EMI Shielding Delivers ''Quiet'' Systems

Feb. 1, 2001
EMI shielding gaskets protect against radiation from metal interfaces.

EMI shielding gaskets effectively control electromagnetic fields with no performance degradation - demanding little consideration from the designer.

Recently updated international standards tighten the electromagnetic compatibility (EMC) requirements for electronic systems, particularly power supplies and other power electronic systems. Virtually all electronic systems need to control spurious electronic noise (EMI, electromagnetic interference) through the use of EMC. These regulations are forcing the power electronics industry to qualify more of their products for EMC. Current developments in shielding technology allow electromagnetic interference control at the board level as well as on system enclosures. One of the primary devices to control EMI are gaskets of several different types.

First, we must understand what components and elements in a system are EMI sources. Many components can contribute, for example:

- At the board level, buck and boost switching regulators create noise as their current changes over time (di/dt).

- Enclosures may have several areas that can cause EMI or allow passage of troublesome signals, which include connector interfaces and metallic enclosure seams.

Incorporating the correct products into these and other applications can control EMI without the need for rigorous mathematical analysis or extensive engineering. If these EMI controls can be designed in at the early stages of cabinet or board design, then the concern for EMI further downstream is not an issue.

Electromagnetic Fields An understanding of the EMI shielding process requires a short discussion on electromagnetic theory. Electromagnetic waves consist of two essential components: a magnetic field (H) and electric field (E). These two fields are perpendicular to each other, and the direction of wave propagation is at right angles to the plane containing these two components. An H-field occurs at a low frequency range, up to 200 kHz, and has a large wavelength. In contrast, an E-field source has a high voltage and a small current flow and operates at a higher frequency with smaller wavelengths. At very large distances from the source, the ratio of E to H is equal for either wave - regardless of its origination. When this occurs, the wave is said to be plane wave, and the wave impedance is equal to 377W.

Wave impedance is an important factor in EMI shielding. This can be illustrated by considering what happens when an electromagnetic wave encounters a discontinuity, such as the interface between metallic elements of an enclosure. If the magnitude of the wave impedance is greatly different from the intrinsic impedance of the discontinuity, most of the energy is reflected, and very little is transmitted across the boundary. Most metals have an intrinsic impedance of only milliohms. For low impedance H-fields, less energy is reflected and more is absorbed, because the metal is more closely matched to the impedance of the field. That's why shielding H-fields is so difficult. On the other hand, the wave impedance of E-fields is high, so most of the energy is reflected and shielding is less difficult.

Consider the theoretical case of an incident wave normal to the surface of a metallic structure, as shown in Fig. 1. If the conductivity of a metal wall is infinite, an E-field equal to and opposite to that of the incident E-field components of the wave is generated in the shield. This satisfies the boundary condition that the total tangential E-field must vanish at the boundary. Under these ideal conditions, shielding is perfect because the two fields cancel one other. However, the H-fields are in phase, so current flow in the shield is doubled.

Shielding effectiveness of metallic enclosures is not infinite because the conductivity of all metals is finite, but it can possess very high values. Because metallic shields have less than infinite conductivity, part of the field is transmitted across the boundary and supports a current in the metal.

Because electromagnetic waves reflect when encountering a discontinuity, we can create an ideal cage to preserve the current flow through the box and conversely, we can enclose a component at the board level to cage these same electromagnetic fields.

Electromagnetic leakage through a seam or discontinuity can occur in two ways. First, the material can leak through the material directly. In the case of dissimilar materials where one has lower conductivity than the other, the rate of current decay in the lower conductivity material will also be lower. Therefore, more current will appear on the far side of the shield. Second, leakage can occur at the interface between two pieces of metal, as is illustrated in Fig. 2.

If an air gap exists in the seam, the flow of current will be diverted to those points. Also, a high resistance seam will behave much like an open seam, simply altering the current distribution slightly. This emphasizes the importance of maintaining a high degree of electrical conductivity at the enclosure interfaces to avoid air gaps or gaps of high resistance. Otherwise, these gaps provide pathways for electromagnetic leakage, resulting in EMI problems.

EMI Gaskets Products that provide EMI shielding are readily available for both enclosure and board-level applications. Photo 1, on page 20, shows an extruded conductive gasket with clip attachment for enclosures. This easily applied gasket enables a quick application for a wide range of cabinet flanges that ensure seam continuity.

These gaskets are useful for doors and seams, such as the enclosures that may encase a power generator or transformer. These materials are also available in custom and standard shapes and sizes with a variety of attachment systems. However, the clip attachment is the most desirable for its ease of installation. Ni/C-filled silicone elastomer material offers excellent conductivity and mating characteristics to many commonly used metals.

Overmolded covers are suitable for shielding at the board level. These covers eliminate air gaps with the use of a conductive silicone elastomer molded directly to the metal cover, as shown in Fig. 3.

This silicone elastomer fills gaps between the cover and the board, creating an isolating cage around the radiating component. This can be an effective way to protect circuits from the electromagnetic fields emitted by components, such as switching regulators. A distinct advantage is the low deflection force required to obtain an optimum seal. Other design advantages include space savings, low-added weight, high level shielding performance, and low cost.

These two shielding solutions are most useful when shielding E-fields. Low impedance H-fields are considerably more difficult to shield.

Conductive foil tapes are available with a conductive adhesive that allows similar metal-to-metal contact. This technology is extended to create conductive foil over foam connector gaskets. This addresses another area of EMI emission: connectors mounted on backplanes. You can accomplish this by placing conductive foil over foam gaskets that are cut to the configuration of the connector plate, as shown in Photo 2, on page 26.

This is a simple method for using a similar metal to connect a connector to a panel and allow continuous current flow. This solution is more effective at shielding H-fields, due to the low-impedance characteristics of aluminum and copper. Also, foil products can be easily die cut to shapes to fit the inside enclosures or between electrically "hot" components.

Corrosion-resistant gaskets incorporate particle plating and elastomer technology. This material is based on a silver-plated-aluminum filler dispersed in a fluorosilicone binder, with corrosion inhibiting additives that contain no chromates. It offers shielding effectiveness of 100 dB at 500 MHz and meets all requirements of MIL-G-83528 Type D (initial and aged).

Reinforced conductive elastomer seals consist of a corrosion-resistant elastomer base, reinforced with a woven or knitted fabric material. These seals are intended for use in airframe shielding applications. The integrally molded reinforcing material provides improved mechanical properties, resulting in seals that withstand high levels of wear and abuse, while maintaining the electrical properties of the conductive elastomer base material.

Die-cut conductive elastomer gaskets provide shielding and environmental sealing when between a connector flange and their mating surfaces.

Metal Gaskets Another form of gasket is the all-metal type that consists of compressible strips knitted into rectangular or round cross sections. The knitted monel or Ferrex (tin-plated, copper-clad steel) wire forms many spring-like interlocked loops, making it highly resilient. An elastomer core type of gasket consists of two layers of wire mesh knitted around a rectangular or round core of neoprene or silicone. The mesh provides EMI shielding, and the core provides excellent compressibility with a high level of resilience.

Some gaskets combine metal gasketing in parallel with an integral elastomer weather sealing strip. They are available with or without adhesive backing in a broad range of standard cross sections. They're recommended for applications requiring a weather seal in addition to EMI shielding.

Another type of gasket features a highly resilient, hollow knit, tin-plated steel mesh that provides recovery from high compressive forces. Compression set is less than 30% at 80% compression. These round profile gaskets are suited for gasket-in-a-groove applications, such as in cast housings for outdoor Cat. 5 boxes, NEMA enclosures with outboard environmental seals, and industrial enclosures (Photo 3).

Compressed mesh EMI gaskets are designed for applications requiring small round or rectangular EMI seals, such as waveguide choke flanges, or for shafts. They're made by die-compressing wire mesh into a range of shapes, resiliencies, and metals.

Beryllium copper gaskets combine high levels of EMI shielding effectiveness with spring-finger wiping and low closure force properties. High tensile strength, anticorrosion properties, and good electrical conductivity makes them ideal for shielding over a broad frequency range. They're available in bright tin, copper, and nickel finishes at standard-length (16 in.) and custom-length strips in a variety of cross sections. For low compression grounding contacts, fingers are available with pressure sensitive adhesive for pick-and-stick application.

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