Conductive coatings

Many electronic products are enclosed in plastic cases for aesthetic or cost reasons. These can be made to provide a degree of electromagnetic shielding by covering one or both sides with a conductive coating. There are several ways to do this; the most popular are compared in the table below.

A large part of the comparison has to do with the nature of the application process rather than with the screening performance. Screening depends mostly on the conductivity of the material used, and this can be selected for each process. But other aspects, particularly the quantity of parts to be coated, the availability and cost of the processes, the complexity of the mechanical design, and the need (or not) for masking, have a greater impact on the choice of process.

Environmental factors, particularly abrasion and corrosion resistance and adhesion, are critical in the selection of the correct coating. Even a narrow scratch will act as an unintended aperture, and drastically reduce the shielding effectiveness. If the conductive material flakes off in use, under thermal or mechanical stress, this can create a real hazard of internal short circuits in the equipment. Therefore the material chosen for the coating must be guaranteed to be compatible with the plastic used for the substrate. This limits the choice of plastics.

The issue of masking often creates difficulties. Masking the piece-part moulding to prevent some areas from being coated always increases the price of the total part, and can be incompatible with certain processes. It may also place severe requirements on the mask tolerance, if the edge position of the coating is critical. But it's often a marketing requirement to have only the inside of the housing coated, so that the conductive finish doesn't spoil the appearance of the outside of the unit.

Mechanical design of the moulding that is to be coated is also crucial, to enable the lowest-cost and most effective coating process to be used. You should anticipate conductive coating from the start and ensure that it doesn't compromise the other important factors.

In summary, issues with conductive coatings are:

Good against HF E-fields,  ineffective against LF H-fields: because the skin effect confines RF currents to the surface of a screen, thin coatings will be as effective against electric fields at high frequencies as solid metal cases; but they are ineffective against LF magnetic fields

The lower the conductivity of the coating (or the higher its resistivity), the less its efficiency

Mechanical/environmental factors, particularly abrasion and corrosion resistance and adhesion, are critical in the selection of the correct coating

Even a narrow scratch will be an unintended aperture, and drastically reduce the shielding effectiveness. The same dimensional considerations apply to apertures and seams as before, and gaskets may be necessary to join separate parts; although you may have an opportunity for creative moulding design to eliminate the need for gaskets

Design of the base moulding is critical, to enable use of the lowest-cost and most effective coating process

If you are coating as a retro-fix, watch out for creepage and clearance surprises: many plastic enclosures are originally designed with the understanding that they are entirely insulating, both to the outside world and within the enclosure. When you apply a conductive coating, this property is lost, and therefore creepage and clearance distances that may be essential for safety purposes have to be re-established. This may mean masking certain areas of the coating or adding extra insulating components.

WEEE

Another constraint on conductively coated plastics is the need to consider end-of-life recyclability with the impact of the WEEE Directive. This places a limit on the proportion of material in a product which cannot be recycled; because the conductive coating and the plastic form a homogeneous assembly which can't be separated, the enclosure's recyclability is limited or impossible. This may steer you away from coatings towards individual piece-part screens which are assembled separately from the plastic case, or towards partitioning of the product or system so that only part of it needs to be screened.

Conductive windows

Viewing windows normally involve a large open area in the enclosure. You have a number of options:

cover the window with a transparent conductive material, which must make good contact to the surrounding shield. Shielded windows can be obtained which are laminated with fine blackened copper mesh, or which are coated with an extremely thin film of indium tin oxide (ITO) or gold. In either case, there is a tradeoff in viewing quality over a clear window, and screening effectiveness over a solid shield. A near field shielding effectiveness of 50-60dB at 10MHz, dropping to 20-30dB at 100MHz, is obtainable from an ITO coating which gives a sheet resistance of 10 ohms/square and allows a light transmission of around 80%. But shielded windows are relatively expensive, both in parts cost and installation cost.

alternatively to retain a clear window, shield behind the display with a sub-shield, which must of course make good all-round contact with the main panel. The electrical connections to the display must be filtered to preserve the shield’s integrity, and the display itself is unshielded and must therefore not be susceptible nor contain emitting sources. This alternative is frequently easier and cheaper than shielded windows. Using typical 6 to 12 inch TFT display modules, provided the frame of the module is carefully bonded to the surrounding enclosure and the connections are configured to minimise clock and video noise, commercial emissions and immunity standards are easily met, although more rigorous military and aerospace requirements are difficult.

accept that the display won't be shielded and you're not going to use a full shielded enclosure; shield only those parts of the product that really need it, by partitioning the system at an early stage, and make sure (as above) that the display module doesn't need it. This is a much preferable solution as long as your performance requirements aren't too onerous.