Four-laser bending beam measurements and FEM modeling of underfill induced wafer warpage

In recent years, the market share of wafer level packages has been growing steadily because of the low cost and small form factor of the packages. Wafer level underfill, as a convergence of wafer level CSPs and flip-chip underfill packages, presents a solution for high performance and low cost packaging. A novel B-stageable wafer level underfill material has been developed and a successful wafer level underfill process and assembly has been demonstrated. In wafer-level packages, typically a thick polymer film is applied on the wafer. One potential problem with the polymer application is the warpage and stress introduced by the underfill curing in the B-stage process. This paper presents the experimental evaluation and a finite-element modeling (FEM) of the wafer warpage introduced by underfill curing at wafer level. A novel four-laser bending beam system is used to measure the curvature of the wafer after the B-stage curing and the complete curing of the polymer underfill. In addition to the measurement at the end of underfill curing, the wafer warpage during thermal cycling has also been measured. Theoretical calculation and finite-element modeling of the wafer warpage are performed. Material properties of the underfill are characterized and presented as different material models in FEM analysis. The effect of underfill material properties on wafer warpage is evaluated.