The fatigue life of an underfilled flip-chip package has been evaluated using the Coffin-Manson relation and finite element modeling (FEM)-computed solder shear strain for typical flip-chip structures. In the course of this effort, numerical simulations were performed for underfill materials of varying thermo-structural properties, two chip sizes, and two solder bump heights. The results were used to examine the parametric sensitivity of the thermal strain in the solder joints and the axial, as well as shear stress in the underfill material. The predicted improvement in the number of cycles-to-failure of the underfilled flip-chip was found to agree with empirical observation. However, the maximum improvement achievable by underfilling was found to be limited by the adhesion strength of the underfill material.
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Gektin et al. (1997) studied this question.
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