SMT锡膏详解Part 2.docx
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SMT锡膏详解Part2
3.1.4Solderpowder
Itiscommonlyunderstoodthatsolderparticleshapeandsizedistributionarequiteimportantfactorstosecuresuccessfulprintingoffinelinework.
Whensolderpastewasstartedtouse,‘irregular’shapesolderpowderwasquitepopular.
Thiswasbecauseitwaslessexpensivetoproduceandtherewerenofinepitchapplication.
Butastimewentby,alongwithdramaticinnovationonsemiconductortechnology,useoffinepitchapplicationhasbeenacceleratedfrom0.8→0.4mmorevensmaller.
Sphericalsolderparticle
Insuchfinelineapplication,improvementofprintingqualityisessentialtoreducesolderingdefects,andthen,sphericalsolderparticlehasreplacedthe‘irregular’shapedsolderparticlesasitprovidesbettertheology.
Thefinerthepatternpitchandstencilaperturebecome,thesmallertheparticlesizeisrequired.Asoneofreferencestodeterminesuitablesolderpowdersize,itisrecommendedthatthemaximumparticlediametershallbe1/4~1/5ofaperturewidth.
Untiltoday,Kokiofferstwopowdersizes,eitherSE(S)4-typepowder;20-50micronformainly>0.5mmpitchorSE(S)5-type;20-40micronfor0.4mmpitchapplication.
Inordertobothimproveprintingqualityandreduceproductioncosts(of20-40micronpowder),wehavestartedtoimplement20-45micronpowder,SE(S)48-typewithlatelydevelopedproducts,suchasSE(S)48-M855,SE(S)48-M1000-2.
Inthefuture,wewillhaveonlyonetypeofsolderpowderSE(S)48-fornormalprintingapplication.
Forsuperfinepatternapplication,suchasCSPbumping,flipchipsoldering,wehavenow15~25micronsolderpowder.
Becauseofitsrelativelyhighoxidecontent,itishighlyrecommendedtocombineitwithhalidecontainingflux.
3.1.5Fluxforsolderpaste
Propertiesoffluxmediumarealsoquitedeterminativeforprintabilityasasolderpaste.
Fluxmediumforsolderpasteisaverycomplexmaterialanddifferentpropertiesofsolidscontent,fluxcontent,thixotropy,viscosity,solventbehaviorandetc.synergizeeachother.
Optimizationofeachpropertyiscriticaltorealizequalityprinting.
Viscosityofoursolderpastesforordinaryprintingapplicationaregenerallyrangedfromapprox.1000to2600Ps.
ViscosityofKokisolderpastesaremeasuredbyMalcomviscometerModel:
PCU-200series.
Viscosityofsolderpasteshallhaveaninfluenceoveritsrollingbehavior,smearing,andetc.
Malcomvisocometer
Thixotropicagenthelpsmakethesolderpasteresistanttosharestressduringprintingprocessmakingcontinualprintingpossibleandrecoverviscosityafterthepasteisdepositedonsubstrates.
Thixotropicagentnormallyadoptediscalled‘wax’.
Itnotsimplymakesthepasteresistanttomechanicalstressbysqueegees,buthelpsreducefrictionbetweenthepasteandaperturewallsenablingsmoothreleaseofthepaste.
Solventevaporationrateshallberesponsibleforstencillifeandstencilidletimeatprintingstage.
Normally,1~4differentboilingpointsolvents(220~290°C)areformulatedinthepastemainlytocontroltacktimeandslumpproperty.Whenlowboilingpointsolventisused,inotherwords,whenfastevaporationratesolventisformulatedinapaste,thepastemaydryoutonthestencilinshortperiodoftime.
Or,iftheprintingenvironmenttemperatureisratherhigh,likeover30°C,assolventevaporatesveryquick,itstartssticktosqueegee,clogsinstencilaperturesandetc.
Moredetailsfollowat‘Workability’section.
3.2Workability
Acrucialfactor,inadditiontoprintability,solderabilityandreliabilitythatyoujustcannotneglectwhenselectingthebestsuitedsolderpasteforyourcustomer’sproductioniswhetherithasagoodworkability.
3.2.1Stencilidletime
Assubstratepatternbecomesincreasinglyfinerincluding0.4mmpitch,microBGA(0.3mmdia.)and0402chipcomponent,customersarepayingmoreattentiononthisfeature.
Definitionofstencilidletimeistheabilityofthesolderpastetobeleftidleonthestencilandthenrecover,withoutkneading,todeliveracceptableprints.Therheologicalpropertiesofthesolderpaste,anditsinherentabilitytotransitionfromits“atrest”viscositytoits“working”
viscosity,influenceasolderpaste’sabilitytorespondwelltoidletimesorpauses.
Weshallsaysolderpastewhichhasratherhighthixotropy,suchasSE(S)4-M953imayhaverelativelyshortstencilidletime,becausewhenitisleftatrestforsome30min.,thixotropymayrecoveranddeterioratesrheology,anditwouldresultinpoorprintingoffirst4~5printsafterprintingisresumedisresumed.
3.2.2Stencillife
Thelengthoftimethatasolderpastecanbeworkedonthestencilandstillmaintainitsproperties(printability,printdefinition,tack,etc.)followingmultipleprinting/idlecycles.
Factorswhichdeterminestencillifearemainlythixotropyandsolventevaporation.Forexample,ifasolderpasteisnotprovidedwithgoodthixotropy(Line1)andiskeptbeingprinted,viscositykeepsfallingandendsupwithlotsofsolderingdefectslikebridgingandsolderbeading.
Or,ifsolventusedforthesolderpasteisofquickevaporation(Line2),viscosityofthesolderpastebecomesexcessivelyhighandstartsstickingtosqueegeeandclogginginstencilapertures.
Stirring
Thus,Line3examplesideallongstencillifesolderpaste.Aviscosityvariation(A)incontinualprintingisquitesmall.
ThoughmostofKokisolderpasteshaverathersmallviscositydropincontinualprinting,itisstillhighlyrecommendedtocontrolambienttemperaturetobebelow25°Ctopreventexcessevaporationofsolvents.
3.2.3Tackiness
Tackinesscanbeclassifiedintotwoterms,tacktimeandtackforce.
Tacktimeislengthoftimethatasolderpastecanstaytackyenoughtoholdacomponentinpositionafterprinting.Tackforceindicatesstrengthoftackinessthatthesolderpasteholdsacomponent.
WedeterminetacktimeofoursolderpastesbyusingMalcommadeTackinesstester.
5depositsofsolderpasteareprintedonaceramicplatebyusing200μmthickness,6.5mmdiameteraperture.
Then,atestprobeispressedontoeachdepositfor0.2sec.withaloadof50ganditispulledbackupat100mm/sec.andtensilestrengthismeasured.
Byrepeatingthistest,wedeterminehowlongthepastestaysstickyenough(westandardizeminimumstickinessneededtoholdacomponentmorethan100gf.).
Pullupat100mm/sec.
MalcomTackinesstester
Whatlargelycontributetotacktimecharacteristicsissolvents.Tackinessitselfiscorrelatedwithstencillifeandslumpresistivityastheevaporationrateofsolventisthemaindetermination.Itisnormallydesiredtousehigherboilingpointsolventasitevaporatesatslowerrateandpreserveslongtacktime.It,,however,requiressomeconcernwhattypeofsolventtousebecauseit,inturn,tendstocauseslumpingofthesolderpastewhenhighboilingpointsolventisformulated.Solventswenormallyformulatesareofboilingpointrangedfrom230~300°C.Further,acombinationof2~3differentsolventsisquitnormalpracticetoobtainbestcompromiseintermsoftackperformance,viscosity,odor,compatibilitywiththerestofcompositions.
3.2.4Slumpresistivity
Therearemainlythreedifferentmodeofslump.
1.Shearslump:
Slumpwhichoccursovercontinualprintingbyshearstress.
2.Staticslump:
Slumpwhichoccursatroomtemperaturewithoutanyshearstress.
3.Heatslump:
Slumpwhichoccursduringheatingprocess,i.e.pre-heatingandreflowing.
Asthespacebetweenpadsisdecreasingduetothedemandforhigherpackagingdensity,slumppropertyofthesolderpasteisnowregardedoneofmostimportantfactorsforthefinepitchsolderpaste.
Incasethesolderpasteslumpsafterprintingandspreadsaroundpads,itcausesbridgingwithneighboringpads,microsolderballs,andetc.
Therearevariousfactorswhichshallaffectslumppropertyofsolderpastes,suchasfluxcontent,viscosity,softeningpropertyofrosinsandthixotropicagents,boilingpointofsolventsandetc.
Heatslumpresistivityisthemostcriticalfeatureamongtheabovethreemodesforqualitysolderingasmostofslumpingtendtooccurduringheatingprocessduetosofteningofflux.
Whenreflowing,thesolderpasteisexposedintemperatureasbelow.
Temperature(°C)
Peaktemp.
210~225°C5sec.
2500
Viscosity(Ps)
Time(sec.)
300
240
180
60
120
250
Viscosityof
solderpaste
Dropofviscosityof
solderpaste
Pre-heattemp.
140~170°C60~120sec.
Zonewhereslumptendstooccur.
Softeningpointof
solidsofflux.
Ramp-uptemp.
1.5~3.0°C/sec.
Asthetemperatureelevatesatfirstrampupstage,solidsmaterials(rosins,thixotropicagents,activators)softendramaticallyincombinationwithsolventleftintheflux.Then,inproportionwithevaporationofsolvent,theviscositystartsincreasing.
Generally,solderingdefects,suchasbridging,solderbeading,duetoslumpingoccuratthisstage.
Inordertoreduceslumping,itisnecessarytoselectpropercombinationofdifferentboilingpointsolventsandthixotropicagents.
Whenevaluatingslumpingpropertyofsolderpastes,wenormallyuseISO(orIPC)standardwhichisindicatedinTechnicalInformationofoursolderpastes.
3.3Goodsolderabilty
3.3.1Wettability
Inproductionprocess,repairofproductsgetsmoredifficultandcostlywhendefectshappenatdownstreamoftheproductionline.Poorwettingornonwettingisverycriticaldefectasitisn