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EMC Basics #14: How to make plastic housing work for EMI

Posted: 21 May 2012 ?? ?Print Version ?Bookmark and Share

Keywords:EMI? shielding? gasketing?

There are two factors combining to create EMI shielding problems: increasing clock frequencies and the shift to plastic housings. The trend to higher clock frequencies continues. Modern laptop and desktop computers run in the GHz range, a thousand times faster than the early personal computers, but even modest applications are running clocks in the 20- to 100MHz range. CISPR 22 calls for emission testing up to six GHz, depending on maximum clock frequency.

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 100MHz clock, you will be testing to 1GHz, so expect to limit openings to about 2 cm, and 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, such as availability, cost, durability, and ease of application.

Herein lies the problem: it's difficult to get conductive closure at the seams, and the problem is 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.

Figure: a) Conductive coating brought up to mating surfaces; b) EMI gasketing placed in groove.

How to make the shield work
You need to bring the conductive coating right up to the mating seams, then ensure the surfaces conductively mate pretty-much continuously along the entire seam. This requires the plastic enclosure be designed so as to facilitate proper coating.

A reliable method is to use tongue and groove, making sure the mating surfaces are stiff enough to ensure continuous contact. Better, yet, the groove can provide a "nesting" place for conductive EMI gasketing (figure).

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