Guidelines to select underfills for flip chip on board assemblies

The effect of thermo-mechanical properties of underfill materials, such as coefficient of thermal expansion (CTE) and Young´s modulus, on reliability of Flip Chip on Board (FCOB) under thermal cycling stresses is investigated in this study. Quasi two-dimensional finite element modeling (FEM) viscoplastic stress analysis is combined with an energy partitioning (EP) model for creep-fatigue damage accumulation, to predict the fatigue durability of the flip chip assembly for the given accelerated thermal cycle. Parametric FEM simulations are performed for five different CTEs and five different stiffnesses of the underfill material. The results show that the stiffness of the underfill material plays a crucial role in influencing the fatigue life of FCOB assemblies. The CTE does not have a significant effect on the fatigue life. The eventual goal is to define the optimum design and process parameters of the flip-chip-on-board assemblies in order to maximize the fatigue endurance of the solder joints under cyclic thermal loading environments