Interfacial Strength Characterization and Simulation of the Stacked Copper-Polymer Structures in Fan-out Packages

Stacked Copper-polymer structures are essential components in fan-out packages for the electrical and thermal connectivity of package-to-system. The interfacial strength of copper-polymer structures, such as adhesion, plays a significant role in determining package performance and reliability. This paper evaluates the impact of interfacial strength of copper-polymer structures on fan-out package reliability by utilizing a fracture mechanics approach. The adhesion of copper with different polymer materials is characterized by using the four-point bending (4PB) test, from which the critical strain energy release rate (Gc) can be determined. This paper also investigates the effects of cleaning processes and moisture precondition. The results indicate that some types of polymer are highly sensitive to moisture precondition, which result in the degradation of interfacial strength due to hydrolysis. Furthermore, a practical design of stacked Cu-polymer structures is simulated based on fracture modeling methodology, where the VCCT (Virtual Crack Closure Technique) is applied to estimate the strain energy release rate (G) and stress intensity factors at a specific Cu-polymer interface. The results show that a predictive modeling approach can be developed to evaluate the risk of delamination compared to Gc and that the optimum layout design of Cu redistribution layer (RDL) can be determined. More importantly, the material selection strategy of polymer and Cu structure design optimization for ensuring product reliability can be achieved.