Electromigration study of eutectic SnPb flip-chip solder joints on ceramic substrates

This study investigates electromigration study of eutectic SnPb flip-chip solder joints on ceramic substrates. The under bump metallization (UBM) structure consists of 5-mum Cu / 3-mum Ni under bump metallization (UBM). Under the current stressing by 0.9 A at 150degC, we did not find void formation but a large amount of intermetallic compound (IMC) of Cu₆(Sn,Ni)₅ were formed when the bump resistance increased 10 mOmega. Three-dimensional electrical simulation by finite element analysis was carried out to simulate the current density distribution in solder joints with slit Cu traces. It is found that the current density was almost uniformly distributed in solder joint for this structure. It is found that the bump resistance only increases 0.07 mOmega when half of the solder bump was transformed into Cu₆Sn₅ IMC. The reason for the low crowding effect in the solder joints will be discussed in the conference.