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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.Index Terms-Fatigue, finite element analysis, solder, underfill, viscoelastic model.

The generation of thermal stresses is a major cause for mechanical failure in encapsulated electronic components. In this study numerical modeling is employed to analyze thermal stresses in a high-voltage transformer encapsulated with filled epoxy. The transformer assembly consists of materials with an extremely disparate range of thermomechanical properties. The thermal histories considered mimic those in the operational condition. It is found that, upon thermal cooling from elevated temperature, the ceramic core can be under local tensile stress although it is entirely surrounded by materials with much greater coefficients of thermal expansion. The unique aspect of this paper originates from the fact that the volume shrinkage of the viscoelastic encapsulant during physical aging contributes to an increase in stress over time, thus increasing the tendency of fracture. This counter intuitive result (stress increase due to nonlinear viscoelastic physical aging) can now be predicted using constitutive models recently developed at Sandia National Laboratories. When a silicone coating between the core and the encapsulation is included, the stress is significantly reduced. The modeling result is shown to corroborate with the actual performance of the transformer.