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In this work, the viscoplastic mechanical response of a typical underfill encapsulant has been characterized via rate dependent stress-strain testing over a wide temperature range, and creep testing for a large range of applied stress levels and temperatures. A specimen preparation procedure has been developed to manufacture 80 × 5 mm uniaxial tension test samples with a specified thickness of .5 mm. The test specimens are dispensed and cured with production equipment using the same conditions as those used in actual flip chip assembly, and no release agent is required to extract them from the mold. Using the manufactured test specimens, a microscale tension-torsion testing machine has been used to evaluate stress-strain and creep behavior of the underfill material as a function of temperature. Stress-strain curves have been measured at 5 temperatures (25, 50, 75, 100 and 125 C), and strain rates spanning over 5 orders of magnitude. In addition, creep curves have been evaluated for the same 5 temperatures and several stress levels. With the obtained mechanical property data, several viscoelastic and viscoplastic material models have been fit to the data, and optimum constitutive models for subsequent use in finite element simulations have been determined.

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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 are epoxy based, they have the propensity to absorb moisture, which can lead to undesirable changes in their mechanical and adhesion behaviors. In this study, the effects of moisture adsorption on the stress-strain behavior of an underfill encapsulant were evaluated experimentally and theoretically.A novel specimen preparation procedure has been used to manufacture 60 x 3 mm uniaxial tension test samples, with a specified thickness of 0.5 mm. The test specimens 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 them from the mold. The fabricated uniaxial test specimens were then exposed in an adjustable thermal and humidity chamber to combined hygrothermal exposures at 85 C and 85% RH 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 (T = 25, 50, 75, 100 and 125 C). 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 moisture exposure.From the recorded results, it was found that the moisture exposures strongly affected the mechanical properties of the tested underfill 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.

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