Design Techniques for EMC
Part 4 — Shielding (Screening)(This article has been split in three, this is the last part)
By Eur Ing Keith Armstrong C.Eng MIEE MIEEE, Cherry Clough Consultants
This is the fourth in a series of six articles on basic goodpractice electromagnetic compatibility (EMC) techniques inelectronic design, to be published during 2006-7. It is intended
for designers of electronic modules, products and equipment,
but to avoid having to write modules/products/equipment
throughout – everything that is sold as the result of a design
process will be called a ‘product’ here.

4.3.5 SE is compromised by leakage from apertures
4.3.6 The resonant/antenna behaviour of a single
aperture in the far field
4.3.7 Multiple apertures in the far field
4.3.8 Cavity resonances and aperture leakage
4.3.9 Near-field leakages through apertures
4.3.10 Designing to reduce aperture leakages
4.3.11 Waveguide-below-cutoff techniques
Published in Issue 70, May 2007
4.3.12 Shielding of displays (and the like)
4.3.13 Shielding membrane switch panels
4.3.14 Shielding ventilation apertures
4.3.15 Shielding rotating metal shafts that penetrate
an enclosure
4.3.16 Combining heatsinking with enclosureshielding
4.3.17 Preventing shield degradation from conductor
penetrations
4.4
Shielding at PCB level
4.4.1 Reasons for shielding at PCB level
4.4.2 Overview of techniques for PCB level
shielding
4.4.3 Types of PCB shielding-can
4.4.4 Methods for bonding shielding-cans to PCB
planes
4.4.5 Suitable materials for PCB shielding-cans
4.4.6 Apertures in PCB shielding-cans
4.4.7 Waveguide-below-cutoff methods in PCBlevel shielding
4.4.8 Cavity resonances in PCB shielding-cans
4.4.9 Shielded cables and traces entering/exiting
PCB shielding-cans
4.4.10 Combining filtering with shielding at PCB
level
4.4.11 Combing heatsinking with shielding at PCB
level
4.5
Estimating shielding effectiveness with PC-based
simulators
In this Issue
4.6
EMC gaskets
4.6.1 Volume-conductive elastomers
4.6.2 Conductively coated or wrapped elastomers,
see Figure 4BH
4.6.3 Metal (wire) meshes, see Figure 4BJ
4.6.4 Spring fingers (‘finger stock’), see Figure
4BK
4.6.5 Some other types of gaskets
4.6.6 Mechanical design techniques for gaskets
4.6.7 Gasket clamping
4.7
Materials useful for shielding
4.7.1 Metals and their surface finishes
4.7.2 The problems of polymer passivation
4.7.3 Metallised papers and fabrics

This series is an update of the series first published in the UK
EMC Journal in 1999 [1], and includes basic good EMC
practices relevant for electronic, printed-circuit-board (PCB)
and mechanical designers in all applications areas (household,commercial, entertainment, industrial, medical and healthcare,
automotive, railway, marine, aerospace, military, etc.). Safety
risks caused by electromagnetic interference (EMI) are notcovered here; see [2] for more on this issue.
These articles deal with the practical issues of what EMC
techniques should generally be used and how they should
generally be applied. Why they are needed or why they work is
not covered (or, at least, not covered in any theoretical depth)– but they are well understood academically and well provenover decades of practice. A good understanding of the basics
of EMC is a great benefit in helping to prevent under- or overengineering, but goes beyond the scope of these articles.
The techniques covered in these six articles will be:1) Circuit design (digital, analogue, switch-mode,
communications), and choosing components
2) Cables and connectors
3) Filtering and suppressing transients
4) Shielding (screening)5) PCB layout (including transmission lines)
6) ESD, surge, electromechanical devices, power factor
correction, voltage fluctuations, supply dips and dropouts
Many textbooks and articles have been written about all of theabove topics, so this magazine article format can do no morethan introduce the various issues and point to the most importantof the basic good-practice EMC design techniques. References
are provided for further study and more in-depth EMC design
techniques.

Table of contents for this article
4. Part 4 – Shielding (screening)
Published in Issue 69, March 2007
4.1
Introduction
4.2
Shielding with metal plates
4.3
Volumetric shielding of products
4.3.1 Nested shielding and cost-of-manufacture
4.3.2 General concepts in volumetric shielding
4.3.3 Skin effect and absorption
4.3.4 Very low frequency shielding
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The EMC Journal July 2007

4.8
4.9

4.7.4 Paints and lacquers
4.7.5 Painted or plated plastics
4.7.6 Shielding with volume-conductive plastics
4.7.7 Alternatives to shielding plastic enclosures
4.7.8 Environmental considerations
4.7.9 Preventing corrosion
References
Acknowledgements

4. Part 4 – Shielding (screening)
4.6 EMC gaskets
EMC gaskets are conductive and compressible, and used toprevent apertures at joints, seams, doors and removable panelsfrom compromising SE. They are also used for ensuring correct
RF-bonding for connectors and filters. To function as intended
they require a good electrical contact all along both sides ofthe seam, door, joint, etc., so metal contact surfaces usuallyneed a conductive plating.

Figure 4BF Examples of some volume-conductive
elastomeric EMC gaskets
They have environmental sealing properties, and can suffer fromcompression-set if over-compressed. Compression-set isgenerally prevented by designing a groove which helps to retain
the gasket during assembly, and has mechanical features (like‘bump stops’) which prevent over-compression. (See later forgroove design).

Gaskets must meet a number of often-conflicting mechanicaland electrical requirements, not to mention chemical (e.g. toprevent corrosion). Shielding gaskets are sometimes required
to be environmental seals too, adding to the compromise. Wherea gasket does not return to its original shape when the pressure
is removed, it is suffering from ‘compression-set’, so is notsuitable for doors and removable panels. Considerations when
designing or selecting gaskets include:

The conductivity of these gaskets is not very high, even when
they are compressed to their optimum, so the SE they canachieve is not as good as metal mesh or spring finger types.
Hollow-core extruded elastomers are more suitable forgasketting plastic or sheet metal enclosures where highcompressive forces might distort the mounting areas anddegrade the SE of the enclosure, but they have even lower
conductivity than solid types.

 Mechanical compliance Compression-set
 Impedance over a wide range of frequencies
 Resistance to corrosion (galvanic compatibility with its
mating surfaces, appropriate for the intended environment,
see 4.7.9)

PTFE (Teflon) foam types filled with carbon particles areavailable (e.g. from W. L. Gore) and may be useful in combining
EMC shielding with environmental sealing in especiallyaggressive environments.

 Ability to withstand the expected rigours of normal use

A special type of volume-conductive gasket is supplied as aliquid and cured after being applied, often called ‘form-in-place’gasketting. Application can be manual (e.g. with a glue gun)but in high-volume manufacture it is usually robotically applied,
as shown in Figure 4BG.

 Shape and preparation of mounting surface
 Ease of assembly and disassembly Environmental sealing, smoke and fire characteristics

Some of these liquid gaskets can also be used as adhesives, forapplications such as that shown in Figure 4AH, and many of us
are familiar with the use of ‘conductive epoxy’ to repair
connections to the rear screen heaters in motorcars.

There are many types of EMC gaskets, and the main types are
discussed below.
4.6.1 Volume-conductive elastomers
These are elastomers with metal particles in them (usually tiny
metal-plated glass spheres), available in tape (extruded) formor as cast or die-cut materials, in a very wide variety of shapes
and sizes, see Figure 4BF. Solid elastomers can require quite
large pressures to compress adequately, making them difficult
to use in manually-opened doors without power assistance orlevers. Extruded types are available with hollow cross-sections,making them much ‘squashier’. If compressed overmuch they
can also suffer from compression-set.

The EMC Journal July 2007

The elastomer is usually silicone, which is quite stiff – making
it quite difficult to achieve adequate compression along its
length, especially with plastic parts. Recently, types that foamup after application have been developed, making much softerand more compliant gaskets that are easier to design with. These
are the types of materials used in the ‘mold-in-place’ gaskets
shown in Figure 4AY.

36

They have poor environmental sealing performance, but some
types are available bonded to an environmental seal, so that
two types of gasket may be applied in one operation. Also,
some types are available filled with an uncured silicone, which
provide good environmental sealing.

Figure 4BG Example of Form-In-Place (FIP) gaskets
being robotically applied
4.6.2 Conductively coated or wrapped elastomers, see
Figure 4BH
These are elastomer foams or tubes with conductive outercoatings or coverings of metallised fabric, with a low
compression-set in general. The elastomer is not conductive,
and merely provides a support function for its conductive
covering. They can have hollow cross-sections or be foam, andcan be very soft and flexible and only require low compressive
forces. However, they do not – in general – make the best
environmental seals, and their conductive layers may bevulnerable to wear.

Figure 4BJ Examples of some metal mesh EMC gaskets
Commonly used mesh materials include:

Phosphor Bronze
Tin-coated Copper Clad SteelSilver plated Brass
Monel

Wire mesh gaskets are available in tape, round or rectangularcross-section gaskets. Because they generally require highcompressive forces they are best mounted in a slot or a flangeof a stiff enclosure (see later). They tend to suffer fromcompression-set, so are not the best choice where a joint may
be repeatedly opened and closed, but they are a good choice
for sturdy metal parts that are permanently fastened, or where
replacement of the gasket each time the joint is opened would
not be a problem.
4.6.4 Spring fingers (‘finger stock’), see Figure 4BK
These are traditionally made from beryllium-copper or stainlesssteel and can be very compliant. Because some people are
becoming concerned about the possible health hazards ofberyllium, other materials are being developed, such as Laird
Technology’s ‘clean copper’.
Figure 4BH Examples of some conductively-coated or
wrapped polymer EMC gaskets

Spring fingers have very low compressive forces and nocompression-set, even if squashed flat for years, so are very
suitable for modules, doors and panels, that must be easy to
manually insert/extract and open, and which have a high levelof use. Their wiping contact action helps to maintain a good
RF bond by removing oxide and corrosion films and dirt, andthey have a good impedance match with metal surfaces.

Coatings and wrappings for these gaskets include:

metal films
knitted wire mesh ‘stockings’
metallised fabrics
metallised foils

Spring fingers are quite vulnerable to accidental damage, suchas snapping off by getting caught in a coat sleeve. The
dimensions of spring fingers and the gaps between them causes
inductance, so for high frequencies or critical use a double row
may be required, such as is often seen on the doors of most
EMC or RF test chambers.

4.6.3 Metal (wire) meshes, see Figure 4BJ
These can be random meshes or knitted types. They are
generally very stiff but match the impedance of metal enclosuresbetter and so provide better SEs than the above types. Some
types of gaskets use a thin knitted mesh ‘stocking’ over a foamcore (see later) to reduce the force required.
37

The EMC Journal July 2007

For shielded rooms with spring-finger door gaskets, the usual
instructions are to smear them with petroleum jelly once every
year, but this is rarely a requirement in equipment user
instructions.

examples are shown in Figure 4BL, which also shows somespring-finger gaskets for D-Type connectors, and for the
expansion card slots of PCs.
It can be easy to design spring finger gaskets into the metalwork
of a product, so that they do not require an additional assembly
step. Figure 4BM shows the example of a Sun Microsystems
server, where the attractive plastic cover was fitted with a plain
springy-steel sheet underneath, with spring fingers around theedges to make connection with the metal box in which the server
electronics was housed. Metal sheets like this can be cut andbent from plain sheet in one stamping operation, and are usuallytinned to provide a lower resistance contact.

Figure 4BK Examples of some spring finger EMC gaskets
Spring fingers can be mounted by a variety of methods:






gluing (often with a self-adhesive strip)
riveting
solderingweldingclipping onto a mechanical feature like an edge or a flangeon, or into a slot in the metalwork.

Figure 4BM Example of D-I-Y spring fingers

They need a flat contact area on both sides, plated with a highlyconductive material that is galvanically compatible with theplating of the fingerstock to help prevent corrosion. Materials
with tough oxide skins (like plain aluminium) or polymer
passivation are unsuitable. Although the best RF-bonds require
area contact rather than sharp points, there are types of spring
fingers with sharp points that can give better results with lessthan-perfect contact areas.

4.6.5 Some other types of gaskets
The four main types of gaskets have been described above, butthere are many other types, some of which are shown in Figure
4BN, for example…
 Graphite (best used under very high compression, between
machined surfaces) Oriented wires in silicone (good results with poor surfaces,
but require high pressure) Spiral foil (can be combined with cured or uncured silicone
to provide an environmental seal) Canted coil spring gaskets (often used in connectors andglands, see Figures 2T, 2V of [4])
 Metal fibre gaskets, using woven metal wire, sintered metal
fibre or expanded metal (used on flanged mating surfaces
where compressive forces are very high) Metal or metallised ‘velcro’ (mostly used for RF-bonding
seams in metallised fabrics)

Figure 4BL Examples of some special types of spring
finger EMC gaskets
Like some other types of gaskets, spring fingers can be madecircular, for use in RF-bonding the mating halves of circularshielded connectors together (e.g. as in Figure 2T of [4]). Some
The EMC Journal July 200738

Figure 4BN Some examples of other types of conductive
gasketFigure 4BQ More examples of gasket mechanical design
information

4.6.6 Mechanical design techniques for gaskets
Some gaskets require a low compression force, some a medium
force and others a high force. But even very soft gaskets canrequire a surprising amount of pressure overall, sufficient tobend quite sturdy items of metalwork, so careful mechanicaldesign is always required. The shielded enclosure must be
capable of achieving sufficient pressure to achieve the required
contact resistance, all along the length of the gasket. Somegaskets need up to 0.7MPa (100 psi) to achieve a low-enoughcontact resistance. Hollow elastomers generally need less than
180kg/metre (10 pounds/inch), whilst foam cored and spring
finger types might only need 20kg/metre (1 pound/inch).

Softer gaskets ease mechanical design, but are less likely to
break through films or oxide, corrosion or dirt to achieve goodelectrical contact over the life of the equipment, so they needhigh-quality corrosion-protected conductive surfaces for theircontacts on both sides.
Figure 4BR shows a typical gasket design for the door of anindustrial cabinet, using a conductive rubber or silicone
compound to provide an environmental seal as well as an EMC
shield. Spring fingers are also often used in such applications,but in this case are fixed to the side so that they wipe whenbeing closed or opened, as the photograph of the stainless-steelcabinet in Figure 4BR shows.

Designing lids, covers, doors, etc, so that they have sufficient
stiffness and fixings to compress the chosen type of gasket is
not easy (see 4.6.7) and is beyond the scope of this article.
Some gasket manufacturers supply very useful application notesthat provide a great deal of technical assistance with mechanicaldesign, such as the advice on using gaskets in a sheet metal
enclosure.
Figures 4BP and 4BQ show examples of the data and otherdesign information provided by gasket manufacturers to help
shielded enclosures be designed to compress their conductive
gaskets correctly. For more information, see [11], [12], [39],[40], [41] and [42].

Figure 4BR Examples of gaskets on industrial cabinet
doors
It is worth noting in passing that the green/yellow wire used forsafety earthing of a door or panel has no benefits for EMC,
above a few hundred kHz. This might be extended to a few
MHz if a number of short wide earthing straps are used, spreadalong a hinge, instead of a single wire.
Gaskets need appropriate mechanical provisions to be easy toassemble whilst also effective at maintaining SE. If they aresimply stuck on to a surface and squashed between mating parts
they may not work as well as was hoped – the more the fixing

Figure 4BP Some examples of gasket mechanical design
information
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The EMC Journal July 2007

screws are tightened in an effort to compress the gasket themore the gaps between the fixings can bow, opening up leaky
gaps.

The force required per fixing is determined by dividing thetotal compressive force required to compress the gasket
optimally (see Figure 4BP), and of course the fixings chosen
should be rated for at least their maximum tension. Where
groove design is inadequate for gasket protection, and bumpstops are not used, assembly should be carried out with torquecontrolled tools to ensure that gaskets are compressed correctly.

This is because of inadequate stiffness in the mating parts ofthe enclosure, but it is difficult to make the mating parts rigid
enough without a groove for the gasket to be squashed into.
This groove also helps correctly position and retain the gasket
during assembly. The dimensions of the groove ensure that thegasket is compressed optimally to give a low contact resistancewhen the mating half is correctly fitted.

Thinner or more flexible materials will require the fixing pitch
to be reduced to prevent distortion, which generally takes theform of bowing, creating apertures which can reduce SE as
shown in Figure 4BT. This problem is not uncommon whentrying to add gaskets to enclosures that were not designed totake them. Distortion becomes very obvious when, during
emissions testing, fixings are tightened to try to improve SE –but beyond a certain torque the SE is worsened instead.

The groove should be designed so that so that if the fastenings
are over-tightened, the gasket will not be compressed so muchas to suffer compression-set or other damage, and the matinghalf will not become distorted. Where grooves are not used,bump-stops and similar mechanical features should be used.
Figure 4BS sketches this type of design, and some partial
examples of manufacturer’s design information is shown in
Figure 4BQ.

Figure 4BT Fixings, bowings, and SE

4.7 Materials useful for shielding
4.7.1 Metals and their surface finishes
Steel and aluminium are often used to construct enclosures,
because they are relatively cheap and easy to cut, machine,
bend and join. Aluminium has a high conductivity but is very
reactive so its surfaces are always oxidised, and the oxide is agood insulator and very tough, making it difficult to RF-bond
to. The thickness of the oxide grows with time, so the reflectivityof plain aluminium decreases and RF-bonding becomes moredifficult.

Figure 4BS Example of gasket groove design
Figure 4BS sketches an enclosure that is either die-cast or milledfrom solid, but gasket manufacturers’ application notes describe
how to design sheet-metal enclosures for gasket retention andoptimal compression, as (partially) shown in Figure 4BR.
All gasket details and measures must be shown on
manufacturing drawings, and all proposed changes to themassessed for their impact on shielding and EMC. It is notuncommon, when painting work is transferred to a different
supplier, for gaskets to be made useless because masking
information was not put on the drawings. Changes in the painting
processes used can also have a deleterious effect (as candifferent painting operatives) due to varying degrees ofoverspray into gasket mounting areas which are not masked
off. The use of special conductive tapes with a masking layer isdiscussed in 4.7.4.

Anodising is a very common surface treatment for aluminium,
but works by increasing the thickness of the oxide layer, creating
an insulating surface. It is not an appropriate surface treatment
for a shielded enclosure, but I have seen products in which ananodised front panel was considered essential for its scratch
resistance, so at all the shield bonding points it was removed
by machining.
Alochrom, Alodine, Iridite, Oakite and tin plating are all names
for commercial high-conductivity passivating finishes. These
have good reflectivity, and help achieve good RF bonds, helping
to achieve good SE. Some of these methods rely on hexavalentchromium, which is being outlawed in the European Union by
the Restriction on Hazardous Substances directive (2002/95/
EC) so alternatives based on trivalent chromium and otherchemicals are being developed (see 4.7.8).

4.6.7 Gasket clamping
For high values of SE when using a metal enclosure that has
stiff flanges at its joints (e.g. cast metal box and lid, see Figure4BS) – the fixing pitch should not generally exceed 50mm (2inches). When using sheet metal enclosures, high values of SE
will generally require a fixing pitch not more than 19mm (0.75inch). Lower values of SE allow larger spacings between fixings.
The EMC Journal July 200740

Aluminium can also be tin-plated, although this is not a simple
process. Reasons for tin-plating are to make it solderable, or to
reduce galvanic potentials at metal contacts (e.g. with tin-plated
steel or copper) and so reduce corrosion (see 4.7.9).

Castings often use aluminium/zinc alloys, but magnesium alloys
are increasingly popular. Aluminium/zinc casting alloys canoften be polished to a high gloss, and if their zinc content ishigh they can retain a good surface conductivity for many years– but they do scratch easily. Chromate (trivalent) passivationor similar is generally required, and always required formagnesium alloys.

Mild steel has a reasonable conductivity and also has significant
relative permeability that helps shield low-frequency magnetic
fields (see 4.3.4 and Figure 4G of [20]). Steel surfaces alsooxidise (rust) so for good RF bonds in shielded enclosures steel
is usually plated with zinc or tin. Zinc can be plated as a metal,
or as ‘galvanising’, and the metal is better for EMC purposes.
Tin can also be plated in two forms, dull (matt) or bright – andthe dull finish is better for EMC as it is easier to make RF
bonds and solder to. Zinc and tin have higher conductivity thansteel, so plating with them improves reflectivity.

Apart from the EMC benefits (which make cast or machined
enclosures almost mandatory in some high-performance
applications), suitable design of cast or machined enclosures
can make them quicker and easier to assemble than sheet metal,
helping offset their generally higher material and tooling costs.
4.7.2 The problems of polymer passivation
Beware of ‘automatic’ passivation with polymers. Many buyers,
suppliers and metal platers assume that polymer passivation isalways required – even when not specified on the drawing –and it is no good specifying the surface conductivity to beachieved as some metal platers do not seem to understand theconcept and apply polymer passivation regardless.

Sheet steel that is galvanised or already zinc plated (e.g.
‘Zintec’) helps prevent rusting, but it will still rust at its cut
edges. Another problem with plated sheet metals such as Zintec
is that they are often supplied already passivated with a polymer
coating, which is an insulator, making it impossible to create
the RF-bonds required for good SE without using highcompressive-force conductive gaskets with all their mechanicaldesign difficulties. So it is best to fabricate the metal parts fromplain metals, and only then plate them with zinc or tin.

If the traditional ‘yellow passivation’ is applied it is obviousthat the plater has done something to the metal parts – but ifthey use a clear polymer it is impossible to tell it from baremetal by eye or touch.

Stainless steel has lower conductivity and permeability thanmild steel, is harder to work and more expensive, so is generally
only used in specialised areas such as in food preparation andother areas which are subject to regular wetting. But stainlesssteel cabinets are usually made of metal that is quite thick, andseam welded to a high quality (for hygiene purposes), so theyoften make excellent shielded enclosures (see the one
photographed in Figure 4BR).

Many product manufacturers suffer shielding problems until
they discover that their shiny metal parts actually have aninvisible insulating passivation layer. Sometimes, althoughmetal parts have been supplied with a perfectly good surfaceconductive finish for years, they can suddenly and without any
warning start to arrive with polymer passivation layer – thismay be because the company buyer has changed suppliers, orthe supplier has unilaterally decided to make the change.

Copper, brass, tin and similar metals have a high conductivity
and are easily worked but their high cost means they are
generally only used for small enclosures, such as PCB shieldingcans, and even then tin-plated steel is more common.

Based on the costly experiences of numerous manufacturers, I
now always recommend employing a surface conductivity testat goods-in, and including this test and its specifications in the
manufacturing drawings and purchasing order. It is best to havean agreement with the supplier that any deliveries that do notpass this test will be rejected back to the supplier at his own
cost, who must then replace them within a specified time withparts that pass the test.

Zinc metal plating can suffer from a heavy white ‘bloom’ inhigh humidity. A chromate conversion process can passivate
the zinc surface to prevent this, leaving a conductive surface(unlike polymer passivation). As discussed above, chromate
passivation processes are currently being modified to replacehexavalent chromium with its trivalent form, but not all metals
can be passivated as successfully so some further development
is required.

It is easy to make a suitable test device with a low-voltage
electronic buzzer mounted in a hand-held device fitted with
two spring-loaded contacts that press smooth conductive pads
onto the metal surface to be tested. For the best sensitivity tosurface conductivity, the conductive pads can be made of tinplated copper onto which are stuck some soft conductive
gaskets, such as the conductive-fabric-over-foam types (seebelow). The soft gaskets press against the sample, and apply auniform low pressure – helping to avoid the possibility thatany sharp edges, grit or swarf would cause an erroneously low
reading.

Where metal or metal parts are for use in constructing a shielded
enclosure, it is very important to specify ‘no passivation’ or‘chromate passivation only’ on metal drawings. And even so, asurface resistance test (using very smooth probes and low
pressure) is always recommended before accepting any batchesof sheet metal or metal parts into a manufacturer’s stores.
Enclosures made from cast or ‘machined-from-solid’ metal havesome advantages over those made from folded sheet metal,
including...

Because an alochromed aluminium surface looks much like ananodised one, and because anodising is the more common
treatment, it is easy for errors to be made in the design andpurchasing process. A product that had good SE when madewith alochromed aluminium can suddenly become noncompliant when constructed with anodised aluminium that looks

 fewer joints and seams (apertures) to degrade SE easier to include grooves for EMC gaskets
 stiffer, making it easier to compress lengths of gasket
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The EMC Journal July 2007

just the same. The solution is to check all parts that are supposed
to be conductive, before they are accepted into a manufacturer’s
stores, as discussed for polymer passivation above.

Compared with metal, metallised papers and fabrics have lower
conductivity, and so have lower values of reflectivity andabsorption.

4.7.3 Metallised papers and fabrics
Short fibres of polyester and similar materials can be metallised
and bonded into a paper-like material with a random alignmentof fibres. The coating on the individual fibres is very thin but
the paper can be made to various thicknesses to improve its
absorption. It is a low-cost material that makes good RF-bonds,
and is easy to cut and glue. It can even be pasted onto walls toconstruct a shielded room, when it is often called ‘EMC
wallpaper’. Figure 4BU shows some examples of metallised
paper materials being used to shield a product.

4.7.4 Paints and lacquers
Non-conductive paints and lacquers do not reduce thereflectivity of a metal surface. But they can create SE problemsby overspraying onto areas where they increase the impedanceof RF-bonds. Painting is often a manual process, so overspraycan vary as can the degree of skill used to mask off criticalareas. Changes in painting methods or technology can affectoverspray, so where masking was not previously required, newpainting techniques might make it essential, to maintain thedesired SE.
One solution that avoids having to control the painting process
is to use special metal tapes with a masking tape layer on top,that affix to the metal surface with a conductive pressuresensitive adhesive, available from 3M and others. Before themetal is painted, the tape is stuck onto the areas where thegaskets must make contact. After painting, the masking tapelayer is peeled off to reveal the bright shiny metal tape. The
metal under the tape is protected from oxidation and corrosion
by the conductive glue.
Conductive paints and epoxies consist of a binding agent andconductive filler. They have a lower conductivity than metal,
so have lower reflectivity. Silver-loaded epoxy is sometimes
used to reduce corrosion, and although its reflectivity is not as
good as the metal when it is new, it will probably be better than
if it was allowed to corrode.

Figure 4BU Example of the use of metallised paper
4.7.5 Painted or plated plastics
Plastics can be conductively coated by painting with conductive
paints (see 4.7.4); flame spraying; thermo spraying; plasma
flame spraying; or electroless plating (a chemical deposition
process). Suitable materials include graphite; silver; copper;
and nickel, but they do not usually achieve the same conductivity
as their bulk materials. Conductive paints require much thicker
layers for their SE to compare with metallised finishes, because
they are mostly binding agent so have low conductivity.

Conductive fabrics are made in a similar way, except that themetallised fibres are longer and are twisted into threads andthen woven to create a fabric. Conductive fabrics are often used
to make shielded tents, as shown in Figure 4BV, which havethe advantage of light weight and portability because they canbe folded for transport. Where a product needs to be shielded
and the appearance of the shielding does not matter, for example
a missile during transport, or a mobile phone that has been
seized by police or security forces, metallised fabrics are oftenvery appropriate materials.

The most important issue with conductively-coating plastics is
to ensure that the coating remains firmly stuck to the plastic
over at least the intended operational lifetime. Different typesof plastics require different coating materials and coatingprocesses, and flaking or peeling conductive films cancompromise reliability, and even increase safety risks.
Accelerated lifecycle tests are strongly recommended to ensurethat the conductive coatings don’t crack or flake off over the
anticipated life of the product despite its environmental
exposure (temperature, condensation, salt spray, etc.).
A problem with some conductive coatings, especially paints, isthat they can flake off if sufficient mechanical pressure isapplied, or rub off due to friction.
Metallised coatings are very thin, so have poor absorption at
frequencies below a few hundred MHz, Nickel is often used to
improve absorption at low frequencies, because it isferromagnetic. But despite the shortcomings of painting or
plating plastics, the SE of an enclosure made from such materials
is usually limited by apertures and conductor penetrations, justFigure 4BV Example of a shielded tent made frommetallised fabrics
The EMC Journal July 200742

as for enclosure made of solid metals.

but where it is used the final EMC testing should be done on
products that have used the automatic process, because theresults can be very different from the hand-applied coatings on
prototypes.

See 4.3 in [20] for the basic issues of shielding, which apply
equally to the conductive coatings on shielded plastic
enclosures.

As well as an internal conductive coating, an additional external
conductive coating can improve the SE of a plastic enclosure.
Conductively coating all sides of plastic parts can also help
overcome the problems with RF-bonding sketched in Figure
4BW. The external coating could be overpainted with something
more aesthetically pleasing, as long as overspray did notcompromise any RF bonds.

It can be difficult to get good RF bonds between the conductive
surfaces of a plated or conductively-painted plastic enclosure– especially if the coating was a retro-fit to an existing enclosurethat was not originally designed to be shielded. Figure 4BW
shows the typical problem. The parts to be RF-bonded havebeen conductively coated on their inside surfaces, but these do
not come into contact when assembled – creating an aperturein the shield.

Care should be taken, when retrofitting shielding to a plastic
enclosure by adding conductive coatings, not to increase safety
risks by decreasing creepage distances and clearances. It often
happens that immunity to electrostatic discharge (ESD) is
compromised, because air-discharge can be more likely to occur
due to the conductive coatings.

Module 4

4.7.6 Shielding with volume-conductive plastics
Volume-conductive plastics or resins generally use distributed
conductive particles or threads in an insulating binder thatprovides the mechanical strength. Typical conductive fillers
include: carbon fibres; carbon black; metal-coated glass beads;
nickel coated carbon fibres; and stainless steel fibres.
They often suffer from a ‘skin’ of the basic plastic or resin that
forms over the surfaces, making it difficult to achieve good RF
bonds without machining the surface, using helicoil inserts orsimilar methods. This insulating skin makes it difficult to prevent
long apertures being created at joints, and also makes it difficult
to provide good bonds to the bodies of connectors, glands, andfilters.

Figure 4BW A typical problem with RF-bonding coatedplastic enclosures
Even where the conductive coatings wrap around the joints,
contact never occurs along the full length of the seam – just as
with metal parts, contact only occurs at a few ‘high spots’.
Temperature variations could even cause the contact points tomove, changing the size of the shield apertures and making
enclosure SE unpredictable.

Other problems include the consistency of mixing the
conductive particles in the polymer or resin, which can makeenclosures weak in some areas (too much conductive filler),and lacking in shielding in others (too little conductive filler).
This is especially a problem at corners, which tend to suffer
form too little filler.

One way of dealing with this problem is to design shielded
plastic enclosures with ‘built-in’ plastic spring fingers. When
conductively-coated, these can make good RF-bonds to theirmating part’s shielding surface.

Materials based on carbon fibres (which are themselves
conductive) and self-conductive polymers are starting tobecome available, but they do not have the high conductivityof metal and so do not give as good an SE for a given thickness.

Conductive gaskets can be used at seams and joints, as describedfor metal enclosures above – but the lower Young’s modulus ofplastics means that achieving the compressive forces theyrequire, without causing mechanical distortion, is more difficult.
However, it is possible to design so that gaskets can be usedsuccessfully – although it may be very costly to retrofit suchdesign characteristics. So – where it is possible that a plastic
enclosure might need to be shielded using a conductive coating– it is strongly recommended to design it from the first with thenecessary ‘built-in’ spring fingers or fixings suitable for
conductive gaskets.

The conductivity of conductive plastics is generally much lower
than that of bulk material of the filler, so reflectivity andabsorption are both reduced.
The low conductivity of many conductive plastic coatings, andvolume-conductive plastics, prevents them from being able tohandle high fault currents or high levels of surge currents fromlightning. These high current events generate significant heat,
which damages the materials.
4.7.7 Alternatives to shielding plastic enclosures
Because of the Waste Electrical and Electronic Equipment in
the Environment directive (2002/96/EC) conductively-coatedor volume-conductive plastic parts are falling out of favour,because they are so difficult to recycle. Instead, products are
increasingly being designed using PCB-level shielding (see 4.4).

Prototypes usually use conductive coatings that are handapplied, and their quality and thickness will vary depending on
the skill of the operator. Where manual application is used inserial manufacture, less care might be used, and the SE suffer.
Automatic coating processes should give better repeatability,
43

The EMC Journal July 2007

Another alterative technique that is often employed is to fit athin metal (or metallised card or plastic) shielding box within
the plastic box, but around all the electronics. This metal box
does not have to look nice, or provide any mechanical support– so it can be low-cost. Retrofitting such a box to an existing
design is often very difficult indeed, if it is even possible. Ifsuch an internal shielding box might possibly be required – itis strongly recommended to design the product so that the box
can be fitted later in the project, if found to be necessary.

connectors, antennas and grounding structures is especially aproblem for RF transmitters, which have tight specifications
on their harmonic and spurious emissions so as not to interferewith other radio frequencies.
Gases such as oxygen, sulphur dioxide or similar pollutants
are usually dealt with by plating with a less reactive metal (e.g.
zinc plating on steel) or a number of other surface treatments,
discussed earlier. Multipoint RF-bonding using small, hard,contact points can generate pressures that are so high that coldwelding occurs, creating a small but gas-tight bond that is lesssusceptible to corrosion from gasses. Star washers are often
used for this purpose, but – as mentioned in the section on
spring fingers - RF bonds at higher frequencies benefit fromarea contacts instead of points.

Magnesium (or zinc) alloy castings can be used instead of plastic
mouldings, and of course are much easier to use as shields andmore easily recycled. Magnesium alloys can be as light as plastic
for greater strength, or thinner and lighter for the same strength,
but they cost more – so tend to be used for improving
ruggedness, reducing size and weight, or for fashionable items.

Liquids that bridge joints between dissimilar metals can causevery rapid corrosion due to the galvanic effect. Environmental
sealing gaskets can help keep certain gases and liquids awayfrom a joint, but it is also good practice to use similar metals incontact, because condensation can occur inside an equipment(unless anti-condensation heaters are used).

4.7.8 Environmental considerations
Two European Directives concerning the protection of theenvironment (known as WEEE and RoHS) are now in force inthe European Union. These directives will influence the typeof shielding used, and the materials used in their construction,
as has already been mentioned in 4.7.7 and other sections above.
The volatile chemicals used in some conductive coating andelectro-plating processes have a negative environmental impact,
and these processes tend to make the coated materials difficult
to recycle [38].

Corrosion is most likely when dissimilar metals are in close
proximity, or in contact, in the presence of electrolytes such as
water (e.g. condensation), beer, food and drink, jet fuel, bloodor tissue fluids, or a variety of other liquids. Two different metals
plus an electrolyte creates an ‘accidental battery’, and current
will flow from the cathode to the anode all the time the liquid
bridges between the two metals. The anodic end of theaccidental battery (the most positive in galvanic potential) has
its metal turned into corrosion products, whereas the cathodicend (the most negative) is hardly affected at all.

Chromate passivation using hexavalent chromium (‘Chrome
6’ or ‘Hex-Cr’) has been a marvellous technique for decades,
but when Hex-Cr gets into water supplies very serious cancer
outbreaks can occur. There are no direct replacements forsurface treatment with Hex-Cr that provide all of its good
properties on all metals (but without the cancer risks), but for
specific applications/metals there are replacement coatings thatcan be as good, maybe even better on some tests [43]. The US
Military has developed a trivalent chromium coating called TCP
for aluminium, which is also effective on some other metals,
and is available commercially. TCP applied to zinc-plated steel
also looks promising.

Metals can be divided into a number of groups according totheir galvanic potential, and five typical groups, withapproximately 0.3V range within each group, are shown
below….
Group 1 (Most anodic) magnesium, and magnesium alloys
Group 2 Aluminium and its alloys, cadmium, galvanised steel,and zincGroup 3 Tin, tin-lead solder, lead, duralumin alloys, iron, andlow alloy steels
Group 4 Nickel, monel, copper, brass, bronze, stainless steels,chromium, chrome steels
Group 5 (Most cathodic) silver, gold, platinum, graphite, andtitanium

Vacuum metallisation is ‘eco-friendly’, and tin and aluminium
are non-toxic and easy to recycle - so vacuum-metallised plastic
shielding-cans that are pressed, clipped or soldered into place
may have fewer environmental disadvantages thanconductively-coating the plastic enclosures themselves, or using
volume-conductive plastics. [25] describes how thermo-formed
shielding inserts can aid the recycling of plastic enclosures,and [29] describes how surface-mounted metal cans are easy
to remove and recycle.

Monel (in Group 4) is often claimed to be a metal that does notcorrode, and is often used in conductive gaskets. It does notoxidise readily in the air, but has no special resistance to galvanic
corrosion.

4.7.9 Preventing corrosion
Corrosion replaces metals with oxides, sulphides, chlorides,
etc., increasing the resistance at RF bonds and reducing SE.
Corrosion products are bulkier than their original metal, so tend
to force joints apart, opening up apertures and reducing SE.

Figure 4BX shows the recommended relationships in [41]between the groups the joint metals come from, the environment
the joint is in, and the additional protective measures (like greaseor painting). ‘Protected’ means indoors, inside a housing, notexposed to liquids and free from condensation almost all of thetime.

Some corrosion products behave as semi-conductors (non-linear
resistance), which can generate harmonics of any AC flowing
through them. They can also demodulate RF waveforms, andcan intermodulate two or more AC signals creating new
frequencies at their sum and difference frequencies. These can
add to emissions, or cause immunity problems. Corrosion in
The EMC Journal July 200744

with, for enclosure SE to be maintained reasonably well overthe life of the equipment. Figure 4BY shows the effect of astandard 144 hour salt spray test on two aluminium discs
separated by three different kinds of gasket. The least suitablegasket was sample A, whilst the best was sample C. Even thougha joint might not have to weather salt spray, 144 hours is not along time, so this test is an indication of how well a joint mightlast in more normal environments over a period of years.
Good gasket manufacturers should be able to supply a wealthof test data on the compatibility of their products with different
metals.
There are modern corrosion protection materials that mightprove useful, such as ‘vapour-phase corrosion inhibition’ (visit
www.cortecVpCI.com). I have no experience of this technique,but understand that this is based on pellets of a solid material
that slowly sublime, coating everything nearby with a molecularlayer that is a barrier to gasses and liquids, but is easily displacedby mechanical pressure, for example at an RF bond.

Figure 4BX Corrosion prevention guidance fromNAVAIR AD 115 [44]Dissimilar metals can be plated to reduce their galvanic effect,
for example tin-plated copper is a better partner for aluminium
than copper would be. When plated, the dissimilar metal joint
is protected from any liquids, so will not experience galvanic
corrosion. However, scratches or pinholes in the plating can
expose the underlying metal so the plating must be thick enoughfor the expected mechanical stresses, and also of sufficientquality.

4.8 References[1] Keith Armstrong, “Design Techniques for EMC”, UK EMC Journal, a 6-partseries published bi-monthly over the period February – December 1999. An
improved version of this original series is available from the “Publications& Downloads” page at www.cherryclough.com[2] The Institution of Engineering and Technology (IET, was the Institution ofElectrical Engineers, IEE), Professional Network on Functional Safety, “EMCand Functional Safety Resource List”, from the “Publications & Downloads”
page at www.cherryclough.com[3] Keith Armstrong, “Design Techniques for EMC, Part 3 – Filtering andSuppressing Transients”, The EMC Journal, September and November 2006,
and January 2007, from the EMC Journal archives at www.complianceclub.com.
[4] Keith Armstrong, “Design Techniques for EMC, Part 2 – Cables andConnectors”, The EMC Journal, May and July 2006, from the EMC Journalarchives at www.compliance-club.com.
[5] John R Barnes, “Robust Electronic Design Reference Book, Volume I”, Kluwer
Academic Publishers, 2004, ISBN: 1-4020-7737-8. See Chapter 34 forshielding and shielding design.
[6] Keith Armstrong, “Design Techniques for EMC, Part 0 – Introduction, andPart 1 – Circuit Design and Choice of Components”, The EMC Journal,
January 2006 pp 29-41, plus March 2006 pp 30-37, available from http://
www.compliance-club.com.
[7] “Shielding Design and Calculation”, Ron Brewer, Laird Technologies Tech
Notes #105, from www.lairdtech.com[8] RF Café, Skin Depth, http://www.rfcafe.com/references/electrical/
skin_depth.htm[9] “Shielding Methods”, Chomerics, http://www.chomerics.com/tech/
Shielding_methods.htm[10] “EMI Shielding Engineering Handbook”, Chomerics, available from: http://www.chomerics.com/tech/Shielding_methods.htm[11] The chapter entitled “EMI Shielding Theory & Gasket Design Guide” in the
Chomerics “EMI Shielding Engineering Handbook”, available from: http://
www.chomerics.com/products/emi_lit_contents.htm[12] “Technical Notes”, Laird Technologies, technical information and guidanceon shielding and gasketting, available from: http://www.lairdtech.com/pages/
catalogs/emi.asp (scroll down the page to the “Technical Notes” section).
[13] William A. Radasky, “The Threat and Detection of Electromagnetic Weaponsat Airports and on Aircraft”, 2005 IEEE Conference on Technologies forHomeland Security, Boston, Massachusetts, 26 April 2005, available fromhttp://.ieeexplore.ieee.org[14] “An Aperture Attenuation Modeling Program to Determine Shielding
Effectiveness of Printed Circuit Board Shields and Other Enclosures”, freespreadsheet from MAJR Products, Inc., download from http://www.majr.com/
freetool.htm[15] A free SE calculator (for far-field SE): www.emc.york.ac.uk/examples/
screening/index.html[16] A free SE calculator (for far-field SE): http://www.amanogawa.com/
index.html (click on ‘EMC/EMI’, then ‘Java Applets’, then on ‘Shielding
Effectiveness’)[17] Shabista Ali, Daniel S Weile, Thomas Clupper, “Effect of Near Field Radiatorson the Radiation Leakage Through Perforated Shields”, IEEE Transactions
on EMC, Vol 47, No. 2, May 2005, pp 367-373.
[18] Keith Armstrong, “Design Techniques for EMC, Part 3 – Filtering, andSuppressing Transients”, The EMC Journal, September and November 2006,
January 2007, available from http://www.compliance-club.com

AC or DC current through a dissimilar metal joint hastensgalvanic corrosion, even when a liquid is not present to act as
an electrolyte (this is why car battery terminals are always kept
very heavily greased). So it is best to use metals that are in thesame group, or the same metal, where currents could besignificant. Plating dissimilar metals with the same metal is agood way to prevent galvanic corrosion, for example tin-plated
aluminium with tin-plated copper, as long as the plated surfaces
are robust enough and don’t have pinholes.
Appendix E of [45] has a great deal of useful information anddesign guides on preventing corrosion, and a wealth ofreferences for further study.

Figure 4BY Example of a corrosion test on three different
gaskets
As mentioned in 4.6, conductive gaskets must also begalvanically compatible with the material they are in contact
45

The EMC Journal July 2007

[19] Tim Williams and Keith Armstrong, “EMC Testing”, a series in seven parts
published in the EMC & Compliance Journal 2001-2, available from the‘Publications & Downloads’ page at www.cherryclough.com.
[20] Keith Armstrong, “Design Techniques for EMC Part 4 – Shielding (Screening)
first half”, The EMC Journal, March 2007, pp 31-46, www.complianceclub.com[21] M K Armstrong, “GigaHertz Shielding at Printed Circuit Board Level —Eliminate or Reduce Enclosure Shielding Costs”, IEE “New EMC Issues in
Design” event held at Qinetiq, 28th April2004.
[22] Keith Armstrong, “EMC for Printed Circuit Boards (PCBs) – Basic andAdvanced Design Techniques”, Armstrong/Nutwood UK Ltd. January 2007,
view details at http://www.cherryclough.com, order frompam@nutwood.eu.com[23] The basic standard on microvia PCB technology is IPC-2315, from the Instituteof Printed Circuits at www.ipc.org.
[24] Steve Owen, “The Shielding Effectiveness of In-Mould Labelled Enclosures”,
EMC York 2000 Conference, 10-11 July 2000
[25] Chris Hills, “Formed EMI Shields, the Green Replacement for Conductive
Paint and Plating for Plastic Housings”, EMC Compliance Journal, June
2000, pp 36-39[26] Maria Sabrina Sarto et al, “New Light-Weight Formable Shields”, EMC
Compliance Journal, Aug. 2001, pp 17-19[27] Maria Sabrina Sarto et al, “An Innovative Shielding Concept for EMI
Reduction”, IEEE EMC Society Newsletter, Summer 2001, Issue No. 190,
pp 22-28[28] “EMC Shielding with Carbon Fibres”, IEE EMC Professional Network, EMC
Industry News 2002-11-21[29] Jack Black, “Automated Installation of Surface Mountable EMI Shields Using
Tape and Reel Packaging”, BMI Inc., Palatine, IL, USA[30] Darryl Yarborough, “EMI Shield Mounting Methods, Some Tried and Some
New, Through-Hole, Surface, and the New Hybrid”, Conformity, Jan. 2001,
pp 32-34[31] Gore-Shield GS5200 Gaskets, www.gore.com[32] Roy Bjorlin, Gary Shawhan, “Conformal Board-Level Shielding Products”,
Interference Technology Annual Guide 2003, pp 226-230[33] Thomas Clupper, “Improve PCB Shielding for Portable Devices”, Microwaves
and RF, May 2003, pp 72-84[34] Thomas Clupper, “A New PCB-Level Shielding Technology”, Interference
Technology Annual Guide 2003, pp 187-195[35] Eisuke Hanada et al, “Electromagnetic Attenuation with Stainless-Steel FiberMesh”, IEEE EMC Society Newsletter, 2003[36] Laird Technologies, www.lairdtech.com. To access their “Knowledge Base”,
click on “Technical Resources”.
[37] Tim Williams and Keith Armstrong, “EMC Testing”, a series of seven articles
in the EMC & Compliance Journal, Feb 2001 – Apr 2002, www.complianceclub.com/KeithArmstrongPortfolio[38] Rocky R Arnold, “Electronic Product Trends Drive New EMI/RFI Shielding
Solutions”, Interference Technology, EMC Test and Design Guide 2003, pp
48-53[39] J P Quine, A J Pesta, “Shielding Effectiveness of an Enclosure Employing
Gasketted Seams – Relation between SE and Gasket Transfer Impedance”,
IEEE 19995 International Symposium on EMC, Atlanta, 14-18 August 1995,
ISBN: 0-7803-2573-7, pp 392-395, available via http://ieeexplore.ieee.org[40]Gary Fenical,“Ten Considerations When Specifying an RF Gasket”,
Interference Technology, EMC Test & Design Guide 2005, pp 64-72,
www.interferencetechnology.com[41] Ed Nakauchi and Mike Oliver, “EMI Shielding with Thermal Management”,
EE – Evaluation Engineering, August 2002, www.evaluationengineering.com[42] W Drew Perigrim, “Plating to Obtain Maximum Shielding Performance”,
Laird Technologies Tech Notes #103, from www.lairdtech.com[43] Richard A Anderson, “Reviewing RoHS Compliance for Harsh Environments”,
Microwaves & RF, February 2007, pages 66-75. www.mwrf.com.
[44] NAVAIR AD 115 “Electromagnetic Compatibility Design Guide for Avionics
and Related Ground Equipment”, 3rd Edition June 1988, Naval Air Systems
Command, Department of the Navy, Washington DC, USA. This document
does not appear to be easily obtainable via the Internet, and has probably
been superseded by a more recent US military standard.
[45] John R Barnes, “Robust Electronic Design Reference Book, Volume II,
Appendices”, Kluwer Academic Publishers, 2004, ISBN 1-4020-7738-6

4.9 Acknowledgements
I am very grateful to the following people for suggesting a number ofcorrections, modifications and additions to the first series publishedin 1999 [1]: Feng Chen, Kevin Ellis, Neil Helsby, Alan Keenan, Mike
Langrish, Tom Liszka, Tom Sato, and John Woodgate.
Eur Ing Keith Armstrong CEng MIEE MIEEE
Partner, Cherry Clough Consultants, www.cherryclough.com,
Member EMCIA
Phone: +44 (0)1785 660 247, Fax: +44 (0)1785 660 247,
keith.armstrong@cherryclough.com; www.cherryclough.com
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