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PCB Thermal Relief Guidelines for Effective Layouts

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Thermal reliefs are the spoked connections from a thru-hole or surface mount pad to an area fill or plane. Their purpose is to provide a robust ... Skiptomaincontent Togglemenubar CadencePCBDesign&Analysis Togglesubmenufor:LearnByTopic 3DECAD/MCADandRigidFlex DesignDataManagement DesignReliability DesignReuseandProductivity HDIandMiniaturization HighSpeedDesignandAnalysis LayoutandRouting Manufacturability RF/MicrowaveDesign SchematicCaptureandCircuitSimulation Signal/PowerIntegrity Togglesubmenufor:DesignInsights E-books&WhitePapers MigrationGuides PCBDesignfromStarttoFinish TechnicalPapers Webinars Togglesubmenufor:ProductVideos AllegroPCBEditor AllegroSystemCapture OrCADPCBDesigner PSpice TrainingVideos Togglesubmenufor:TrainingVideos AllegroPCBEditor OrCADCapture OrCADPCBEditor PSpice LatestBlogs ContactUs Opensearchbox Searchsitewide Closesearchbox RecentSearches SharethisPost SharethisPost Shareonfacebook Shareontwitter Shareonlinkedin Shareonemail Home Blog PCBThermalReliefGuidelinesforEffectiveLayouts KeyTakeaways WhatarethermalreliefpadsinPCBlayout? Guidelinesforcreatingthermalreliefpads. HowtouseyourPCBCADtoolswhencreatingthermalreliefpads. PCBthermalreliefguidelinesareusedincreatingthesepadstoaidinmanufacturabilityAthermalreliefonacircuitboardensuresthattheproperamountofheatisfocusedonapinforsoldering.Withoutthermalreliefsoncircuitboards,therewouldbeawiderangeofassemblyproblemsassociatedwithsoldering.Somepinswouldn’tgetenoughheatforthesoldertoformaproperjoint,andotherpartswouldbedifficultfortechnicianstounsolderduringrework.Inversely,thiscouldturnthereliefintoanunplannedfuse.ItisessentialforlayoutengineerstounderstandgoodPCBthermalreliefguidelineswhensettinguptheCADsystemparametersoftheirdesigntoavoidthermalproblems.TheImportanceofThermalReliefsinPCBLayoutThermalreliefsarethespokedconnectionsfromathru-holeorsurfacemountpadtoanareafillorplane.Theirpurposeistoprovidearobustelectricalconnectionfromthepadtotheplanewhilepreventingtheheatneededforsolderingfromdissipatingintothemetalplane.Thegreencolorrepresentsasolidplaneofcopperonacircuitboardinthepicturebelow,andyoucanseefourthermalreliefconnectionsforeachouterpin.Inthecaseofasurface-mountpinthatconnectstoalargemetalarea,multipletracesmaybeusedfromthepadtothecoppertoaccomplishthesameresults.Thepurposeofthethermalreliefistoaidinthemanufacturabilityofthecircuitboard.Whileadirectfloodedconnectiongivesthebestelectricalperformance,italsoservesasaheatsinkforpinsconnectedtotheplane.Thissinkcouldeffectivelypulltheheatawayfromthepinduringautomatedsolderingoperationsandproducepoorsolderjointsintheconnection.Simultaneously,theheatsinkingattributesofadirectconnectionwouldmakemanualsolderingoperationsdifficultfortechniciansduringrework.Additionally,toomuchmetalononepadversustheotheronsmalltwo-pindiscretepartscancauseathermalimbalanceinthecomponent.Thisimbalancecanresultinamanufacturingphenomenonknownas“tombstoning.”However,itisalsoimperativethatthespokesofthermalreliefsbethickenoughtohandlethecurrentexpectedtopassthroughthem.Balancingthenumberandwidthsofthermalreliefspokesiscrucialforagoodthermalreliefdesign.Spokesthataretoosmallmaynotconductenoughcurrentforpowercircuitsorprovideinadequateconnectionsforpowerintegrity.Spokesthataretoolargemayendupdrawingtoomuchheatawayfromthepad.Again,inthepicturebelow,youcanseehowwidethespokesofthesethermalreliefpadsaretoprovidethecorrectamountofcoppertoconnectthepintotheplane.Next,we’lllookcloserattheguidelinesofwhereandhowdesignersshouldusethesereliefsinPCBlayout.Standardthru-holepinthermalreliefpadsPCBThermalReliefGuidelinesforLayoutTheneedforathermalreliefpadcancomeupduringdifferentaspectsofPCBlayout,andherearefourareaswheredesignersshouldconsidertheiruse.1.Thru-HolePinsHolesdrilledthroughtheboardforsolderingofthru-holepinsareprimecandidatesforthermalreliefpads.Asmallmetalarea,suchasasplitplane,canactasaheatsinkandpullheatawayfromthepin.Inlargemulti-layerboardswithmultiplegroundplanes,thethru-holepinsmaybeconnectedtoseveralsquarefeetofmetal,requiringanexcessiveamountofheattocreateagoodsolderjoint.Thisheatrequirementopensupthepossibilityofnearbytracesandcomponentsbeingdamagedwhenapplyingextraheatduringsolderingoperations.Thru-holepinsliketheseneedathermalreliefpadtoprovideagoodelectricalconnectionwithouttheheatsinkingcharacteristicsofadirectconnection.2.SurfaceMountPinsForelectricalreasons,manydesignerswillembedasurfacemountpadinametalfill.Whilethiswillgivethebestelectricalperformance,italsopullsheatawayfromthatpad.Ifthesepadsbelongtoasmalltwo-pindiscretepart,thesolderontheotherpadmaymeltfasterthantheembeddedpad.Theresultingsurfacetensionofthemeltingsolderwillpullthepartupandawayfromtheotherpad,creatingatombstone.Byusingmultipletracestoconnectasurfacemountpadinsteadofembeddingitintoanareaofmetal,youcanbalancetheamountofheatappliedduringsolderingbetweenbothpads.3.ViasMostviasthatconnecttopowerandgrounddonotrequireathermalreliefpadbecausetheydon’thaveapinsolderedintothem.However,inthecaseofnetsthatconducthighcurrentlevels,someshortpowertracesmaystillconducttoomuchheattotheirsurfacemountpins.Inthesecases,athermalreliefpinshouldbeconsidered. 4.MetalRoutingWhileathermalreliefpadisnecessaryforpinsconnectingtopowerandgroundplanesorlargeareafills,theiruseonpowertraceroutingmustbeconsideredforeachcase. Somepowertracesthatconductlowerlevelsofcurrentwillberoutedwithnarrowertraces.CADsystemsthatautomaticallyaddathermalreliefconnectiontopinsonthesenetsmayendupdoingadisservicetothedesignbytryingtoforcethereliefpad.Thespokesmaynothaveenoughroomtoconnect,resultinginaconnectionwithoutenoughmetal.Rememberthatthenumberandwidthsofthethermalreliefpadspokesshouldequalthewidthofthetracedictatedbyitspowerrequirements.Ifyouareroutinga50milpowertrace,butyoucanonlygettwotenmilspokestoconnect,yourisklackingmetalfortheconnection.Thislackcanleadtoexcessiveheatduringthecircuitboard’soperation,ultimatelyresultinginaburned-outconnection.Additionally,moremetalintheconnectionequalslowernetinductance,whichwillhelpkeepEMIundercontrol.Soyoumustcreatethermalreliefswiththerightamountofmetalintheirconnections,andusually,yourPCBCADtoolswillhaveutilitiesthatcanhelp.SettingupthermalreliefpadparametersinCadenceAllegroUsingYourDesignToolstoCreateThermalReliefsThermalreliefpadsareusuallycreatedautomaticallybyyourPCBCADsystemafterenteringthedesiredconfigurationforthepadintothesystem.Whenpadstacksarebuilt,manyCADsystemswillallowyoutoenterthethermalrelief’ssize,layers,andotherparametersintothepadstack.Thermalreliefparameterscanalsobeconfiguredforeachnet,plane,orfillinwhichtheywillbeused.Inthepictureabove,youcanseehowtheusersetsupthermalreliefattributesforaspecificfillinCadence’sAllegroPCBEditor.Theusercansetupdifferentparametersforthru-holepins,surfacemountpins,andvias,includinghowtheyareconnectedandhowmanyspokes.Withthethermalsadequatelyconfigured,theywillautomaticallyformwhentheirnetisroutedwithatraceorafill.Inthepicturebelow,youcanseeanexampleofhowAllegrocreatesthermalsinthissimplelittlecopperpourconnectingathru-holepintoasurfacemountpin.Theuseroutlinesthedesiredarea,andthenthesystemautomaticallyfillsinthecopperpour,creatingthethermalreliefpadsaspartoftheprocess.Thethru-holepinisconnectedwithtwothermalspokes,whilethesurfacemountpinhasspokesonallfoursides. UsingCadenceAllegrotocreateasmallcopperpourandtwothermalreliefpadsThermalManagementforPCBLayoutsThermalreliefpadsarejustoneaspectofthethermalandotherpower-relatedissuestoconsideranddesignwhenlayingoutaprintedcircuitboard.Fortunately,CADsystemslikeAllegro’sPCBEditorcontainmanydifferentutilitiesandtoolsthathelp.Forinstance,atthetopofthisarticleisavideodescribinghowCadence’sIRDropAnalysiscanenabledesignerstocreateastablepowerdeliverynetworkintheirdesign.Allegroalsofeaturesbuilt-indesignrulesandconstraintstoensurepowertracesareroutedatthenecessarywidthsthroughoutyourboarddesign.Allegroalsoallowsdesignerstoviewandchecktheirdesignsin3Dtohandilyspotareasontheirboardwithrestrictedairflowandotherthermalcoolingproblems.FormoreinformationonPCBroutingandplanes,checkoutourE-bookaboutit.Ifyou’relookingtolearnmoreabouthowCadencehasthesolutionforyou,talktoourteamofexperts.  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