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EMI Shielding: Components / Installation



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Components' adequacy and installation conditions are key to any successful shielding installation, and these factors are in strict correlation. Components affect shielding performance, and they can create a discontinuity in the shield's integrity; installation conditions are important for both the main shielding structures and the components. Apart from functional apertures, such as those due to viewing needs, air ventilation, or pass-through cables, the most important and tricky aspect is represented by the joints between adjacent parts of the main shielding structure and between the structure and the installed components (switches, lamps, etc.).



Joints are classified as permanent, semi-permanent, or frequently operated joints. The last two types are usually provided with gaskets as described in the following section; the permanent joints are often fabricated by means of soldering, screws, or rivets. Continuous soldering should be preferred over (in order of performance) spot welding, screws with cap nuts or in blind holes, and rivets. Any protrusion of metallic parts that radiates toward the shielded region must be absolutely avoided. If use of such parts is unavoidable, adequate grounding is necessary.



For all the components briefly described in the following, the reader is invited to analyze the huge amount of data available from manufacturers.

Ill. 1 Different types of shielded gaskets

Ill. 2 Equivalent circuit for a portion of a shielded gasket.

GASKETS

Electromagnetic-interference gaskets are an important class of components installed in the joints between panels to reduce the EM-field penetration. They are manufactured in a very wide variety of materials and shapes to meet their assigned levels of performance. Gasket performance is usually specified in terms of their EM sealing capability and their mechanical strength and resistance to chemical agents, depending on the function and application. Several considerations need to be accounted for in the choice of the most suitable version for any specific application, as follows (not in order of importance):

  • Severity of environmental conditions (temperature, presence of corrosive
  • agents, pressure conditions, vibrations, etc.)
  • Amount of shielding required
  • Uneveness of the mating surfaces to be joint
  • Class of use (i.e., number and frequency of operation)
  • Cost

Among the main types of constructive materials are conductive polymers or rubbers, wire-mesh flexible fabrics, and finger-shaped gaskets. Beryllium-copper, steel, and tin-coated phosphor bronze are among the most used metallic materials. Figure 9.1 shows several types of gaskets.

The EM performance of a gasket relates to its capability of restoring the electrical continuity between the two adjacent mating surfaces. Therefore it's very useful to have knowledge of the correct equivalent circuit and to understand the frequency behavior of any gasket configuration. Additionally it's useful to be aware of uncertainties in the values of components in an equivalent circuit (shown in ill 2) that are due to installation conditions, together with their degradation over time. So a reliable approach depends mainly on graphs and measurement that describe the EM performance of any gasket type. The IEEE Standard 1302 gives guidelines for the EM characterization of conductive gaskets in the frequency range of dc to 18 GHz.

A very approximate method for a rough estimate of the gasket's SE is based on the TL approximation and leads to the following approximate expression:

SE R 20 log [h_0 / 4 pi g ] (1)

… where a uniform plane wave normally incident onto the gasket is assumed as the incident field, h0 is the free-space impedance, and hg is the intrinsic impedance of the gasket, assumed to be equal to the total impedance (resistance, in the cited reference) offered by the gasket to a current ?owing through it from one surface to the other:

hg Rc1 Rg Rc2 (eq 2)

In (eq 2), Rc1 and Rc2 are the contact resistance (impedance) existing between the gasket and the two mating surfaces, while Rg is its internal resistance (impedance). The modeling of inductive behavior is not a simple task, and in an oversimplified method it's generally neglected. Two other flaws are present in such an approximation: first, (1) has been deduced for a planar shield of infinite extension and second, the gasket impedance given by (2) is generally different from the intrinsic impedance of the gasket material to be used in (1), which does not depend on gasket shape and dimensions. For all these reasons, the use of expression (1) is not recommended.

Another approximate approach is based on the introduction of a transfer impedance that relates the per-unit-length current density JS on the gasket side exposed to the incident uniform plane wave to the voltage drop V0 across the gasketed seam on the opposite side:

Zt V0/JS [3]

The units of Zt are V m . Because of the assumption of a normally incident plane wave, the amplitude Hinc of the incident magnetic field is related to the amplitude of the induced current density as

JS 2Hinc

…and to the amplitude Einc of the incident electric field as …

Einc h0Hinc

On the other hand, the voltage drop on the non-exposed side is almost equal to the product of the gasket thickness t by the transmitted electric field Etr

For gasket lengths much smaller than the operating wavelength, it can be assumed that the following approximation holds for the SE of the gasket:

If the incident EM field is not a plane wave and its wave impedance is known, expression (4) can still be used by adopting the appropriate values Zw1;2 instead of h0. In most cases, however, the wave impedance is not unique and /or is not known. Given this drawback, together with the limitation to the gasket length in comparison with the wavelength, a more general method is called for to characterize gasket behavior, namely a radiated testing technique performed by means of different antenna types, like magnetic-field loop antennas, electric-field monopole antennas, and plane-wave radiators. These antenna techniques usually adopt a metal cavity for the EM power to penetrate through a gasketed seam; the measurement performed in the absence of the gasket offers a reference value for the evaluation of performance. It should be noted that such measurement methods based on standardized apertures are strongly influenced by several factors such as the mutual positions of the antennas-seam system and the surrounding environments on the two sides of the gasketed aperture. The values of the aperture dimensions are obviously also crucial.

Nested reverberation chambers can also be used to estimate the power beyond the gasketed components and that in the source region. More sophisticated models of perforated gaskets (knitted-mesh type, finger-shaped, etc.) are based on the evaluation (analytical or numerical) of the EM-field penetration through the holes of the gasket [5]. This is an open field of research. The reader is reminded to carefully check the conditions under which the gaskets have been tested by manufacturers, and additionally not to neglect two especially important considerations: the pressure agent on the gasket and its potentially evolutive performance because of corrosion, galvanic action, and so forth.

SHIELDED WINDOWS

Shielded windows are very common components in shielding applications. The visibility of displays or status lamps and LEDs is needed but is often accompanied by the introduction of a discontinuity in the shield, with the consequent detriment of performance. Shielded windows are a way to reduce such performance degradation and are usually applied through a gasket on their contour, although other techniques are also available, such as those based on permanent soldering or welding.

Various non-electromagnetic constraints are critical to the choice of shielded windows: transparency in the frequency spectrum corresponding to wavelengths between 400 and 700 nm, anti-glare and anti-scratch tractability, mechanical and thermal properties, resistance to chemical agents, and physical dimensions of the window. Other elements that influence the choice are the refractive index of the transparent material and the conductive busbar and gasketing available or necessary for the termination of the shielded window.

Unfortunately, window dimensions affect performance measurements. Therefore all the manufacturers' data can be compared only when the sample dimensions are equal. In general, the smaller the dimensions of the sample under test, the higher is the performance that it exhibits.

Materials available for shielded windows range from conductive glasses or plastics (conductivity is achieved through embedded metallic particles or thin films on the surfaces) to composites, frequently in the form of wire grids, woven or knitted, embedded in an optically transparent host.

Performance is strongly dependent on frequency, so reference should be made to manufacturers' data. Additionally some general expressions suitable for a rough estimation of the expected capabilities exist, usually based on the assumption of infinite extension of the structure. For instance, a shielded window realized by means of a woven wire grid and excited by a plane-wave field offers a SE given by the expression [1]

SE 164 20 log df ; (5)

... where d is the distance between adjacent wires and f is the operating frequency.

However, this expression is useful only for an idea about the most important quantities affecting performance; it's not adequate for an accurate prediction, which involves dimensions of the shielded window, material characteristics, wires diameter, and so forth. Typically 80 to 200 openings per inch (OPI) are considered for woven grid meshes, consisting of wires whose diameter is in the range of 20 to 100 mm. The SE that can be achieved (as per manufacturers' declarations) is generally between 50 and 80 dB under plane-wave excitation at the frequency of 1 GHz. Sample dimensions are usually between 50 50mm and 300 300mm, and the optical transparency is often between 60% and 90%.

Knitted wire meshes generally offer worse SE performance than woven grids, with a typical SE ranging between 20 and 40 dB under plane-wave excitation at the frequency of 1 GHz. The number of OPI is generally between 10 and 30 for wire diameters being the same as those adopted in woven wire grids.

For a conductive glass or plastic, the expressions -- given earlier -- may be considered. Typical values (i.e., those declared by manufacturers) of SE achievable by means of conductive glasses or plastics are in the range of 40 and 60 dB under plane wave excitation at the frequency of 1 GHz. The thickness of the compact thin films is often in the range of 50 to 200 nm, with an optical transparency in the order of 90%.

ELECTROMAGNETIC ABSORBERS

Electromagnetic absorbers are used for a number of applications. Most common are the anechoic chambers used in compliance testing of apparatus and systems in lieu of open area test sites. Other applications range from the reduction of radar cross sections of objects to the improvement of the radiation pattern of antennas, to resonances damping in shielding applications. Of course, in the framework of EM shielding, the last application is of evident usefulness in the improvement of the enclosure's performance. The high conductivity of shield walls works to trap the EM energy penetrated through the shield discontinuities (which may be viewed as traveling back and forth within the shielded volume). Materials capable of effectively dissipating such energy in the shielded region can considerably help in the improvement of shielding performance.

The shape and material of EM absorbers vary according to EM constraints, environmental peculiarities, and manufacturer design. Generally, they are used to cover a metallic surface in such a way that:

1. the reflected field is as low as possible;

2. the reflected field phase is opposite, as much as possible, to that of the incident field;

3. the transmitted field is attenuated as much as possible while traveling through the absorber material and before being reflected by the conductive surface over which the EM absorber has been installed.

The first and third conditions are those that offer the best opportunities in broadband unknown polarization situations. The second is more suitable for frequency and polarization selective performance because of its resonant nature.

The most common shapes for EM absorbers are pyramidal cones, possibly truncated or twisted, whose dimensions may range between few centimeters and few meters, and tiles of homogeneous or multilayer material whose thickness is in the order of few millimeters. Material compositions are often not fully publicized by manufacturers, but graphites, iron oxides, ferrites, urethanes, and conductive foams are among the most used.

The prediction of absorber performance is rather complex, depending on the geometrical shape and on the physical parameters of the materials adopted.

Approximations based on equivalent TLs or homogenization approaches have been considered to circumvent the need of a numerical analysis for accurate predictions.

Information on the frequency dependence of permittivity and permeability of materials is generally not available from manufacturers, and for this reason prediction of EM absorber behavior under conditions different from those used to test it could be tricky.

Under plane-wave illumination the comparison of EM absorber performance is generally based on the reflection coefficient G (which is a complex quantity), defined as

G E r E_inc; (6)

where E r is the (complex) amplitude of the (linearly polarized) reflected electric field.

Alternatively, the reflectivity Ra (sometimes termed in other ways) can be used:

R_a 20 log G (7)

Typical values of reflectivity of commercially available absorbers range between 10 and 50 (or better) dB for normally incident plane waves, at frequencies between about 30MHz and well above the GHz. For oblique incidence, performance is generally worse.

SHIELDED CONNECTORS

Connectors are generally installed to allow the entrance of cables and optic fibers into the shielded region. They are especially important in determining the shielding performance. Several types of shielded connectors are commercially available, and the reader will find plentiful information from manufacturers. The best connectors for EMC purposes are those with a shielded backshell. The use of a shielded cable whose shield is adequately terminated at the connector input is often advisable as well as the installation of a filter to limit the antenna behavior of the portion of cable present in the shielded region. Ways of properly grounding the cable shield are found in [2].

AIR-VENTILATION SYSTEMS

Almost all the electric and electronic systems present some thermal requirements, generally satisfied by means of air-ventilation ducts with or without fans. Such apertures represent serious shield discontinuities if not adequately shielded. They are usually of two types: those obtained by covering the apertures by means of meshed panels and those obtained by the use of honeycomb apertures exploiting the waveguide below cut-off attenuation. The first solution has the advantage of keeping out dust at the cost of an extra resistance to the air flow, while the second has better performance in terms of SE and drop of air pressure, but it presents the drawback of :

requiring an adequate dust filter. Typical performance (at the frequency of 1 GHz) range between 50 and 75 dB against a uniform plane-wave field for the first type, and between 75 and over 100 dB for the second one. Various shapes of honeycomb openings have been designed for the improvement of performance, and their description is beyond the scope of this sect.. The interested reader will find all the relevant information in manufacturers' data sheets. Various models exist for the estimation of the SE provided by a metallic honeycomb, either approximate [1,9], such as

SE = 27 ' a 20 log N; (8)

where ' is the length of the waveguide (i.e., the thickness of the honeycomb cell) and a is the radius of the N apertures. Or, more accurate [10], is

SE 27:3 ' a 20 log 2ka p cos f; (9)

which accounts also for the angle of incidence f of the external TM-polarized plane wave field.

FUSES, SWITCHES, AND OTHER SIMILAR COMPONENTS

Control and protection devices directly accessible from the exterior of the shielded housing represent another class of coupling paths that may deteriorate shielding performance. Actual shielded components for control, protection, and operation are generally reliable as concerns their performance. However, a gasket between the main body of the component and the internal side of the panel and an adequate nut are usually recommended to limit the discontinuity in the shield integrity. Moreover nonconductive shafts for switches, potentiometers, and the like are useful to avoid a guided path between the source region and the shielded one.

Also see: EMI Shielding: Design Guidelines

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Updated: Friday, 2012-01-27 17:56 PST