conduct

ViaInPad-ConductiveFillorNon-ConductiveFill?

“WhichistheBestChoiceforMyDesign?”

Oneofthemostcommonlyaskedquestionswhendecidinghowtofillmechanically

drilledvia-in-padholesiswhetherornottospecifyconductiveornon-conductiveepoxies.

Thisbriefpaperwilltakeupwhereourprevious“TechTalkforTechies”leftoff,withalook

intothebestpracticesandmanufacturabilityoffillingviasforvia-in-padstructures.

SmallerisBetter

Intheearly2000sthefirstfine-pitchball

gridarraydevicesbecamepopularwithdesigners

lookingtopackasmuchhorpowerintoassmall

aspaceaspossible.“Smallerisbetter”becamethe

ruleandwiththatthemechanicaldrillingworld

becameverelyimpactedbyavailabledrillbit

sizes,aspectratios,andplatingmethodologies.

Firstofall,thediameterofthedrillneededtobe

inthe0.006”orsmallerrangeduetothereduction

ly,theaspectratio(depthtodiameter)becamelimited

bydrillflutelength,positionalaccuracy,rigidityofthetools(topreventbreakage),andthe

tly,theplatingneededtocloup

theholeasmuchaspossible,whichledtoproblemswithvoiding,incompletefill,and

gas/solutionentrapment.

Lardrillingtechnologyandmicroviaplatingsystemsrapidlyadvancedtotakethe

lowedanatural

parationofmechanicalandlardiametersbadonwhichwerebestintheir

icaldrilldiametersweredesignateddowntowherelarvia

technologytookover—typicallyat0.006”aytheprocesshas

maturedfor0.5mmpitchBGAsandtighterusingmicroviasandlargerdeviceshaving

greaterdiameterpadsthatallowmechanicaldrilling(withthelimitationsonannularring

andaspectratioaccountedfor).

FinishedHoleDiameterandAspectRatioThebestpracticeforepoxyfilledviasistohaveaFINISHEDviaholediameter

betweenthesizesof0.008”and0.018”(easytoremember!).Themainreasonisthatthe

viscosityoftheepoxyfill(accountingforbothconductiveandnon-conductivevarieties)

allowsforittobepushedcompletelythroughaholeassmallas0.008”,butwillnotreadily

runoutofaholeaslargeas0.018”.Theassociatedaspectratioisbestifthedepthto

diameterislessthan10:1(tomaintainaminimum‘industrystandard’fillof97%).

AnnularRingForannularringconsiderations,theIPCstandardpersistsforClass2andClass3

minimums,sotheassociatedpaddiametermustbesufficientlylargetoaccommodatethe

mech’slookat

anexample:Foran0.008”FHS(finishedholesize)thedrillwilllikelybe0.010”(toallow

0.001”/sideplatingintheholeminimum).ThatmeansthatforaClass2designthepad

wouldtypicallybe0.018”orlarger,andforaClass3design0.020”orlarger(toallow

minimumannularringinthefinishedboard).Asyoucane,mechanicallydrilledvia-in-padstructuresareforlargerpitchdevicesduetothenecessarypaddiameterto

meetminimumannularringperspecification.

TheBIGDecision:ConductiveorNon-ConductiveEpoxyViaFillMaterial?

WhataretheavailableVIPfillmaterialoptions?

Conductive:Conductiveviafillmaterialispopularizedbytwomaincompanies’products—ebothofthe‘silvercoatedcopper

particle’filledepoxymatrix,whichprovidessomeelectricalandthermalconductivityina

ferencebetweenthetwomaterialsisintheoverallparticulatesizeand

finishedCTE(coefficientofthermalexpansion)—theCB100hasalargerparticlesizewitha

higherCTE,andhasbeenavailableformuchlongermakingitabetterknownmaterial.

Figure1-DupontCB100ConductiveViaFill

Non-Conductive:Thenon-conductiveproductsaresimilarlypopularizedbytwoproducts—cathePeters

producthasbeenavailableforalongertimebutinrecentyearstheSan-Eiproducthas

overwhelmingdemand,AdvancedCircuitsusonlytheSan-EiKagakumaterialforin-

houVIPprocessing.

Figure2-San-EiKagakuPHP-900Non-conductiveViaFillinBlindVia

MakingYourDecision

MatchtheCTEoftheVIPfillmaterialandLaminateThefirstconsideration(andonlyconsiderationinsomedesigns)istotrytomatch

hatimportant?Becauasthe

boardlivesoutitslifeinanon/offandheating/coolingstatetheexpansionandcontraction

ofthepadstackstructuresmoveeitherwithoragainsttheadjacentboardlaminate

material,whichcanleadtostressfracturesandpossibleelectricalopensintheveryworst

cas.

Thisconsiderationnaturallyfavorsthenon-conductiveepoxiesastheirCTEisa

betterm

industryhasfollowedsuit—likely90%ormoreproductstodayspecifynon-conductive

epoxy,makingitamorepopularandthereforelesxpensiveoptionaswell.

Butthisbegsthequestion—whyorwherewoulditbenecessarytolecta

conductiveepoxyfill?

lvias(viasudtoconductheatawayfromacomponent)havebeenshowntomarginallybenefit

fromaconductivefillastheentirestructurehelpstransmitthermalenergyoutandaway

ncountBGAsthatrunhotcouldbenefitfromtheuofconductive

rcommonapplicationistoaddthermalviasinagridmatrixdirectly

beneathaflat-packSMD.

Legacyorolderestablisheddesignsfromcompaniesthathavebeenrunningfora

decadeormoreoftenspecifiedconductiveDuPontCB100astheviafillbecauitwasthe

oardasmblieshavelongagobeen

characterizedandlifecyclepropertiesarewellknown.

FactorFiction?Onemythtoputtorestisthataconductivefillisrequiredforasolidvia-in-pad

structure—someassumeincorrectlythatitmustbeconductiveinordertoplateoverthe

nottheca!Theboard

fabricatoralwaysrunsthecured/planarizedepoxyfilledholesthroughelectrolesscopper

followedbyanelectrolyticstrikeplate,whicheffectivelyalstheholeandpreparesitfor

thefinalpatternplatingoperationwhenthesurfacepadisplated.

WrapItmustbestatedthatforallofthevia-in-padstructuresacontinuousWRAPof

copperplatingisrequired(byIPCandcustomerspecification)fromthebarrelofthehole

outontothesurfacepad,thusprovidingamlesscontinuityofthestructurewhichhelps

holdittogetherduringnormalexpansion/

howeverisanotherverycomplexstoryaltogether,andisbeyondthescopeofthisarticle

(willlikelybecoveredinafutureeditionof“TechTalkforTechies”!).

Conclusion:DesignandIntentofthePCB

SointheenditisthedesignandintentofthePCBthatdeterminestheneedfor

edCircuits’ProductEngineerscanassist

onally,

thespecificationsincludingCTEcanbeobtainedeitheronthemanufacturer’swebsitesor

byrequestingthemfromourengineerswhocanprovidespecificinformationonhowbest

toproceed!

Wehopethisbriefpaperhelpsguideyouinthelectionoftheavailable

optionsforyourdesign—pleacontactusifyouhavefurtherquestions.