Electroplating.docx
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Electroplating.docx
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Electroplating
ElectrochemistryEncyclopedia
(http:
//electrochem.cwru.edu/ed/encycl/)
ELECTROPLATING
MordechaySchlesinger
DepartmentofPhysics
UniversityofWindsor
Windsor,ONN9B3P4,Canada
(September,2002)
Electroplatinghas,overrecentdecades,evolvedfromanarttoanexactscience.Thisdevelopmentisseenasresponsiblefortheever-increasingnumberandwideningtypesofapplicationsofthisbranchofpracticalscienceandengineering.Someofthetechnologicalareasinwhichmeansandmethodsofelectroplatingconstituteanessentialcomponentareallaspectsofelectronics:
macroandmicro,optics,opto-electronics,andsensorsofmosttypes,tonameonlyafew.Inadditionanumberofkeyindustriessuchastheautomobileindustry(thatusesforexamplechromeplatingtoenhancethecorrosionresistanceofmetalparts)adoptthemethodsevenwhereothermethods,suchasevaporation,sputtering,chemicalvapordeposition(CVD)andthelikeareanoption.Thatissoforreasonsofeconomyandconvenience.Bywayofillustrationitshouldbenotedthatthatmodernelectroplatingequipsthepractitionerwiththeabilitytopredesignthepropertiesofsurfacesandinthecaseofelectroformingthoseofthewholepart.Furthermore,theabilitytodepositverythinmultilayers(lessthanamillionthofacm)viaelectroplatingrepresentsyetanewavenueofproducingnewmaterials.
Fig.1.Schematicsofanelectrolyticcellforplatingmetal"M"fromasolutionofthemetalsalt"MA".
Electroplatingisoftenalsocalled"electrodeposition",andthetwotermsareusedinterchangeably.Asamatteroffact,"electroplating"canbeconsideredtooccurbytheprocessofelectrodeposition.Electrodepositionistheprocessofproducingacoating,usuallymetallic,onasurfacebytheactionofelectriccurrent.Thedepositionofametalliccoatingontoanobjectisachievedbyputtinganegativechargeontheobjecttobecoatedandimmersingitintoasolutionwhichcontainsasaltofthemetaltobedeposited(inotherwords,theobjecttobeplatedismadethecathodeofanelectrolyticcell).Themetallicionsofthesaltcarryapositivechargeandarethusattractedtotheobject.Whentheyreachthenegativelychargedobject(thatistobeelectroplated),itprovideselectronstoreducethepositivelychargedionstometallicform.Figure1isaschematicpresentationofanelectrolyticcellforelectroplatingametal"M"fromanaqueous(water)solutionofmetalsalt"MA".
Tofurtherillustratetheforegoing,letusassumethatonehasanobjectmadeofoneofthecommonmetals,likecopper,andthatithasbeenproperlypre-cleaned.Weshouldwanttoplateitwith,say,nickel.Awirewillhavetobeattachedtotheobjectwhiletheotherendofthewireshouldbeattachedtothenegativepoleofabattery(orapowersupply).Tothepositivepoleofthebattery(orpowersupply)weconnectanotherwirewithitsotherendconnectedtoarodmadeofnickel.Nextwefillthecellwithasolutionofthemetalsalttobeplated.Itispossibletouseamoltensaltandinsomenotsocommoncases,suchasthedepositionoftungsten,thatiswhatisdone.Inmost,morecommon,casesthoughthesaltissimplydissolvedinwater.Inourpresentexamplethenickelchloridesaltdissociatesinwatertopositivelychargednickelcationsandnegativelychargedchlorideanions.Astheobjecttobeplatedisnegativelychargeditattractsthepositivelychargednickelcations,andelectronsflowfromtheobjecttothecationstoneutralizethem(toreducethem)tometallicform.Meanwhilethenegativelychargedchlorideanionsareattractedtothepositivelychargednickelrod(knownastheanodeoftheelectrolyticcell).Attheanodeelectronsareremovedfromthenickelmetal,oxidizingittothenickelcations.Thusweseethatthenickeldissolvesasionsintothesolution.Thatishowreplacementnickelissuppliedtothesolutionforthatwhichhasbeenplatedoutandoneretainsasolutionofnickelchlorideinthecell.
Nickelchlorideisusedheretoexemplifytheprocessofelectroplatingforanumberofreasons.Firstamongthoseissimplicity.Itisnotrecommended,however,thatnickelbeusedfor,say,schoolsciencedemonstrationsbecausesomeindividualsarequiteallergictoit.Wefurtherdonotrecommendthatchloridesaltsbeusedbecausethoseareamenabletoreleasechlorinegas.Forschooloramateurtypedemonstrationwerecommendplatingcoppercoinswithzincornickelcoinswithcopper.
History
Beforecontinuingwithmoredetaileddiscussionofthesubjectathand,abriefhistoryofelectroplatingwillbepresented.Theearlyhistoryofelectroplatingmaybetracedbacktoaround1800.Auniversityprofessor,orinmodernterms:
achemist,LuigiBrugnatelliisconsideredasthefirstpersontoapplyelectrodepositionprocesstoelectroplategold.BrugnatelliwasafriendofAllisandroVolta(afterwhomtheelectricunit"volt"hasbeennamed)whohadjustashorttimebeforediscoveredthechemicalprinciplesthatwouldmakepossiblethedevelopmentof"voltaic"electricalcells.Volta'sfirstactualdemonstrationofthatwascalled"VoltaicPile".Asaconsequenceofthisdevelopment,Brugnatelli'searlyworkusingvoltaicelectricityenabledhimtoexperimentwithvariousplatingsolutions.By1805hehadrefinedhisprocessenoughtoplateafinelayerofgoldoverlargesilvermetalobjects.HewroteinalettertotheBelgianJournalofPhysicsandChemistry(laterreprintedinBritain),whichreads:
"Ihavelatelygiltinacompletemannertwolargesilvermedals,bybringingthemintocommunicationbymeansofasteelwire,withanegativepoleofavoltaicpile,andkeepingthemoneaftertheotherimmersedinammoniuretofgoldnewlymadeandwellsaturated".
UnfortunatelyforBrugnatelli,adisagreementorfallingoutwiththeFrenchAcademyofSciences,theleadingscientificbodyofEuropeatthetime,preventedBrugnatelli'sworkfrombeingpublishedinthescientificjournalsofhisday.HisworkremainedlargelyunknownoutsideofhisnativeItalyexceptforasmallgroupofassociates.By1839,however,scientistinBritainaswellasinRussiahadindependentlydevisedmetaldepositionprocessessimilartothoseofBrugnatelli'sforcopperelectroplatingofprintingpressplates.By1840,thisdiscoverywasadaptedandrefinedbyHenryandGeorgeElkingtonofBirmingham,Englandforgoldandsilverplating.CollaboratingwiththeirpartnerJohnWrightandusingformulaedevelopedbythelatterforpotassiumcyanideplatingbaths,theElkingtonswereabletohavethefirstviablepatentsforgoldandsilverelectroplatingissuedontheirname.FromGreatBritaintheelectroplatingprocessforgoldandsilverquicklyspreadthroughouttherestofEuropeandlatertotheUnitedStates.
Withtheburgeoningknowledgeandunderstandingofthesubjectofelectrochemistryanditsimportanceinunderstandingtheprocessesof"electrodeposition"cametheabilitytodepositothermetals.Bythe1850'selectroplatingmethodsofbrightnickel,brass,tin,andzincwerecommercializedandwereappliedforengineeringandspecificcommercialpurposes.Intime,theindustrialageandfinancialcapitalhadexpandedfromGreatBritaintotherestoftheworld.Asaresult,electrodepositionprocesseswereexpandinginscopeandfoundmoreandmoreusageintheproductionofavarietyofgoodsandservices.Whilethisexpansionwastakingplace,nosignificantscientificdiscoveriesweremadeuntiltheemergenceoftheelectronicindustryinthemidfortiesofthelastcentury.Theonlyexceptiontothiswereimprovementsmadetodirectcurrentpowersupplieswhichwere/areusedinsteadofbatteries.Thusitissaidthattheyearsfrom1870to1940wereaquietperiodasfaraselectroplatingwasconcerned,significantonlyingradualimprovementinlargerscalemanufacturingprocesses,anodicandcathodicreactionprinciplesandplatingbathformulae.Duringthelateryearsoftheforties,rediscoveryofheavygoldplatingforelectroniccomponentstookplace.Incomparisontothat,duringthemidtolaterfiftiestheusageofnewandmore"userfriendly"platingbathsbasedonacidformulae(ratherthanstronglypoisonouscyanidebasedones)weredevelopedandintroducedforlargescalecommercialuse.Inclosingthisbriefhistoricsummaryoneistonotethattodayanumberofregulatorylaws(enactedmainlyinthenineteen-seventies),forinstanceconcerningwastewateremissionandwastedisposal,setthetone/directionfortheelectrodeposition/electroplatingindustryforthenextthirtyyears.
Today,withtheimpressiveprogressanddeeperunderstandingoftheunderlyingelectrochemicalprinciplesofelectrodeposition,sophisticatedplatingbathsformulaehavebeendevelopedandarebeingroutinelyemployed.Thoseprovidemuchgreatercontrolovertheworkingcharacteristicsofthedepositionprocessthanhitherto.Layerthickness,performanceofelectroplatedfinishesisamongtheattributesthathavebeenbroughtunderstrictcontrol.Newdevelopmentsenablegreaterplatingspeed,betterthrowingpower(theabilityofaplatingsolutiontoproducearelativelyuniformdistributionofmetaluponacathodeofirregularshape),aswellasreliableplatedfinishes.Inaddition,electroplatingofmaterialssuchasplatinum,osmium,andrutheniumarenowbroadlyusedinelectronicsforconnectors,circuitboards,contactsetc.Thewriterbelievesthatnewandinnovativeelectroplatingtechnologywillfacilitatetherapidexpansionofthetelecommunicationindustry.Ingeneral,thegrowthoftheelectronicsindustryasawhole,andthedemandtosupporttheexpansionofitsunderlyinginfrastructurewillcontinuetodriveimprovementsworldwideint
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