Microstructural Development and Possible Whiskering Behavior of Thin Sn Films Electrodeposited on Cu(Zn) Substrates

The aging behavior at room temperature of thin Sn films, electrodeposited on top of Cu(Zn) substrates containing 15 wt.% and 36 wt.% Zn, was investigated, using focused ion beam microscopy and x-ray diffraction analysis to evaluate the microstructural and (residual) stress development in the specimens. For comparison, parallel experiments and similar analyses were performed with Sn films electrodeposited on pure Cu substrates. Whereas Sn whiskering was observed for the Sn films deposited on the Cu substrates, such whiskering was not observed for the Sn films deposited on the Cu(Zn) substrates. It was found that alloying the Cu substrates with Zn strongly slows down the formation rate of the intermetallic compound Cu6Sn5 at the Sn/Cu(alloy) interface. The Sn films on the Cu(Zn) substrates remained whisker free for the entire time of investigation even though an overall compressive state of stress has developed after several weeks of aging. It was concluded that a homogeneous, compressive stress in the Sn film does not lead to whisker formation: the presence of negative stress gradients is essential for Sn whisker growth.