Conformal coating problems WHITE...

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.


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.


BlisteringBlisteringistheformationofdome-orcircular-shapedbubblesoftheconformalcoatingfilmheldawayfromthesubstrate.

Blisteringcanbethestartofconformalcoatingdelaminating.

However,itcanalsohappenwithintheconformalcoatingitselfandnotjusttheinterfaceoftheconformalcoatingandthelaminatesurface.

Examples

CausesPoorcleanlinessThecleanlinessofthesurfaceiscriticalforgoodadhesion.Ifthesurfaceiscontaminatedacrossawideareathenblisteringcanoccurduetothepooradhesionofthecoatingtothelaminateasthecoatingpeelsaway.

Thiscouldalsobemadeworsebyhighpermeability(seebelow).

PoorcompatibilityFortheconformalcoatingtoadheretothesurface,bothneedtobecompatibletobondtogether.Iftheyareincompatiblethentheywillnotbond.Thisincompatibilitymaybyduetosurfaceenergiesmismatch.

Iftheconformalcoatingisnotmatchedtothelaminatethenblisteringcanoccur.


HighcoatingpermeabilitytomoistureIfthecoatingallowsthemoisturetopermeateintotheregionbetweenthecoatingandthelaminate,thenadifferenceinvaporpressureoccurs.Thiscouldleadtoosmoticpressuresbuildingduetoachangeintemperatureandpressure,whichliftthecoatingawayfromthelaminateandcauseablister.

IncorrectdegreeofcoatingcureIftheconformalcoatingisnotcuredcorrectlythenthiscanleadtoadhesionproblems.

PreventionImprovecleanlinessCleanthesurfaceoftheboardtoimprovetheadhesionofthecoating.Removecontaminationsuchasmoldreleaseagentsoncomponentsbeforeapplication.

CheckcompatibilityMatchtheconformalcoatingtothesolderresistandlaminatesothatgoodadhesionisachieved.Changethecoatingand/orsurfacefinishsothesurfaceenergiesaremorefavorable.

ReducepermeabilitytomoistureChoosealesspermeableconformalcoatingtolimitmoisturepenetrationtothesurfaceofthelaminate.

ReducethecoatingthicknessReducingthecoatingthicknessreducestheproblemsassociatedwithcoefficientofthermalexpansion(CTE)mismatchwiththecomponentsandthelaminatethatleadtoflexingofthecoating.

LowerthecuretemperatureReduceforcedrying(thermalprofile)duringcuringtoavoidexcessstressandstrainsontheconformalcoating.

PrimethesurfaceApplyaprimertotreattheboardsurface.Thiswillincreasethebondingoftheconformalcoatingtothesubstrate.


BlushingBlushingiswheretheconformalcoating,onceappliedtothecircuit,willappeartobeblotchy,milky,orcloudy.

Someoftheproblemscanincludesurfacetackinessorgreasiness,incompletecure,pooradhesion,pooradhesiononover-coating,coatingdiscolorationovertimeandpoorglossretention(mattfinish).

Examples

CausesEnvironmentalconditionsIftheconformalcoatingisdriedorcuredduringconditionsofcoolambienttemperatures,droppingtemperatures,orhighhumidity,someconformalcoatingscandevelopblushing.Thiscanbecausedbytheabsorptionofcarbondioxideandwaterintotheconformalcoatingfilm.

PreventionEnvironmentalcontrolControlthetemperatureandhumidityintheenvironmentduringprocessingandthiscouldreducetheblushingeffectintheconformalcoating.


Bubbles,pinholesandfoamBubbles,pinholesandfoamaresimilardefects.Bubblesaretrappedpocketsofairwithintheconformalcoating.PinholesareburstbubblesthatreachtothesurfaceofthePCBfromthesubstrate.Foamisextremebubbling.

Examples

CausesPrematuredryingWhenapplyingtheconformalcoatingthewetfilmsurfacecanskinover,entrappingsolventsunderthesurfacethatcanpreventthebubblesescaping.

CoatinglayersappliedtooquicklyApplyingmultiplelayersofcoatingcanentrapsolvent.

Solventpoppingisanapplicationphenomenonthatoccurswhenasecondcoatisappliedwithoutsufficientflashtimeofthefirst.Solvent,whichisstilltryingtooutgas,istrappedunderneaththesecondcoat,andasitmigratestothetopitbursts,leavingasmallcrater.

Iftheconformalcoatingisappliedtoothickortooviscousanybubblesthatarecreatedintheprocessofapplicationcanbecomeentrappedinthecoatingbeforetheysettleout.

AirentrapmentAirentrappedundercomponentsbythecoatingprocesscanbeforcedoutduringthedrying&curingprocessandcausebubbles.


ChampagneeffectsPressurepotswithconformalcoatinginsidecanabsorbairthatcanmanifestaschampagnebubbleswhenapplied.

WrongviscosityBrushcoatingwithtooviscousamaterialorworkingthecoatingcancausebubblesduringapplication.

ToomuchspraypressureIncorrectsprayequipmentorpressurescancauseexcessivebubbles&foam.

PreventionApplythecorrectfilmthicknessWhenapplyingtheconformalcoatingensurethewetfilmsurfaceisnottoothick.Thisisparticularlyimportantwhenspraycoating.Itisbettertoapplymultiplethinlayersthanonethicklayer.

UsethecorrectdryingprocessEnsuretheindividualcoatinglayersaresufficientlydrybeforeapplyingthenextcoatinglayer.Thiswillavoidsolventpopping.

ChangethedipprocesssettingsToavoidairentrapmentunderPCBcomponentstrydippingslowerintotheconformalcoatingdiptank,addingadwelltimetoallowpenetrationofthecoatingand,orusingalowerviscosityversionoftheconformalcoatingtounderfilltheareamoreeffectively.

TurnoffpressureinpotswhennotusedWhenusingpressurepotswithconformalcoatinginsidetrytoavoidleavingthepotspressurisedforlongperiodsoftimewhennotbeingusedintheprocess.

Also,avoidrunningthepressurepotslessthan50%fullandlettingthemstand.Thiswillminimisethechampagneeffectinthecoating.

BrushthecoatingcorrectlyWhenbrushcoatingtrytoblendtheviscosityoftheconformalcoatingsoitflowseasilyand“flow”thematerialontothePCB.DonotworkthecoatingintothePCBasitcreatesbubbles.

AdjustthepressureduringsprayingReducingthepressureonsprayingreducesthemechanicalagitationandreducesbubbles.


Capillaryflow(scavenging)CapillaryflowinconformalcoatingiswherethecoatingpullsorrunsawayfromcertainareasofaPCB.Oritcanmoveunderneathdevices.

Thecoatingmovestoamorefavorableareaoftheboardduetoacombinationofeffects.Thisleavesapatchyfinishonthesurfaceoftheboard.

Examples

CausesLowviscosityofcoatingIftheconformalcoatinghasaverylowviscosityitwillpenetrateintodevicesmucheasierandmorescavengingwilloccur.

VolumeofcoatingAnabnormallyhighamountofwetconformalcoatingappliedduringtheapplicationprocesscanmeantheexcessmaterialcanwickintocomponentsbefore“gelling”ofthematerialisachieved.

SubstratecleanlinessIfthecircuitboardisnotcleanorhasalowsurfaceenergythentheconformalcoatingwillnotbondtothesubstrate.

HighsurfacetensionIftheconformalcoatinghasahighsurfacetensionthenitwillalsoliketoenergeticallypenetrateundercomponentsleavinguncoatedareasbehind.


PreventionIncreasetheviscosityIncreasingtheviscosityoftheconformalcoatingwilllowerthemobilityofthecoatingandaidtheadhesiontotherightareasoftheboard.

ReducethevolumeofcoatingReducingtheamountofconformalcoatingappliedineachapplicationwillreducetheamountoffreematerialtocapillary.

SubstratecleanlinessCleaningtheboardbeforeprocessingwillaidthebondingofthecoatingduringtheapplication.

LowerthesurfacetensionUseasolventbasedmaterialratherthanwaterbasedor100%solidconformalcoating.

HeattheboardPriortocoatingtheboardraisingthecoretemperatureofthecircuitcanallowthecoatingtodryquickerandbelessmobileonthecircuitboard.


ContaminationContaminationisdebrisfoundunder,trappedinorontopoftheconformalcoatingonthecircuitboard.Thiscontaminationmaybedetrimentaltothelong-termreliabilityofthecircuit.

Examples

CausesContaminationcancomefromavarietyofareas.Thisincludespre-productionstagesoflaminateandcomponentmanufacture,theprocessstagesduringapplicationandthedrying&curingstages.

PreventionImprovecleanlinessBeforeproductionstarts,allpartsofthecircuitshouldbefitforpurposeintermsofcleanliness.

ChangeprocessmaterialsUselesscontaminatingprocessmaterialssuchasflux.

CleantheproductionandapplicationlineOncereadyforassembly,theboardproductionprocessshouldnotaddunwantedcontaminationtoacircuitboard.

Theconformalcoatingapplicationprocessshouldnotaddcontamination.Thisincludesallthestagesaftercleaningincludingmasking.Theareasshouldbecontaminationfreetoalevelthatdoesnotproducesignificantdefects.


CorrosionCorrosionisthechemicalorelectrochemicalreactionbetweenacircuitboard,usuallyametal,anditsenvironment,thatproducesadeteriorationofthecircuitanditsproperties.

Corrosioncanoccurwithoutaconformalcoatingbeingpresent.Aconformalcoatingcouldhelppreventorslowdowncorrosionasacompleteprocess.

Examples

CausesFactorsthatinfluencecorrosionand,orelectrochemicalmigration,usuallyrevolvearoundioniccontaminationandthecleanlinessofacircuit.Thesecontaminantscomefromelectronicprocessessuchas:

BoardfabricationThelaminategoesthroughalotofchemicalprocesses.Thecleaningofthechemicalsonthecircuitiscrucialtoensurethecontaminantsareremoved.

ComponentsComponentsalsocangetcontaminated.Again,cleanlinessiscritical.

AssemblyequipmentTheequipmentusedtomakethecircuitboardmayaddcontamination.


SolderingprocessThefluxresiduescanhaveamajorinfluenceonthelong-termreliabilityofthecircuit.

OperatorhandlingPeoplecanaddcontaminationinavarietyofwaysduetodirtyhands,hairlossetc.

IncorrectcleaningThecleaningprocessmaynotcleantheboardscorrectlyorleavecleaningfluidresiduesonthecircuitduetoafailuretorinsetheboard.

PreventionIntroduceincominginspectionAfullinspectionofincomingboardsandcomponents,forsolderabilityandcleanliness,canhelp.

ImprovetheprocesscontrolIfyouareusinganocleanprocessthentighteninguptheproductionprocessiscritical.Adjustthesolderingwindowandfine-tunetoensuretheminimumoffluxisusedandisbeloworequaltotherecommendedamount.

ImproveoperatorcleanlinessAlloperatorswearglovesandothersuitableclothingtominimisecontamination.

SelecttherightmanufacturingmaterialsUse“lowresidue”nocleanflux/pastethatislessharmfultothecircuit.Usetheminimumamount.

CleanthecircuitboardItispossibletominimisethemajorityofcorrosioneffectsbyusinganeffectivecleaningprocessbeforeconformalcoating.

ControlthesolderreworkprocessLowresiduewire/fluxusedforrework.


CrackingCrackinginconformalcoatingiswherethesmoothsurfaceofthecoatingfracturesintosections.Thiscanoccurintheshort,mediumandlongterm.

Thecracksintheconformalcoatingleavethesurfaceareabelowexposedtopotentialcontaminationpenetration.Thisallowsmoisture,wateranddebristoreachtheboardlevelwhereotherdefectscouldthenoccursuchascorrosion.

Examples

CausesCuretemperaturetoohighWhentheconformalcoatingisacceleratedcuredwithinanovenforexamplethetemperatureofcurecanleadtoprematuredryingofthetopsurfacebeforetheremainderofthecoatingisdry.Crackingcanoccur.

CuretemperaturetooquickHeatingthecoatingtooquicklywithoutallowingenoughtimeforroomtemperature(RT)dryingcancausecracking.

FilmthicknesstoogreatIfthedryconformalcoatingistoothickonthecircuitboardthenthiscancausecoefficientofthermalexpansion(CTE)mismatchandcracksoccurringinthecoating.


EnvironmentextremesIftheoperatingtemperatureofthecircuitboardistoohighortoolow,thenthiscancausetheconformalcoatingtoflextoomuchandcrackinthefieldduetothermalstressesandshock.

PreventionReducethecuretemperatureLoweringtheinitialcuretemperatureoftheovencanreducethecrackingeffect.

SlowthecureprocessAllowaninitialdryingtimeatroomtemperaturebeforeexposuretoelevatedtemperature.Thiscanhelpminimisethecrackingofthecoating.

LowercoatingthicknessReducethecoatingthicknesstoavoidCTEmismatchissuesandthermalshockeffects.

ImprovematerialselectionSelectacoatingwithawidertemperaturerangeperformancecanhelp.Also,selectamoreflexibleconformalcoatingandthatmatchesclosertotheothercomponentsforlessCTEmismatchproblems.


DelaminationDelaminationinconformalcoatingiswheretheconformalcoatingliftsfromthesurfaceofthePCB.Thisleavesthecircuitboardareabelowexposed.

Theliftingcanbepartialorcomplete.Itcanoccurnaturallyduetoareactioninthematerials.Itcanoccurduringtheproductionprocesssuchasde-masking.

Examples

CausesPoorcleanlinessThecleanlinessofthesurfaceisimportantforgoodadhesion.Ifthesurfaceiscontaminatedacrossawideareathendelaminationcanoccurduetothepooradhesionofthecoatingtothelaminate.

IncorrectcompatibilityTheconformalcoatingandthesurfacehavetobondtogether.Iftheyareincompatiblethentheywillnotbond.Thisincompatibilitymaybyduetosurfaceenergiesmismatch.Iftheconformalcoatingisnotmatchedtothelaminatethendelaminationcanoccur.

HighpermeabilitytomoistureIftheconformalcoatingallowsthemoisturetopermeateintotheregionbetweenthecoatingandthelaminate,thenadifferenceinvaporpressureoccurs.Thiscouldleadtoosmoticpressuresbuildingduetoachangeintemperatureandpressure,whichliftthecoatingawayfromthelaminate.


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.


De-wettingDe-wettingisthetendencyoftheconformalcoatingmaterialtorefusetowetthesurfacethatithasbeenappliedtoevenly.

Theeffectcanactlocallyduetolocalcontamination.Itcanbeaglobaleffectrightacrosstheboard.

Examples

CausesBoardmanufacturingprocessResiduesfromboardmanufactureincludingsiliconesurfactantsfromsolderresist&HASLrinsecontaminationcancausede-wetting.

ComponentresiduesMaterialslikemold-releaseagentsusedtoalloweasyreleaseofcomponentsfromtheirmoldswhenfabricatedcouldcauseconformalcoatingstode-wet.

SiliconeoilOilsfromadhesivesinproductionandfrommaintenanceprocessescanbelethaltocircuitboards.Also,siliconeresiduesfromRTVantivibrationandstakingcompoundscanalsocauseproblems.

SolderingprocessesTheresiduesfromthesolderingproductionlineandprocesscancausede-wettingissues.


CleaningbathcontaminationSomecleaningsolventsmaycausede-wettingascananycontaminantspickedupinthecleaningprocess.Whererinsinghasfailedthentheseresiduescanbepresentontheboard.

OperatorhandlingPeopleonthelinecanaddcontaminantsfrombarriercreams,foodandothermaterials.

PreventionControltheprocessMinimisethecontaminantsthatareonthecomponentpartsbeforeassembly.Thisincludeslaminate&componentcleanlinesscontrolbeforeassemblyandselectionoflowresidueprocessmaterialsincludingfluxesandpastesduringassembly

CleantheboardsImprovingthecleanlinessoftheboardsbeforeconformalcoatingcouldminimisede-wettingdefects.

CompatibilityofsolderresistEnsurethesolderresistiscompatiblewiththeconformalcoatingoritmaynotwetcorrectly.

RepairwhileprocessingAfterapplyingthecoating,locallystripthede-wettedareaandre-applythecoatingtothecorrectspecification.


DiscolorationAchangeinthecolourorlossoftransparencyoftheconformalcoatingthatadverselyaffectstheperformanceortheidentificationofthecomponentsbelowthecoating.

CausesUVlightexposureDamagefromUVradiation,likesunlight,fromprolongedexposurecanchangethecolourofacoating.

ContaminationProlongedexposuretoparticulatesanddustthatsticktoatackysurfacecancausechangesincolour.

PreventionEliminatetheUVlightDon’texposetheconformalcoatingtoprolongedUVlightradiation.

CorrectconformalcoatingUseaUVstableconformalcoating.

CleanEnvironmentPreventthecircuitfromgettingcoveredincontamination.


FishEyesAfisheyeisalocalisedde-wettingdefectthatlookslikeafisheyecrateronthesurfaceoftheconformalcoating.

Examples

CausesFisheyesarenormallycausedbyacontaminantonthesurfacepriortoapplicationoftheconformalcoating.Typically,thatcontaminantisasubstancelikeasiliconparticle.

Whenconformalcoatingisappliedontopofthecontaminant,theconformalcoatingisunabletoadheretotheoilysurfaceanda“fisheye”iscaused.

PreventionImprovethecleanlinessofthesurfaceThoroughlycleanthesurfaceoftheboardbeforecoating.Fisheyesareasurfacepreparationdefect.Ifthesurfaceofthecircuitbardhasbeenproperlycleanedanddegreased,fisheyecanusuallybepreventedcompletely.


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.


TimethecoatingapplicationlayerscorrectlyToavoidlaminationofcoatinginsolvent-basedcoatings,ensurethattheoptimumtimebetweencoatinglayersisselected.

Thistimeisdependentonseveralfactorsincludingthesolventflashofftime,thetemperatureandhumidityinthearea,theextractionrateandthechemicalnatureofthematerial.

Thekeytosuccessistodrythecoatingenoughwherethesecondlayerbondstothesurfaceofthefirstlayerbutdoesnottrapexcessivesolventinthelowerlayer.Thistrappingcanleadtoexcessivebubbles.


MealingMealingisaformofdelaminationoftheconformalcoatingthatoccursacrossthecircuitboard.Itisablistering(vessication)affect.

Examples

CausesSurfacecleanlinessPoorcleanlinessofthesurfacereducesadhesion.Ifthesurfaceiscontaminatedacrossawideareathenmealingcanoccurduetothepooradhesionofthecoatingtothelaminate.

MaterialscompatibilityCompatibilitybetweentheconformalcoatingandthesurfaceadheredtoduetosurfaceenergiesmismatchisimportant.Iftheconformalcoatingisnotmatchedtothelaminatethenmealingcanoccur.

PermeabilitytomoistureofthecoatingIftheconformalcoatingallowsthemoisturetopermeateintotheregionbetweenthecoatingandthelaminate,thenadifferenceinvapourpressurecouldoccur.Thiscouldleadtoosmoticpressuresbuildingduetoachangeintemperatureandpressure,whichcouldliftthecoatingawayfromthelaminate.

DegreeofcoatingcureIfthecoatingisnotcuredcorrectlythenthiscanleadtoadhesionproblems.


PreventionCleantheboardCleanthesurfaceoftheboardtoimprovetheadhesionofthecoating.Removecontaminationsuchasmoldreleaseagentsoncomponentsbeforeapplication.

MaterialcompatibilityMatchtheconformalcoatingtothesolderresistandlaminatesothatgoodadhesionisachieved.Changethecoatingand/orsurfacefinishsothesurfaceenergiesaremorefavorable.

CoatingthicknessReducetheconformalcoatingthickness.ThisreducestheproblemsassociatedwithCTEmismatchwiththecomponentsandlaminatethatleadtoflexingofthecoating.

LowercuringheatReduceforcedryingoftheconformalcoatingafterapplication(changethethermalprofile)toavoidexcessstressandstrainsonthecoating.

PrimethesubstrateApplyaprimertotreattheboardsurface.Thiswillincreasethebondingoftheconformalcoatingtothesubstrate.


OrangePeelThisdefectisanuneventexturedsurfaceformedontheconformalcoating,oftenappearingdull,whichlooksliketheskinoftheorangeoragolfballwithsmallcratersanddimples.

Example

CausesSpraysetupOrangepeelisoftencausedbecauseofpoorgunadjustmentastheconformalcoatingisbeingapplied.

ApplicationincorrectAnincorrectapplicationtechniquecanproducetheorangepeeleffect.ThismanifestsinaerosolorspraygunapplicationwherethecoatingisappliedtoofarawayfromthePCBandthecoatingistoo“dry”onthesurfacetolevelout.

IncorrectviscosityAnincorrectviscosityoftheconformalcoatingbeforeapplicationcancauseorangepeel.

CoatingthicknessTheincorrectthicknessofthecoatingisdepositedsothatthereisinsufficientconformalcoatingtoself-level.


Curing

Theincorrectcureprofilewhenusinganacceleratedcuringmechanism.Thiscouldbethattheflashoffzonebeforeheatcuringisinadequateortheovenprofileisincorrect.

PreventionGunsetupSetthespraygunupcorrectly.Gunsetupandadjustmentarecriticaltoasmoothandflatappearanceofaconformalcoating.

Iftheneedle/nozzlecombinationisnotspecificallymatchedtothecoatingbeingapplied,theairpressure(especiallywhenusinganhighvolumelowpressure(HVLP)gunmaynotbesufficienttocorrectlyatomisetheconformalcoating.

ViscosityReducetheviscosityoftheconformalcoating.Ifacoatingismixedatthewrongviscosityanditistoothick(toohighaviscosity),orangepeelislikely.

SprayingtechniqueOptimisethedistanceforsprayapplicationtothemanufacturerspecificationwhenapplyingtheconformalcoating.

MaterialvolumeappliedEnsureenoughlayersofcoatingareappliedtoformalevelcoating.Thisisespeciallytrueforaerosolsthataresusceptibletothisissue.

DryingIncreasetheflashofftimeforthesolventstoevaporatebeforeincreasingthetemperatureforelevatedcuringoftheconformalcoating.Re-profilethecuringovenusingtheconformalcoatingmaterialsuppliersrecommendedprofile.


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).


UnevencoatingUnevenconformalcoatingasadefectiswherethethicknessofthematerialisoutsidetherangeofthespecifiedrequirement.

Whenthecoatingistoothinthiswillgiveinadequateprotection.Whenthecoatingistoothickthecoatingcanleadtootherdefectssuchasbubbles,crackinganddelamination.

Examples

CausesCapillaryeffectsWickingcanpullthecoatingundercomponents,fillingtheareaunderneaththedevicewithverythickmaterialandleavingotherareasclearofcoating.Thiscanalsoleadtolargefilletsofcoatingbetweenleadsandbridgingeffects,whichcanleadtoCTEmismatchproblemsinthelongterm.

EffectsofGravityGravitycanleadtocoatingrunningdownthesidesofcomponentsandpoolingatthebase.Itcanalsoleadtopoortipcoverageofsharpedges.Thiscanalsobeafactorwithsurfacetensioneffectsofconformalcoatingthatcanmovetheliquidaroundthecircuit.

OverlappingmateriallayersAnautomaticspraypatternfromarobotmayinadvertentlyapplysomeareaswiththickeramountsofcoatingbyoverlappingthespraystripes.


ShadowingAmanualsprayprocessmaybepoorlyapplied,leadingtoshadowingandareasofthickcoating.

EntrappedcoatingAcircuitboardwhenwithdrawnfromadiptankinadipprocessmayhaveentrappedconformalcoatinginpocketsofthecomponentsorboarditselfthatcanleadtoverythickareasofcoating.

DryingtooquickAconformalcoatingmaynotleveloutcorrectlywhenappliedmaybeprematurely“gelling”tooquickly.

PreventionCapillaryeffectsAvoidwicking(scavenging)andtheproblemsassociatedwithit.

EffectsofgravityEnsurethecoatingtechniquetakesintoaccounttheeffectsofgravity.Forexamplewhenspraying,usemultiplethinlayersofcoating.Thisreducestheamountoffreeliquidtoflowontheverticalwallsandensuresthecoatingstickswhereapplied.

OverlappingmateriallayersProgramtheselectiverobottoavoidcrossingtheflowstripeswherepossible.

ShadowingAvoidshadowingeffectproblemswithconformalcoating.

EntrappedcoatingEnsuretheoptimumdrainanglewhendippingtoensureminimalcoatingistrapped.Otheroptionsincludingbuildinginatiltorrotatemechanismintothefixturesofthedipprocesssothatthedraincanbemoreeffective.Finally,designthecircuitwithdrainholesandflowpathstoachievegoodcoatingcoverage.

DryingtooquickUsealowerviscositymixofthecoatingifthecoatingisprematurelydryingandnotleveling.Also,reducetheextractionrateandincreasetheambienttemperatureforbetterflow.

Conformal coating problems WHITE...

Documents

Transcript of Conformal coating problems WHITE...