EMC & Circuit Protection


Mind the gap


While electronics engineers are familiar with addressing the issue of electromagnetic compatibility, in terms of good board layout, shielding of the enclosure is just as important. David Ward explains


T


he EMC directive has now been in force for 20 years and the latest version, being Directive


2004/108/EU, is due to be updated and harmonised in 2016 to Directive 2014/30/EU. EMC is the acronym for electromagnetic compatibility which means the ability of equipment to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to other equipment in that environment. Electronics engineers are very familiar with this and will consider in their design good board layout, filtering, grounding and signal integrity in order to try resolving EMI at its source. However, shielding of the enclosure is just as important and will help to solve the problems of radiated emissions and susceptibility.


Mating surfaces on an enclosure can look very flat and you can think there is full metal to metal contact but in reality in a mass production process nothing can be


that flat and gaps will and do exist. These gaps are slots and they can become radiating antennas. This joint unevenness can be addressed by using more fixings to get better contact between the mating surfaces and at lower frequencies this can work.


For example at 100MHz to achieve 20db shielding, gaps of 150mm are acceptable, this reduces to 15mm at 40db – where more and more fixings are required. However, at 1GHz it is impossible, as to achieve 40db the gap is now just 1.5mm. As a result these seams need to be filled with a conductive gasket and this will take up all the joint unevenness and depending on the type of gasket will also provide an environmental seal against dust and moisture.


Types of gasket


Shielding is a mechanical fix for an electrical problem and the enclosure design engineer should always be aware of the types of gaskets available and their different attributes and ensure there is enough land area on the enclosure seams, doors etc. to fit the gasket. In the quest to reduce the size and weight of electronic equipment there is a lot a pressure to reduce these land areas which makes this more important than ever.


There are 4 main gasket types available:


• Knitted wire mesh • Electrically conductive elastomers • Conductive fabric over foam • Metal: Beryllium copper/stainless steel


bonded to a carrier, however, it offers a good dust and moisture seal. It is often preferred for rugged military applications as it provides EMP protection as the gasket can carry high currents. Conductive elastomers are conductive particles loaded into silicone or fluorosilicone. Many different particles are available from carbon up to pure silver. The most popular are nickel coated graphite and silver plated aluminium, both have a low specific gravity making them more cost effective when compared to copper of nickel based fillers. All, apart from carbon, offer good all round shielding and high performance at all frequencies.


Nickel coated graphite is very cost


effective being up to five times less expensive than silver plated aluminium


Knitted mesh gaskets Conductive fabric over foam


and yet still able to offer shielding effectiveness similar to that of silver based products. The conductive elastomer compound can be extruded in continuous lengths in many different profiles and can be moulded as sheet and die cut or moulded as a component.


Conductive elastomer "O" rings make a very cost effective EMI seal and they also offer dust and moisture sealing. They are popular in defence and aerospace applications due to their small profile size, low weight and high performance.


Form in place Conductive elastomers


Knitted wire mesh was the original conductive gasket. It is and remains cost effective and can be fitted in grooves or surface mounted when bonded to a carrier. There are a number of wire options available. Monel which is an alloy of nickel and


copper, tin plated copper clad steel, aluminium and stainless steel. These different wire types give choice for galvanic compatibility and corrosion issues. Wire mesh which offers high performance in magnetic fields, particularly TCS as it is a high permeability material. It is easy to fabricate in order to provide complex gasket shapes. Knitted wire mesh is less effective at


Berylllium copper finger stock 44 September 2014


high frequencies as shielding effectiveness starts to drop off after 1GHz unless more layers of wire mesh are added. When


Components in Electronics


Form in place is a conductive elastomer, being conductive silicone in liquid form for depositing directly on to the enclosure hardware. This is particularly suited for use on small enclosures due to narrow land widths. Different fillers are available that are similar to that of conductive elastomers. This type of gasket offers a dust and moisture seal. It is not suitable for opening and closing of the enclosure. There are also logistical issues as the customers hardware needs to be sent to a FIP supplier. Oriented wire in silicone, also classed as a conductive elastomer are Monel or Aluminium wires vertically oriented in solid or sponge silicone, up to 140 wires per CM² (900/sq inch). Available as flat sheet material for surface mounting, the gasket can be die cut to shape or fabricated from strips to make a picture frame gasket and provides a good EMI and environmental seal. Again this type of gasket is very popular in defence applications as it offers very low contact resistance as the wires penetrate the mating surface. This material is not recommended for opening and closing applications such as doors. Conductive fabric over foam gaskets


are good for commercial applications, most are stick on surface mount and are soft and compliant making good cabinet door seals. These gaskets are very popular in lap top computers and gaming machines, for example, where they are used for grounding. They are also popular when used in shear/wiping applications


on doors for shielded rooms, they offer good all round high performance and are available in many plated finishes to enhance shielding and address galvanic concerns. Large gasket profiles are prone to damage and fingers can get broken so regular visual inspection is recommended. It should be noted that BeCu is falling out of favour due to health and safety issues and stainless steel is available as an alternative but stainless steel does not offer as good shielding and makes a poor spring.


Corrosion prevention When two different metals are in contact with each other in the presence of moisture or more so in salt spray, galvanic corrosion will occur and shielding will be reduced. It is therefore important to try and match as closely as possible the conductive gasket metal to the enclosure metal on the galvanic scale. For example a nickel coated (plated or painted) enclosure in contact with a nickel coated graphite in silicone elastomer gasket will be a perfect match, however at the other end of the scale a silver or copper based gasket (more noble metal) in contact with


such as card frames. Fabric over foam offers limited dust seal and no water seal. Shielding is good at high frequencies up to 10Ghz. Many profiles are UL94V0 qualified.


Beryllium copper fingers are good in shear/wiping applications and will be seen


Oriented wire in silicone


aluminium (less noble metal) in a salt spray environment corrosion will rapidly take place, causing a breakdown in the conducive joint as the potential difference between them is great. In harsh salt spray environments it is often better to use an outboard non- conductive seal to protect the EMI seal, this will ensure long term shielding performance as any corrosion issues will be eliminated. Other areas when choosing a EMI gasket that need need to be taken into consideration are: environmental sealing; compression/load forces; fixing/mounting methods; bend radius and reusability.


Kemtron | www.kemtron.co.uk


David Wall is managing director at Kemtron


www.cieonline.co.uk


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