Coffin-Manson based fatigue analysis of underfilled DCAs

The continuing drive toward high-density, low-profile Integrated Circuit packaging has accelerated the spread of flip-chip technology to laminated substrates, creating direct chip attach (DCA) configurations. However, the substantial difference in the coefficients of thermal expansion (CTE) between the chip and the laminated substrates makes DCA configurations vulnerable to thermally-induced strains and the resulting solder joint fatigue. The reliability of flip-chip technology is dramatically improved by “underfilling” the gap between the chip and substrate with epoxy. The present effort is aimed at exploring the benefits of underfilling in DCA configurations. The thermo-structural behavior of an underfilled DCA is evaluated using FEM and employing an axisymmetric model of a typical DCA structure. Numerical simulations are performed for different sets of underfill material properties. The results are used to determine the parametric sensitivity of the thermal strain in the solder joints and the axial and shear stresses in the underfill material and to define the desirable range of underfill material properties. These results together with the Coffin-Manson relation are used to predict the theoretical improvement in cycles to failure. The results suggest that to minimize fatigue failure, the CTE of an underfill material should match that of solder material and its Young´s Modulus should be as high as the adhesion strength of the underfill allows.