Impelementatoin of a viscoelastic model for the temperature dependent material behavior of underfill encapsulants

In this work, the viscoelastic mechanical response of a typical underfill encapsulant has been characterized via rate dependent stress-strain testing over a wide temperature range, and via creep testing for a large range of applied stress levels and temperatures. The 60 × 3 × 0.5 mm 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 is required to extract them from the mold. The manufactured test specimens were used to evaluate the stress-strain and creep behavior of the underfill material as a function of temperature through testing in a microscale tension-torsion testing machine. Stress-strain curves have been measured at 5 temperatures (25, 50, 75, 100 and 125 C), and strain rates spanning over 4 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, 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 has been obtained. The viscoelastic model for underfill has also been implemented in finite element analysis. A quarter model of a flip chip on laminate assembly has been developed for the analysis, and the underfill was modeled as both an elastic-plastic material and as a viscoelastic material. The time dependent variations of the stresses in the underfill and silicon die obtained with the viscoelastic model have been compared to the time-independent results from the conventional elastic-plastic material model.