There are many benefits to using plastic enclosures to house your electronics. With a multitude of suppliers serving the market, typically offering a wide range of standard products in different styles and sizes at economical prices, resulting in a reduction in the time to market for new projects. Products are readily available from the manufacturers themselves and from their broad-line and specialised distributors. The units are lightweight with potentially complex shapes and features are easily incorporated into the design, they are easily then modified to accept displays, switches, connectors and other hardware.
Moulded enclosures, however, have one specific shortcoming by virtue of the intrinsic properties of the material itself. Plastic, unlike metal, offers no inherent attenuation to electric or magnetic fields. In many applications, this is of no consequence, but if radiation emitted by the enclosed electronics or their susceptibility to external fields is a potential problem, the lack of screening could become an issue.
Enclosure manufacturers have addressed this issue typically by adding a conductive coating to the inside of the enclosures. There are however, some products on the market that are moulded from conductive plastic material. Whichever approach is used, and the coating method is by far the most popular, the design of the mouldings can have a significant impact on the screening effectiveness of the conductive coating.
The majority of plastic enclosures are constructed from a top and base moulding; in some designs there may be a battery compartment or removable end panels in addition. One of the key requirements for effective screening is that the internal surface of the enclosure should be as continuous as possible to ensure electrical conductivity between all its parts. In particular, long slots should be avoided as these act as a radiating slot antenna. To prevent slots between the mating halves of a typical enclosure, a tongue and groove structure forms an effective complex path, improving the attenuation performance. Obviously, the higher the frequency, the shorter the wavelength, so even very small gaps can have a detrimental effect on EMC performance. If there are removable end panels, they need to be secured into an interference fit slot or, if they are secured using fixings, a conductive gasket should be fitted to the mating surface. The rear face of plastic panels will have to be conductively coated, or if aluminium panels are used, the front surface will normally be anodised and the rear left with a natural finish or iridised, a RoHS-complaint conductive finish. The best methods of providing suitable continuity in enclosures with dedicated battery compartments will depend on the design. If the battery box is constructed with solid partitions between it and the main internal space of the enclosure, the only precautions that need to be taken are to ensure that the hole for the wires into the enclosure is as small as possible. If the battery is just clipped into mouldings in the enclosure without a partition, then the lid of the battery box will form part of the overall screening and will be a potential weak spot in the screening as there will typically only be a flat surface interface with the body of the enclosure itself.
Suppliers of conductive coatings have developed several different preferred materials for spray coating the inside of enclosures to achieve different levels of attenuation, balanced against cost. Vero Technologies has worked closely with its supplier partners, R F Solutions and Polymer Coatings, to offer three alternative coatings that will suit more than 95% of possible applications; for highly specialised uses, other coatings are available.
Meeting general commercial level requirements, a nickel based colloid offers acceptable attenuation at a competitive cost with a 50-micron thick film.
The material is COSHH, RoHS and REACH Compliant and provides attenuation of 60-65 dB at 50 microns when tested to ASTM ES7-83, a standard test method for measuring the electromagnetic shielding effectiveness of planar materials. After temperature ageing of seven days at 29.4°C at 95% RH there was no degradation of properties after the environmental testing.
For more severe requirements, a copper colloid, formulated with silver-coated copper particles and conductive resins provides a conductive layer. It provides an effective shield against RFI and EMI and can act as a ground plane to protect against electrostatic discharge (ESD).
As can be seen, high frequency performance is much better than that of nickel; inevitably, given the relative costs of the base materials, the higher attenuation costs more. While not shown on the above graph, the material has been tested up to 10 GHz to Military Standard MIL STD 285, typically providing 78 dB at 10 GHz. The material is COSHH, RoHS and REACH Compliant. After temperature ageing for days at 85°C at 85% RH and 10 cycles of temperature cycling of 75°C for 1hr, ambient for one hour and -30°C for one hour, and 56 days of high humidity testing at 35°C and 95% RH for 56 days, there was no degradation of the attenuation properties. Striking the optimum balance between cost and performance, the copper colloid is Vero Technologies’ default coating material, although higher or lower performance coatings can be specified as required.
The highest level of attenuation is achieved using a silver colloid, formed from silver flakes and conductive resins. It provides a highly effective shield against RFI and EMI and can act as a ground plane to protect against electrostatic discharge (ESD).
The material is COSHH, RoHS and REACH Compliant. After temperature ageing for days at 85°C at 85% RH and 10 cycles of temperature cycling of 75°C for 1hr, ambient for one hour and -30°C for one hour, and 56 days of high humidity testing at 35°C and 95% RH for 56 days, there was no degradation of the attenuation properties.
Many applications require a display to be incorporated into the enclosure, normally viewed through a transparent window. Two main alternatives are available to preserve the integrity of the internal conductive coating when a window is required. A wire mesh will provide continuity at the expense of clarity. The preferred option, originally developed for use on military helicopters, is a clear conductive coating that provides the required electrical conductivity without obscuring the display. Vero technologies can apply the clear coating to any window material as required by the needs of the project.
Electrostatic Discharge is a possible problem that can be minimised by the application of graphite or carbon-based high conductivity coatings. ESD events can occur without a visible or audible spark at low voltages around 10 V, sufficient to damage sensitive electronic components, causing immediate failure or degrading their long-term reliability and performance.
Several different types of coating can be applied for specific applications. Whilst they are obviously mutually exclusive, for medical use anti-bacteriological coatings inhibit the growth of e-coli and MRSA as well as other bacteria. Fire retardant low-smoke coatings can be applied for aerospace applications, protective coatings that resist harmful substances found in petrochemical, pharmaceutical and similar industries are available, low friction coatings reduce surface wear and high visibility florescent, luminescent and iridescent coatings have benefits in safety-critical applications.
Moulded enclosures are a popular choice of housing. Available in sizes from key fobs, through hand-held units up to desktop instrument housings, they provide style and functionality at low cost. With the addition of high performance coatings to the internal and external surfaces of the units, they also provide enhanced protection against damage to the housed electronics from a wide range of external threats.