In this paper, an experimental approach is presented to investigate the influence of second level underfill on the thermomechanical behavior of two BGA packages during thermal cycles. Two different flip chip packages with two major underfill reinforcement methods (corner bonding and full bottom surface bonding) and no-underfill were studied. To quantitatively measure the deformation of solder balls, all the BGA packages were cross-sectioned before thermal cycles. The two-dimensional digital image correlation (DIC) technique was used to capture the in-plane deformation of the critical solder ball in thermal cycling intervals. The accumulated plastic strain of the BGA solder was calculated after every 10 thermal cycles. The temperature of each cycle was set from −40 to 100 °C at a 20°C/second rate. The experiment results showed that Package A with fully underfilled and corner underfilled both alleviated the averaged plastic strain on the critical solder ball in comparison with the no-underfilled Package A. However, Package B with corners underfilled had a larger plastic strain than the package without underfill. The material properties of underfill applied in the two reinforcement methods are identical. The results indicate that inappropriate underfilling methods can adversely affect the thermomechanical reliability of the packages. The underfill material and reinforcement methods are associated with the stiffness rigidity and the compact CTE of the package itself. In the respect of thermomechanical reliability, second level underfilling should be individually specified for varied packages.