In this paper, Sn films were electrodeposited from a slightly acidic electrolyte onto pure Cu substrates and aged at room temperature. Tin whiskers were observed after about 20 days. Transmission electron microscopy observations of foils prepared by focused ion beam methods revealed the following distribution of the Cu6Sn5 intermetallic phase: the largest grains were present at the Sn/Cu interface (typically several microns or larger in size); the next largest grains were located along the grain boundaries between as-deposited Sn grains and between whiskers and as-deposited Sn grains (200-500 nm in size); the finest grains (< 70 nm in size) precipitated within Sn grains. Precipitate-free zones (PFZs) were also observed in the vicinity of grain boundaries. The precipitates along grain boundaries between as-deposited Sn grains had a triangular or wedge shape with their long dimension parallel to the boundaries and tips along the whisker growth direction. The finest precipitates hardened the grains, making them less susceptible to deformation. The mechanism and necessary conditions for whisker growth are discussed. It is suggested that the whiskers grow through the movement of PFZs.