Eric, I'll chip in my two cents here, having had some experiences with conductive coatings. I've seen emission problems from products with both nickel coatings and copper. Problems have been due to incomplete coverage, failure to fill details, and unauthorized vendor mechanical changes.
You can build testers for quick estimates how effective a coating is. My favorite is two, one-turn, shielded loops with adjustable spacing. While magnetic coupling is perhaps an unfair test for chassis shielding, I feel it gives a good handle on how well a coating will prevent nearby current-carrying conductors from generating external fields. (I've been able to follow a computer's clock lines through the bottom of a chassis by using a small -- 2mm -- loop; a result which rather startled the people who saw it done.) I generally "calibrate" a two-loop probe at a standard distance, say, an inch or two cm, setting air as zero, and galvanized iron as perfect. Because the coupling is rather localized, this will usually find small defects in coverage which multiple resistance tests might miss. The failures have mostly been related to quality of the coating, not necessarily its material, however,some materials require different techniques. Nickel has a relatively high resistivity and must be quite thick for good shielding. Copper can be MUCH thinner, but is, as a result, more prone to failure to fill in chassis details than nickel, where multiple passes are more likely to cover everything. I had to press a vendor to run three or four sprayer passes for a copper coating which replaced a nickel one, and it took a good deal of cajoling to make it happen. However, once this was done, performance was better even that a nickel one meeting emission requirements in use. You will need to coordinate among mechanical, electrical and QC departments when using coatings. Too may details in the casting will result in a hard-to-cover enclosure, and you will need to know here the electrical design expects ground pits so these areas can be treated with special care. All these are possible,but (as with most other things in our business) they are most often done in isolation, which makes our job more difficult! ====================== Original Message Follows ==================== >> Date: 24-Feb-98 13:08:27 MsgID: 1058-10464 ToID: 72146,373 From: Eric Henning >INTERNET:[email protected] Subj: conductive coatings Chrg: $0.00 Imp: Norm Sens: Std Receipt: No Parts: 1 anybody have any opinions on conductive coatings on plastic for rfi/emi protection? we've been very successful at designing our products to be quiet and immune even in plastic boxes but i've been asked to look into conductive coatings for further protection. i've ordered some spray/brush on coatings but is it a waste of time? I was hoping to find spray can samples but haven't, anybody know of spray bomb coatings? also looking into vacuum application. I guess i'd like to try spray/brush stuff first just to see if it makes any difference. thanks eric [email protected] ====================== End of Original Message =====================
