Circuit Protection
Managing the dark art of shielding protection
Electromagnetic interference presents a growing challenge for product designers. Bruno Chaigneau considers some of the diverse options that are available to engineers but warns that working with EMI shielding specialists from an early stage is crucial
T
he prominence of electronics in both the personal and work lives of the industrialised world’s population has
grown beyond what anyone would have imagined twenty or thirty years ago. Sectors including automotive, medical, military & aerospace and communications have seen electronic content proliferate with benefits such as increased functionality, greater convenience, and smaller form factors.
Whilst digital and analogue technology has evolved with the integration of multiple devices into single packages and increasing performance from smaller geometries, the shielding challenges that arise from densely packed circuitry that puts power and sensitive digital in close proximity have increased. The use of elastomers combined with various fillers to enhance their performance, has given engineers a wide and diverse range of materials that offer the dual function of providing a gasket between two mating surfaces and shielding from electromagnetic interference (EMI). Shielding technology, often considered something of a dark art, has evolved over many years. Early materials utilised fillers such as silver and silver-plated copper in silicone elastomers, while more recently, cost-effective silver-plated aluminium and nickel-plated graphite composites have become a popular choice for product design engineers.
Diverse applications
The diverse range of applications and environments in which electronic equipment can be used means EMI shielding materials need to be carefully selected. For example, an automotive application might see exposure to a range of oils and lubricants as well as temperature excursions as low as – 55ºC and as high as +100ºC. Meanwhile, applications in the food industry may see contact with an even wider range of substances that could include coffee, sauces and beverages.
EMI shielding materials used on equipment designed for indoor use are placed under limited demands from an environmental perspective, whereas
30 September 2012
outdoor applications will typically see greater extremes of temperature and be affected by factors such as corrosive sea salt, airborne pollutants, and high humidity. Regardless of the application or the
environment in which it is designed to be used, the provision of EMI shielding ideally needs to be part of the engineer’s thought process from an early stage of the design. Careful enclosure design and relative positioning of particular components can, to a degree, mitigate the need for large amounts of shielding. However, the crowded nature of modern product designs means that EMI issues are much more likely to cause problems than in the past.
attributes that make it suitable, or indeed unsuitable, for specific applications. In terms of the cost of the finished conductive elastomeric material, the main contributor, and by a significant margin, is the filler used rather than the base elastomer.
Silicone is well suited to applications
where there are extremes of both high and low temperature. Its suitability for outdoor applications is further enhanced by excellent weathering, ageing and ozone resistance characteristics Silicone also exhibits good dielectric properties. Fluorosilicone exhibits similar high and low temperature characteristics to silicone as well as excellent weathering, ageing, ozone resistance and dielectric properties. In addition very good resistance to fuels, oils, aliphatic solvents, water, dilute bases & acids makes the use of fluorosilicone in harsh or unpredictable indoor and outdoor applications a good choice. Fluorocarbon has excellent compression set making it useful in assemblies that may need to taken apart and re-assembled at times during their working lives. Excellent resistance to oil, fuels, aliphatic, aromatic
Silver/Copper (Ag/Cu) is able to resist the highest levels of electromagnetic pulse (EMP) induced current. It is ideal for high- end commercial applications in non- corrosive (usually indoor) environments. Nickel/Aluminium (Ni/Al) is the optimum filler for delivering high shielding performance in the most challenging environments. It is usually combined with silicone or fluourosilicone elastomers. Silver/Aluminium (Ag/Al) when combined with a fluorosilicone elastomer, is ideal for military grade applications where high shielding performance and resistance to corrosion are high priorities. Silver (Ag) is an expensive option, but
provides a very high level of shielding and through conductivity performance. This filler is suited to use in non-corrosive environments. When combined with silicone it provides a low closure force, high shielding effectiveness solution. The compromise is low tear strength and limited fluid resistance.
Moulded versus extruded Moulded gaskets can be made with very narrow tolerances for precision applications. However designers need to factor in the cost of tooling when deciding whether to opt for a moulded rather than extruded approach. Extruded gaskets have larger tolerances and are offered in a wide range of cross-sections. Compared to moulded gaskets they require longer set- up and secondary processes to cut and splice the gasket to the desired finished length.
Conclusion
Even with careful design, it is usual for some EMI issues to not become apparent until a fully working packaged prototype of the product is ‘powered up’. It is then a case of utilising the diverse range of materials available from suppliers such as Parker Chomerics to manage EMI and so ensure the long-term reliable operation of the equipment. Working closely with EMI material specialists from the early stages of a project can help potential problem areas be anticipated. Invariably, early recognition and management of such issues in a design will lead to a more cost-effective, reliable and elegant end design.
Elastomers
The choice of elastomer bases used in extruded and moulded EMI shielding materials include silicones, fluorosilicones or fluorocarbons, and Ethylene Propylene Diene Monomers – better known as EPDM. Each elastomer has particular
Components in Electronics
and chlorinated solvents and acids, plus very low gas permeability are the key attributes of fluorocarbons. They also have good high and low temperature resistance. EPDM exhibits excellent resistance to weathering, ozone, dilute bases and acids. It can also be used effectively in applications where water and steam are present.
Fillers
The metals that are combined with the base elastomer – also known as fillers - are what determine the shielding performance of the finished extruded or moulded part. Their precise, uniform dispersion within the binder produces materials with stable and consistent electrical and physical properties. Some of the commonly used materials and their primary properties and degree of performance are described below. Nickel/Carbon (Ni/C) provides low-end shielding and ESD protection with very limited corrosion and fluid resistance.
Once used mainly to shield critical defence and aerospace electronic systems, extruded and moulded shielded gaskets are now required in a much broader spectrum of both military and commercial applications; this reflects the growing complexity of everyday electronics equipment that many of us take for granted. Conductive elastomers from materials specialists have become popular with packaging designers of a wide and diverse range of products and equipment in the consumer, telecommunications, business, industrial equipment, automotive and medical device markets. Working with EMI shielding specialists from an early stage of the design process can help reduce the cost and complexity of the solutions employed to address EMI problems.
Chomerics |
www.parker.com/chomerics
Bruno Chaigneau is an Applications Engineer at Parker Chomerics
www.cieonline.co.uk
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