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EMC Basics #14: Making plastic coatings work for EMI

[Editor's note : we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known experts Daryl Gerke and Bill Kimmel of Kimmel Gerke Associates. There are links to all previous entries in the EMC Basics series here.]

Two factors are 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 100-MHz 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 100-MHz clock, you will be testing to 1 GHz, 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.

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 1 .

 

Figure 1: a) Conductive coating brought up to mating surfaces;

b) EMI gasketing placed in groove.

However, we encounter strong resistance when discussing this with the mechanical designers. They want to mask off the coating back from the seam to ensure the coating doesn't show on the outside. The fact is, if the coating doesn't get into the seams and close the gaps, the shield will not work .

In fact, if you don't close the gaps, the measured emissions may well increase ! Why? The shield may well collimate the available RF energy, resulting in emission hot spots.

You also need to make provision to terminate cable shields and filters. In most cases, these need to be grounded firmly to the shield, as casual contact will not do. The worst case is to run the cable shield without terminating it to the enclosure shield: that's a guaranteed test failure.

Summary

Don't fool yourself. If you want the shield to work, design the enclosure to close the seams. Tongue and groove works well, preferably with EMI gasketing. Also make sure your cable shields and cable filters are well terminated to the coating.

Also relevant to this topic:

Debugging: The 9 Indispensible Rules for Finding Even the Most Elusive Software and Hardware Problems (Chapter 5, Part 3 of 3)(and see its preceding sections, which are linked within)

About the author

William Kimmel , an EMI/EMC consultant with Kimmel Gerke Associates since 1987, along with business partner, Daryl Gerke, focuses on design and troubleshooting (not test and regulations). He and Gerke have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still relevant and in demand. He can be reached via http://www.emiguru.com.


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