Shielding techniques--Cabinets, cubicles and chambers

Bonding structural components

Enclosures are normally built out of a number of components. Typical larger enclosures will use structural supports - pillar and frame members - and panels which bolt to them.

Various standards for enclosures, such as the IEC 60297 series give the mechanical dimensions and other requirements to enable such enclosures from different manufacturers to integrate successfully. Such standards do not have anything to say about the electrical integrity of these enclosures, beyond the need to ensure that parts are galvanically connected for safety purposes.

The safety bond- a length of green-and-yellow wire interconnecting panel and frame, or different panels - is familiar to system builders. It is vital to realize that this is not adequate for EMC bonding. The purpose of a wired safety bond is to prevent different parts of the structure from assuming different potentials and hence presenting an electric shock hazard at power frequencies: it has no other purpose. It cannot give a low impedance connection at RF. This is not to say that the safety bond is forbidden for EMC purposes: it can coexist quite happily with a proper EMC bond. But the one is no substitute for the other. If you are intent on building an adequate enclosure for RF purposes, then full metal-to-metal connection at all joints is required. Section 5 (gives greater detail on bonding and you are referred there for a proper discussion. In the context of enclosure design and construction, the following points should be noted:

++ any un-bonded joints between panels and structural members present discontinuities in current flow and as such will degrade shielding effectiveness and transfer impedance

++ bonds are best made by surface-to-surface conductive contact at frequent intervals, or preferably continuously along a seam; "bonding" straps, although necessary in many circumstances and preferable to wire connections, are a second best option

++ bonding between parts requires removal of insulating layers, e.g. paint or anodizing, and often the treatment of mating surfaces to ensure conductivity, e.g. zinc plating or chromate conversion

++ positive pressure is required to make a bond; fasteners will provide this but usually the gap between two fasteners does not allow pressure to be maintained, hence the use of conductive gaskets

++ once a bond is made between two surfaces, it should be protected from corrosion by being made gas-tight or by applying some type of overall coating.

Shielding hardware

To improve the bonding of various aspects of a conductive enclosure, many types of hardware are available. These can be classified either as conductive gaskets which are intended to be applied between mating surfaces, and other conductive hardware products which are for application at different sorts of intentional aperture.

Conductive gaskets

When two conductive surfaces are mated, electrical contact is unlikely to exist along the whole length of the seam or join. However accurately machined the components may be, some areas will be under greater pressure than others, and these will carry the current across the joint in preference to the poorly-mated areas. The majority of commercial enclosures are not machined at all, and contact across the joint in between those points of pressure provided by fasteners is uncertain and unlikely. Thus current will be diverted or channeled through the assured contact points and not elsewhere; this in turn creates discontinuities in the current flow and hence degradation in the EMC effectiveness of the structure. Even if the two surfaces are very close together and so have an appreciable capacitance between them, the impedance of this capacitance is orders of magnitude greater than that which results from actual contact, and it does not significantly improve the effectiveness of the joint. If no contact can be made at all, then some capacitance due to overlap of two non-contacting surfaces is better than none, but this situation is rare in conventional enclosures.

The purpose of conducting gaskets is to remedy the situation of occasional 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. The optimum situation is where the gasket provides the same impedance as the bulk impedance of the metal; different types of gasket approach this ideal more or less closely. If no pressure were applied between two surfaces, they would only make contact at the three highest points; as pressure is applied by fasteners or other means, the irregular high spots are flattened resulting in a greater number of contact points and more surface area. A gasket provides this effect more widely at lower closing pressure. The ideal gasket will bridge all irregularities within its designed deflection without losing its resilience, conductivity or stability.

The effect of a conductive gasket -- (a) no gasket (b) gasket in place.

Surveys the various types of conductive gasket available and compares their properties and applications. The methods of using these gaskets on typical doors and panels.

Mechanical properties are at least as important as the electrical shielding performance; the latter can usually be adjusted by choice of conductive material, silver, copper and monel being the preferred variations. Compression set (the gasket takes up a particular shape after repeated compression/relaxation cycles, leading to reduced contact force) is an important consideration for gaskets in applications such as doors and panels which will be subjected to repeated mating and opening. Compression force (the mating force needed to achieve the specified electrical performance) and required deflection to achieve it is also more important in these applications, and interacts with the mechanical design of the housing. These factors will dictate the primary choice of type of gasket.

--- Gaskets on doors and panels of shielded cabinets

-- -----: Types of conductive gasket

Material Conductive elastomer , , Oriented or woven wire in elastomer Be-Cu spring finger Knitted wire mesh Conductive fabric over foam core -- Shielding: Variable depending on filler, poss. up to 120dB

.... Medium 40-80dB, wires will puncture surface films

...... Good, up to 130dB, poorer above 1GHz Up to 100dB, poor above 1GHz, wires will puncture surface film

...... Good, 60-110dB broadband depending on fabric material

- Mechanical High compression forces, possible problems with compression set, brittleness and ageing Fragile, limited range of compression forces; Problems with compression set, significant compression once; Low compression force, very little compression set -- Environmental seal; Yes; Partial; No; No Partial -- Variations/ application

Wide choice of fillers, elastomers, moldings or extrusions Suitable for thin, flat- formed gaskets e.g. connectors Best used for wiping contacts , Can be stand alone or over elastomer core to improve compression properties , , General purpose

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Electrochemical grouping:

Group I, Group II, Group III, Group IV, Magnesium and alloys | Aluminum and alloys | Cadmium plate | Brass Aluminum and alloys Beryllium; Beryllium Zinc, Zinc plate Carbon steel Iron Stainless steel Copper, copper alloys Zinc, Zinc plate Chromium plate Nickel, nickel plate Nickel/copper alloys Chromium plate Cadmium plate Tin, tin plate, solder Monel Carbon steel Brass Silver Iron Stainless steel Nickel, nickel plate Copper, copper alloys Tin, tin plate, solder Monel

----

Electrochemical compatibility

Corrosion caused by galvanic action is a major problem, both between mating surfaces directly and between gasket material and its adjacent surfaces. As a rule of thumb, electrochemical potential difference should not exceed 300mV to prevent excessive corrosion, although this is not the only criterion: surface resistivity is also important.

Galvanized sheet metal develops a coating of high resistance oxidization, and cadmium or chromate passivation also results in a fine surface protective film. For these surfaces you will need a gasket construction that has hard points, such as wire mesh or oriented wires, which penetrate these films to contact the underlying conductive surface.

Tin and solder coatings are compatible with most metals, but are soft and easily corroded by vibration and abrasion. Silver oxide is as conductive as the metal itself, but may be affected by sulphur emissions under some atmospheric conditions; silver is more expensive than other finishes. Untreated aluminum is wholly unsuitable since aluminum oxide is highly insulating, and anodizing the aluminum is worse since this deliberately grows a thick layer of surface oxide. Alochroming (chromate conversion)

is a technique much used by the military for providing a conductive and non-corroding finish for aluminum enclosures. It costs about the same as anodizing, but does not provide such a hard-wearing and scratch-resistant finish so is less suitable for aluminum front panels and the like.

The optimum metals for gaskets and for surface treatment are nickel, monel, or stainless steel. ----- the common grouping of metal types by electrochemical compatibility. Selecting adjacent materials (gasket and mating surface) from a single group in the table will give the least galvanic corrosion. Materials in adjacent groups can also be used together, with appropriate protection. Materials from separated groups (e.g., aluminum and brass) should not be used together.

--- Construction of a typical shielding cable gland assembly (Elkay Electrical)

Other conductive hardware

Whilst conductive gaskets form a large class of products for mating two surfaces, there is also a growing range of more specialized products for particular shielding applications.

Cable penetrations

As is described elsewhere, cables entering or leaving an enclosure should have their screens properly terminated to the enclosure wall.

(Unscreened cables should enter or leave via a suitable filter if the shielding effectiveness is to be maintained.) This means that full 360 dgr. contact should be maintained around the outer surface of the cable screen.

Mechanisms for ensuring this are similar to conventional cable glands for environmental sealing, except that the appropriate parts are fully conductive. Most of the traditional manufacturers of cable accessories are now aware of the importance of EMC aspects, and provide EMC-specific cable glands as part of their stock range. A typical construction using tapered washers to compress an iris-type spring against the cable screen outer surface . This is one of the most common methods of clamping to the screen, but others are possible, including collet or other clamping mechanisms, elastomeric compression modules, or folding the screening braid back over a conductive tube.

Aspects which you need to consider when specifying a screening cable gland system are:

• ++ mechanical compatibility: the cable screen outer diameter must match the gland's construction, often within quite tight tolerances;
• ++ electrochemical compatibility: the materials used for screen, gland and enclosure panel should discourage corrosion;
• ++ ease of assembly, especially if unskilled or poorly trained technicians are expected, or the working area is restricted;
• ++ conductivity across the joints, which directly affects shielding effectiveness;
• ++ least disturbance of the screen;
• ++ whether or not an environmental seal is also required.

Waveguide tubes

When a hole in a thin panel is extended so that the depth of the panel is greater than the hole diameter, it becomes a tube. A tube acts as a waveguide for electromagnetic energy. A waveguide will allow propagation of an electromagnetic wave along its length only if its diameter (or maximum cross- section dimension, if rectangular) is above the minimum required to support the dominant electromagnetic mode. This is directly related to wavelength, and therefore there is a "cut-off" frequency below which energy is attenuated rather than propagated down the waveguide. For a square cross- section tube with air as the medium, the cut-off frequency is f_c = 150/0 MHz where d is the side dimension of the tube in meters So e.g. a 3cm tube will have f_c = 5GHz. If the frequency is substantially lower than f_c, then the attenuation or shielding effectiveness due to one such tube is given by the simple expression SE = 27.3. l/d dB where l is the length of the tube and d is the side dimension as above Thus a tube of length-to-width ratio of 4 will give an attenuation exceeding 100dB, more than enough for most purposes.

Waveguide tubes are widely used in shielded enclosures to carry non-electrical services and metal-free fiber optic cables through the walls of the enclosure without the penetration compromising the shielding effectiveness. The usual form of construction is a simple metal tube welded into the wall or fixed to it in the same manner as a cable gland. It rarely matters whether the tube projects into or out of the enclosure, or equally on both sides of the wall.

What is vital is that no conductors are passed through the tube. It is emphatically not suitable for passing metallic cables of any sort, or metal pipes. These destroy the waveguide properties of the tube and render it as useless for shielding integrity as would passing a cable through an ordinary hole.

Ventilation panels

One application for waveguide tubes is to provide ready-made ventilation panels for shielded enclosures. Since the walls of the waveguide can be made quite thin and the tubes can be stacked together with little effect on the electromagnetic attenuation, an assembly of such tubes can be built to provide a high degree of through airflow at the same time as a high level of attenuation. These are available as pre-packaged units known as "honeycomb panels" which can simply be fitted into the wall of an enclosure (observing the proper precautions regarding bonding all around the periphery of the assembly) to give any reasonable level of ventilation. These are much more effective at screening than a mesh of holes of the same open area in a thin panel, but of course are more costly and require some thickness in addition to the panel. The wire mesh approach may be adequate for many low-performance applications.

To avoid variations in attenuation due to wave polarization, it is common to stack two layers of honeycomb panel against each other, with different orientations of the waveguide tubes in each layer.

--- Honeycomb ventilation panel

Shielded windows

Viewing apertures can represent the largest size hole in the apparatus. If the display behind the window is a serious source or victim of disturbances, then the entire aperture needs to be shielded. Special conductively treated windows are available for this purpose; they need to be installed, as always, with great care to ensure that they are bonded to the surrounding panel all the way around the edge. The conductive treatment must be brought out to the edge of the window in such a way that good contact can be made to it with no breaks - a metal or metallized frame is often the best way of ensuring this.

The material itself tends to be available in two varieties:

++ glass, polycarbonate or acrylic which has been coated with a very thin coating of a conductive film, usually indium tin oxide (ITO), which is substantially transparent. The coating is usually quite fragile and needs some form of protection, and its poor conductivity prevents it achieving high levels of shielding effectiveness. Its visual properties are good, though, and it does not degrade image resolution noticeably;

++ glass or polycarbonate laminated with a metallic mesh. These are more rugged, and can achieve better shielding than the coating type, but the mesh can substantially degrade image resolution due to Moir6 fringing effects, although these can be avoided with careful design.

In both cases there is a trade-off to be made between electromagnetic protection and transparency or light transmission; generally, the more transparent a window, the less shielding effect it can give.

Because shielded windows are expensive, especially for large areas, wherever possible it is better design practice to find other ways than shielding to protect equipment (such as monitors) with large displays. Fortunately, most types of display are not serious victims or emitters of RF fields (active matrix LCDs can be an exception), at least at the levels required from commercial standards.

Installation and maintenance of screened enclosures

As apertures and seams in screened enclosures such as racks and cabinets can affect the screening performance drastically, it is very important to ensure that measures which are taken to control their effects at the system design stage are not degraded by installation and maintenance procedures. Bonding integrity must be maintained continuously. Any surfaces which are intended to mate must not be allowed to corrode and must never be painted until after they have been assembled. Normally a robust conductive finish such as zinc, alodine or alochrome is used, but if the environment is corrosive (such as in a naval installation) then more specialized measures, and more frequent maintenance, will be needed.

Where fastenings provide a conductive path they must all be kept in place and at the correct tightness. Replacement of short, wide bonding straps by loops of wire is unacceptable as their inductance is excessive.

Doors, panels and hatches which make contact via gaskets or spring finger stock must be installed and treated with care so as not to damage or distort the contact surfaces, which should be regularly checked and cleaned if necessary. Filtered inlets and shield penetrations must make assured 360-dgr contact to their host panels; a DC continuity check is rarely adequate to confirm that this is present.

It should be clear by now that requiring a cabinet or other enclosure to exhibit good shielding is not a simple or inexpensive option. The requirement affects all aspects of the installation throughout its intended life cycle. Also, maintenance and installation personnel need to be trained in the principles and techniques involved, since they may otherwise unwittingly compromise shielding integrity just by following their own established practices. For these reasons, shielding is often best regarded as a means of last resort if other, lower-cost EMC options are unavailable.

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