Most equipment manufactures choose to design custom housings for their electronics products
in order to differentiate them in the market place. Usually enclosure design is driven by the two
beasts of beauty and cost, with EMC requirements an irritating afterthought.
The ideal enclosure from an EMC point of view is the perfect Faraday cage. Take the PCBs
that make up a typical product, wrap them in a seamless monolith of copper without apertures
and bingo! You’ve fixed it. Unfortunately this solution is impractical.
The art of good enclosure design is therefore to get “as close as is necessary” to the Faraday
cage, without sacrificing the aesthetic appeal of the product or significantly increasing its cost.
For most designers “As close as is necessary” is usually defined as meeting standard or type
specific requirements. This usually involves at least:
(i) Containing Radiated Emissions
(ii) Providing Radiated Immunity
(iii) Providing ESD Immunity
(iv) Providing Fast Transient Immunity
In designing a compliant enclosure the designer needs to look at the following:
(i) Mechanical components - their material composition and coatings
(ii) Electrically bonding the enclosure components
(iii) Electrically bonding the internal hardware and its interfaces to the enclosure
(iv) Dealing with apertures in the enclosure
(v) Screening and partitioning inside the enclosure
(vi) Earthing the product
Component Material and Coatings
Generally the enclosure designer will work with components made from folded steel or
aluminium alloy or bulk extruded alloy and possibly vacuum formed aluminium or plastic.
Where the volumes of finished product justify the tooling expense, the designer might also use
injection moulded plastic or rubber or die cast aluminium.
Often housings made entirely of conductive metal components form poor EMC
enclosures simply because the individual components have been painted or coated in a
non-conductive material.
There are however a wide range of affordable conductive coatings available. For example:
= For aluminium components - alochrome plating rather than non-conductive anodising.
= For steel components - zinc galvanising or passivated zinc plating (zinc and chromate
passivated) for corrosion resistance. Where aesthetic appeal is important - bright nickel
or chromium plating. Note: Zintec is a sheet steel with conductive zinc coating and can
be used as an alternative to plating where corrosion at the cut edges of a component is
not an issue.
= For plastic components - internal metalisation or conductive carbon coating. Partially
conductive carbon loaded plastics can be used but often the strength and flexibility of a
loaded plastic is inferior to an unloaded component.
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