Effective elastic modulus of underfill material for flip-chip applications

Most underfill materials are two phase composites, e.g., epoxy matrix filled with ceramic particles. The primary purpose of loading ceramic particles is to reduce the coefficient of thermal expansion (CTE) and to increase the elastic modulus. These two thermal mechanical properties are critical parameters to the thermomechanical reliability of a flip-chip package. Developing underfill materials with desired values of the CTE and elastic modulus is a key enabling technology for the next generation low-cost, high-reliability flip-chip packages. In this paper, a micromechanics model is developed to estimate the elastic modulus of underfill materials based on the properties of the matrix and the fillers. To validate the prediction from the micromechanics model, samples of underfill materials with various filler compositions are made. The moduli of these samples are measured using a Thermal Mechanical Analysis system (TMA). Satisfactory results are obtained between the theoretically predicted and experimentally measured values.