StandardTestMethodfor
PorosityinGoldCoatingsonMetalSubstratesbyNitricAcidVapor1ThisstandardisissuedunderthefixeddesignationB735;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginaladoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscriptepsilon(e)indicatesaneditorialchangesincethelastrevisionorreapproval.
1.Scope
1.1Thistestmethodcoversequipmentandproceduresforusingnitricacidvaporfordeterminingporosityingoldcoatings,particularlyelectrodepositsandcladmetalsusedonelectricalcontacts.
1.2Thistestmethodisdesignedtoshowwhethertheporositylevelislessorgreaterthansomevaluethat,byexperience,isconsideredbytheusertobeacceptablefortheintendedapplication.
1.3AvarietyofotherporositytestingmethodsaredescribedinGuideB765andintheliterature.2,3OtherporositytestmethodsareB741,B798,B799,andB809.
1.4ThevaluesstatedinSIunitsaretoberegardedasstandard.Thevaluesgiveninparenthesesareforinformationonly.
1.5Thisstandarddoesnotpurporttoaddressallofthesafetyconcerns,ifany,associatedwithitsuse.Itistheresponsibilityoftheuserofthisstandardtoestablishappro-priatesafetyandhealthpracticesanddeterminetheapplica-bilityofregulatorylimitationspriortouse.Specificprecau-tionsaregiveninSection8andNote3.
2.ReferencedDocuments2.1ASTMStandards:
B374TerminologyRelatingtoElectroplating4B542TerminologyRelatingtoElectricalContactsandTheirUse5B741TestMethodforPorosityInGoldCoatingsonMetalSubstratesByPaperElectrography5B765GuidetotheSelectionofPorosityTestsforElec-trodepositsandRelatedMetallicCoatings4ThistestmethodisunderthejurisdictionofASTMCommitteeB02onNonferrousMetalsandAlloysandisthedirectresponsibilityofSubcommitteeB02.11onElectricalContactTestMethods.
CurrenteditionapprovedApril15,1995.PublishedJune1995.OriginallypublishedasB735–84.LastpreviouseditionB735–92.2Forexamplesee:Nobel,F.J.,Ostrow,B.D.,andThompson,D.W.,“PorosityTestingofGoldDeposits,”Plating,Vol52,1965,p.1001.3S.J.Krumbein,PorosityTestingofContactPlatings,Proceedings,ConnectorsandInterconnectionTechnologySymposium,Oct.1987,p.47.4AnnualBookofASTMStandards,Vol02.05.5AnnualBookofASTMStandards,Vol02.04.
1B798TestMethodforPorosityinGoldorPalladiumCoatingsonMetalSubstratesbyGel-BulkElectrography5B799TestMethodforPorosityinGoldandPalladiumCoatingsbySulfurousAcid/Sulfur-DioxideVapor5B809TestMethodforPorosityinMetallicCoatingsbyHumidSulfurVapor(“Flowers-of-Sulfur”)43.Terminology
3.1Definitions:ManytermsusedinthistestmethodaredefinedinTerminologyB542andtermsrelatingtometalliccoatingsaredefinedinTerminologyB374.
3.2DefinitionsofTermsSpecifictoThisStandard:
3.2.1corrosionproducts—thosereactionproductsemanat-ingfromtheporesthatprotrudefrom,orareotherwiseattachedto,thecoatingsurfaceafteravaportestexposure.
3.2.2measurementarea(or“significantsurface”)—thesurfacethatisexaminedforthepresenceofporosity.Thesignificantsurfacesormeasurementareasoftheparttobetestedshallbeindicatedonthedrawingofthepartorbyprovisionofsuitablymarkedsamples.
3.2.3Discussion—Forspecificationpurposes,thesignifi-cantsurfacesormeasurementareasareoftendefinedasthoseportionsofthesurfacethatareessentialtotheserviceabilityorfunctionofthepart,suchasitscontactproperties,orwhichcanbethesourceofcorrosionproductsortarnishfilmsthatinterferewiththefunctionofthepart.
3.2.4metalliccoatings—includeplatings,claddings,orothermetalliclayersappliedtothesubstrate.Thecoatingcancompriseasinglemetalliclayeroracombinationofmetalliclayers.
3.2.5Porosity—thepresenceofanydiscontinuity,crack,orholeinthecoatingthatexposesadifferentunderlyingmetal.3.2.6Underplate—ametalliccoatinglayerbetweenthesubstrateandthetopmostlayerorlayers.Thethicknessofanunderplateisusuallygreaterthan0.8µm(30µin.).
4.SummaryofTestMethod
4.1Thistestmethodemploysnitricacid(HNO3)vaporatlowrelativehumidity.Reactionofthegasmixturewithacorrodiblebasemetalatporesitesproducesreactionproducts
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1
thatappearasdiscretespotsonthegoldsurface.Individualspotsarecountedwiththeaidofaloupeorlow-powerstereomicroscope.
4.2Thistestmethodissuitableforinlaysorcladdingscontaining75%ormoreofgoldorforelectroplatingscon-taining95%ormoreofgoldonsubstratesofcopper,nickel,andtheiralloys,thatarecommonlyusedinelectricalcontacts.4.3Thenitricacidvaportestistooseveretobeusedforgoldcoatingslessthan0.6µm(25µin.)inthickness.Itisalsonotsuitableforcoatingsthatarelessnoblethangoldorplatinum,suchaspalladiumanditsalloys,orgold-flashedpalladiumanditsalloys.
4.4Thisporositytestinvolvescorrosionreactionsinwhichtheproductsdelineatedefectsitesincoatings.Sincethechemistryandpropertiesoftheseproductsmaynotresemblethosefoundinnaturalorserviceenvironments,thesetestsarenotrecommendedforpredictionoftheelectricalperformanceofcontactsunlesscorrelationisfirstestablishedwithserviceexperience.
5.SignificanceandUse
5.1Goldcoatingsareoftenspecifiedforthecontactsofseparableelectricalconnectorsandotherdevices.Electrode-positsaretheformofgoldthatismostusedoncontacts,althoughitisalsoemployedascladmetalandasweldmentsonthecontactsurface.Theintrinsicnobilityofgoldenablesittoresisttheformationofinsulatingoxidefilmsthatcouldinterferewithreliablecontactoperation.
5.2Inorderthatthenobilityofgoldbeassured,porosity,cracks,andotherdefectsinthecoatingthatexposebase-metalsubstratesandunderplatesmustbeminimalorabsent,exceptinthosecaseswhereitisfeasibletousethecontactsinstructuresthatshieldthesurfacefromtheenvironmentorwherecorrosioninhibitingsurfacetreatmentsforthedepositareemployed.Thelevelofporosityinthecoatingthatmaybetolerabledependsontheseverityoftheenvironmenttotheunderplateorsubstrate,designfactorsforthecontactdeviceliketheforcewithwhichitismated,circuitparameters,andthereliabilityofcontactoperationthatitisnecessarytomaintain.Also,whenpresent,thelocationofporesonthesurfaceisimportant.Iftheporesarefewinnumberandareoutsideofthezoneofcontactofthematingsurfaces,theirpresencecanoftenbetolerated.
5.3Methodsfordeterminingporesonacontactsurfacearemostsuitableiftheyenabletheirpreciselocationandnumberstobedetermined.Contactsurfacesareoftencurvedorirregularinshape,andtestingmethodsshouldbesuitableforthem.Inaddition,theseverityofporosity-determiningtestsmayvary.Thistestmethodisregardedassevere.
5.4Therelationshipofporositylevelsrevealedbyparticularteststocontactbehaviormustbemadebytheuseroftheseteststhroughpracticalexperienceorbyjudgement.Thus,absenceofporosityinthecoatingmaybearequirementforsomeapplications,whileafewporesonthecriticalsurfacesmaybeacceptableforanother.Suchacceptance(orpass-fail)criteriashouldbepartoftheproductspecificationfortheparticularproductorpartrequiringtheporositytest.
5.5Thistestmethodishighlysensitiveandiscapableofdetectingvirtuallyallporosityorotherdefectsingoldcoatings
thatcouldparticipateinsubstratecorrosionreactions.Thetestisrapid,simple,andinexpensive.Inaddition,itcanbeusedoncontactshavingcomplexgeometrysuchaspin-socketcontacts.However,itispreferredthatdeeplyrecessedsocketsbeopenedtoexposetheircriticalsurfacespriortotesting.
5.6Thistestmethodisconsidereddestructiveinthatitrevealsthepresenceofporositybycontaminatingthesurfacewithcorrosionproductsandbyundercuttingthecoatingatporesitesorattheboundariesofunplatedareas.Anypartsexposedtothesetestsshallnotbeplacedinservice.
5.7Thistestmethodisintendedtobeusedforquantitativedescriptionsofporosity(suchasnumberofporesperunitareaorpercontact)onlyoncoatingsthathaveaporedensitysufficientlylowthatthecorrosionsitesarewellseparatedandcanbereadilyresolved.Asageneralguidelinethiscanbeachievedforporedensitiesuptoabout100/cm2orper100contacts.Abovethisvaluethetestsareusefulforthequalita-tivedetectionandcomparisonsofporosity.
6.Apparatus
6.1TestChamber,maybeanyconvenientsizeglassvesselcapableofbeingsealedwithaglasslid,suchasaglassdesiccatorof9to12Lcapacity.Theratiooftheairspaceinthechamber(incubiccentimetres)tothenitricacidsurfacearea(insquarecentimetres)shallnotbegreaterthan25:1.
6.2SpecimenHoldersorSupports—Supportsorhangersshallbemadefromglass,polytetrafluoroethyleneorotherinertmaterials.Itisessentialthattheholdersbesodesigned,andthespecimenssoarranged,thatthecirculationofthevaporisnotimpeded.Specimensshallbeatleast75mm(3in.)fromtheliquidsurfaceandatleast25mm(1in.)fromthevesselwalls.Also,themeasurementareasofthespecimensshallbeatleast12mm(0.5in.)fromeachother.
6.2.1Donotuseaporcelainplateoranyotherstructurethatwouldcovermorethan30%oftheliquidsurfacecross-sectionalarea.Thisistoensurethatmovementofairandvaporwithinthevesselwillnotberestrictedduringthetest.
6.3Stereomicroscope,havinga103magnification,shallbeusedforporecounting.Inadditionamovablesourceofilluminationcapableofgivingobliquelightingonthespecimensurfaceisalsouseful.
7.Reagent
7.1NitricAcid,ReagentGradeConcentrated7061%HNO3,spgr1.415to1.420.
8.SafetyHazards
8.1Carryoutthistestmethodinachemicalfumehood,sincethegasesthatarereleased,mainlywhenthereactionvesselisopenedattheendofeachtest,areverycorrosive.8.2Usecaution,however,toensurethatdraftsthatareoftenfoundinfumehoodsdonotcausesignificantcoolingofthechamberwalls,thatcouldleadtoariseintherelativehumidityandaccelerationofthetest(see9.3).Itisoftenconvenienttoenclosethereactionvesselinaboxwithaloose-fittingcover.8.3Observenormalprecautionsinhandlingcorrosiveacids.Inparticular,weargogglescompletelyenclosingtheeyeswhenhandlingnitricacid,andmakeeyewashfacilitiesreadilyavailable.
9.Procedure
9.1Handlespecimensaslittleaspossible,andonlywithtweezers,microscope-lenstissue,orclean,softcottongloves.Priortothetest,inspectthesamplesunder103magnificationforevidenceofparticulatematter.Ifpresent,suchparticlesshallberemovedbyblowingthemoffwithclean,oil-freeair.Ifthisisnotsuccessfuldiscardthesample.Then,cleanthesampleswithsolventsorsolutionsthatdonotcontainchlori-natedhydrocarbons,CFC’s,orotherknownozone-destroyingcompounds.TheprocedureoutlinedinNote1hasbeenfoundtogivesatisfactoryresultsforplatingswithmildtomoderatesurfacecontamination.
NOTE1—Suggestedcleaningprocedure:
(1)Keepindividualcontactsseparatedifthereisapossibilityofdamagetothemeasurementareasduringthevariouscleaningsteps.(2)Cleansamplesfor5mininanultrasoniccleanerwhichcontainsahot(65–85°C)2%aqueoussolutionofamildlyalkaline(pH7.5–10)detergent(suchasMicroorSparkleen).
(3)Afterultrasoniccleaning,rinsesamplesunderwarmrunningtapwaterforatleast5s.
(4)Rinsesamplesultrasonicallyfor2mininfreshdeionizedwatertoremovethelastdetergentresidues.
(5)Immerseinfreshmethanolorisopropanol,andultrasonically“agitate”foratleast30sinordertoremovethewaterfromthesamples.(6)Removeanddrysamplesuntilthealcoholhascompletelyevapo-rated.Ifanairblastisusedasanaidtodrying,theairshallbeoilfree,clean,anddry.
(7)Donottouchmeasurementareaofthesampleswithbarefingersaftercleaning.
(8)Reinspectsamples(under103magnification)forparticulatematteronthesurface.Ifparticulatesarefound,repeatthecleaningsteps.Surfacecleanlinessisextremelyimportant;contaminants,suchasplatingsalts,organicfilms,andmetalflakesmaygiveerroneousindicationsofdefects,andareunacceptable.
NOTE2—Iflargeareasofexposednon-noblemetalarepresent,maskingtheseareasmaybenecessary.However,whenmaskingisdonewithplater’stape,takecarenottoinhibittheflowofacidvaportothemeasurementarea.
9.2Theambienttemperatureandthetemperatureofthespecimensandsolutionare2363°Catthebeginningofthetestandmaintainedthroughoutthetestperiod.
9.3Therelativehumidityintheimmediatevicinityofthetestchambershallbenogreaterthan60%,6although55%orbelowispreferred.Iftherelativehumidityisgreaterthan60%,donotrunthistest.
9.4AddfreshHNOchamber,andimmediately3tothebottomofthecleananddrytestclosethecover.After3065min,loadthesamples,usingsuitablefixtures,andreplacethecover.Theambientrelativehumidityshallbenogreaterthan60%duringboththeadditionoftheHNO3andtheinsertionofthesamples.6NOTE3—Caution:Donotgreasetherimofthedesiccatornoritscover.Ifdesired,pressaminimumofthreestripsofpressuresensitivepolytetrafluoroethylenetape(adhesivesidedown)atequalintervalsaroundthedesiccatorrim.
6Forexamplesee:Holden,C.A.,andLuik,R.,“CriticalNitricAcidVaporTestParametersAffectingtheApparentPorosityofThinandSelectivelyPlatedGold,”Plating,Vol69,May,1982,p.110.
9.5Unlessotherwisespecified,theexposuretimetonitricacidvaporshallbe6065min.Anexposuretimeof7565minisalsocommonlyusedforgoldthicknessesinthe2to2.5µm(75to100µin.)range.Atableofconvenientexposuretimesisgivenintheappendix.
NOTE4—Variationsinexposuretimewiththicknessareoftenrecom-mendedbecauseporesinthickercoatingsaredeeperandtheiraveragesizesaresmallerthanthoseinthinnercoatings.Thenitricacidmediumwouldthereforetakelongertopenetrateanaverageporeinthickercoatingscomparedtothinnerones.Ontheotherhand,whenexposuretimesaretoolong,thecorrosionproductswilloverlapandimpairporedelineation.AdetaileddiscussionoftheseeffectsisgiveninFootnote3.
9.6Removethesamplesattheendofthetestanddryinanovenat12565°Cfor3065min.Thenremovefromtheoven,andplacedirectlyintoadesiccatorcontainingactivedesiccant,andallowtocooltoroomtemperature.
NOTE5—DiscardtheHNO3inasafe,legally-acceptablemanner.
9.7Leavesamplesindessicatoruntilactuallyreadytostarttheexamination.Thenopendesiccatorslowly,sincecoolingspecimensmaycreateapartialvacuum.
9.8Theexaminationshalltakeplacewithin1.5hofremovalfromtheoven.
9.9Countindividualporesat103magnificationusingcollimatedincandescentilluminationatanobliqueanglebelow15°.Theporesiteswillbedelineatedbythecorrosionproductsprotrudingfromthesesites.Thesesolidsmaybetransparentinthecaseofgold-platednickelornickelunderplate,andexercisegreatcareincounting,particularlywithroughorcurvedsurfaces.
9.10Measureandcountacorrosionproductwhenatleast3⁄4ofthecorrosionproductfallswithinthemeasurementarea.Donotcountcorrosionproductsthatinitiateoutsidethemeasure-mentareabutfallwithinit,andthatareirregularinshape,(seeFig.1).
9.11Occasionallyblistersmayformwithgoldplatedonnickeloranickelunderplate.Thismayoccuratporesitesandmaybeduetoentrapmentofcorrosionproductsbeneaththeplating.Counttheblistersaspores.
9.12Itisalsogoodpracticetomakeduplicatereadingsofeacharea,andthenaveragethetworeadings.
9.13Defineporesizebythelongestdiameterofthecorro-sionproduct.Corrosionproductssmallerthan0.05mm(0.002in.)aregenerallynotresolvableat103.Donotcountthemaspores,eveniftheyareobserved(athighermagnification).
NOTE6—Ifthecorrosionproductsarebeingtabulatedaccordingto
FIG.1CorrosionProductCounting
size,ausefulsizingtechniqueistotabulatetheporesinaccordancewiththreesizeranges.Theseare(approximately):0.12mm(0.005-in.)diam-eterorless,between0.12and0.40mm(0.005and0.015-in.)diameter,andgreaterthan0.40mm(0.015-in.)diameter.Agraduatedreticleinthemicroscopeeyepieceisusefulasanaidtocountingandsizing.
10.PrecisionandBias
10.1Precision—Theprecisionofthistestmethodisbeinginvestigatedwithgold-platedelectricalcontactshavinganickelunderplate.Round-robinresultsfromaseriesoffourrunsineachlaboratorywithatestofsimilarseveritygavecoefficientsofvariationoflessthan20%ineachofthreeparticipatinglaboratories.7However,poorerprecisionwasobtainedwhentheresultsofthedifferentlaboratorieswerecompared.
Krumbein,S.J.andHolden,C.A.Jr.,“PorosityTestingofMetallicCoatings,”inTestingofMetallicandInorganicCoatings,ASTMSTPNo.947,W.B.HardingandG.A.DiBari,eds.,ASTM,1987,p.193.
710.2Bias—Theporosityofcommerciallyproducedcontactplatingsisapropertywithpotentiallylargesample-to-samplevariability.7Sincethereisnoacceptablereferencematerialsuitablefordeterminingthebiasforporositytesting,nostatementonbiasisbeingmade.11.Keywords
11.1goldcoatings;goldplatings;nitricacidvapor(poros-ity)test;platingporosity;porecorrosiontest;porositytesting;porositytesting;nitricacidvapor
APPENDIX
(NonmandatoryInformation)
X1.TIMEOFEXPOSURETONITRICACIDVAPOR
X1.1Theexposuretimesrecommendedinthistestmethodarethosethataresufficientlylongtoproducecorrosionproductsgreaterthan0.05mm(0.002in.)indiameter,butshortenoughtopreventsignificantoverlapoftheseproducts.Iftheexposuretimeistooshort,thecorrosionproductsatmanyporesitesmaybetoosmalltobeeasilyseenat103magnification.Ontheotherhand,iftheexposuretimeistoolong,thecorrosionproductsmayoverlaporcoalesce,therebyimpairingtheabilitytodelineatetheindividualporesites.3X1.2TableX1.1maybehelpfulinspecifyingexposure
TABLEX1.1ExposureTimeforGoldCoatings
GoldThickness,µm(µin.)<0.6,(<25)
0.6to2.0(25to80)1.8to2.5(75to100)
ExposureTimetoNitric
AcidVapor,min
...60657565
timesforthosegoldcoatingsthatarenormallyappliedtoelectricalcontacts.
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