Reflow soldering profiles - CompuPhase

The standard reflow profile has four zones: preheat, soak, reflow and cooling. The profile describes the ideal temperature curve of the top ... ReflowsolderingprofilesSkiptomaincontent(skipnavigationmenu) Youarehere: Home> Articles> Electronics> Reflowsolderingprofiles Navigation: Home Projects Products Documentation Shop Overview Publications Applicationnotes USB&WirelessButtons Audioplayers/controllers 2DAnimation(EGI&AniSprite) EGOforWindows Programming Desktopapplications Embeddedsystems Computergraphics Miscellaneous Miscellaneous Reflowsolderingprofiles  Theseguidelinesapplyspecificallyfor"batch"ovens(alsocalledbench-top ovensorfurnace-styleovens).Thetemperatureprofileisinpartmandatedbythe (mechanical)designoftheoven,andbatchovenshavearadicallydifferent designthanconveyorbeltovens. Inparticular,theseguidelinesapplytothe“eC-reflow-mate” reflowovenfromEurocircuits. Aparticulardifferenceistheinfluenceofthermalinertia.Thermalinertia existsintheovenitselfandinthePCB(andthecomponents).Inaconveyor beltoven,thePCBphysicallymovesthroughthedifferentheatingzonesinthe oven,andtherefore,thethermalinertiaoftheseheatingzoneshavelittle implicationonthePCB(onceaPCBisoutoftheoven,itdoesnotmatterany morehowlongtheovenstayshot). Inaconveyorbeltoven,thedistancebetweentheheatingzonesisfixedand theconveyorbeltrunsthroughallzones—andtherefore,thePCBsmove throughallzoneswiththesamespeed.Thedifficultyofcalibratingaprofile foraconveyorbeltovenisthatachangeintheparameterforonezonetypically influencesthecharacteristicsoftheotherzones.Incomparison,abatchoven allowsforsomeflexibilityintheprofiles.Thatsaid,becauseoftherelatively largethermalinertia,theactualtemperaturecurvedeviatesfromthe (theoretical)profile. Thesolution,then,istoanticipatethethermalinertiaintheprofile.The knowledgerequiredtomakethesedecisionsmustbeobtainedwithactual measurements. Characteristicsofthe“eC-reflow-mate” TheEurocircuitseC-reflow-mate isabatchovenwithquartzinfra-redheatersonthetopandbottomplusconvection (controlledairflow).Theovenhastwofloatingtemperaturesensors(topandbottom) andaprovisionforonesensortoclamp/attachontothePCB(the“external” sensor). Eurocircuitsdocumentsthattheinfra-redlampstake20to30secondstoheatup tothedesiredtemperature.Theovenfirmwareswitchestheinfra-redlampsoff 8 °Cbeforereachingthedesiredtemperature,tocompensateforthe thermalinertiaofthelamps. Themaximumtemperatureramp(forheatingup)isroughly1.5 °C/s.The cooldowntemperaturerampfortheovenisroughly0.8 °C/s,butthe PCBcoolsdownslower(duetothermalinertia).Atapre-settemperature,the ovendooropensforacceleratedcooling(freeaircooling). ThePCBtemperaturesensor(the“external”sensor)isaK-type thermocouplewithsemi-rigidwires.ThesensorcanbetapedontoaPCB,but shouldnotbesoldered. PCBframeversusgridplateversusPCBspacers Insomereflowovens,thePCBsaresnappedinaframethattouchesthePCBonly ontheedges.Thepurposeoftheframeistominimizethecontactareabetween thePCBandtheoven,toavoidheatfromthePCBtoflowaway.Otherreflowovens haveagridplateontowhichyouplacethePCBs.Thedisadvantageofagridis thatitletsheatflowawayfromthePCB,anadvantageisthatheatisspreadmore evenlyacrossthebottomofthePCB. BacktopagePCBspacers(stand-offs) TheeC-reflow-matehasagridplateandcomeswithasetofspacers.When usingthespacers,theeC-reflow-matehasthesamecharacteristicsofanoven thatclampsthePCBinaframe.Originally,thespacerswereof(bluecoated) aluminium;morerecently,Eurocircuitsprovidesspacersfroma(white)synthetic compound,tofurtherimprovethethermalinsulation.APCBframeisavailableas anoptionfortheeC-reflow-mate. Asideeffectofusingspacersonagrid(orusingaframe)isthatthePCB coolsdownslower.Sincecoolingistypicallyalreadyslowinabatchoven,this shouldbecompensatedintheprofile. Obviously,whenaPCBhascomponentsonbothsides,youhavenootheroptionthan tousespacersoraframe. PositioningthePCBtemperaturesensor Manybatchreflowovensuseathermocouplesensortomeasurethetemperatureof thePCB.Thissensorshouldmeasurethetemperatureoftherelevantcold spotonthePCB.Ideally,therefore,thesensorissolderedonthegroundpinof alargecomponent,orclampedinsideavianearthisgroundpin. ThecoldestspotonaPCBneednotbeaspotwherecomponentsaresoldered.For example,alargecopperarea(e.g.agroundzone)butotherwiseunpopulated, maywellbethetrue"coldspot"ofthePCB.However,sincetherearenocomponents atthatvacantspottobesoldered,itstemperaturedoesnotmatter.The relevantcoldspotonthePCBisthecoldestspotat(ornear)asolder joint. Notallthermocoupleswithstandsoldering,andevenwhenusinganappropriate solder-resistingthermo­couple,itistypicallyimpossible—oratleast impractical,tosolderthetemperaturesensoronthePCBafterthepick&place process,ortofindaviathatisnearby,withaholethathastherightsize, andthatisnotcoveredwithsolder-resist. Inpractice,thesensoristaped(orclamped)onavacantspotonthePCB.The bestspottotapeorclampthesensorisstillnearacoldspot,whichwouldbe onagroundtrace/zonenearalargecomponent.DenselypopulatedareasonthePCB arealsocolderthanunpopulatedareas. Solderpasteproperties Commonsolderisanalloyfrommetalstin,silver,leadandcopper.The meltingpointsofthesemetalsare: MetalMeltingtemperature tin(Sn)232 °C silver(Ag)961.8 °C copper(Cu)1,084.6 °C lead(Pb)327.5 °C bismuth(Bi)271 °C Thealloys,however,havelowermeltingtemperaturesthanthepuremetals: AlloyMeltingtemperature Sn63Pb37183 °C Sn62Pb36Ag2179 °C Sn96.5Ag3Cu0.5(SAC305)217–219 °C Sn96Ag4221 °C Sn92.5Ag3.5Cu1Bi3213 °C Sn42Bi58138 °C Forsolderingwithlead,theSn63Pb37alloyisalmostuniversallyused.For lead-freesoldering,thealloySn96.5Ag3Cu0.5isthemostcommon.Thealloyis alsoknownastheabbreviationSAC305. Puremetalshaveasharptransitionfromsolidtoliquid:belowthemeltingpoint, thematerialissolid,abovethemeltingpoint,itisliquid.Analloymayhave a“plasticrange”wherethematerialisbetweensolidandliquid phases.Alloyswithameltingpointinsteadofameltingrange arecalledeutecticalloys.TheSn63Pb37alloyisaneutecticalloy. The“shelflife”ofsolderpasteisgenerally6to9months.Pots ofsolderpasteshouldbekeptinacoolstorage(1 °Cto6 °C),but allowedtoacclimatetoroomtemperaturebeforeuse. Solderpasteisusuallyappliedviaastencil—typicallyfromlaser-cut stainlesssteelsheets.Thethicknessofthestencilischosendependingonthe densityclassofthecomponentsandthesurfacefinishofthePCB.Forultra-fine pitchcomponents(pitch≤0.4mm),stencilscanbe0.10mm(0.004inch)thick, whileforcomponentswithapitch≥1.27mm,0.2mm(0.008inch)isappropriate. PCBswithaHALfinish(“hotairleveled”)requirelesssolderpaste thansilverorgoldplatedfinishes—andthereforethestencilcanbe thinner.Withamixofcomponentsoffineandcoarsepitch,onecanchooseathick stencil,butmaketheaperturesforthefine-pitchcomponentsupto20%smaller thanthepadsizes. Profilecharacteristics Thestandardreflowprofilehasfourzones:preheat,soak,reflowandcooling. TheprofiledescribestheidealtemperaturecurveofthetoplayerofthePCB. BacktopageStandardreflowsolderingprofile Thepreheatingzoneshouldincreasethetemperatureatamaximumrateof 3 °C/s.Thepurposeofpreheatingistoallowthesolventstoevaporate andtoactivatetheflux. Thesoakzonebringsthetemperatureofallcomponentsandboardareastoan equallevel.Duetodifferencesinthermalinertia,componentsdonotheatupat thesamespeed—thisisespeciallythecasewithinfra-redheating,due tounevenabsorptionofinfra-redenergybycomponents(dependingoncolourand reflectivity). Inthereflowzone,thetemperaturerisesatarateofapproximately2 °C toatemperatureabovethemeltingpoint(thetemperatureaboveliquidus, orTAL).Thepeaktemperatureforthereflowzoneisatleast25 °C aboveTAL,becausesolderbothcoalescesbetterwiththecopperandwets thepadsandcomponentpinsbetterwhenitishotter—therebycreating betterjoints.Thisisespeciallyimportantforlead-freesolders(SAC305), becausethissolderislesseffectiveatwettingthanleadedsolders. TheTAL(temperatureaboveliquidus)shouldbeheldfor40to80seconds, sothatanintermetalliccompound(IMC)isabletoform.Notethattheduration ofthereflowzoneisshorterthantheTALperiod,becausethetemperatureisstill aboveliquidusatthestartofthecoolingzone. BacktopageDecisionpointsinaprofile Thecoolingzoneideallyhasarampof4 °C/s(tocooldown quickly,butavoidthermalstress).Freeaircoolingissufficient. Thefluxinthesolderpastegetsactivatedatapproximately100 °C.After itisactivated,theTALshouldbereachedwithinroughly3minutes—or lessforno-cleansolderpaste,asasthesepasteshavelessactiveflux. Exceedingthattimedeterioratestheeffectivenessoftheflux.Inotherwords, thefluxinthesolderpastesetsanupperboundonthedurationofthesoakzone. Refiningthesoakzone BacktopageProfilewithaslantedsoakphase Forlead-freereflowsoldering,adisadvantageofthestandardprofileisthe relativeheightofthepeakofthereflowzone.Withthesoakzoneat150 °C andthereflowzoneat245 °C(typicalvalueforlead-freereflowsoldering), thetemperaturerisefromsoakzonetoreflowzoneis95 °C.Dueto thermalinertia,suchalargequicktemperaturerisemay,again,leadto temperaturedifferencesacrossthePCB.Ratherthanincreasingthetemperature ofthesoakzoneonthewhole,analternativeprofileistomakethesoakzone temperatureaslowrampfrom150 °Cto180 °C.Thelower relativepeakofthereflowzoneresultsinamoreequalizedtemperatureacross thePCBandcomponents. Aswrittenabove,youmaywanttoreducethedurationofthesoakzone,especially whenusingno-cleansolderpaste(no-cleanpastehaslessactiveflux).The optimaldurationofthesoakzonealsodependsonthePCB,thereforesome experimentationisrecommended. Refiningthereflowzone Duringreflow,an“intermetalliccompound”formswherethetin touchesthepadsonthePCB.Theformingofthisintermetalliccompound(IMC), whichconsistsprimarilyofcopperandtin,isarequirementforagoodsolder joint;thespeedatwhichitformsisafunctionofthetemperature. AlthoughtheformingoftheIMCisarequirementforagoodsolderjoint, excessivegrowthshouldbeavoided.Thecopper/tinintermetalliccompoundis brittleandthereforethelargeritgrows,thegreatertherisksforcrackswhen thejointispointunderstress—suchasstressduetotherepeated expansionandcontractionofcomponentsthatwarmupandcooldownduringtheir operationallife). For(lead-free)reflowsoldering,theadvisedsolderingtemperatureis245 °C, whichiswellbelowtheadvisedtemperatureofasolderingironforhandsoldering. Therationaleforthisadviseisthattheintermetalliccompoundgrowsmoreslowly atlowertemperatures,andthereforeavoidsexcessivegrowth.If,foraparticular PCB,itturnsoutthatthecoldspotshavenotbeensufficientlyheatedfora properreflow,theadviseistolengthenthereflowzone(butkeepthepeak temperatureat245 °C). Asanaside:withhandsoldering(usingasolderingiron),the“timeabove liquidus”(TAL)ofthesolderismuchshorterthanduringreflow,and thereforeahighersolderingtemperatureisadmissible. WiththeeC-reflow-mate,thereflowdurationcanbesetintwoways:eitherby adjustingthepointsintheprofilecurve,orbyenablingandsettingthe “reflowholdtime”option.Withbothmethods,theovenstartsholding thetemperaturewhenitreachesthereflowtemperature(typicallysetat 245 °C);thedifferencebetweenthetwooptionsisthestartofthe cool-downphase.Whenusingtheprofile,thecool-downphasestartsatthetime intheprofile;whenusingthe“reflowholdtime”option,cool-down startsthespecifiednumberofsecondssincefirstreachingthereflowtemperature. Thereflowholdoptionisofteneasiertouseanditgivesbettercontrolover thetotalreflowperiod. Refiningthecoolzone BacktopageCooldownphase Aswithmanybatchovens,theeC-reflow-matehasaslowcool-downtemperature ramp.Tocooldownmorequickly,theovenopensthedooratsomepre-settemperature; thistemperatureshouldbechosensothatthesolderisguaranteedtohavesolidified. Thedefaulttemperaturesettingforthisis190 °C(forleadfreesoldering); thisisonthesafeside,formanyboardsthedoor-opentemperaturecanbeincreased to195–200 °C. Thetemperaturerampforcoolingdownislargelydictatedbythermalinertiaand thedecisiontoopenthedoorisdeterminedbythePCBtemperature(thehighest valuemeasuredbythethreesensorsisusedforthedecisionwhethertoopenthe door).Therefore,thetimesetforcoolingdownisbarelyrelevant.Itiscommon practicetosetthecool-downperioduptoopeningthedoortobejust1second. Therampforcoolingdownwiththedooropenisirrelevantaswell:thePCBis subjecttofreeaircooling. Bottomheating TheeC-reflow-matehasheatersonthetopandthebottomandtheseareindividually controlled.Thebottomheaterhelpstoreducethedifferencebetweenhotspots andcoldspotsonthePCB.ForPCBswithcomponentsonlyonasingleside,the bottomtemperaturecanbemadeequaltothetoptemperature(“Follow 0 °C”).ForPCBswithcomponentsonbothsides,thebottomheater caneitherbesettofollowthetopheaterat-40 °C,ortobesetto afixedtemperatureofapproximately180 °C. Profilerecommendations Thetablebelowisastartingpointforaprofileforabatchoven.Theoptimal profilealsodependsonthePCB,andthereforethesuitabilityofaprofilemust beverifiedwithactualmeasurements. ZoneLead(Sn63Pb37)Lead-free(SAC305) Preheatto150 °Cin60 sto150 °Cin60 s Soakfrom150 °Cto165 °Cin120 sfrom150 °Cto180 °Cin120 s ReflowPeaktemperature225 °Cto235 °C,holdfor20 sPeaktemperature245 °Cto255 °C,holdfor15 s Cooling-4 °C/sorfree-aircooling-4 °C/sorfree-aircooling Itshouldbeverified(bymeasurements)thatthepeaktemperaturedoesnotexceed thespecificationsofthecomponents(inbothtemperatureandtheperiodatthis temperature),andthatthedurationoftemperatureaboveliquidusisin a40 sto80 swindow. Idealprofileversusmeasuredprofile Togiveanindicationofatemperatureprofileinpractice,hereisascreenshot ofthesoftwarefortheeC-reflow-mate,withtheprescribedprofilesuperimposed. Inthisgraphic,theredcurverepresentsasensorthatfloatsabovethePCB,the yellowcurveisforthesensorthefloatsbelowthePCB,thegreencurverepresent asensorthattouchesthePCBandthebluecurveistheprescribedprofile. ClearlyvisibleistheeffectofthermalinertiaandhowtheeC-reflow-mate compensatesforitbyovershootingthetargetcurve(seeforexampletheyellow curvenear1:30fromthestart).AlsovisibleishowthePCBisstill heatingupwhiletheovenitselfisalreadycoolingdown(seethegreencurvenear 4:15fromthestart).ThePCBcoolsdownslowly,untilthedooropens—which happenswhenthegreencurvereaches200 °C. BacktopagePCBspacers(stand-offs) ThecurvescorrespondtoaPCBwithasizeof100×150mmthatwasputon PCBspacersinthemiddleoftheoven(seetheimageat theright).Thebottomheatingissettofollowthetopheating(“Follow 0 °C”). Closingremarks DifferentkindsofPCBsneeddifferentprofiles.Thisapplicationnoteassumes PCBsfromcommonFR4material.Forpowerelectronics,andnotablypowerLEDs, PCBsbasedon“InsulatedMetalSubstrate”(IMS)areoftenused. IMSconsistsofametalcarrier(usuallyaluminium)withathinepoxy-based layergluedontopofitandanetchedcopperlayerforthecircuitontheepoxy layer.Bynecessity,IMSPCBsaresingle-sidedcircuitswithonlytracesonthe toplayer(the“component”layer),andtheyareonlysuitablefor surface-mountcomponents(SMDs).Forthesolderprofile,themaincharacteristic ofIMSisthataluminiumhaslowthermalinertiaandgoodthermalconductivity. Thetemperaturedifferencesareminimalacrosstheboard(nohotspotsversus coldspots).Thesoakzonecanthereforebeshortened(orberemovedaltogether). Mostreflowovenscanrunasanindependent,self-containedunit.Allreflow ovensthatweareawareof,canbemonitoredwithacomputer.Weadviseto monitorthetemperaturecurvesandtochecktheseagainsttheprofile —evenifnoproblematicsolderjointshavesurfaced.Wealsoadviseyou tokeeplogsofthereflowruns,bothforsuccessfulandunsuccessfulruns,so thatthesecanbecomparedandanalysed.Forexample,followinguponthemeasured graph(picturedabove),wesubsequentlycompensatedfortheslowcool-downafter reflowbyreducingthereflowtimeintheprofilefrom30secondsto 10seconds(sincethemeasuredtimeaboveliquidusturnedouttobe easily30secondslongerthantheprescribedreflowtime—forthisPCB). Furtherreading Lee,Dr.Ning-Chen,OptimizingReflowProgileviaDefectMechanismsAnalysis, SolderingandSurfaceMountTechnology,vol.11nr.1,1999. Asurveyoftheeffectsoftemperatureanddurationofvariouszonesonthe solderabilityandthequalityofsolderjoints.Theconclusionsareonly partiallyrelevantfortheeC-reflow-mate,becausethepaperassumestheuse offorcedconvectionovens(AlthoughtheeC-reflow-matehasaventilatortocreate airflow,itbehavesprimarilyasaninfra-redoven.). Peo,Marc,HowChallengingConventionalWisdomCanOptimizeSolderReflow, SurfaceMountTechnologyMagazine(PennwellCorp.),May1998. Thisarticlefocusesonsixconventionalwisdoms.Itdiscussesthe newperspectivesbroughtbymodernreflowovensandmodernsolderpastes. J-STD-020D.1,JEDEC,March2008 AjointIPC/JEDECstandardonthemoisture/reflowclassificationofSMDdevices. Thoughoftcited,notethatthestandardprescribestheprofileinsection5.6 forthepurposeofclassificationofcomponents.Itisnotnecessarily suitableasaproductionprofile.Inparticular,therecommendedhold timeofthetemperatureaboveliquidusof60to150seconds,isfar longerthanotherrecommendations. Copyright©2018–2022,ThiadmerRiemersma,CompuPhaseUpdated:2018-03-03Thisworkislicensedundera CreativeCommonsLicense