Process analysis of electroless nickel deposition baths using optical spectroscopies

Summary The analytical techniques of Vis-spectrophotometry and FTIR spectroscopy have been applied to the quantitative determination of important plating bath constituents of an electroless nickel-phosphorus electrolyte under realistic bath operation conditions. In combination with multivariate calibration methods (partial-least squares (PLS)- and principal component (PCR) regression) Ni²⁺, the reducing agent H₂PO ₂ ^– and its oxidation product HPO ₃ ^– could be directly determined even in the presence of other bath constituents like malic, lactic and adipic acids. For Ni²⁺, absorbance spectra in the 500–900 nm wavelength range were used to develop a PLS calibration model that allows to compensate matrix influences on the Ni²⁺ spectrum. PLS regression was carried out in the concentration range 0.5–6 g l^–1 Ni²⁺ with independently varying bath parameters (concentration of other bath constituents, pH and temperature). Evaluation of the model by predicting the concentrations of test samples which had been drawn from real process solutions of a Ni plating bath yielded a root-mean-squared error of 0.06 g l^–1. The metal concentration of the nickel electrolyte in a compact electroplating unit was monitored in-line by measuring spectra by means of a Vis-spectrometer adapted to the process via quartz glass fiber optical cables of 50 m length. A comparison of the in-line data with potentiometric off-line reference analyses showed good agreement of both data sets with a root mean-squared error of 0.15 g l^–1. The potential of FTIR spectroscopy for process analysis of plating bath solutions could be demonstrated by measuring off-line the concentrations of H₂PO ₂ ^– and HPO ₃ ^– . For these two components calibration samples containing all other main bath constituents were used to set up a PCR calibration model with IR spectra measured in the range from 850 to 1800 cm^–1 with an ATR trough technique. The data showed — within the error of the method of about 10% — generally a good accordance with the stoichiometry of the Ni²⁺/H₂PO ₂ ^– reduction process. The correlation of both parameters which was confirmed in these experiments could allow effective bath control with minimum analytical instrumentation, e.g. only a Vis-spectrophotometer.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday