Tips for Preventing Electromagnetic Interference
Wireless baby monitors top the gift lists of many expectant parents. These devices allow parents to hear their baby crying no matter where they are in the house. It's ironic, then, that a device designed to give parents peace of mind could also be a source of anxiety for airline pilots. But, that was exactly the case 3 years ago in Britain.
Pilots approaching Luton Airport, about 30 miles north of London, reported occasional electromagnetic interference (EMI) in their radio communication with air traffic control. Investigators pinpointed the source of the interference as wireless baby monitors in nearby homes.
In another bit of irony, the interference problem had nothing to do with the wireless technology itself. The real culprit was a faulty plug-top power supply. Although the power supply functioned well enough, it sometimes emitted electromagnetic radiation in the same wavelength as the VHF radio channels that pilots use to communicate with air traffic control. The emissions were caused by a spurious oscillation that occurred when the internal cables were in a certain position.
The power supply used a switch-mode power converter. These devices switch at very high rates to maximize efficiency and minimize heat losses. As a result, they can emit electromagnetic radiation at up to 1,000 times their basic switching rate. Such power supplies must include filters and shielding components to prevent their emissions from exceeding statutory limits and interfering with other electronic equipment.
How could such "noisy" power supplies reach the market? Maybe the manufacturer only performed EMI testing on a sample of power supplies that did not exhibit the spurious oscillation. Or, perhaps engineers only tested the power supply for emissions up to 30 megahertz, in the mistaken belief that the device could not generate emissions above that frequency.
Regardless of the reason, the case illustrates the importance of preventing EMI and addressing the issue early in the design stage.
"Roughly half the time, engineers wait too late in the design of a product to deal with shielding or EMI problems," says Gary Fenical, technical sales representative for electromagnetic compatibility at Laird Technologies (Delaware Water Gap, PA). "Once the board has been laid out and the case has been designed, it's too far down the road to deal with the problem in the most cost-effective way."
Because most electronic assemblies don't need EMI shielding to work correctly, these components are seldom designed into a product from the beginning. Instead, says Fenical, EMI problems are discovered during compliance testing, and "engineers are forced to scramble around to get the thing to meet requirements."
Countering EMI problems upfront is even more important now, given the size of today's electronic products. As portable electronics get smaller and more powerful, problems with EMI become more common and difficult to mitigate.
Fortunately, assemblers have myriad options to prevent electronic assemblies from causing or receiving EMI. These run the gamut from custom metal stampings to carbon nanotubes. Which to choose depends on the mechanical, electrical, environmental, safety, cost and end-of-life requirements of the assembly.
Take the Case
For weight and styling reasons, many electronic devices have plastic cases. However, because it's nonconductive, plastic is no barrier to EMI.
There are three ways around this problem. One is to mold the case from a plastic filled with conductive particles. However, a filled plastic may be unable to meet requirements for moldability, assembly or aesthetics. Another option is to line the interior of the case with a die-cut metal foil or conductive fabric. This is a viable option as long as the geometry of the case is relatively simple. A third option is to metallize the inside surfaces of the housing.
The latter option can be accomplished through vacuum deposition, electroless plating, arc spray or, most commonly, a conductive polymer applied with a spray nozzle. The polymer can be a two-part epoxy, a one-part urethane or a one-part acrylic. To provide conductivity, fine particles of nickel, silver, copper or carbon are mixed into the polymer. Nickel-filled polymers are relatively inexpensive and provide moderate levels of EMI shielding over a wide frequency range. Silver-filled polymers provide better shielding performance, but are more expensive.
The coatings are applied at a thickness ranging from 0.003 to 0.08 millimeter. Depending on the formulation, the coatings provide EMI shielding levels of 40 to 100 decibels over a frequency range of 30 megahertz to 1 gigahertz. Maximum surface resistivity ranges from 0.03 to 2 ohms per square.
In the future, nanotechnology may obviate the need for conductive coatings. For example, a major cell phone manufacturer plans to produce phone cases from nanotube-reinforced, recyclable plastic within the next 2 years. The new material will be stronger and lighter than conventional plastics, and it will also be impervious to electromagnetic radiation.
Don't Blow a Gasket
Shielding gaskets prevent electromagnetic waves from leaving or entering through seams in an electrical enclosure. These gaskets are available in a wide variety of designs and materials. Engineers should select a gasket that best meets requirements for mechanical compliance; compression set; impedance; environmental and corrosion resistance; shape and preparation of the mounting surface; and ease of assembly and disassembly.
There are five main types of shielding gaskets: prefabricated conductive elastomers, formed-in-place conductive elastomers, wire meshes, spring fingers and metal spirals.
Prefabricated conductive elastomers are molded or extruded prior to assembly. They are available as narrow strips with standard cross-sections that are cut to length and pressed or bonded into a groove in a flange. They can also be custom-molded in circles, rectangles or any other contiguous shape to match a specific assembly.
These gaskets are made of silicone, neoprene, butadiene-acrylonitrile, natural rubber or urethane foam. The elastomer is filled conductive particles, coated with metal, or wrapped with a conductive fabric. These materials have a low compression set and do not require significant contact pressure to create a seal. However, they do not provide the best environmental seal, and their conductive coating may be vulnerable to wear, says Keith Armstrong, president of Cherry Clough Consultants (Brocton, UK), an engineering firm specializing in EMI issues.
Formed-in-place gaskets are silicone adhesives filled with particles of silver, copper, aluminum, tungsten-carbide or other metal. With automated equipment, these materials can be dispensed on flanges as narrow as 0.5 millimeter. The primary advantage of formed-in-place gaskets is that they are created on demand. They can be dispensed in any amount, along any path. If the product design changes, engineers simply reprogram the dispensing robot.
Formed-in-place gaskets have a low compression set and adhere well to both metal and plastic. They tolerate a wide range of temperatures and provide a good environmental seal. However, they require significant contact pressure to achieve a good seal, so they may be inappropriate for use with doors unless there's a lever or other device to push against the material, says Armstrong.
Wire mesh gaskets consist of thin metal wire knitted into narrow strips that are cut to length and inserted into a groove in a flange. The gaskets can be pressed, bonded, spot-welded or riveted in place.
Any metal that can be made into wire can be used for this type of gasket. Some common metals include tin-plated phosphor bronze; tin-coated, copper-clad steel; silver-plated brass; Monel; and aluminum. Wire mesh strips are typically made in four basic cross-sections: rectangular, round, round with a fin, and double-cored.
These gaskets are very stiff, and they match the impedance of metal enclosures. Alone, they can't provide a pressure or weather seal. However, some wire meshes can be bonded to a polymer so that they can provide some environmental sealing.
In addition, wire mesh can be knitted into wide strips and backed with pressure-sensitive adhesive for use as shielding tape. And, like molded elastomers, wire mesh can be custom-made into circles, rectangles or any other flat, contiguous shape.
Spring fingers are thin, curved strips of beryllium copper or stainless steel. They can be bonded, clipped or riveted to an assembly. They are very compliant and are ideal for use with doors and other seams that see a high level of opening and closing, says Armstrong. They have the same impedance as metal enclosures, but they may require more maintenance than other types of gaskets.
Wound from spring-temper beryllium copper, spiral gaskets are cut to length and squeezed into a groove in the flange. Expansion of the spiral keeps the gasket in the groove. These gaskets have excellent spring memory and compression resistance. The spiral is plated with a tin-lead alloy to enhance its conductivity and shielding properties.
The gasket can be made in any diameter, ranging from 0.034 to 1.5 inches. The standard gasket is hollow, but moderate- and low-force versions include a cord insert made from PVC, silicone or rubber. This cord acts as a mechanical stop to prevent overcompression during handling and use.