Effect of Ag content on Cu6Sn5 growth behavior at Sn-Ag/Cu solder interface during multiple reflows

Influenced by Ag content in multiple reflows, the growth behavior of Cu6Sn5 in Sn, Sn3Ag and Sn3.5Ag solders was studied respectively in this work. Parameters, such as the intergranular angle between two adjacent Cu6Sn5 grains, aspect ratio of interfacial grains and distance between copper substrate and bulk solder in grain boundary region, were defined and measured to character the interfacial Cu6Sn5 grains in context of different Ag concentration in solder. At the same time, the two opposite or competing effects, Sn-activity reducing of Ag atoms and boundary-diffusion improving of Ag3Sn particles, on Cu6Sn5 size growth were contrastively analyzed. Actually, the Sn-activity governs the Cu6Sn5 growth rate in the early stage of soldering whereas boundary-diffusion plays a predominant role in late process. As the mechanical properties or the linking strength of solder joints is closely related to grain growth, the results of this research can assist in assessing the quality of Sn-Ag electronic solders highly.