CPI Parametric Investigation of UBM-Al Interface for Cu Pillar Flip-Chip Application

Under bump metallization (UBM)-Al delamination is a high-value chip-package interaction reliability problem in flip-chip Cu pillar packaging. We devised a test vehicle for quick reliability assessment. We demonstrate that preclean is a critical step to ensure reliable UBM-Al adhesion and achieve bump shear strength >12 g/mil
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for 15-μm polymer opening. Key parameters affecting bump shear strength, such as UBM critical dimension (CD), polyimide (PI) CD, SiN CD, and PI thickness, were reviewed from both simulation and experimental standpoints and good agreement was achieved between the two. Based on the observed failure mode, a 1-D model was also proposed to correctly predict the experimental trend. Optimum design rules for Cu pillar were outlined. Chip-substrate assembly was demonstrated on 80-μm bump pitch with staggered bump configuration using mass reflow. Progressive thermal cycling was performed to evaluate package performance. Failure mode was found to be solder crack, verifying the improvement in UBM-Al adhesion.