Nov. 15, 2002
Effective EMI control prevents spurious signals from entering or leaving an enclosure.

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.

Processing method is the fourth factor. Improper processing can decrease fill aspect ratio. Finally, the conductivity of the fill itself also affects compound conductivity.

Several EMI blocking thermoplastics are available. One uses nickel-coated PAN carbon fibers. Nickel coating increases conductivity by up to 50 times. At 15% loading, the fibers have a conductivity equal to that of a 50% loading of uncoated fibers.

Another is based on stainless-steel fibers. These composites have conductivities equivalent to those of PAN, but at significantly lower percent weight concentration. As a result, the mechanical or processing characteristics of the resin base remain relatively unchanged. Polymers with 5% stainless steel can provide up to 40 dB of attenuation. And unlike other additives, stainless-steel fill allows enclosures to easily meet color specifications.

A common problem in enclosures is large area openings needed for displays or ventilation. While heavy screens can provide effective EMI shielding for ventilation holes, they often interfere with the optical characteristics of displays. Thus, manufacturers have developed special shielding windows. One type of window is a fine conductive screen laminated between glass or plastic sheets. Another is a screen cast within a plastic sheet. The third consists of glass or plastic with a transparent conductive coating such as indium-tin oxide.

Until recently, screen mesh density ranged from 30 openings/in. for 0.001-in.-diameter tungsten wire to 10 openings/in. for 0.0045-in. wire. The screens are knitted on machines originally developed for the textile industry and modified to handle wire. Shields of this type provide about 60 dB of attenuation at frequencies below 10 MHz.

Higher density screens have greater high-frequency attenuation, typically 60 dB at 1 GHz. These screens are woven rather than knitted, and the wires are often smaller than 0.005 in. in diameter. The screens use silver-plated stainless-steel wires; copper-plated stainless-steel wires; and copper wires. Meshes range from 80 to 150 mesh.

Shielding considerations extend beyond the enclosure itself. EMI can be radiated from cables entering or leaving the case.

Sometimes, cable-related EMI problems are not found until a system is installed. In the past, the only way to eliminate such problems was to replace the cable. Now several shields use a zipper closure to minimize interference and crosstalk. A dielectric spacer is inserted between layers of stacked ribbon cable, forming a sandwich. A shielded jacket covers the sandwich. Some jackets use a tinned-copper grounding braid attached to the inside of the jacket overlap. Zippertubing offers an almost leakproof seam. Together, the techniques can attenuate crosstalk by reducing capacitive coupling and serving as a current sink.

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