Edited by Martha K. Raymond
It is no secret that electronic equipment is becoming ubiquitous, smaller, and ever more closely packed. Digital circuitry operates at increasingly high clock rates. It is increasingly difficult to keep the resulting electromagnetic radiation in its place. A complicating factor is that as digital circuits supersede analog, the type of radiation changes from primarily electric-field mode to mostly magnetic-field mode. Magnetic-field mode energy is less easily reflected by a conductive surface than electric-field mode, making shielding more difficult.
Shielding is not the only answer, but often it is the most cost effective one. Cabinets that incorporate shielding keep internally generated radiation from disrupting other equipment and can keep out externally generated EMI that might otherwise garble signals within the enclosure.
A typical enclosure falls short of the ideal Faraday cage primarily around door edges and through cut outs for handles, latches, hinges, ventilation, and cables. This is known as aperture leakage. It is an increasing problem with higher frequencies.
Shielding (attenuation) is usually measured in decibels, denoted dB. This is a log scale, and some of the approximate reductions in intensity of electromagnetic radiation are shown in the table.
Low-frequency emissions can better penetrate shielding material whereas higher frequencies with shorter wavelengths infiltrate through even small openings. Thus shielding materials are not uniformly effective across the whole frequency range. So material suppliers show attenuation qualities as graphs.
Examples of the frequency ranges include electrical devices such as circuit breakers that typically emit noise at frequencies between 100 kHz and 300 MHz. In addition, relay noise can run from 10 kHz to 200 MHz, and motor noise from 10 to 400 kHz. Telecom equipment requires shielding at higher frequencies, over 1 GHz in some applications.
Most gaskets don’t seal securely over a large range of gasket deflections unless they use a high closure force.
The more force a gasket requires to form a seal, the more the door bends. If the door bends appreciably, it may not compress the gasket at points remote from the latch or latches. This has been a problem for those who build cabinets to NEMA-12, NEMA-4, or to other standards with similar environmental requirements. This is why a good gasket must seal at low compression forces and over a wide range of compression distances. Emka bubble-type gasketing supplies those two properties and is widely used. In contrast, enclosure manufacturers that employ other gasket types must either build in more latching points than would be necessary, or stiffen the door.
EMI shielding has many of the same requirements as environmental sealing. It must have good conductivity to work effectively over the whole range of frequencies. Moreover, there must be good conductivity between the gasket and the door as well as between the gasket and frame. Obviously, the gasket must intimately contact both the frame and the door of the enclosure to insure this conductive path.
For example, suppose an enclosure has a latch installed in a lower corner. This exaggerates and clarifies the interaction between enclosure frame, gasket, and door. There’s a force on a cam inside the door frame to keep the door closed. This force comes from compression on the gasket when the door is pushed closed and locked. However, the gasket pushes back on the door with the same force.
With a perfectly stiff door, the gasket would push against the door equally at all points. But the latch placement is such that there is a longer length of door above it than below it. This geometry twists the door, away from the enclosure at the top and towards it at the bottom.
Compression on the gasket drops as the door twists away. So the top of the door may not even touch the gasket. Gasket material at the bottom of the door is probably over compressed and will fail prematurely. It is the compression on a gasket that forms the all important seal.
In cabinet designs with one latch in the center, the door tends to bow out at the top and bottom. Doors with latches on the top and bottom will bow out in the middle. Exactly the same happens on the hinge side at the top and bottom door edges.
Designers can solve the problem in several ways. One is to stiffen the door. Typical methods use heavier-gauge metal, welded in stiffeners, or flanges made deeper or bent back. All these tactics cost money and add weight. They would probably be prohibitively expensive on a large cabinet.
Another approach might be to use another latch and possibly another hinge. The system would have more uniform gasket compression. This is the main reason for three and multipoint locking systems. A third hinge not only improves the consistency of compression on the door’s hinged edge, but stiffens the whole door. Again, extra hardware and assembly time boost cost.
A third option would be a gasket that compresses at low forces. The bubble-type gasket, for example, typically needs only 25% of the compression force for a foam gasket. This translates into approximately 75% less deflection in the door. Therefore. the door can be less stiff or incorporate fewer latching points.
Gasketing that works over a wide range of compression can handle more twist without leaking. Bubble-type gaskets have an operational range of compression of up to 11 mm. This capability tolerates more twist thus further reducing the need for stiff doors and multipoint latching systems.
All in all, gasketing shouldn’t be an afterthought. The type of gasketing should be a factor in deciding on how to design flanges, doors, and other features. Designs aiming for maximum cost effectiveness should consider them from the beginning.
An additional advantage of the bubble-type gasketing is its availability in EMI shielding versions. Cabinets employing this method can be easily upgraded to EMC standards.
Several factors can effect gasket shielding reliability. For example, paint and powder coatings are insulators, and should be eliminated from the gasket area. Unfortunately this allows corrosion. Corrosion can reduce EMI shielding by 20 to 32 dB over several years, depending on humidity, without special measures. Loss of attenuation arises from non or semiconductive oxides which are products of corrosion. They increase electrical resistance at the metal-to-gasket junctions.
Ironically, the very conductivity of conductive gaskets means they support galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals make electrical contact in the presence of an electrolyte, which can be moisture.
However, corrosion does need to be prevented. Though plating is often ineffective against galvanic corrosion, there are proprietary coatings which can be successful with the proper care.
Conductive tape can be an economical and reliable way to provide a conductive circuit and eliminate corrosion. This tape goes on the cabinet prior to painting. Production personnel apply it to the metal where the gasket will touch. The cabinet then gets coated and baked. Workers later remove an outer layer of release paper on the tape to reveal a permanent, corrosion resistant, conductive surface for gasket installation.
Effective shielding is important not just when the cabinet is new, but throughout its life. Besides experiencing corrosion, gasket shielding can degrade in other ways. One is through repeated flexing from door openings and closings. Here conductive particles become electrically isolated from the surrounding substrate reducing effectiveness.
Another source of degradation comes from gaskets taking set when they repeatedly compress and relax. They effectively stop springing back to their original shape. This phenomenon can increase resistance or even cause an air gap between the metal and the gasket.
The sliding motion of a door across a gasket can wear the gasket surface, eventually causing conductivity loss. The obvious way to solve this problem is with a door design that avoids any sliding action
Finally, designers must be aware that the physical qualities of elastomers change with age. The presence of heat, ozone, water, sea water, UV light, oils and other chemicals exacerbates the effects. Most commonly, elastomers become hard and crack, causing problems in both environmental and EMI sealing.
The elastomeric substrate of choice for most applications is EPDM, which works well in harsh environmental conditions. Compression set is minimal and can be further reduced through profile design. In addition, the rubber flexes instead of compressing, mitigating the effects of repeated opening and closing.
Vulcanizing a complete wrap of conductive nylon fabric to a bubble type EPDM substrate achieves the optimum in EMI shielding. A complete 360° wrap gives two layers of shield. Emka uses a woven fabric with silvered fibers further coated with an organic anticorrosion layer. This ensures permanent conductivity throughout the surface of the gasket.
Handle and hinge cut outs
It is possible to virtually eliminate radiation leakage through handle and hinge cut outs in the doors: just design the latching system to work outside the gasket. This approach is simple and costs about the same as an “inside the gasket” system. There is however, a trade-off. For a given cabinet width, the opening will be slightly narrower. The difference can be as little as a half inch with careful design, and the use of handles specifically designed for this purpose. Of course, it is impractical in some designs to keep latches outside the gasket. Here the first precaution is to make cut outs as small as possible. For example small quarter-turn latches can often serve in place of larger flush-swing handles.
Second, metal or metal-plated handles can help. They should be grounded to the door. As an example, the Emka 1150 standard zinc die-cast handle has an attenuation of up to 70 dB. A Nylon 1125 handle attenuates 60 dB or less for most of its range. Chrome plating the 1125 boosts attenuation to over 100 dB. The cut out is larger on the 2100 handle, limiting peak attenuation to about 95 dB even with chrome plating and good grounding.
A few guidelines for hinge selection can help ensure a well-shielded enclosure. Well-grounded metallic hinges with minimum-sized cut outs work well. Weld-on types would be the first choice, but clamp-on or screw-on types with grounding plates or nuts are usually successful and cost less to install.
A successfully shielded enclosure must also work reliably in practice. Flexibility is important as well. Latches and hinges should be able to accommodate both right-hand and left-hand opening doors. In addition, interchangeable hardware can make for easy upgrades. Hardware commonality, for example, can let designers change a standard cabinet to an EMI-shielded version, or to a shielded NEMA version without reengineering and using the same inventory. In all, designers can reduce the effects of EMI at reasonable cost with some thought during initial design.