Accelerated Solder Interconnect Testing Under Electromigratory and Mechanical Strain Conditions

Continuous power density increases and interconnect scaling in electronic packages raises risk of electromigration (EM) induced failures in high current interconnects. Concurrently, thermal cycling fatigue also places interconnects at risk of reliability failure during electronics' operating lifetime. These two differing failure mechanisms are historically treated separately, but in operation, the combination of EM effects and thermal cycling can act synchronously in accelerating failure. Presently, there is no model to predict the complexity of reliability estimation arising from these interacting failure modes but is certainly important for high current density applications. In this work, a novel testing system has been employed to facilitate the estimation of the reliability of solder interconnects under the combined influence of EM and mechanical strain. The system subjects solder interconnects to high current density, elevated ambient temperature, and a constant tensile stress while recording the change in electrical resistance and change in length of the solder over time. The solder samples were created using two copper wires connected by a eutectic Pb/Sn solder ball to imitate flip-chip or BGA packaging interconnects, allowing for controlled testing conditions in order to demonstrate the combined effects of a mechanical load and EM on the lifetime of a solder joint. A significant reduction in lifetime was observed for samples that endured the coupled accelerating factors. Comparing the experimental results of different current densities at different stress levels provided a new outlook on the nature of coupled failure acceleration in solders. This novel test methodology can inform model generation for better anticipating the failure rate of solder interconnects which naturally experience multiple stress inputs during their lifetime.