Potentiometric Analysis of Hypophosphite in Electroless Nickel Plating Baths N. Feldstein, T. S. Lancsek, and J. A. Amick R C A Corporation, David Sarnoff Research Center, Princeton, N . J .
ELECTROLESS PLATING of metals and metal alloys is widely used in industry for the inexpensive deposition of metal films, In many plating baths, especially for the deposition of nickel, hypophosphite is employed as the reducing agent. For nickel baths, both the plating rate and the phosphorus content of the deposit are dependent upon the hypophosphite concentration, with the former generally having a first order dependency on hypophosphite concentration. It is therefore important to control the hypophosphite concentration so that reproducible plating characteristics can be obtained. Although there are several methods ( I , 2) described in the literature for the determination of hypophosphite in electroless plating baths, they generally suffer from two disadvantages: the procedure is lengthy, and the phosphite anion, a byproduct of the hypophosphite oxidation, interferes with the analysis. In this paper, a new electrochemical method for the analysis of hypophosphite that circumvents these limitations is proposed. BASIS FOR ANALYTICAL TECHNIQUE
In a recent study (3) it was observed that the addition of certain oxy-anions such as nitrate, bromate, iodate, or arsenite, to a room temperature electroless nickel plating bath results in a rapid decrease in the plating rate, once a critical concentration is reached. Furthermore, this critical concentration is related to the hypophosphite concentration. If the potential of a Ni electrode in contact with the solution is monitored, the potential is observed to become more positive as the plating rate decreases. To account for this behavior, a model was proposed in which interfacial adsorption of the oxy-anions interferes with the hypophosphite oxidation at the. electrode-solution interface, decreasing the plating rate and changing the Ni electrode potential from the normal mixed potential ( 4 ) to the potential characteristic of a solution free from hypophosphite. Because of the abruptness of the potential decrease, it was conceived that an analytical technique could be designed based on this behavior. The procedure and characteristics of the analytical technique using nitrate as a titrant are describedin this paper. EXPERIMENTAL
Chemicals. All chemicals employed in this investigation were reagent grade; the water used is deionized and double distilled. In specific, nickel chloride, nickel sulfate, sodium hypophosphite, sodium pyrophosphate, sodium nitrate, sodium acetate, and thiourea were “Baker Analyzed” Reagent (J. T. Baker Chemical Co., Phillipsburg, N. J.) while
( 1 ) ASTM “Symposium of Electroless Nickel Plating,” Special
Technical Publication No. 265, American Society for Testing and Materials, Philadelphia, Pa. (2) K . M. Gorbonova and A. A. Nikiforova, “Physicochemical Principles of Nickel Plating,” National Science Foundation, Washington, D. C. (3) N. Feldstein and P. R. Amodio,J. Electrochem. Soc., in press. (4) M. Paunovic, Plating, 55, 1161 (1968).
sodium phosphite and ammonium hydroxide were Fisher Certified Reagent (Fisher Scientific Co., Fair Lawn, N. J.) and Transjst AR grade (Mallinckrodt Chemical Works, New York, N. Y.), respectively. Measurements. All titration experiments were carried out using conventional potentiometric analysis procedures. The titrants employed were 0.50M NaN03, 1.5M NaNOa, and 1.66 X 10-4M (NH& CS solutions. Electroless plating solutions having known hypophosphite content were analyzed after they had reached a constant temperature in a water bath. As indicator electrodes, freshly deposited nickel from the same electroless plating bath plated on 1-inch diameter ceramic wafers was used. A saturated calomel electrode was employed as the counter electrode. Potential measurements were carried out using a differential voltmeter (John Fluke Mfg. Co., Model 825A). RESULTS AND DISCUSSION
Figure 1 shows the results of titrating 200-cm3 aliquots of a specific room temperature electroless nickel plating bath with sodium nitrate. The electroless nickel bath employed had the following composition ( 5 ) : NiS04,9.5 X 10-*M; NaH2P02, variable; Na4P207, 1.1 X 10-’M; and NHIOH, 2.9 X 10-l M. As seen from Figure 1, upon the addition of sodium nitrate, the potential changes from an initial level of about -900 mV us. SCE to a final level of about -550 mV us. SCE. These two potential levels (-900 and -550 mV), correspond, respectively, to the mixed potential of the bath and the equilibrium potential for the electroless bath in the absence of hypophosphite. Apparently, the oxidation of hypophosphite at the nickel surface is inhibited. Although the data shown in Figure 1 were obtained at 25.0 + 0.2 “C, no apparent effect on the titration was noted for variations of as much as =t1 about the nominal 25 “C value. Furthermore, reproducibility of better than 5 was obtained for repeat titrations. From Figure 1, it should be noted that at the higher hypophosphite concentrations, above 2.8 X lO-lM, the end points converge so that the analytical technique is less precise at these concentrations. This same effect has been noted for other plating baths and titrants as will be described below. Since the phosphite anion, a by-product of the oxidation of hypophosphite, interferes with conventional analyses for hypophosphite, its influence on this titration method was examined. Solutions containing up to 2 X 10-’MNa2HP03. 5 H 2 0 were prepared and titrated in the same manner as described above. Results for a given hypophosphite concentration were identical to those for baths from which phosphite was absent (Figure Id). For solutions having a phosphite content above 3 X lO-lM, however, the amount of nitrate required for a given hypophosphite concentration is increased. It is thus concluded that even though the concentration of the phosphite anion will increase as electroless plating proceeds, no interference from the phosphite by-product will be en(5) N . Feldstein, unpublished data, 1970. ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
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Figure 1. Variations of potential nitrate Titrant 0.50M NaNOa NaH2P02concentrations, a. 0.945 X 10-lM 6. c. 1.89 X 10-lM d . e. 2.84 X 10-IM f.
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Figure 2. Variations of potential with addition of thiourea Titrant 1.66 X 10-4M thiourea NaH2P02concentrations, a. 0.473 X lWIM 6. 0.945 X 10+M c. 1.42 X 10-lM
countered at phosphite concentrations below 2 X 10-lM. To ensure that this condition is satisfied, it may be necessary to dilute the aliquot before titration. It is well documented that a wide variety of impurities, (e.g., thiourea, lead, and cadmium ions) above a critical concentration level, will inhibit the plating process. Hence it is anticipated that these impurities may also be used as titrants in the potentiometric determination of hypophosphite. Figure 2 shows the titration results obtained at 25 "C employing thiourea (1.66 X lO-4M) as the titrant. The electroless plating
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bath employed is the same as that used for the titrations in Figure 1 . In using thiourea as the titrant, it was found that the volume of titrant required did not increase with increasing hypophosphite concentration for concentrations above about 10-lM. This lower limiting value for thiourea titration is probably due to the stronger adsorption forces for thiourea on the nickel surface. Comparison of the results in Figure 1 with those in Figure 2 indicate that nitrate is the preferred titrant, because higher hypophosphite concentrations can be analyzed. Other electroless plating baths gave similar results, but each bath apparently has its own characteristic critical concentration of nitrate required for inhibition at a given hypophosphite concentration. For example, an acidic bath composed of NiCl?, 8.75 X lO-*M; CH3COONa, 1.22 X lO-IM, and variable NaH2P02was analyzed. Figure 3 shows the titration curves for this acidic bath titrated with 1.5M sodium nitrate solution. The analyses were carried out at 62 f 1 'C using 200-cm3 aliquots. In this case, it was found that the nitrate concentration required to reach the end point did not change for hypophosphite concentrations above 1.4 X 10-lM. For convenience in titration, it may be desirable to modify acid baths by the addition of ammonium hydroxide to give an alkaline medium. In so doing the baths become sufficiently active at room temperature so that the titration can be carried out at 25 OC rather than at the elevated temperatures. Furthermore, the maximum hypophosphite concentration that can be determined is increased for the alkaline medium titrated at room temperature. Although this investigation was concerned with electroless nickel baths containing hypophosphite, the technique should be equally applicable to the analysis of other reducing agents employed in electroless plating compositions. Whenever one of two reactions contributing to a mixed potential can be inhibited abruptly by the addition of an agent in a critical concentration, the potential at a catalytic electrode will also change, permitting a simple and reliable end point to be determined.
RECEIVED for review March 27, 1970. Accepted April 30, 1970.
