Medical Device Link.

DESIGN

Screening materials, gaskets and ventilations have already been discussed in this series. However, on numerous occasions an enclosure’s design will need to incorporate intentionally large apertures for displays, keypads or switches. If a display or touchscreen needs to be incorporated into the design of the enclosure, the aperture will need to be screened using some form of conductive window. The two common types of screened optical window are conductive-coated and wire mesh. Both types will ultimately offer a reduction in light transmission and may require the use of a higher contrast display.

Conductively coated windows. These often take the form of a glass substrate onto which a primary layer of silicon oxide and a secondary layer of indium tin oxide (ITO) are placed. The ITO typically offers a surface resistance of 12 Ω/square. Because this layer is generally no more than approximately a few hundred nanometres and can be easily damaged by abrasion, it is recommended that the window is connected around its perimeter with a suitable conductive fabric over foam gasket or a silver-loaded silicon conductive gasket. Copper tape can also be used providing it uses a pressure-sensitive conductive adhesive.

Immersed metal wire gaskets or folded copper beryllium gaskets should be avoided because they will damage the delicate coating, especially if the enclosure is subject to vibration or rough handling. Laminated options that offer greater strength are also available. These can be ordered with antireflective or nonglare surface treatments and manufactured with custom edge profiles.

Touchscreens often use a plastic polyester alternative with a sputter-deposited ITO layer. The conductive coating tends to be extremely thin in comparison with the wavelength; it only works as a reflective screen (electric field) and will not offer much absorption attenuation for lower frequency magnetic fields.

Wire mesh windows. These offer excellent transparency, good screening and are usually manufactured using an extremely fine 304-grade stainless steel woven mesh, which has excellent optical properties. Not only can this be manufactured into the display, it can also be used to produce screened windows. This style of mesh window will offer superior magnetic low frequency because of its superior absorption loss.

The window can be made in polycarbonate or glass, and would normally be manufactured complete with a conductive silicon gasket around its perimeter. When held between the aperture and bezel it will give a continuous screened joint. The mesh weave can sometimes cause refraction of light compared with the less transparent coated window.

A lower-grade copper mesh can also be used and, although it is not of optical quality, it may be acceptable. The copper mesh could be laminated using a simple thermal laminator, which may be useful during research and development work. Once again, attention must be paid to ensure that a continuous bond between the screen enclosure and the woven mesh is made along its entire perimeter.

Membrane switches and keypads

A flexi shield has been developed that is available with a conductive adhesive, which can be used for membrane switches or splash-proof keypads. It normally uses a vacuum-deposited ITO coating and can achieve a surface resistivity of 12 Ω/square. This type of shield can be die cut and is supplied with a pressure-sensitive adhesive. It is imperative that a continuous electrical bond is made around the complete perimeter of the keypad or switch to avoid a slot radiator and subsequent RF leakage.

Plastic enclosures

Often, a plastic enclosure is favoured for items that require double insulation, that are electrostatic sensitive or for reasons of cost or ease-of-manufacture. In all these cases, a level of attenuation can be obtained using conductive coatings. Generally, a conductive coating will be thin, typically 10 μm, and the manufacturer of the coating will normally specify the surface resistivity in Ω/square. These coatings can be painted, but are usually sprayed on, or specialised companies can offer a zinc arc spray or an electroplating service in copper or nickel.

Conductive paints. Several types of RF-conductive paint coatings are commonly used such as copper and nickel or a silver and copper particle conductive spray. Copper and silver will offer superior surface conductivity compared with nickel and are likely to offer improved reflection- loss screening of higher frequencies. Conductive paint is also available in an aerosol, which can be useful during research and development work. The coating is normally applied to the inside of the enclosure. Limitations include the following:

• Unless it is applied thickly, it is unlikely to offer much absorption loss and thus it is not good for magnetic screening; this can be improved if a nickel-particle spray is used to provide a higher material permeability.
• The paint finish is easily scratched off and damaged, which will cause problems if a gasket is designed to bite into a material such as a beryllium copper twist.
• Conductivity will never be as good as that provided by an equivalent solid metal because of the multitude of individual contacts that need to be made between each metal particle suspended within the coating.
• Unless extremely accurate masking is used, over-spray will compromise the plastic insulation at seams and apertures.
• Styrene-based plastics can bloom unless the spray is water-based.
• It can be more demanding to engineer a good seam with little RF leakage. If a gasket is needed, a less abrasive silver-loaded “O” ring style gasket should be positioned around the flange perimeter; the conductive spray will also need to be applied to the gasket retaining groove to ensure continuity; the “O” ring seal will also offer added protection against moisture ingress.

Electro-plating. Although this will offer considerably higher conductivity than paints, it is still applied as a thin coating and therefore will not provide good magnetic screening. As with paints, magnetic screening can be improved if a nickel is chosen, albeit at the expense of higher frequency electrical screening. Electro-plating will require the use of a specialist company and tooling to ensure untreated areas are masked correctly.

Zinc arc spray. As a thermal process, the arc spray will penetrate into the plastic surface resulting in a rough finish. It will require the use of a specialist company. The accuracy of the applied coating is inferior to plating. Zinc arc spray should not be used with a copper gasket because galvanic corrosion is likely to occur.

Cables

Parts I and II of this article have dealt with the design of an RF-screened enclosure, but all this effort will be futile if the screen integrity is compromised by poor consideration of cables entering and exiting the enclosures. If the equipment is powered from a mains supply, use of an AC power filter is to be expected, perhaps built within an International Electrotechnical Commission socket. This filter must be bonded using a low inductive connection and is normally produced to enable the filter to be bolted directly to the chassis.

Unless noise is filtered just prior to the exit point on the chassis, screened cables should be used to contain the RF energy and prevent the cable radiating or picking up RF energy to and from the enclosure. Screened cables will only work if the outer-screened braid is connected to chassis ground around its entire circumference using an appropriate gland.

Where connectors are used, as in the case of a seam, all bulkheads, should be electrically bonded directly to the enclosure, if necessary using pre-cut finger stock. The mating plug or socket must be of the type designed for screened applications (normally identified by the use of a screened metal hood) and, once again, the braid must be connected around its entire circumference. Connectors with their outer braid terminated using pigtails must be avoided because they offer too much inductance.

Summary

The engineer will be able to design a successful screened enclosure using careful selection of materials, gaskets and, if necessary, screened optical windows. Many of the techniques covered in this series of articles can be employed during the research and development stage to arrive at a cost-effective solution that offers the required mechanical, aesthetical, environmental and electrical properties.

Jon Bearpark is Principal Engineer at RFI Global Services Ltd, Ewhurst Park, Ramsdall, Basingstoke RG26 5RQ, UK, tel. +44 1256 855 434, e-mail: jon.bearpark@rfi-global.com, www.rfi-global.com.