A Quantitative Evaluation of the Inelastic Energy Absorptions in Cu-Polyimide Interconnect and the Effect on Interface Debond Driving Force

Interface debonding is one of the critical failure modes in fan-in or fan-out Cu-polyimide (PI) redistribution interconnect. Because of the time-dependent inelastic constitutive characteristics of Cu and PI, the energy absorptions through the inelastic deformations of these materials would affect the debond driving force, and should be considered in the analysis of the risk of interface debonding under process or in-service conditions. In this study, a numerical procedure was developed for evaluating the debond driving force for the Cu-PI interface. The procedure focuses on the quantitative estimation of each energy absorption modes including viscoelastic damping, viscoplastic dissipations, and debond separation. The procedure was applied to investigate the driving forces of a Cu-PI interface crack under either mechanical or thermal loads. It was shown that the dominant modes of energy dissipations are significantly different under Mode-I and mixed-mode loading conditions. In particularly, the energy dissipation through viscoplastic deformation of Cu is insignificant under Mode-I condition, but it is the dominant energy dissipation mode under mixed-mode condition. On the other hand, the contribution of viscoelastic damping on energy dissipation during debond growth is limited for both Mode-I and mixed-moded condition. Because typical thermal processes for redistribution interconnect lead to mixed-moded stress condition for the Cu-PI interface, the viscoplastic deformation of Cu plays an important role in the overall debond energy dissipation, and should be considered in the design optimization for interconnect reliability.