> Subject: Effective EMI coatings for steel, aluminum, plastic.
> Greetings to all viewers. I have been monitoring this forum for some time
> and would like to take this opportunity to ask a question concerning
> coatings
> or platings for steel, aluminum and plastic. Does anyone know of any
> studies
> that may have been performed comparing various coatings on the three
> mentioned
> substrates that considers cost, conductivity and/or shielding
> effectiveness.
> Apparently some of the tried and true materials are coming under attack for
> environmental friendliness and suitable alternatives must be identified.
>
> I am aware of the use of zinc chromates, electroless nickel and copper as
> well as various paints. Any experiences out there with practicality,
> durability, cost and environmental friendliness. Thanks in advance for
> your comments.
>
> -Regards, ------------------------------------------------
> | M.J. Sitarski, Environmental & Regulatory Compliance |
> |*********************************************************|
> ----- "Knowledge is Power" --------------------------------
Steel: Only worked once with it. Was not impressed.
Believe it had to do with the oxide layer that
forms. Using star washers to get around this
did not work well. The steel Cold Rolled C1010
happens to be harder than the material of the
star washer. Never used it again.
EMI issues: other people say it's great. Listen to
what they have to say. I've had people tell me the
high freq characteristics are terrible. I suppose
alot has to do with the carbon content of the steel.
Safety issues: Only compatibility of metals which
could be minor.
Cost: Depends upon the grade and treatment. Can be expensive.
Aluminum: Worked with it alot. Avoid anodizing processes
unless you have sound reasons not to avoid.
Anodizing by its very nature produces an oxide layer.
Untreated Al will produce an oxide layer.
Al 50-52-H32 is the alloy I used.
Alochrom 1000
Leaves a corrosion resistant finish that's conductive
provided there is good (not necessarily high) contact
pressure. So bolting gives good bonding. The finish
is prone to fingerprints. It appears Silver.
Alochrom 1200
Almost the same as the 1000. Less prone to fingerprints.
Used more for EMC specific applications. More expensive.
Looks gold.
EMI issues: Fairly good track record. E-field attenuation
is generally twice that of steel.
Safety issues: Only compatibility of metals which
could be minor.
Cost: Depends upon the alloy and treatment. Can be expensive.
Plastic: Order of increasing conductivity (ohms per square)
Nickel - Copper - Silver
Stay below 1.0 ohm per square. Multicoat if needed.
If you can reasonably stay below 0.5 ohm per square,
all the better.
There is what I call a water conductive paint and an
oil base (not really sure if that's correct) conductive
paint. The water base is more conductive by a factor of
2:1 over the oil.
Is not stable in places where there is alot of use,
i.e. shearing and mating of surfaces. Once you screw
down onto it, don't mess with it.
EMI issues: Lowest of the other two. E-field attenuation
completely dependent upon upon vendor application of
coating and wear of srufaces.
Safety issues: Here's the catch with conductive coated
plastic.
You need 4 approvals for your coated cover:
1. 94V-0 approved plastic, since it primary use is as a fire
enclosure.
2. Vendor approval to make the cover if the vendor has to
mix materials toget the type of plastic you need.
3. Approved conductive coatings.
4. Vendor approval to APPLY the coating.
Cost: Depends upon all of the above.
Hope this has helped.
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The comments and opinions stated herein are mine alone,
and do not reflect those of my employer.
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