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外文翻译--引线键合的现状与发展趋势.doc

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外文翻译--引线键合的现状与发展趋势.doc

附录1WIREBONDINGPRESENTANDFUTURETRENDSABSTRACTGoldwirebondinghasbeenastandardoperationinplasticICpackageassemblyforseveraldecades.However,asthepackagesgrowlarger,thinner,anddenser,thecapabilitiesofequipment,andofthewireitself,arebeingtakentothelimit.Recently,specialtywireshavebeenintroducedwithdifferentdopantsandprocessingproceduresgivinghigherstrengthandstiffness.Theseimprovedpropertiescanofferbetterwirewashprotectionaswellascapabilityforlongspansandlowloops.Disadvantagescomefromhighercostandpotentialreliabilityissuesfromthehighdopantlevels.Advancesinthewirebondersallowbettermachinemotioncontrolwithpreciseplacementofthewires.Improvedcontrolsoftwarecanproducelowloopsandlongstraightspansbysimultaneouslyregulatingmotionalongallthreeaxes,fromballplacementtothewedgebondontheleadtip.Somemanufacturersevenbelievethatallimprovementscanbeachievedthroughtheequipmentalone,withouthavingtoresorttothespecialtywires.Thecombinationofbetterwiresandbettermachinesshouldbeabletoproducethetypeofwirebondsdesiredinthinfinepitchplasticpackages.Nevertheless,thephysicallimitsofballbondingmayeventuallybereached,ifthecurrenttrendoffinerandfinerpitchcontinues.Inthiscase,othertypesofinterconnectionwillstartlookingattractive,suchasTAB,flipchip,orthedevelopmentofhighspeed,flexiblewedgebonding.1.0CURRENTSTATUSThegoldAubondingwiregenerallyusedintheindustryforautomatedthermosonicbondingisprimarilyberylliumBedoped,atabout3to8partspermillionppm.Inthisrange,berylliumgivesthewireincreasedstrengthbyprecipitationhardeninginthematrixoftheintermetallicsformedfromthetwoelements.Italsoallowsthegoldtobemoreeasilydrawn,asverypuregoldistoosoftforshapingintoafinewire1,2.Strongerandharderwireshavebeenintroducedintothemarketrecentlyinresponsetotheneedsforfinepitchandlongerspans.Thesewiresarealsoformulatedtoproducelowerloopsforthinoutlinepackages3.Whileadequateformostplasticpackagingbondingneeds,thepreviousgenerationofbondersdonothavetheabilitytobondaccuratelyforthenextgenerationofhighdensitypackages.Thesemachinestendtohaveanalogmotioncontrol,whichdoesnotlendtoprecisetravelandplacementofthewires.Also,beinganalog,eachmachineisuniquewithrespecttoitssetup.Parametersoptimizedononebondercannoteasilybetransferredtoanother.2.0TRENDSINGOLDBONDINGWIRE2.1MATERIALREFINEMENTSTable1comparesthepropertiesofsometypicalcommerciallyavailablewires.Generallyallthenewwireshaveimprovedproperties,suchasgreaterbreakingload,lowerloopheight,andincreasedpullandshearstrengths.DopantsincreasethestrengthofAuwires.Forinstance,Beincreasesgrainrefinementandstrengthbyintroducingintermetallicprecipitates,sinceitisasmallatomcomparedtogoldandiselectronicallyissimilar.Eveninminuteamounts,theatomswilldistortthelattice,makingBeaneffectivehardeningagent.Thisiscurrentlytheprimarydopantusedintheindustry.However,increasingtheamounttoabove12ppmwillcauseBetocomeoutofsolution,andinduceintergranularfractureduringbonding1.Othermetalsarealsousedtomaximizemechanicalproperties.SilverAg,palladiumPd,copperCu,andplatinumPtareaddedforsolidsolutionstrengtheningandchemicalstability.Threeofthefourelementscitedareinthesamegroupintheperiodictableasgold.Inaddition,justaswithgold,allfourelementsarefacecenteredcubicFCCwithsimilarlatticeconstantsandelectronicstructure.Strengtheningisbysubstitutionoflatticeatomswithdopantatomstoprovidethechemicalstability4.Othermetals,likecalciumandlanthanum,canalsobeusedforstrengtheningbyprecipitationhardening,inmuchthesamemannerasBe.Inadditiontothedopants,theproductionprocessalsoinfluencesthewireproperties.Carefulrolling,annealing,anddrawingimprovethewirepropertiesbyinsuringgrainuniformityandevendispersionoftheprecipitates1.2.2LOWLOOPINGCHARACERISTICSAnotherpurposefortheserefinementsistoincreasestrengthandhardnessandtoreducethelengthoftheheataffectedzoneHAZ.TheHAZisthelocationwheretheloopisformed,andshouldideallybebetween25pmto100pmfromtheballbond.Throughrecrystallizationandgraingrowth,theHAZcaneasilybedeformed.Largergrainsaregenerallyweakerfromalackofdislocationsduetoworkhardening.Opantprecipitatestieupthegrainboundariesduringheatingandpreventgraingrowth.Onereasonforthehighdopantlevelistoinsurethatanadequateamountofprecipitatesexistsintheatomiclattice.Thus,thequalityofthewireloopingcapabilitiesrequiresgoodcontroloftheHAZ.FigureIillustratesthehardnessprofileofastandardAuwireafterballformation6.Hardnessismeasuredfromtheneckpositiontowardtheballbondwitha2gmload.ThedistributionisUshapedwithaminimumacertaindistanceawayfromtheballbond.Thewirebendsattheminimumhardnessposition,whichusuallyisinthe00.10.20.30.40.5centeroftheHAZ.Thispointcorrespondstothepositionoflargestcurvatureandaffectstheloopheight.ShorteningtheHAZshouldproducealowerloop.Fortheexamplegiven,wireAisharderandhasaHAZclosertotheneckthanwireBresultinginalowerloop57.2.3LONGSPANCHARACERISTICSAsthewirespanfromthebondpadsonthechiptotheleadfingersincreases,theriskofwiresagging,andsweepingduringmoldingalsogrows.calciumandlanthanum,canalsobeusedforstrengtheningbyprecipitationhardening,inmuchthesamemannerasBe.Inadditiontothedopants,theproductionprocessalsoinfluencesthewireproperties.Carefulrolling,annealing,anddrawingimprovethewirepropertiesbyinsuringgrainuniformityandevendispersionoftheprecipitates1.Figure2showshowthemoldflowaffectswirewashing8a.I.istheratiooftheamountofwiredeflection,6,overthebondpadpitchs.Thus,if6sisequalto1,thewirebondhasdrapedovertotheadjacentpad.Thisratioisusedasametricfordescribingtheextentofwiresweepinapackage.Inpractice,however,sweepisdefinedasanydeformationthatexceedsaratiospecifiedbytheICmanufacturer.Typically,thisvaluecouldbefromonetothreewirediametersdependingontheassemblydesignrulesofthemanufacturer.Thepolarplotillustratesthelocationofthewiresonthetestdevice,withtheanglemadebyeachwirewithrespecttotheflowdescribedbythepositionofthedatapoint.Comerwiresexhibitthehighestamountofsweepcomparedtothecenterwiresduetotheirlongerspans.Asthefrontvelocityofthemoldingencapsulantisincreased,higherflowinducedforcesareobservedresultinginmoresweep.Towithstandthemoldcompoundflow,theAuwiremustbestronger.Theincreasedstrengthandstiffnessofthespecialtywires,indicatedbythehighbreakingloadandelongationvalues,providemoreresistancetomoldsweep.Table2comparesthetypicalsweepobservedbetweenthestandardwiresandthenewerhighstrengthwiresfordifferentwirespans.ThesamerefinementswhichgivetheAuspecialtywireslowloopingcapabililitiesalsoprovidethemeansforlongspans.Itshouldbepointedoutthatwiresweepresultsfromacombinationoffactorssuchasmoldcavitydesign,moldingparameters,anddevicelayout8b,c.Thesefactorscontroltheextentofsweepjustasmuchasthepropertiesofthewiresandtheprofilesofthebonds.Thus,inordertoreduceoreliminatemoldinginducedwiresweep,a

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