ENIG 化学镀镍-镀金过程中 IMC 的异常生长和开裂

深圳市福英达工业技术有限公司/一站式锡膏解决方案供应商AbnormalGrowthandCrackingofIMCinENIGElectrolessNickel-goldPlatingENIG(ElectrolessNickelImmersionGold)isawidelyusedprocessforPCBsurfacetreatment,alsoknownasimmersiongoldorelectrolessnickelgold.ThegoldlayerofENIGisthinnerandlessdensethanthatofelectrolessgoldplatedboards.Generally,thethicknessofthenickellayerisrequiredtobe3-5μm,andthethicknessofthegoldlayeris0.05-0.15μm(usuallycontrolledintherangeof0.07-0.1μm).WhenitisnecessarytocarryoutthesolderingprocessonthesurfaceoftheENIG,itisnecessarytoincreasethethicknessofthenickellayerto4-7μm.ENIGtreatmentofthepadsurfaceflat,intheclose-pitchdeviceprintingandmountingprocesshasagooduseofresults.Thegoldlayerhasgoodconductivityandwearresistanceandcanbeuseddirectlyasacontactsurface.Inaddition,thegoldlayeralsohastheabilitytoresistoxidationandcorrosion,andcaneffectivelyprotecttheunderlyingnickellayerfromoxidation,soastomaintaingoodsolderability.Figure1.ENIGnickel-goldpadsHowever,whensolderingwithENIGelectrolessnickel-goldplates,theproblemofabnormalgrowthandcrackingofintermetalliccompounds(IMC)onthepadsmayoccur.Thispaperwilldiscussthecausesofthisproblemandpossiblesolutions.IMCgrowthandthermalembrittlementTin-copperintermetalliccompounds(IMCs)arethermallybrittleathightemperaturesforlongperiodsoftime.ThisisduetoliquidsolderandcopperformedbytheIMCispebbled(slicedscalloped),therearegapsbetweenthegrainboundaries,sothattheliquidsolderandpadcopperfoilcontinuestogenerateCu6Sn5,andthesmallergrainsandintothelargergrains.Whenthegraingrowthtoacertainsize,thebottomofthesubstratehasbeenunabletowithstand,thegrainswillbreaktheirownfallintothesolderjoints,thisphenomenonisknownasdrop.Therefore,itisveryimportanttocontrolthethicknessoftheIMClayer,toothickIMClayerwillleadtothermalembrittlementandcrackingproblems.IMCLayeringandFallingTheproblemoftin-nickelIMCdelaminationanddropoutarisesfromthemultiplegrowthofIMC.TheSn-NiIMClayerformedduringthefirstreflowprocesswillbepushedupbythenewlygeneratedIMClayeratthebottomduringthesecondreflowprocess.IfthefirstIMClayeristoothick,andthesecondIMClayerisalsoverythick,thenthelatergeneratedIMCwillcrowdouttheoriginalIMClayer,resultingintheseparationoftheintrinsicIMClayerandsheddingintothesolderjoint.Therefore,itisnecessarytocontrolthethicknessofbothSn-CuIMCandSn-NiIMCduringthesolderingprocess.theIMClayershouldbeasthinaspossible,butmustbepresent,otherwiseitwillnotformareliablesolderjoint.FactorsaffectingIMCgrowth（a）Excessivesolderingtimeandtemperature:ExcessivesolderingtimeandhighsolderingtemperaturewillacceleratethegrowthrateofIMC,resultinginIMClayerthicknessexceedingthenormalrange.（b）Excessiveroughnessoftheelectrolessnickellayer:AroughelectrolessnickellayeracceleratesthedissolutionofIMCandthuspromotesthegrowthofIMC.（c）Presenceofnickeluplift:Ifthenickellayerisuplifted,thefreenickelwilldissolvedirectlyintothesolder,furtherincreasingthethicknessoftheIMC.Bystrictlycontrollingthesolderingtimeandtemperature,controllingtheroughnessoftheelectrolessnickellayer,andavoidingthephenomenonofnickellayeruplift,thethicknessofIMCcanbereduced,andthereliabilityandmechanicalstrengthofsolderjointscanbeimproved.Furth