Viscoelastic properties of underfill for numerical analysis of flip chip packages

This paper considers the viscoelastic properties of underfill in the numerical stress analysis of FCOB packages. The 'master curve' is obtained for a typical underfill resin based on the time-temperature superposition (TTS) technique. The finite element method (FEM) was employed for a FCOB package consisting of die, solders, underfill and FR-4 printed circuit board, which was subjected to a thermal cycle, and the evolution of residual stresses were examined. The results indicate that in the elastic model the stresses at the solder joints are significantly underestimated. The maximum creep strain accumulated in the solder is two to three times higher in the viscoelastic model than in the elastic model. Significant relaxation takes place in the underfill resin, especially at elevated temperatures and during extended dwell time, which deteriorates significantly the original functions of underfill. The implication is that the viscoelastic nature of underfill material must be properly taken into account if the failure of solder joints and thus the lifetime of the package are to be accurately estimated. The normal and shear stress components at the solder joint near the copper stud were also much higher in the viscoelastic model. This is despite the fact that the von Mises stresses along the solder-copper stud interface were basically similar between the two models, except at the copper stud corners. However, the stresses generated in the underfill adjacent to the die corner were more than five times higher in the elastic model than in the viscoelastic model. This observation arises from the constantly high elastic modulus of the underfill material assumed in the elastic model.