In this study, we investigated mechanical deformation-induced Sn whisker growth, which is frequently encountered in advanced flexible substrate packaging. Concentrated compressive stresses are introduced around the leads and solder surface finish joints connected by compression fixing. Six types of pure Sn thin films were electroplated on Ni-protected Cu substrates. These were 2- and 6-μm-thick Sn films electroplated with three different current densities: 2, 10, and 20 A/dm2. These films were compressed at room temperature and ambient humidity. The surface and cross-sectional grain morphologies of the films were examined by scanning electron microscopy and focused ion beam spectroscopy, respectively. The grain orientations of the electroplated Sn films were analyzed by x-ray diffraction and electron backscatter diffraction. After compression, nodule hillocks and whiskers were found around the indents. Beneath the indents, the original columnar Sn grains were deformed, and recovery and recrystallization processes occurred. Rapid whisker formation was observed. The whiskers induced by mechanical deformation are closely related to the grain microstructures, and the initial compression stresses are critical to the types and distribution of whiskers as well.