Viscoelastic Modeling of Microelectronic Packaging Polymers Including Moisture Effects

Abstract

Reliable, consistent, and comprehensive material property data are needed for microelectronics encapsulants for the purpose of mechanical design, reliability assessment, and process optimization of electronic packages. Since the vast majority of contemporary underfills and other microelectronic packaging polymers used are epoxy based, they have the propensity to adsorb moisture, which can lead to undesirable changes in their mechanical and adhesion behaviors. In this study, the mechanical behavior of microelectronic packaging polymers including moisture effects has been evaluated experimentally and theoretically. A novel specimen preparation procedure has been used to manufacture uniaxial tension test samples. The materials were dispensed and cured with production equipment using the same conditions as those used in actual flip chip assembly, and no release agent was required to extract the specimens from the mold. The fabricated uniaxial test specimens were then exposed in an adjustable thermal and humidity chamber to combined hygrothermal exposures for various durations. After moisture preconditioning, a microscale tension-torsion testing machine was used to evaluate the complete stress-strain behavior of the material at several temperatures. The viscoelastic mechanical response of the underfill encapsulant has also been characterized via creep testing for a large range of applied stress levels and temperatures before and after moisture exposure. From the recorded results, it was found that the moisture exposures strongly affected the mechanical properties of the tested underfill and other polymers including the initial elastic modulus and ultimate tensile stress. With the obtained mechanical property data, a three-dimensional linear viscoelastic model based on Prony series response functions has been applied to fit the stress-strain and creep data, and excellent correlation had been obtained for samples with and without moisture exposure. The effects of moisture were built into the model using the observed changes in the glass transition temperature within the WLF Shift Function. Moreover, the surface morphologies of the fractured polymer specimens have also been analyzed in some cases using optical microscopic view to understand the effect of moisture exposure. The viscoelastic model for underfill has also been implemented in finite element analysis. Quarter models of flip chip on laminate assembly have been developed. The first model was used to analyze the time dependent variations of the stresses in the underfill and silicon die obtained with the viscoelastic model which have then been compared to the time-independent results from the conventional elastic-plastic material model. The second model has been developed to study the effects of moisture exposure in underfill layer on the mechanical behavior of other components of the assembly.