 
|
  
   |
| About
CE-Mag Free Subscriptions Current Issue Article Archives ESD Help Mr. Static Web Gallery Staff Info Contact us
|
|
|
|||||||||||||||||||||||||||||||||
|
|
|||||||||||||||||||||||||||||||||
|
A Guide to EMI Shielding Gasket TechnologyOptions in materials of construction, shielding effectiveness, and closure force abound, as this primer outlines. EMI shielding gaskets have been with us for more than 50 years,
ever since those first electromagnetic waves began to wander where
they were not wanted. An EMI shielding gasket is a conductive
medium designed to fill equipment apertures and to provide a continuous
low-impedance joint. It is technically definable as a flexible
connection between two electrical conductors with a fixed resistance
to current passing through it. The EMI gasket began as a randomly
placed ground point, but gasket designs have evolved and diversified
to match the growing proliferation of electronic equipment over
the past half century.
Using Gaskets to Contain EMI Gaskets are among the most useful tools available to the design engineer in following the common guidelines for EMI containment, which, in most commercial applications, are
The Design Problem. Any good circuit designer should pay attention very early in the design stage to suppression, isolation, and desensitization. Today's designer has computer-aided design (CAD) systems to assist in board layout, such as SPICE, ORCAD, and NEC-2 modeling software, as well as PCB-layout design tools like Boardmaker, all of which make attacking interference at the source easier than ever. But as helpful as the CAD systems are, there remain challenges that the design engineer must address. As clock and system speeds have increased, designers have worked to reduce timing problems and radiating loops by shrinking the board size and incorporating VLSI circuits and surface-mount components. Increasing system speed requires an increase in system bandwidth, however, which in turn increases the potential for susceptibility. EMI radiating from the circuitry also increases in strength as a function of the frequency squared. The use of additional suppression components, such as filters, ferrite beads, and bypass capacitors, can help decrease harmonic amplitudes and bandwidth but they also reduce the overall system speed. EMI shielding is the designer's other option in this situation. Shielding is an attractive solution because it is noninvasive to the circuitry. It has no effect on system speed while containing emissions and providing good immunity from outside sources. Shielding techniques also require no 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. These must be able to withstand such external environmental challenges as mechanical shock and vibration and provide durability for products designed for long-term service. Items such as doors, access panels, connectors, or windows that must be incorporated into the case have to be considered when enclosure materials are selected. However, seams and openings in the shielded housing lower the shielding effectiveness of the case. The principal difficulty in designing an EMI-proof enclosure relates to these openings that are unavoidable in manufacturing the structure and that are necessary for operation of the enclosed device. Selecting the Gasket Type. A gasket is required if the gaps, slots, and holes around the metal plate interface allow excessive radiation of the internally generated electromagnetic frequencies to escape and if such emissions cause the equipment to radiate outside of the permitted levels. The gasket is used to limit the size of any gaps, slots, or
holes within the metal or metallized enclosure so as to cause
attenuation by reducing gaps, slots, or holes below the cutoff
wavelength of the problem frequencies. (Incidentally, the wavelength
( A wide array of shielding gasket products are available. These include beryllium copper (BeCu) fingers, wire mesh, elastomeric core, conductive elastomers, and metallized fabric bonded to foam. Selection of an EMI gasket to seal openings is governed by performance criteria, including:
Shielding Effectiveness The combination of the shape, material of 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, capacitative, and inductive elements that are characteristics derived from the base materials. Over a range of frequencies, a gasket's shielding performance can change as the influence of one or more of these elements is predominant. All EMI gaskets are constructed so as to provide a low-impedance path for conducting EMI current efficiently between two mating surfaces. At low frequencies the resistive elements of a gasket tend to be the dominant factor. As frequencies rise, the materials used in constructing the gasket become dominant. A gasket made of wire mesh, oriented wire, fabric-clad foam, or 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. As frequencies increase through the megahertz region, a greater proportion of the gasket's shielding effectiveness can be attributed to its absorption-loss factor than to the reflection-loss factor. The absorption-loss factor, an important measure of the shielding effectiveness of materials, is directly proportional to the thickness of the conductive material. However, looking at this factor in isolation may be misleading. Experiments have shown that current distribution over the cross section of a conductor at high frequencies is limited to the conductor's outer surface and that depth penetration is a function of the frequency. The resistance of the conductor is also a function of the frequency, because the resistance will vary not only with the conductor cross section but also with the skin depth. The skin depth is the layer of metal thickness in which the RF current is flowing at any given frequency. A phenomenon known as skin effect results from an increase in the interior inductance of a conductor to a point where more than 99% of the current is flowing near the surface. Skin effect explains why, beyond certain frequencies, it is more efficient to carry RF energy through hollow conductors. The phenomenon can be characterized by the equation:
where d = skin depth, f = the frequency in hertz, s = conductivity of the metal relative to copper, and µ = permeability of the metal relative to copper. From this it can be seen that simply relying on dc or low-frequency resistivity measurements to qualify a gasket is misguided. Shielding-effectiveness test standards provide relative performance data that can be used to evaluate the attenuation characteristics of different gasket technologies. These test standards must be viewed with the understanding that no single test can fully measure all the factors that affect a gasket's performance. IEEE Standard 1302 is a good reference guide for EMI gaskets characterization, but it identifies four commonly referenced standards and five alternative testing techniques that all can be used to characterize the shielding performance of a gasket. Unfortunately, each of these common standards and alternative techniques has its particular biases, and results acquired with one test method cannot be compared quantitatively to those gathered via another method. Mounting Methods and Closure Forces The designer needs to know the minimum contact resistance required by a gasket in order to maintain conductivity with the housing flanges. For consistency of electrical contact, EMI shielding gaskets flex and/or compress to some degree to follow the dimensional variations of the surfaces to which they are attached. Poor conductivity between opposing flanges through the gasket will diminish shielding effectiveness. A total lack of contact along any part of the joint results in a thin gap capable of acting as a slot antenna. Such an antenna can transmit energy at wavelengths shorter than about four times the gap length. Increasing compression of the gasket improves contact resistance but also adds to the closure force, or compression load deflection, and can raise a host of other problems. Placing some gaskets under loads can result in hard-to-operate doors, case bowing, or a premature compression set. The compression load deflection varies between gaskets used in a similar application in accordance with their material composition and profile shape. Most gaskets work effectively when compressed to between 30 and 70% of their freestanding height. Table I shows the relationship of compression load deflection to contact resistance for a typical fabric-bonded-to-foam type of gasket.
The spring effect of a gasket is a critical characteristic, particularly in applications where the unit is repeatedly opened and closed. These cases call for a resilient material formulation with low compression set. Compression-set test information supplied by most gasket manufacturers is generally independent of part shape and is useful for appraising the efficiencies of different types of gasket. The most common conductive gaskets in use today are
types made of conductive fabric cladding bonded to a foam core;
beryllium copper fingerstock or other stamped metal; conductively
loaded polymers; and wire mesh. The remainder of this article
examines and compares the characteristics and performance values
of these EMI shielding technologies.
Fabric-Clad-Foam Gaskets
As previously mentioned, at higher frequencies a greater percentage of the shielding effectiveness achieved by an EMI gasket is attributable to its absorption loss than to its reflection-loss factor. Because the absorption loss factor is directly proportional to the thickness of the conductive material deposited on the substrate, it is tempting to consider nickel copper the best material for fabric-clad-foam gasketing, because its several metal layers make it the thickest fabric coating currently available. Plating fabric substrates with identical base fabrics to increase metal thickness can improve the measured dc contact resistance and, to a slight degree, the shielding effectiveness of the material over a portion of the RF range. However, the skin effect will limit the thickness seen by the current beyond a certain frequency range and so minimizes the advantages offered by thicker conductive metal layers. Conductive-fabric-clad foam gaskets, offer the lowest compression force of any type of gasket. Their foam cell structures and flexible conductive fabrics make compression easy. Low-closure-force gaskets, such as those with C- or V-shape profiles, provide compression rates as low as 1.3 lb per linear foot. Fabric-clad EMI gaskets feature a usable deflection
range of 10 to 75% of the free height dimension. Compression set
values range from about 5 to 35% at 70°C, depending 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.
Stamped and Formed Metal Gaskets
Stainless steel provides a low-cost alternative to the beryllium copper fingerstock gasket. The commercial electronics industry, including PC manufacturers, uses stamped stainless-steel gaskets to keep costs down. But stainless steel lacks the resilient spring properties of BeCu and is not recommended for high-cycling applications. It is suited for use in static joints and limited-access panels. The shielding effectiveness of stamped and formed metal gaskets ranges from a high of 120 dB BeCu parts down to 60–70 dB for stainless-steel parts. Shielding performance of low-grade stainless-steel and even BeCu fingerstock gaskets tends to fall off at high frequencies, owing to the slots or openings intrinsic to their construction. The lower levels of conductivity will affect the shielding of stainless-steel products to a higher degree than other gasket types. Compression forces range from 10 to 20 lb per linear
foot for standard fingerstock metal thickness and profiles. Usable
compression height for BeCu gaskets ranges from 20 to 80% of the
free height dimension. Compression set values for this gasketing
technology range from an industry-leading figure of less than
1% for BeCu up to 25–40% for some grades of stainless steel
when used within the gaskets' specified range of compression.
Overcompressing any stamped metal gasket greatly increases its
compression set.
Conductively Loaded Elastomers Loaded elastomers are another long-standing EMI gasketing technology. They consist of polymeric binders, such as silicon, fluorosilicon, EPDM, and carbon, which are loaded with conductive fillers, such as silver, copper, carbon, nickel, and aluminum. Loaded elastomers not only provide EMI shielding but also can function as a pressure and moisture seal. Elastomers qualified to MIL-G-83528 are manufactured
to tight quality guidelines. These are the elastomeric products
that have been preferred by the U.S. military for years, and they
tend to be more expensive than wire mesh, stainless-steel fingers,
and fabric-clad foam. Most manufacturers of conductive elastomers
supplement their military-grade offerings with commercial-grade
materials that feature the same elastomeric binders with similar
conductive fillers, such as silver on glass or silver on copper.
Gaskets are easily produced when profiles are extruded or molded
into sheets, tubes, or complex geometries.
A variation of the fully loaded conductive elastomer is the coextruded version, which features a conductively loaded outer liner over a nonconductive hollow inner core.  The
shielding effectiveness of conductively loaded polymers ranges
from 120 dB for military-grade products down to about 60 dB for
the commercial grades. Variation in their shielding effectiveness
is tied to the compression force applied and the amount of conductive
loading. Loaded elastomers require a minimum deflection of 10–30%
from free height in order to function properly, and their maximum
deflection is limited to 50%. These values are of course dependent
on the geometry of the part. Compression load values tend to be
among the highest of all gasketing products, ranging from about
25 lb per linear foot up to 100 lb per linear inch. Compression
set values at 100°C range from 5% for hollow-core extrusions
up to 30% or more for some solid-core constructions. Wire Mesh Gaskets
BeCu wire mesh, which was developed relatively recently,
offers shielding performance and spring qualities that nearly
match those of its fingerstock cousin. BeCu wire can also be plated
to improve its galvanic compatibility with flange surfaces under
harsh environmental conditions.
Another variation of wire mesh product takes the form of knitted wire over a sponge or nonconductive elastomer core. This provides the mesh gasket with better spring qualities and supplies dust- and moisture-seal functionality that is lacking in standard material. Wire mesh has been most commonly used in static-seal or limited-access applications because of limitations in the spring properties of the original materials and the mounting methods available. Wire mesh, such as loaded elastomers, is best used in a groove-mount application. The shielding effectiveness of wire mesh gaskets ranges
from 40 to 105 dB, depending on the material and the construction
of the profile. Hollow-core gaskets achieve a usable contact resistance
with about 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 their maximum compression deflection
near 40%. Compression set for BeCu wire mesh is under 1%; for
other hollow- or elastomer-core meshes this value reaches 20%.
Solid-core wire mesh gasket compression set can be as high as
30%. Compression load forces range from 5 lb per linear foot for
hollow-core gaskets up to 20 lb/sq in. for solid-core meshes.
Conclusion Ultimately, the deciding factor in selection of an EMI
shielding gasket is the end application. Each of the gasketing
technologies offers advantages and disadvantages. Differences
in their compression force, compression set, shielding effectiveness,
cost, and other specific characteristics should be kept in mind
when choosing a gasket to fulfill a particular need. The values
given in this article are averages of what is commonly available
in each gasket category. Test information provides a gauge of
relative performance. Potential purchasers are advised to consult
with gasket suppliers to obtain actual performance data for the
specific gasket profile and construction under consideration.
Shane Hudak is a product manager for Schlegel Systems (Rochester, NY)
Back to 2000 Annual Reference Guide Table of Contents
|
|||||||||||||||||||||||||||||||||
) in meters is c—the speed of light, approximately 300,000,000
m/sec—divided by the frequency in hertz.) The key is to design
the gasket into the case or housing properly. The basis for
any gasket application is to make sure the flange faces are
smooth, clean, and treated as necessary to provide conductive
surfaces.
Fabric-clad-foam
gaskets perform well from the low megahertz region up into the
centimeter wave range of the RF spectrum (3–30 GHz). This
class of EMI shielding devices employs the newest of the gasketing
technologies. Their low cost, mechanical functionality, attachment
mechanisms, and shielding values make fabric-clad-foam gaskets
very popular in the commercial electronics industry. Shielding
effectiveness ranges from about 60 dB to just over 100 dB. This
level of performance is attributable mainly to the fact that
a high-quality fabric-clad-foam gasket is made from a uniformly
metallized fabric, such as silver on nylon, nickel copper on
nylon or polyester, or another combination, including tin copper
or nickel silver. The tightly woven fabric acts as a continuous
metallized shield against an impinging electromagnetic wave.
Differences in shielding performance with this type of EMI gasketing
technology are based not only on the material of construction
but also on the quality of the plating process used to produce
the metallized fabric.
Stamped
and formed metal gaskets date back to the 1940s when spring
clips were used to ground chassis in electronic equipment. They
are popular with the military as well as with commercial industries.
The appeal of these gaskets is due to their robust construction
and the level of shielding effectiveness that can be achieved.
Gaskets made of BeCu fingerstock have been most preferred. Of
all the materials that can be converted into a spring, BeCu
has the highest conductivity. BeCu offers more than twice the
conductivity of such materials as phosphor bronze and stainless
steel. Because of the thickness of the metal and their large
cross-sectional dimension, stamped metal gaskets can provide
good shielding effectiveness down to the 50-kHz range.
To
round out this review of the technologies, wire mesh EMI gaskets
have been around since EMI shielding began. This cost-effective
gasketing technology features knitted solid-core or hollow-core
profiles. Popular wire materials include Monel (a nickel copper
alloy), SnCuFe (tin-plated, copper-clad steel), aluminum, and
beryllium copper. Monel wire has been most often used, owing
to its strength and good aging and spring properties. SnCuFe
offers good shielding performance, especially in the upper magnetic-field
region where the shielding capabilities of other gasket materials
are virtually nonexistent. However, poor corrosion resistance
has been a factor limiting the utility of the wire material.