Whisker Formation in Pb-Free Surface Finishes

摘要

Tin-lead (Sn-Pb) alloys have been used extensively for surface finishing of electronic components in part because Pb was found to be effective in retarding Sn whisker growth in electrodeposits. Although whiskers appear to be a local response to the existence of residual compressive stress in the electrodeposited film, questions remain as to the dominant source of stress and the precise growth mechanism. Using both deflection measurements of plated cantilever beams and x-ray diffraction methods (siN_2Ψ), we have determined the in-plane residual stress in Sn, Sn-Cu, and Sn-Pb alloys electrodeposited from both bright- and matte-finish methanesulfonate-based electrolytes. Stress measurements were made soon after deposition and as a function of time. The appearance of hillocks and/or whiskers following plating is influenced by several factors such as intrinsic growth stress, stress induced by Cu_6Sn_5 formation at the Sn-Cu interface, and the deposit microstructure.