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Nexus:SolvingConformalCoatingProblemsintheproductionprocess1
SolvingconformalcoatingproblemsintheapplicationprocessDrLeeHitchensNexuswww.conformalcoatinghelp.com
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 2
IntroductionNormally,problemsrelatingtoconformalcoatingcanbebrokendownintotwoareas.
Thoseareasareproblemsassociatedwithconformalcoatingprocessingandproblemsintheshort,mediumandlong-termreliabilityofthecircuitboardorproductcoated.
Ultimately,therearemanyfactorsthatcanaffecttheshort,mediumandlong-termreliabilityofacircuitboardthatarenotassociatedwithconformalcoatingandarenotconsideredhere.
Thescopeofthispaperisreallytofocusontroubleshootingtheproblemsassociatedwithconformalcoatingprocessingandproduction.
DefiningaconformalcoatingfailureinprocessConformalcoatingfailuresduetotheprocesscomefrommanydifferentareas.
However,theycanusuallybedividedintothreemaintypes.
Theseare:
• Failuresduetotheincorrectselectionofthematerialand,ortheapplicationmethodatthebeginning.
• Failuresintheconformalcoatingmaskingprocessduringproduction• Failureintheconformalcoatingproductionprocessitself.
Thefirstarea,selectionofmaterialandapplicationmethod,relatestotheholisticapproachtoconformalcoatingandisstudiedelsewhere.
Also,theconformalcoatingmaskingisconsideredasaseparateproblemandhelpcanbefoundhere.
Thisleavesthefinalsectionthatrelatestofailurestousetheconformalcoatingmaterialandtheapplicationprocessescorrectlythatwewillfocuson.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 3
TypicalfailuremechanismsinconformalcoatingprocessingTypicalfailuremechanismsarelistedbelow.
Blistering
Blushing
Bubbles,pinholesandfoam
Capillaryflow(Scavenging)
Contamination
Corrosion
Cracking
De-wetting
Delamination
Discoloration
FishEyes
Layering
Mealing
OrangePeel
Shadowing
Unevencoating
Itisn’texhaustivebutcoversallofthepointsraisedintheIPCstandardssuchastheIPCHDBK-830andIPC-A-610documents.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 4
BlisteringBlisteringistheformationofdome-orcircular-shapedbubblesoftheconformalcoatingfilmheldawayfromthesubstrate.
Blisteringcanbethestartofconformalcoatingdelaminating.
However,itcanalsohappenwithintheconformalcoatingitselfandnotjusttheinterfaceoftheconformalcoatingandthelaminatesurface.
Examples
CausesPoorcleanlinessThecleanlinessofthesurfaceiscriticalforgoodadhesion.Ifthesurfaceiscontaminatedacrossawideareathenblisteringcanoccurduetothepooradhesionofthecoatingtothelaminateasthecoatingpeelsaway.
Thiscouldalsobemadeworsebyhighpermeability(seebelow).
PoorcompatibilityFortheconformalcoatingtoadheretothesurface,bothneedtobecompatibletobondtogether.Iftheyareincompatiblethentheywillnotbond.Thisincompatibilitymaybyduetosurfaceenergiesmismatch.
Iftheconformalcoatingisnotmatchedtothelaminatethenblisteringcanoccur.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 5
HighcoatingpermeabilitytomoistureIfthecoatingallowsthemoisturetopermeateintotheregionbetweenthecoatingandthelaminate,thenadifferenceinvaporpressureoccurs.Thiscouldleadtoosmoticpressuresbuildingduetoachangeintemperatureandpressure,whichliftthecoatingawayfromthelaminateandcauseablister.
IncorrectdegreeofcoatingcureIftheconformalcoatingisnotcuredcorrectlythenthiscanleadtoadhesionproblems.
PreventionImprovecleanlinessCleanthesurfaceoftheboardtoimprovetheadhesionofthecoating.Removecontaminationsuchasmoldreleaseagentsoncomponentsbeforeapplication.
CheckcompatibilityMatchtheconformalcoatingtothesolderresistandlaminatesothatgoodadhesionisachieved.Changethecoatingand/orsurfacefinishsothesurfaceenergiesaremorefavorable.
ReducepermeabilitytomoistureChoosealesspermeableconformalcoatingtolimitmoisturepenetrationtothesurfaceofthelaminate.
ReducethecoatingthicknessReducingthecoatingthicknessreducestheproblemsassociatedwithcoefficientofthermalexpansion(CTE)mismatchwiththecomponentsandthelaminatethatleadtoflexingofthecoating.
LowerthecuretemperatureReduceforcedrying(thermalprofile)duringcuringtoavoidexcessstressandstrainsontheconformalcoating.
PrimethesurfaceApplyaprimertotreattheboardsurface.Thiswillincreasethebondingoftheconformalcoatingtothesubstrate.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 6
BlushingBlushingiswheretheconformalcoating,onceappliedtothecircuit,willappeartobeblotchy,milky,orcloudy.
Someoftheproblemscanincludesurfacetackinessorgreasiness,incompletecure,pooradhesion,pooradhesiononover-coating,coatingdiscolorationovertimeandpoorglossretention(mattfinish).
Examples
CausesEnvironmentalconditionsIftheconformalcoatingisdriedorcuredduringconditionsofcoolambienttemperatures,droppingtemperatures,orhighhumidity,someconformalcoatingscandevelopblushing.Thiscanbecausedbytheabsorptionofcarbondioxideandwaterintotheconformalcoatingfilm.
PreventionEnvironmentalcontrolControlthetemperatureandhumidityintheenvironmentduringprocessingandthiscouldreducetheblushingeffectintheconformalcoating.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 7
Bubbles,pinholesandfoamBubbles,pinholesandfoamaresimilardefects.Bubblesaretrappedpocketsofairwithintheconformalcoating.PinholesareburstbubblesthatreachtothesurfaceofthePCBfromthesubstrate.Foamisextremebubbling.
Examples
CausesPrematuredryingWhenapplyingtheconformalcoatingthewetfilmsurfacecanskinover,entrappingsolventsunderthesurfacethatcanpreventthebubblesescaping.
CoatinglayersappliedtooquicklyApplyingmultiplelayersofcoatingcanentrapsolvent.
Solventpoppingisanapplicationphenomenonthatoccurswhenasecondcoatisappliedwithoutsufficientflashtimeofthefirst.Solvent,whichisstilltryingtooutgas,istrappedunderneaththesecondcoat,andasitmigratestothetopitbursts,leavingasmallcrater.
Iftheconformalcoatingisappliedtoothickortooviscousanybubblesthatarecreatedintheprocessofapplicationcanbecomeentrappedinthecoatingbeforetheysettleout.
AirentrapmentAirentrappedundercomponentsbythecoatingprocesscanbeforcedoutduringthedrying&curingprocessandcausebubbles.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 8
ChampagneeffectsPressurepotswithconformalcoatinginsidecanabsorbairthatcanmanifestaschampagnebubbleswhenapplied.
WrongviscosityBrushcoatingwithtooviscousamaterialorworkingthecoatingcancausebubblesduringapplication.
ToomuchspraypressureIncorrectsprayequipmentorpressurescancauseexcessivebubbles&foam.
PreventionApplythecorrectfilmthicknessWhenapplyingtheconformalcoatingensurethewetfilmsurfaceisnottoothick.Thisisparticularlyimportantwhenspraycoating.Itisbettertoapplymultiplethinlayersthanonethicklayer.
UsethecorrectdryingprocessEnsuretheindividualcoatinglayersaresufficientlydrybeforeapplyingthenextcoatinglayer.Thiswillavoidsolventpopping.
ChangethedipprocesssettingsToavoidairentrapmentunderPCBcomponentstrydippingslowerintotheconformalcoatingdiptank,addingadwelltimetoallowpenetrationofthecoatingand,orusingalowerviscosityversionoftheconformalcoatingtounderfilltheareamoreeffectively.
TurnoffpressureinpotswhennotusedWhenusingpressurepotswithconformalcoatinginsidetrytoavoidleavingthepotspressurisedforlongperiodsoftimewhennotbeingusedintheprocess.
Also,avoidrunningthepressurepotslessthan50%fullandlettingthemstand.Thiswillminimisethechampagneeffectinthecoating.
BrushthecoatingcorrectlyWhenbrushcoatingtrytoblendtheviscosityoftheconformalcoatingsoitflowseasilyand“flow”thematerialontothePCB.DonotworkthecoatingintothePCBasitcreatesbubbles.
AdjustthepressureduringsprayingReducingthepressureonsprayingreducesthemechanicalagitationandreducesbubbles.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 9
Capillaryflow(scavenging)CapillaryflowinconformalcoatingiswherethecoatingpullsorrunsawayfromcertainareasofaPCB.Oritcanmoveunderneathdevices.
Thecoatingmovestoamorefavorableareaoftheboardduetoacombinationofeffects.Thisleavesapatchyfinishonthesurfaceoftheboard.
Examples
CausesLowviscosityofcoatingIftheconformalcoatinghasaverylowviscosityitwillpenetrateintodevicesmucheasierandmorescavengingwilloccur.
VolumeofcoatingAnabnormallyhighamountofwetconformalcoatingappliedduringtheapplicationprocesscanmeantheexcessmaterialcanwickintocomponentsbefore“gelling”ofthematerialisachieved.
SubstratecleanlinessIfthecircuitboardisnotcleanorhasalowsurfaceenergythentheconformalcoatingwillnotbondtothesubstrate.
HighsurfacetensionIftheconformalcoatinghasahighsurfacetensionthenitwillalsoliketoenergeticallypenetrateundercomponentsleavinguncoatedareasbehind.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 10
PreventionIncreasetheviscosityIncreasingtheviscosityoftheconformalcoatingwilllowerthemobilityofthecoatingandaidtheadhesiontotherightareasoftheboard.
ReducethevolumeofcoatingReducingtheamountofconformalcoatingappliedineachapplicationwillreducetheamountoffreematerialtocapillary.
SubstratecleanlinessCleaningtheboardbeforeprocessingwillaidthebondingofthecoatingduringtheapplication.
LowerthesurfacetensionUseasolventbasedmaterialratherthanwaterbasedor100%solidconformalcoating.
HeattheboardPriortocoatingtheboardraisingthecoretemperatureofthecircuitcanallowthecoatingtodryquickerandbelessmobileonthecircuitboard.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 11
ContaminationContaminationisdebrisfoundunder,trappedinorontopoftheconformalcoatingonthecircuitboard.Thiscontaminationmaybedetrimentaltothelong-termreliabilityofthecircuit.
Examples
CausesContaminationcancomefromavarietyofareas.Thisincludespre-productionstagesoflaminateandcomponentmanufacture,theprocessstagesduringapplicationandthedrying&curingstages.
PreventionImprovecleanlinessBeforeproductionstarts,allpartsofthecircuitshouldbefitforpurposeintermsofcleanliness.
ChangeprocessmaterialsUselesscontaminatingprocessmaterialssuchasflux.
CleantheproductionandapplicationlineOncereadyforassembly,theboardproductionprocessshouldnotaddunwantedcontaminationtoacircuitboard.
Theconformalcoatingapplicationprocessshouldnotaddcontamination.Thisincludesallthestagesaftercleaningincludingmasking.Theareasshouldbecontaminationfreetoalevelthatdoesnotproducesignificantdefects.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 12
CorrosionCorrosionisthechemicalorelectrochemicalreactionbetweenacircuitboard,usuallyametal,anditsenvironment,thatproducesadeteriorationofthecircuitanditsproperties.
Corrosioncanoccurwithoutaconformalcoatingbeingpresent.Aconformalcoatingcouldhelppreventorslowdowncorrosionasacompleteprocess.
Examples
CausesFactorsthatinfluencecorrosionand,orelectrochemicalmigration,usuallyrevolvearoundioniccontaminationandthecleanlinessofacircuit.Thesecontaminantscomefromelectronicprocessessuchas:
BoardfabricationThelaminategoesthroughalotofchemicalprocesses.Thecleaningofthechemicalsonthecircuitiscrucialtoensurethecontaminantsareremoved.
ComponentsComponentsalsocangetcontaminated.Again,cleanlinessiscritical.
AssemblyequipmentTheequipmentusedtomakethecircuitboardmayaddcontamination.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 13
SolderingprocessThefluxresiduescanhaveamajorinfluenceonthelong-termreliabilityofthecircuit.
OperatorhandlingPeoplecanaddcontaminationinavarietyofwaysduetodirtyhands,hairlossetc.
IncorrectcleaningThecleaningprocessmaynotcleantheboardscorrectlyorleavecleaningfluidresiduesonthecircuitduetoafailuretorinsetheboard.
PreventionIntroduceincominginspectionAfullinspectionofincomingboardsandcomponents,forsolderabilityandcleanliness,canhelp.
ImprovetheprocesscontrolIfyouareusinganocleanprocessthentighteninguptheproductionprocessiscritical.Adjustthesolderingwindowandfine-tunetoensuretheminimumoffluxisusedandisbeloworequaltotherecommendedamount.
ImproveoperatorcleanlinessAlloperatorswearglovesandothersuitableclothingtominimisecontamination.
SelecttherightmanufacturingmaterialsUse“lowresidue”nocleanflux/pastethatislessharmfultothecircuit.Usetheminimumamount.
CleanthecircuitboardItispossibletominimisethemajorityofcorrosioneffectsbyusinganeffectivecleaningprocessbeforeconformalcoating.
ControlthesolderreworkprocessLowresiduewire/fluxusedforrework.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 14
CrackingCrackinginconformalcoatingiswherethesmoothsurfaceofthecoatingfracturesintosections.Thiscanoccurintheshort,mediumandlongterm.
Thecracksintheconformalcoatingleavethesurfaceareabelowexposedtopotentialcontaminationpenetration.Thisallowsmoisture,wateranddebristoreachtheboardlevelwhereotherdefectscouldthenoccursuchascorrosion.
Examples
CausesCuretemperaturetoohighWhentheconformalcoatingisacceleratedcuredwithinanovenforexamplethetemperatureofcurecanleadtoprematuredryingofthetopsurfacebeforetheremainderofthecoatingisdry.Crackingcanoccur.
CuretemperaturetooquickHeatingthecoatingtooquicklywithoutallowingenoughtimeforroomtemperature(RT)dryingcancausecracking.
FilmthicknesstoogreatIfthedryconformalcoatingistoothickonthecircuitboardthenthiscancausecoefficientofthermalexpansion(CTE)mismatchandcracksoccurringinthecoating.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 15
EnvironmentextremesIftheoperatingtemperatureofthecircuitboardistoohighortoolow,thenthiscancausetheconformalcoatingtoflextoomuchandcrackinthefieldduetothermalstressesandshock.
PreventionReducethecuretemperatureLoweringtheinitialcuretemperatureoftheovencanreducethecrackingeffect.
SlowthecureprocessAllowaninitialdryingtimeatroomtemperaturebeforeexposuretoelevatedtemperature.Thiscanhelpminimisethecrackingofthecoating.
LowercoatingthicknessReducethecoatingthicknesstoavoidCTEmismatchissuesandthermalshockeffects.
ImprovematerialselectionSelectacoatingwithawidertemperaturerangeperformancecanhelp.Also,selectamoreflexibleconformalcoatingandthatmatchesclosertotheothercomponentsforlessCTEmismatchproblems.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 16
DelaminationDelaminationinconformalcoatingiswheretheconformalcoatingliftsfromthesurfaceofthePCB.Thisleavesthecircuitboardareabelowexposed.
Theliftingcanbepartialorcomplete.Itcanoccurnaturallyduetoareactioninthematerials.Itcanoccurduringtheproductionprocesssuchasde-masking.
Examples
CausesPoorcleanlinessThecleanlinessofthesurfaceisimportantforgoodadhesion.Ifthesurfaceiscontaminatedacrossawideareathendelaminationcanoccurduetothepooradhesionofthecoatingtothelaminate.
IncorrectcompatibilityTheconformalcoatingandthesurfacehavetobondtogether.Iftheyareincompatiblethentheywillnotbond.Thisincompatibilitymaybyduetosurfaceenergiesmismatch.Iftheconformalcoatingisnotmatchedtothelaminatethendelaminationcanoccur.
HighpermeabilitytomoistureIftheconformalcoatingallowsthemoisturetopermeateintotheregionbetweenthecoatingandthelaminate,thenadifferenceinvaporpressureoccurs.Thiscouldleadtoosmoticpressuresbuildingduetoachangeintemperatureandpressure,whichliftthecoatingawayfromthelaminate.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 17
DegreeofcoatingcureIftheconformalcoatingisnotcuredcorrectlythenthiscanleadtoadhesionproblemsandmismatchofCoefficientofThermalExpansion(CTE).
Thicknessandcurelevelofconformalcoatingduringde-maskingIftheconformalcoatingistoothickandwellcured,thecoatingmayliftofftheboardbeforetearingwhende-maskingwithtapeanddots.
PreventionImprovecleanlinessCleanthesurfaceoftheboardtoimprovetheadhesionofthecoating.Removecontaminationsuchasmoldreleaseagentsoncomponentsbeforeapplication.
CheckcompatibilityMatchtheconformalcoatingtothesolderresistandlaminatesothatgoodadhesionisachieved.Changethecoatingand/orsurfacefinishsothesurfaceenergiesaremorefavorable.
LowerpermeabilitytomoistureChoosealesspermeableconformalcoating.Thereareseveraloptionsavailable.
ReducethecoatingthicknessThisreducestheproblemsassociatedwithCTEmismatchwiththecomponentsandlaminatethatleadtoflexingofthecoating.
LowerthecuretemperatureReduceforcedrying(thermalprofile)duringcuringtoavoidexcessstressandstrainsonthecoating.
PrimethesurfaceApplyaprimertotreattheboardsurface.Thiswillincreasethebondingoftheconformalcoatingtothesubstrate.ThisoccursallthetimeforParyleneandothervapordepositioncoatings.
Reducethicknessandde-maskquicklyIfthecoatingisthinenoughthenitwillteareasily.Also,de-maskingbeforefullytheconformalcoatingiscompletelydryorcuredalsohelps.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 18
De-wettingDe-wettingisthetendencyoftheconformalcoatingmaterialtorefusetowetthesurfacethatithasbeenappliedtoevenly.
Theeffectcanactlocallyduetolocalcontamination.Itcanbeaglobaleffectrightacrosstheboard.
Examples
CausesBoardmanufacturingprocessResiduesfromboardmanufactureincludingsiliconesurfactantsfromsolderresist&HASLrinsecontaminationcancausede-wetting.
ComponentresiduesMaterialslikemold-releaseagentsusedtoalloweasyreleaseofcomponentsfromtheirmoldswhenfabricatedcouldcauseconformalcoatingstode-wet.
SiliconeoilOilsfromadhesivesinproductionandfrommaintenanceprocessescanbelethaltocircuitboards.Also,siliconeresiduesfromRTVantivibrationandstakingcompoundscanalsocauseproblems.
SolderingprocessesTheresiduesfromthesolderingproductionlineandprocesscancausede-wettingissues.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 19
CleaningbathcontaminationSomecleaningsolventsmaycausede-wettingascananycontaminantspickedupinthecleaningprocess.Whererinsinghasfailedthentheseresiduescanbepresentontheboard.
OperatorhandlingPeopleonthelinecanaddcontaminantsfrombarriercreams,foodandothermaterials.
PreventionControltheprocessMinimisethecontaminantsthatareonthecomponentpartsbeforeassembly.Thisincludeslaminate&componentcleanlinesscontrolbeforeassemblyandselectionoflowresidueprocessmaterialsincludingfluxesandpastesduringassembly
CleantheboardsImprovingthecleanlinessoftheboardsbeforeconformalcoatingcouldminimisede-wettingdefects.
CompatibilityofsolderresistEnsurethesolderresistiscompatiblewiththeconformalcoatingoritmaynotwetcorrectly.
RepairwhileprocessingAfterapplyingthecoating,locallystripthede-wettedareaandre-applythecoatingtothecorrectspecification.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 20
DiscolorationAchangeinthecolourorlossoftransparencyoftheconformalcoatingthatadverselyaffectstheperformanceortheidentificationofthecomponentsbelowthecoating.
CausesUVlightexposureDamagefromUVradiation,likesunlight,fromprolongedexposurecanchangethecolourofacoating.
ContaminationProlongedexposuretoparticulatesanddustthatsticktoatackysurfacecancausechangesincolour.
PreventionEliminatetheUVlightDon’texposetheconformalcoatingtoprolongedUVlightradiation.
CorrectconformalcoatingUseaUVstableconformalcoating.
CleanEnvironmentPreventthecircuitfromgettingcoveredincontamination.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 21
FishEyesAfisheyeisalocalisedde-wettingdefectthatlookslikeafisheyecrateronthesurfaceoftheconformalcoating.
Examples
CausesFisheyesarenormallycausedbyacontaminantonthesurfacepriortoapplicationoftheconformalcoating.Typically,thatcontaminantisasubstancelikeasiliconparticle.
Whenconformalcoatingisappliedontopofthecontaminant,theconformalcoatingisunabletoadheretotheoilysurfaceanda“fisheye”iscaused.
PreventionImprovethecleanlinessofthesurfaceThoroughlycleanthesurfaceoftheboardbeforecoating.Fisheyesareasurfacepreparationdefect.Ifthesurfaceofthecircuitbardhasbeenproperlycleanedanddegreased,fisheyecanusuallybepreventedcompletely.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 22
LayeringLayeringiswheremultiplelayersofconformalcoatingareappliedtoacircuitboardanddonotadheretoeachotherorcauseincorrectcuringoftheindividuallayers.
CausesCoatingadhesionSomeconformalcoatingsdonotsticktothemselves.ForexampleyoucannotapplymultiplelayersofParylenetoacircuitboard,astheywillnotadheretoeachother.
InadequatelevelofcoatingcureSomecoatingsmustbefullycuredbeforeyouapplyasecondlayer.Otherwise,incorrectcuringcanoccurduetothesecondlayerinhibitingthefirst.
IneffectivecoatingkeyingFinally,somecoatingsmustnotbefullycuredbeforethesecondcoatinglayerisappliedtoensuregoodadhesion.Thiscanbethecasewithmanysolventbasedcoatingprocesseswhichrequiremultiplelayersofcoatingtoachievethefilmthicknessdefined.Thesecondlayerbonds(keys)intothefirstlayer.
PreventionApplytheconformalcoatingcorrectlyItispossibletominimisethemajorityoflayerproblemsbyfollowingthemanufacturersguidelines.
GetitrightfirsttimeForParylene,ensurethecorrectfilmthicknessisachievedinonecoatapplication.Thisisachievedthroughtrialsandcalculations.IfrepairofParyleneisrequired,thennormalpracticeistouseasolventorwaterbasedfilm-formingcoatingasa“patch”overtherepairedarea.
MakethecoatingadhereToavoidcoatinginhibition,followthemanufacturerscureschedules.Also,useadhesionpromotionandsurfacepreparationtechniquestohelp.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 23
TimethecoatingapplicationlayerscorrectlyToavoidlaminationofcoatinginsolvent-basedcoatings,ensurethattheoptimumtimebetweencoatinglayersisselected.
Thistimeisdependentonseveralfactorsincludingthesolventflashofftime,thetemperatureandhumidityinthearea,theextractionrateandthechemicalnatureofthematerial.
Thekeytosuccessistodrythecoatingenoughwherethesecondlayerbondstothesurfaceofthefirstlayerbutdoesnottrapexcessivesolventinthelowerlayer.Thistrappingcanleadtoexcessivebubbles.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 24
MealingMealingisaformofdelaminationoftheconformalcoatingthatoccursacrossthecircuitboard.Itisablistering(vessication)affect.
Examples
CausesSurfacecleanlinessPoorcleanlinessofthesurfacereducesadhesion.Ifthesurfaceiscontaminatedacrossawideareathenmealingcanoccurduetothepooradhesionofthecoatingtothelaminate.
MaterialscompatibilityCompatibilitybetweentheconformalcoatingandthesurfaceadheredtoduetosurfaceenergiesmismatchisimportant.Iftheconformalcoatingisnotmatchedtothelaminatethenmealingcanoccur.
PermeabilitytomoistureofthecoatingIftheconformalcoatingallowsthemoisturetopermeateintotheregionbetweenthecoatingandthelaminate,thenadifferenceinvapourpressurecouldoccur.Thiscouldleadtoosmoticpressuresbuildingduetoachangeintemperatureandpressure,whichcouldliftthecoatingawayfromthelaminate.
DegreeofcoatingcureIfthecoatingisnotcuredcorrectlythenthiscanleadtoadhesionproblems.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 25
PreventionCleantheboardCleanthesurfaceoftheboardtoimprovetheadhesionofthecoating.Removecontaminationsuchasmoldreleaseagentsoncomponentsbeforeapplication.
MaterialcompatibilityMatchtheconformalcoatingtothesolderresistandlaminatesothatgoodadhesionisachieved.Changethecoatingand/orsurfacefinishsothesurfaceenergiesaremorefavorable.
CoatingthicknessReducetheconformalcoatingthickness.ThisreducestheproblemsassociatedwithCTEmismatchwiththecomponentsandlaminatethatleadtoflexingofthecoating.
LowercuringheatReduceforcedryingoftheconformalcoatingafterapplication(changethethermalprofile)toavoidexcessstressandstrainsonthecoating.
PrimethesubstrateApplyaprimertotreattheboardsurface.Thiswillincreasethebondingoftheconformalcoatingtothesubstrate.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 26
OrangePeelThisdefectisanuneventexturedsurfaceformedontheconformalcoating,oftenappearingdull,whichlooksliketheskinoftheorangeoragolfballwithsmallcratersanddimples.
Example
CausesSpraysetupOrangepeelisoftencausedbecauseofpoorgunadjustmentastheconformalcoatingisbeingapplied.
ApplicationincorrectAnincorrectapplicationtechniquecanproducetheorangepeeleffect.ThismanifestsinaerosolorspraygunapplicationwherethecoatingisappliedtoofarawayfromthePCBandthecoatingistoo“dry”onthesurfacetolevelout.
IncorrectviscosityAnincorrectviscosityoftheconformalcoatingbeforeapplicationcancauseorangepeel.
CoatingthicknessTheincorrectthicknessofthecoatingisdepositedsothatthereisinsufficientconformalcoatingtoself-level.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 27
Curing
Theincorrectcureprofilewhenusinganacceleratedcuringmechanism.Thiscouldbethattheflashoffzonebeforeheatcuringisinadequateortheovenprofileisincorrect.
PreventionGunsetupSetthespraygunupcorrectly.Gunsetupandadjustmentarecriticaltoasmoothandflatappearanceofaconformalcoating.
Iftheneedle/nozzlecombinationisnotspecificallymatchedtothecoatingbeingapplied,theairpressure(especiallywhenusinganhighvolumelowpressure(HVLP)gunmaynotbesufficienttocorrectlyatomisetheconformalcoating.
ViscosityReducetheviscosityoftheconformalcoating.Ifacoatingismixedatthewrongviscosityanditistoothick(toohighaviscosity),orangepeelislikely.
SprayingtechniqueOptimisethedistanceforsprayapplicationtothemanufacturerspecificationwhenapplyingtheconformalcoating.
MaterialvolumeappliedEnsureenoughlayersofcoatingareappliedtoformalevelcoating.Thisisespeciallytrueforaerosolsthataresusceptibletothisissue.
DryingIncreasetheflashofftimeforthesolventstoevaporatebeforeincreasingthetemperatureforelevatedcuringoftheconformalcoating.Re-profilethecuringovenusingtheconformalcoatingmaterialsuppliersrecommendedprofile.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 28
ShadowingShadowingiswherecorrectconformalcoatingcoverageisnotachievedonasectionofaboardduetoacomponentorpartoftheboardisblockinganotherareaofthecircuitboard.
Thiscanoccurwhencoatingisappliedbyspraymethodtoacircuitboard.Thiscaneasilyoccurwith3Dcomponentsandtallcomponents.
CausesIfthecircuitboardhasanarchitecturestructurethatis3Dinnaturewithlargecomponentssuchasheatsinks,capacitorsandthrough-holecomponentsonthecircuitthenthisismorelikelytooccur.
PreventionAvoidingshadowingcanbeeasilydoneifyouusethecorrectsprayapplicationmethodforconformalcoating.
Themethodthatshouldbeadoptedformostcasesisasfollows:
1. Anglethespraygunat45degreessotheconformalcoatingcanpenetrateeasierontothecircuitboard.
2. SpraythePCBinalightcoatinginarasteredpattern.3. DONOTapplythefullfilmthicknessonthisfirstpass.Athinlayeris
appropriate.4. Rotatethecircuitboardby90degrees5. Repeattherasteredpatternacrossthecircuit.6. Rotateagainandrepeatuntil4coatshavebeenappliedtothecircuit.7. ExaminethePCBforcoverage.Ifthecoatingcoverageisgoodthenthe
boardiscoated.Ifthecoatingispatchyortoothinthenrepeattheprocessafterthecorrecttimeforthecoatingtodryenough(toavoidlayeringissues).
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 29
UnevencoatingUnevenconformalcoatingasadefectiswherethethicknessofthematerialisoutsidetherangeofthespecifiedrequirement.
Whenthecoatingistoothinthiswillgiveinadequateprotection.Whenthecoatingistoothickthecoatingcanleadtootherdefectssuchasbubbles,crackinganddelamination.
Examples
CausesCapillaryeffectsWickingcanpullthecoatingundercomponents,fillingtheareaunderneaththedevicewithverythickmaterialandleavingotherareasclearofcoating.Thiscanalsoleadtolargefilletsofcoatingbetweenleadsandbridgingeffects,whichcanleadtoCTEmismatchproblemsinthelongterm.
EffectsofGravityGravitycanleadtocoatingrunningdownthesidesofcomponentsandpoolingatthebase.Itcanalsoleadtopoortipcoverageofsharpedges.Thiscanalsobeafactorwithsurfacetensioneffectsofconformalcoatingthatcanmovetheliquidaroundthecircuit.
OverlappingmateriallayersAnautomaticspraypatternfromarobotmayinadvertentlyapplysomeareaswiththickeramountsofcoatingbyoverlappingthespraystripes.
Nexus:Solvingconformalcoatingproblemsintheproductionprocess 30
ShadowingAmanualsprayprocessmaybepoorlyapplied,leadingtoshadowingandareasofthickcoating.
EntrappedcoatingAcircuitboardwhenwithdrawnfromadiptankinadipprocessmayhaveentrappedconformalcoatinginpocketsofthecomponentsorboarditselfthatcanleadtoverythickareasofcoating.
DryingtooquickAconformalcoatingmaynotleveloutcorrectlywhenappliedmaybeprematurely“gelling”tooquickly.
PreventionCapillaryeffectsAvoidwicking(scavenging)andtheproblemsassociatedwithit.
EffectsofgravityEnsurethecoatingtechniquetakesintoaccounttheeffectsofgravity.Forexamplewhenspraying,usemultiplethinlayersofcoating.Thisreducestheamountoffreeliquidtoflowontheverticalwallsandensuresthecoatingstickswhereapplied.
OverlappingmateriallayersProgramtheselectiverobottoavoidcrossingtheflowstripeswherepossible.
ShadowingAvoidshadowingeffectproblemswithconformalcoating.
EntrappedcoatingEnsuretheoptimumdrainanglewhendippingtoensureminimalcoatingistrapped.Otheroptionsincludingbuildinginatiltorrotatemechanismintothefixturesofthedipprocesssothatthedraincanbemoreeffective.Finally,designthecircuitwithdrainholesandflowpathstoachievegoodcoatingcoverage.
DryingtooquickUsealowerviscositymixofthecoatingifthecoatingisprematurelydryingandnotleveling.Also,reducetheextractionrateandincreasetheambienttemperatureforbetterflow.