Metal fatigue in copper pillar Flip Chip BGA: A refined acceleration model for the aluminium pad cracking failure mechanism

Abstract

The increasing complexity of automotive-oriented Application Specific Integrated Circuits (ASICs) pushed the industry to innovative packaging solutions that are sometimes challenged by the severe automotive quality targets. This study is related to the characterization of an advanced Flip Chip Ball Grid Array (FC-BGA) package with copper pillar interconnections, specifically to its reliability performance versus the Temperature Cycling (aligned to AEC-Q100 Grade 1 perimeter).The thermo-mechanical stress – due to the different Coefficients of Thermal Expansion (CTEs) of the materials in the system – is the root cause of a failure event never discovered before: cracked pads at the aluminium layer level, just below the Cu pillar, showing the symptoms of metal fatigue.Starting from the consolidated Coffin–Manson law, a refined acceleration model for this new failure mechanism is here proposed. It assumes the existence of a stress-free temperature range, which is defined as the portion of the entire thermal load that does not produce fatigue on the pillar stack. The stress-free temperature range hypothesis and the estimate of its extent were validated through a good fitting of the data coming from the experimental trials performed on the aforementioned copper pillar FC-BGA package.The model obtained was employed to allow a more accurate failure rate prediction for the studied Flip Chip package for automotive applications.