The effects of the grain size and thickness of Sn films on the kinetics of spontaneous Sn whisker growth have been quantitatively analyzed separately. The results reveal that the length and diameter of the whiskers are related to the parameters of the Sn film. A thicker film has a stronger influence on the whisker morphology than does the grain size. When the biaxial stress in the film reached a steady state, the growth rate was dependent on the grain size and thickness of the film. An equation is proposed to calculate the spontaneous growth rate of Sn whiskers in correlation with the dimension and microstructure of the Sn film. Spontaneous Sn whisker growth can be suppressed by increasing the thickness and grain size of the film.