EMI Shielding Theory & Gasket Design Guide
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EMI Shielding Theory & Gasket Design Guide SECTION CONTENTS PAGE Theory of shielding and gasketing 192 Conductive elastomer gasket design 196 Gasket junction design 196 Corrosion 198 Selection of seal cross section 202 General tolerances 204 Gasket mounting choices 205 Fastener requirements 206 Designing a solid-O conductive elastomer gasket-in-a-groove 209 Mesh EMI gasketing selection guide 214 Glossary of terms 218 Part number cross reference index 220 US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 191 EMI Shielding Theory Theory of Shielding wave impedance is greatly different transmitted across the boundary and Gasketing from the intrinsic impedance of the and supports a current in the metal Fundamental Concepts discontinuity, most of the energy will as illustrated in Figure 2. The be reflected, and very little will be amount of current flow at any depth A knowledge of the fundamental transmitted across the boundary. in the shield, and the rate of decay concepts of EMI shielding will aid Most metals have an intrinsic is governed by the conductivity of the designer in selecting the gasket impedance of only milliohms. For the metal and its permeability. The inherently best suited to a specific low impedance fields (H dominant), residual current appearing on the design. less energy is reflected, and more opposite face is the one responsible All electromagnetic waves consist is absorbed, because the metal for generating the field which exists of two essential components, a is more closely matched to the on the other side. magnetic field, and an electric field. impedance of the field. This is why These two fields are perpendicular it is so difficult to shield against Ei to each other, and the direction of magnetic fields. On the other hand, wave propagation is at right angles the wave impedance of electric Et to the plane containing these two fields is high, so most of the energy Jo components. The relative magnitude is reflected for this case. between the magnetic (H) field and Jt Consider the theoretical case the electric (E) field depends upon of an incident wave normal to how far away the wave is from its the surface of a metallic structure Figure 2 Variation of Current Density source, and on the nature of the as illustrated in Figure 1. If the with Thickness for Electrically Thick generating source itself. The ratio conductivity of the metal wall is Walls of E to H is called the wave infinite, an electric field equal and Our conclusion from Figures 2 impedance, Z . w opposite to that of the incident and 3 is that thickness plays an If the source contains a large electric field components of the important role in shielding. When current flow compared to its potential, wave is generated in the shield. skin depth is considered, however, such as may be generated by a This satisfies the boundary condition it turns out that thickness is only loop, a transformer, or power lines, that the total tangential electric field critical at low frequencies. At high it is called a current, magnetic, or must vanish at the boundary. Under frequencies, even metal foils are low impedance source. The latter these ideal conditions, shielding effective shields. definition is derived from the fact should be perfect because the two The current density for thin shields that the ratio of E to H has a small fields exactly cancel one another. is shown in Figure 3. The current value. Conversely, if the source The fact that the magnetic fields are density in thick shields is the same operates at high voltage, and only in phase means that the current flow as for thin shields. A secondary a small amount of current flows, the in the shield is doubled. reflection occurs at the far side of source impedance is said to be the shield for all thicknesses. The high, and the wave is commonly x only difference with thin shields is referred to as an electric field. At Perfectly Ei that a large part of the re-reflected very large distances from the Conductive Plane z=0 wave may appear on the front source, the ratio of E to H is equal surface. This wave can add to or for either wave regardless of its H E i subtract from the primary reflected origination. When this occurs, the Hr wave depending upon the phase wave is said to be a plane wave, Er relationship between them. For this and the wave impedance is equal H reason, a correction factor appears to 377 ohms, which is the intrinsic z y in the shielding calculations to impedance of free space. Beyond account for reflections from the this point all waves essentially lose Figure 1 Standard Wave Pattern of a far surface of a thin shield. their curvature, and the surface Perfect Conductor Illuminated by a A gap or slot in a shield will allow containing the two components Normally Incident, + X Polarized Plane electromagnetic fields to radiate becomes a plane instead of a Wave through the shield, unless the section of a sphere in the case current continuity can be preserved of a point source of radiation. Shielding effectiveness of metallic across the gaps. The function of an The importance of wave enclosures is not infinite, because EMI gasket is to preserve continuity impedance can be illustrated by the conductivity of all metals is finite. of current flow in the shield. If the considering what happens when an They can, however, approach very gasket is made of a material electromagnetic wave encounters a large values. Because metallic identical to the walls of the shielded discontinuity. If the magnitude of the shields have less than infinite conductivity, part of the field is US Headquarters TEL +(1) 781-935-4850 FAX +(1) 781-933-4318 • www.chomerics.com Europe TEL +(44) 1628 404000 FAX +(44) 1628 404090 Asia Pacific TEL +(852) 2 428 8008 FAX +(852) 2 423 8253 192 South America TEL +(55) 11 3917 1099 FAX +(55) 11 3917 0817 gap exists in the seam, the flow of The reflection term is largely E i current will be diverted to those dependent upon the relative points or areas which are in contact. mismatch between the incoming Et A change in the direction of the flow wave and the surface impedance of Jo Jt of current alters the current distribution the shield. Reflection terms for all in the shield as well as in the gasket. wave types have been worked out Current on 3 Rereflection A high resistance joint does not by others. The equations for the front wall due from rear wall to reflection behave much differently than open three principal fields are given by from rear wall seams. It simply alters the distribution the expressions: Figure 3 Variation of Current Density of current somewhat. A current G RE = 353.6 + 10 log10 (2) distribution for a typical seam is 3 2 with Thickness for Electrically Thin Wall f µr1 shown in Figure 4. Lines of constant 0.462 µ fG enclosure, the current distribution in current flow spaced at larger intervals RH= 20 log10 + 0.136r1 +0.354 (3) ( r √Gf √ µ ) the gasket will also be the same indicate less flow of current. 1 assuming it could perfectly fill the G x 106 It is important in gasket design RP= 108.2 + 10 log10 (4) µ slot. (This is not possible due to to make the electrical properties of f mechanical considerations.) the gasket as similar to the shield where The flow of current through a as possible, maintain a high degree RE, RH, and RP are the reflection terms for the shield including a gasket interface is of electrical conductivity at the electric, magnetic, and plane wave fields illustrated in Figure 4. Electromagnetic interface, and avoid air, or high expressed in dB. leakage through the seam can occur resistance gaps. G is the relative conductivity referred to in two ways. First, the energy can copper, leak through the material directly. Shielding and The gasket material shown in Gasket Equations1 f is the frequency in Hz, Figure 4 is assumed to have lower The previous section was devoted µ is the relative permeability referred to conductivity than the material in the to a physical understanding of the free space, shield. The rate of current decay, fundamental concepts of shielding r1 is the distance from the source to the therefore, is also less in the gasket. and gasketing. This section is devoted shield in inches. It is apparent that more current will to mathematical expressions useful for general design purposes. It is The absorption term A is the same for all three waves and is Lines of helpful to understand the criteria constant current for selecting the parameters of a given by the expression: σg < σm shielded enclosure. (5) In the previous section, it was A = 3.338 x 10–3 x t √µfG shown that electromagnetic waves Metal Shield where incident upon a discontinuity will be A is the absorption or penetration loss Ei partially reflected, and partly trans- expressed in dB, and t is the thickness Et mitted across the boundary and into of the shield in mils. the material. The effectiveness of the shield is the sum total of these two The factor B can be mathematically effects, plus a correction factor to positive or negative (in practice it is Gasket account for reflections from the back always negative), and becomes surfaces of the shield.