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The purpose of this study is to investigate the thermomechanical behaviors of flip chip ball grid array packages during reflow process, underfilling, underfill curing, and under thermal cycling tests (TCT). The emphasis is placed on the deformations and stress state (especially for die stresses) of the packages. The real-time Twyman-Green interferometry is used for measuring the out-of-plane thermal deformations of the specimen subjected to thermal loadings before and after underfilling. A finite element method (FEM) and an available theoretical solution have been employed to validate experimental observations and provided stress results. The TCT has been carried out, and its result indicates that the die-cracking failure dominates the life of thermal reliability. The full-field deformation data shows that the thermomechanical behavior of the packages before underfilling is creep-governed due to solder joint creep, but becomes, after underfilling, more or less creep-independent and acts as a three-layer structure with zero stress at underfill-curing temperature. However, it is also shown that no warpage status (or free-stress state) for the package could shift from the underfill-curing temperature to the glass transition temperature (Tg) of the underfill material, once the package goes through solder reflow in which the temperature is usually higher than the Tg of underfill material. For the stress analyses, the FEM model and the corrected Suhir solution validated by experimental results present the deformations and stress state of the packages under the TCT. It is suggested that the adequate die/substrate thickness ratio can reduce the die stress significantly and, as a result, increase the life of the TCT. In addition, the die cracking in the packages during the TCT could be also eliminated by using a lower Tg underfill material which allows the tensile-compressive cycling loads at the critical point during the TCT, instead of higher tensile-lower compressive or purely high tensile loads.