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Designing Enclosures to Minimize EMI/RFI

The new European Electromagnetic Compatibility regulations, which require a significant reduction in electromagnetic radiation emission, have prompted controls manufacturers to turn to enclosures for help.

BRIAN HINTON, Hoffman Engineering, Anoka, Minn., and ROBERT BAXTER, Schroff Inc., Warwick, R.I.

New European standards, which affect virtually every manufacturer of electrical and electronic devices who sell products in the European market, are the result of several trends that have been occurring as such devices become a part of almost every facet of our daily lives. Since both industry and commerce rely on electronic devices to provide a functional and reliable infrastructure, steps must be made to comply with European standards, reduce emissions of electromagnetic interference (EMI), and shield devices from EMI in the environment. Enclosure manufacturers can help significantly by working EMI shielding into their designs.

In general, enclosures perform five basic functions: provide a physical structure and support for electrical and electronic components; maintain components at the proper operating temperature and humidity; provide protection against physical damage, contamination, and/or tampering; distribute power and accommodate input/output cabling; and provide proper shielding to make electric/electronic devices compatible with each other.

While an enclosure made of electrically conductive material provides a certain amount of EMI shielding, the ideal enclosure for electromagnetic compliance would be a seamless sphere manufactured from highly conductive material, such as copper, aluminum, silver, or gold. Unfortunately, this solution is impractical and inhibits the other functions the enclosure must perform. In special applications where nonconductive composite materials are used for enclosures requiring EMI protection, special steps can be taken during the manufacturing process to ensure proper shielding.

Shielding effectiveness

The measure of an enclosure's ability to protect devices from EMI is known as shielding effectiveness (SE). Because electromagnetic energy behaves differently at different frequencies, the SE of a given enclosure is "frequency dependent." Therefore, any measure of SE should be in relation to a device's operational frequencies (see Figure 1).

Typically, SE is measured in decibels (dB). The dB is a logarithmic measure of efficiency and is described by the following ex- pression: SE 20log (power measure outside the enclosure) (power radi- ated from inside the enclosure)

It is important that all results describing the SE of the enclosures under consideration be derived using a standard method. In the U.S., MIL STD-285 is the standard test procedure that suppliers of electronic enclosures use to evaluate the SE of their packaging.

Electromagnetic emission factors

If an enclosure were a completely sealed conductive sphere, EMI would not be much of a problem. However, enclosures must provide for power and input/output cabling, doors, inspection panels, various holes and cutouts for switching, lights, or meters. Panels, doors, and modular attachments must be designed to provide for a continuous electrical continuity.

Doors on enclosures used in a clean-room environment can usually be electromagnetically sealed through the use of a continuous metal finger-style gasket such as those fabricated from beryllium copper (BeCu). If the enclosure is destined for the outdoors or a harsh, industrial environment where there is dust, oil, moisture, or corrosive chemicals, however, then a silicone or urethane gasket with molded-in fabric-based shielding is needed. Additionally, the mating surfaces of the door and the frame will be subject to corrosion over time, often requiring special treatments, such as plating, to provide maximum galvanic compatibility between structural and gasket materials.

When absolute watertightness is required, two gaskets can be employed on the perimeter of the door. An outer seal of foamed-in-place urethane will provide a watertight seal, and an inner conductive gasket will provide EMI shielding.

Cable entry and exit ports can be sources of EM emissions, as well as the cables themselves. Flexible silicone or urethane shielding material can be used to seal these holes, and multiple cables can often be twisted to promote signal cancellation. In industrial applications where exposed cables also need physical protection, continuous metallic wireways effectively seal openings, protect cabling, and provide shielding against EMI.

Openings for cooling vents, air conditioning, or heat exchangers also require treatment to prevent EMI. Often a simple metal mesh with openings no greater than one-tenth the wavelength of the highest applicable frequency provide adequate shielding. In general, however, EMI shielding treatments increase cooling requirements due to restricted airflow. In contaminated industrial environments, this may necessitate the use of heat exchangers.

Shielding enclosures

With the growing use of delicate process control equipment in harsh industrial applications, there is a need for EMI shielded enclosures made of nonconductive composites such as plastics or fiberglass. Since composites are not normally conductive, they provide no barrier to the transmission of electromagnetic energy and must be enhanced with a variety of treatments.

The interior surface of the composite enclosure can be sprayed with a carbon-based coating to create a continuous conductive barrier. A coating of approximately 6-10 mil thick is usually effective in EMI suppression over a broad range of frequencies. In general, suppression of the lower radio frequencies usually requires the thickest possible coatings.

Silicone or urethane EMI/RFI gasketing material is required around doors, and special effort must be made to create a meeting surface that is both conductive and durable. While a conductive spray is a practical solution, it is subject to wear on mating surfaces and, over time, the electrical continuity may be broken and allow emissions to enter or escape. Adhesive EMI gaskets on both mating surfaces are a practical solution.

Shielding 19-inch rack enclosures is accomplished at several levels of electronics packaging, and slightly different techniques are used for different types of electronics. For commercial 19-inch rackmounted electronics, it is often desirable to use a shielded rack.

It is helpful to remember that some commercially available solutions have acceptable SE ratings, but may compromise other enclosure functions or turn out to be cost-prohibitive. The challenge is to develop a cost-effective shielded enclosure that is easily cooled and lends itself to uncomplicated I/O point solutions. Only a bottom-up design effort, where all factors can play an equal role in the development of the application, will result in a solution that maximizes each packaging function.

Worldwide standards are increasing the EMI shielding requirements for all electrical and electronic devices. At the same time, there is a growing demand for more rugged, EMI-shielded electronic enclosures for use in harsh industrial environments. While there are ample design solutions for controlling most EMI, it is important that enclosure designers become involved early in the process to help lower costs, reduce design cycle time, and provide the best product for end-users.