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Product category: Gaskets, Seals and Screens
News Release from: Laird Technologies | Subject: RF gaskets
Edited by the Electronicstalk Editorial Team on 11 October 2005

Ten considerations when specifying an RF
gasket

Compiling a list of essential requirements is an ideal way to reduce cost and improve the performance of RF gaskets in specific applications says Gary Fenical from Laird Technologies.

For decades we have been asked to provide "an RF gasket that will give me 100dB from 10kHz to 18GHz" Although there are such gaskets, except for rare applications, no one needs such a gasket

A list of essential requirements needed to properly specify just what is required is the way to reduce cost and improve the performance of RF gaskets in specific applications.

The list is not in order of importance because the entire list is of equal importance.

Not considering any one of the factors will potentially cause the gasketed joint to give problems either immediately or in the future.

The first consideration is the operating frequency.

Trying to specify and RF gasket without knowing the frequency or frequencies that need to be attenuated is like shooting in the dark.

As mentioned in the introduction, there are gaskets that can provide high shielding effectiveness over vast frequency ranges.

However, it is more economical and efficient to shield the frequencies that are exceeding the limits required or causing susceptibility problems.

Many times, gaskets made of less costly materials may be used.

Gaskets may be placed at strategic locations and at calculated distances rather than using continuous lengths, again saving money by using less material.

As operating frequencies continue to go higher, traditional RF gaskets may not provide the desired results.

RF absorbers are now being used in conjunction with RF gaskets or alone in many applications.

Remember that it is not just the fundamental frequency that may be causing problems, but the harmonics as well.

The FCC requires unintentional radiators to be tested to five times the highest used frequency within the device and for intentional radiators it is increased to 10 times the highest frequency used within the device.

If you are having susceptibility or crosstalk problems, again don't look only to the fundamental frequencies but remember to look at the harmonics as well.

The next consideration is attenuation.

Shielding effectiveness values in excess of 100dB and even in excess of 120dB across wide frequency ranges can be achieved by many gasket materials and geometries.

It must be understood, that to achieve such results, the gasket must be installed exactly to specifications and on very conductive materials.

Installing high-performance RF gaskets on less conductive surfaces reduce their shielding effectiveness.

Materials or platings such as nickel, zinc, chromate, and others reduce maximum the shielding effectiveness available from more conductive RF gasket materials and platings.

The attenuation of RF gasketed joint is also greatly effected by the mechanical characteristics of the joint and ultimately, the mechanical "fit" of the RF gasket.

In over 20 years of working with RF gasket materials, I have never seen the gasket fail or not provide its specified shielding effectiveness.

Any failures have always been poor design of the application or the RF gasket being installed on lower conductive surfaces.

Designing within the specified parameters of the gasket is essential.

Factors that will be discussed later such as load and force, and compressions height are of the utmost importance to achieve the required and/or specified attenuation of RF gasketing materials.

Next come materials compatibility/corrosive considerations.

The driving force for galvanic corrosion reactions is the difference in equilibrium electrical potential between two different metallic materials in an electrolyte solution.

Many experimental measurements have shown the differences among common metals.

The metals with the highest potentials are unlikely to corrode and they are described as noble.

The metals with lower potentials are described as active.

When two dissimilar materials are in contact, the more active one will tend to have accelerated corrosion while the corrosion rate of the more noble metal will decrease.

For example, copper is much more noble than aluminium but both are stable in water by themselves, with little or no corrosion.

If aluminium is put in contact with copper in the presence of water, however, the aluminium (active) will tend to corrode rapidly whereas the copper (noble) will not corrode.

The driving force for the electrochemical corrosion reaction is proportional to the voltage difference between materials.

Zero or very small differences in potential will have little or no effect on corrosion rates of either material.

A large potential difference, 0.5V or more, can accelerate the corrosion rate of the more active material by several orders of magnitude.

It is easy to see that it is desirable to use a gasket material that is galvanically similar to the material it will contact.

It is necessary to choose an RF gasket that is closely compatible with the metal of the conductive surface on which it is installed.

For metal RF gaskets, plating the gasket is the preferred way to reduce the galvanic potential.

For electrically conductive elastomers (EcE), choosing a conductive filler that is closely compatible with the base material is the preferred method.

For other types of gaskets, again, matching the conductive material of the RF gasket with the conductive material of the installation is the method.

Environmental considerations are important.

Electrically conductive elastomer (EcE) is the only inherent RF and environmental gasket.

Available in many different elastomer compounds, fillers and geometries, the selection is endless.

However, if EcE materials are not the gasket of choice, there are many combination products available.

Nonconductive elastomers can be combined with metal, wire mesh, and other types of gaskets.

Many times, a nonconductive elastomer installed outboard of the conductive RF gasket is the choice.

The decision depends on the application and available space as well as load/force considerations.

If only a dust seal is required, fabric-over-foam (FoF) can be a good choice.

Load and forces must be considered.

Metal RF gaskets are produced form a vast array of alloys allowing for precise control of forces.

Thickness, geometry, heat treating, and to a lesser degree, plating can effect the compression force of metal gaskets.

Metal RF gasket can be produced with wide ranges of load/deflection characteristics.

The compression forces of EcE RF gaskets are primarily controlled by the elastomer compound, the percentage of filler loading and to a great degree the geometry of the gasket.

Over the years, various innovations in geometry have reduced the compression force of EcE material by more than 100% over solid gaskets.

New EcE materials such as form-in-place (FiP) and mould-in-place (MiP) types can be produced with varying degrees of compression/deflection characteristics as well.

The forces of knitted wire mesh gaskets are controlled by the type and size of the wire, the number of layers, which can be anywhere from one layer to a solid knitted profile, and the nonconductive elastomer core.

Copper-beryllium knitted wire mesh gaskets (Patent 5,294,270) are custom heat treated and do not require a nonconductive elastomer core to produce the spring properties and have very low compression force.

The compression forces of fabric-over-foam (FoF) gaskets are determined by the centre profile or geometry and the core material of the gasket.

Generally, FoF gaskets provide very low compression forces.

Produced of metallised open-celled foam with a layer of metallised polyester mesh on each side, generally, conductive foam (CF) gaskets provide the lowest compression forces.

Fastening and mounting methods must be considered.

This category is limited only by the application and your imagination.

Mounting methods for RF gaskets include, but are not limited to: pressure sensitive adhesive (PSA); clip-on with or without lances; slot mounted; riveted; welded; soldered; fitted die cut profiles; snap-in; slot mount; Rite Trak snap-in; snap-in rivet track; conductive and nonconductive adhesives; fastener or screw mounted; form-in-place; mould-in-place etc.

The actual application and the mechanics of the device on which the gasket must be mounted will determine the best mounting method.

One important factor is to design the device to accept the necessary gasket and mounting method from the beginning.

Trying to install a gasket in a joint that was never meant to accept a gasket will result in reduced performance, possible gasket failure and even failure of the structure itself because it was not designed to deal with the forces to properly compress the gasket.

Nuclear, biological and chemical (NBC) considerations may be important.

Usually this category is only considered to be for the military.

However, many times this is a consideration for use in the food and medical industries as well as others.

Many machines in these industries are required to withstand harsh washdowns and the materials of which they are manufactured, including gaskets, must remain functional and also not promote the growth or retention of chemical or biological agents.

Also, the gaskets may have to perform in harsh manufacturing or processing environments.

EPDM (ethylene/propylene/diene monomer) based elastomeric gaskets are commonly used in military hardware that could be exposed to nuclear, biological and chemical (NBC) threats in the field.

This is also the choice for other industries potentially exposed to harsh chemical and cleaning treatments.

It is important that the materials used in this hardware be resistant to these chemical agents, which is known as "chemical hardening".

NBC hardenability test data have shown that the mechanical and electrical properties of ElectroSeal 96 stay well within 20% of their operational performance values after exposure to the chemical agents and decontamination solution.

Of the three chemical agents, most had little impact on the material properties relative to a control sample.

The only chemical agent to show a noticeable level of change was the VX (nerve agent).

The properties of samples exposed to a decontamination solution have almost identical properties values to the control.

This data has shown that the chemical agents and the decontamination solution have a minimal affect on the functionality of this compound.

Decontaminability test data have shown that the desorption levels for ElectroSeal 96 are significantly lower than the acceptance criteria level for HD (mustard gas), and VX chemical agents.

When it was exposed to the GD (nerve agent) chemical agent it had a desorption value that was slightly higher than the acceptance criteria.

Overall, the decontaminability performance of ElectroSeal 96 is excellent.

The desorption criteria are based on the risk to unprotected personnel working on or in close proximity to contaminated components for a 12 hour shift.

The actual risk posed to unprotected personnel is probably much lower than the values indicated by this study due to the limited exposed surface area of this material when used as a gasket.

The NBC survivability of ElectroSeal 96 was measured using a very aggressive test methodology that represents the worse case scenario for a gasket material.

In tests, the chemical agents were applied directly to the gasket material in high concentrations.

Historically the gaskets exposed to NBC testing have been assembled in a component that is then placed in test.

In an assembled component, the gasket material has little direct exposure to the chemicals.

Therefore, it is expected that because ElectroSeal 96 performed well in this type of NBC test, then it would perform even better when assembled in a component and then exposed to the chemical agents.

Cycle life should be considered.

Will the joint into which the gasket is installed be opened frequently such as a cabinet door?.

Will it be opened occasionally such as for maintenance or will it never be opened?.

These questions need to be answered to determine if cycle life is an issue.

For shear or sliding applications, metal products are generally the choice; however, FoF materials have been used in shear applications with exceptional results.

For compression applications, almost any of the RF gasket types will do the job.

However, compression set (the ability of a gasket to return to its original height after compression) can be an issue.

Metal gaskets used within their tolerances generally have no or little compression set.

Copper-beryllium RF gaskets used within their tolerance have no compressions set at all.

Materials such as stainless steel and phosphor bronze have some initial compression set but after the initial compression set they will not continue to set further.

Copper-beryllium knitted mesh gaskets also exhibit no compression set.

Other knitted wire mesh gaskets may have some compression set that is determined by the combination of the wire material and the elastomer core.

EcE, FoF and other elastomer/rubber based gaskets will exhibit some compression set, but again, if used within their recommended tolerances, this is minimal.

Conductive foam exhibits the most compression set of available gaskets and is recommended for applications that will be installed permanently or opened infrequently.

Space and weight considerations are important.

This category not only considers the size and weight of the actual gasket but involves the compression forces of the gasket.

When weight is of the utmost importance such as for aerospace applications, the compression force that an RF gasket has will determine how robust the structure on which it is applied needs to be.

Gaskets that have lower forces can allow for thinner and lighter-weight materials because they do not have to fight high compression forces.

The basis for geometry and compression force is discussed elsewhere in this article.

And finally comes recyclability.

Who has not heard of WEEE or RoHS?.

I'm sure that everyone in manufacturing today has been confronted with meeting these two very important directives.

WEEE (Directive on Waste Electrical and Electronic Equipment) and RoHS (Restriction of Hazardous Substances) are two EU (European Union) Directives having great impact on manufacturing.

Generally, RoHS requirements are dealt with during the manufacturing process and WEEE requirements come into play during the recovery and recycling phase of a product.

The first widely misunderstood bit of information is that copper-beryllium RF gaskets do not meet these requirements or are a recycling problem.

Copper-beryllium alloys were not included in any special end-of-life requirements or restrictions in the final EU Directives on End-of-Life Vehicles (ELV) (2000/53/EC and 2002/525/EC), on the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS) (2002/95/EC), or on waste electrical and electronic equipment (WEEE) (2002/96/EC).

Major European equipment manufacturers, and their associations, such as VDA (German automobile manufacturers) and C4E (electrical, telecomms and computer manufacturers), have approved the continued use of copper beryllium in their published materials guidance documents.

Copper beryllium does not impact end of life disposal issues as it does not threaten the environment and is recyclable.

Generally, with a few exceptions for some platings, all Laird Technologies' RF gaskets comply with the RoHS and WEEE directives.

Although 10 selection criteria were discussed here, there are others, such as: standards compliance; operating environment; cost; storage environment; shielding, grounding, other; electrical requirements; materials thickness, alloy; and product safety.

Generally, gasket manufacturers have expert staff that have a great deal of experience in all of the selection criteria.

Use this guide to get some general ideas on which types of gaskets may be useable in your particular application then contact an applications engineer to fine-tune your gasket selection.

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