Spontaneous growth mechanism of tin whiskers

A study has been made of the spontaneous growth of tin whiskers from tin electrodeposits on phosphor bronze sheet. The driving force for the evolution of tin whiskers is a biaxial compressive stress of about 8 MPa developed in tin deposits by the formation of an intermetallic compound of Cu6Sn5, especially in grain boundaries of tin films. The biaxial compressive stress gives rise to strains normal to the film plane, which are dependent on the tin grain orientations. The shear stresses due to differences between strains of different grains along the thickness direction make the tin surface oxide film sheared approximately along boundaries of grains with particular orientations which are different from the major texture of the film. Tin extrudes from the grains, whose surface oxide films are sheared. The extrusion takes place continuously by expansion of the dislocation loops by climb and their subsequent glide toward the surface in the slip direction to form whiskers. (C) 1998 Acta Metallurgica Inc.