Today’s higher-speed devices produce ever higher frequencies that must be controlled to meet global emissions and susceptibility requirements. By applying prudent engineering design practices, you can often resolve potential EMI problems.
However, enclosures that house electronic products often need doors, removable panels, electronic displays, air ventilation, or cable openings—and all these allow signals to escape. One of the best ways to seal the signals in the enclosure is to use EMI shielding gaskets.
But how do you select the right shielding gasket? To start, define the working environment of the gasket, said Tony Sosnowski, technical development manager at Instrument Specialties. The characteristics to investigate include:
The operating frequencies of the equipment in the enclosures.
The enclosure construction, including the compatibility of the metals or the need for a conductive coating if plastics are used.
Market and product regulations such as the commercial, military, medical, product safety, and foreign requirements.
The operating environment including special needs for extreme temperatures, outdoor use, or wet conditions.
The space and weight considerations for size and load force.
Additional criteria may help you refine your search for the best gasket. For example, when you specify an EMI gasket for a new product, said Norm Quesnel at Chomerics, also investigate and understand:
Shielding effectiveness.
Closure force or compressive force required to provide the gasket deflection.
Deflection range.
Resistance to corrosion.
Flammability rating.
Chemical resistance properties.
Compression set after long-term loading.
Lightning and EMP survivability.
Friction forces.
Hardness of conductive elastomer gaskets.
Outgassing.
Consider using a gasket that creates a highly conductive, long-term, low-resistance coupling maintaining mechanical and electrical integrity over time, said Mark Hansen at Vanguard Products. It should be easy to install and withstand installation handling and possible elongation without being damaged. Typically, there are very few gaskets that meet all these criteria, and most are compromises between mechanical and electrical performance.
Each application affects the performance of the gasket. Construction features such as openings, vents, connectors, and covers vary with the application, said Mr. Sosnowski of Instrument Specialties. They all influence test results and can produce a large number of concerns that need to be analyzed before the proper gasket is selected.
Openings allow unwanted high-frequency RF to enter or escape from enclosures. These openings or slot lengths must be reduced in size as much as possible. The smaller the slot the less the chance these high frequencies will pass (Figure 1).
To reduce the slot length in an enclosure, the mating surfaces must be electrically connected. Typically, this is accomplished with shielding gaskets to connect the mating surfaces and to make the surface appear as a continuous RF absorber with a high shielding-effectiveness ratio (Figure 2).
Most enclosure materials provide all the shielding needed to meet commercial EMC specifications, said Shane Hudak, EMC product manager at Schlegel. But as soon as you introduce slots and holes into the enclosure, the need for gaskets becomes apparent.
The electromagnetic energy that leaks through the gaps, slots, and holes depends on the fundamental operating frequency of the equipment and the size of the openings in the enclosure, continued Mr. Hudak. For example, at 150 MHz, a 100 mm-wide slot has 20- dB attenuation; but at 1 GHz, a slot of 75 mm provides only 6-dB attenuation. Adding a gasket decreases the gap size and reduces the energy entering and exiting the enclosure.
When you are ready to select a gasket, consider how and where the gasket will attach, with mechanical fasteners or adhesives, and the type of surface the gasket will contact, added Mr. Hudak. Before you proceed with your design, your best bet is to use published data about EMI gaskets generated by a legitimate test method and reputable test house.
Additionally, all flanges or areas where the gaskets are mounted should be clean, smooth, and conductive. Nonconductive paint must be removed from the mating surfaces. Next, select the gasket with compression-load characteristics that match the flange, because the gasket will bow or flex on the flange surface.
If you are putting screws on your enclosure at 3/4″ to 1″ centers, you probably will not need a gasket, stated Bonnie Kunkel at Spira Manufacturing. But, there are gaskets that will provide high levels of shielding along with 15″ to 20″ screw spacing.
Compression set is another quality of gaskets and inherent in many gasket types, said Ms. Kunkel. When some gaskets are compressed, they stay that way even after the pressure decreases. When new, the gasket fills up the gaps between chassis parts. But as it ages, it may no longer fill the gaps. The resulting spaces or holes will be the source for EMI leakage.
Compression set is the decrease in the height of a gasket deformed under specific load, time, and temperature conditions. It is expressed as a percentage of initial deflection:1
(Hi – Hf)/(Hi – Hd)
where: Hi = initial height of gasket.
Hd = deflected height (normally 75% of Hi).
Hf = final height after load is removed.
Compression set is an important characteristic for joints that are cycled, such as doors and access panels, said Schlegel’s Mr. Hudak. If a gasket is prone to take a compression set, the shielding performance will decrease with each opening and closing of the door. The gasket will require progressively higher compression to achieve the shielding levels equivalent to new gasket material.
Materials susceptible to compression set or that degrade over time are vulnerable to interference, especially if access or replacement creates a maintenance burden, said David Castro, EMI gasket product manager at Bal Seal Engineering. A design that passes shielding requirements during qualification testing may not maintain shielding capability in the field.
Longevity is another consideration when specifying EMI gaskets, said Phil Thomas at Tech-Etch. Select EMI gaskets that provide constant attenuation levels over the expected life of the product. Compression set, surface contamination, shielding-material conductivity, and contact force all affect attenuation over the life span of the gasket.
How much will gasket shielding diminish over time? It is a tricky and difficult question to answer. Some gaskets age very quickly, losing their shielding properties in as little as six months, said Spira’s Ms. Kunkel. Ask the manufacturer what the span life of the gasket is and if you can receive shielding test results for this parameter.
Many questions arise about the interpretations of gasket performance specifications, observed Mr. Hudak of Schlegel. Unfortunately, there is not an official gasket measurement standard today. There are groups of standards developed by different industries; for example, the military and the automotive industries. But none of them can accurately predict how the gasket will perform in use.
Consider the performance data as a relative measurement of the shielding effectiveness of gaskets and use the same test specification to compare one gasket to another. However, because test procedures among standards are different, a gasket measured with the military’s MIL-G-83528 shielding-effectiveness procedure cannot be compared reliably to the result of the automotive’s SAE 1705 test used on another gasket.
The published test data for gaskets is a good comparison between the conductive and mechanical properties of the shielding products, said Mr. Hansen of Vanguard Products. The true performance of the materials, however, should be identified through testing in the specific application. Use the published data only as a guideline for relative quality and performance.
The engineering community needs just two types of tests, one for shielding quality and another for evaluation testing.2 The first test must verify that the shielding quality in a specific application is equal to or better than the EMC needs of the system.
The other test determines the shielding quality of a gasket when paired with a specific joint surface. Often this includes environmental testing such as humidity or salt-spray exposure. It measures the shielding degradation as a function of exposure to the environment.
The tests for gaskets are based on either radiated or conducted methods. The radiated methods include:
A modified MIL-STD-285 per the MIL-G-83528 standard.
A dual transverse electromagnetic cell.
A dual stir-mode reverberation chamber.
The conducted method states that electric and magnetic fields emanating from a seam or gasketed joint are proportional to the transfer impedance of the joint. The test results are obtained by measuring the impedance of the joint and normalizing the measured impedance for a meter-long length of gasket.
Gasketing for Doors
It may be obvious, but doors need to open and close consistently with relative ease while protecting the electronics inside. Typically, if you tighten down one area of a sheet-metal door, another part will deflect, creating apertures for EMI, said Mr. Hudak of Schlegel. These undulations are your biggest problem when you apply gasketing to a door.
To avoid this, ensure that the mating surfaces are constructed with adequate space between the door frame and the door to accommodate gaskets, continued Mr. Hudak. Without the proper amount of space, the gaskets will compress beyond the recommended maximum, and the doors will be difficult to open and close.
Be sure to provide space for EMI gaskets on doors and access areas early in the design cycle, said Vanguard’s Mr. Hansen. You can incorporate slots or grooves for installation of gaskets to help pass a shielding test.
To provide a conductive contact surface, make sure that the mating surfaces are masked before painting, said Schlegel’s Mr. Hudak. Special tapes can be used on painted or powder-coated equipment and with flush-mounted doors.
Foil tapes with conductive adhesives provide a good mating surface for many gaskets, added Instrument Specialties’ Mr. Sosnowski. Before painting, some foils can be installed with a protective surface that later is removed to expose the conductive surface. Foils also can be used around port and I/O openings.
Material compatibility is another aspect to consider. The mounting surface, gasket,
and mating-surface materials must be compatible to avoid a galvanic reaction or corrosive condition.
Achieving longevity is the main challenge when installing EMI gaskets on shielding doors, said Mr. Thomas of Tech-Etch. Beryllium-copper gaskets offer excellent performance over the life of the product and are not subject to compression set. The material is highly conductive, and its spring characteristics provide a consistent contact force to ensure low contact resistance for the life of the product.
Leaf seals adapted from the building industry address compression, conductivity, and reliability concerns said Mr. Hudak from Schlegel. They combine an environmental seal with a low-closing-force property, and they self-mount or attach with adhesives.
Today, a wide variety of gasket materials is available, said Mr. Sosnowski of Instrument Specialties. Some provide an environmental seal and EMI protection. Others offer a long cycle life and a wide functioning range that allows for variations in tolerances and door construction. The gaskets also differ in attenuation performance and application limitations.
The constantly changing face of the shielding industry makes it imperative that you
take advantage of the application specialists, consultants, and engineering services provided by the shielding suppliers, noted Mr. Sosnowski. The vendor you choose should have a large selection of products and services as well as testing capabilities. Early vendor involvement may be the key to a successful low-cost design.
References
1. EMI Shielding Engineering Handbook, Chomerics, 1990, p. 140.
2. How to Test EMI Gaskets, EMI Info Series, Spira Manufacturing, 1993.
Gaskets
Foam Gasket Offers Shielding
For Low-Compression Uses
The FoamTite M60 EMI Gasket provides 60-dB broadband shielding for applications that need low compression force to achieve product compliance. It is constructed of a nickel-plated conductive mesh fabric bonded to the company’s proprietary thermoplastic elastomer or urethane foam core. The gasket material resists corrosion, has a non-fray mesh, and incorporates a self-terminating feature that does not require end treatment. It attaches using an adhesive strip. The Easy-Peel™ Release Liner helps in installation and removal. Advanced Performance Materials, (800) 217-5757.
Gaskets Provide 100-dB
Shielding Effectiveness
Independent test results in Technical Report TR-92 show the shielding effectiveness of the company’s shielding gaskets is >100 dB to 1 GHz. The gasket is a patented design of coiled beryllium-copper wire with inclining coils that deflect independently when compressed. The coils close gaps and improve shielding at discontinuations in an enclosure or connector interface. The gaskets resist compression set and fatigue during cycling tests to 60% compression. They are plated with gold, silver, tin, or nickel. Sizes range from 1/32″ to 3/8″, and the gaskets are available in continuous or cut lengths and welded rings. Bal Seal Engineering, (800) 366-1006.
Soft Gasket Strip Provides
Shielding for Enclosures
Zemrex ES is a soft gasket shielding strip for commercial enclosures, cabinets, and panels that require minimum closure force. The gasket has a single layer of fine monel or aluminum wire knitted over a neoprene sponge. Up to 50% compression can be applied. Shielding effectiveness ranges from >130 dB @ 100 kHz to 42 dB @ 10 GHz. The gasket is applied with a pressure-sensitive adhesive. Cambio International, (603) 524-3714.
Gasket Meets Mechanical
Requirements for Enclosures
The SOFT-SHIELD® 5000 Series Gasket meets the shielding and mechanical performance requirements for commercial electronic enclosures. It is made with conductive cloth over soft urethane foam and provides >90 dB shielding from 30 MHz to 1 GHz. At 10 GHz, it provides >75 dB. The gasket deflects 40% and requires <1 lb/in. of closure force. Gasket profiles include rectangular, square, and D shapes. Chomerics, (617) 935-4850.Gasket With 0.032″ Width
Provides 100-dB Shielding
A Shielding Micro-Gasket with an assortment of profiles and sizes provides 100-dB shielding up to 1 GHz. It is available in widths as small as 0.032″. The gasket is a conductive silicone in continuous lengths for enclosure doors, panels, and bezels. Custom designs and installation mounting options are available. Fujipoly America, (908) 298-3850.
Gasket Combines Shielding
And Corrosion Resistance
Soft Knit Gaskets feature metallized nylon yarn over a UL94 VO-approved neoprene sponge elastomer. The surface resistivity for the silver-plated gasket is 0.045 W /in.2 at 20% deflection and 0.304 W /in.2 for tin-plated versions. Deflection cycling tests indicate no change in shielding effectiveness after 10,000 cycles. The compression set is <22% at 50% deflection. The gaskets are available in round, rectangular, square, and D- and C-fold profiles. Instrument Specialties, (717) 424-8510.
80 Gasket Profiles Offered
For Shielding Applications
More than 80 profiles of EMI shielding gaskets are offered for shear, wiping, and compression applications. Installation options include riveted-leaf seal gaskets mounted to flanges on electronic cabinets, D-shape and leaf-seal gaskets with plastic clips for attachment on cabinets, and magnetic gaskets embedded in foam for sealing steel cabinets. The shielding performance is 60 dB for silver, 70 dB for copper, and 80 dB for nickel-copper. Schlegel, (716) 427-7200.
Flexible Gasket Uses
Silicone in Spiral Design
The Flexi-Shield EMI Gasket bonds a spiral-shaped stainless steel and beryllium-copper material around a silicone tube or cord for EMI shielding, and rain, wind, and dust sealing. It resists compression set and is easy to handle. The material shows no visible wear after 1,000 insertions. It provides 130-dB shielding effectiveness @ 1 GHz. The groove-mounted gasket flexes to conform to uneven joint surfaces. Cross-sectional diameters for the gasket range from 0.063″ to 0.125″. Spira, (818) 764-8222.
Gasket Hooks Over Flange
To Provide 100-dB Attenuation
Two sizes of EMI/RFI shielding gaskets provide 100-dB attenuation. The leading edge of the gasket hooks over the mounting flange, and the opposing side attaches by pressure-sensitive adhesive. The 125LP60HO60 closes gaps from 0.03″ to 0.07″ and is 0.09″ high × 0.60″ wide. The 125LP45HO60 is used for gaps from 0.02″ to 0.045″ and measures 0.06″ high × 0.45″ wide. Tech-Etch, (508) 747-0300.
Die-Cut Gasket Fits
Irregular Surfaces
A custom die-cut EMI/RFI shielding Gasket is used for I/O back panels. Notching and die-cutting provide the correct fit around irregular surfaces. The conductive outer layer is coextruded over an elastomer base for shielding effectiveness of >100 dB to 1 GHz. Vanguard Products, (203) 744-7265.
Nickel Graphite Material
Ideal for Commercial Uses
The new NC-Concil is made of nickel-coated graphite particles in a silicone elastomer. It provides high electrical conductivity, broadband shielding, and environmental sealing. Shielding effectiveness is >100 dB @ 1 GHz. The material is available in molded sheet/rule die-cut parts, extruded profiles, and molded shapes. A compatible nickel-graphite, conductive, one-compound, RTV silicone adhesive sealant also is offered. Tecknit, (908) 272-5500.
Copyright 1997 Nelson Publishing Inc.
August 1997