Anisotropy of curing residual stress of underfill in the encapsulation under three-dimensionally constrained condition based on in-situ characterization

To improve the reliability of microelectronic packages, underfills are widely used in flip-chip packages to protect solder bumps from hazard environment. Nevertheless, the residual stress caused by the curing shrinkage of underfill has always an important issue for the reliability. Currently, there is a lack of comprehensive understanding of the curing residual stress in the constrained state of packaging structure. In this work, a novel in-situ characterization was conducted on the flip-chip test vehicle (FCTV) to analyze the curing behavior of underfill in the 3-dimensionally constrained state. Interestingly, the curing shrinkage exhibited anisotropy under 3-dimensionally constrained state, and the actual shrinkage in Z-axis direction was much larger than that in X/Y-axis direction. The shrinkage in Z-axis direction can produce residual compressive stress (or prestress) in the solder bumps, while the shrinkage in X/Y-axis direction was mainly residual shrinkage, which contributed obviously to warpage. Therefore, except for CTE mismatch, curing residual stress also played an important role in the package warpage and deformation during thermal processes. In addition, an abnormal negative expansion behavior (NEB) in the laminar underfill after release from the constrained state also seemed to be related to the curing residual stress and needs to be further studied. This work provided a new perspective on the influence of underfill curing residual stress on the reliability of packaging structures.