Predicting the Effect of Underfill Filler Volume Fraction on Solder Fatigue Life and Residual Stress for Surface Mount Components Using Nonlinear Viscoelastic Analyses

Glassy thermoset polymer underfills are commonly used for reliability enhancement in modern electronics. By adding filler to the polymer, underfill mechanical properties, such as bulk and shear moduli and coefficient of thermal expansion, can be altered. Addition of underfills can affect the solder reliability and component failure during dynamic environments. By modifying the nonlinear viscoelastic simplified potential energy clock model, a generic computational tool was created for analyzing filled polymers. Together with a unified creep plasticity model for solder and the Coffin-Manson fatigue criterion, solder fatigue life for underfilled surface mount components was investigated for various underfill filler materials and filler volume fractions (FVFs) using finite element analyses. By creating models of representative components with very different geometries, the effect of adding an underfill and increasing the FVF of hard and glass micro-balloons (GMB) fillers was analyzed. For a large stiff component, the addition of an unfilled underfill reduced the localized tensile stress in the component. Underfill filler volume fractions greater than 10% for hard filler and 15% for GMB filler resulted in a positive effect on the fatigue life. The results were different for a small flexible component. The addition of an unfilled underfill slightly increased the localized tensile stress in the component, but a positive effect on the fatigue life was still demonstrated if the underfill FVFs were greater than 15% for hard filler and 30% for GMB filler.