Geometry Effects on the Electromigration of Eutectic SN/PB Flip-Chip Solder Bumps

This work investigates the effect of passivation opening diameter and underbump metallization (UBM) diameter on the electromigration (EM) resistance of Sn/Pb eutectic solder bumps. For the bump geometries studied, the electromigration lifetime depends strongly on the UBM area but weakly on the passivation opening area. The applicability of Black´s model for extrapolating lifetime from accelerated currents to operating currents is investigated. The thermal activation energy, E_A, is approximately 1.0 eV and the current density exponent, n, is approximately 2.0. While Black´s model for current density acceleration appears to apply, the current density exponent may not be transferable between bumps of different geometries. A physical model of voiding within the bump using finite element analysis is proposed to explain the observed results