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Detection of Low Copper Concentrations in Nickel Plating Baths
Keywords: Analysis, Plating , Packaging
Nickel electroplating is widely used in semiconductor manufacturing primarily during the packaging stage [1]. It is not used as a final coating, but instead as a barrier layer, to prevent formation of copper–tin intermetallic compounds that affect the reliability of solder joints. The nickel is deposited from baths containing nickel salt (in relatively high concentrations), boric acid, and other ions [1]. The quality of the deposited nickel is highly dependent on the composition of the plating bath. In addition to monitoring the main bath components, the contaminants must also be monitored. Metallic contaminants are acceptable while their concentrations are below approximately 20 ppm [2]. Copper, lead, zinc and cadmium, even in relatively small quantities (higher than 20 ppm) produce a dull, black or skip plate condition in the low-current-density areas. Copper is considered a main contaminant due to its higher concentrations in the bath and the most detrimental effect on the nickel deposit. Traditionally, low concentrations of metals in plating solutions can be monitored using super sensitive polarography methods [3]. These methods can detect parts per trillion of copper and other metals. However, utilization of mercury electrodes makes this method less desirable due to safety and environmental concerns. In nickel-plating solutions, concentrations of copper are high enough that it would be more appropriate to measure copper with a solid electrode. Mercury electrodes are more sensitive than solid electrodes, but in the case of nickel-plating baths, such sensitivity is not required. Prior publications describe methods for determination of low copper concentrations using solid electrodes [4]. These methods work well when copper is present in low (ppm) concentrations and other compounds are present in low amounts or are non-existent. In the nickel-plating bath, the main challenge is that nickel and other component concentrations are very high, while the concentrations of copper are much lower. For that reason, the new analytical method had been carefully verified for possible interference with other bath components. This article provides results of copper analysis in various commercial nickel baths. The response of copper is linear when Rotating Platinum Disc Electrode is used. The methods to eliminate interferences with other bath components are also discussed. One of these methods is to perform dilution with a pH buffer. The pH of this buffer should be close to the pH of the nickel bath. Dilution of analyzed solution with pH buffer reduces the concentration of analyzed copper, but helps to prevent precipitation of boric acid as well as to eliminate interference caused by different concentrations of boric acid in the plating bath. The changes in the concentration of sulfamate ions are also compensated if the bath is diluted with the same pH buffer. Since copper is more electropositive than nickel, the nickel effect on analysis of copper can be easily eliminated by careful selection of the deposition voltage during the copper analysis.
Michael Pavlov, R&D Manager
ECI Technology
Totowa, NJ
USA


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