Secondary Current Crowding Effect during Electromigration of Flip-Chip Solder Joints

In flip-chip interconnects under current stressing, the primary current crowding effect occurs at the entrance edge of the contact interface with the highest current density. In this study, an increased current density also occurred at the other edge of the contact interface, followed by a selective dissolution of under bump metallization. After primary current crowding, the rest of electrons flow to the metallization edge, followed by an abrupt change in direction toward the anode. Primary current crowding is attributed to the electrical field change whereas the secondary crowding effect is due to the physical blocking of the electron flow. Because this effect is not as great as that of primary current crowding, it must be assisted by thermal diffusion.