外文翻译--引线键合的现状与发展趋势.doc

附录1WIREBONDING:PRESENTANDFUTURETRENDSABSTRACTGoldwirebondinghasbeenastandardoperationinplasticICpackageassemblyforseveraldecades.However,asthepackagesgrowlarger,thinner,anddenser,thecapabilitiesofequipment,andofthewireitself,arebeingtakentothelimit.Recently,specialtywireshavebeenintroducedwithdifferentdopantsandprocessingproceduresgivinghigherstrengthandstiffness.Theseimprovedpropertiescanofferbetterwirewashprotectionaswellascapabilityforlongspansandlowloops.Disadvantagescomefromhighercostandpotentialreliabilityissuesfromthehighdopantlevels.Advancesinthewirebondersallowbettermachinemotioncontrolwithpreciseplacementofthewires.Improvedcontrolsoftwarecanproducelowloopsandlongstraightspansbysimultaneouslyregulatingmotionalongallthreeaxes,fromballplacementtothewedgebondontheleadtip.Somemanufacturersevenbelievethatallimprovementscanbeachievedthroughtheequipmentalone,withouthavingtoresorttothespecialtywires.Thecombinationofbetterwiresandbettermachinesshouldbeabletoproducethetypeofwirebondsdesiredinthinfinepitchplasticpackages.Nevertheless,thephysicallimitsofballbondingmayeventuallybereached,ifthecurrenttrendoffinerandfinerpitchcontinues.Inthiscase,othertypesofinterconnectionwillstartlookingattractive,suchasTAB,flip-chip,orthedevelopmentofhighspeed,flexiblewedgebonding.1.0CURRENTSTATUSThegold(Au)bondingwiregenerallyusedintheindustryforautomatedthermosonicbondingisprimarilyberyllium(Be)doped,atabout3to8parts-per-million(ppm).Inthisrange,berylliumgivesthewireincreasedstrengthbyprecipitationhardeninginthematrixoftheintermetallicsformedfromthetwoelements.Italsoallowsthegoldtobemoreeasilydrawn,asverypuregoldistoosoftforshapingintoafinewire(1,2).Strongerandharderwireshavebeenintroducedintothemarketrecentlyinresponsetotheneedsforfinepitchandlongerspans.Thesewiresarealsoformulatedtoproducelowerloopsforthinoutlinepackages(3).Whileadequateformostplasticpackagingbondingneeds,thepreviousgenerationofbondersdonothavetheabilitytobondaccuratelyforthenextgenerationofhighdensitypackages.Thesemachinestendtohaveanalogmotioncontrol,whichdoesnotlendtoprecisetravelandplacementofthewires.Also,beinganalog,eachmachineisuniquewithrespecttoitsset-up.Parametersoptimizedononebondercannoteasilybetransferredtoanother.2.0TRENDSINGOLDBONDINGWIRE2.1MATERIALREFINEMENTSTable1comparesthepropertiesofsometypicalcommerciallyavailablewires.Generallyallthe“new”wireshaveimprovedproperties,suchasgreaterbreakingload,lowerloopheight,andincreasedpullandshearstrengths.DopantsincreasethestrengthofAuwires.Forinstance,Beincreasesgrainrefinementandstrengthbyintroducingintermetallicprecipitates,sinceitisasmallatomcomparedtogoldandiselectronicallyissimilar.Eveninminuteamounts,theatomswilldistortthelattice,makingBeaneffectivehardeningagent.Thisiscurrentlytheprimarydopantusedintheindustry.However,increasingtheamounttoabove12ppmwillcauseBetocomeoutofsolution,andinduceintergranularfractureduringbonding(1).Othermetalsarealsousedtomaximizemechanicalproperties.Silver(Ag),palladium(Pd),copper(Cu),andplatinum(Pt)areaddedforsolidsolutionstrengtheningandchemicalstability.Threeofthefourelementscitedareinthesamegroupintheperiodictableasgold.Inaddition,justaswithgold,allfourelementsarefacecenteredcubic(FCC)withsimilarlatticeconstantsandelectronicstructure.Strengtheningisbysubstitutionoflatticeatomswithdopantatomstoprovidethechemicalstability(4).Othermetals,likecalciumandlanthanum,canalsobeusedforstrengtheningbyprecipitationhardening,inmuchthesamemannerasBe.Inadditiontothedopants,theproductionprocessalsoinfluencesthewireproperties.Carefulrolling,annealing,anddrawingimprovethewirepropertiesbyinsuringgrainuniformityandevendispersionoftheprecipitates(1).2.2LOWLOOPINGCHARACERISTICSAnotherpurposefortheserefinementsistoincreasestrengthandhardnessandtoreducethelengthoftheheataffectedzone(HAZ).TheHAZisthelocationwheretheloopisformed,andshouldideallybebetween25pmto100pmfromtheballbond.Throughrecrystallizationandgraingrowth,theHAZcaneasilybedeformed.Largergrainsaregenerallyweakerfromalackofdislocationsduetoworkhardening.Opantprecipitatestieupthegrainboundariesduringheatingandpreventgraingrowth.Onereasonforthehighdopantlevelistoinsurethatanadequateamountofprecipitatesexistsintheatomiclattice.Thus,thequalityofthewireloopingcapabilitiesrequiresgoodcontroloftheHAZ.FigureIillustratesthehardnessprofileofastandardAuwireafterballformation(6).Hardnessismeasuredfromtheneckpositiontowardtheballbondwitha2gmload.ThedistributionisU-shapedwithaminimumacertaindistanceawayfromtheballbond.Thewirebendsattheminimumhardnessposition,whichusuallyisinthe00.40.5centeroftheHAZ.Thispointcorrespondstothepositionoflargestcurvatureandaffectstheloopheight.ShorteningtheHAZshouldproducealowerloop.Fortheexamplegiven,wireAisharderandhasaHAZclosertotheneckthanwireBresultinginalowerloop(5-7).2.3LONGSPANCHARACERISTICSAsthewirespanfromthebondpadsonthechiptotheleadfingersincreases,theriskofwiresagging,andsweepingduringmoldingalsogrows.calciumandlanthanum,canalsobeusedforstrengtheningbyprecipitationhardening,inmuchthesamemannerasBe.Inadditiontothedopants,theproductionprocessalsoinfluencesthewireproperties.Carefulrolling,annealing,anddrawingimprovethewirepropertiesbyinsuringgrainuniformityandevendispersionoftheprecipitates(1).Figure2showshowthemoldflowaffectswirewashing(8a).&I.istheratiooftheamountofwiredeflection,6,overthebondpadpitchs.Thus,if6sisequalto1,thewirebondhasdrapedovertotheadjacentpad.Thisratioisusedasametricfordescribingtheextentofwiresweepinapackage.Inpractice,however,sweepisdefinedasanydeformationthatexceedsaratiospecifiedbytheICmanufacturer.Typically,thisvaluecouldbefromonetothreewirediametersdependingontheassemblydesignrulesofthemanufacturer.Thepolarplotillustratesthelocationofthewiresonthetestdevice,withtheanglemadebyeachwirewithrespecttotheflowdescribedbythepositionofthedatapoint.Comerwiresexhibitthehighestamountofsweepcomparedtothecenterwiresduetotheirlongerspans.Asthefrontvelocityofthemoldingencapsulantisincreased,higherflow-inducedforcesareobservedresultinginmoresweep.Towithstandthemol