Stencils and Paste Glue Screening.docx
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Stencils and Paste Glue Screening.docx
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StencilsandPasteGlueScreening
4.1StencilsandPaste/GlueScreening
CreatedBy:
MikeBerryon02/12/99at05:
39PM
Category:
4.0SMT
ReferenceDocuments
MPIGENERIC3000SCREENPASTEm
MPIGENERIC3000SCREENADHESIVE
OEM-2013MPMAPSERIESSCREENPRINTER
OEM-2014MPMSPSERIESSCREENPRINTER
OEM-2016MPMUP3030
OEM-2017DEK265GSXSERIESSCREENPRINTER
OMP-6030WASHFORMIS-SCREENEDGOLDPLATEDCARDS
OMP-3001PASTEANDADHESIVELABELPROCEDURE
OMP-6000SOLDERPASTE&ADHESIVETRANSPORTATION
http:
//belle/proceng/people/Russell/gluescreening/Glue_main.html-VariableHeightGlueScreening
TORLN4\apps\ScreenerLearingCommunity-LotusNotesDatabasewithdiscussionsfrombi-weeklymeetings->
SolderPaste
Solderpasteisaconsistenthomogenousmixtureofsolderpowderinastableviscousfluxvehiclewhichisusedtojoin2metalsurfacesuponheatingabovereflowtemperature.Pasteis50%fluxand50%metalbyvolume.Therefore,whenusinga6milstencil,3milsofsolderwillbeleftafterreflow.Thefluxperforms2mainfunctions.First,itsuspendsthepowdertomaintainthehomogeneousmixture.Second,itchemicallyremovesoxidefromthecomponents,thepcbpadandthepowdertoallowagoodmetallicbondtoform.ThemostcommontypeofsolderpasteisEutectic.Itis63%tin(Sn)and37%lead(Pb).AnexampleofhighmeltsolderisSn10/PB88/Ag2withameltingpointbetween268oCand302oC.
Therearedifferenttypesoffluxavailable.Theyare:
∙NC:
Nocleanorlowresidue
∙WS:
Watersoluble/Organicacids
∙RA:
RosinActivated
∙RMA:
Rosinmildlyactivated
Thesefluxesaremadeupof4mainingredients:
∙Rosin/Resin-Flowproperties,activity
∙Solvent-Dissolverosin/resinandactivator
∙Activators-Oxidescrubbingandcleaning
∙Modifiers-Thickeners,rheoligicalagents,viscositymodifierstokeeppowderfromsettling,tackifiersandcolouring.
SolderPasteViscosity:
SyringeDispensing:
200-400kcps
StencilPrinting:
400-1200kcps
Stencils
Thefollowingdiagramshowsthe3differentmethodsofmakingastencil:
Chemicaletch,laser,andE-Fab.E-Fabprovidesthebestqualityaperatures.E-Fabstencilsare"grown"withlayersofnickel.Chemicaletchhasaphotoresistaddedoverthestencilandtheacideatsawaythemetalthatisexposed.Lasercutstencilshaveeachaperaturecut1atatime.Chemicalandlasercutstencilsaremadeofstainlesssteel.Wehavetriedlasercutplasticstencilsbuttheresultswerenotasgoodasstainlesssteel.
Thefollowingshouldbetakenintoaccountwhendesigningthestencil:
Thestencildesignistohaveastencilheaderblock.Inthisblockisinformationaboutthestencildesignthatgetstransferredtothestencilvendorandwillappearontheartwork.Thewordingshouldbereadableasoneviewsthetopsidestencildesignfromthetopsideandreadableasoneviewsthebottomsidestencildesignfromthebottomside.
STENCIL#=
ASM#=
RAWCARD#=
ID=xxxxDATE:
mm/dd/yy
THICKNESS:
xxMILS
READABLE/SQUEEGEESIDE
where:
∙STENCIL#-stencilpartnumber
∙ASM#-assemblypartnumber
∙RAWCARD#-rawcardpartnumber
∙ID-designsystemfilename
∙THICKNESS:
xxMILS-stencilthickness
StencilOrderingConsiderations
∙ForallSIMMcardapplications,unlessotherwisespecifiedbyProcessEngineering,stencilscanbeofthefollowingthickness:
∙8mils(.008")
ForproductshavingSOJmemorymodulesonly.Thesestencilscan
eitherbechemicallyetchedorlasercut.
∙7mils(.007")
Forproductshaving:
1.finepitch(<50mil)&SOJmodulescombined
2.fineTSOPonly
Thesestencilsmustbelasercut.
∙6mils(.006")
Forproductshaving:
1.finepitch(<50mil)&TSOPmodulescombined
2.(<=16mil)finepitchonly
Thesestencilsmustbelasercut,orE-fab.
∙Stepdownstencilsarenotrecommendedsincerubbersqueegeesrequiredforthistypeofstencildonotproducesamescreenaccuracy.
∙Nickel-platedstainlesssteelstencilsarepreferredforbetterreleaseandprintcharacteristics.Theadditionalcostandleadtimeofnickelplatingmaynotalwaysbejustified.Here,stainlesssteelisacceptable.Thescreeningengineershouldadviseonthisrequirement.Nickel-platedbrassisacceptablebuthasbeenshowntostretchearlyinhighvolumeapplications.Thereforeitisonlyrecommendedforshortlife,prototypeassemblies.BrassfoilisnotacceptableforuseatCelestica.BrassissusceptibletopittingandcorrosionbythesaponifiersolutionsusedtocleanthefoilintheSMTprocess.
∙Theuseofhightensionbordermeshisrecommended.
∙Toaidthepastereleaseprocess,trapezoidalaperturesshouldbespecified.
∙Allframesaretobecoatedforsaponifiersolutioncompatibility.(e.g.powderepoxycoating)
∙Inthecasewherethestencilframeismadeofsquaretubularaluminum,itshouldbeensuredthattheyareeitherwatertightorthatadequatedrainageholeshavebeenprovidedtopermitwatertoescapeafterthestencilhasbeencleaned.
∙StencilframesshouldbemadetofitMPMandDEKsolderpastescreeningmachines.SeeFigure2forframedimensions.
∙Dependingontheproduct'spanelization,twostencillayoutsarepossible:
1.1Up-SingleImage
Bestsuitedforcurrent'mini-panel'panelizations.Doublesidedproductswillrequire2stencils:
1forthefrontsideand1forthebackside.
2.2Up-MultipleImage
Bestsuitedforsome'ladder'panelizations.Boththefrontandbacksideimagescanbeplacedonthestencilatonetime,savingthecostof1stencil.Seethediagramthatfollowstodeterminewhetherthisisaviableoptionforthecurrentproduct.ThedecisionshouldbebasedonwhethertheMINdimensionsshowninthe'MultipleImage'layoutinFigure2areattainablegiventhecurrentproduct'spanelization.
Thetwolayoutsareshowninthediagramthatfollows:
∙Allstencilsmusthavevisionalignmentfiducials;thelocationofwhichmustcorrespondtotheglobalfiducialsonthehandlingedgesofthepanel.Theyaretobehalf-etchedfromthe"boardside"(asopposedto"squeegeeside")ofthestencilandback-filledwithblackepoxyorpaintforimprovedopticalcharacteristics.Therecommendedstencilfiducialdiameteris1.00mm(.039")Besidesthesefiducials,all2Up-MultipleImagestencillayoutscanhaveoptional0.25mm(.010")halfetchedepoxyfilledsquareholescorrespondingtowherethetoolingholesareonthepanelhandlingedge.TheyareoptionaldependingonwhetherCelesticasendsthe2Uppanelizationdirectlytothestencilmanufacturerorsendsthetopandbottomimagesseparately;requiringthemanufacturetoperformthe2Uplayout.Theoptionalfiducialsshouldbeprovidedinthelattercasetohelpensuretheproperorientationofthetopandbottomimagesonthestencil.Here,thetoolingholeindicatorsarealwayslocatedonthesidenearestthestencilframe.
Thelocationofthe1.00mm(.039")diameterfiducial(Figure3)correspondtothelocationoftheglobalfiducialsonthepanel/card.
Step-DownStencils
StencilAperatures
ThestencilapertureiswhatultimatelycontrolsthefirstpassSMTyieldifalltheproperclearanceandfootprintdimensionshavebeenincorporatedintothedesign.Itisthereforecriticalthattherulesonoptimumstencilaperturesforthevariouscomponenttypesbefollowed.Theaspectratio(Figure4)shouldbegreaterthan1.5ifpossible,butforE-Fabcanbeaslowas1.3.Inaddition,astencilshouldalwayshaveasurfacetensioncalculationof0.6orgreater:
PadPullingTension(P)Aperature(LengthxWidth)
---------------------------------=------------------------------------------------->=0.6
RetainingWallTension(R)StencilThicknessxAperaturePerimeter
StencilAperatureReductions
ThestencilaperturesofagivencomponentmaybemodifiedtocontrolthevolumeofsolderdepositedduringSMTassembly.Thisisknownasmicro-modification.Thereare4reasonsfordoingthis:
∙Providebettergaskettingofthestencilonthepad
∙Preventbridgingonfinepitchcomponents
∙Preventsolderballing
∙ForlargepadsunderpartssuchasD2paks,wirebonddies,etc.:
tominimizevoidingandallowoutgassing.
Thefollowingillustrateshowthemodificationsaredone.TheyareusuallysymmetricalandunlessotherwisespecifiedbyProcessEngineering,havethesamecentroidastheSMTpad.The"PASTE.XLS"spreadsheetisavailablethroughtheCORELABdesignservices,andisatoolusedtocalculateaperaturereductions.Specificstencilaperturereductionsareprovidedaspartofeachcomponentfootprintdefinitionwhichfollows.
DiscreteComponentAperatureReductions
Stencilaperturesshouldnotbecenteredonthepadsbutshiftedtotheoutsideedgesasshown.Thishelpsreducethenumberofsolderballswhenusingno-cleansolderpaste.Withtheexceptionof0402'sand0201's,thisworksfine,however,withthesesmallcomponentstheyaremorelikelytotombstone(especiallycapacitors)duetothemomentforcethatpasteexertsontheterminations.For0402'sithasbeenfoundtoreducetombstoningbymovingthepasteclosertotheinsideofthepadsbutnottoofarastocausesolderballs.
FromPASTE.XLS:
1.1.1QFPandSOICAperatureReductions
FromPASTE.XLS:
Note2:
Makesurethattheactualpadsizeisthesameaswhatiscalledoutonthedesign.Sometimesboardshopswilloveretchthecopperpads.
PLCCandSOJStencilAperatureReductionsFromPASTE.XLS:
Combinationfootprints
Herearetheguidelinesfororderingstencilsforcardswhichutilizecombinationfootprints.CombinationfootprintsarepadgeometriesthatcanaccepteitherSOJorTSOPcomponents.
1.(.300")SOJ/TSOPcombination:
Order2stencils:
(.300")SOJonlyand(.300")TSOPonly.
2.(.300")and(.400")SOJ/TSOPcombination:
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