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, 1 4, 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.