The integrated theory of whisker formation - a stress analysis

Whisker formation is generally recognized to be a stress relief mechanism common to electrodeposited films of tin (Sn), cadmium (Cd), zinc (Zn), and silver (Ag). This paper constructs a theoretical framework for stress creation within tin films and compares the theoretical construct with actual stress measurements. The relationship between film microstructure and stress was developed by melding finite element analysis (FEA) with film microstructure observations and stress measurements. Additionally, an analysis is presented that shows a relationship between film stress and individual grain orientation. This paper has concentrated on the relationship between microstructure and stress states for Sn/Cu and Sn/Ni/Cu couples. The authors believe that whisker formation is part of a stress relief scenario and that to understand the fundamental aspects of whisker formation and growth it is absolutely necessary to understand stress states within the film structure. The FEA analysis presented herein should be considered a preliminary assessment of the relationship between zonal structures and film stress states and the experimental data herein presented is in basic agreement with the theoretical predictions.