Nucleation and Growth of Tin Hillocks by In Situ Nanoindentation

For several decades, tin whiskers have been a major reliability issue in the microelectronics industry. These single crystalline tin filaments can grow long enough to cause short circuiting and device failure. Although tin whisker/hillock growth is driven by compressive stresses, a mechanistic model of their formation, evolution, and microstructural influence has not been fully developed. In this work, the growth of mechanically induced tin whiskers/hillocks was studied using an in situ nanoindenter and electron backscatter diffraction in a dedicated scanning electron microscope. Electroplated Sn-on-Cu samples were indented and monitored in vacuum to study their growth behavior without the influence of atmosphere. Aging experiments were conducted to study the effect of intermetallics on hillock growth. The grain orientation of the hillocks and the plastically deformed area surrounding the indentation were studied on slabs lifted out of the sample with the use of focused ion beam. High-angle grain boundaries were seen to favor the formation of Sn hillocks. A finite element model was developed to study the evolution of the compressive stress state in the Sn plating and the results showed good agreement with the experimental results.