We have simultaneously measured the evolution of intermetallic volume, stress, and whisker density in Sn and Pb-Sn alloy layers on Cu to study the fundamental mechanisms controlling whisker formation. For pure Sn, the stress becomes increasingly compressive and then saturates, corresponding to a plastically deformed region spreading away from the growing intermetallic particles. Whisker nucleation begins after the stress saturates. Pb-Sn layers have similar intermetallic growth kinetics but the resulting stress and whisker density are much less. Measurements after sputtering demonstrate the important role of the surface oxide in inhibiting stress relaxation.

Although the driving force for the growth of Sn whiskers from the surface of Sn coatings on copper is thought to be internally generated stress due to the formation of Cu 6 Sn 5 at the Cu/Sn interface, little is known about the nature of this internal stress and how it cracks the surface Sn oxide (an important precursor to whisker formation). Arguments based on elasticity alone do not appear to be sufficient and suggest an important role for plastic deformation. Direct observations, made by transmission electron microscopy of cross-sectioned bimetallic Cu/Sn thin-film specimens, confirm plastic deformation of the Sn grains due to the formation of Cu 6 Sn 5 . Dislocation motion and pile-up at the surface Sn oxide, rotation associated with subgrain boundary formation, interaction of the subgrain boundaries with the Sn surface, and diffusional processes are various mechanisms that can produce stress at the Sn surface and crack the Sn oxide.

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