nexus how to measure the conformal coating … how to measure the conformal coating thickness on...
TRANSCRIPT
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.
Nexus:Howtomeasuretheconformalcoatingthicknessonyourprintedcircuit3
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.
Nexus:Howtomeasuretheconformalcoatingthicknessonyourprintedcircuit4
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.
Nexus:Howtomeasuretheconformalcoatingthicknessonyourprintedcircuit5
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.