The effects of underfill on the thermal fatigue reliability of solder joints in newly developed flip chip on module

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.