INTRODUCTIONTOCONNECTORDESIGNFUNDAMENTALSINTRODUCTIONSection1:ContactInterfaceSurfaceRoughnessElectricalPropertiesMechanicalPropertiesSection2:EngineeringMaterialsPlasticsMetalsPlatingsSection3:ContactNormalForceandContactGeometryRelationshiptoHertzstressEffectsoncontactperformanceSection4:ConnectorDesignOverviewEngineeringapproachtodesignprocessCustomerandmanufacturingrequirementsDesignFMEAConceptdevelopmentapproachSection5:ConnectorTestingPurposeLLCRmeasurementCommontypesoftestsandpurposesSection6:RecommendedAreasforFurtherTrainingSection1:ContactInterfaceSection1:ContactInterfaceThefirst3sectionswilldealwithsomeofthebasicunderlyingtheoryforconnectors.Thecontactinterfaceistheheartoftheconnector,sowewilllookatthisareafirst.Inordertounderstandwhathappensatthecontactinterface,itisnecessarytofirstlookatthecontactsurfacefromthemicroscopicpointofview.Thesurfaceisnotasmoothsurface,regardlessofwhetheritisontherolledsurfaceofthematerialorthestampededge.Ananalogybetweenthecontactsurfaceandahillyterraincanbeusedtohelpvisualizethetopologyofthecontactsurface.Thecontactsurfaceismadeupofanumeroushillsandvalleys.Thehighpointsofthehillswhereelectricalcontactismadewiththeoppositematingsurfacearecalledasperitiesora-spots.a-spotswhereelectricalconnectionismadecontactacontactbSection1:ContactInterfaceTSection1:ContactInterfaceThecurrentflowlinesmustbendandconstricttogothruthesea-spots.Thus,theterminologyconstrictionresistanceisusedfortheresistanceresultingfromthecurrenthavingtoflowonlythruthea-spotswhenitreachesthematinginterface.Thisisoftenalsocalledcontactresistance,butcontactresistanceisactuallyconstrictionresistanceplustheresistanceofanythinoxidefilmsorothercontaminantsattheinterface.Ananalogymightbetothinkofitasalargegroupofpeoplemovingdownahallwayandthensuddenlyencounteringasmallexittoleavethebuilding.Therewilloccurabunchinguppeopleattheentryway,reducingtheflowofpeople(i.e,restrictingthenumberofpeoplewhocanpassthruthisexitareaatonetime.currentflowlinesSection1:ContactInterfaceTSection1:ContactInterfaceTheformulaforconstrictionresistance,Rc,forasinglea-spotis

Rc=kr/awherer=resistivityofthemateriala=contactspotdiameterk=aparameterdependentonthemodeofdeformationandgeometricfactorsAdditionally,thevariableaisrelatedtohardness,H,andnormalforce,FN,inthefollowingmanner

a~(FN/H)1/2

Thus,asnormalforceincreasesorhardnessdecreasesthea-spotsizeincreases.Inactuality,therearenumerousa-spotsatanycontactinterface,sotheconstrictionresistanceisactuallyacombinationofamacroconstrictiondeterminedbytheoveralldistributionofcontactspotsandaparallelresistanceofmicroconstrictionsduetotheindividualspots.Thus,Rcbecomes

Rc=RM+Rm=r/D+r/nae

whereD=diameterofthedistributionofcontactspots(apparentcontactarea)ae=effectivesizeoftheindividualspotsSection1:ContactInterfaceTSection1:ContactInterfaceHowever,ithasbeenshownthatadistributionofcontactspotsbehavesinthesamemannerasasinglespotofdiameter,De,whereDeisaneffectivediameterthatrepresentsthetotala-spotarea.Thustheconstrictionresistancecanberepresentedbytheformula:

Rc=kr/DeSowhatdoesallthistellusinanutshell?Basicallythatthereare4factorsthatsignificantlyeffectthevalueofconstrictionresistance:1)resistivityofthematerial2)normalforce3)hardnessofthematerial4)geometryofthecontactareaHowever,withcontactsthereisanotherfactorthatmustbeconsidered.Contactsareforthemostpartalwaysplated.Sohowdoesthiseffectthepreviousdiscussion?Well,sincetheplatingthicknessisverysmallinthe15to200microinches,wewouldexpectthatthebulkoftheconstrictiontooccurinthebasemetalandthedeformationofthea-spotstooccurintheplating.basemetalplatingSection1:ContactInterfaceHSection1:ContactInterfaceThus,wewouldexpecttheconstrictionresistancetobeafunctionoftheresistivityofthebasemetalandthehardnessoftheplating.Forexample,tinplatedbrasshasamuchlowerconstrictionresistancethanpurebrassorpuretin(note:theresistivityofbrassandtininWcmx104are7.5and16.9,respectively,andthehardnessinlbs/in2are172,000to185,000and18,000,respectively).Also,notethatasthethicknessoftinincreasestheconstrictionresistancealsoincreases,duetosomeoftheconstrictionoccurringinthetin,insteadofthebrass.However,whatwearereallyinterestediniscontactresistance,whichismadeupofbothconstrictionresistanceandfilmresistance,asmentionedearlier.Filmresistanceisafunctionofthetypeofplatingusedandenvironmentalexposure.ThisdiscussionwillbepostponeduntilSection3,whenengineeringmaterialsarecovered.Section1:ContactInterfaceTSection1:ContactInterfaceWewillnowmoveintoamoredetaileddiscussionofthemechanicalaspectsofthecontactinterface.Themechanicalaspectsareconcernedwithfrictionandwear.Frictionandwearareactuallytwodifferentwaystodescribewhathappensatthea-spotswhentheinterfaceisdisruptedbyanappliedstress.Thediscussionwillfocusonasinglea-spot,sincetheargumentcaneasilybeexpandedtomultiplea-spots.Friction,asweknow,istheforcethatopposestherelativemotionoftwosurfacesincontactunderashearstress.

Ff=mFnwhereFf=frictionforce,m=coefficientoffriction,andFn=forceholdingthetwosurfacestogether(inthecaseoftheconnectorthisisthecontactnormalforce)Forcontacts,thecoefficientoffrictioncanrangefrom0.05to>1.Lowvaluesofcoefficientoffrictionreflectsituationswherethecontactareaiscoveredbysomechemicallybondedfilm(suchasoxides),adsorbedfilms(suchaswaterororganic),orintentionallyappliedlubricants.Highvaluesofcoefficientoffrictionreflecttheeffectsofplasticdeformationofthea-spotsandthecreationofmetallicallybondedjunctionsofhighershearstrengththanthebasemetal.Shearforcesinthislattercaseresultinfractureoccurringawayfromthesurface,resultinginweardebris.Section1:ContactInterfaceWSection1:ContactInterfaceWhathappensatthea-spotsisthatsomethemcanplasticallydeformundersmallloadsduetotheirphysicalsize,whichresultsinlocalizedworkhardeningandcoldwelding(creationofabondbetweentwometallicsurfacesthatoccurswhentheyareinintimatecontact).Whenthecoldweldingjointisstrongerthanthecohesivestrengthofthebasemetal,fractureoccursawayfromthejunction,resultinginawearparticleandmetaltransfer.Iftheplasticdeformationisnotassevere,lessworkhardeningandcoldweldingwilloccurandthedisruptionofthejunctionwilloccuratornearthesurface,i.e.littlewearormetaltransferoccurs.Inthepicturebelow“a”representsthefirstcaseand“b”representsthesecondcase.a:largeplasticdeformationata-spotb:smallplasticdeformationata-spotwearparticle(adhesivewear)Noappreciablewear(burnishingwear)Section1:ContactInterfaceWSection1:ContactInterfaceNowifthetransferredmetalparticleat“a”breaksloosethereisathirdwearmechanismthatoccurs,whichiscalledabrasivewear.Thesebrokenparticlesareharderduetoworkhardeningandthuswillgougeatbothcontactsurfacesduringmovement,resultinginincreasedwearratesatthecontactsurface.Thecoefficientoffrictionvalue,alongwiththelead-ingeometry,affectstheconnectormatingforceandthewearcharacteristics(burnishing,adhesive,andabrasive)affectthedurabilityoftheconnectorcontactinterfaces.Insummationforthissection,wehavefoundoutthatsurfaceroughness,contactnormalforce,andthegeometryofthesurfacesincontactarethekeyparameterseffectingthecontactinterface.Surfaceroughnesseffectsthenumberandsizeofa-spotscreated.Thenormalforceeffectsthetotalcontactareaattheinterface,andgeometryeffectstheareaoverwhichtheasperitieswillbedistributed.Section1:ContactInterfaceNSection2:EngineeringMaterialsThereare3basicmaterialsusedinconnectordesign:engineeringplastics;conductive,non-ferrous,springmetals;andplatingmaterial,bothnobleandnon-nobletocoatthemetals.Thefirstmaterialswewilldiscussaretheengineeringplasticsusedinconnectors.PlasticsPlasticsareoneofthebasiccomponentsusedinmakingconnectors.Theyareusedoverothermaterials,becausetheyarereasonablyhighstrength,dimensionallystabledielectricmaterialthatcanbemoldedintointricate,thinwalledshapesatalowcost.Plasticscanbedividedintotwogroups:thermosetsandthermoplastics.Thermosetsareplasticsthattakeapermanentshapeafterheatingduetocross-linkingofmolecules.Thus,thermosetscannotberecycledasadditionalheatingwillcausetheplastictodegraderatherthansoftenorreflow.Ananalogywouldbetocomparethermosetstohardboilinganegg.Onceanegghasbeenhardboiled,additionalheatingwillnotreturntheeggtoaliquidstate,butonlycauseittoburnifenoughheatisapplied.Thermoplastics,ontheotherhand,hardenatlowertemperaturesandsoftenandbecomemoldableathightemperatures.Somedegradationwilloccurwhenthermoplasticisrecycled,butgenerallymostthermoplasticscanberecycledwithoutanyseriousconsequences,ifmixedasapercentagewithvirginmaterial.Ananalogytobuttercouldbeusedtoexplainthermoplasticbehavior.Asthebutterisheateditsoftensandallowsyoutomolditintoadifferentshape.Whenitcoolsitagainbecomeshard.Thisprocesscanberepeated,withonlyminordegradationunlessthebutterisoverheated.Thisdifferenceinbehaviorbetweenthetwogroupsisthemainreasonwhythermoplasticsarethepredominantgroupusedintheconnectorindustry.Section2:EngineeringMateriSection2:EngineeringMaterialsPlastics(continued)Itisimportanttorealizewhendesigningwithplasticsthattheirpropertiesdonotallowthemtobetreatedasinert,isotropic,rigidmaterial.Additionally,therearedifferentclasseswithinthethermoplasticgroup,eachwithdistinctstructuresthateffecttheirmechanicalandphysicalproperties.Thermoplasticscanbegroupedinto3classesbytheirpolymerstructure:amorphous,crystalline,andliquidcrystalline.Polymerchainswithoutanyorganizedstructuralregionsareknownasamorphous.TypicalexamplesareABS,polycarbonate,PVC,andpolystyrene.Polymerchainsthathaveregionsoforganizedstructure,whichbehavelikecrystals,connectedbyamorphousregionsareknownascrystallineplastics.Typicalexamplesarenylon,polyester(PBT,PET),polypropylene,andpolyethylene.Finally,polymerchainsthatarestiff,rod-likestructuresthatareorganizedinlargeparallelarraysinboththemeltedandsolidstatesareknownasliquidcrystallinepolymers(LCP).Section2:EngineeringMateriSection2:EngineeringMaterialsPlastics(continued)Someofthepropertydifferencesbetweenthe3classesofplasticsareshowninthefollowingtwotables:Amorphous Crystalline LiquidCrystallineflexible,weaker,lessimpact stiff,strong,impactresistant stiff,strong,impactresistantresistant,softengradually& floweasily,highmelttemp softengradually&continuouslycontinuously resistanttoheat,creep,& floweasily,highmelttemp, chemicals resistanttoheat,creep,& chemicals lowviscosity,warpage,& shrinkageSection2:EngineeringMateriSection2:EngineeringMaterialsPlastics(continued)GeneralrelativepolymerpropertiesProperty Amorphous Crystalline LiquidCrystallineSpecificgravity lower higher higherTensilestrength lower higher highestTensilemodulus lower higher highestDuctility,elongation higher lower lowestResistancetocreep lower higher highMaximumusagetemp lower higher highShrinkageandwarpage lower higher lowestFlow lower higher highestChemicalresistance lower higher highestSection2:EngineeringMateriSection2:EngineeringMaterialsPlastics(continued)Thephysicalandmechanicalpropertiesoftheseplasticscanalsobemodifiedwiththeadditionoffillers,fibers,andotherchemicalcompounds.Whenthemechanicalpropertiesareimproved,themodifiedplasticresiniscalledareinforcedresin.Whentheadditivedoesnotsignificantlyimprovethemechanicalproperties,butdoesaffectthephysicalnatureofthematerial,themodifiedplasticresiniscalledafilledresin.Somepropertiesofplasticsthatshouldberememberedduringthedesignprocessare:1)Underloading,plasticmaterialwillcreep(geometrychangeunderfixedload).Therateofcreepincreasesnonlinearlywithincreasedtemperature.2)Plasticsshrinkastheycoolandtheshrinkageratevarieswiththetypeofplastic.Wallthickness,flowdirection,andmoldingconditionswillalleffecttheamountofshrinkagethatoccurs.Bowandsinkmarksoccurduetosomesectionscoolingslowerandshrinkingmorethanothersections.3)Mostplasticsusedintheconnectorindustryarereinforcedresins.Glassfibersarethenormalfillmaterialused.Thus,fiberorientationinthepartisimportantasthestrengthofthepartwillvarygreatlywithorientation.4)Knitlinesformwheretwofrontsofflowingplasticmeet.Theseareasarenotasstrongasotherareas,soitisimportanttocontrolthelocationofknitlines.5)Regrindmaterialcanberemoldedwithvirginmaterial.Sincethereissomedegradationofmaterialproperties,itisimportanttodeterminetheacceptableamountofregrind.Section2:EngineeringMateriSection2:EngineeringMaterialsPlastics(continued)6)Plasticsareaffectedbyvarioussolvents.Someplasticsaremoresolventresistantthanothers,soitisnecessarytodeterminewhatsolventstheplasticwillbeexposedtoinitsexpectedapplicationinordertodetermineacceptableplasticsthatcanbeusedinthedesign.7)Plasticsalsoabsorbmoisturetovaryingdegrees.Waterabsorptionwillaffectmechanicalandelectricalproperties,aswellas,dimensions.8)Plasticsvaryintheirdegreeofthermalstability.Someplasticschangedimensionallymorethanothersathighertemperatures.Somelosetheirphysicalpropertiesfasterwithincreasingtemperatures.Thetemperaturesatwhichthesechangesaresignificantcanoftenbeintherangeoftemperaturetheconnectorsmayseeeitherduringprocessing,transporting,orinoperation.Thus,thisshouldbecarefullyconsideredduringthedesignprocess.9)Themoldingprocessleavesinternalstressesintheplasticpart.Thesestressesmayresultinwarpage,notpresentintheasmoldedcondition,occurringinthepartduringlaterassemblyoperations,suchassolderreflow.Thus,itisimportantduringthedesignprocesstoconsiderifthesepossibilitiesexist.Section2:EngineeringMateriSection2:EngineeringMaterialsPlastics(continued)Afewbasicrulestorememberwhendesigningwithplasticsare:1)Avoidundercutswheneverpossible.Keepsthemoldtoolingsimplerandmorecosteffective.2)Sharpcornersshouldbeavoided.Sharpcorners,particularlyinteriorcorners,causepoorflowpatterns,reducemechanicalproperties,increasetoolwear,andincreasemolded-instresses.Ifpossible,insideradiishouldbeequaltoatleasthalfthewallthickness.Outsidecornersshouldbeequaltotheinsideradiiplusthewallthickness.3)Theminimumvolumeofplasticthatsatisfiesthestructural,functional,appearance,andmoldabilityrequirementsisusuallybest.4)Usinggradualtransitionsfromthicktothinwallsisbetter.5)Plasticmaterialsaregoodinsulatorsofelectricalandheatenergy.Thisshouldberememberedwhenconsideringwallthicknessforcomponents.Section2:EngineeringMateriSection2:EngineeringMaterialsMetalsCopperalloysarethematerialofchoiceforcontacts,becausetheyhavegoodelectricalconductivity,strength,formability,andcorrosionresistance.Inthetablebelow,someofthefactorsthatcomeintoplaywhenchoosinganalloyareshown.Copperalloyshavebeengroupedaccordingtotheirprincipalalloyingconstituents,becauseofthedominantroletheyhaveinaffectingalloyproperties.Thealloyinggroupsconsistofthehighpercentagecoppers;zinc-containingbrasses;bronzesbasedontin,aluminum,orsilicon;brassvariationswithtin;andnickelincombinationwithotherelementssuchaszinc,silicon,andtin.CopperalloyswithineachgroupcanbeidentifiedbytheirUNS(UnifiedNumberingSystem)designation.CopperalloysareprefixedbytheletterCfollowedbyaanextensionoffivedigits.Morecommonly,thefirst3orfourdigitsoftheUNSdesignationareused.Whenthetrailingdigitsarezeros,theyoftenareomittedtofacilitatealloyrecognition.ProductRequirements DesignFactors MaterialPropertiesFunctionSignalvspower Current ConductivityNormalandinsertionforce Contactforce Strength,bendingmodulusReliabilityTemperature Stablecontactforce StressrelaxationEnvironment Tolerance CorrosionresistanceManufacturing Dimensioncontrol Formability,springbackCost/performance Alloychoice Acceptabletrade-offSection2:EngineeringMateriSection2:EngineeringMaterialsMetals(continued)Themajorcopperalloysusedinconnectorsareshowninthefollowingtable:Alloygroup UNSdesignation PrincipalalloyelementsCoppers1 C101-C1579` Ag,As,Mg,P,ZrHighcoppers2 C162-C199 Cd,Be,Cr,FeBrass C210-C280 ZnTinbrass C404-C465 Sn,ZnTin(phosphor)bronze C501-524 Sn,PAluminumbronze C608-C642 AlSiliconbronze C647-C661 Co,SiModifiedbrass C664-C697 Zn,Al,CoCoppernickel C701-C729 Ni,FeorNi,Si,SnSilvernickel C735-C798 Ni,Zn__________________________________________________________________________________________1)Containsaminimumof99.3%copper2)Containsaminimumof96%copperSection2:EngineeringMateriSection2:EngineeringMaterialsMetals(continued)Whenchoosingametalforacontactthemostimportantmaterialpropertiesarestrength,elasticmodulus,andelectricalconductivity.Resistancetostressrelaxation,formability,corrosionresistance,andtoughnessareotherimportantpropertiesthatmustbeconsidered.Theoffsetyieldstrengthandelasticmoduliofalloysdeterminethemagnitudeofthecontactnormalforce.Theyformthebasisfordeterminingthecontact’snormalforce,deflection,andinsertion/withdrawalforces.Mostoftenitisbendingstressthatisresponsiblefordevelopingtheforce.Connectorreliabilityrequiresthenormalforcetoremainaboveathresholdvaluethroughoutthelifeoftheconnector.ElectricalconductivityofcopperalloysisexpressedasapercentageofapurecopperstandardIACS(InternationalAnnealedCopperStandard).Purecopperhasavalueof100%atthepuritylevelthatwasavailablewhenthestandardwassetover100yearsago.Advancessincethattimehavecreatedcopperwithbetterconductivityandthusahighervaluethan100%.Thefundamentalmeasurementisresistivityinmicrohm-cm,whichisconvertedintoIACSbydividingtheresistivityby172.4Alloyingdecreasesconductivity,withtheamountofdecreasedependingontheelementorelementsaddedandthetheiramount.Thermalandelectricalconductivityarelinearlyrelated.However,therelationshipistemperatureandalloydependent.Thecombinationoftwoisimportantindeterminingthetemperaturerise(T-rise)ofacontactduetocurrentflow.ThefollowingformulacanbeusedtoestimateT-riseinacontactbeam.

DT=(J2L2)/2ge

2gt2A2Section2:EngineeringMateriSection2:EngineeringMaterialsMetals(continued)whereDTisthetemperaturerise,Jisthecurrent,Listhebeamlength,Aisthecross-sectionalarea,ge

istheelectricalconductivity,andgtisthethermalconductivityMicroplasticprocessesmayoccurinthecontactspringoverprolongedperiodsoftimeevenifthestressiselastic.Thecontactnormalforceisreducedwhensomeoftheelasticstressandassociatedstrainisreplacedbyplasticdeformation.Thisprocessisknownasstressrelaxation(reductioninstressatafixeddisplacement).Increasingtemperatureandlevelofappliedstress,alongwithtimewillincreasetheamountofstressrelaxationthatoccurs,withsomealloysbeingmoreresistantthanotherstothisphenomenon.Bendingisacommonformingoperationinthemanufactureofcontacts.Thelimitingradiusofwhichthematerialcanbeformedwithoutcrackingiscentraltochoosinganalloy.Thisradiusvaluemayvarywithdirectionofforming.Formingwiththeaxisofbendperpendiculartotherollingdirectionofthematerialispreferred,becauseitusuallyhasthebetterformability.Thisisalsoknownaslongitudinalbending.Bendingperpendiculartothisisknownastransversebending.Theamountofelasticspringbackinthematerialisanotherfactorthatmustbeconsideredwhenformingthematerial.Section2:EngineeringMateriSection2:EngineeringMaterialsMetals(continued)Stresscorrosioninacopperalloyoccurswhentheactionofcorrosionfromaharshenvironmentandahighelastictensilestressacttocausecrackingandeventualfailureinthematerial.Alloysvaryintheirsusceptibilitytostresscorrosion.Thetablebelowcomparestherelativesusceptibilityofasmallselectionofalloys,withthehighernumberbeingmoresusceptible.UNSDesignation SusceptibilitytostresscorrosionC260 1000C510 20C521 10C172 <10C194 0Section2:EngineeringMateriSection2:EngineeringMaterialsMetals(continued)Toughnessreferstotheabilityofthealloytoabsorbtheenergyofmechanicaldeformationwithoutfailure.Itismeasureofthematerial’sabilitytoresistpermanentset.Toughnessofamaterialcanbeestimatedbythefollowingformula:

Toughness=(YS)(US)+1/2(TS-YS)(US)where

YS

isthe0.2%offsetyieldstrength,TSistheultimatetensilestrength,andUSistheuniformstrainThetablebelowcomparesafewofalloysusingtheaboveapproximation.

C172(290TM02) C172(1901/2HM) C510(spring) C521(spring)TS(ksi) 119.2 125.4 99.2 109.7YS(ksi) 110.8 100.0 91.9 101.7US(%) 15.5 16.7 8.2 8.4Elongation 16.0 17.0 11.0 13.0Toughness(ksi) 1882 1782 783 887Generally,alloysexhibitinghightoughnessvaluesarebetterabletoresistpermanentset.ThisisduetotheirhighyieldstrengthandabilitytopermitlargescaledeflectionbystrainhardeningSection2:EngineeringMateriSection2:EngineeringMaterialsPlatingsPlatingsareusedprimarilytoretardcorrosionandoxidation,actasbarrierstointermetallicgrowth,decreasecontactresistance,andimprovesolderability.Platingscanbeclassifiedaseitherprecious/nobleornon-precious/non-noble.Gold,silver,andpalladium-nickelwouldbeintheformercategory.Nickel,copper,tin,andtin-leadwouldbeinthelattercategory.Goldisuniqueamongtheplatingmaterials,asitistheonlyonecompletelyfreeofoxidefilms,whichissignificantbecausemetallicoxidesgenerallyhaveahighelectricalresistance.However,goldisnormallyalloyedwithothermaterialssuchascobalttoincreaseitshardness,aspuregoldisusuallytoo,softtouseasaplatingmaterialthatwillbematednumeroustimes.Platingsareusedintwoways:asasurfacefinishandasanunderplatebetweenthesurfacefinishandthebasemetal.Underplates:Wewillfirstdiscussunderplates.Underplatesareusedforfourprimaryreasons:

1)Preventalloyingelementsinthebasemetalfromgraduallydiffusingto thesurfaceanddegradingthecontactinterfaceresistance 2)Improvethewearcapabilityofthesurfacefinish 3)Actasalevelingfinishtoreducethesurfaceroughnessofthebasemetal 4)Reducetheeffectsofporosity

Section2:EngineeringMateriSection2:EngineeringMaterialsPlatingsThemostcommonandeffectiveunderplatehasbeenproventobenickel.Itisespeciallyusefulundergold.Usingnickelundergoldpermitslessgoldtobeused,becauseitimprovesthewearcapabilityofgold,reducestheeffectsofporosityinthegoldplate,andallowsselectiveplatingtobeused,sincethenickeloxidelayeronexposednickelinhibitscorrosiongrowth.Exposednickeloxidealsoiseffectiveinpreventingsolderwicking.Aneffectiveplatingthicknessfornickelhasbeenfoundtobeintherangeof1.2to2mm(50to80minches).Ifthenickelunderplateismadetoo,thick,problemswithcrackingcanoccurduringanysubsequentformingorbending.Copperisalsousedasanunderplate,butismuchlesscommon.Itisnotaseffectiveadiffusionbarrierandwillitselfdiffuseintothesurfaceplating.Ifitistobeusedasadiffusionbarrieraminimumthicknessof2.5mmshouldbeused.Copperisprimarilyusedlevelingagenttoimprovethesurfacefinishofthemetal.Surfacefinishes:Forsurfacefinishplatings,goldistheoverwhelmingchoiceforsignallevelcontacts.Goldsurfacesarealmostcompletelyfreeofoxidefilmsundervirtuallyallenvironmentalconditions,whichmeanslesscontactnormalforceisrequiredtomakecleanmetaltometalcontact.However,thereareapplicationswheregoldisnotusedordesirable.Forexampleinarcingcontacts,golderodesrapidlyandhasatendencytoweldtogether.Itsuseonhighpressure,semi-permanentconnectionsorincrimptypeconnectionsisoflimitedornovalue.Puregoldhaspoordurabilitycharacteristics,sinceittendstostick,gall,smear,andrub-offfast.Thatiswhyitisnormallyalloyedtomakeitharder.Section2:EngineeringMateriSection2:EngineeringMaterialsPlatingsPalladiumdoesnotoxidize,exceptathightemperatures,andresistscor