Electromigration reliability and morphologies of Cu pillar flip-chip solder joints

The Cu pillar is a thick under bump metallurgy (UBM) structure developed based on the consideration of alleviating current crowding in a flip-chip solder joint in operation conditions. We present in this work electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated Cu at ~62 mum onto Cu substrate pad metallization through the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled averaged current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150degC,10 kA/cm² at 160degC, and 15 kA/cm² at 125degC.Electromigration reliability of this particular solder joint turns out to be much enhanced compared to a conventional solder joint with thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52 mum thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general distributed on the cathode side of the solder joint.