Influence of copper diffusion on the shape of whiskers grown on bright tin layers

• The growth mechanism of a nodule/hillock on bright tin layers was explained.
• Large amount of voids were observed under the developed hillocks and nodules.
• The hillock/nodule fragments due to additional impurity atoms creating subgrains.
• Copper atoms from the intermetallic accumulate around whisker grain boundaries.
• Whisker subgrain size and location is responsible for nodule or filament growth.

Whisker growth on electroplated bright (grain size < 0.5 μm) tin layers is examined and discussed in terms of the variety of shapes formed. Whiskers may be categorized by three basic morphological types of crystal structure which are usually observed on tin-based finishes (hillocks, nodules and filaments). In this paper, analysis of different whisker shapes and the reason for their development are defined. Constant elevated temperature in ambient humidity (105 °C/15% relative humidity) tests were used to accelerate the growth of very large whiskers (>50 μm nodule/hillock base) through enhanced diffusion, and to minimize the effect of corrosion on the samples. Cross sections from the layers and whiskers were etched and observed using a Focused Ion Beam (FIB). Transmission Electron Microscopy (TEM) was used to analyze the developing whiskers and to provide insight on the development of hillocks, nodules and filaments. Results indicate that fragmentation of a hillock/nodule, and later mass migration and segregation of copper, are critical steps in the evolution of these large whiskers. Copper impurities segregating at the whisker grain boundaries are responsible for the development of various whisker shapes.