Relationship between the Reduction Behavior of Copper and Tin Oxide Films by Formic acid and Solder Wettability

Abstract

Void-less solder mounting ensures optimal electrical and heat conduction of soldered components. This requires removal of the oxide films on the metal surfaces. The purpose of this study is to clarify the relationship between the oxide films of the soldering components and solder wettability. The surface oxide films of the copper plate and solder foil were analyzed by transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. The respective oxide films were determined as Cu₂O and SnOx, and the thicknesses were approximately 4.0 and 5.3 nm, respectively. Cu₂O and SnOx were both reduced by formic acid and the changes in film thicknesses upon the introduction of formic acid were measured in real-time using an ellipsometer. The reduction rate of SnOx was higher than that of Cu₂O. Furthermore, the solder foil and solder ball were melted on a copper plate whose thickness was adjusted to an arbitrary oxide film thickness. Although the contact angles of the solder balls decreased by reducing the thickness of the copper oxide film, it was necessary to completely remove the oxide film to obtain good solder wettability.