Guidelines to select underfills for flip chip on board assemblies

The effect of thermo-mechanical properties of underfill, such as coefficient of thermal expansion (CTE) and stiffness (Young's modulus), on reliability of flip chip on board (FCOB) under thermal cycling stresses is investigated in this study. 3-D and quasi three-dimensional viscoplastic stress analysis using finite element modeling (FEM) is combined with an energy partitioning (EP) model for creep-fatigue damage accumulation, to predict the fatigue durability for a given thermal cycle. Parametric FEM simulations are performed for five different CTEs and five different stiffnesses of the underfill. The creep work dissipation due to thermal cycling is estimated with quasi 3-D model, while 3-D model is used to estimate the hydrostatic stresses. To minimize the computational effort, the 3-D analysis is conducted only for the extreme values of the two parameters (CTE and stiffness) and the results are interpolated for intermediate values. The results show that the stiffness of the underfill material as well as the CTE play important role in influencing the fatigue life of FCOB assemblies. The fatigue durability increases as underfill stiffness and CTE increase. The eventual goal is to define the optimum design parameters of the FCOB underfill, in order to maximize the fatigue endurance of the solder joints under cyclic thermal loading environments.