Advanced-PCB-design-and-layout-for-EMC---Part-2

AdvancedPCBdesignandlayoutforEMC.Part2–SegregationandInterfaceSuppressionByEurIngKeithArmstrongC.EngMIEEMIEEE,CherryCloughConsultantsThisisthesecondinaseriesofeightarticlesongood-practiceEMCdesigntechniquesforprintedcircuitboard(PCB)designandlayout.ThisseriesisintendedforthedesignersofanyelectroniccircuitsthataretobeconstructedonPCBs,andofcourseforthePCBdesignersthemselves.Allapplicationsareasarecovered,fromhouseholdappliances;commercial,medicalandindustrialequipment;throughautomotive,railandmarinetoaerospaceandmilitary.ThesePCBtechniquesarehelpfulwhenitisdesiredto…•Savecostbyreducing(oreliminating)enclosure-levelshielding•Reducetimetomarketandcompliancecostsbyreducingthenumberofdesigniterations•Improvetherangeofco-locatedwirelessdatacomms(GSM,DECT,Bluetooth,IEEE802.11,etc.)•Useveryhigh-speeddevices,orhighpowerdigitalsignalprocessing(DSP)•UsethelatestICtechnologies(130nmor90nmprocesses,‘chipscale’packages,etc.)Thetopicstobecoveredinthisseriesare:1.Savingtimeandcostoverall2.Segregationandinterfacesuppression3.PCB-chassisbonding4.Referenceplanesfor0Vandpower5.Decoupling,includingburiedcapacitancetechnology6.Transmissionlines7.Routingandlayerstacking,includingmicroviatechnology8.AnumberofmiscellaneousfinalissuesApreviousseriesbythesameauthorintheEMC&ComplianceJournalin1999“DesignTechniquesforEMC”[1]includedasectiononPCBdesignandlayout(“Part5–PCBDesignandLayout”,October1999,pages5–17),butonlysetouttocoverthemostbasicPCBtechniquesforEMC–theonesthatallPCBsshouldfollownomatterhowsimpletheircircuits.Thisseriesispostedonthewebandthewebversionshavebeensubstantiallyimprovedovertheinterveningyears[2].Otherarticlesandpublicationsbythisauthor(e.g.[3][4]andVolume3of[5])havealsoaddressedbasicPCBtechniquesforEMC.Thisserieswillnotrepeatthebasicdesigninformationinthesearticles–itwillbuilduponit.Liketheabovearticles,thisserieswillnotbotherwithwhythesetechniqueswork,theywillsimplydescribethetechniquesandwhentheyareappropriate.Butthesetechniquesarewell-proveninpracticebynumerousdesignersworld-wide,andthereasonswhytheyworkareunderstoodbyacademics,sotheycanbeusedwithconfidence.Therearefewtechniquesdescribedinthisseriesthatarerelativelyunproven,andthiswillbementionedwhereappropriate.Partofthisarticleisverysimilartothepaper:“GHzShieldingatPCBlevel”presentedbytheauthorattheIEE’sevent:“NewEMCIssuesinDesign”,heldatQinetiq,Farnborough,onthe28thofApril2004.TableofContents,forthisPartoftheseries1Introductiontoadvancedsegregation2PCB-levelshielding2.1ReasonsforshieldingonthePCB2.2OverviewofshieldingatPCBlevel2.3TypesofPCBshielding-can2.4Attachingshielding-canstoPCBs2.5PCBshielding-canmaterials2.6Aperturesandgapsinshielding-cans2.7Waveguide-below-cutoffmethods2.8Nearfieldeffectsonshielding2.9Cavityresonances3Interconnectionsandshielding3.1CombiningPCBshieldingwithfiltering4Combiningshieldingwithheatsinking5Environmentalissues6PCB-levelfiltering6.1ReasonsforfilteringonthePCB6.2OverviewofPCBfiltering6.3High-performancefilteringrequiresagoodqualityRFreference6.4Designofsingle-stagelow-powerandsignalPCBfilters6.5PowerfilteringonPCBs6.6Filteringforshieldedconnectors7Placementofoff-boardinterconnections8Compromises9References1IntroductiontoadvancedsegregationSegregationisoneofthemostcost-effectivePCBtechniques,andwasdescribedinPart5of[1];Part5of[2];[3]andVolume3of[5].Theadvancedsegregationtechniquesdescribedinthisarticleemploythesameanalysesoftheinterfacebetweenthe‘inside’and‘outsideworlds’,andbetweenthe‘noisy’and‘sensitive’zonesonthePCB,butemploymoreattentiontothedetailsoftheshieldingandfiltering.Itisalwaysagoodideatoplanfortheworstcaseshieldingandfilteringthatmightberequiredaroundeachsegregatedcircuitzone,incaseitisrequired.Equipmentcanbe‘pre-compliance’EMCtestedwithoutit’sshieldingandwithminimalPCBfilteringfitted,withtheextrashieldingandfilteringparts(thatitishopedwillnotberequired)readilyavailableincasetheyareneededtopassthetests.IftheEMCtestisfailed,theshieldingandfilteringcanquicklybeupgradedwiththeaidofasolderingiron,andtheequipmentretestedwithoutsignificantdelay,tofindtheminimumconfigurationthatpassesthetestoratleasttohelpidentifywheretheproblemarealies.Thiscansavemonthsoftimecomparedwiththeusualscenario,inwhichPCB‘respins’arerequiredeachtimeitissuspectedthatadditionalshieldingand/orfilteringisrequiredtopassanEMCtest.Shieldingandfilteringworktogethertocreatehigh-performancesegregation.Highlevelsofshieldingwillnotbeachievedwithoutfiltering,andhighlevelsoffilteringwillnotbeachievedwithoutshielding,especiallyatfrequenciesabove100MHz.2PCB-levelshieldingGeneralshieldingissuesaredescribedinPart4of[1];Part4of[2];Volume2of[5]andChapter8of[21].PCBshieldingemploysthesamegeneraldesign,butissuesanddesigndetailsmorerelevanttoadvancedPCBsegregationaredescribedinthissection,andinsections3,4and5below.2.1ReasonsforshieldingonthePCBTherearemanygoodreasonsforusingshieldingtechniquesatPCBlevel.Themostobviousoneisthatshieldingatthelowestlevelofassembly–thePCB–coststheleastandaddstheleastweight.Shieldingatthelevelofthemoduleorenclosurecostsatleasttentimesmoret