Gaskets are major weapons in the noise-free arsenal of products EMC design engineers use to contain and deter EMI. They maintain the EMI silence by providing an electrical bond between moving and stationary mating surfaces for a variety of systems, ranging from mobile phones to shielded rooms.
To select the right gasket, first you need to fully understand the test method used by the manufacturer to rate the gasket, such as MIL-STD-285, MIL-G-83528 or transfer impedance; the environment in which the end product will be used; and design constraints. The next step is to learn the benefits and drawbacks of the different types of gaskets. And there are so many choices, from a metal spring gasket to a tubular wire mesh or a rubber-type gasket.
Spring gaskets provide you with many sizes and mounting configurations, said Stephanie Arnold of Instrument Specialties. Flexible spring fingers guarantee contact and shielding effectiveness of uneven surfaces by compensating for variations in construction or generous tolerances.
Beryllium-copper (BeCu) gaskets are a good example of this type of product. They also exhibit high tensile strength, exceptional stability and excellent thermal and electrical conductivity.
Beryllium-copper gaskets are used in low closure force applications where a wiping or sliding action is applied parallel to the mating surface, reported James Pean of Tecknit. They have a high resistance to relaxation of shape and virtually eliminate compression set. However, they do not offer any sealing capability against environmental actions, such as humidity or salt spray.
Applications best served by metal, spiral or spring gaskets include operations requiring frequent openings and closings of a cover or where a high level of shielding is needed with very low closure force, said Bonnie Kunkel, General Manager of Spira.
Spiral gaskets also are appropriate where joint surfaces are uneven, said Ms. Kunkel. These gaskets accommodate the unevenness by making contact along all portions of the chassis. For applications requiring shielding in excess of 90 dB, tin-plated spiral gaskets are best because they have low contact resistance and an inherent conductive stability.
If the application has a complex geometry or requires rapid installation, rubber-type gaskets may be more suitable, said Joe Rowan of W. L. Gore & Associates. Rubber gaskets can be easily die-cut to shape.
Silicone rubber gaskets filled with silver and other conductive fillers also offer environmental sealing and shielding, said Mr. Pean. They can be extruded, molded or die-cut into a variety of cross sections. Shielding effectiveness is typically 100 dB at 1 GHz. Rubber gaskets, however, should not be compressed beyond their limit because they are susceptible to compression set.
Rubber gaskets can be combined with fingerstock and wire mesh or act as stand-alone gaskets, said Ms. Arnold. Examples include oriented wire, impregnated woven wire and electrically conductive elastomers.
Environmental seal-type gaskets combine neoprene or silicone rubber gasketing bonded to a variety of wire-mesh materials such as BeCu, Monel or tin-plated copper steel. Rubber-type gaskets also provide protection against dust, dirt and moisture, and are provided in a wide range of sizes.
Oriented wire gaskets are used in harsh weather applications and can be die cut or used in strip or sheet form. Wire materials are made from aluminum and Monel. These gaskets accommodate temperature ranges from -80°F to +500°F, and provide shielding effectiveness to 100 dB in the E-field and 50 dB in the H-field.
Electrically conductive elastomers are the most versatile gasketing method, said Ms. Arnold. They are supplied in sheet form or die cut to shape, are offered in an O-ring version, or can be molded or extruded. These gaskets provide excellent environmental sealing and EMI shielding, handle temperature ranges from -85°F to +390°F, and have a shielding effectiveness as high as 120 dB at 10 GHz.
Appropriate Choices
The best way to determine which gasket is best for a particular application is to test the gasket on the product design. This is true no matter what gasket or test standard you use, because correlation between standards is extremely difficult.
Work done by Freyer, Rowan and Hatfield and presented at the 1994 IEEE EMC Symposium demonstrated the lack of consistency of grading among the mode stirred, transfer impedance, MIL-STD-285 and modified ASTM D 4935-89 test techniques. Additionally, shielding levels expressed in decibels do not translate from one test method to another.
Part of the selection process for the right gasket must include where it will be used and what requirements it must meet, said Richard Dawson, Applications Engineer at Bal Seal Engineering. For example, if the gasket must function for a long period of time and cannot be replaced easily, then a gasket resistant to compression set should be considered.
Other considerations are operating frequency, material compatibility and worldwide compliance to commercial or military standards, said Ms. Arnold. Load forces, operating environment, cost, attenuation performance, the fastening and mounting method and the gasket life cycle also should be studied.
A systems approach to gasket design considerations generally yields the best result, said Dennis Perry, EMC Product Manager at Schlegel. The matrix for choosing a gasket includes material type, gasket size, shape, durability and a method to test the integrity of the sealing system design.
How can you determine which gasket is most appropriate for your application? Here are a few rules of thumb from Mr. Perry:
Choose a material that conforms easily to irregular mating surfaces.
Err on the side of a low-closure force material because closing the cover or door will be much easier if the gasket is not stiff as steel.
Make sure the gasket manufacturer can supply a UL rating, if necessary.
Select a versatile strategy, especially in places where the gasket real estate is limited. Dual and triple gaskets for EMI, environmental and fluid-resistance applications require excessive space. Consider combining functions into a single, integrated gasket whenever possible.
But the best advice, especially if you are not comfortable choosing an EMI strategy, is to call a gasket manufacturer. They have qualified engineers who are experts at designing cost-effective EMI solutions and they are just a phone call away.
New Products
Gaskets Provide Shielding
In Microwave Connectors
A line of canted-coil springs provides EMI shielding in microwave coaxial connectors. The patented wire spring design has an inclining coil that deflects independently when compressed. The springs resist compression set and fatigue and adjust to dimensional variations in mating parts. The springs are made of beryllium copper or stainless steel, and are available with gold, silver, nickel and tin plating. Coil heights range from 0.015″ to 0.450″. Ring diameters begin at 0.025″. Bal Seal Engineering Co., Inc., (800) 366-1006.
D-Shaped Wire-Mesh Gaskets
Used for Narrow Flanges
The UltraFlexR line of beryllium-copper knitted wire-mesh gaskets is offered in a D-shaped profile for applications where the mounting flange is not wide enough to accommodate a single-fin gasket. The gasket recovers 100% of its original height up to 75% deflection. It is available in widths from 0.200″ to 0.500″ and heights from 0.130″ to 0.375″. The gaskets can be plated with tin, nickel or cadmium. They are supplied with pressure-sensitive adhesive in spools of 25′. Instrument Specialties Co., Inc., (717) 424-8510.
Conductive Laminates
Absorb Shock, Shield to 1 GHz
Conductive laminates combined with foam absorb impact shock and provide an average shielding effectiveness >80 dB from 10 MHz to 1 GHz. The foam and fabric mixture yields a compressible structural support for electronic products, such as cellular phones or portable instruments. The laminates can be custom shaped into flexible grounding straps for backplane shielding of circuit boards. Schlegel Corp., (800) 204-0863.
Gasket and Shielding Catalog
Includes Engineering Help
A new EMI catalog for spiral gaskets and shielding products has sections to help engineers choose the appropriate EMI gasket. It includes an expanded data-sheet chapter, shielding quality data, groove and surface mounting guidelines, corrosion control facts and custom gasket information. Spiral gaskets are offered in diameters starting at 1/32″ in tin/lead-plated beryllium copper and a stainless steel version. The gaskets can be groove mounted or attached to various substrates. Spira Manufacturing Corp., (818) 764-8222.
Shielding Gasket
Offers Low Closure Force
Consil-V is an extruded silicone elastomer filled with silver-plated inert particles. The V-shape cross section accommodates applications requiring low closure force gasketing. The gasket is available with pressure-sensitive adhesive or push rivets. It provides 70 dB of shielding effectiveness at 1 GHz. Continuous lengths are available up to 25′. Tecknit, (908) 272-5500.
Conformable Shielding Material
Provides Environmental Seal
GORE-SHIELDTM EMI gasketing material suppresses radiated EMI and RFI while maintaining an environmental seal. The expanded PTFE material provides protection in applications as high as 18 GHz and remains conductive under vibration. The material is pliable and can be formed around corners. It supplies shielding effectiveness to 100 dB when tested to MIL-G-83528. The operating temperature range is -200°C to +200°C. It is available in thicknesses from 0.01″ to 0.125″ and widths from 0.125″ to 10″. W.L. Gore & Associates, Inc., (800) 231-4EMI.
Silver-Filled Gasket
Available in Four Shapes
Three profiles have been added to the CHO-SEAL® 3000 line of conductive elastomer EMI gaskets. P, V and D configurations are available in addition to the original O profile. The gasket consists of a silver-filled silicone elastomer applied over a nonconductive silicone tube. All gasket forms offer low closure force and a conductive surface. They provide 60 dB of shielding effectiveness from 200 MHz to 10 Hz. The gaskets stretch 150% without affecting their coating or electrical conductivity. Applications include shielding and grounding in cellular phones and small electronic packages. Chomerics, Inc., Division of Parker Hannifin, (617) 935-4850.
EMI/RFI
Gaskets/Seals
Copyright 1995 Nelson Publishing Inc.
August 1995