EMI Shielding Principles
The importance of wave impedance is shown by an electromagnetic wave encountering an obstacle such as a metal shield (see Figure 3). If the impedance of the wave differs greatly from the natural impedance of the shield, much of the energy is reflected and the rest is transmitted across the surface boundary, where absorption in the shield further attenuates it. Because most metals have an intrinsic impedance of only milliohms, less low impedance H-field energy is reflected and more is absorbed. This is because the metal is more closely matched to the impedance of the field. This is also why it is difficult to shield against magnetic fields. On the other hand, the wave impedance of electric fields is high, so most of the energy is reflected for this case. At higher frequencies, typically over 10 MHz, EMI shielding is governed mostly by absorption
Figure 3
Attenuation of EMI by a Shield
Shielding effectiveness of metallic enclosures is not infinite, because the conductivity of all metals is finite. They can, however, approach very large values. Because metallic shields have less than infinite conductivity, part of the field is transmitted across the boundary and supports a current in the metal, as illustrated in Figure 4. The amount of current flow at any depth in the shield, and the rate of decay is governed by the conductivity of the metal, its permeability, and the frequency and amplitude of the field source. The residual current appearing on the opposite face is the one responsible for generating the field which exists on the other side.
*
The current density in a metal shield is not affected by the shield’s thickness. A secondary reflection occurs at the far side of the shield for all thicknesses. The only difference with thin shields is that a large part of the re-reflected wave may appear on the front surface. This wave can add to or subtract from the primary reflected wave depending upon the phase relationship between them. For this reason, a correction factor appears in shielding equations to account for reflections from the far surface of a thin shield.
Figure 4
Variation of Current Density
with Thickness for Electrically Thin Wall
E = Electric Field Strength
J = Current Density
i = initial
t = transmitted
*
EMI Gasketing
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A gap or seam in a shield will allow electromagnetic fields to radiate through the shield, unless the current continuity can be preserved across the gaps. The function of an EMI gasket is to preserve continuity or current flow in the shield. If a gasket is made of material identical to the walls of the shielded enclosure, the current density in the gasket will be the same. (This assumes it could perfectly fill the slot, which is not possible due to mechanical considerations.)
The flow of current through a shield including a gasket interface is illustrated in Figure 5. Electromagnetic leakage through the seam can occur in two ways. First, the energy can leak through the material directly. The gasket material shown in Figure 5 is assumed to have lower conductivity than the material in the shield. The rate of current decay, therefore, is less in the gasket, resulting in more current flow on the far side of the shield. This increased flow causes a larger leakage field to appear on the far side. Second, leakage can occur at the interface between the gasket and the shield. If an air gap exists at the interface, the flow of current will be diverted to the points or areas in contact. A change in the direction of the flow of current alters the current distribution in the shield as well as in the gasket, which lowers shielding performance. A high resistance joint does not behave much differently than an open seam. It simply alters the distribution of current somewhat. A current distribution for a typical seam is shown in Figure 5. Lines of constant current spaced at larger intervals indicate less flow of current. It is important in gasket design to make the electrical properties of the gasket as similar to the shield as possible, maintain low impedance interface surfaces, and avoid air gaps which also increase joint resistance.
*
Shielding and EMI Gasket Equations
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As described above, electromagnetic waves incident upon a discontinuity will be partially reflected, and partly absorbed by the material. The effectiveness of the shield is the sum total of these two effects, plus a correction factor to account for reflections from the back surfaces of the shield.
Reductions in field strength are determined by the frequency, the shielding material’s conductivity, thickness and permeability, and by the distance between the radiating source and the EMI shield.
How well a shield reduces the energy of (attenuates) a radiated electromagnetic field is referred to as its shielding effectiveness, or SE. The standard unit of SE measurement is the decibel, or dB. The decibel value is the ratio of two measurements of electromagnetic field strength taken before and after shielding is in place. Every 20 dB increase in SE represents a tenfold reduction in EMI leakage through a shield. A 60 dB shield reduces field strength by a factor of 1,000 times (e.g., from 5 volts per meter to 5 millivolts/meter).
The overall expression for shielding effectiveness is written as:
SE = R + A + B (1)
where
SE is the shielding effectiveness
R is the reflection factor
A is the absorption factor, and
B is the correction factor to account for reflections from the far boundary
All values are expressed in dB (decibels) Reflection loss (R) includes reflections at both surfaces of the shield, and is dependent upon the relative mismatch between the incoming wave impedance and the frequency of the impinging wave, as well as upon the electrical parameters of the shielding material itself. The equations for the three principal fields are given by the following expressions:
where:
RE, RH, and RP are the reflection terms for the electric, magnetic, and plane wave fields expressed in dB
G is the relative conductivity referred to copper
f is the frequency in Hz
m is the relative permeability referred to free space
r1 is the distance from the source to the shield in inches
The absorption term (A) is the same for all three waves and is given by the expression:
where:
A is the absorption or penetration loss expressed in dB, and t is the thickness of the shield in mils.
The correction factor (B) can be mathematically positive or negative (in practice it is always negative), and becomes insignificant when A>6 dB. It is usually only important when metals are thin, and at low frequencies (i.e., below approximately 20 kHz).
*
A plot of reflection and absorption loss for copper and iron is shown in Figure 6. This illustration gives a good physical representation of the behavior of the component parts of an electromagnetic wave. It also illustrates why it is so much more difficult to shield magnetic fields than electric fields or plane waves. Note: in Figure 6, copper offers more shielding effectiveness than iron in all cases except for absorption loss. This is due to the high permeability of iron. These shielding numbers are theoretical, hence they are very high (and unrealistic) practical values.
When only electric field or plane wave protection is required, reflection is the important factor to consider in the design. If magnetic shielding is required, particularly at frequencies below 10 kHz, it is customary to neglect all terms in equation (1) except the absorption
term A.
Nomographs are available from Chomerics to aid designers in determining absorption and magnetic field reflection losses directly. These nomographs are based on the previously shown equations used in determining shielding effectiveness. Commercial equipment typically needs shielding of 40-60 dB from 30 MHz to 1 GHz, and often 10 GHz. (See also Chomerics’ EMI Shielding for Military/Aerospace Electronics Engineering Handbook.)
EMI Gasket Types
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Conductive EMI gaskets must conform intimately to enclosure mating surfaces. They provide current continuity in shielding systems by reducing resistance across the seams. EMI gaskets are generally made of metal, metal combined with elastomer materials, or metallized fabric over foam cores. All-metal gaskets include knitted wire mesh made in different metals for cost-performance choices. By knitting wire mesh around elastomer cores, metal gaskets can better meet mechanical design needs such as enhanced compressibility. Another type of metal gasket, typically used in door seams (where shear forces are present), features linear rows of beryllium copper spring fingers, or spirals.
Foam-core gaskets covered by conductive fabric, yarn or metal foil provide a large deflection range with modest closure force. Another approach for low closure force applications is a hollow silicone tube with a conductive surface coating.
Conductive elastomer gaskets, which contain metal-plated particles, provide excellent shielding characteristics. Such gaskets can be extruded and cut to length, typically for preventing EMI leakage around the perimeters or between sections of electronic enclosures. Co-extruded EMI gaskets combine a conductive elastomer with a non-conductive silicone environmental seal. Conductive elastomers can also be die-cut from sheet material, or molded into intricate shapes. Robotically applied form-in-place gasketing systems precisely dispense conductive elastomer compounds on electronic housings. Specially formulated conductive elastomers can be molded onto thin-wall plastic spacer frames to provide grounding of circuit boards in small enclosures such as cellular phone housings. Conductive elastomers are also being molded directly onto the inner surface of large plastic housing covers in place of plating or other types of metallizing. These shielded covers feature integral conductive elastomer walls, which eliminate the need for EMI gaskets. When form-stable gaskets are impractical for an application, conductive adhesives and sealants can often be applied. A variety of formulations are available for providing either rigid or flexible shielding solutions. These materials can also be used for bonding EMI gaskets to flange surfaces.
EMI Gasket Selection
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The most critical features to consider when evaluating EMI gaskets are: shielding effectiveness, operating environmental conditions, fitting the physical properties of the gasket with the packaging design, electrical stability, and the installed cost. Chomerics’ inPHormTM product selection software provides a convenient, comprehensive system for engineers to create an application profile. The inPHorm program recommends one or more products that will best fit the application needs. On-line technical data is provided for hundreds of EMI shielding products (see Inside Front Cover).
Shielding Effectiveness
As described in the preceding pages, determining a system’s overall shielding needs involves understanding the radiated emission spectrum of the equipment, and the specifications the unit must meet. Various EMI gasket types provide different levels of shielding effectiveness across the frequency range of 10 kHz to 20 GHz.
Operating Conditions
One reason so many gasket variations exist is a result of trying to provide a best fit for the different operating environmental conditions. Exposure to high and low temperatures, wind and rain, salt spray from the ocean, solvents used in cleaning and plating, and other conditions can severely affect the life of a gasket.
Mechanical Requirements
The primary goal of a shielding gasket is to seal openings in an electronic enclosure to prevent transmission of EMI. Improper design of the seal or enclosure mating flanges can result in a failure to meet this goal. Several mechanical design issues must be considered for the proper mating of an EMI gasket with the flanges of an electronic enclosure. Among the most important are compression-deflection and compression set.
* Compression-Deflection
EMI gaskets require some amount of compressive force to function properly. As a result of this load the material will decrease in height (deflect). The magnitude of the decrease is proportional to the applied load up to the elastic limit or the point at which the material yields or ruptures. Many commercial-grade gaskets must deflect 30-40% under low closure forces to properly maintain contact with mating flanges.
* Compression Set
If a gasket material is subjected to a compressive force for an extended time, some deflection remains when the load is removed. Compression set is an important property in designs where the gasket will be compressed and released regularly while in service, such as in enclosure doors and access panels.
Electrical Stability
EMI gaskets provide conductive pathways that electrically bond system components to a common ground. The gaskets serve as low impedance conductors to ensure the reliability of an enclosure’s shielding. For example, an EMI gasket will provide continuity between the housing shield and other system ground points. Gasket conductivity and electrical stability are critical to system performance and shielding integrity.
Installed Cost
The method of installing an EMI gasket can be a major factor in determining EMI shielding costs. Gaskets can be installed using pressure-sensitive adhesives, fasteners, or epoxies, or by press-fitting into grooves. Some gaskets are molded in place on the enclosure flange or on a plastic spacer frame.
The installed cost for an EMI gasket includes the cost of the gasket, together with labor and other manufacturing costs. A gasket applied without fasteners or adhesives, for instance, may offer lower installed cost than an EMI gasket purchased at a lower price.
Where EMI Shielding is Needed
Requirements for EMI shielding abound in computers, medical devices, telecommunications, and many other types of electronic equipment. As new emission and immunity requirements are placed on these devices, the importance of shielding grows.
Among the typical applications for EMI shielding are the following:
Enclosures
Plastic and other non-conductive materials used for lightweight housings can be metallized with sprayable conductive paints, thin-film metal coatings or plating. Laminates of metal foil and plastic film can be formed and die-cut into shadow shields, ground planes or Faraday cages. Metal housings for electronic systems provide inherent levels of EMI shielding, dependent on factors such as metal type, flange design, and seam and aperture treatment.
Apertures
Doors, cable ports, vents, windows, access panels and other openings in an otherwise shielded electronic package are pathways for radiated EMI. A variety of gaskets and specialized conductive materials are available for preserving shielding around door seams and the perimeters of other openings. Shielding vents and windows are designed to reduce the amount of EMI passing through such apertures.
The amount of EMI leakage through an opening is a function of the maximum dimension of the opening at a given frequency. A long, narrow slit, regardless of width, like the gap around the edge of a door, will leak much more radiation than a round hole of the same area. The imperfect joints between panels or covers and enclosure walls are typical “slots” where EMI can efficiently escape or enter a shielded enclosure. Conductive EMI gaskets inserted between panel mating surfaces will provide low resistance across the seam, preserve current continuity of the enclosure, and provide the necessary shielding.
Cables
Signal-carrying cables can act as antennas to radiate EMI. Conversely, false signals can occur when EMI couples into a cable. A number of shielding products are available for reducing EMI problems on both internal and external cables.
Grounding Issues
Shielding against EMI emissions is commonly provided by a conductive enclosure. The separate parts of the enclosure must be electrically bonded together and grounded for the shielding to work. Disruptions in the electrical continuity between parts adversely affect shielding performance. Proper grounding of PCBs and shielding enclosure components is also a method for reducing board-generated EMI. However, improper or ineffective grounding may actually increase EMI emission levels, with the ground itself become a major radiating source. Many Chomerics shielding materials can be used for providing conductive grounding paths.
Gasket Mounting Choices
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Our various EMI gasket mounting techniques offer designers cost-effective choices in both materials and assembly. These options offer aesthetic choices and accommodate packaging requirements such as tight spaces, weight limits, housing materials and assembly costs. Most Chomerics gaskets attach using easily repairable systems. Our Applications Engineering Department or your local Chomerics representative can provide full details on EMI gasket mounting. The most common systems are shown here with the available shielding products.
Pressure-Sensitive Adhesive
Quick, efficient attachment strip
* Conductive Elastomers
* SOFT-SHIELD
* POLASHEET
* SPRING-LINE
* POLASTRIP
Friction Fit in a Groove
Prevents over-deflection of gasket Retaining groove required
* Conductive Elastomers
* MESH STRIP
* POLASTRIP
* SOFT-SHIELD
* SPRINGMESH
Adhesive Compounds
Conductive or non-conductive spot bonding
* Conductive Elastomers
* MESH STRIP
Robotically Dispensed
Form-in-Place
Conductive Elastomer
Chomerics’ Cho-Form™
automated technology applies
high quality conductive elastomer gaskets to metal or plastic housings. Manufacturing options include Chomerics facilities, authorized Application Partners, and turnkey systems.
Friction Fit on Tangs Accommodates thin walls, intricate shapes
* Conductive Elastomers
Spacer Gaskets
Fully customized, integral conductive elastomer and plastic spacer provide economical EMI shielding and grounding in small enclosures. Locator pins ensure accurate and easy installation, manually or robotically.
Clip-On Gaskets
Require knife edge mounting flange.
* Conductive Elastomers
* METALKLIP
* CLIP-SHIELD
* SOFT-SHIELD 5000
Rivets/Screws
Require integral compression stops Require mounting holes on flange
* Conductive Elastomers
* SHIELDMESH
* SPRING-LINE
* COMBO STRIP
Frames
Extruded aluminum frames and strips add rigidity. Built-in compression stops for rivets and screws.
* Conductive Elastomers
* Mesh Strip
We can supply any quantity and any kind of Copper Beryllium Finger Strips and Industrial Enclosure Shielding from stock.would you please inform us how many you need and your target price, then we will confirm ASAP. We are sincerely hope to do business with you and establish long term business relationship with your respectable company.
Look forward to hearing from you soon.
Best regards,
Sam Xu
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Beryllium Copper Finger Strips and Industrial Enclosure Shielding
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Conductive fabric over foam makes sense where no environmental seal, complex profile, or demanding mechanical durability is needed (Figure 1). It can be the lowest-cost option.
For very small cross sections, a formed-in-place silicone-based silver-copper-filled or nickel-graphite-filled conductive elastomer is dispensed robotically (Figure 2). The robot can deposit the elastomer on a surface as narrow as 0.025 to 0.030 in.
When you need mechanical durability, such as frequent direct compression actuations or shearing forces, BeCu strips make the best choice (Figure 3). BeCu fingerstock withstands lateral shearing forces as well as perpendicular forces, where other shielding materials are best suited for perpendicular, direct-compression applications.
The first material is a silicone or fluorosilicone rubber strip coextruded with wire mesh (Figure 4). Limited to a square or rectangular profile, it can be used in enclosures that have grooves. In the groove, the silicone faces the environment for environmental sealing; the wire mesh faces inside toward the electronics to provide the electromagnetic attenuation.
