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In this paper, hygro-thermally induced deformations and stresses in a flip chip ball grid array (FCPBGA) package are evaluated. Comprehensive warpage and stress analyses are performed on the FCPBGA package by means of experimental warpage measurement using a 3D optical testing system, characterization of hygrothermal expansion and viscoelastic properties of mold compounds, and inelastic finite element analysis of FCPBGA packages. The hygroscopic expansion of a fully cured molding compound and 3D deformation of the FCPBGA package due to moisture absorption were measured using the 3D optical testing system, which was developed by in our previous study (Cheol-Hyun Han and Eun Sok Kim, 2000; Blickenstaff et al., 2003). The study shows that moisture may cause severe stress development in the FCPBGA package. The warpage of the FCPBGA package can even be reversed by moisture absorption. A finite element model was employed to analyze the stresses in the packages and in solder balls. The experimental results are agreeable with the finite element solutions.