Driving force for Sn whisker growth in the system Cu–Sn

The evolution of residual stress gradients in Sn thin films on Cu substrates upon aging at ambient temperature has been investigated, for specimens which do exhibit and which do not exhibit Sn whisker growth, by performing x-ray diffraction stress measurements at constant penetration depths. Comparison of the measured near-surface stress-depth profiles for both types of specimens, as function of aging time at ambient temperature, showed that a significant negative stress gradient from the surface toward the Sn∕Cu interface is decisive for Sn whisker growth in the system Cu–Sn.