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ReflowsolderingforFine-PitchComponentsCriticalfactorsinreflowingsoldering•1Theheattransferfromaheatingsource.•2.Theheatabsorptionrateanduniformitybyassembly.•3.Thesolderabilityofthemetalstobejoined.•4.Themeltingpointofthesolder.•5.Theheattoleranceofthematerialontheassembly.•6.Thefluxchemistryandthecleanlinessofthemetalstojoined.DynamicsoftheReflowProcess•PhaseOne-SolventEvaporation•PhaseTwo-FluxActivation(OxideReduction)•PhaseThree-SolderMelt•PhaseFour-CompleteSolderMelt•PhaseFive-CoolDownTemperatureandtimeprofileTin/Leadsolderoxidegrowthrates-thicknessvstimeIncreasingFluxRateProbabilityofICpackagecrackTemperatureandtimeprofileTheHeatSources•1.Infra-red(includingIRandIRmixedwithconvection.•2.Vaporphase•3.Convection-hotgas•4.Laser•5.Thermosonic•Infra-redreflow–Theinfra-redheattransferisdonebyemittinglightfromtheinfra-redregionofthespectrum.IRemissionisthathavingwavelengthof0.72micronsto1000microns–0.72-2.5micron,shortIR–0.25-5.0micron,mediumIR–longerthan5.0micron,longwaveIR•TheIRemittertemperaturedeterminestheemittedwavelengthofIRradiation•Thehigherthetemperature,theshortertheemittedwavelength.•ThetemperatureofthePCBAisafunctionoftheabsorbitivityoftheassembly.•TheabsorbitivityisIRwavelengthandmaterialmassdependentiftheassemblysurfaceisnonlinearandhasmulti-colourbodies.–q=oAFEFa(TS-TA)–q=Theaverageradiativeheattransferfromthesourcetotheassembly–FE=Theemissivityfactorforthesourceandproduct–Fa=Theconfigurationorgeometricviewfactorofthepcba–o=TheStefan-Boltzmanconstant–A=Areabeingheated–TA=Temperatureoftheareabeingheated–TS=Temperatureofthesource–Infra-redreflow•InthecommonlyusedIRsystem,thePCBSabsorbsheatradiatedfromthesurroundingheatedair.ThisthermalenergyisabsorbedbythePCBAsbeforesolderingoccurs.•Advantages:–Precisecontrolofreflowtemperature–Fasterenergytransferthanconvection–BetterradiatedenergypenetrationIR-Reflow•Disadvantages:–Temperatureisthermalmassdependent–Colourselectivity–Shadowingeffect•IRreflowoptimization–Componentareplacedwithuniformmassdistribution.–Locatelargercomponentsnearthecornersoredges–AdjustthebeltspeedSchematicviewofanin-lineIRreflowsolderingsystemTypicaltemperatureprofileforarea-sourceIRreflowsystemT-3tungstenlampT-3quartzlampatfullandhalfratedpowerDual-emitterT-3lampsystemEmissionspectraofdual-emitterT-3lampsystemPanelIRemitterComparisonofemittertypes•heattransfer•Temperatureadjust•Coloursensitivity•shadoweffect•tempuniformity•timetostabilizeafterchange•heattransferby–radiation–convection•LampPanel•highlow•fastslow•yesless•yesless•fairbetter•shorterlonger•80-90%20-40%•10-20%60-80%LightAbsorptionandReflectionPropertiesformaterials•Vapourphasereflowsoldering•Whenmostfluidsareheatedtotheirboilingpoint,theyexistintwophase,vaporandliquid.•Inthevaporphase,theseexistasaboilingfluidandasaturatedvaporatthetemperatureoftheboilingfluid.•Ifanassemblyatalowertemperatureisplacedinthesaturatedvapor,thevaporimmediatelycondenseontheassemblyandtransferlatentheatextremelyrapidlytotheassembly.Therateheattransferis:•q=hAA(TS-TA)•whereq=theheattransfertotheassembly•h=thevaporheattransfercoefficient(varies)•TS=thesaturatedvaportemperature(constant)•TA=theassemblytemperature(varies)•AA=theassemblyarea.VapourPhaseReflowSoldering•Vapourphasesolderingorcondensationsolderingisbytransferringthelatentheatofaboilingvapourtothesurfacesuponwhichitcondenses.Itisasurfaceheatingphenomena.Objectsinthereflowenvironmentareheatedonthesurfacefirst.•Advantages–Rapidrateofheattransfer(typical6to10degC/sec–Inertatmosphere–wellcontrolledmaximumtemperatureVapourPhaseReflowSoldering•Advantages:–Insensitivetovariationstoboardthermalmass–Insensitivetovariationinboardgeometry•Disadvantages–Highoperationcost(FC-70)–Longheatuptime–Littlecontrolovertemperatureprofile–Unfavourabletolowthermalmasspart(solderwickingDual-vapourreflowsystemBatchtypeproductionIn-linesingle-vaporsolderingsystemBoilingTemp.CdegreeDensityg/mlMfgr’sDesignationManufacturer1742152152302532601.882.032.011.821.901.84FC-43FC-5311APF–215LS/230FC-71HS./2603M3MAirproductsGalden3MGaldenConvectionreflowsoldering•Theprocessofheattransferfromahotgasflowingoveracoolersurfaceiscalledconvectionheating.•Ifthegascirculationiscausedbydifferentdensitiesofmassesbeingheatedthenthetermfreeconvectionisapplied.•Ifgascirculationisdonebymechanicalmeanssuchasafanorpumpthenitistermedforcedconvection•q=hcAS(TG-TS)•q=theheattransferratefromthegastothesurface•hc=thermalconvectionconductancebasedonfluiddynamics•As=surface(theassembly)area•TG=thegastemperaturenearthesurface•TS=thesurface(assembly)temperatureConvectionovenclassification•SMEMAclassifiestheconvectionsystemsclassIIIandthemixedconvectionandIRsystemsasclassIIAirflowinaconvectiondominantreflowsystemComparisonofheatflowtransferNitrogenReflowSoldering•Reasonstousenitrogeninreflowsoldering:–1)Morefinepitchpackageused–2)theincreasinginuseoflowactivityfluxes•Advantage:–canreducesolderball–avoidthefluxfromhardeningintheabsenceofoxygenHotbarandthermodebonding•Hotbarbondingusesmetalbarsthatcontactalltheleadsonapackagesimultaneously.•Thechoice