Investigation of non-uniform moisture distribution on determination of hygroscopic swelling coefficient and finite element modeling for a flip chip package

Abstract

The coefficient of hygroscopic swelling is a material property used to measure the volumetric change with moisture absorption under given humidity/temperature conditions. Current hygroscopic swelling characterization techniques use an averaged approach based on the averaged moisture content. However, the moisture distribution is not uniform across the test specimen during measurement. This introduces analysis errors in determining the material property. In this paper, an exact analytical solution is derived to obtain the accurate coefficient of hygroscopic swelling, with the consideration of 3-dimentional moisture diffusion across the specimen. The correlation between the locally defined hygroswelling coefficient and the averaged hygroswelling coefficient is obtained analytically. The results show that the coefficient of hygroswelling obtained based on the previous method using the averaged approach overestimates the property up to 250%. The methodology and formulation developed in this paper is applied to analyze a set of existing experimental data, and results are compared to the current approach. Based on the locally accurate coefficient of hygroscopic swelling, this paper also investigates the reliability of a flip chip ball grid array package under HAST condition (120/spl deg/C/100%RH). FEA simulation results revealed the significance of contribution of hygroswelling-induced tensile stresses under bump region. The finite element results give an insight of the failure mechanism associated with moisture absorption.