A while ago, I was the Marketing Communications Manager for Schlegel Systems, Inc. which is now Schlegel Electronic Materials. I wrote a white paper with content from the Project Manager, Shane Hudak, who now is an Executive Partner at Nazareth-Hanover Associates, LLC.
EMI Gaskets: A Product Overview
by Shane Hudak, Product Manager
Schlegel Systems, Inc.
Abstract: Shielding gaskets have been used since the discovery of Electromagnetic Interference problems in the 1940s. This article is a short primer on the development and applications for shielding gaskets and where we are headed in the new millennium.
EMI Gaskets – Then and Now
EMI shielding gaskets have been with us for over 50 years now, ever since those first electromagnetic waves began to wander where they were not wanted. An EMI gasket can be described as a conductive medium designed to fill apertures and provide a continuous, low impedance joint. The EMI gasket has evolved from its original roots as a randomly placed ground point, and gasket designs have expanded to match the growing proliferation of electronic equipment surrounding us in the twenty-first century. And even though EMI gaskets have been pronounced dead more times than the PC, they remain very much alive and plan to stick around for a little while longer.
EMI gaskets are generally defined as a flexible connection between two electrical conductors with a fixed resistance to current passing through it. Gaskets are one of the most useful tools available to the design engineer as they follow the common guidelines for EMI containment. In most commercial applications, these guidelines include:
Reduce interference at the source,
Isolate the offending circuits by shielding, filtering, or grounding, and
Increase the immunity of susceptible circuits.
Stated simply, suppression, isolation, and desensitization should be the goals of any circuit designer and should be implemented very early in the design stage. Today’s circuit designer has computer aided design systems to assist in board layout, such as SPICE, ORCAD and NEC-2 modeling software along with PCB layout design tools, such as Boardmaker, which makes attacking interference at the source easier than ever.
As promising as the computer aided design systems are there are still challenges the design engineer must address. As clock and system speeds increase, designers have worked to reduce timing problems and radiating loops by reducing the board size and incorporating Very-Large-Scale-Integrated circuits and Surface Mount Technology components. However increasing system speed requires an increase in system bandwidth, which in turn increases the potential for susceptibility. In addition, radiated EMI coming off the circuitry will also increase in strength as a function of the frequency squared. Additional use of suppression components, such as filter, ferrite beads and bypass capacitors, will help to decrease harmonic amplitudes and bandwidth but reduces the overall system speed.
EMI shielding is a designer’s other option in this situation. Shielding is an attractive solution since it is considered noninvasive to the circuitry. Shielding does not have any effect on the systems speed and works to contain emissions as well as provide increase immunity from outside sources. Shielding techniques also do not require alteration to the board layout and, if well thought out, will not be affected by future system upgrades. The use of shielding materials is a cost-effective way for the design engineer to reduce EMI.
The most popular enclosure options are sheet metal boxes and metallized plastic enclosures. They need to withstand various external environmental conditions including mechanical shock and vibration, as well as durability for long term service products. Selection of the enclosure materials must take into consideration items such as doors, access panels, connectors or windows which must be incorporated into the case.
Seams and openings all penetrate the shielded housing which effectively lowers the shielding effectiveness of the case. The difficulty in designing an “EMI-proof” enclosure structure is openings are unavoidable in manufacturing and needed for operation of the device.
EMI Gaskets- Selection
A gasket is required when the gaps, slots and holes around the metal plate interface allow excessive radiation of the internally generated Electro-magnetic frequencies to escape and if such radiation causes the equipment to radiate outside of the permitted levels. A gasket is used to limit the size of any gaps, slots, or holes within the metal or metallized enclosure such as to cause attenuation by reducing gaps, slots, or holes below the cut-off wavelength of the problem frequencies. Incidentally, the wavelength (Lambda) in meters is C divided by Frequency (in Hz) where C is the speed of light (approx. 300,000,000 M/s).
The key is to properly design the gasket into the case or housing. The basis for any gasket application is to is to make sure the flange faces are smooth, clean and treated as necessary to provide conductive surfaces. A wide array of shielding and gasket products is available. They include beryllium-copper fingers, wire mesh, elastomeric core, conductive elastomers, and metallized fabric bonded to foam. An EMI gasket to seal openings can be selected according to a number of performance criteria, namely:
- Shielding effectiveness over the specified frequency range and in accordance with test limits,
- Mounting methods and closure forces,
- Galvanic compatibility with the housing structure and corrosion resistance to the outside environment,
- Operating temperature range,
- Cost.
Most shielding manufacturers supply estimates of shielding effectiveness that can be achieved by their gaskets. Be aware that shielding effectiveness is a relative function, which depends on the aperture and the shapes needed to fit mechanical criteria including wiping, sliding and hinged actions.
For consistent electrical contact, EMI shielding gaskets flex and/or compress to a certain amount to follow the dimensional variations of the surfaces where they are attached. Poor conductivity between opposing flanges through the gasket will result in lower shielding effectiveness. A total lack of contact along any part of the joint would result in a thin gap capable of acting as a slot antenna. Such an antenna would transmit energy at wavelengths shorter than about 4 times gap length.
| % Compression CLD CR 10 2.01 0.101 20 2.07 0.076 30 2.11 0.077 40 2.11 0.076 50 2.13 0.072 60 5.81 0.049 70 19.81 0.035 80 49.33 0.018 Recommended Minimum Compression = 30% Maximum Compression = 70% Figure 1 |
The designer must know the minimum contact resistance a gasket needs to maintain conductivity with the flanges. Lowering the contact resistance of many gaskets by increasing the compression on the gasket increases the closure force or Compression-Load-Deflection (CLD) force and raises the chances of bowing in the case. Most gaskets work effectively compressed between 30 and 70 percent of their freestanding height.
Because of material composition and profile shape, the compression load deflection(CLD) will vary between gasket types in a similar application. In addition to the need to maintain the required amount of compression to keep a unit in compliance, the spring effect of a gasket is a critical characteristic, specifically in those applications where considerable force is exerted on the gasket. The more the unit is opened and closed, the greater the need for a resilient material formulation is required. Most manufacturers can supply test information on the amount of compression set their gaskets take, and here, these numbers are relatively independent of the part’s shape.
Shielding Effectiveness
The combination of the shape, material construction and the interface of the gasket to the flange or joint achieve the shielding effectiveness of a gasket. An EMI shielding gasket has resistive, capacitive and inductive elements which are characteristics of the base materials. Over a frequency range a gaskets shielding performance can change due to the predominance of one or more of these elements at any given moment. In general, all EMI gaskets are constructed to provide a low impedance path in order to conduct EMI current efficiently between two mating surfaces. At low frequencies the resistive elements tend to be the dominant factor. As frequencies rise the materials used in constructing the gasket will become the dominant element. Gaskets such as wire mesh, oriented wire, fabric-clad-foam and stamped fingers will function as an inductor in series with a resistive load. Conductively loaded elastomers will function as shunt capacitors in parallel to a resistive load. We can see by considering these elements that simply relying on DC or low frequency resistivity measurements as a method to qualify a gasket can be erroneous.
As frequencies increase through the MHz region, a greater percentage of the shielding effectiveness achieved by an EMI gasket can be contributed to its Absorption Loss factor rather than the Reflection Loss factor. The Absorption Loss factor is an important parameter for shielding effectiveness of materials and is directly proportional to the thickness of conductive material deposited on the substrate, but as with DC resistivity, using this parameter alone may be misleading. Experiments show that the current distribution over the cross section of a conductor at high frequencies is limited to its outer surface with a depth penetration that is a function of the frequency.
=1/fHzµ
= Skin depth (defined at 38.8 % of the surface value)
µ = permeability of metal relative to copper
= conductivity of metal relative to copper
f = frequency in Hertz
The resistance of the conductor is also a function of the frequency since the resistance will vary with the conductor cross section, which is equal to the skin depth multiplied by the width of the conductor. This phenomenon is called the ‘Skin Effect’ and explains why, beyond certain frequencies, it is only possible to carry energy through hollow conductors (waveguide).
Shielding effectiveness test standards must be viewed with the understanding that no single test can fully measure all the factors that effect a gasket’s performance. IEEE Std.1302 is a good reference guide for understanding how EMI gaskets can be characterized. But even this guide identifies four commonly referenced standards and five alternative testing techniques, which can be used to characterize the shielding performance of a gasket. Unfortunately, each of these common standards and alternative techniques has its own unique biases and results from one method can not be compared quantitatively to another method.
Shielding effectiveness as selection criteria ultimately depends upon the application. Use of a manufacturer’s tests will provide reference data on how a gasket works under standardize test procedures, but ultimately the uniqueness of each case design hampers any direct connection between test data and reality. Certain conductive materials have their niches in providing shielding at different portions of the frequency spectrum.
What are some of the underlying differences in materials that make up EMI gaskets? Let’s look at the most common conductive gaskets used today: Conductive fabric cladding bonded to a foam core, beryllium copper fingerstock or other stamped metal gaskets, wire mesh gaskets and conductively loaded polymers.
Fabric-Clad-Foam/Fabric-Over-Foam Gaskets
As a product line, Fabric-Clad-Foam gaskets perform well from the low MHz region up into the “centimeter wave” region of the RF spectrum (from 3 GHz to 30 GHz). This class of EMI shielding products is the newest of the gasket technologies and is very popular in the commercial electronics industry due to its low cost, mechanical functionality, attachment methods and shielding values. Shielding effectiveness values range from about 60dB to just over 100dB. This performance is due mainly to the fact that a high quality Fabric-Clad-Foam gasket is constructed with a uniformly metalized fabric, such as Silver on nylon, Nickel-Copper on nylon or polyester, and other combinations such as Tin-Copper and Nickel-Silver. The tightly woven fabric acts as continuous metalized shield to an impinging electromagnetic wave. Differences in the shielding performance of this type of EMI gasket technology are based not only on material, but also on the quality of the plating process used to produce the metalized fabric.
As was mentioned earlier, at higher frequencies a greater percentage of the shielding effectiveness achieved by an EMI gasket can be contributed to its Absorption Loss factor rather than the Reflection Loss factor. The Absorption Loss factor is directly proportional to the thickness of conductive material deposited on the substrate. Therefore, when considering Fabric-Clad-Foam gaskets the temptation to consider Nickel Copper as the best material since several layers make it the thickest fabric currently available. However, the Skin Effect will limit the thickness seen by the current beyond a certain frequency range and limits any advantages of thicker conductive fabrics.
Mechanically, conductive fabric clad foam gaskets have the lowest compression force of any type of gasket. These gaskets have foam cell structures and flexible conductive fabrics that makes compression easy. Low closure force gaskets, such as a “C” or “V” shape profile design provide a compression rate as low as 1.3 lbs./lineal foot.
Compression set values for this gasket line range from about 5% to 35% at 70oC. This range of values is dependent on the foam core material used in the construction of the gasket. Generally speaking, open-cell polyurethane foams are less susceptible to compression set than their closed cell counterparts. Generally this type of EMI gasket has usable deflection range of 10% to 75% of the free height dimension.
Stamped/Formed Metal Gaskets
Stamped or formed metal gaskets have a heritage dating back into the 1940s when spring clips were used to ground chassis in electronic equipment. These gaskets have been popular with the military as well as the commercial industry and are appealing due to their robust construction and level of shielding effectiveness that can be achieved. Within this class of gaskets, the Beryllium-Copper fingerstock has been the most popular. Out of all the materials that can be converted into a spring, BeCu has the highest conductivity. BeCu is more than 100% higher in conductivity compared to materials such as phosphor bronze or stainless steel. Due to the metal thickness and the large cross sectional dimension, stamped metal gaskets can provide good shielding effectiveness down to the 50 kHz range.
Stainless steel is the low cost alternative to the beryllium copper finger stock gasket. The commercial electronics industry, such as PC manufactures, has utilized this stamped metal gasket method to reduce cost. However, stainless steel does not have the resilient spring properties of the beryllium copper part and is not recommended under high cycling applications. Static joints or limited access panel are the ideal applications.
Shielding effectiveness for stamped or formed metal gasket range from a high of 120 dB for the beryllium copper parts down to 60 to 70dB for stainless steel parts. High frequency shielding performance of the low-grade stainless steel and even the BeCu fingerstock tends to fall off in value due to the slots or openings inherent to their construction. These lower levels of conductivity will effect the shielding of the stainless steel products to a higher degree than other gasket types.
Compression forces range around 10 to 20 lbs./Lin. Ft for standard fingerstock metal thickness and profiles. Usable compression height for the BeCu gaskets range from 20% to 80% of the free height dimension. Compression set values for this gasketing technology range from an industry leading <1% for beryllium copper to 25% to 40% for some grades of stainless steel when used within the gaskets’ specified range of compression.
Conductively Loaded Elastomers
Loaded elastomers are another of the senior EMI gasketing products on the market. These products feature polymeric binders, such as silicon, Fluorosilicone, EPDM or carbon, that are loaded with conductive fillers, such as silver, copper, carbon, nickel and aluminum. Loaded elastomers not only provide EMI shielding but can function as a pressure and moisture seal as well. Elastomers that have been qualified to MIL-G-83528 are manufactured to tight quality guidelines. These are the elastomeric products that have been preferred by the US military over the years. These products also tend to be expensive when compared to other materials, such as wire mesh, stainless steel fingers and fabric-clad-foam.
Most conductive elastomer manufactures have supplemented their military grade products with commercial grade materials. These commercial grade materials feature the same elastomeric binders with similar conductive fillers, like silver on glass or silver on copper. Extruding or molding profiles into sheets, tubes or other complex geometry easily produces the gaskets. A variation of the fully loaded conductive elastomer is the co-extruded version, which features a conductively loaded outer liner over a non-conductive hollow inner core.
Shielding effectiveness ranges from a high of 120dB for the military grade products down to about 60dB for the commercial grades. Variation in shielding effectiveness of conductively loaded elastomers is tied to the compression force applied and the amount of conductive loading.
Loaded elastomers require a minimum deflection of anywhere from 10% to 30% from free height to function properly and have restrictive maximum deflection values as low as 50%. This, of course, is dependent on the geometry of the part. Compression load values tend to be near the highest of all gasket products, ranging from about 25 lbs./Lin. Ft. to a high of 100 lb./Lin. In. Compression set value, taken at 100oC, range from a low of 5% for hollow core extrusions up to 30% or more for some solid core constructions.
Wire Mesh
Wire mesh EMI gaskets round out the range of gasket technologies. Wire mesh has been around since the beginning of EMI shielding gaskets. This gasket technology is one of the more cost-effective gasketing solutions which features knitted solid core or hollow core profiles. Popular wire materials include Monel(a nickel-copper alloy), SnCuFe(Tin-plated, Copper-clad Steel) along with aluminum and beryllium copper wire. Monel wire has been the most common due to its good aging, strength and spring properties. SnCuFe has good shielding performance especially in the upper magnetic field region where the shielding of other gasket materials is virtually non-existent. However, corrosion resistance has been the limiting factor in SnCuFe wire.
BeCu wire mesh is relatively new with the distinct advantage of offering the shielding performance and spring qualities near that of the fingerstock cousin. The BeCu wire can also be plated to improve its galvanic compatibility to flange surfaces under harsh environmental conditions. Another variation in the wire mesh product is the knitted wire over a sponge or non-conductive elastomer core. This provides better spring qualities to the mesh gasket and can supply dust and moisture seal functions, which are lacking in standard material.
Wire mesh has been most popular in static seal or limited access applications. This may be attributed to the limited spring properties of the original materials and to the limited mounting methods available. Wire mesh, like loaded elastomers, is best used in a groove mount application.
Shielding effectiveness values of wire mesh gasket products range from 40dB to 105dB depending on the materials and construction of the profile. Hollow core gaskets achieve a usable contact resistance with around 20% deflection and can be compressed up to 75% depending on the material. Solid core mesh gaskets need less deflection to function efficiently but reach there maximum compress deflection around 40%. Compression set values for BeCu wire mesh is <1%, other hollow or elastomer core meshes reaches 20%. Solid core wire mesh gaskets compression set values can be as high as 30%. Compression load forces range from 5 lbs./Lin. Ft for hollow core gaskets up to 20 lbs./sq. in. for solid core meshes.
Conclusion
The ultimate deciding factor in the selection of an EMI shielding gasket is the end application. Each of these gasket technologies has advantages and disadvantages with specific characteristics (compression force, compression set, shielding effectiveness, cost, etc) which should be kept in mind. These values listed in this article are averages of what is commonly available in each gasket category. Test information provides a gauge of relative performance. Consult with each gasket supplier to obtain the actual performance data for the specific gasket profile and construction under consideration.
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