[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.]
Continuing with our shielding theme, we'll look at EMI gaskets. As discussed previously, seams and other openings can be a "weak link" in EMI shielding. Two key concerns are gasket choice and gasket mounting. We'll look at the former now, and save gasket mounting for a future article.
Years ago, EMI gaskets were widely used in military systems and radio communications equipment, but rarely seen in commercial equipment. Thanks to increasing processor speeds and increasing EMI threats, gaskets are now common in a wide range of electronic equipment. As a rule of thumb, we recommend gaskets whenever shielding needs exceed 60 dB, although gaskets can still help at lower levels.
We regularly encounter EMI problems due to poor shielding and gasketing. While you might be tempted to leave EMI gasket choices solely to the mechanical engineers, don't do that. You need to work with your mechanical colleagues. Like many EMI problems, both disciplines need to be involved. Fortunately, the solutions are usually simple once you understand some basic principles.
How gaskets work
EMI gaskets perform their magic by providing a conductive path across seams and other discontinuities in an electronic enclosure. This “shorts out” any potential difference across the shield surface while maintaining smooth current flow. As a mechanical analogy, gaskets simply plug the leaks.
In a perfect EMI shield (a “Faraday cage”), the EMI currents induced on the shield remain inside (or outside) the shield. In the real world, however, seams or other joints present a discontinuity. Shield currents are diverted, and a voltage appears across the seam or joint. As a result, time-varying voltages and currents can launch an electromagnetic wave, just like a wire antenna.
In fact, seams in shields are often modeled as “slot antennas.” The only difference between a wire antenna and a slot antenna is that a wire antenna is metal surrounded by space, and a slot antenna is space surrounded by metal. That means even a thin slot (such as two metal surfaces separated by paint) can radiate if it is long enough.
A critical parameter is length, not thickness. Most of us in the EMI business worry when slots are longer than 1/20 wavelength (e.g. 5 cm at 300 MHz, 1.5 cm at 1 GHz.) The secret to success is to minimize impedance across the joint with clean, continuous metal-to-metal contact.
An alternate would be to reduce the current flowing in the shield, but this usually isn’t practical. However, don’t overlook this. We once had a case where hundreds of amps of high-frequency power-return currents were flowing in the cabinet. That case gave us two options to explore -- either improve the gaskets, or reduce the currents -- we ended up choosing the latter with good success.
Types of Gaskets
There are several types of popular EMI gaskets. All will work well when properly installed, so the choice is often usually based on overall mechanical issues. Here are some pros and cons on different EMI gaskets:
•Beryllium-Copper: These gaskets provide very-high EMI performance. The material has high conductivity and is very springy, which makes it ideal for doors and panels. The material can be formed into many shapes, such as fingerstock, serrations, and spirals, and can be plated for corrosion protection. The drawbacks are cost, mechanical vulnerability (such as snagging of fingerstock), and the lack of an environmental seal.
•Conductive elastomers: These gaskets provide good performance. They often have metallic particles or wires embedded in them, so they do require pressure to assure an EMI seal. A big advantage is that they can also provide an environmental seal as well as an RF seal. The main drawback is the compression force, but hollow elastomer gaskets often overcome this issue.
•Wire mesh: Like fingerstock, these gaskets can provide very high levels of EMI performance. A drawback is that many mesh gaskets take a set, and thus can not be reused. Those are fine for permanent seals, but are not suitable for doors or access panels. They also lack an environmental seal.
•Conductive cloth over foam: These gaskets are very popular in commercial applications, and are quite cost effective. Most use a silver-plated cloth over foam to create a soft gasket that can take up a lot of mechanical slack. The major drawback is a lack of an environmental seal.
•Conductive epoxies: these are permanent gaskets, formed from a metal impregnated caulk. Silver loading is very common, and the seal is usually also watertight. The major drawback is that any repaired joint must be cleaned and recaulked to maintain a seal.
•Form-in-place gaskets: These gaskets often make sense for high volume applications that can automate the creation of a gasket right on an enclosure. These are similar to conductive elastomers. When cured, these gaskets are resilient and thus may be reusable. The major drawback is that they are not practical for low volumes.
No discussion of gaskets would be complete without a few comments on corrosion. Even a small amount of corrosion can render a good gasket ineffective. Equipment used in harsh environments, such as medical, military, vehicular, or industrial are often subject to corrosion. Fortunately, corrosion is not usually a concern for commercial products.)
To fight corrosion, plated gaskets can be used to minimize the effects of dissimilar metals. Another option is to seal out moisture at the gasket interface. Hybrid gaskets are available that incorporate both an environmental seal and an EMI seal. In those cases, be sure to install the environmental seal to the outside to protect the EMI gasket from external moisture.
In conclusion, consider gaskets in your shield designs, particularly if you need high levels of shielding. And work with your mechanical colleagues to assure the best choice.
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
Daryl Gerke, an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel 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 or his other blog at http://www.jumptoconsulting.com/.