The Effects of Time-Dependent Inelastic Behaviors on the Debonding of Cu-Polyimide Interface

Multilayered fan-in or fan-out (FO) redistribution interconnects are used extensively in advanced package designs for many state-of-theart high-performance computing and portable applications. A common feature of the various redistribution interconnect designs is the high density of materials interfaces between metal conductors and ceramic or polymeric dielectrics. While the design offers significant benefit in electrical performance, the lack of strong bonds between the dissimilar materials leads to a higher risk in delamination failure under process and reliability test conditions. In the estimations of the mechanical or thermomechanical debond driving forces, an important factor to be considered is the time-dependent inelastic behaviors of metals and polymers. Under thermal or mechanical loading conditions, the energy dissipation in the layered structure is not only through the breaking of the weak chemical bonds at the dissimilar materials interface, but also through the viscoelastic and viscoplastic deformations of the polymer and metals around the interface. From the perspectives of the thermomechanical reliability and structural design of the redistribution interconnect, it is important to evaluate the contributions of the inelastic energy absorptions of the metal and polymer and their effects on the interface debond growth.In this study, the influences of the viscoelastic behavior of polyimide (PI) dielectric and the viscoplastic behavior of Cu metallization on the debonding driving force of the redistribution interconnect is considered. The thermoviscoelastic constitutive behavior of the PI thin film was modelled by using a generalized Maxwell model with time-temperature superposition scheme. The viscoplastic behavior the Cu interconnect was considered by using the Anand model. A finite-element based numerical model was developed to evaluate the debonding growth at the Cu-PI interface. The model was first applied to evaluate the energy dissipations of the inelastic materials and their influences on the debonding strain energy release rate under either Mode-I or mixed-mode loading conditions. The effects of temperature and loading rate on the partition of energy dissipation through interface separation, viscoelastic and viscoplastic deformations were also discussed. The model was then applied to investigate an interface crack in redistribution interconnect under thermomechanical load. The numerical procedure developed in this study can be implemented to enable a quick evaluation of the interconnect geometry and materials selection for package design and process development.