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TechnicalBulletinSettingupaConformalCoatingProductionFacility

Thesetupofaturnkeyconformalcoatingproductionline,whetheritisabatchorinlineprocess,hasmanysimilarcharacteristics.

Whetherthecoatingapplicationisbasedaroundahighvolumeselectivecoatingrobotorabatchdipcoatingprocesstherulesandrequirementstendtobethesame.

Thisbulletinreviewsthepotentialneedsofthefacility.

GeneralRequirements

Anycoatingfacilitywillneedthebasicrequirementsputinplacethatwouldbestandardforanypieceofelectronicmanufacturingprocess.TheseincludeESDsystems,facilitiesforthemachines,theenvironmentalrequirementsandthenormalHealth&Safety(HSE)considerations.

Thefirsttwoarestraightforwardandarenotconsideredhere.However,thelasttwoneedtobeconsideredinmoredepth.

Health&Safety(HSE)

HSEtendstobemoreimportantforconformalcoatingssinceingeneralthecoatingsthemselvesarehazardous,orthewaytheyareappliedmakesthempotentiallyharmfultooperators.Iftheyaresolventbasedcoatings,thenextractiontoremovethefumesisveryimportant.Mostequipmentwillhaveminimumrequirementsforextractiontobefittedandtheseguidelinesshouldbefollowed.

Ifthecoatingsarenotsolventbased,thentherecanbeotherhazards.ApopularconformalcoatingmethodofapplicationisUVcure.However,twohazardsimmediatelycomeoutinthistechnology.ThefirstistheUVlightitselfsinceitishighlyharmfultopeopleifseenandthereforeshieldinghastobegoodaroundtheUVcuresystem.ThesecondisthegroundlevelozonethatcanbecreatedbytheUVcureprocess.Again,thisishighlydangeroustooperatorsandextractionisneededtoremoveitfromthearea.

BeyondUVcurethereisalsosprayingofconformalcoating.Atomisingaliquid,evenifitisnotsolventbased,canbehazardoustooperatorsandtheirlungs.Therefore,theequipmentused,andmoreimportantly,thewaytheventilationworksaroundthisequipmentisveryimportant.

So,ifextractionisrequiredthenitiscriticaltofollowthelocalHSEadviceonhowthisisfittedandthetypeofextraction.Machinemanufacturerssupplydatabaseduponthesystemtheyratetheirsystem.Forexample,theCEsystemisusedinEurope.But,thismayvaryinNorthAmerica.Atypicalmistakemadeistousenormalextractionratherthananexplosionproofversion.InEuropeandmanyotherareasthiswouldbeillegalsocarehastobetakeninthisselection.

Environment

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Conformalcoatingsaresensitivetotheenvironmentthattheyareprocessed.Thecoatingroomareashouldbeclean&dustfreesincePCBsaresusceptibletoparticlesstickingtothedryingcoating.Whatlevelofcleanlinessisprovidedisdowntotherequirementsofthecustomerormanufacturer.However,thecleanerthefacility,thefewertheissuesthereareinmeetingmanufacturingstandardsliketheIPC-A-6101.

Thetemperaturerangeshouldbereasonablycontrolledwithinsensiblelimitssincetheviscosityoftheconformalcoatingwillvarywithtemperature.Thiscanreallydisruptacoatinglinewhereachangeinviscosityduetoroomtemperaturechangesadjuststhecoatingperformance.

Anothercriticalpointisthehumidity.Thismayneedtobecontrolledtobeabove35%(forESDreasons)and<55%duetomoistureissues.Thisisbecausehavingahighhumiditycanaffectthecoatingintegrityandtheapplicationmethodproducingdullcoatingthicknessandentrappingmoisturewithinthecoatingitself.

Productionlinerequirements

Theproductionline,whetherthisisanoperatorbrushcoatingprintedcircuitboards(PCBs)oraninlinespraycoatingprocesscanbemadeupofseveralstages(notallaremandatory).

Theseare:

Boards In

Cleaning

Inspection

Pre Coating Inspection

Masking

Coating Application

Drying / Curing

De-MaskingDe-Masking

Inspection

Boards Out

Asstatednotallofthesestagesmayberequiredanditmaybeaverysimpleprocess.

But,wewillexploreeachofthestagesinturntoreviewtheoptionsforanengineerwhensettingupacoatingproductionline.

BoardsIn/BoardsOut

Thisisverystraightforward.ThePCBsdeliveredtothecoatingareareadyforprocessing.

Inspection

InspectingthePCBsbeforestartingtheprocesshelpsstopthelineifthereisaproblembeforetoomuchworkhasbeendoneupstream.ItalsoidentifieswhichPCBsaretobecoated,thematerialtobeusedandhowtheyaretobecoated.Thiscanbedoneautomaticallyaspartoftheproductionprocessorcompletedbytheoperator.

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Cleaning

Cleaningcircuitboardsbeforecoatingmayormaynotberequired.Thisisadecisionthathastobemadeearlieronperhapsinthedesignstagebeforeproductionactuallystarts.Thecleaningprotocolisfortheengineerstodecide.

However,ifyoudocleanitisitmaybeworthconsideringafewguidelines:

• EnsurethatthecleaningprocessdoesnotaddmorecontaminantstothePCBthanitstartedwith.Forexample,makesuretherinsingofthecleaningfluidfromthePCBissuccessfulandthattherinsewaterisDIwaterandhighenoughpuritynottoaddioniccontaminantstothePCB.

• ThePCBsaredryenoughforthecoatingprocessandnotleavewaterentrappedundercomponentswhichcouldimpedethecoatingapplication.

• Iftheintentionistocleanthenensurethecleaningprocessactuallyremovesresiduesthatcausedefectslikede-wettingtominimisefinishingattheendoftheprocess

Again,thisisfortheengineerstodecideandtheconsiderationofwhetheryouneedtocleanyourcircuitboardsforconformalcoatingisawholetopiconitsown.

Masking

Maskingmaynotberequired.However,thereareveryfewcircuitboardsintheworldthatcanbecompletelycoatedalloverandhavenoareasthatmustnotbecoated.SomecomponentsonPCBsgenerallyneedtoavoidhavingconformalcoatingappliedtothemandthereforethisgivestheengineerstwooptions:

1. Youcanavoidcoatingthecomponentorareausingaselectivesprayordipsystem.

2. Youcanapplysomeformofmaskingthatthecoatingcanbeappliedtoandremovethemaskingmaterialsafterapplication.

Again,engineersneedtoconsidertheiroptionsonwhethertouseconformalcoatingmasking.

Pre-CoatingInspection

InourcoatingservicesaroundtheworldwehavefounditismoreefficienttodoublecheckthemaskingprocessiscorrectbeforecoatingapplicationratherthanrepairthePCBaftertheprocessgoeswrongbecausemaskingwasincorrect.Thischeckcanbemanualorautomatedbutitishighlyvaluable.

CoatingApplication

TheonestagethatcannotbeavoidedisthecoatingappliedtothePCB.Therearemanydifferentmethodsforapplyingconformalcoatingsandprobablythemostimportantfactorinallofthemistraining.Theoperatorneedstounderstandtheprocesstobeabletoworkcorrectly.

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Hereareafewkeypointsonthemostpopularmethods:

Brushing

Brushcoatingappearstobeoneofthesimplestmethodsinconformalcoatingapplication.However,itisnoteasytodowell.WithgoodtechniqueitispossibletobehighlyeffectiveandmeetalltherequirementsofstandardsliketheIPCspecification.

Keypointsinclude:

• Createanenvironmentsuitableforbrushingthatissimilartothefinishingandinspectionstages.GoodUVlightingandextraction(solventbasedcoatings)canmakeamassivedifferenceinperformanceofoperatorsoveraperiodoftime.

• Traintheoperatortodevelopthebrushcoatingtechniquetominimisedefectsthatareeasilycreatedifthecoatingisappliedwrongly.

• Controltheviscosityoftheconformalcoatingasthismaychangewithtimeandagainleadtopoorerapplicationperformance.

Dipping

Dipcoatingcanbehanddippinginatubofconformalcoatingoramoreformalcontrolledprocessusingadippingsystem.

Intermsofprocessthekeyfactorsforcontrolofcoatingapplicationareviscosityandwithdrawalspeedofthedippingmachine.Ifyoucontrolthesefactorsthenyoucancontrolthecoatingthicknessonthecircuitboard.ConsiderthatifyouareusingadipsystemhowyouhandlethedrippingPCBsandthecontroloftheevaporatingsolventsifthecoatingsgiveofffumes.

BatchSpraying/Aerosol

Whensprayingwithanaerosolorasprayguninaspraybooththereareseveralkeypointstoconsider.

• Thedominantfactorisoperatortraining.Thisisasemiskilledprocessandtheapplicationofconformalcoatingbyhandsprayisnotsimple.

• Ingeneral,itisfarbettertoapplyseveralthinlayersofcoatingthanonethicklayer.Thisisduetoproblemslikebubblesentrapmentandskinningofthecoating.

• Youcanachieveagreatfinishwithsprayingiftheoperatorisskilful.Youcanalsominimiseyourprocessingafterapplicationinde-maskingandtouchupifthecoatingisappliedcorrectly.

• Theconformalcoatingthicknessiscontrolledbytheoperator.Ultimatelygoodtrainingiscrucialandamethodformonitoringcoatingthicknessapplicationisdeveloped.

SelectiveRobot

Roboticcoatingisthedominantapplicationmethodinconformalcoatingforhighvolumeproduction.Itcaneliminateorminimisemaskingneeds,especiallyifthePCBisdesignedfortheprocess.Again,keyfactorsincludeoperatortrainingintheuseofthesystemsinceultimatelyitcanbeacomplexmachineandtogetthebestfromitrequirestheoperatortounderstandtheprocess.

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Drying/Curing

Aconformalcoatingisawetprocess(unlessyouuseParylene)andthereforethecoatinghastodry.Thedifferencebetweendryingandcuringisexploredelsewhere.However,theprocessrequireddependsonthecoatingitself.

Forexample,asolventbasedacryliccoatingwilldrybeevaporatingthesolventsawayfromthePCB.DuringthistimethePCBiswetandcouldgetcontaminatedbyairborneparticulatesorbeaccidentlytouchedbyoperators.So,eitherthemachineitselfortheconveyorsneedtoshielditfromtheenvironmentorthePCBsneedtobestoredawayuntildry.

However,ifthecoatingneedscuringfast,withUVlightorheatforexample,thenanextraprocessandproductionsystemssuchasUVlightconveyorsorinlineinfraredconvectionovensneedtobeincluded.

De-masking

Likemasking,thereisskillinremovingthemaskingmaterialsandensuringthefinalfinishedPCBisfittopasstheinspectioncriteria.Workinstructionsareneededalongwithtrainingonthespecificmethodsoffinishingtoensurethiscanbeachieved.Again,theworkenvironmentiscriticalandcreatingthecorrectareareallyhelps.

Inspection

Ultimately,whenthePCBiscoatedandfinished,youwanttoknowiftheproductmeetstheinspectioncriteria.ThisconformalcoatinginspectioncanbedonemanuallyorautomaticallydependingonthevolumeofPCBsandthelevelofsophisticationrequired.Again,itisdowntooperatortrainingandusingtherightequipmenttoensurethatthisispossible.

Also,atthisstageitispossibletomeasurefactorslikecoatingconformalthicknesstocheckthatthecriteriaaremet.Therearecoatingconformalthicknessmeasurementsystems2availabletodothisthatifusedcorrectlywillgiveyouveryaccuratereadings.

Summary

Settingupaconformalcoatingfacilityisastraightforwardprocessaslongasallfactorsareconsidered.Ingeneral,akeyfactorisoperatortraining.

Getthisrightandmanyoftheproblemsthatcouldoccurduringproductionwillbeavoided.

Aftertraining,settingthefacilityupcorrectlywiththerightsystems,toolsandprocesscontrolsalsoensuresasmoothrunningproduction.