| Radiocommunications Agency
|Shielding of products|
What this technique is used for
Shielding and filtering are complementary practices. There is little point in applying good filtering and circuit design practice to guard against conducted coupling if there is no return path for the filtered currents to take. The shield provides such a return, and also guards against direct field coupling with the internal circuits and conductors. But shielding by itself has little benefit if wires leaving the enclosure are not filtered to it, or shielded themselves.
Shielding prevents internal electromagnetic energy from coupling to the environment and so reduces emissions; it also prevents external electromagnetic energy from the environment from coupling into the system and disrupting its operation, so improving immunity. It involves placing a conductive barrier around the critical parts of the circuit (not necessarily around the whole product) so that the electromagnetic field which couples to it is attenuated by a combination of reflection and absorption. The shield can be an all-metal enclosure if protection down to low frequencies is needed, but if only high frequency (> 30MHz) protection will be enough then a thin conductive coating deposited on plastic is adequate.
Practical shields are compromised by apertures and other discontinuities. The layout of structures within the shielded enclosure, particularly cables, has a significant effect on the shielding effectiveness of such real enclosures.
How this technique is used
Shielding almost always uses metal, sometimes in very thin layers (e.g. plated on to plastic) or meshes, sometimes as sheets, castings, or milled-from-solid. Volume-conductive paints or plastics generally use very fine metal powders or wires distributed throughout their volume, in sufficient density that they touch their nearby particles or wires.
The electromagnetic wave impedance of free space is 377 ohms (120 π); pure electric fields have a much higher impedance, while pure magnetic fields are much lower. A change in the wave impedance cause electric and magnetic fields to be reflected. A metal barrier of infinite extent provides good shielding to electric fields because its wave impedance can be made very low.
That part which is not reflected, will induce a current flow in that side of the wall, which reduces in intensity as it penetrates into the wall. The remaining current flow which reaches the far side is then reflected back into the barrier, and the surface current density results in an attenuated transmitted field. The total shielding effectiveness – that is, the ratio of incident to transmitted field – is the sum of reflection and absorption losses.
The thicker the wall, the greater the attenuation of the current through it to the other side. This absorption loss depends on the number of “skin depths” through the wall. The skin depth is an expression of the electromagnetic property which tends to confine AC current flow to the surface of a conductor, becoming less as frequency, conductivity or permeability increases. Fields are attenuated by 8.6dB (1/e) for each skin depth of penetration.
Key issues in employing this technique
The type and thickness of the metal
For good absorption, the metal needs to have a minimum thickness of at least six ‘skin depths’ – this will give at least 50dB absorption. The skin depth in a metal depends upon the wavelength of the signals to be shielded from, and the type of metal used. However, even thin films will give good reflection loss, which may be sufficient in many applications.
Limitations on theory
Shielding theory for an infinite barrier, as outlined above, cannot easily be applied to real-world enclosures. This is for a number of reasons:
enclosures are a finite size, so that theory which assumes infinite dimensions does not cope with the edges of the barrier.
real enclosures have many discontinuities, due to shape and construction, and because of internal structures. Seams and apertures have a large effect on the shielding performance.
the enclosure structure causes resonances which cannot be modelled using simple barrier theory but which also have a large effect.
internal components modify the effects of resonances and apertures, already hard to model, and also affect the wave impedance within the enclosure in an unpredictable fashion.
if the shield is located in free space then the incident wave impedance can be modelled, but real situations do not normally equate to free space and so the incident wave is also unknown.
With all these factors affecting real shielding performance, calculating the SE theoretically is largely irrelevant, and simple rules of thumb are used instead to give guidelines to achieve adequate performance.
Apertures and conductor penetrations
Practical shielding effectiveness (SE) is limited by necessary apertures and discontinuities in the shield, and not by the intrinsic properties of the shielding material. Apertures are needed for ventilation, control access and for viewing indicators.
A simple model for shielding degradation assumes that SE is directly proportional to the ratio of longest aperture dimension and frequency, with zero SE when λ= 2L: SE = 20log(λ/2L). Thus the SE increases linearly with decreasing frequency up to the maximum determined by the barrier material, with a greater degradation for larger apertures.
Any aperture therefore must be limited in size. Ventilation holes can be covered with a perforated mesh screen, or the conductive panel may itself be perforated. Windows may use optically transparent screening material, or the relevant displays may be located outside the shielded boundary. But the shielding effectiveness can be reduced by seams as much as it is by apertures.
An electromagnetic shield is normally made from several panels joined together at seams. Unfortunately, when two sheets are joined the electrical conductivity across the joint is imperfect. This may be because of distortion, so that surfaces do not mate perfectly, or because of painting, anodising or corrosion, so that an insulating layer is present on one or both metal surfaces. The effect of a joint discontinuity is to force shield current to flow around the discontinuity and thereby to create a field coupling path through the shield. A long aperture or narrow seam will have a greater effect on current flowing at right angles to it than on parallel current flow.
An improvement in shielding effectiveness can be made by reducing the spacing of contact points, such as fasteners, between different panels. Where this is inadequate or impractical, then you can ensure a conductive path between two panels or flanges by using any of the several brands of conductive gasket, knitted wire mesh or finger strip that are available.
The purpose of conducting gaskets is to remedy the problem of irregular contact at pressure points, by providing a continuous contact path across a joint in a conformal manner. There is then much less variation in joint impedance along the length of the joint and hence less diversion of current paths.
Low frequency shielding
Conductor penetrations - Any conductors that penetrate a shielding surface can carry unwanted field energy from one side of the shielding barrier to the other side. Therefore all conductor penetrations should either be shielded, with the conductor screen bonded to the shielding surface, or filtered, with the filter capacitors bonded to the shielding surface.