EMI Shielding with Coated Plastics
- Author: William D. Kimmel
For shielding effectiveness, you need to keep openings in your enclosure less than 1/20 wavelength of the highest applicable frequency, typically to the tenth harmonic of the fastest clock. If you have a 100 MHz clock, you will be testing to 1 GHz, so expect to limit openings to about 2 cm - even less if you have higher clock frequencies. It has now reached the point where continuous closure is necessary - occasional contact is not sufficient. The second aspect is the increasing use of plastic enclosures. As we know, plastic provides no shielding unless provided with a conductive coating. The conductivity of the coating is not the driving factor in high frequency shielding effectiveness - it’s the longest dimension of the opening, which almost always occurs at the mating seams. So select your coating for criteria other than EMI - availability, cost, durability, ease of application, etc. Herein lies the problem - it’s difficult to get conductive closure at the seams, and the problem lies with the design of the mold. Done right, the shield works very well. Unfortunately, most of the molded plastics are poorly designed to contain EMI. Radiated emissions failure is almost a foregone conclusion, if the plastic enclosure is not properly designed.
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