The reliable operation of modern microelectronic systems depends on the control of electromagnetic interference. Some forms of interference can be controlled by suppression of components and good circuit design. Radiated interference, however, is best controlled by enclosing the circuitry in a shielded enclosure. Metal enclosures generally offer very good shielding. Metals, being highly conductive, will absorb or reflect electromagnetic waves.
Plastics provide the enclosure designer with several advantages over metal. Reductions in weight and cost, coupled with increased flexibility and aesthetic appeal have led to increasing use of plastics for enclosures. However, being non-conductive, plastics are transparent to electromagnetic waves and hence offer no protection against EMI.
SHIELDING PLASTIC ENCLOSURES
To allow plastics to be used as enclosures for computers, mobile phones and other electronic products, several techniques have been developed over the past few years. To make plastic enclosures effective in protecting against EMI/RFI, there are essentially three routes the designer can consider: polymers can be made conductive by adding fillers; thin metal plates or foils can be bonded to the enclosure; or the surface of the enclosure can be coated with a conductive film.
Plastics can be filled with conductive media, and graphite particles or metal flakes are commonly used. However, these materials are difficult to mould and only provide moderate levels of shielding. In recent years the use of nickel-coated carbon fibres has improved the performance of these materials.
Enclosing the RF components in a metal can or bonding thin metal plates to the plastic enclosure will in some instances provide a cost effective method for shielding. In-mould foiling would provide an even lower cost option but the process is limited to fairly simple geometries.
By far the most common means of shielding an enclosure is to apply a conductive film to the surface of the enclosure. IVC can apply EMI shielding coatings using either vacuum deposition, electroplating or conductive paint spraying techniques.
APPLICATION OF CONDUCTIVE PAINTS
A large range of conductive paints has been developed by various companies for EMI shielding applications. Most of the formulations are based on adding either one or a combination of metal flakes of nickel, silver plated copper or pure silver.
Nickel loaded paints offer the cheapest solution but provide the lowest level of shielding while silver paints offer the highest degree of shielding at a premium. Although water based paints are available, solvent based coatings make for easier processing especially in high volume applications. Solvent based coatings are available in conventional solvents (MEK, MBK) as well as very mild solvents (ethyl alcohol) for use on solvent sensitive substrates and thin wall applications.
Developed some twenty years ago, nickel loaded paints were widely used in the early eighties. However, their use has declined in recent years as copper and silver pigment technology has developed. With increased usage, the price of silver loaded paints has fallen significantly in recent years and now most products such as mobile phones are shielded with silver loaded paints. Silver plated copper paints are widely used for shielding mouldings such as computer housings.
All paints can be applied manually or using robotic spraying equipment. In recent years the development of robotics and HVLP (high volume low pressure) guns has significantly improved the efficiency of spraying processes.
IVC now has several robotic spraying lines. These are used for spraying both high and low volume products.
One of the main advantages of conductive paint spraying is the wide range of coatings available. By careful evaluation, coatings that best fit a customer’s needs can be designed.
Shielding effectiveness (SE) is a measure of how effective the coating is as a barrier to the transmission of electromagnetic waves.
Typically the shielding effectiveness of coatings ranges from 40dB to 90dB. There is good correlation between the conductivity of the coating and the level of shielding: the lower the resistance, the better the shielding properties.
Shielding effectiveness can be varied by choosing the metal pigment in the paint. For a given paint formulation the sheet resistivity can also be varied by controlling the thickness of the coating. Figure 1 illustrates the variation of sheet resistivity with thickness for three standard formulations. It is interesting to note that, although the cost of silver paints may be more than copper and nickel loaded paints, for high shielding levels, a coating of silver paint may actually be cheaper. This is because a much thinner layer and, hence, lower paint usage is required.
Paints adhere well to most engineering plastics. However, as with all coating techniques, to ensure the best results, mouldings should be free from dirt, grease and mould release agents. Some plastics, ie glass reinforced polyester, glass reinforced phenolics, SMC, DMC and Polyurethane foam mouldings, may need an adhesion promoting primer prior to the application of conductive paint.
The compatibility of solvents in the paint system with the substrate material needs to be examined. The impact strengths of certain plastics can be reduced significantly on exposure to commonly used solvents. Current technology allows the selection of coatings that contain mild solvents that result in the elimination of craze or solvent attack caused by high stress levels or solvent sensitive substrates.
One of the main advantages of conductive paint spraying is the ability to coat mouldings of virtually any size and geometry.
To achieve optimum properties, most paints are cured at 60 degrees C to 70 degrees C for 30min. However, for mouldings which are not thermally stable, paints can be air dried.
Suitable for high or low volume production, conductive paints provide a method for EMI shielding of plastic enclosures, offering several advantages over other techniques. These include a wide range of formulations allowing coatings to be designed to meet customer requirements, excellent adhesion and environmental stability, relatively low cost, very high levels of shielding available with silver paint, ability to coat selective areas, relatively low tooling costs and short lead times to produce tooling.