what frequencies are you emitting at? This will affect which EMI mitigation methods you use since allowable slot length is directly determined by the wavelength. In this case slot length is defined as distance between electrical contacts along a seam or size of openings. It may be distance between fingers on a gasket, size of punched holes on a sheet metal face, distance between fasteners, etc. General rule of thumb is max slot length of 1/20 of the wavelength for something like 30 dB attenuation (don't quote me on the dB value). Higher frequency -> shorter wavelength -> smaller acceptable slot length.

Doug

 

[bobgroh]

I'm afraid to say that you should really find some expert help in your EMI problem. This is not something that can be tackled effectively with a few simple forum exchanges. As others have indicated, the whole situation has to considered - the regulations (i.e. exactly what the EMI limits that your product is facing), considerations of the design of your product itself, who and how your product will be tested for compliance to whatever regulations there are,etc, etc. There are all kinds of considerations that need to be identified - the nature of the radiations (e.g. magnetic, electro-magnetic or ??).

To answer your question a bit more directly, spray on shielding will probably not do the job - the sprayed material is too thin for much effectiveness at even very high frequencies and, if the problem is very low frequencies, probably has no measureable effect.

Lot's more I could say but the bottom line is 'get some professional help'. Yes, it will cost the company something (I would guess less than $10K for a short investigation) but the costs of failing may be much higher than that! By the way, RF engineering is where I spent most of my career of over 50 years duration.

 

[var10]

Thanks everyone for your suggestions. I am presently working on various areas of the product to achieve emi shielding.

@IRstuf - I tried placing a Indium tin oxide film on the screen to stop emi. I bought a small piece of sample 0.15mm thick and stuck it over the touch screen (capacitance). Doesn't really work well for obvious reasons but it was worth a try. Still waiting on copper fingers.

Sorry for such delayed response.

 

[IRstuff]

Yeah, ITO needs to be custom deposited. Slapping on a slab just isn't going to work.

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[Tunalover]

Injection molded plastic suppliers provide polymers with conductive strands mixed in. It provides some attenuation but is only a partial solution I'm sure. It's cheap and easy to do though as it requires little design change.

Tunalover