Role of Internal Stress in Whisker Growth of Electrodeposited Tin Thin Films
Hiromi UENO, Keisuke TANAKA, Hirohisa KIMACHI and Daisuke SHIMADA
Abstract:Tin (Sn) thin films with various thicknesses were electrodeposited on Cu substrate with and without Ni undercoat. The change of the internal stress in Sn films was measured by the X-ray diffraction method. For the case of 2.2ƒÊm thick film without Ni undercoat, the initial internal stress was tensile and then gradually changed into compression. After 15h, the compressive stress became below -25MPa and remained constant afterward. Filament-type whiskers were grown on the surface after the stress changed into compression. For Sn films thicker than 4.3ƒÊm, the initial stress was compression and only nodule-type whiskers were grown on the surface. For the case of 2.2ƒÊm thick film with Ni undercoat, the stress was maintained in tension and no whisker was observed. For the cases of thicker films with and without Ni undercoat, the stress was compression around -30MPa just after deposition, and did not change with time. Nodule and mount-type whiskers were formed on the surface, but no filament-type whisker was observed. The compressive stress introduced by bending in 2.2ƒÊm thick film with Ni undercoat resulted in the formation of mount-type whisker. The postbake process induced the tensile stress in 2.2ƒÊm thick film and eliminated the formation of whiskers. Key Words:Tin whisker, Residual stress, Electrodeposited tin film, X-ray stress measurement, Ni undercoat, Postbake