An MD&DI February 1998 Column
EMI FIELD NOTESSelecting the proper EMI gasket is a complex task that requires careful examination of all criteria.
T he medical device industry clearly understands how to use EMI gaskets to address the design issues associated with electromagnetic compatibility of electronic products and to meet regulatory requirements. However, what is not universally grasped are the specific design considerations necessary for selecting the proper gasket for a particular product. This article describes different types of EMI gaskets and their applications. Moreover, it examines the specific issues that determine the appropriate gasket choice.
EMI gaskets can be grouped according to their material composition.
Wire Mesh. Wire mesh gaskets are composed of tin-plated, copper-clad steel or Monel wire that has been knitted on industrial knitting machines and formed into circular or rectangular cross sections. In other versions, the wire is knitted over a silicone or neoprene sponge core or over a solid silicone core with a hollow cross section. Other variations include attaching the wire to nonconductive elastomers for environmental sealing to metal frames. Compressed mesh gaskets are also available. The most recent addition to this class of gaskets is a tin-plated steel wire mesh gasket with an air core.
An array of typical EMI shielding gaskets used in protecting medical electronics equipment. Photo courtesy of Chomerics Div., Parker Hannifin Corp. (Woburn, MA)
Finger Stock. These gaskets are formed beryllium-copper spring fingers in various cross sections, including spiral wrapped. They can be tin or nickel plated.
Wires Oriented in Elastomer. Such gaskets are spring-loaded, Monel wires oriented in one direction in either a solid or a sponge silicone matrix.
Metal Screens Impregnated with Elastomer. A woven aluminum screen impregnated with silicone or neoprene or an expanded Monel foil impregnated with silicone elastomer characterizes these gaskets.
Conductive Yarn, Conductive Fabric, or Foil over Nonconductive Foam. Nonconductive foam serves as a common base for gaskets that add the following conductors: silver-plated nylon yarn; nickel-, copper-, or silver-plated fabric; or reinforced aluminum foil. These types of gaskets are most prevalent in commercial electronics because of their low cost, low closure force deflection requirements, and relatively high shielding effectiveness.
Conductive Elastomer over Nonconductive Elastomer. These gaskets usually feature a silver-filled conductive elastomer over a nonconductive silicone core.
Conductive Elastomer. Typically, these gaskets are silicone elastomer filled with electrically conductive particles such as carbon, nickel, nickel-plated graphite, silver-plated aluminum, silver-plated copper, silver-plated nickel, silver-plated glass, or pure silver. Other conductive elastomers include fluorosilicone, fluorocarbon, and ethylene-propylene terpolymer (EPDM) elastomer. It is possible to mold or extrude the elastomer into many cross sections or to robotically apply it as a form-in-place gasket. The latter option is the most recent development in this area.
EMI gaskets generally have uses in two areas: low-impedance grounding and EMI shielding. In low-impedance grounding applications, gaskets provide a low-impedance ground so that the metal structural parts that form the chassis won't be affected by internal electromagnetic fields and therefore won't contribute to the radiated electromagnetic fields within an enclosure.
Using a gasket for EMI shielding is the more traditional application. The gasket seals an otherwise shielded enclosure to prevent electromagnetic leakage from the seams of mating flanges. In addition, EMI gaskets can also protect a device from airflow and other environmental conditions.
Electrical, mechanical, and environmental issues can all affect the design of a device and therefore the selection of device components, including gaskets.
For enclosure flange design, the flange width, fastener types and spacing, flange surface treatment, and means of gasket attachment must all be factored into the gasket selection process. EMI gaskets must be properly deflected to perform. They must also be mated against a conductive surface treatment to avoid being electrically insulated.
Further electrical requirements include conductivity of the gasket itself; shielding effectiveness against electric, magnetic, and plane wave electromagnetic fields; and transfer impedance. The most important thing to remember about electrical requirements is that there is no universally accepted means of measuring electrical performance. Each manufacturer is likely to measure performance using a slightly different procedure, especially when determining shielding effectiveness. It is important that designers understand the gasket performance data presented, and in particular how they relate to a specific design issue. For example, shielding of a handheld medical electronics enclosure must meet certain radiated emission requirements for the European Union. It is often necessary to rely on a manufacturer's technical personnel, EMI test personnel, or EMI consultants for such information. It is also important to note that not all gasket types perform the same way against lower-frequency magnetic fields or at plane wave frequencies above 1 GHz.
Designers also must consider requirements for compression and deflection, the difference in height achieved when a soft gasket is pushed down. EMI gaskets must be properly deflected to perform as intended. Each type of gasket and each cross section of that gasket has a unique compression and deflection curve, which defines the gasket's range of deflection. A design that does not affect the minimum deflection does not allow the gasket to provide the necessary electrical conductivity. In the case of over-deflection, however, the gasket may be permanently damaged or may have an unacceptable compression set.
Flange designs must allow for the effect of compression set on EMI gaskets. Certain gasket types, specifically wire mesh gaskets, can exhibit severe compression set, which in turn prevents them from providing a repeatable sealing function.
Specific mechanical requirements such as tear resistance, elongation, du-rability, or performance in shear force applications must also be taken into account. If a design subjects the gasket to repeatable shear, it may not survive very long. Therefore, designers should examine gasket solutions that include finger stock. For applications that require increased durability, designers should consider elastomer gaskets reinforced with Dacron.
Of course, environment plays a key role in how a gasket will perform. Designers should consider the temperatures and any liquids, such as saline solution, a gasket may encounter. In the medical field, fluids can include saline solution or other chemicals. Further, electronic products exposed to high humidity or salt air may corrode. To seal a product for use in a corrosive environment, wire mesh gaskets or conductive elastomer gaskets must be designed either with environmental seals or as comolded or coextruded parts. Conductive elastomers can be used with an acknowledged hierarchy of gasket materials that resist corrosion in flanged joints. The three materials in descending order of performance are: a passivated silver-plated aluminum in fluorosilicone, silver-plated aluminum in silicone, and nickel-plated graphite in silicone. For single gasket designs, it is important to choose the materials based on available corrosion resistance data. Designers again face the problem that the industry has no universally accepted corrosion test for EMI gaskets, and therefore must rely on manufacturers' personnel and consultants for guidance. Designers must carefully examine corrosion test data to ensure that the test method properly replicates the gasket design.
Selecting the appropriate EMI gasket is relatively complicated, and the array of gasket choices reflects this complexity. Choosing the correct gasket for a particular problem requires an awareness of gasket design criteria. Although this article has provided some basic guidance for selecting the proper EMI gasket, designers must remember that EMI gasket performance can be verified only when installed in a system and tested accordingly.
Joseph Butler is the market manager for Chomerics Div. of Parker Hannifin Corp. (Woburn, MA).