Eutectic bump evaluation with various passivation and polyimide structures [flip chip interconnects]

A set of critical parameters, namely passivation opening and polyimide pull-back/push-forward (i.e. covering the passivation edge and extending on to the aluminum pad) have been studied. The test vehicle used for this analysis was an F35S daisy chain on a laminated substrate. In this study, the packages were subjected to environmental stress test and bumped bare dice subjected to autoclave testing. The major failure modes seen were solder ingression into the incomplete underfill around the bumps, and cracks in the passivation layer and substrates. The investigation revealed that the bond pad designs with larger passivation opening and polyimide push-forward yielded a more reliable structure. The simulation study showed that there is a 600% reduction in the bump formation and reflow induced stress on the structure where the polyimide extending on to the aluminum compared to no polyimide on to the passivation.