Time-dependent material modeling for finite element analyses of flip chips

Finite element analysis (FEA) has been established as an effective method for reliability assessment of flip chip assemblies. The simulation results are significantly dependent on the selected material models. Regarding flip chip assemblies, this statement mainly applies to the tin lead solder of the flip chip joints and the encapsulant-the so-called underfill. Comprehensive material data of eutectic solder were determined on real flip chip joints. Based on these data, three modeling approaches were evaluated (target platform was the FEA code ANSYS): viscoplasticity (Anand´s model), power law creep (with 2 terms)+plasticity, and sinh law creep+plasticity (as user defined model in ANSYS). Underfills are often modeled as very simple elastic materials. Tensile tests were performed on underfill samples to study its real behavior. Two modeling approaches of a representative underfill were evaluated: linear elasticity and linear viscoelasticity. The properties of all above mentioned approaches are discussed in terms the simulated material behavior at various temperatures and deformation rates. For each combination of approaches, temperature cycling tests on a flip chip module were simulated by ANSYS. Different combinations of modeling approaches for solder and underfill led to different simulation results although each model was based on the same measurement data. The differences are discussed and conclusions are drawn about which modeling approach is preferable for typical applications