EMI Shielding Theory & Gasket Design Guide - Sealing Devices

1 SECTION CONTENTSPAGET heory of Shielding and gasketing192 Conductive elastomer Gasket design196 Gasket junction design196 Corrosion198 Selection of seal cross section202 General tolerances 204 Gasket mounting choices205 fastener requirements206 Designing a solid-O conductive elastomer Gasket -in-a-groove 209 Mesh EMI gasketing selection guide214 Glossary of terms218 Part number cross reference index220 EMI Shielding Theory & Gasket Design Guide191US HeadquartersTEL+(1) 781-935-4850 FAX+(1) 781-933-4318 TEL+(44) 1628 404000 FAX+(44) 1628 404090 Asia PacificTEL+(852) 2 428 8008 FAX +(852) 2 423 8253 South America TEL+(55) 11 3917 1099 FAX +(55) 11 3917 0817192US HeadquartersTEL+(1) 781-935-4850 FAX+(1) 781-933-4318 TEL+(44) 1628 404000 FAX+(44) 1628 404090 Asia PacificTEL+(852) 2 428 8008 FAX +(852) 2 423 8253 South America TEL+(55) 11 3917 1099 FAX +(55) 11 3917 0817 EMI Shielding TheoryTheory of Shieldingand GasketingFundamental ConceptsA knowledge of the fundamentalconcepts of EMI Shielding will aidthe designer in selecting the gasketinherently best suited to a electromagnetic waves consistof two essential components, amagnetic field, and an electric two fields are perpendicularto each other, and the direction ofwave propagation is at right anglesto the plane containing these twocomponents.

2 The relative magnitudebetween the magnetic (H) field andthe electric (E) field depends uponhow far away the wave is from itssource, and on the nature of thegenerating source itself. The ratioof E to H is called the waveimpedance, the source contains a largecurrent flow compared to its potential,such as may be generated by aloop, a transformer, or power lines,it is called a current, magnetic, orlow impedance source. The latterdefinition is derived from the factthat the ratio of E to H has a smallvalue. Conversely, if the sourceoperates at high voltage, and onlya small amount of current flows, thesource impedance is said to behigh, and the wave is commonlyreferred to as an electric field. Atvery large distances from thesource, the ratio of E to H is equalfor either wave regardless of itsorigination.

3 When this occurs, thewave is said to be a plane wave,and the wave impedance is equalto 377 ohms, which is the intrinsicimpedance of free space. Beyondthis point all waves essentially losetheir curvature, and the surfacecontaining the two componentsbecomes a plane instead of asection of a sphere in the caseof a point source of importance of waveimpedance can be illustrated byconsidering what happens when anelectromagnetic wave encounters adiscontinuity. If the magnitude of thewave impedance is greatly differentfrom the intrinsic impedance of thediscontinuity, most of the energy willbe reflected, and very little will betransmitted across the metals have an intrinsicimpedance of only milliohms.

4 Forlow impedance fields (H dominant),less energy is reflected, and moreis absorbed, because the metalis more closely matched to theimpedance of the field. This is whyit is so difficult to shield againstmagnetic fields. On the other hand,the wave impedance of electricfields is high, so most of the energyis reflected for this the theoretical caseof an incident wave normal tothe surface of a metallic structureas illustrated in Figure 1. If theconductivity of the metal wall isinfinite, an electric field equal andopposite to that of the incidentelectric field components of thewave is generated in the satisfies the boundary conditionthat the total tangential electric fieldmust vanish at the boundary.

5 Underthese ideal conditions, shieldingshould be perfect because the twofields exactly cancel one fact that the magnetic fields arein phase means that the current flowin the shield is effectiveness of metallicenclosures is not infinite, becausethe conductivity of all metals is can, however, approach verylarge values. Because metallicshields have less than infiniteconductivity, part of the field istransmitted across the boundaryand supports a current in the metalas illustrated in Figure 2. Theamount of current flow at any depthin the shield, and the rate of decayis governed by the conductivity ofthe metal and its permeability. Theresidual current appearing on theopposite face is the one responsiblefor generating the field which existson the other conclusion from Figures 2and 3 is that thickness plays animportant role in Shielding .

6 Whenskin depth is considered, however,it turns out that thickness is onlycritical at low frequencies. At highfrequencies, even metal foils areeffective current density for thin shieldsis shown in Figure 3. The currentdensity in thick shields is the sameas for thin shields. A secondaryreflection occurs at the far side ofthe shield for all thicknesses. Theonly difference with thin shields isthat a large part of the re-reflectedwave may appear on the frontsurface. This wave can add to orsubtract from the primary reflectedwave depending upon the phaserelationship between them. For thisreason, a correction factor appearsin the Shielding calculations toaccount for reflections from thefar surface of a thin gap or slot in a shield will allowelectromagnetic fields to radiatethrough the shield, unless thecurrent continuity can be preservedacross the gaps.

7 The function of anEMI Gasket is to preserve continuityof current flow in the shield. If thegasket is made of a materialidentical to the walls of the shielded Figure 1 Standard Wave Pattern of aPerfect Conductor Illuminated by aNormally Incident, + X Polarized PlaneWavezErHrHiEiEHPerfectlyConductiveP lane z=0yxFigure 2 Variation of Current Densitywith Thickness for Electrically ThickWallsEtJtJoEiUS HeadquartersTEL+(1) 781-935-4850 FAX+(1) 781-933-4318 TEL+(44) 1628 404000 FAX+(44) 1628 404090 Asia PacificTEL+(852) 2 428 8008 FAX +(852) 2 423 8253 South America TEL+(55) 11 3917 1099 FAX +(55) 11 3917 0817193((()))(6)enclosure, the current distribution inthe Gasket will also be the sameassuming it could perfectly fill theslot.

8 (This is not possible due tomechanical considerations.)The flow of current through ashield including a Gasket interface isillustrated in Figure 4. Electromagneticleakage through the seam can occurin two ways. First, the energy canleak through the material Gasket material shown inFigure 4 is assumed to have lowerconductivity than the material in theshield. The rate of current decay,therefore, is also less in the is apparent that more current willappear on the far side of the increased flow causes a largerleakage field to appear on the farside of the shield. Second, leakagecan occur at the interface betweenthe Gasket and the shield. If an airgap exists in the seam, the flow ofcurrent will be diverted to thosepoints or areas which are in change in the direction of the flowof current alters the current distributionin the shield as well as in the high resistance joint does notbehave much differently than openseams.

9 It simply alters the distributionof current somewhat. A currentdistribution for a typical seam isshown in Figure 4. Lines of constantcurrent flow spaced at larger intervalsindicate less flow of is important in Gasket designto make the electrical properties ofthe Gasket as similar to the shieldas possible, maintain a high degreeof electrical conductivity at theinterface, and avoid air, or highresistance andGasket Equations1 The previous section was devotedto a physical understanding of thefundamental concepts of shieldingand gasketing. This section is devotedto mathematical expressions usefulfor general Design purposes. It ishelpful to understand the criteriafor selecting the parameters of ashielded the previous section, it wasshown that electromagnetic wavesincident upon a discontinuity will bepartially reflected, and partly trans-mitted across the boundary and intothe material.

10 The effectiveness of theshield is the sum total of these twoeffects, plus a correction factor toaccount for reflections from the backsurfaces of the shield. The overallexpression for Shielding effectivenessis written = R + A + B(1) is the Shielding effectiveness2expressed in dB,R is the reflection factor expressed in dB,A is the absorption term expressed in dB, andB is the correction factor due to reflections fromthe far boundary expressed in reflection term is largelydependent upon the relativemismatch between the incomingwave and the surface impedance ofthe shield. Reflection terms for allwave types have been worked outby equations for thethree principal fields are given bythe expressions:whereRE, RH, and RPare the reflection terms for theelectric, magnetic, and plane wave fieldsexpressed in is the relative conductivity referred tocopper,f is the frequency in Hz, is the relative permeability referred tofree space,r1is the distance from the source to theshield in absorption term A is thesame for all three waves and isgiven by the expression.