The growth and segregation of intermetallic compounds in the bulk of flip chip Sn2.4Ag solder joint under electrical current stressing

This paper reports a peculiar growth and segregation behavior of IMC in a flip chip joint with Sn2.4Ag solder bump under electrical current stressing. A set of two neighboring solder joints were adopted for investigation at 1.5~3.0×10⁴ A/cm² of current stressing at various temperatures up to 180°C. One of the joint experiences current flow from substrate to the Si chip while opposite for the other joint. The size analysis with SEM imaging shows that the Ag₃Sn IMCs formed in the bulk during current stressing grows following the empirical equation Δr (radius change) = ktⁿ with n =1.1, indicating a reaction controlled kinetics. The activation energy of the Ag₃Sn IMC growth was estimated to be 11.5 (at 3.0 × 10⁴ A/cm²) ~14.0 (at 1.0×10⁴ A/cm²) kJ/mole. Both Cu₆Sn₅ and Cu₃Sn were formed at the joint/metallization interface, while only Cu₆Sn₅ were found in the bulk. The SEM investigation at various cross sections from center to one end of the hemisphere for a joint, current stressed with 1.5×10⁴A/cm² at 180°C for 330 hours, shows a serious segregation of the Cu₆Sn₅ IMC closing to the end portion of the bump. A mechanism incorporating IMC dissolution, electromigration, and thermomigration was proposed to explain the segregation behavior.