Experimental Characterization and Viscoplastic Modeling of the Temperature Dependent Material Behavior of Underfill Encapsulants

Abstract

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.

Effects of moisture exposure on the mechanical behavior of flip chip underfills in microelectronic packaging

“…The glass transition region for T > +100 C is clearly evident. In our prior work [24][25]32], we have found that an empirical four-parameter hyperbolic tangent model can be used to accurately model the observed nonlinear stress-strain data at any temperature. The general representation of this relation is…”

“…After the Prony coefficients were determined, the master curve at a reference temperature was shifted to the various temperatures for comparison with experimental data. Chhanda, et al [24][25] have also used a Prony series viscoelastic model to represent the time dependent mechanical behavior of underfill encapsulants. They showed the model could accurately represent creep and stress-strain for a variety of temperatures.…”

“…Free standing underfill samples for mechanical testing were prepared using a novel procedure described in previous work of the authors [13][14][24][25][26][27][28]. A stacked assembly view of the "mold assembly" utilized in this investigation is pictured in Figure 2a, and Figure 2b shows example uniaxial specimens after curing.…”

“…shear modulus vs. time and bulk modulus vs. time data, curve fitting can be used to get the Prony coefficients and two methods have been implemented [23][24][25]. In the first method, the temperature shift factor for each temperature can be determined by constructing a master curve, where the shear moduli or bulk moduli at all temperatures are shifted to the curve for a selected reference temperature.…”

Nanomechanical characterization of SAC solder joints - reduction of aging effects using microalloy additions

“…In our work, the thermal drifts were kept smaller than 0.05 nm/sec, and the thermal drift effects were excluded from the resulting displacement data. The method for calculating the creep strain rate proposed by Mayo and Nix [38] has been adopted in this study, as well as log hyperbolic tangent creep model proposed by Chhanda, et al [39]. In addition, the approach presented by the authors in reference [20] has been utilized to predict (extrapolate) tensile creep strain rates for low stress levels using nanoindentation creep data measured at very high compressive stress levels.…”