HowtomeasuretheconformalcoatingthicknessonyourprintedcircuitboardDrLeeHitchensNexuswww.conformalcoatinghelp.com

Nexus:Howtomeasuretheconformalcoatingthicknessonyourprintedcircuit2

IntroductionOneofthekeychallengesinthesuccessfulapplicationofconformalcoatingtoaprintedcircuitboard(PCB)isachievingauniformandconsistentcoatingcoverage,atthespecifiedcoatingthickness.

Toothinacoatingwillprovideinadequateprotectionagainstcorrosion,orinsulationbetweenconductors.Iftheconformalcoatingistoothick,itwillbepronetocoatingdefectssuchasbubbles,insufficientcuring,andcrackingduringthermalshockcyclesetc.

Thereareseveralcommonlyusedmethodstomeasuretheappliedconformalcoatingthickness.However,thereiscurrentlynoreliable,non-destructivemethodtomeasurethecoatingthicknessalloveranactualcoatedcircuitboard.

Allthecommonlyusedmethodsrelyeitheronusingaflatcontrolsubstrate,coatedinthesamemannerastheproductionparttomeasurethethicknessonaflatpanel,ormeasuringacertainflatareaofthecircuitboard.

Theobviousdrawbacktothismethodisthatyoucannotguaranteethicknessonsharpedgesofdifficulttocoatdevices,whicharecommonlyamongstthemostcriticalareasforpreventingcorrosionoftheassemblyinservice.

DestructiveconformalcoatingthicknessmeasurementsItmaybeusefultoestablishaconformalcoatingthicknessbaselineforanassembly/processbythecreationof‘golden’referenceboards,usingtheexactproductionapplicationmethod.

Thisboardwillthenusuallybeencapsulatedinanepoxymoldingresin,beforebeingcutandpolished.Theepoxyresinactsasa‘binder’tokeepeverythinginplace.

Itisthenpossibletoopticallymeasuretheconformalcoatingthicknessatany(ormultiple)pointchosen.

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Theobviousadvantagetothismethodisitenablesexactthicknessmeasurementstobemadeonrealassemblies.

Thedownsidesarethatitisadestructive,timeconsumingandlabourintensive(readrelativelyexpensive)process,notreallysuitedtoproduction.

Sotheadvantageinthismethodthenliesinperformingthisattheprocessset-upstage.

Itthenenablesastatementthatsay,‘50micronsonaflat,unencumberedareaoftheboardequatestoanapproximatethicknessof10micronsonthetopofacomponentlead.

Therefore50micronsonaflatunencumberedareaofboardortestsampleistheminimumcoatingthicknesswecanacceptfromourprocesschecks,whichwillbecoverednext.

InprocessconformalcoatingthicknessmeasurementsConformalcoatingthicknesscanbemeasuredineitherthewetappliedstateorinthedry,curedcondition.

Thisisforthepurposeofprovidingaprocessgo/nogocheck,ortocreatesomekindofSPCprogramduringtheconformalcoatingprocess.

Wetfilmthicknessmeasurement

Wetfilmthicknessmeasurementofaconformalcoatingcanbeusedasaprocesschecktoensurethecoatingthicknessiscorrectpriortodryingand/orcuring.

Inthistechnique,asimpledevice,commonlyknownasawet-filmgaugeisusedtodeterminethethicknessofappliedcoating.

Thewet-filmgaugeconsistsofaseriesofaccuratelymachinedcombs,withadifferingdistancebetweenthemarkedpointsandthebaseline.

Thecombcontainingmarkingsforthecoatingthicknessrequired(say7-12mils)isplacedintothewetcoatingfilmandatleastoneoftheteethshouldremaincoatingfree.

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Ifyoucouldseethecoatingtouchingtheteeth7-10mils,butthe11and12milteethwerefreeofcoating,youwouldknowthatthewet-coatingthicknesswassomewherebetween10and11milsthick.

Oncecompleted,thewetfilmmeasurementismultipliedbythesolidscontentofthematerialblendtodeterminethedryfilmthickness.

Thiscorrelationwouldusuallybeperformedwithoneofthedrymeasurements,tocreateago/nogoprocesscheck.

Dryfilmmechanicalmeasurement

Thismethodisbyfarandawaythelowestcosttechniquealthoughislimitedinaccuracy.

Thesubstrateismeasured(withcalipers)priortoconformalcoating,inknownandmarkedlocations,usuallyinanumberofplaces.

Thecouponsarethencoatedandprocessedintheusualproductionprocess.

Whentheconformalcoatingisdriedandissufficientlyhardenough,thethicknessisre-measuredinthesameplaces,

Thereadingsarethenaveragedtogiveanestimatecoatingthicknesswithstandarddeviationsbeingusedtoassessuniformity.

Itisimportanttoknowthatiftheboardiscoatedonbothsides,thenthethicknessmeasurementsmustbedividedbytwo,toaccountforthefactthattherearetwocoatingthicknessesbeingmeasured.

Thisisclearlythecasewithclassicverticaldipping,butsinceitintroducesmoresourcesoferror,forsprayprocessesitisusualtojustcoatonesideofthepanel.

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Directeddy-currentmeasurement

Eddycurrentmeasurementtechniquesareusedtomeasurethethicknessofnon-conductivecoatingsonnon-ferrousmetalsubstrates.

Whentheprobeisbroughtnearaconductivesurface(forexamplecopperinnerlayersincircuitboards),magneticeddycurrentsarecreatedonthesurfaceandcanbemeasuredtodeterminetheconformalcoatingthickness.

Typically,thedeviceswillhaveoptionstostoremeasurementresultsorperforminstantanalysisofreadingsandoutputtoaprinterorcomputerforfurtherexamination.

Thetypicaltoleranceis±1%withinacertainthickness.

Testingissensitivetosurfaceroughness,curvature,substratethickness,typeofmetalsubstrateanddistancefromanedge.

Themeasurementsystemsneedtobezeroedonthesubstratebeinginvestigated,tosubtractthethicknessofthenon-conductivematerialsifthemeasurementisbeingmadeonacircuitboard.

Itisquiteusualforaprogramtouseallthreeofthesetechniquesatvariousstagesofprocessdevelopmentandproduction.

Otheralternatives?Opticalmeasurementsshowpromiseforanon-destructive,thicknessmeasurementtotaketheplaceofcross-sectioninganddryfilmmeasurements,.

But,buttherearemanychallengestoovercome,nottheleastthehighreflectanceofthesolderjointsandtheconformalcoatingthemselves.

Untilthistechnologyhasmatured,thereisstillnogoodalternativetotheoldtriedandtestedmethodsdescribedabove.