Electromigration in Copper-Core Solder Ball Joints During Thermal Cycle Tests

Electromigration current densities in Cu and Al lines on a silicon die exceed 1.0 × 10⁶ A/cm². However, solder joints can only withstand electromigration current densities below about 1.0 × 10⁴ A/cm². Thus, electromigration in solder joints will become a problem in semiconductor packages in the near future. Previous studies demonstrated that Cu-core solder balls increased the electromigration lifetime and led to better current stability at temperatures below 423K. This is because electrons flow through the Cu cores, reducing the current density on the cathode side, which is where electromigration occurs. In the present study, we forcused on the reliability of solder joints in a combined environment by examining the effect of thermal cycle tests on the current in a new test sample. A new test sample for the evaluation of joining reliability by using Cu-core solder balls in a combined enbironment was made. In initial tests, this test sample exhibited similar results to those observed in previous studies. Cu-core solder balls subjected to cyclic testing at 233/398K and a current density of 1.0 × 10⁴ A/cm2 exhibited lower reliabilities than when there was no current. Examination of cross-sections of the solder balls after reliability testing revealed that the combined environment accelerated growth of intermetallic compounds and cracks in the joining region. In a combined environment, Cu-core balls were converted into intermetallic compounds on the anode side. This phenomenon is thought to occur due to the different electrical resistivities of Cu-Sn intermetallic compounds.