Suppression of External-Stress-Induced Formation of Tin Whisker by Annealing of Electroplated Sn-Ag Alloy Films

Abstract

  Sn-Ag alloy films, which have been electroplated on thin phosphor bronze plates and annealed at a temperature between 483-523 K, are deterred from tin whisker formation even under a normal mechanical stress. The structure, morphology and chemical analysis studies indicate that large Ag₃Sn crystals precipitate and β-Sn(110) planes grow parallel to the film surface. In the as-plated films, Ag₃Sn crystals have been already distributed. In annealed films, Ag₃Sn crystals increase their sizes to extend from the free surface side to the Sn-Ni interfaces owing to the enhanced diffusion of Ag and Sn atoms at around the eutectic temperature. These characteristic structures and morphologies play key roles to prevent tin whisker formation at ambient temperature probably because they reduce crystal-crystal interfaces, suppress Sn atomic diffusions, decrease the crystal nucleation sites, and release stress and strain retained in as-plated films.