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Thermal fatigue damage of flip chip solder joints is a serious reliability concern, although it usually remains tolerable with the flip chip connections (of smaller chips) to organic boards (FR4) as practiced by many industries for over a quarter century. Here, as there is more demand in higher I/O, smaller package size and lower cost, flip chip mounted on module level of board is considered. However, bonding large chips to organic module means a larger differential thermal expansion mismatch between the module and the chip. To reduce the thermal stresses and strains at solder joints, a polymer underfill is added to fill the cavity between the chip and module. This procedure has typically at least resulted in an increase of the thermal fatigue life by a factor of 10, as compared to the non-underfilled case. Yet, our particular case is to deal with flip chip mounted both side of PCB module symmetrically. Thus, thermo-mechanical reliability of solder bump with different underfills is reexamined in this study. Also, using modified Darveaux's model with consideration of solder bumps' size and shape, thermal fatigue life of solder bump is predicted and verified by experimental data.