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The purpose of this paper is to experimentally, numerically, and theoretically investigate the thermomechanical behaviors of flip-chip ball grid array (BGA) packages with low- and high-Tg underfills under thermal loads, after 85°C/85%RH conditions, and after solder reflow. The low- and high-Tg underfills all cured at 150°C are with glass transition temperatures (Tg) at 84°C and 142°C, respectively. The emphasis of the study is placed on the deformations and die stresses of the packages under these loading conditions. This paper has presented that Suhir's solution of a trimaterial assembly and finite-element method (FEM) can effectively predict the thermomechanical behaviors of both flip-chip BGA packages. It is also found from Twyman-Green experiments that the stress-free temperature is at underfill-curing temperature (Tc) for both packages before solder reflow, but not after solder reflow. Furthermore, the relation between curvature and temperature is nearly linear for packages with high-Tg underfills, while bilinear (with turning point near Tg) for those with low-Tg underfills. It is also discovered that the packages with low-Tg underfills during cooling processes suffer stress relaxation, and, thus, their deformations depend on cooling rate, but not for those with high-Tg underfills. The die stresses can be directly determined by Suhir's die stress solution associated with the curvature data from the experiments. The coplanarity of the packages with and without fillets has also been addressed by the FEM analyses combined with experiments. It is found that the fillet effect on the warpage is negligible for this flip-chip BGA and the 2-D axis-symmetrical model can be approximately used for addressing the global warpage. Regarding 85°C/85%RH environmental effect, it is shown that the packages with high-Tg underfills are insensitive to this environmental effect, while those with low-Tg underfills are minor sensitive (about 10% decrease). It is also found that solder reflow process does affect the curvature of the packages by increasing 18% and 15% of the average curvature for those with high- and low-Tg underfills, respectively.