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EMC Basics #15: Solve leaky RF seam problems with gaskets

Posted: 24 May 2012 ?? ?Print Version ?Bookmark and Share

Keywords:EMI? shielding? gasket. seal?

As we continue with�our shielding theme, we will 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 300MHz, 1.5 cm at 1GHz.) 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 exploreeither improve the gaskets, or reduce the currentswe 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.

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