填充工艺在倒装芯片上的应用

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Bottom-fillprocessforflip-chipapplicationsFlipchip(FC)technologyisapackagingmethodinwhichthechipisdirectlyattachedtothesubstrate,whichhastheadvantagesofhighdensity,highperformanceandlowcost.However,duetothecoefficientofthermalexpansion(CTE)mismatchbetweenthechipandthesubstrate,thesolderjointsaresubjectedtolargethermalstresseswhenthetemperaturechanges,leadingtofatiguedamageandfailure.Toimprovethereliabilityofsolderjoints,acommonmethodistoinjectapolymermaterial,calledunderfill,betweenthechipandthesubstrate.Underfillimprovesthestressdistributioninthesolderjoint,reducesthestrainamplitudeinthesolderjoint,andextendsthethermalfatiguelifeofthesolderjoint.Figure1.UnderfillprocessUnderfillisaliquidencapsulant,typicallyanepoxyresinheavilyfilledwithSiO2,thatisusedbetweenthechipandthesubstrateafterflipchipinterconnections.Aftercuring,thehardenedunderfillhasahighmodulus,lowCTEtomatchthesolderjoints,lowmoistureabsorption,andgoodadhesiontothechipandsubstrate.Thermalstressesonthesolderjointsareredistributedbetweenthechip,underfill,substrateandallsolderjoints,ratherthanbeingconcentratedontheperipheralsolderjoints.Theapplicationofunderfillhasbeenproventoreducethemostimportantsolderjointstrainlevelsto0.10-0.25ofthestrainofanunencapsulatedsolderjoint.asaresult,underfillcanincreasethefatiguelifeofsolderjointsbyafactorof10to100.Inaddition,itprotectsthesolderjointsfromenvironmentalattack.Underfillisapracticalsolutionforexpandingtheuseofflipchiptechnologyfromceramictoorganicsubstratesandfromhigh-endtocost-sensitiveproducts.Figure2.FlipChipProcesswithUnderfillAdvancesinflipchiptechnologyhaveledtothedevelopmentofunderfillprocessesandunderfillmaterials.Figure2illustratestheprocessstepsofaflipchipusingtheunderfillprocess.Separatefluxdispensingandcleaningstepsarerequiredbeforeandafterthechipisassembled.Afterthechipisassembledonthesubstrate,theunfilledmaterialisdispensedanddraggedintothegapbetweenthechipandthesubstrate.ForeignmaterialcontrolbeforeunderfillapplicationBottomfilleradhesivebeforesizingneedtoconfirmthattheboardandfillersurfacewithoutforeignobjectsandalargenumberoffluxresidue,morefluxresiduewillleadtoadhesiveattachedtothefluxresidue,subsequentuseoftheprocessoffluxresiduevolatilization,softening,mutationdirectlyaffectthemechanicalpropertiesoftheadhesive,whichaffectsthereliabilityoftheproduct.ThestandardbottomfillingsizingprocessrequiresPCBAcleaninganddrying,andthendispensingcuring.Theadvantagesofthebottomfillingprocessare:1.Improvethereliabilityofthesolderjointstoextendtheservicelifeoftheproduct;2.Protectthesolderjointsfromenvironmentalerosion,improveproductcorrosionresistance;3.Reducethethermalstressbetweenthechipandthesubstrate,improvetheproduct'sresistancetothermalcycling;4.Enhancetheadhesionbetweenthechipandthesubstratetoimprovetheimpactresistanceandvibrationresistanceoftheproduct.Disadvantagesofthebottomfillingprocess:1.Increasethecostandcomplexityofpackaging,requiringadditionalequipmentandmaterials;2.Needtoselectappropriatebottom-fillingmaterialsandparameterstomatchthecharacteristicsofthechipandsubstratetoavoidfailuremodessuchasresidualstress,cracks,corrosionandvoids;3.Difficulttorepairorreworkthepackage,requiringtheremovalofthebot