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TechnicalBulletinHowtoSelectaConformalCoatingMaterial

Choosingaconformalcoatingmaterialisacomplicatedandinvolvedprocessusuallybalancinganumberoftrade-offsandcompromises.Thisisbecausethereisnosuchthingasauniversallyapplicableconformalcoating.Theaimofthisarticleistogiveyouanideaofthequestionstoaskyourselfandyoursupplierstoshortlistsuitablematerialsfortheendapplication.

FactorstoConsider

Whenselectingaconformalcoatingyouaregoingtoneedtoconsiderseveralfactorswhichifyouareluckywillalladduptoatleastonematerialchoice.Ifnot,thencompromisesarerequired.

Factorstobeconsideredare:

• TemperatureOperatingRange• International,CustomerorNationalStandards• EnvironmentalConsiderations• ChemicalResistance• RequirementtoReworktheConformalCoating• ApplicationProcessConsiderations• Curingtheconformalcoating• National,State,RegionalLegislationandInsurance• SupplierPerformance• Price

Eachofthesefactorsisexaminedbrieflytoallowaninsightintothecomplexityofselectingaconformalcoatingmaterial.

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TemperatureOperatingRange

Conformal-coatedassemblieswilloftenbeexposedtoawiderangeofoperatingtemperaturesduringtheirservicelife.Itisimportanttounderstandthemaximumandminimumtemperaturesthattheunitwillbeexposedto.Thisneedstoaholisticandrealisticappraisal.

Forexample,ifyourboardsareoperatinginanunheatedpartofanaircraft,theycouldeasilysee-65ºCwhenairborne.Whenontherunway,theycouldeasilysee+100ºC.Thenyouneedtoconsidertheheatingeffectsofthecircuit,certainlocalhotspotsontheassemblycouldreachlocalmaximumsof125ºCsay.Thenyouroperatingrangebecomes-65to+125ºC.

Ifyouroperatingrangegoesbelow-65ºCorabove150ºCthenyourchoiceismadesimple,becausesiliconeconformalcoatingsaretheonlymaterialsthatwillsurviveextendeduseatthisrange.Ifyouroperatingrangeiswithinthiswindow,thenkeepreading.

International,CustomerorNationalStandards

Thenextquestiontoaskiswhetheryourcustomerorendapplicationrequirestheconformalcoatingtobequalifiedorapprovedtointernational,nationalorcustomerspecificstandards.

ExamplesofthesewouldbeUL746E(WhichincludesUL94)NorthAmericanSafetyStandard,MIL-I-46058C,orBMWGS95011-5.TheQualifiedProductLists(QPL)willsignificantlyreducethenumberofcoatingsavailabletoyou.TheSpecificationssuchastheIPC-CC-830Standarddoesnothaveaqualifiedproductlist.However,anyproductthatappearsontheMIL-I-46058CQPL,isautomaticallysaidtomeettherequirementsofIPC-CC-830.Therearealotofmaterialsthatstatethatthey‘meettherequirementsofIPC-CC-830’.AnymaterialmakingthisclaimthatdoesnotappearontheMILQPLmusthaveapubliclyavailabletestreporttobackuptheseassertions.Youwillneedtoseethisreportinordertokeepamaterialinyourshortlist.

EnvironmentalConsiderations

Aconformal-coatedprintedcircuitboardcanbeexposedtomanydifferenttypesofdeleteriousenvironments.Foranyunitthatwillbeplacedinapartiallysealedprotectivehousing,theconformalcoatingwouldbeconsideredsecondaryprotectiononlyandthereforelikelytobesubjectedonlytocondensationandhumidityasthetemperaturewithinthehousingchanges.Acrylicmaterialsarewidelyusedinsuchapplications,althoughsiliconematerialsgenerallyofferthebestprotectionincondensingenvironments.Thefollowingdescribesthemostcommonlyencounteredenvironmentalconditionsthatmayneedtobemet:

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HighHumidity

Whenhighhumidityistheissue,SiliconeMaterialscanbeanorderofmagnitudemoreporoustowatervapourthanothercoatingchemistries.AcrylicsandUrethanechemistriesallhavegoodresistancetohighhumidityenvironments.Two-component(two-part)materialsandmaterialsthatchemicallycross-linktoasignificantdegreewouldallbeexpectedtoprovideabetterbarriertohumidity.

Salt-SprayEnvironments

Thisisaverycommonrequirementinmanycustomerspecifications,butnotwithinanyofthecommongeneralspecifications.ThekeytosuccesswithSaltSprayenvironmentsareevenanduniform,void-freecoverageoftheassembly.Chemically,salt-waterisbenigntomostcoatings.However,thetestsandtheenvironmentswilltestworkmanshipandfinddefectsintheappliedcoating.Siliconematerialsandurethanechemistriesareoftenusedintheseapplications,althoughacrylicscanpassmosttestregimes.

Corrosivegases

Theseenvironmentsareperhapsthetoughestofall,sincethecorrosivegaseswillexposeanydefectsorvoidsintheappliedcoating.Inaddition,thereisarealdifferentiationbetweencoatingchemistriesinprotectivecapability.Acrylicsandsingle-componentalkyd,orurethanechemistriesareallextremelysusceptibletocorrosivegases.Two-partpolyurethaneandepoxychemistrymostlyshowgoodresistance,butsomesinglecomponentUVcurablematerialsandsinglecomponentsiliconechemistriesprovideverygoodresistancetoo.UVcurablematerialsaresignificantlycross-linkedandsothisisnotsurprising.Siliconematerialsseemtoworkbyreactingtoformchemicalbondswiththemetalsurfaceswhichlargelyinertthemetalsubstratefromcorrosion.

ImmersionEnvironments

Whentheassemblyislikelytobeimmersedinwater,thereisonlyonerealchoice,andthatistouseParylene,avapourdepositedconformalcoatingthatprovidesexceptionalcoverageandinsulation.Liquidappliedsiliconecoatingscanprovidedecentprotectionifappliedatagenerousthickness.

Corroded PCB not protected by conformal coating

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However,thekeyisqualityworkmanshipandprocessqualitycontrolsinceimmersionwillfindanyweakareasofcoverage.

Forimmersion-inorsplashbysolvents,theresultsareverydependentupontheparticularcoatingchemistryandsolventofinterest.Ingeneral,two-partmaterialswouldbethebestchoice,althoughsomeUVcurablematerialsprovideoutstandingchemicalresistance.

RequirementtoReworktheConformalCoating

Itispossibletoreworkmosttypesofconformalcoatingifnecessary,althoughsomeconformalcoatingscanbeverydifficulttorework.Ifyourassemblyhasalongservicelife,willbeplacedinahousingandbesubjectedtomultipleupgradesandrepairsoveritslife(commoninmilitaryandaerospaceapplications)thenacrylicsarewithoutadoubttheeasiesttypeofcoatingchemistrytoremoveandrework,duetotheirverylimitedchemicalresistance.Singlepartacrylic,urethaneandalkydmaterialscanbe

removedbychemicalmethodsinmachineslikeaconformalcoatingwetstrippingsystem.

Anytwo-partchemistry,Silicones,Parylene’sandUVcurablematerialsarebestremovedlocallybymediablastingwithanESDsafemediablaster.Providedthecoatingislesshardthanthesoldermask,thenagooddegreeofselectivitycanbeachievedwiththismethod,whichwillbecoveredinaforthcoming‘HowTo’guide.

ApplicationProcessConsiderations

Withenoughengineeringexpertiseandbudget,anymaterialcanbemadetoworkinanytypeofapplicationmethod,withtheexceptionofParylene,whichcanonlybeappliedbyvapourdepositioninadedicatedapplicationsystem.

However,therearecertaincombinationsthatleadtoextremeprocessdifficultyand/orcostimplicationsandareworthconsidering:

Solvent-Basedmaterials

Allsinglecomponentsolvent-basedmaterials,withtheexceptionofsomemoisturecureurethanechemistriescanbesuccessfullyappliedbyanyofthecommontechniques.Two-partsolventbased

Stripping Conformal Coating from a PCB

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materialsworkbestwithsprayorbrushapplicationsandcanbeappliedthroughroboticapplicationswiththerightsystemspecification.Unlessyouarewillingtothrowawayadiptankofmaterialwhenthepot-lifeisexceededthendippingisfarfromideal.

UVcurablematerials

UVcureconformalcoatingsareideallydesignedforhighvolume,highthroughputenvironmentsandarebestappliedbyselectivecoatingequipmentwithanimmediatein-line,conveyorisedcureoperation.However,duetotheiroutstandingprotectiveproperties,theyfindapplicationinlowvolumeapplicationstoo.Inthiscase,manualspraycoatingisacceptable,aslongasthecoatingthicknessandcureprocessarecarefullycontrolledtoensurethethicknessiswithinspecificationandtoavoidover-cure.Also,whencuringUVmaterialsandmaskingisinvolved,considerationmustbegiventothis.

Water-basedmaterials

Workbestinamanualapplicationprocess,beitdip,sprayorbrush.Water-basedmaterialsaredifficulttoapplywithrepeatabilityandreproducibilitywithselectivesprayequipment,andcanbeimpossibletocureinanin-lineprocessduetotheexcessivethicknessofmaterialappliedinthesemethods.

Siliconebasedmaterials

RTVsiliconesarenotreallysuitablefordipcoatingapplicationswithoutinertgassystemsinplace,butheat-curesiliconematerialsareideal.Withthisexception,siliconechemistrycanbesuccessfullyappliedbyanymethodology.

SelectiveRoboticCoating

Nearlyalloftheroboticmanufacturersmakeequipmentcanapplyconformalcoatingssuccessfullyifcorrectspecified.Thekeyistounderstandthevalvetechnologiesandhowthatrelatestothematerial,thequalityofthesprayprocessandthespeedofprocessing.Forinstance,whenconsideringswitchingfromasolventbasedcoatingusinganon-atomisedvalvetoanenvironmentallyfriendlycoatingsuchasaUVcureorsiliconecureproductthenitshouldberealisedthatachangeofvalvetechnologyisprobablyneededandtherewillbespeedlimitations,andthereforethroughputpenalties.

Dipping

Generally,allconformalcoatingscanbedippedinadipcoatingsystem.However,awiseruleofthumbistoavoiddip-coatingmethodsforanymaterialcontainingamoisture-curemechanismunlessyouwish

Robotic System applying conformal coating

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toaddinexpensiveprocesscontrolssuchasinertgassystems.Also,althoughperfectlypossibletodipUVcurecoatingsitiscriticalthatthesystemisdesignedtodothiswithparticularsafeguardsinplace.

HandSpraying

Allconformalcoatingscanbehandsprayedwithinasprayboothandtheprocessofconsiderationisthesameasfortherobots.Theeasiestarethesinglepartcoatings.TwopartcoatingswillhaveashelflifetoconsiderandproductssuchasUVcureandmoisturecurewillneedtobehandledcarefully.

Ifyouareconstrainedbyexistingcapitalequipmentthenyourchoicesmaybemorelimited,butifnewequipmentcanbepurchased,thentherearefarmorechoices.

Curingtheconformalcoating

Whentalkingaboutcuretimes,thereareseveraldefinitionstobeawareof:

1. DryingTimetohandle.Thisisthetimeatroomtemperatureatwhichthecoatingisdrytothetouchandcanbehandledormovedontoanotheroperation.

2. Timetofullcure.Thisisthetimeuntilanyphysicaldryingprocesshasbeencompletedandanycross-linkingreactionhastakenplace.Elevatedtemperaturesorothermethodsofcatalysiscanusuallyreducethetimetofullcure.

3. Timetooptimumproperties.Intuitionwouldtellyouwouldthinkthiswouldbethesameastimetofullcure.However,evenwhenthecoatingisfullycured,otherpropertiessuchasadhesioncandevelopovertime

Fromaproductionflowpointofview,itisdesirabletominimizethetimetohandleandtimetofullcureasmuchaspossible,takingintoaccountthefactorspreviouslydiscussed.Fromaleanproductionpointofview,minimisingthetimetooptimumpropertiesenablesyoutoshipproductyouknowisatitsoptimum,orenablesthesepropertiestodevelopduringtransporttimetotheendcustomer,withouthavingtostoreasignificantnumberofdaysofinventory.

National,State,RegionalLegislationandInsurance

Mostcountries,statesandevenregionshavetheirownlegislationrelatingtotheemissionofVolatileOrganicCompounds(VOCs),withdifferingdefinitionsandinterpretations.Wheninstallinganewprocessyouneedtobemindfulofthese,andifpossible,useacompliantmaterial.Forexistingprocesses,thereareoptionsincludingcarbonfiltration,solventcollection,oroxidising(afterburner)unitsthatcanbeinstalledatsignificantexpense,althoughthiscanbemoreeffectivethanre-qualifying

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newmaterialswithmultipleendcustomers.Sometimes,processand/orperformanceconsiderationswillrequirethistypeofsolution,evenfornewqualifications.

Shippingflammable,hazardousmaterialsisexpensiveandrequiresaspecialisedfreightfirminmostcountries.Usingasafermaterialcansimplifylogisticsandreducecost.Usingflammable,hazardousmaterialsisinherentlymoredangerouswithagreaterriskofseriousinjury,long-termexposureeffectsorcatastrophicincidents,andthisisreflectedininsurancepremiums,whichisanotheropportunitytosavecost.Sometimes,incidentswithinplants,suchasfireswillnecessitateachangeinmaterial,regardlessofprocess,lackofbusinesscaseoranyotherdriver,otherthancompliance.

SupplierPerformance

Choosingaconformalcoatingmaterialisalongandexhaustiveprocess,withmanyconsiderationsandconflictingchoicesandtradeoffstobemade.Mostsupplierswouldwalkyouthroughasimilarframeworktothistodecideonthemostsuitableconformalcoatingforconsiderationonyourshortlist.

Thisconsultationprovidesavaluableinsightintoyourpotentialmaterialsupplier.Iftheyareunwillingorunabletohelpyouthroughtheselectionprocess,thentheyareunlikelytobemuchhelptoyouwithanyongoingissuesthatneedtoberesolved.Ifyougetgreatsupportandback-uppriortoqualification,thenthisisanindicatorthatyoursupplierknowsyourbusinesswellenoughtobecomeavalue-addedsupplierduringproduction.

Thesamplingprocesswillhelpyouunderstandthegeographicreachofyourpotentialsupplier,andlocalavailabilityetc.Ifyoursamplehastoshiphalfwayacrosstheworld,thenyoursupplychainislikelytobeunsuitableordifficultinproduction.

Finally,akeyfactorisqualitycontrolofthemanufacturingprocess.Considerthecostsforwithdrawingaproductthathasbeencoatedinaconformalcoatingthathasbeenproducedincorrectly.Traceabilityandqualitycontroloftheprocessiscriticaltoavoidadisasteratalaterpoint.

Price

Manymaterialsareselectedonprice.Althoughthisisavalidpoint,considerationofalltheotherfactorsisarguablyasimportantifnotmore.Ultimately,thecostofthecoatingisfractionsofthetrueofmanufactureandthewrongmaterialspecifiedatthedesignstagecancostmanytimesthesavingoriginallyconsidered.

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Conclusions

SelectingaConformalCoatingisacrucialstageofthedesignprocess.However,itshouldbeconsideredinthecontextofallthefactorsoutlinedabove.Ifnot,thenitshouldnotbeasurprisetofindproblemslaterdowntheprocessstreamthatcouldhavebeenavoidedfromthestart.

Ifindoubt,checkwiththesupplierandindependentexperts.Donotneglectthisstageoryourprocessandfinalproductcouldbecompromised.

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