The Essentials of Electronics and Electrical Equipment: Shielding

Effective EMI control prevents spurious signals from entering or leaving an enclosure. Shields and filters are the predominant techniques for controlling EMI.

Shields can involve many combinations of foils, conductive inks, paper, and adhesives. For example, there are now shields that consist of silver ink printed on 5-mil-thick polyester. After printing, a curing process removes nonconductive solvents. The result is a homogeneous shield that does not crack or delaminate when bent. The shield can be mechanically fastened and grounded with solder tabs. The cost is about the same as that for conventional laminated designs.

Carbon and stainless-steel fibers, combined with thermoplastics, provide an effective shield in many applications. Carbon fibers are usually classified as either PAN (polyacrylonitrile) or pitch. PAN fiber composites are selected for their high strength. Also, because PAN fiber has a higher aspect ratio (length to diameter ratio) than pitch, less is needed to provide a given conductivity.

Pitch-based fibers are not as strong as low-modulus PAN fibers. However, pitch fibers process easily into high-modulus products, making them attractive for stiffness-critical and thermally sensitive applications. The third carbon additive commonly used is carbon black. Carbon-black plastics are inexpensive, and are primarily for applications requiring high surface conductivity that allows dissipation of static charge.

Five factors affect plastic conductivity. The first is fill aspect ratio, which is proportional to conductivity. Second is loading level, also proportional to conductivity. The lowest fill loading needed to produce conductivity (generally defined as 10⁵Ω/sq) is called the critical concentration.
The third factor is resin type. The amount of fill needed for the critical concentration depends on the resin. For example, because nylon has a crystalline structure, its surface becomes conductive at lower fill concentrations than materials such as amorphous polycarbonate.


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