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www.conformalcoating.co.uk
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.