Text version of the page

INTRODUCTION TO KNITTED CONDUCTIVE EMI GASKETS

FIGURE 7. SHIELDING EFFECTIVENESS VS. COMPRESSION FORCE Class C — Marine Environment — Shipboard exposure or land exposurewithin two miles of salt water where conditions of Class A are not met. Class D — Space Environment — Exposure to high vacuum and high radiation. Class E — Nuclear,Biological,Chemical (NBC) — Applications subject toexposure to solvents used to break down NBC toxic substances.Choice of an elastomer used in a knitted wire product for a resilient core is afunction of the class in which the specific equipment will be operated or stored.The elastomer materials listed below are commonly used in conjunction with EMI gasketing materials,as bonded constructions such as the Enviro-Seal product,or the elastomer core products,defined in this catalog.
IncreasingShielding Effectiveness Neoprene This elastomer is commonly used in conjunction with EMI gaskets in the Class A environment and will withstand temperatures ranging from –65°F to 212°F (–54°C to 100°C) for the solid material and –25°F to 212°F (–32°C to 100°C) for the closed cell sponge elastomer.The neoprene materials are the least expensive of the synthetic rubber materials and are best suited for commercial applications from a cost standpoint.
P MIN P MAX Pressure — Increasing Silicone This material has outstanding physical characteristics and will operate continuously at temperatures ranging from –80°F to 500°F (–62°C to260°C) for solid and –103°F to 401°F (–75°C to 205°C) for closed cellsponge silicone.These materials remain flexible and are highly resistant to water and swelling in the presence of hydrocarbons. Compression Set As previously mentioned,use of compression stops built into an enclosure orsupplied with the EMI gasket aids in controlling the amount of deflection,thus minimizing the amount of compression set.Selection of a gasketingmaterial for a joint which must be opened and closed is,to a large extent,determined by the compression set characteristics of the gasket material. Most resilient gasket materials will recover most of their original height after being deflected no greater than 25% – 30%.The difference between the original height and the height after the compression force is removed,is thecompression set.Compression set increases as the deflection pressure isincreased. Note:For materials compatibility in a corros i ive environment, see the MetalsGalvanic Compatibility Chart on the inside back cover. Environmental Considerations The choice of gasket materials and surface finish of an enclosure is a function of the operating or storage temperature of the electronic package. Most electronic packages,however,must be designed for one of the following five general environments: Class A — Controlled Environment — Temperature and humidity are controlled,generally indoor applications. Class B — Uncontrolled Environment — Temperature and humidity are notcontrolled,exposed to humidities at 100% with occasional wetting,outdoor exposure or exposure in uncontrolled warehouses.
www.lairdtech.com8