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EMC shielding for plastic enclosures

25 November 2010

Plastics have overtaken metals as the material of choice for portable electronics equipment enclosures, but that poses a fundamental problem of electromagnetic compatibility (EMC). Fortunately, conductive paints are now broadening the choice of coatings for plastic enclosures, as Christophe Loret explains

Plastics have overtaken metals as the material of choice for portable electronics equipment enclosures. But despite a host of benefits, including greater design freedom, and weight and cost savings, plastics, unlike metals, are unable to provide shielding for sensitive electronic components and circuits. This means designers need to understand and utilise shielded coating technologies.

Selecting the most appropriate conductive coating depends on a number of factors including: production volumes, size of parts, profile and complexity, degree of shielding required, reliability and the nature of the operating environment. Before we move on to conductive paints, let’s look at two well-proven processes and their limitations.

The vacuum metallisation process - sometimes referred to as ‘physical vapour deposition’ (PVD) – involves a metallic coating material being placed in a vacuum chamber along with the plastic component to be coated. The material that is being applied is then heated until it starts to evaporate; this vaporised metal condenses on the exposed product or surfaces as a thin metallic film.

Aluminium is typically used in this process as it has one of the highest conductivity levels apart from silver and copper; using silver in this type of process would be too expensive and copper is susceptible to corrosion. Because there is a need for parts masking and surface preparation, vacuum metallisation is best suited to small or medium batch quantities of small, limited complexity parts.

Electroless plating is a chemical process that applies duplex coatings consisting of a layer of electrolessly-deposited pure copper with an overcoat of electrolessly-deposited nickel-phosphorus alloy. Electroless shielding can be applied over the entire plastic part, or selectively to metalise only specified areas.

Conductive paints
Conductive paints offer an effective and easy-to-use approach to the shielding of plastic enclosures, particularly larger parts. Epoxy, polyurethane and acrylic coatings are available to meet the needs of various substrates, and these contain fillers of silver (highest performance), silver-plated copper, copper and nickel (lowest performance) to provide a range of shielding performance/cost options.

A significant advantage of conductive paints over other approaches is their ease of application using either a high volume/low pressure spray system or more conventional propeller-agitated pressure pots. These paints can be applied manually or via robotic systems for higher throughput and more consistent results. Paint types can also be matched to the plastic enclosure material to avoid time consuming and expensive surface treatments prior to their application.
 
As well as the type of plastic used, there are other factors affecting the choice of conductive paint, including the level of shielding required, the size of the part, the manufacturing volume, and whether the end product will be used indoors, or in a more challenging outdoor environment.

Conductive paint manufacturers are fully aware of the wider availability of higher temperature rated plastics, including reinforced PEEK and epoxy composites for harsh environment and high-specification applications, so new generations of conductive paints are being formulated to meet special requirements such as an ability to work at temperatures of up to 600oC.

Conductive plastics
Conductive plastics offer an alternative to coatings, the materials commonly consisting of nickel-plated carbon fibres integrated into an injection mouldable thermoplastic resin. Conductive plastics provide more design freedom, allowing complex features and geometries to be achieved and an enclosure wall thickness of just 0.8mm (depending on wall height) without compromise to the shielding performance. As injection mould tooling is involved, conductive plastics are better suited to the production of high volume, small sized components.

Compared with their metal-based shielded counterparts, enclosures made from conductive plastics can achieve total cost of ownership savings of up to 65%, while space and weight savings of up to 75% can have a big impact in areas where these factors are critical design goals.

Christophe Loret is conductive compounds product manager, Chomerics Division Europe Seal Group. The author’s company can supply a range of products with properties similar to those described, including its ‘Cho-Shield’ conductive paints and ‘Premier’ conductive plastics


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