Creep and its effect on Sn whisker growth

A comprehensive quantitative whisker growth theory has been developed. In this theory whisker growth is controlled by the ratio of the long range grain boundary diffusion creep relaxation rate to the short range creep relaxation rate. The theory relies solely on solid state diffusion effects and does not require additional constraints or assumptions to explain whisker growth. It explains it number of heretofore relatively poorly explained and puzzling observations reported over the past 6 decades, including: (i) Sn films that are 1-10 mu m thick tire more susceptible to whisker growth than thinner or thicker films. (ii) Sn film contamination in the form of hydrocarbons, Cu, hydrogen. etc., tend to increase the whisker propensity; (iii) as-deposited Sn films tend to be more susceptible to whisker growth than fused films; (iv) postplate annealing tends to improve the whisker resistance; (v) whisker growth rate tends toward zero as the temperature exceeds 100 degrees C; (vi) whisker growth propensity depends upon the crystallographic orientation of the Sn film; and (vii) the large statistical variations reported in whisker density and incubation time. The theory predicts: (i) the magnitude of the as deposited film stress state is not fundamental to whisker growth; (ii) at high stresses where power law creep relaxation dominates. whiskers do not form.; (iii) whiskers occur in those regions of the film that are exposed to stresses lower than that required for power law creep; (iv) whiskers form when a quasiequilibrium stress state is achieved as the stress in the film tends toward zero; (v) anything that affects (limits or restrains) power law creep and its associated stress dissipation, such as grain boundary pinning, increases the propensity for whisker growth; (vi) pinning tends to increase the whisker propensity and increase the whisker growth rate. and (vii) films with certain crystallographic orientations and grain sizes are less prone to whisker growth, independent of the anisotropic thermomechanical properties. as a result of the anisotropic diffusion behavior of Sn (C) 2009 American Institute of Physics. [doi: 10.1063/1.3248277]