A product designer works long and hard to develop a new piece of high-frequency electronic equipment. But much of that work could be wasted unless the choice of gaskets to prevent RF leakage is addressed early in the design process.
Selection of a gasket can be complicated since so many elements must be factored into the decision. To help focus on the important topics, three basic questions must be answered.
What is the governing standard for acceptable levels of EMI? Countries and, in some cases, agencies within a country have different test methods and expect the results to be compatible with each of their requirements. In cases where a product is to be used in different nations, designing and testing to the most stringent standard are the most cost-effective approaches. What is the range of operating frequencies? Shielding and gasketing needs are different for high-frequency applications than for low frequencies.
What are the physical limitations? This encompasses the type of enclosure, the projected sizes of the openings to be gasketed, and the loads and forces of the application. Environmental conditions that impact the predicted lifetime of the product, such as internal and ambient temperature, humidity, and corrosive atmosphere, must be considered so the life of the gasket will match that of the product.
Once these three questions are answered, the designer is ready to address gasket selection. The criteria include shielding effectiveness, compression characteristics, corrosion resistance, special design considerations, and recyclability.
Shielding Effectiveness
The primary function of an EMC gasket is to provide a conductive path across the seam between the housing and its cover or door, preventing radiation from leaking at that seam. The gasket prevents internally generated RF from escaping to disturb nearby equipment and externally generated RF from entering your product. Logically then, the first important characteristic to be considered is the gasket’s shielding effectiveness. There are several methods to measure this characteristic.
The current-injection test in the Society of Automotive Engineers (SAE) Aerospace Recommended Practice (ARP) 17051 helps evaluate corrosion effects but not shielding effectiveness.1 The current in this test is the supposed image of current injected by EMI, and measuring the voltage across the gasket during this test gives the characteristic impedance of the gasket. This is useful information but is not sufficient for selecting a gasket.
Two test methods evaluate the all-important shielding characteristic. The more widely used of these, shielding effectiveness, is based on MIL-G-83528, where the attenuation of a covered hole is measured with and without the gasket. Figure 1 depicts tests run by Rome Laboratory on some typical silver-fabric gaskets, copper-fabric gaskets, and nickel-copper-fabric gaskets, showing shielding effectiveness over the range of 50 MHz to 10 GHz.
The second method is transfer impedance, where the impedance of the gasket is measured over a wide range of frequencies and expressed in W /meters. These tests go to 4 GHz with a probable extension to 10 GHz.
Test results vary with test setup and other conditions, so it is wise to turn to an independent laboratory for consistent results before choosing a gasket.
The techniques involved in providing shielding effectiveness have been impacted dramatically in the past decade as high-speed digital circuits now generate such high EMI frequencies. In the past, 200 MHz to 300 MHz was the frequency range of primary concern. Now frequencies of 1 GHz are not uncommon, and designers must find ways to keep these frequencies contained. Some help comes with suppression at the circuit-board level, but the housing and gaskets provide the last point at which EMI can be controlled.
At high frequencies, gaskets made of conductive fabric over open-celled foam provide a higher level of attenuation than conventional metallized-textile gaskets. For example, a nickel/copper nylon-woven cladding for use in harsh environments or with higher frequencies provides more than 100 dB attenuation to 2 GHz, 95 dB to 10 GHz, and 70 dB to 18 GHz.
Compression Characteristics
In actual use, a gasket must be compressed to ensure a low-impedance contact between the chassis and the opening and accommodate minor dimensional variations of the surfaces. The designer must evaluate the total compression force required to close an opening when the gasket is in place. There can be as much as a 10:1 variation between gaskets, depending on the type of material and the construction, and this may be significant when gasketing a large opening.
Compression forces of gaskets can be compared through a measurement technique which derives the number of pounds required to compress one linear foot of a gasket a total of 1% of its free height. Conductive fabric over an open-celled urethane foam offers the lowest compression force due to its open structure and flexible, conductive fabric. Typically, the force required to compress 1 ft of this type gasket by 1% is just 0.16 lb.
Corrosion Resistance
To avoid galvanic erosion, the metals in a gasket must be compatible with those in the surfaces to be gasketed. The quality of electrical contact between dissimilar metals will degrade in time, possibly stabilizing at some point.
The amount of attenuation after this degradation still must be suitable for EMI control. There are several good methods for predicting the degradation that will result from galvanic action of different materials. SAE ARP 1481 lists compatible metallic couples with low-contact resistance.2
Design Considerations
The equipment design engineer should select a gasket early in the design phase because there are many interrelated factors involved. Give special attention to compression forces of the gasket configuration. The use of a mechanical compression stop may be desirable. Possibly the action of the gasket will involve sliding contact and self-cleaning but this impacts the design of the enclosure and should be given careful consideration.
The gasket manufacturer can make valuable recommendations concerning the use of mechanical barriers to electromagnetic radiation, such as grooves or channels to supplement the gasket action.
Mounting methods include pressure-sensitive adhesives, rivets, or clips.
Recyclability Concerns
The European Union is working on a policy that addresses safe recyclability and prohibits the use of certain materials. This may impact the gasket selection process in Europe since beryllium copper, one of the popular gasket materials, now is considered difficult to recycle and therefore potentially undesirable.
Conductive-Fabric Gasket Material
Conductive fabrics used for gasketing generally are woven metallized polyamid fibers. Some common metals deposited on textile material are silver, coated silver, coated copper, nickel/copper, and coated nickel/copper. Each has its advantages and disadvantages.
Silver
The bonding of silver to polyamid is excellent. Conductivity is very good, and even oxidized silver is conductive. The material is very ductile. However, it has a very low abrasion resistance, and it is not suitable for use in a dynamic application where the surfaces slide against the gasket.
Coated Silver
When a layer of protective coating is applied, the gasket can be used in dynamic applications.
Coated Copper
Since it is less ductile than silver, copper is sometimes coated with silvered oxide fiber to protect it from the environment.
Nickel/Copper
In this alloy, nickel protects copper from oxidation. Sometimes a silver coating is applied to the fiber prior to the nickel/copper to get better adhesion and guard against delaminating problems. Unfortunately, nickel is less flexible than copper. When it is compressed continually, it cracks, ejects conductive particles into the surrounding area, and exposes the copper.
Coated Nickel/Copper
A coating on the nickel/copper gasket prevents the nickel from ejecting particles when it cracks, and conductivity is preserved.
Conclusion
The choice of EMI gaskets for a given application involves many factors. A designer should answer the basic questions about product usage early in the design phase and use these answers in the search for an optimum solution. No specific type of gasket is best for all applications. As a result, the designer should discuss requirements with an EMI expert early in the design phase.
Acknowledgement
Much of the background material for this article came from documents prepared by Christian Brull, the EMC product manager at Schlegel Systems, a UniPoly Co.
References
1. “Coaxial Test Procedure to Measure the RF Shielding Characteristics of EMI Gasket Materials,” ARP 1705, Society of Automotive Engineers, 1994.
2. “Corrosion Control and Electrical Conductivity in Enclosure Design,” ARP1481, Rev. A., Society of Automotive Engineers, 1998.
Gaskets and Shields
Low-Profile Strips
The Silicone Elastometer is a pick-and-place, low-compression gasket that provides up to 80-dB shielding. This fabric-over-foam protection is available in several shapes and styles and in sizes as small as 0.04″ wide. It offers an alternative to the typical conductive elastometer gasket for PCMCIA cards, cell phones, and other small products. Arc Technologies, (978) 388-2993.
Fabric Gasketing
The Thermshield Fabric Gaskets are low-compression-force round, D-shaped, L-shaped, or rectangular strips. They offer up to 105-dB attenuation at 10 MHz, with environmental protection up to IP53. The self-terminating fabric is nickel-copper, silver, or tin-copper. Special high-strength adhesive can be provided. Cambio International, (603) 524-3714.
Surface-Mount Gaskets
The Gore-Shield® SMT Gasket is a series of small GS 5200 pads, each with a solderable shim attached, packaged in standard tape-and-reel format. With the physical characteristics of a standard surface-mount component, it can be installed with pick-and-place equipment. After solder is reflowed using conventional paste, it provides EMI shielding. W.L. Gore and Associates, (800) 445-4673.
On-Board Shielding
The EZ Peel Shielding Cans with removable lids are used on surface-mount or through-hole circuit boards and do not require changes in normal assembly procedures. They can be supplied on tape-and-reels for pick-and-place installation. Solid tops are scored, allowing them to be peeled off and resealed without damage to the board or components. Instrument Specialties, (570) 424-8510.
Hook-On Gaskets
The 125LP-Series Hook-On Gaskets offer a low profile with up to 100-dB attenuation. The beryllium-copper shielding is self-locating, with one side hooked over the flange of the product and the opposite side attachable by pressure-sensitive adhesive. Two sizes are available to close gaps from 0.02″ to 0.07″. Tech-Etch, (508) 747-0300.
Soft Gaskets
The SOFT-SHIELD® 5000 Low-Closure-Force EMI Gaskets require 90 dB of shielding from 30 MHz to 1 GHz and >75 dB of shielding at 10 GHz. They also comply with the UL flammability requirements specification 94V-O. Gasket profiles include rectangular, square, and D shapes. Chomerics/Parker Hannifin, (781) 939-4163.
Nickel-Copper Gasket
The C12 Nickel-Copper Fabric Gasket consists of a layer of nickel over a layer of copper, both of which cover a nylon woven fabric. The coating provides galvanic compatibility with yellow and clear chromate, aluminum, steel, and zinc. The gasket has 105-dB shielding effectiveness from 20 MHz to 2 GHz. Schlegel Systems, (800) 204-0863.
Tin-Plated Gasket
The Flectron® Tin-Plated Gasket is a tin-plated fabric bonded to a low-compression-force foam. This surface treatment exhibits galvanic compatibility or low electromotive force differential with metals typically used in electronic enclosures. With a surface resistivity of <0.07 W and shielding effectiveness >100 dB from 20 MHz to 10 GHz, it operates satisfactorily in temperatures from -40°F to 158°F. AMP, (800) 843-4556.
Copyright 2000 Nelson Publishing Inc.
February 2000
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