Solder paste general informationPart 2.docx

Solder paste general informationPart 2.docx

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SolderpastegeneralinformationPart2

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.Poorwettingornonwettingisveryc