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EMC coatings and antimicrobial doping for moulded enclosures

Author : John Morton is with Vero Technologies

05 May 2017

By using Vero Technologies’ standard electronic enclosures, customers can reduce their Time to Market.

Vero antimicrobial enclosures

The enclosures can also be quickly modified to incorporate I/O and other connectors, displays, light pipes etc. Thus the customer has a semi-custom design in only a couple of weeks. 

As well this being a very cost effective route to market, other issues such as EMC & MRSA problems can be addressed as well:-

EMC

Moulded enclosures have one specific shortcoming: by virtue of the intrinsic properties of the material itself, plastic, unlike metal, offers no inherent attenuation to the passage of electric or magnetic fields. In many applications, this deficiency is of no consequence, but if radiation emitted by the housed electronics or their susceptibility to external fields is a potential problem, the lack of screening could be an issue.

Enclosure manufacturers have addressed this issue by adding conductive coatings to the inside of the enclosures, although 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 design there may be battery compartments and removable end panels as well. 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, and in particular long slots should be avoided. A tongue and groove structure forms an effective complex path, improving the attenuation 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.

Suppliers of conductive coatings have developed several different main materials for spraying the inside of enclosures. Vero Technologies offers three alternative coatings to achieve different levels of attenuation at differing price points to suit more than 95 percent of possible applications; for highly specialised uses, other coatings are available.

For general commercial level requirements, a nickel based colloid offers acceptable attenuation at a competitive cost with a 50-micron thick film.

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).

The high frequency performance is much better than that of nickel; inevitably, given the relative costs of the base materials, the higher attenuation costs more. 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.

Veronex copper

The highest attenuation is achieved using a silver colloid, formed of silver flakes and conductive resins. It provides an effective shield against RFI and EMI and can also act as a ground plane to protect against electrostatic discharge (ESD).

Transparent windows

Many applications require a display to be incorporated into the enclosure, normally viewed through a transparent window. For several recent projects, Vero has developed a technique by which a transparent conductive film is sprayed onto the window in the same way as the body of the device is coated. While there are some trade-offs against vacuum deposition in terms of coating uniformity, opacity and conductivity, the results are totally acceptable for a normal 'commercial' application. Across the visible light spectrum, wavelengths from roughly 390 to 700 µm, the transmission is around 90 percent at thicknesses of up to 6 µm, falling slightly to 85 percent at 12 µm. The EMC attenuation is directly proportional to the thickness of the coating.

Medical applications

Hospital acquired infections have become a serious problem in recent years. While improved housekeeping and new guidelines for staff have reduced the problem to a certain extent, the underlying problem of bacterial growth on surfaces remains. To help address the issue, Vero Technologies is now able to offer its ranges of plastic and metal enclosures and panels with Biomaster, a silver-based antimicrobial additive in the material, which provides an effective means of reducing bacteria by up to 99.99 percent for the active life of the product. The key feature is that the silver ion material is added to the raw ABS before the moulding process. As the additive becomes an integral part of the material, the efficacy of the protection does not diminish over time, giving far superior protection to external coatings that will inevitably degrade through normal wear and tear, routine cleaning and accidental damage.

External coatings

Several different types of coating can be applied for specific applications. 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.

Conclusion

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.


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