Flip-chip assembly has been widely adapted to various electronic devices due to advantages, such as miniaturization of electronic devices and high density integration. The chip-on-flex (COF) package used in this paper is a flip-chip package with an anisotropic conductive adhesive flim (ACF) interconnection and shows flexible features and reduced thickness compared with chip-on-board (COB) packages. All electronic packages experience temperature variation during service conditions and under environmental changes. Under temperature variation, stresses emerge due to the differences in the coefficient of thermal expansion among components. In order to evaluate the thermomechanical reliability of a COF package, a thermal cycling (TC) test was conducted. A moire experiment using Twyman/Green interferometry was performed to observe the warpage behavior of the package under a TC condition. Through the experiment, the rate of change of chip warpage with respect to temperature (dw/dT) as a parameter of the thermal damage model was obtained. A finite element analysis (FEA) was also performed to calculate the maximum shear stress at the ACF layer as another parameter of the model. From the experiment and FEA results, the thermal damage model can accurately represent the TC life of the COF package. However, based on observations of different warpage behavior of the COF package compared with a COB package from the moire experiment, a modified thermal damage model that can predict the TC life of both packages more accurately was proposed.