The spontaneous growth of whiskers from Pb-free Sn solder films on Cu
substrates poses a serious threat to the reliability of electronic circuits.
Researchers at Brown are using experiments and computer simulations
to understand the mechanisms for the formation of Sn whiskers. As part
of this effort, a new computer model has been developed that couples
stress-driven grain boundary diffusion with elastic and plastic
deformation in a 3-dimensional polycrystalline microstructure. This
model has been used to show how stresses develop in the Sn film due to
reactions with the Cu substrate, and how these stresses lead to
deformation and whisker formation in the film.
Figure 1 shows a simulation of whisker formation. Cu diffuses into the
Sn film and reacts to form an intermetallic compound, resulting in
volumetric expansion within the film. The volumetric expansion is
accommodated in much of the film by dislocation plasticity (red). A
few grains, however, are assumed to have a flow stress slightly lower
than their neighbors. This relaxes the stress near the soft grain, and the
resulting stress gradient drives Sn to diffuse towards the soft grain. The
soft grain is extruded from the film, forming a whisker.
Based on results of these simulations, an analytical model has been
developed that predicts whisker growth rates which are in good
agreement with representative experiments (see Figure 2).
These and future results will be used to guide efforts to develop Sn films
with whisker-resistant microstructures.