This paper discusses how the microstructures of Sn films affect the kinetics of spontaneous Sn whisker growth. Thin films and those with small grains grew longer whiskers at higher rates than did thick films and those with large grains. Biaxial stresses in the films were measured using synchrotron radiation X-ray; the stress evolution during annealing was correlated with the growth kinetics. An incubation period was observed, in which the compressive stresses in the films built up and the whiskers nucleated. The results indicate that thickness has a greater effect on whisker growth than grain size has.