Effects of UBM Thickness and Current Flow Configuration on Electromigration Failure Mechanisms in Solder Interconnects

Abstract

This paper investigates the effects of the under bump metallization (UBM) on Electromigration (EM) reliability of SAC solder joints and presents evidence suggestive of hidden mechanisms controlling the EM failure. This conclusion is based on the observation that the EM resistance does not show a monotonic increase with UBM thickness, but rather decreases above a certain critical limit. Since a thicker UBM would provide a greater supply of Cu, our observation contradicts the conventional view on the role of UBM on prolonging EM failure. We initially ascribed it to the increased of EM-prone microstructures or weak- links at the SAC/UBM interface active in thicker UBM layers. This is considered to be due to a more even distribution of EM flux across the joint by reducing current crowding at the corners of the joint. However, switching the test configuration, reducing current crowding, yielded results that disagreed with our proposition. It is found that the impact of the current configuration, and thereby the level of current crowding, on EM resistance does not vary much with the UBM thickness. This defies initial prediction that there would be a smaller impact in samples with thicker UBMs when the current configuration is switched. These results suggest that the EM failure mechanism is affected by UBM in a more complicated manner. Considering hidden factors such as the change in thermal stress with UBM thickness that act against the growth of EM void is necessary in order to better understand the mechanism.