The amount of complex and sensitive electronic circuitry present in appliances is continually increasing in order to support the levels of functionality expected by end users in commercial, consumer and medical applications. This has made shielding against potentially damaging electromagnetic interference (EMI) a much more important element of design than ever before.

Effective approaches to shielding are needed to ensure that not only is a piece of equipment protected against malfunction caused by spurious EMI from external sources, but also that it does not radiate noise that could damage other appliances in close proximity.

In terms of design engineering responsibility, taking care of the provision of adequate shielding has always tended to sit uncomfortably between the electromechanical and electronic disciplines within OEMs. Regardless as to who assumes the responsibility, they should consider a number of key issues when choosing a shielding solution, and they should also fully explore the different types of materials currently available to appliance designers.

Soft-Shield 3500 consists of nickel-plated nylon wrapped around a urethane foam core.

The amount of complex and sensitive electronic circuitry present in appliances is continually increasing in order to support the levels of functionality expected by end users in commercial, consumer and medical applications. This has made shielding against potentially damaging electromagnetic interference (EMI) a much more important element of design than ever before.

Effective approaches to shielding are needed to ensure that not only is a piece of equipment protected against malfunction caused by spurious EMI from external sources, but also that it does not radiate noise that could damage other appliances in close proximity.

In terms of design engineering responsibility, taking care of the provision of adequate shielding has always tended to sit uncomfortably between the electromechanical and electronic disciplines within OEMs. Regardless as to who assumes the responsibility, they should consider a number of key issues when choosing a shielding solution, and they should also fully explore the different types of materials currently available to appliance designers.

Cho-Seal 6371 molded parts.

When addressing the shielding aspect of appliance design, it makes good sense for OEMs to try and work closely with EMI shielding material manufacturers. This will enable the OEMs to draw upon the knowledge and experience of people whose focus is entirely on shielding solutions. This is especially true if custom parts are being considered, but still valid even when standard, off-the-shelf parts are able to do the job adequately.

An individual appliance is likely to have several areas that require some form of EMI shielding. In order to make the right choice of material for each, the designer will need to consider several factors, including the shielding performance required, mechanical considerations such as the closure forces and tolerance build-up, whether the mating parts are dynamic or static, and physical aspects. For example, the material may be exposed to high shear or compressive forces, environmental concerns such as exposure to moisture and humidity, and perhaps resistance to extreme heat or flame.

Thought should also be given to the manufacturability and ease of assembly of a particular shielding solution. This is especially true for appliances produced in high volume. The trend for manufacturing to move to low labor cost countries in Asia and Eastern Europe is a clear indicator of how important reducing assembly costs is to appliance OEMs.

Material options

Soft-Shield 4850 is composed of electrically conductive fibers integrated into a low-density foam.

The EMI shielding materials that appliance designers have at their disposal can be broken into five broad groups: metal gaskets; molded or extruded elastomers; foam-based gaskets; conductive plastics; and adhesives, inks and coatings.

Metal Gaskets. All-metal EMI shields and gaskets have been in existence the longest of all materials developed to combat electrical noise. They are still relevant for a large number of applications, particularly in rack-mounted equipment where individual circuits may need to be periodically removed and replaced.

Beryllium-copper fingerstock gaskets combine high levels of EMI shielding effectiveness with spring-finger wiping and low closure force properties. Beryllium-copper’s high tensile strength and superb electrical conductivity are ideal for shielding over a broad frequency range. Supplied in either standard or custom lengths for high-volume applications, strips are available in a wide variety of cross-sections. For low-compression grounding contacts, individual fingers are available with pressure-sensitive adhesive for convenient peel-and-stick application.

Chomerics’ Premier conductive plastic material.

Although the standalone electrical conductivity of fingerstock is unrivaled, other factors such as the dimension and number of slots in the fingerstock and surface contact area in an assembly can affect the final shielding performance with the result that elastomer approaches may actually provide better performance.

Other metal gaskets such as PCB card cages, connector and panel shields, and knitted wire mesh exist to provide a solution for static applications that require good shielding performance without any environmental sealing.

Molded and extruded elastomers. By combining an elastomer with a conductive filler such as nickel/graphite, a material can be created that provides both EMI shielding and an environmental seal. These materials lead to the even dispersion of the conductive filler within the elastomer to provide uniform physical and electrical performance.

CHO-BOND 1016 electrically conductive sealant is a single-component RTV silicone filled with nickel-plated graphite particles.

The elastomeric binder and the dispersed conductive filler can be manufactured into almost any shape using molding, extrusion or die-cutting processes. It is also possible to overmold the material onto plastic or metal components to provide a one-piece shielded housing that can speed and simplify assembly of an appliance sub-assembly.

Although several elastomers can be used, silicone is usually the material of choice due its ability to compress at low closure forces and its excellent compression set performance across a very wide temperature range. Designers can expect to achieve shielding effectiveness in the region of 80 dB from 100 MHz to 10 GHz (Chomerics Cho-Seal 6371) using these types of materials.

Extruded conductive elastomer gaskets can be supplied in a wide range of solid and hollow cross-section configurations. With a hollow section, conductive elastomers require even lower force in order for them to be compressed, and they also have larger deflection ranges. This makes them ideal for use in applications such as appliance door sealing where it is often difficult or inconvenient to generate high closure forces and where there may be significant tolerance build up over long lengths.

Various shielding materials.

Foam-based gaskets. A more recent development in the EMI shielding market has been the emergence of wrapped foam gaskets. These materials have been developed to provide economical shielding in high-volume applications. Typical uses include shielding and sealing of doors, faceplates and backplanes.

Foam-based gaskets such as the Chomerics Soft-Shield range consist of a conductive nickel-plated nylon material that is wrapped around and bonded to a soft urethane foam core. This type of material achieves a shielding effectiveness greater than 90 dB between 50 MHz and 10 GHz and requires a closure force of typically less than 1 lb/in (0.175Nmm).

Ultra-low closure forces like this make foam-based shielding materials suitable for applications such as thin-walled plastic enclosures. Although foam core gaskets don’t offer the same design flexibility as molded elastomer parts in terms of the shapes that can be achieved, they are available in a significant number of profiles such as rectangles, P, C, and D-shapes plus kiss-cut parts from sheet. Light-duty adhesives affixed to one side can help simplify and improve the accuracy of assembly.

For many indoor electronic devices that require shielding, the choice of EMI gaskets will come down to either fingerstock or fabric-over-foam. Historically fingerstock has been the gasket of choice, especially in high-end electronics. Fingerstock is reliable, proven and widely available. However, fabric-over foam gasketing is quickly becoming the preferred choice. This is due to improvements in the plating adhesion, advances in foam formulations, and the introduction of UL94V-0 rated versions. These gaskets are also easier to handle and less prone to damage during assembly and use.

In addition to fabric-over-foam, newly developed conductive foams can be used as an inexpensive alternative. Unlike plated foams, Soft-Shield 4850 is composed of electrically conductive fibers integrated into a low-density foam. Since the foam is not plated, the product offers excellent compression for long-term shielding. These Z-axis conductive foams allow OEMs and CEMs the ability to convert roll stock product into various forms which helps minimize costly tooling and optimize higher speed, lower cost conversion operations.

Conductive plastics. Injection-molded plastics that incorporate EMI shielding provide an elegant, versatile and assembly-friendly solution. They can be used to provide housings for printed circuit board assemblies and they permit details such as internal compartments and integrated mechanical fasteners to be achieved. With their ability to provide an economic solution for many high-volume appliance applications, conductive plastics negate the need for secondary operations, reduce weight, and can cut costs by up to 50 percent compared to die castings, formed metal, machined extrusions, and plated plastic parts.

As an example, Chomerics Premier plastic shielding materials use a blend of polycarbonate and ABS engineered for stable electrical, mechanical and physical performance. Using a proprietary process, the base material is filled with long fibers of nickel-plated carbon. The process ensures dense dispersion, random orientation, and full engagement of the fibers to give optimal shielding performance.

Adhesives, inks and coatings. These materials do not generally lend themselves to high-volume production unless an automated application process is  used to speed throughput and improve repeatability. There are numerous conductive adhesives on the market, these are typically used to bond other conductive materials such as EMI vents, windows, mesh gaskets, or for filling cracks and seams.

Conductive coatings and inks provide EMI shielding, anti-static protection, corona shielding, and surface grounding in a wide range of applications. They can be applied to plastic surfaces by painting or for higher volumes, by using conventional spraying equipment.

In summary, choosing the right gasket requires knowledge of both electrical and mechanical requirements. Shear forces, environmental effects, compression set, and pricing are just some of the factors that influence which gasket is best for an application. Materials must be both cost-effective as well as compliant with appropriate environmental regulations (RoHs, WEEE, etc.) that require their safe disposal or recycling at the end of the product’s life. For appliance design engineers who do not have to deal with EMI shielding on a day-to-day basis, the learning curve can be very steep unless they seek application support from the shielding material manufacturers themselves.

For more information, email: chomailbox@parker.com