Metal-filled electrically conductive organic paints and coatings are used on electronic chassis and enclosures to absorb and attenuate electromagnetic energy, thus, shielding the internal electronics. Electrically conductive coatings are also used to dissipate electrostatic charges (ESD) that build up on plastic or glass parts, to ground surfaces, and to prevent corona discharge. Conductive coatings allow static charges to be distributed evenly over the surface and allow charges to be bled off to ground. Generally, a surface resistance of <105 ohm/sq is required, but the most effective compositions have surface resistances of 0.03 to 1.0 ohm/sq/mil. Besides these electrical requirements, EMI shielding coatings must meet a combination of other requirements among which are:
• Ability to be sprayed or otherwise deposited in uniform thicknesses of 1 to 2 mils.
• Good adhesion to a variety of metal, plastic, and glass surfaces of which enclosures may consist. Examples of plastics used for enclosures include ABS, Noryl*, polystyrene, and polycarbonate.
• Rapid room-temperature or low-temperature cure or dry in a reasonable time of 30 to 60 minutes.
• Noncorrosive, especially to aluminum under salt fog and humid environments.
• Abrasion resistant.
• Resistant to jet fuel (JP4), hydraulic fluids, oils, and greases that may be encountered in aircraft applications.
• Electrically stable at temperature extremes.
There are numerous one-part and two-part EMI/ESD compositions on the market that can be cured or dried at room temperature or cured at moderate temperatures. Most of the compositions are sprayable or brushable. Polymer binders include acrylics, polyurethanes, epoxies, silicones, and blends such as vinyl-acrylics and acrylic-urethanes. Fillers include silver, copper, silver-plated copper, blends of silver and silver-plated copper, nickel, and ferrous metals. Table 4.13 presents properties of some commercially available EMI coatings.
Commercial applications of coatings for both EMI and ESD include plastic enclosures for notebook and desktop PCs, routers, servers, medical electronics, and telephone handsets. With the proliferation of wireless electronic devices, EMI shielding has become critical to their performance. Wireless transmitters and transceivers radiate electromagnetic signals. Emissions result from both electric and magnetic fields surrounding current-carrying conductors or wires. These signals radiate into surrounding areas and couple with nearby signal lines, cables, and other conductors. Even though PWBs and PWAs can be designed to minimize or avoid EMI, it becomes increasingly difficult as frequencies increase beyond 800 MHz and into the GHz regions in which most wireless devices operate. At high frequencies, the wavelengths are so short that many circuit conductors have lengths that make them act as efficient quarter- or half-wavelength antennas. Thus, enclosures with EMI shielding become essential.[91]
MIL-HDBK-263 (ESD Control Handbook for Protection of Electrical and Electronic Parts, Assemblies, and Equipment) provides guidelines for the design and application of materials and methods to obviate or mitigate electrostatic effects.