PCB Assembly Guidelines for 0.4mm Package-On-Package …

1 ApplicationReportSPRAAV2A November2013 PCBA ssemblyGuidelinesfor (PoP) packages ,PartIIKeithGutierrezand GeraldColeyABSTRACTO ncethe bottomcircuitboardhas beendesigned,the assemblyguidelinesfor Package-On-Package (PoP)versionsof the OMAP35xxprocessorand the memorydevicemustbe enormousnumberof variablesassociatedwith followingfactorshavea majoreffecton the qualityand reliabilityof finalassembly:the circuitboarddesign,the solderpastecharacteristics,solderpastede position,the reflowprofile,and the fluxingor solderpastedepositionontothe PoP memorypriorto two majortechniquesfor mountingthe PoP memory:one passand two ,the one passprocessis recommendedas it is the mosteconomicaland providesaboutthesameassemblyyieldsas the two BeagleBoard,discussedin the companionarticleto this document,PCBD esignGuidelinesfor (PoP) packages (SPRAAV1) - whichwill be referredto as PartI throughoutthereminderof this document- formsthe basisfor the understandingof applicationreportdescribesonly one of likelyhaveothers.

2 It cannotcoverall of the is stronglyrecommendedthat youperformyourowntrial runsand studiesin conjunctionwithyourassemblyhouseto optimizethe OMAP35xxProcessor(bottomdevice)AndPoPMem ory(top device)StackUp On WordOf November2013 PCBA ssemblyGuidelinesfor (PoP) packages ,Part IISubmitDocumentationFeedbackCopyright 2013, of Figures1 OMAP35xxProcessor(bottomdevice)And PoP Memory(top device)StackUp On 2 - OMAP35xxPoP The OMAP35xxProcessorAnd PoP 1 2 Nitrogen(left image)and Air (rightimage) MajorCausesof Stresson BGAP ackagesand OMAP35xxProcessorAnd PoP of Tables1 DifferentApertureOpeningsand applicationreportis the secondof two I coversthe the bottomboardto whichthe OMAP35xxprocessoris mounteddominatesthe PoP sureto readand applythe guidelinesin Part I beforebeginningPoP document,Part II, coversthe guidelinesfor the assemblyof printedcircuitboardsthat use the assemblyoptionsas well as suggestionsto use whenqualifyingand workingwithyourassemblysites,eitherinter nalor Part2 - OMAP35xxPoPAssemblyGuidelinesAmtechis a trademarkof othertrademarksare the propertyof (PoP)

3 packages ,SPRAAV2A November2013 Part IISubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporatedMemoryP rocessorCircuit WordOf CautionFigure3. Close-upOf The OMAP35xxProcessorAndPoPMemoryStackUp2A WordOf CautionThis sectiondiscussesthe assemblyof the OMAP35xxBGApackagewith its companionmemorymountedon top, hence,the not coverall aspectsofautomatedassemblynor is this a studyof the reliabilityof the PoP PoP devicesand circuitboardsfor fine-pitchBGApackagesat smalleris still verynew for is moreof an art thana ,the materialin this documentwill age and go out of is a rapidlyevolvingtechnology,spendsometime readingthe hugenumberof articles,papers,and companypresentationson all aspectsof lot of time with whatthey know.

4 Whatexperiencethey haveand theirdocumentationof discussingis time well commonthemeemergedas this applicationreportwas developedand afteran extensiveliteraturesearchwas ,especiallyPoP Assembly ,is considereda ,severalassemblyshopsindicatedthey couldor wouldnot handlethe finepitchpackagesand manydid not havethe yourassemblyhousehas neverdonePoP,be preparedto allowtime for severallearningcyclesbeforecommittingto use of the BeagleBoardgerberswill be a big it to helpyourassemblyshopworkthroughthe issuesof bringingup a new yourfindings,you lldiscoverthat yoursupplierscan handlethe deviceor that you mustlocatea moreexperiencedassemblyshopwith mostcommonpairingis an assemblyshopand a providefeedbackto all partiesinvolvedin boarddesign,fabricationand November2013 PCBA ssemblyGuidelinesfor (PoP)

5 packages ,Part IISubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporatedA has exploredseveralcompaniesthat haveworkedtogetherand foundmanycommonpointsofagreementon assemblyguidelinesas well a few minorpointsof suggestionsandrecommendations,plus someprecautionsoriginatedwith advantageof anyrecommendedchangesfroma vendor s DFMstudiesand don t be afraidto changea boardlayoutto fitthe suggestionsof the a final wordof advice,havelots of TeamSportSuccessfuldesignand assemblyof complex,fine-pitchcircuitboardsis aTeamSport. The daysof tossingcircuitdiagramsoverthe cubicalwall to the boarddesignerwho thentossesthemto the circuitboardfabricationshopare s boarddesignrequiresa teamapproachand the entireprocess,fromcomponentselectionto typicalteamis composedof four differentmembersrepresentingthe four majorstepsin productfabrication.

6 The devicesupplier(chips,passives,mechanical ),the PCBdesigner,the PCBfabricationshop,and the be moremembersor somemembersmay do morethanone the teammembersbringstheirown experiencesand designguidelinesto bearon the aresult,it is not uncommonto find resolvedpriorto the startof resultin poorassemblyyieldsat best or 100%failureat frequentcommunicationis the key to resolvingconflictsand everyonemustbe in the to knowyourteammembersand be sureto havefrequentmeetingsas the will be moneyand time well PoPAssemblyTeamwork4 BeagleBoardSchematicsand GerbersAvailableThe BeagleBoardis an open-sourcehardwareplatformbasedon the TexasInstruments' a completesingle-boardcomputersuitablefor softwaredevelopmentand thefollowinglink for additionalinformation,includingthe hardwareschematicsand gerberfiles.

7 Alsoconsidersubscribingto the BeagleBoardwas usedto validatethe assemblyguidelinesdiscussedin this (PoP) packages ,SPRAAV2A November2013 Part IISubmitDocumentationFeedbackCopyright 2013, BeagleBoardTop SideView5 AssemblyOptionsThereare two optionsto considerwhendecidingto buildPoP assemblies:a one passoptionand a a one-passassemblythe OMAP35xxprocessoris first mountedto the board,the memoryis mountedtothe processorand the finishedboardis thenrun throughthe reflowovenin a usesthisprocesswith severalassemblyshops;nonehavereportedany issuesand the November2013 PCBA ssemblyGuidelinesfor (PoP) packages ,Part IISubmitDocumentationFeedbackCopyright 2013, AssemblyOptions- 1 PassThe two-passassemblyhas an intermediatestep in whichthe memoryis first ,thesetwo partsare placedin a carriertray and thenmountedon the circuitboardand the finishedboardis refloweda AssemblyOptions- 2 Pass6 SolderPasteStencilDesignStencildesignhas severalcriticalfactorsthat this discussion,therecommendationsapplyonly to round,solder-mask-definedpadsfor fine-pitchBGApackageswith be consideredas a startingpointfor buildingdesignrulesthat fityourmanufacturingrequirementsand capabilities,assuminga lead-freeprocessis all cases.

8 Youmustcarefullymonitorassemblystartupso as to insurethe desiredresultsare (PoP) packages ,SPRAAV2A November2013 Part IISubmitDocumentationFeedbackCopyright 2013,TexasInstrumentsIncorporated/ = relationshipbetweenthe surfaceof the apertureand the insidesurfaceof the aperturewallsin thestencilis referredto as the arearatio(AR).The arearatiois muchmoresuitablefor shapessuchascirclesthanthe fine pitchBGApads,a roundapertureis arearatioof the stencilwall has beenshownto providethe best transferefficiencyand repeatabilityof the insurea goodpastereleasefromthe biggestimpacton arearatiois the equationis:Where,Ap is the apertureopeningareaand Aw is the wall area.

9 Where,T is the stencilthicknessand r is the showsthe variousvaluesof AR fordifferentapertureopeningsand DifferentApertureOpeningsand StencilThicknessesApertureOpeningStencil ThicknessStencilThickness(mils)ApertureR adius(mils)( m)(mils)AR100 mil mil November2013 PCBA ssemblyGuidelinesfor (PoP) packages ,Part IISubmitDocumentationFeedbackCopyright 2013, OverprintingSomeassemblyhousesspecifya slightlylargerthannormalopeningto overprintthe ,forthe BeagleBoard,thereis no overprintingsincethe padsare roundapertureshapeis recommendedfor allowsfor an equal,uniformamountofpasteddepositionon the resultin unequalamountsof pastebeingdepositedand shouldbe FinishingThe solderstencilshouldbe madefromstainlesssteel,chromiumor aperturesshouldbe lasercut.

10 The finishshouldbe electro-polishedto ,the widerside of the apertureshouldbe againstthe the BeagleBoard,the solderstencilhas the followingparameters:MaterialStainlessste elThickness4milsFabricationLaserCutApert ureShapeRoundApertureDimension10mils(254 um)8 MemoryAttachmentOptionsMemoryattachmentt o the top of the processoris oftenconsideredone of the criticaland mosttroublesometasksin PoP ,this has provento be fact, memorymountinghas provento be the primaryconcernis creatinga strongsolderjoint betweenthe memoryball and the pastecannotbe usedbecausethe die is in the two processesto bedescribedhavebeendevelopedand usedin volumeproduction.