Reflow soldering - Wikipedia

The reflow oven temperature profile is suited for characteristics of a particular circuit board assembly, the size and depth of the ground plane layer ... Reflowsoldering FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Attachmentofelectroniccomponents "Reflow"redirectshere.Forthetypographicalfeature,seeReflowabledocument. Thisarticleneedsadditionalcitationsforverification.Pleasehelpimprovethisarticlebyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.Findsources: "Reflowsoldering" – news ·newspapers ·books ·scholar ·JSTOR(December2010)(Learnhowandwhentoremovethistemplatemessage) ExampleRamptoSpikethermalprofile. Exampleofreflowsolderingthermalprofile. Reflowsolderingisaprocessinwhichasolderpaste(astickymixtureofpowderedsolderandflux)isusedtotemporarilyattachoneorthousandsoftinyelectricalcomponentstotheircontactpads,afterwhichtheentireassemblyissubjectedtocontrolledheat.Thesolderpastereflowsinamoltenstate,creatingpermanentsolderjoints.Heatingmaybeaccomplishedbypassingtheassemblythroughareflowoven,underaninfraredlamp,or(unconventionally)bysolderingindividualjointswithadesolderinghotairpencil. ReflowsolderingwithlongindustrialconvectionovensisthepreferredmethodofsolderingsurfacemounttechnologycomponentsorSMTtoaprintedcircuitboardorPCB.Eachsegmentoftheovenhasaregulatedtemperature,accordingtothespecificthermalrequirementsofeachassembly.Reflowovensmeantspecificallyforthesolderingofsurfacemountcomponentsmayalsobeusedforthrough-holecomponentsbyfillingtheholeswithsolderpasteandinsertingthecomponentleadsthroughthepaste.Wavesolderinghowever,hasbeenthecommonmethodofsolderingmulti-leadedthrough-holecomponentsontoacircuitboarddesignedforsurface-mountcomponents. WhenusedonboardscontainingamixofSMTandplatedthrough-hole(PTH)components,through-holereflow,whenachievablebyspecificallymodifiedpastestencils,mayallowforthewavesolderingsteptobeeliminatedfromtheassemblyprocess,potentiallyreducingassemblycosts.Whilethismaybesaidoflead-tinsolderpastesusedpreviously,lead-freesolderalloyssuchasSACpresentachallengeintermsofthelimitsofoventemperatureprofileadjustmentandrequirementsofspecializedthrough-holecomponentsthatmustbehandsolderedwithsolderwireorcannotreasonablywithstandthehightemperaturesdirectedatcircuitboardsastheytravelontheconveyorofthereflowoven.Thereflowsolderingofthrough-holecomponentsusingsolderpasteinaconvectionovenprocessiscalledintrusivesoldering. Thegoalofthereflowprocessisforthesolderpastetoreachtheeutectictemperatureatwhichtheparticularsolderalloyundergoesaphasechangetoaliquidormoltenstate.Atthisspecifictemperaturerange,themoltenalloydemonstratespropertiesofadhesion.Moltensolderalloybehavesmuchaswater,withpropertiesofcohesionandadhesion.Withsufficientflux,inthestateofliquidus,moltensolderalloyswillexhibitacharacteristiccalled"wetting." Wettingisapropertyofthealloywhenwithinitsspecificeutectictemperaturerange.Wettingisanecessaryconditionfortheformationofsolderjointsthatmeetthecriteriaas"acceptable"or"target"conditions,while"non-conforming"isconsidereddefectiveaccordingtoIPC. Thereflowoventemperatureprofileissuitedforcharacteristicsofaparticularcircuitboardassembly,thesizeanddepthofthegroundplanelayerwithintheboard,thenumberoflayerswithintheboard,thenumberandsizeofthecomponents,forexample.Thetemperatureprofileforaparticularcircuitboardwillallowforreflowofsolderontotheadjoiningsurfaces,withoutoverheatinganddamagingtheelectricalcomponentsbeyondtheirtemperaturetolerance.Intheconventionalreflowsolderingprocess,thereareusuallyfourstages,called"zones",eachhavingadistinctthermalprofile:preheat,thermalsoak(oftenshortenedtojustsoak),reflow,andcooling. Contents 1Preheatzone 2Thermalsoakzone 3Reflowzone 4Coolingzone 5Etymology 6Thermalprofiling 7Seealso 8References Preheatzone[edit] Preheatisthefirststageofthereflowprocess.Duringthisreflowphase,theentireboardassemblyclimbstowardsatargetsoakordwelltemperature.Themaingoalofthepreheatphaseistogettheentireassemblysafelyandconsistentlytoasoakorpre-reflowtemperature.Preheatisalsoanopportunityforvolatilesolventsinthesolderpastetooutgas.Forpastesolventstobeproperlyexpelledandtheassemblytosafelyreachpre-reflowtemperaturesthePCBmustbeheatedinaconsistent,linearmanner.Animportantmetricforthefirstphaseofthereflowprocessisthetemperaturesloperateorrisevstime.ThisisoftenmeasuredindegreesCelsiuspersecond,C/s.Manyvariablesfactorintoamanufacturer'stargetsloperate.Theseinclude:targetprocessingtime,solderpastevolatility,andcomponentconsiderations.Itisimportanttoaccountforalltheseprocessvariables,butinmostcasessensitivecomponentconsiderationsareparamount. “Manycomponentswillcrackiftheirtemperatureischangedtooquickly.Themaximumrateofthermalchangethatthemostsensitivecomponentscanwithstandbecomesthemaximumallowableslope”.However,ifthermallysensitivecomponentsarenotinuseandmaximizingthroughputisofgreatconcern,aggressivesloperatesmaybetailoredtoimproveprocessingtime.Forthisreason,manymanufacturerspushthesesloperatesuptothemaximumcommonallowablerateof3.0 °C/Second.Conversely,ifasolderpastecontainingparticularlystrongsolventsisbeingused,heatingtheassemblytoofastcaneasilycreateanoutofcontrolprocess.Asthevolatilesolventsoutgastheymaysplattersolderoffthepadsandontotheboard.Solder-ballingisthemainconcernofviolentoutgassingduringthepreheatphase.Onceaboardhasbeenrampeduptotemperatureinthepreheatphaseitistimetoenterthesoakorpre-reflowphase. Thermalsoakzone[edit] Thesecondsection,thermalsoak,istypicallya60to120secondexposureforremovalofsolderpastevolatilesandactivationofthefluxes,wherethefluxcomponentsbeginoxidereductiononcomponentleadsandpads.Toohighatemperaturecanleadtosolderspatteringorballingaswellasoxidationofthepaste,theattachmentpadsandthecomponentterminations.Similarly,fluxesmaynotfullyactivateifthetemperatureistoolow.Attheendofthesoakzoneathermalequilibriumoftheentireassemblyisdesiredjustbeforethereflowzone.AsoakprofileissuggestedtodecreaseanydeltaTbetweencomponentsofvaryingsizesorifthePCBassemblyisverylarge.Asoakprofileisalsorecommendedtodiminishvoidinginareaarraytypepackages.[1] Reflowzone[edit] Thissectionneedsadditionalcitationsforverification.Pleasehelpimprovethisarticlebyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.(September2012)(Learnhowandwhentoremovethistemplatemessage) Thethirdsection,thereflowzone,isalsoreferredtoasthe“timeabovereflow”or“temperatureaboveliquidus”(TAL),andisthepartoftheprocesswherethemaximumtemperatureisreached.Animportantconsiderationispeaktemperature,whichisthemaximumallowabletemperatureoftheentireprocess.Acommonpeaktemperatureis20–40 °Caboveliquidus.[1]Thislimitisdeterminedbythecomponentontheassemblywiththelowesttoleranceforhightemperatures(thecomponentmostsusceptibletothermaldamage).Astandardguidelineistosubtract5 °Cfromthemaximumtemperaturethatthemostvulnerablecomponentcansustaintoarriveatthemaximumtemperatureforprocess.Itisimportanttomonitortheprocesstemperaturetokeepitfromexceedingthislimit.Additionally,hightemperatures(beyond260 °C)maycausedamagetotheinternaldiesofSMTcomponentsaswellasfosterintermetallicgrowth.Conversely,atemperaturethatisn’thotenoughmaypreventthepastefromreflowingadequately. Anexampleofacommercialreflowoven. [2] Exampleofamodernthermalprofiler Timeaboveliquidus(TAL),ortimeabovereflow,measureshowlongthesolderisaliquid.Thefluxreducessurfacetensionatthejunctureofthemetalstoaccomplishmetallurgicalbonding,allowingtheindividualsolderpowderspherestocombine.Iftheprofiletimeexceedsthemanufacturer’sspecification,theresultmaybeprematurefluxactivationorconsumption,effectively“drying”thepastebeforeformationofthesolderjoint.Aninsufficienttime/temperaturerelationshipcausesadecreaseintheflux’scleaningaction,resultinginpoorwetting,inadequateremovalofthesolventandflux,andpossiblydefectivesolderjoints.ExpertsusuallyrecommendtheshortestTALpossible,however,mostpastesspecifyaminimumTALof30seconds,althoughthereappearstobenoclearreasonforthatspecifictime.OnepossibilityisthatthereareplacesonthePCBthatarenotmeasuredduringprofiling,andtherefore,settingtheminimumallowabletimeto30secondsreducesthechancesofanunmeasuredareanotreflowing.Ahighminimumreflowtimealsoprovidesamarginofsafetyagainstoventemperaturechanges.Thewettingtimeideallystaysbelow60 secondsaboveliquidus.Additionaltimeaboveliquidusmaycauseexcessiveintermetallicgrowth,whichcanleadtojointbrittleness.Theboardandcomponentsmayalsobedamagedatextendedtemperatureoverliquidus,andmostcomponentshaveawell-definedtimelimitforhowlongtheymaybeexposedtotemperaturesoveragivenmaximum.Toolittletimeaboveliquidusmaytrapsolventsandfluxandcreatethepotentialforcoldordulljointsaswellassoldervoids. Coolingzone[edit] Thelastzoneisacoolingzonetograduallycooltheprocessedboardandsolidifythesolderjoints.Propercoolinginhibitsexcessintermetallicformationorthermalshocktothecomponents.Typicaltemperaturesinthecoolingzonerangefrom30–100 °C(86–212 °F).Afastcoolingrateischosentocreateafinegrainstructurethatismostmechanicallysound.[1]Unlikethemaximumramp-uprate,theramp–downrateisoftenignored.Itmaybethattheramprateislesscriticalabovecertaintemperatures,however,themaximumallowableslopeforanycomponentshouldapplywhetherthecomponentisheatinguporcoolingdown.Acoolingrateof4 °C/siscommonlysuggested.Itisaparametertoconsiderwhenanalyzingprocessresults. Etymology[edit] Theterm"reflow"isusedtorefertothetemperatureabovewhichasolidmassofsolderalloyiscertaintomelt(asopposedtomerelysoften).Ifcooledbelowthistemperature,thesolderwillnotflow.Warmedaboveitoncemore,thesolderwillflowagain—hence"re-flow". Moderncircuitassemblytechniquesthatusereflowsolderingdonotnecessarilyallowthesoldertoflowmorethanonce.Theyguaranteethatthegranulatedsoldercontainedinthesolderpastesurpassesthereflowtemperatureofthesolderinvolved. Thermalprofiling[edit] Thermalprofilingistheactofmeasuringseveralpointsonacircuitboardtodeterminethethermalexcursionittakesthroughthesolderingprocess. Intheelectronicsmanufacturingindustry,SPC(StatisticalProcessControl)helpsdetermineiftheprocessisincontrol,measuredagainstthereflowparametersdefinedbythesolderingtechnologiesandcomponentrequirements.[3][4] Modernsoftwaretoolsallowaprofiletobecaptured,thenautomaticallyoptimizedusingamathematicalsimulation,whichgreatlyreducesthetimeneededtoestablishoptimalsettingsfortheprocess.[5] Seealso[edit] Wavesoldering Reflowoven RestrictionofHazardousSubstancesDirective(RoHS) Thermalprofiling References[edit] ^abcProfilingBasics–ReflowPhases ^Girouard,Roland."Mark5ReflowOven".HellerIndustriesWebsite.HellerIndustriesInc.Retrieved28September2012. ^http://www.ipc.org/TOC/IPC-7530.pdf[bareURLPDF] ^http://www.solderstar.com/files/5214/3567/7718/SolderStar_Reflow_Solutions.pdf[bareURLPDF] ^"AutoSeeker-ReflowThermalProfileOptimisationSoftware".YouTube. http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Electronics_Manufacture_Intrusive_Reflow.pdf Retrievedfrom"https://en.wikipedia.org/w/index.php?title=Reflow_soldering&oldid=1093497163" Categories:PrintedcircuitboardmanufacturingSolderingHiddencategories:AllarticleswithbareURLsforcitationsArticleswithbareURLsforcitationsfromMarch2022ArticleswithPDFformatbareURLsforcitationsArticleswithshortdescriptionShortdescriptionisdifferentfromWikidataArticlesneedingadditionalreferencesfromDecember2010AllarticlesneedingadditionalreferencesArticlesneedingadditionalreferencesfromSeptember2012 Navigationmenu Personaltools NotloggedinTalkContributionsCreateaccountLogin Namespaces ArticleTalk English Views ReadEditViewhistory More Search Navigation MainpageContentsCurrenteventsRandomarticleAboutWikipediaContactusDonate Contribute HelpLearntoeditCommunityportalRecentchangesUploadfile Tools WhatlinkshereRelatedchangesUploadfileSpecialpagesPermanentlinkPageinformationCitethispageWikidataitem Print/export DownloadasPDFPrintableversion Inotherprojects WikimediaCommons Languages CatalàČeštinaDeutschEestiEspañolItaliano日本語Suomi中文 Editlinks