Marketing Manager, Vanguard Products Corp., Danbury, CT
Although the military has been dealing with very-high-frequency (above 20 GHz) shielding issues for years, until recently these frequencies have not been an issue for consumer and industrial applications. This is changing quickly with the advent of high-frequency wireless networking and communications equipment in home and business settings. As a result, many electronics original equipment manufacturers (OEMs) are now faced with shielding and immunity issues in the frequency range of 20–40 GHz and beyond.
Because the mechanical enclosures for consumer-type products tend to be less robust than for an equivalent military application, the mechanical properties of an electromagnetic interference (EMI) shielding gasket must be a key consideration. An EMI gasket for this type of application must possess the inherent ability to maintain consistent continuous electrical contact between the mating surfaces with little compression force, because the allowable gaps at these frequencies are quite small.Gasket Materials
Above 2 GHz, many of the commonly used commercial gasket materials become porous to EMI and radio-frequency interference (RFI). The common finger stock and knitted-mesh gaskets start to experience leakage issues above 1 GHz due to the inherent porosity of their construction. Clad-foam materials have low compression/deflection forces, but they typically experience serious degradation of their shielding values at 1.5 GHz, and at 2 GHz they lose the majority of their shielding performance.
Standard filled elastomers have poor mechanical properties due to the presence of metal fillers throughout. Common limitations include very high compression forces, undesirable compression set, brittleness, poor aging characteristics, and high cost. Some designs lose attenuation properties beyond 50% compression of relaxed height.
High-performance elastomer designs typically rely on silver as the conductive component and have long been used for shielding against EMI and RFI. Silver was chosen because it is an excellent conductor and is one of the few materials that can be put into a rubber matrix while retaining its conductivity. However, standard silver-filled elastomers have intrinsic problems. Typically, the percentage of silver in the elastomer is extraordinarily high—as much as 70% by weight. At these concentrations, the elastomer matrix loses most of its desirable physical attributes. The result does not have the desired conductive and elastomeric properties, but is a compromise between both.Dual-Elastomer Gaskets
Fortunately, new materials, profiles, and application methods are constantly being developed and refined. The dual-elastomer gasket is just one example of the technology currently available. Because silver is a desirable conductor, a dual elastomer with a coextruded thin outer conductive silver membrane over a pure elastomer inner core free from metallic fillers offers a solution (see Figure 1). This combination provides high conductivity and good mechanical properties. The resiliency and low deflection forces can enhance attenuation performance, as well as long-term aging properties. The gasket is not overstressed, and uneven contact between mated contact surfaces is not an issue—both situations are problematic with standard gasket designs.
The shielding effectiveness of an enclosure is a function of the wavelength of the frequencies involved versus the length of the longest opening, or slot length (see Figure 2). A proper gasket installation effectively reduces the slot length. Correct installation of the gasket between mating surfaces of an enclosure slot ensures continuous electrical coupling along its axis and long-term shielding effectiveness. Low compression force is very desirable to facilitate easy closing of mating surfaces (see Figure 3).
Figure 2. Shielding effectiveness as a function of frequency and slot length.
Figure 3. A comparison of the compression forces needed for a variety of standard and dual-elastomer gaskets.
Effectively, the shielding gasket is a connector of one mating surface to another. This allows the electronic enclosure to respond as one continuous absorber of the undesirable high-frequency energy by limiting the passage of these radio waves.
Actually, many of the traditional forms of shielding gaskets may cease to be a choice at high frequencies. Recent independent comparative testing at various OEMs has shown that dual-elastomer designs of various configurations are yielding shielding effectiveness values over 80 dB at up to 40 GHz (see Figure 4).
Figure 4. Shielding effectiveness of dual-elastomer gaskets.
Although many of the high-frequency compliance issues mentioned may still be years away, a very important packaging cost consideration has become evident. Because the electronics are continuously being pushed to higher frequencies at a rapid rate, product life cycles have become very short—many times, a matter of 5–6 months. This leaves little or no time to redevelop the enclosure packaging. Enclosures tend to be specified for longer product life cycles of a few years, while the electronics continue to outpace their shielding integrity. With some foresight, the choice of a higher-performing enclosure and shielding gasket system can alleviate the burden of a full redesign later on. There are many choices of configurations to meet today's shielding needs, and dual-elastomer designs can offer the performance headroom required by the increasing number of high-frequency applications.