SMT锡膏详解Part 1.docx

SMT锡膏详解Part 1.docx

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SMT锡膏详解Part1

GeneralInformationonSolderPaste

GeneralInformationonSolderPaste

1.Kindofsolderpaste

Therearevariousdifferenttypesofsolderpasteavailabletoday.Thoseproductsshallgenerallybecategorizedinthefollowingmannerbytypeofflux.

No-cleantype

Fig.1

Watersolublesolderpasteistobecleanedbypuredeionizedwater.SinceKokino-cleansolderpastes,boththehalidecontainingandhalidefreesolderpastesareallrosinbased,solvent,suchasHCFC,andsaponifiershallberecommendedasacleaner.Mostofourhalidefreesolderpastes,however,arequitedifficulttocleannomatterhowitiscleaned,andmayleavewhiteresidue.Maincomponentwhichtendstoremainonthesubstrateaftercleaningisnormallythixotropicmaterials.Becauseeasy-to-cleanthixotropicmaterialstendtobedifficulttoeffectivelycontrolslumpresistivityofsolderpaste,notmuchattentionispaidforthecleanabilitywhenselectingthixotropicmaterialsasmoreimportanceisattachedtoslumpresistancefeature.

Kokiisspecializedinno-cleansolderpastes,HalidecontainingsolderpasteandHalidefreesolderpaste.

IncreaseofpackagedensityalongwithcompletebanofCFCsgreatlyurgedtheindustryfortheimplementationofno-cleanprocess.Asrequirementsfortheprocessonthebothworkabilityandreliabilityarebecomingmorestrictandprecise,no-cleansolderpastehasbeendiversifiedaccordingly.

Itisoftenregardedthatno-cleansolderpastemustbehalidefree.Itmustbeclearlyunderstoodthatevenhalidecontainingsolderpastecanbeusedforno-cleanprocessaswellbecausetocleanornotmustbedeterminedinaccordancewithreliabilityrequirementsoffinishedproducts.

Infact,theuseofhalidecontaining（0.2%）solderpasteforno-cleanproductionismuchmorepopularinJapanthanthatofhalidefreepaste.

Ingeneralcomparison,halidecontainingpastehasbetterworkability,suchassolderability,butoftenexhibitsinferiorreliabilitytohalidefreetype,suchasinsulationresistance.Thiscanbeattributed

mainlytoitshigheractivationstrength.Relationbetweensolderabilityandreliabilitycouldbeexpressedbytheseesaw.

Reliability

Therefore,sofarasno-cleansolderingprocessisconcerned,inviewofhighreliability,itisdesirableforthepastetobehalidefree,buthopefullywithashighsolderabilityashalidecontainingpastetoensurewideprocesswindow.

Itis,however,truethatmaindifficultyforno-cleanhalidefreesolderpasteisimprovementofactivation.Mostofhalidefreesolderpastescontainorganicacidsasanactivatorinsteadofhalidetoretaincertainactivationstrength.Althoughavarietyoforganicacidsisavailable,generally,thelessmolecularweightorganicacidsitis,themoreactivationithas.Sinceactivationstrengthoforganicacidsthemselvesismuchweakerthanhalide,relativelyactiveandlargeamountoforganicacidstendtobeformulatedinthefluxsystem.

However,suchhighactivationorganicacidstendtohaveapropertytoabsorbmoisture.Then,thereisadangerthatitcouldgetionizedreactingwithwaterwhenitisleftonthesubstrateasfluxresidueanddeteriorateselectricalproperties,suchassurfaceinsulationresistanceandelectromigration.

ActivationsystemofKokisolderpastesemployscombinationoflesshygroscopicorganicacidsandspeciallydevelopednon-ionicactivator.

Althoughthisspecialactivatorsystemdoesnotgetionizedasitdoesnothaveapropertyofdissociationandiselectricallyverystableandsafe,itexhibitsashighactivationstrengthashalogen.Sinceactivationtemperatureofnon-ionicactivatorisrelativelyhigh,itscombinationwithcarefullyselectedorganicacidsprovideslongeractivationtimeatreflowstageenhancingsolderability,andensuresextremelyhighreliabilityatthesametime.

Solderpasteisinvolvedinthefollowingproductionstages.

Storage→Printing→Mounting→Reflow→Inspection→Cleaning

Ateachstage,solderpasteisrequiredtoexhibitgoodperformanceonthefollowingcharacteristics,ingeneral.

Stage

Characteristicstobechecked

Storage

•Stabilityofproperty（viscosity,solderabilityandetc.）

Printing

•Finepitchprinting……fine（0.5mm）/superfinepitch（0.4mm）

•Stencillife

•Stencilidletime

•Rollingproperty

•Squeegeeseparation

•Printspeed（normalspeed20~40mm/sec.→highspeed200mm/sec.）

•Viscositychange

•Clogginginapertures

•Smearing

Mounting

•Tacktime

•Tackforce

•Slumpresistance

Reflow

•Bridge（shortcircuit）

•Solderbead

•Microsolderball

•Tombstoning

•Wettability

Inspection

•Visualcleanliness（fluxresidue）

•Incircuittesterprobetestability

Cleanability

（cleaningtypepastes）

•Visualcleanliness（fluxresidue）

•Ioniccontamination

Table1

Noone,includingKoki,hassucceededtodevelopasolderpastewhichperfectlyfulfillalltherequirements.

Thus,wehaveavarietyofproductline-uptobeabletoofferbestsuitedproductdependingonakeyrequirement（s）fromcustomers.

Thefollowingproductsarewhatwecall‘mainstreamproducts’:

∙SE（S）4-M953

∙SE（S）4-M953i

∙SE（S）48-M954

WenormallyrecommendM953ifortheinitialtrial,butwhentheprintingprocessislikelytobeoftenstoppedforsometime,youmayratherofferM953.Thisisbecause,asM953iisdesignedtobeveryslumpresistiveandthixotropicpropertyisquitehigh,acertaintimeperiodofsuspensionofprintingshallallowM953itorecoveritsviscosity/thixotropyandmakethepastestiffer,,and,asaresult,itmightcausesqueegeestickiness,lessstencilidletime.

Thebothsolderpasteshaveadvantagesanddisadvantages,itiscriticaltoverifyexactrequirementsandapplicationtoofferthebestsuitedproduct.

Inordertomeetspecificrequirements,particularlyhighspeedprinting,betterwetting,ICTprobability,thefollowingproductsareavailable.

∙HighspeedprintingsolderpasteSE（S）4-M850

∙PowerfulwettingsolderpasteSE（S）48-M1000

∙ICTtestablesolderpasteSE（S）48-M600

2.Compositionofsolderpastes

No-cleansolderpastesarebasicallycomposedofAlloyandFlux.

Thixotropic

materials

2.1Alloy

Therearefewdifferentwaysofmanufacturingsolderpowder,suchasgasatomizationandcentrifugalatomizationmethod.Kokimanufacturessolderpowdersbyatomizationmethod.

Solderpowder

recoverytank

Oxygendensimeter

Moltensolder

Featuresofthismethodare;

1）Suitabletomanufacturefineparticles.

Monitorcamera

2）Easytocontroloxidfilmformationonparticlesurface.

3）Lowoxidationlevelofsolderparticles.

Thisisapproximately5mhighand3mdiameterpowdermanufacturingtankwhichisfilledwithnitrogenwithverylowoxygendensity.

Ingotsolderismeltedinthesolderpotsituatedatthetopofthetankandmoltensolderisdrippedrightontoahighspeedspinningspindle.

Whensolderdropshitthespindle,itsplashestowardthewallofthetankandbeforeitreachesthewall,thesolderbecomessphericalandsolids.

Solderpowderobtainedatthisproductionstageisroughly1～100μm.Manyfactors,suchas

solderdrippingspeed,spindlespeed,oxygendensity,determinshape,particlediameter/distributionandoxygencontentdeterminedistributionofparticlesize.

Afterthisstage,solderpowderisbroughttoclassificationstages.

Kokihasadoptedtwodifferentpowderclassificationprocesses.Thefirstclassificationtakesplacebynitrogenblower.Atthisstage,utilizingdifferentweightofeachparticle,particlessmallerthanwhatisrequiredareremoved.Therestofthepowderismovedontoameshsievingtoeliminateparticlelargerthanwhatisrequired.

Aseriesofprocesstakesplaceundernitrogenblanket.

Kokibasicallyofferstwoparticletypes,either20~40μmor20~50μm.Theformarpowderismainlyforsuperfinepitchapplication（0.4mm/16mil）andthelatteroneisforordinarypatternincluding（0.5mm/20milpitch）.

Therearetwokeyissuesonsolderpowder.

Firstly,particledistributiondirectlyaffectsrheology/printabilityofsolderpaste,suchasrollingbehavior,separationfromstencil,slumpresistivityandetc.Properparticlesizeshallbeselectedaccordingtominimumpitchandpopulationdensityoffinepitchcomponents.

Forthepurposeofimprovingtheprintingperformance,weareevaluatingthefollowingnewpowders.

∙

Fig.2

Low

15~25μm

∙20~45μm

Secondly,oxidecontentshouldcarefullybecontrolledaslowaspossible.Inordertominimizetheoxidecontent,particlessmallerthan20μmmustbeaslowaspossible.Thisisbecausewhentheparticlesizedecreasestherelativesurfaceareaincreases.AsitisindicatedinFig.2,particlessmallerthan20μmhaveaquitehighoxidationlevel.

Iflargenumberofhighoxidecontentparticlesarecontainedinthesolderpowder,itcanturnsoutasmicrosolderballswhenreflowed.

Sinceavarietyofsolderpastesareavailabletoday,itisimportantthatarightpastebechosenfortheintendedapplication.Carefulattentionneedtobepaidtotheparticlesizedistributionandtherigidityoftheclassificationastheseseverelyaffectboththeprintabilityandsolderability.

2.2Fluxmedium

Inadditiontosolderparticles,fluxmediumisanothercriticalfactorforsolderpastes,andwhichhaslargeinfluenceoverentirepropertiesandperformancesofthesolderpaste.

Fundamentalrolesoffluxmediumareverysimilartothoseofwavesolderingflux,whichare;

1.Eliminationofoxidefilmofsubstrates

Thefluxchemicallymeltsandremovesoxidefilmformedonthesurfaceofelectricalcomponents,su