Recrystallization as a Growth Mechanism for Whiskers on Plastically Deformed Sn Films

Abstract

Sn whiskers are becoming a serious reliability issue in Pb-free electronic packaging applications. Sn whiskers are also observed in connector parts of electronics as well as on electroplated surface finishes. Sn whiskers found in connector parts are known to behave differently from the typical Sn whiskers reported on electroplated Sn surfaces. In this study, Sn whiskers on plastically deformed Sn-rich films were investigated to understand their growth behavior to establish mitigation strategies for Sn-rich films used in connectors. Therefore, a microhardness indentation technique was applied to plastically deform electroplated matte Sn samples, followed by temperature/humidity (T/H) testing (30°C, dry air). Each sample was examined by scanning electron microscopy at regular time intervals up to 4000 h. Various morphologies of Sn whiskers on plastically deformed matte Sn films were observed, and their growth statistics and kinetics are analyzed in terms of the plating conditions and plastic deformation by using transmission electron microscopy, x-ray diffraction, and the focused ion-beam technique. Sn whiskers were observed on plastically deformed regions of thin (2-μm) and thick (10-μm) matte Sn films, regardless of the current density applied. Plastic deformation was found to promote whisker formation on matte Sn films. A high density of dislocations and newly formed fine Sn subgrains were observed in deformed grains. In addition, the recrystallized grains and Cu<inf>6</inf>Sn<inf>5</inf> intermetallic compound grew further with increasing time. Finally, a growth mechanism for deformation-induced Sn whiskers is proposed based on a recrystallization model combined with the formation of Cu<inf>6</inf>Sn<inf>5</inf>.