Alternative UBM forLeadFreeSolder Bumpingusing C4NP

Microelectronic packaging continues themigration from wirebondtoflip chipfirst level interconnect (FLI) tomeet aggressive requirements forimproved electrical performance, reduced sizeandweight. Theinterconnect pitch isbeing predicted byforecasts like ITRStobereduced to100umand belowforfull array I/0layout. Forwaferbumping, solder electroplating iscommonlyemployed, especially forfine pitchapplications. Waferlevelchipscalepackaging (WLCSP)typically utilizes soldersphereplacement technology tomanufacture thebumps. InWLCSP,pitch and solder ball size areusually muchhigher andthenumberof I/0muchlowerthanforFlipChipinPackage (FCiP) applications. C4NP(Controlled Collapse ChipConnection NewProcess) hasproven tobesuitable forabroad rangeof solder bumppitches, encompassing FCiPtoCSP bump dimensions. As theindustry migrates to300mm wafer processing andlead-free flipchipinterconnect, C4NP is establishing itself asaviable solder bumping alternative. Due toits nature asabumptransfer technology, itisexpected that thebumping yield will beveryhigh, since filled molds canbe inspected prior tosolder transfer tothewafer. Yield isamajor issue forthehighest I/0applications like microprocessors. Theunder bumpmetallurgy (UBM)structure isacritical component ofanysolder interconnect system. TheUBM typically provides three functions: adhesion tounderlying dielectric andmetal, barrier toprotect thesilicon circuitry, andasolder wettable surface. Forlead-free bumps, thebarrier layer iskeytothereliability ofthesolder joint duetothe higher Sncontent intheleadfree solder. A commonbarrier layer usedintheindustry iselectroplated nickel. Thislayer provides goodprotection fromdegradation ofthesilicon metallurgy bytinrichleadfreesolders. C4NPprovides an opportunity toeliminate electroplating, anditsassociated costsforplating chemistry, analysis, supply andwaste treatment. Thispaperanalyzes twoalternative UBM structures: sputtered TiW/Niandelectroless Ni/immersion Au (ENIG), withandwithout Pd.Wafers werefabricated withthese UBM structures, solder applied withC4NP,andchip level stressing performed todetermine therobustness ofthese alternative stack-ups. Analysis ofthese structures following multiple reflows andthermal cycling ispresented. Inaddition, theworkalsodiscusses production cost analysis basedona costmodelspecifically developed to determine manufacturability ofthevarious UBM structures.