Electronics need to be shielded to provide protection from external interference and also to prevent unwanted radiation escaping and causing problems elsewhere. Several alternative coating techniques are available qqqfor plastic components. Paul Stevens reports on qqqrecent technological developments.

One of the most astonishing phenomena in the last decade has been the growth in the application of electronics in consumer goods and industrial products.
Furthermore, the use of radio communications - the most obvious being mobile telephones - has led to a dramatic surge in interest in shielding against radio frequency interference (RFI) and electromagnetic interference (EMI), though, strictly speaking, RFI is simply a subset of EMI, due to radio waves being just part of the electromagnetic spectrum.

External waves

In some cases the need is to improve protection from external electromagnetic waves, while, in other cases, the main driver is to prevent leakage of electromagnetic waves that could affect other equipment.
Metallic casings, on the whole, do a good job of protecting against EMI/RFI, but this does not help the designer who wishes to use plastics to benefit from their lower cost, lighter weight and versatility in production.
Using a metallic shield within a plastic casing is often successful, but the additional cost, weight and space requirement for the separate shielding is seldom acceptable.

Metallic component

There is, however, an alternative that has rapidly gained in popularity: coating plastic components with a conductive material to provide a level of protection similar to that provided by a metallic component (Fig. 1). Three main techniques are available, but it is important to opt for the most cost-effective for each application.
Recent advances in the technologies have also meant that a previous optimum may no longer be the best for the same application now.
Electroplating has traditionally been a popular way of covering a component with a conductive layer.
Being a 'wet' process, electroplating is best used when all surfaces of a component need to be covered; selective plating can be achieved, but it is more costly because of the masking that is required.

Durable coating

Designers are also limited in the range of plastics that can be plated, ABS being the one that most readily accepts plating. But the result can be excellent, with high levels of shielding and a durable coating that can withstand the flexing required when, for example, snap fits are being assembled.
Typical applications for plating, which is usually nickel on copper, are shields and shielding cans that are clamped to PCBs (printed circuit boards) for use inside a product to provide localised protection. Electroplated coatings are also used for some exterior applications, such as for mobile telephone handsets and automotive products.

Electroplating plastics

A relatively recent development in the electroplating of plastics is electroless plating, which gives a far more uniform coating thickness and, therefore, more consistent surface conductivity and EMI shielding performance.
In contrast to electrolytic plating, the electroless plating process requires no external electrical current to sustain the process (Fig. 2).
Instead, catalytic reduction of metal ions on the surface of the substrate takes place, resulting in a uniform coating, even for deep recesses.
Since shielding is primarily achieved by the reflection of electromagnetic waves, a uniform electroless plated coating of 2 microns can provide almost the same protection as a much thicker - typically 22-25 microns - coating deposited electrolytically.
An additional benefit is that the reduced weight resulting from a thinner coating is very welcome for certain applications, especially when compared to spray-painted coatings.
If plated parts need to be recycled, the metallic coatings can be removed by dipping in acid to dissolve the nickel and copper, which can later be recovered.

Vacuum metallisation

Although the vacuum metallisation process gives
a similar coating thickness to electroless plating,
typically in the region of 2.5 to 5 microns, the material used is aluminium, which offers better shielding performance than the copper/nickel combination applied by plating.
Moreover, the use of pure aluminium also results in vacuum metallisation's main advantage: it is the most environmentally friendly. This is because there are no solvents involved in the application, and the aluminium and plastic substrate can both be recycled if the aluminium is first removed by dipping the component in a caustic solution (sodium hydroxide). Vacuum metallisation is therefore often used by manufacturers who are environmentally aware and are producing small components in volumes large enough to justify the cost of the tooling.
Plastics that can be treated by vacuum metallisation include ABS and polycarbonate; although the full choice is wider than for plating, the range is still extremely limited.
Vacuum metallisation chambers usually measure some 2m long by 2m diameter, allowing around 1500 mobile telephone casings or similar-sized components to be loaded.
Processing then takes around 45 minutes, so the volume that can be processed per week is significant. Tooling is initially expensive to purchase, but the high volumes tend to permit the tooling cost to be amortised easily, and careful design of the tooling also provides any masking for areas that do not need to be coated.
The process itself is 'line of sight' only, so there is less freedom to decide which parts of a component are to be coated.
Both electroplating and vacuum metallisation are characterised by high capital costs, meaning that very few manufacturers can justify the huge investment required to install their own processing plant. Sub-contract companies are also limited, leading to a restricted choice for manufacturers who decide to use one of these processes.

Conductive paint spraying

In contrast, a huge number of companies exist in the paint-spraying sector, and many have tried to expand by offering to apply conductive paints. However, achieving a high-performance shielding finish is not as straightforward as achieving a high-quality aesthetic finish, and this has led to conductive paint spraying gaining a reputation as a less effective shielding method than the other alternatives.
Nevertheless, paint spraying remains the most widely used technique, accounting for perhaps 60 to 70 per cent of all shielding coatings.
And thanks to recent developments in the materials that can be sprayed, this proportion is likely to increase at a healthy rate. Traditionally the paints were based on nickel, which, with nickel being a relatively poor conductor, needed to be applied to a thickness of 50 to 70 microns.
In the past three years, newer materials have become available based on silver-plated copper particles. These are some ten-times more conductive than the nickel-based products, and cost less as well! Thanks to the huge take-up by manufacturers of mobile telephone handsets, the price of conductive paint spraying has fallen by around 70 per cent in the last five years.
Recycling is also proving to be economical thanks to the relatively high value of the silver that can be recovered.
Of the three coating techniques described, paint spraying offers the greatest flexibility - in that it can be carried out manually or robotically, on small or large volumes - and with the shortest lead times.
Coating thickness for modern paints is usually around 25 microns, giving excellent shielding performance. The latest generation of paints are also formulated with solvents that have less impact on the environment - often being alcohol-based now instead of MEK.
Virtually any plastic substrate can be sprayed, though care has to be taken to ensure that the solvent in the spray does not affect the impact strength of the substrate or lead to stress cracking.

Vacuum metalisation

Compared with vacuum metallisation, the tooling is both quicker to produce and less costly; whereas tooling for vacuum metallisation will take perhaps six or eight weeks, tooling for paint spraying could be delivered in just three or four weeks and cost one-third of the price.
For large parts or those requiring selective coating of specific areas, paint spraying is almost always the most economical technique.
But no single technique will ever be ideal for all applications, and paint spraying is no exception to this rule. If the paint is being sprayed by hand, the process is heavily operator-dependent.
One problem that can arise, for example, is that loose particles can be left on the surface if the paint is not applied correctly.
Automation can overcome the problems associated with manual application, giving improved consistency, but only if the volumes or quality requirements justify the investment in automation.
Although there is currently a great deal of interest in coating techniques for plastics, there are some other developments that could significantly change the situation in the future.
Research is ongoing into plastics that are inherently conductive, which could negate the need for coatings. Another possibility is the use of conductive fillers that are blended with a base polymer.
Stainless steel and nickel-coated carbon fibre particles are just two that are being investigated, with some already being used in components where the shielding demands are low and aesthetics are of little importance. As the materials are developed further, this market is likely to grow.

In-mould laminating

Other techniques that are worth considering for some applications are the lining of components with metallic foil, and in-mould laminating to create components that effectively have a thin metallic shield integral with the plastic moulding.
Clearly the field of electromagnetic interference/radio frequency interference shielding is developing fast, so designers should ensure that they consider all of the options before specifying a process for a new component - regardless of what was specified before.

With thanks to Dr Yasin Zaka, managing director of Applied Coating Technologies Limited, Tipton, UK, for his assistance in the preparation of this article.