The degradation of a chloride-based solution, used for simultaneous electrodeposition of Au-Sn alloys, has been studied using a combination of turbidity measurements, UV/Vis spectroscopy, XRD and SEM/TEM. Degradation virtually begins immediately after the solution is prepared, due to the gradual reduction of ionic complexes of gold by ascorbic acid. The resulting gold precipitates are initially less than 10 nm in size, but agglomerate after two to three days to form visible particles that settle and coat the container walls. To solve the bath stability problem, a thorough understanding of the mechanism of the Au-Sn plating process and the roles played by each ingredient are required.
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