An improved methodology for determining temperature dependent moduli of underfill encapsulants

Finite element analyses (FEAs) have been widely used to preventively predict the reliability issues of flip-chip (FC) packages. The validity of the simulation results strongly depends on the inputs of the involved material properties. For FC packages Young´s modulus-temperature relationship is a critical material property in predicting of the package reliability during -55°C to 125°C thermal cycling. Traditional tensile tests can obtain the modulus at selected temperatures, but are tedious, expensive, and unable to accurately predict the Young´s modulus-temperature relationship within a wide temperature range. Thus, this paper is targeted to provide a simple but relatively accurate methodology to obtain the Young´s modulus-temperature relationship. In this paper, three commercial silica filled underfill materials were studied. A simple specimen (based on ASTM D638M) preparation method was established using a Teflon mold. A dynamic-mechanical analyzer (DMA) was used to obtain the stress-strain relationship under controlled force mode, storage and loss modulus under multi-frequency mode, and stress relaxation under stress relaxation mode. A simple viscoelastic model was used and an empirical methodology for obtaining Young´s modulus-temperature relationship was established