Efficient sealing against electromagnetic interference

Static seals/materials – Electronics are ubiquitous today and the components used are getting smaller and smaller. This also increases the demands on shielding technology, which has been continuously developed in recent years. The first materials used were fillers such as silver and silver-plated copper in silicone elastomers. Newer materials are based on cost-effective silver-plated aluminum and nickel-plated graphite composites and have become the preferred choice of developers.

In both our private and professional lives in the industrialized world, we are surrounded by electronics to an extent we could not have imagined 20 or 30 years ago. These areas include the automobile, medical technology, aerospace and communications technology. Here, electronics is providing further progress in the form of more functions, more comfort and smaller form factors. While digital and analog technology has evolved by integrating several components into a single housing and with ever higher performance through smaller geometries, the requirements for shielding of ever more densely populated circuits where power electronics are arranged in close proximity to digital logic are also increasing. Elastomers with different filling materials increase the performance here. Developers have a choice of materials available that fulfill a dual function: as a seal between two contacting surfaces and as a shielding of electromagnetic interference (EMI) Fig. 1.

The variety of applications and environments in which electronic equipment is used means that EMI shielding materials must be carefully selected. For example, automotive applications can be exposed to oil and lubricants and temperatures ranging from -55 to +100 °C. Food industry applications can come into contact with even more substances or materials, such as coffee, sauces and beverages.

EMI shielding materials in equipment used indoors are subject to limited environmental requirements. Outdoor applications, on the other hand, are subject to greater temperature fluctuations and environmental influences such as rain, corrosive seawater, air pollutants and high humidity.

Consider EMI shielding early in the development process

Regardless of the application or environment in which EMI shielding is used, such shielding must become part of the development process – preferably from the very beginning of a design. Careful enclosure design and the relative positioning of certain components can reduce the amount of shielding required. However, today’s modern product design with its high level of integration means that EMI aspects occur much more frequently than in the past.

Even with a very careful design, electromagnetic interference can only become noticeable once a finished prototype in a housing is in a functioning system. Then, various materials must be used to contain EMI and ensure long-term, reliable operation.

By working closely with EMI materials specialists, potential problems can be identified very early in the development phase. Early detection and consideration of such aspects can lead to a more cost-effective and reliable final design.

Which elastomer…

The basis of an elastomer in extruded and molded EMI shielding materials are silicones, fluorosilicones, fluorocarbons or ethylene-propylene-diene monomers (EPDM). Each elastomer has specific properties that make it suitable or unsuitable for certain applications. In terms of the cost of the finished conductive elastomer material, the filler is much more expensive than the base elastomer.

Silicone is very well suited for applications where high temperature differences occur. The material is suitable for outdoor applications and offers excellent resistance to weathering, ageing and ozone. Silicone also offers good dielectric properties.

Fluorosilicone has similar temperature characteristics to silicone, as well as excellent weathering, ageing and ozone resistance and dielectric properties. Due to its high resistance to fuels, oils, aliphatic solvents, water, diluted bases and acids, fluorosilicone is well suited for harsh environmental conditions in indoor and outdoor applications.

Fluorocarbon offers excellent compression properties and is suitable for applications that are disassembled and reassembled during their lifetime. The material offers excellent resistance to oil, fuels, aliphatic, aromatic and chlorinated solvents and acids, and very low gas permeability. It also has very good resistance to high and low temperatures.

EPDM offers excellent resistance to weathering, ozone, diluted bases and acids. It can also be used in areas with high water and steam exposure.

… and which fillers for which application?

The metal that is mixed with the base elastomer is the filler and provides the shielding function of the finished extruded or cast material.

Fig. 1 Shielding seals against electromagnetic interference are available for the most comprehensive tasks

The precise, uniform distribution within the binder produces a material with stable and uniform electrical and physical properties. The frequently used materials offer different properties for the respective application:

• Nickel/Carbon (Ni/C) provides low-end shielding and ESD protection with very limited corrosion and fluid resistance.
• Silver/Copper (Ag/Cu) resists even the highest electromagnetic pulse (EMP) induced currents. The materials are suitable for high-end commercial applications in non-corrosive (indoor) applications.
• Nickel/Aluminum (Ni/Al) is the optimal filler for high shielding performance in demanding environments. It is usually combined with silicone or fluorosilicone elastomers.
• Silver/Aluminum (Ag/Al) in combination with a fluorosilicone elastomer is ideal for military applications where high shielding performance and corrosion resistance are required.
• Silver (Ag) is an expensive option, but offers high shielding performance through high conductivity. This fill material is suitable for non-corrosive environments. In combination with silicone it offers low closing forces and high shielding performance. The disadvantage is a low tensile strength and limited resistance to liquids.

Moulded or extruded?

Moulded seals can be manufactured with very tight tolerances for precision applications. However, designers must consider the cost of manufacturing equipment when choosing molded rather than extruded solutions. Extruded gaskets have larger tolerances and are offered in a variety of cross sections. Compared to molded gaskets, they require longer preparatory and secondary processes to cut and adjust the gasket to the desired length.

Conclusion

After being used primarily for shielding military and aerospace electronics systems, extruded and cast shielding gaskets are now being used in other commercial applications. This reflects the growing complexity of today’s electronic systems, which we now take for granted. Conductive elastomers from material specialists are popular with housing developers of various products and market segments across all industries.

Facts for designers

• Working with EMI shielding specialists as early as possible in the development stage can help reduce the complexity of solutions to avoid EMI problems

Facts for buyers

• With the right base elastomers and fillers, solutions can be found for many problems
• The early involvement of EMI shielding specialists reduces costs

Facts for quality managers

• Shielding gaskets are increasingly used in the commercial sector to ensure high product quality

Author Chomerics Europe, Bruno Chaigneau, Application Engineer