Correlation of Intrinsic Thin Film Stress Evolution and IMC Growth with Whisker Growth

This paper explores the notion that the nucleation and growth of Sn whiskers is motivated by net compressive intrinsic thin film stresses. In this view, a threshold level of stress should exist at which Sn whiskers nucleate; furthermore, whiskering will relieve the compressive stress by a measurable amount. We examine the threshold stress for whisker nucleation and measure the amount of stress relieved during Sn whisker growth on brass substrates. The stress evolution has been evaluated by traditional bent beam analysis via novel machine vision techniques. Whisker nucleation and growth of the Cu-Sn intermetallic layer was observed by FIB sectioning, EDX mapping, and electron microscopy. Results show that the measured stress evolution shows little correlation to whisker nucleation and intermetallic growth. Further, we observe whisker population densities under both compressive and near neutral thin film stress conditions.