Stereological Analysis of Microstructural Evolution Due to Aging in SnAgCu Solder Alloys

Due to its high homologous temperature (0.4-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-$$\end{document}0.8 Tm), isothermal aging of Sn3.0Ag0.5Cu solder alloy causes microstructural changes that significantly affect its observed macroscopic creep behavior. In order to characterize such changes in mechanical behavior of the macroscale solder joint, a stereological description of the microstructural changes due to aging is needed. The microstructure of the as-reflowed solder joint mainly consists of primary Sn dendrites surrounded by a eutectic microconstituent consisting of Sn, Ag3Sn, and Cu6Sn5 phases. Upon aging, the microstructure evolves with the growth of the Ag3Sn precipitates within the eutectic microconstituent and change in relative volume fractions of dendritic and eutectic regions. In this study, microstructural changes that occur in Ag3Sn precipitate during aging at temperatures of 25°C, 75°C, and 125°C are monitored for various aging times ranging from 1 to 90 days by employing areal and lineal analysis stereological techniques. The Ag3Sn precipitates are treated as oblate spheroids in shape and their size distributions for all microstructures are determined on the basis of Saltykov–DeHoff analysis. The various microstructures are characterized in terms of Sn dendrite cell size and Ag3Sn particle size. The importance of the developed stereological model for microstructurally adaptive creep modeling is finally discussed.