Conductive coatings for EMI/RFI shielding

Growth in the market for electronic equipment in both industry and the domestic sphere has brought unusual prominence to the problem of electromagnetic shielding.

Much has been said and even more written about electromagnetic compatibility.

However there can be no doubt that the quantitative growth in the market for electronic equipment in both industry and the domestic sphere has brought unusual prominence to the problem of electromagnetic shielding.

Not only are computers, mobile phones and other electronic devices being introduced which require higher levels of shielding, but demand for miniaturization, low weight, short manufacturing lead times and recyclability are providing additional challenges for the equipment designers.

Designing the enclosure in plastic rather than metal offers many advantages.

However plastics have one drawback - being non-conductors they are transparent to electromagnetic radiation.

Applying a conductive coating to the internal walls of an injection moulding can be a fast and efficient method of shielding the product.

There are three major techniques currently used for applying conductive coatings: Vacuum Metallisation, Paint spraying and Electroplating Using the above techniques several coatings have been developed to provide EMI / RFI shielding.

Product designers have to consider a number of criterion to determine the best shielding solution for their product, these include; Level of shielding required, Impact of the coating on the intrinsic properties of the plastic, Adhesion and long-term stability of the coating, Wear and durability, Manufacturing costs, lead-times and capacity, Issues relating to recyclability and environmental compliance Although most of the above issues have long-since been addressed, environmental concerns, as well as the drive by some OEMs towards recyclability have only recently seen prominence.

Recognising these requirements some time ago, ACT embarked on a programme of technology advancement to ensure that all processes can provide an environmentally clean solution without compromising other vital properties.

This led to the following results: The Elamet process is a patented process developed by Degussa in Germany.

It has been specifically developed for coating plastic components with a pure aluminium coating to provide high levels of EMI/RFI shielding.

It is the most environmentally friendly shielding process requiring no solvents and producing no harmful by products.

It is already chosen by many enlightened OEMs specifically for these qualities.

The coatings have a smooth metallic appearance and are highly ductile.

They adhere to most engineering plastics with UL approval of over 65 substrates available.

The process deposits a pure layer of aluminium (99.8%) and as aluminium dissolves in sodium hydroxide parts are easy to recycle.

Coatings produced offer considerable weight saving compared to competitive techniques and very high shielding.

Using multi-impression, high precision masking tools parts can be coated selectively.

A wide range of metal-loaded paints are used to provide EMI / RFI protection, although silver or silver-plated copper loaded paints are the most commonly specified.

Historically all paints contained harsh solvents (usually MEK) to improve adhesion to the substrate.

However the trend towards thin-walled mouldings involving PC / ABS blends has amplified the need to minimise the impact of the coating on the substrate resulting in the development of 'Safe on Substrate' technologies.

These paints are formulated using mild solvents and can be mechanically removed to allow the plastics to be re-cycled.

It is interesting to note that in a number of high volume applications (mobile phones) the value of silver recovered after removing and burning off the coating has been sufficient to pay for the recycling process.

Water-based paints which allow the coating to be chemically removed when recycling are currently being evaluated.

It is expected that these will be introduced later in the year.

Two types of electroplating process are commonly used for EMI shielding of plastic enclosures; electroless and electrolytic plating.

These coatings are unique in their ability to combine aesthetic appeal, wear and corrosion resistance with very high levels of shielding.

A nickel layer is chemically or electrically deposited over a layer of pure copper.

The process does not lend itself to selective finishing; hence it is usually specified on internal components.

For recycling parts are dipped in an acid solution to remove the plated layers.

ACT has pioneered the development of conductive coatings for achieving EMC compliance and can assist OEMs in ensuring that the optimum shielding solution is specified.

All potential customers will be provided with technical advice and support in shielding theory, design and the application of coatings and gaskets.

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