Three-dimensional (3D) Characterization of Electromigration Failure Mechanism of Solder Joints in WLP using X-ray Microscopy

In this study, we investigated electromigration characteristics of wafer-level-packaging solder joint upon temperature cycling stress and failure mechanism will be discussed quantitatively for the first time using non-destructive 3-dimensional X-ray microscopy. Two types of solder-joint resistance shift are found after the stress measurement; one is abruptly (hard) increased resistance and the other is gradually (soft) increased resistance over stress time. Formation of intermetallic compounds and voids that were induced by the consumed Cu are observed at the corner of cathode side in both hard failure and soft failure. However, hard failure has full crack, while soft failure shows only partial crack. When the stress current density or temperature is high, abrupt resistance change was mainly observed. Formation and propagation of defects induced by joule heating is the dominant failure mechanism of WLP solder joint in hard failure, however, higher resistivity of IMC than Sn-based solder led to gradual increase of resistance as amount of IMC increased in soft failure.