Electromigration reliability and morphologies of Cu pillar flip-chip solder joints with Cu substrate pad metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated similar to 62 mu m Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm(2) at 150 degrees C, 10 kA/cm(2) at 160 degrees C, and 15 kA/cm(2) at 125 degrees C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)(6)Sn-5 or Cu6Sn5 intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the similar to 52-mu m-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of similar to 80% Cu-Ni-Sn IMC and similar to 20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.