The stresses occurring in the solder joints during thermal loads have been studied by finite element analysis in order to determine the effect of underfill delaminations on the reliability of FC modules and to evaluate the feasibility of lifetime estimations by the damage integral method. Besides the cases of no underfill and perfect adhesion, underfill delaminations at various interfaces to the solder, to the chip or to the substrate surfaces have been considered. The analysis included four different types of epoxy underfill, three kinds of substrates (alumina, FR-4, and flexboard), and two different tin lead solders (Pb97Sn and eutectic SnPb). The major findings are as follows. (i) The stress analysis has to be based on models that include the inelastic behavior of solder. Otherwise, even the qualitative trends might be wrong. (ii) The major beneficial effect of the underfill is its ability to create a compressive stress within the solder joint that suppresses the tensile stress concentration, which otherwise would lead to opening crack mode conditions at the side of the solder joints. The tensile stress is created by bending, which is a result of the global shear deformation due to the expansion mismatch between chip and substrate. (iii) Since the compressive stress can still be generated, delaminations from solder surfaces are not critical. (iv) When soft underfill is used, an extreme tensile stress concentration is created in the solder at the edge of the bond pad when the underfill is delaminated from the adjacent chip or substrate surfaces. This leads to a complete failure of the FC module very rapidly. (v) Using hard underfill that has a lower CTE than solder, however, the tensile stress in the joint can be kept at a safely low level even when adhesion to chip passivation or substrate is completely lost. Thus, it is possible to design FC modules, for which delaminations are not disastrous. (vi) Since the ultimate failure is always caused by growing of a major crack in the solder or at the underfill/chip interface, the damage integral concept can be applied to all kinds of FC modules, i.e. with or without underfill, to estimate the time to failure.