Advanced-PCB-design-and-layout-for-EMC---Part-8

AdvancedPCBDesignandLayoutforEMC-Part8-AnumberofmiscellaneousfinalissuesByEurIngKeithArmstrongC.EngMIEEMIEEE,CherryCloughConsultantsThisisthelastinaseriesofeightarticlesongood-practicedesigntechniquesforelectromagneticcompatibility(EMC)forprintedcircuitboard(PCB)designandlayout.ThisseriesisintendedforthedesignersofanyelectroniccircuitsthataretobeconstructedonPCBs,andofcourseforthePCBdesignersthemselves.Allapplicationsareasarecovered,fromhouseholdappliances;commercial,medicalandindustrialequipment;throughautomotive,railandmarinetoaerospaceandmilitary.ThesePCBtechniquesarehelpfulwhenitisdesiredto.•Savecostbyreducing(oreliminating)enclosure-levelshielding•Reducetime-to-marketandcompliancecostsbyreducingthenumberofdesigniterations•Improvetherangeofco-locatedwirelessdatacomms(GSM,DECT,Bluetooth,IEEE802.11,etc.)•Useveryhigh-speeddevices,orhighpowerdigitalsignalprocessing(DSP)•UsethelatestICtechnologies(130nmor90nmchipprocesses,’chipscale’packages,etc.)Thetopicstobecoveredinthisseriesare:1.Savingtimeandcostoverall2.Segregationandinterfacesuppression3.PCB-chassisbonding4.Referenceplanesfor0Vandpower5.Decoupling,includingburiedcapacitancetechnology6.Transmissionlines7.Routingandlayerstacking,includingmicroviatechnology8.AnumberofmiscellaneousfinalissuesThisisthefinalpartofthisseries,andIhopeyouhaveenjoyedreadingit,oratleastfoundsomethingsinitthatwereinterestingoruseful.ApreviousseriesbythesameauthorintheEMCComplianceJournalin1999DesignTechniquesforEMC[1]includedasectiononPCBdesignandlayout(Part5-PCBDesignandLayout,October1999,pages5-17),butonlysetouttocoverthemostbasicPCBtechniquesforEMC-theonesthatallPCBsshouldfollownomatterhowsimpletheircircuits.Thatseriesispostedonthewebandthewebversionshavebeensubstantiallyimprovedovertheinterveningyears[2].Otherarticlesandpublicationsbythisauthor(e.g.[3][4]andVolume3of[5])havealsoaddressedbasicPCBtechniquesforEMC.Liketheabovearticles,thisseriesdoesnotspendmuchtimeanalysingwhythesetechniqueswork,itfocusesondescribingtheirpracticalapplicationsandwhentheyareappropriate.Butthesetechniquesarewell-proveninpracticebynumerousdesignersworld-wide,andthereasonswhytheyworkareunderstoodbyacademics,sotheycanbeusedwithconfidence.Therearefewtechniquesdescribedinthisseriesthatarerelativelyunproven,andthiswillbementionedwhereappropriate.Ifyouhavemissedanypartsofthisseries,youcandownloadthemfromthearchiveat://://’sarchives.TableofContents,forthisPartoftheseries1PowersupplyconnectionstoPCBs2Low-Kdielectrics3Chip-scalepackages(CSPs)4Chip-on-board(COB)5HeatsinksonPCBs5.1EMCeffectsofheatsinks5.2HeatsinkRFresonances5.3BondingheatsinkstoaPCBplane5.4Combiningshieldingwithheatsinking5.5Otherheatsinktechniquesthatmayhelp5.6Heatsinksforpowerdevices6Packageresonances7Eliminatethetestpadsforbed-of-nailsorflyingprobetesting8UnusedI/Opins9Crystalsandoscillators10ICtricks11Locationofterminationsattheendsoftransmission-lines12ElectromagneticBandGap(EBG)13SomefinalPCBdesignissues14Bewareboardmanufacturerschanginglayoutsorstack-ups15Future-proofingtheEMCdesign15.1MarkingEMCdesignfeaturesorcriticalpartsonthedesigndrawings15.2Aquality-controlledprocedureforEMCdesign16EMC-competentQA,changecontrol,cost-reduction17Compromises18References19Someusefulsourcesoffurtherinformation1PowersupplyconnectionstoPCBsAllPCBconnectorscarryingpowerand0Vshoulduseadjacentpinsfortheirpowerand0Vconnections.Iftheconnectorislong,thereshouldbeanumberofpower/0Vpinpairsspreadalongitswholelength.Iftheconnectorisalsowidethereshouldbeanumberofpower/0Vpinpairsspreadacrossitswholearea.Ideallythereshouldbeapower/0Vpinpairadjacenttoeachsignalpin,butgivencostandspaceconstraintsthisisusuallyonlydoneforcriticalsignals,suchashigh-speed(e.g.Gb/s)interconnections.UsingdifferentialpairsforGb/sinterconnectionsrelaxesthisconstraintandallowsonepower/0Vpairforeverytwoormoresignals.Thelowertheimbalanceinthedifferentialpairs(intheirdrivers,PCBtraces,connectorpinsandexternalcabling,see[6])thefewerpower/0Vpinpairsarerequiredforagivennumberofsignals,foragivenlevelofEMCperformance.Adecouplingcapacitor(typically10-100nF)shouldbeplacedbetweenthepowerandgroundveryneartoeachpower/0Vpairofconnectorpins.WherethemainpowerentersthePCB,abulkdecouplershouldalsobelocated.Inthepastbulkdecouplingabove470nFwasalwaysdonewithelectrolytics,butmultilayerceramicsarenowavailablewithupto100µF(withlowvoltageratings)andtheyshouldbemuchmoreeffectivethanelectrolytics,aswellasbeingsmaller,morereliable,andreversible.Theaboveguidealsoappliestoconnectionstocables,andalsotointer-boardconnections.Whenusedinabackplanetypeofassembly,spreadingpower/0Vpinpairsacrossthewholeareaofeachconnectorhelpsachievealowimpedanceconnectionatradiofrequencies(R