by P. Bratin, E. Shalyt, and M. Pavlov
ECI Technology, 60 Gordon Drive, Totowa, NJ 07512
The automotive industry is one of the largest consumers of electroplating technology, utilizing virtually any commercially available process from zinc to gold electroplating. The largest by volume electroplating applications are zinc and nickel coatings for corrosive protection of parts. Requirements to uniformity of corrosion protection coating are usually not as strict as in functional plating for electronics. It makes the plating process more forgiving. That is why analytical control in general plating is usually inferior to that in plating for electronics.
Modern trends with continuously increasing requirements on quality, cost, and environmental issues in Automotive Industry calls for tighter control of Electroplating solutions and narrowing of the operation windows, thus diminishing the gap between plating for electronics and general finishing. This creates a demand for high accuracy analytical tools, capable of multi-component analysis of plating solutions.
While determination of metals and acid is a simple routine procedure, determination of proprietary organic additives remains a challenging task. Organic additives are the key ingredients defining the quality of deposit. Too low or too high concentrations of additives can cause problems in grain structure, treeing, burning, whiskers, poor throwing power, surface corrosion, etc.
The additive concentration changes with time as a result of drag-out and electrochemically induced transformations (oxidation, reduction, polymerization, segmentation, and reaction with other components of the electrolyte). Some of the organic additives may adsorb on the cathode surface and get occluded in the deposit - while others are accumulated in solution as breakdown products and must be removed by carbon treatment.(1)
Typically, replenishment of organic additives is based on amp-hours. However, the consumption rate of additives may depend on the operating conditions and even the geometry of the plated parts. Therefore, amp-hour control must be supplemented by analytical determination of additive level.
The absolute concentration of some organic additives in plating solutions can be monitored by either HPLC or spectroscopic techniques. However, the absolute concentration is relevant to the deposit only for very clean solutions which have not undergone extensive electrolysis. During electrolysis, the additives undergo numerous chemical transformations. A fresh solution may have as many as twenty individual substances while a production solution can contain hundreds of stable and intermittent products of the additive transformations. Some of these products can be more active as inhibitors or brighteners than the initial components. For these reasons, it is not unusual for an analytical technique such as HPLC to show zero brightener concentration for an electrolyte producing a bright deposit.
The only viable analytical approach for control of organic additives in electroplating solutions is the measurement of their "effective" concentration. To detennine the "effective" concentration (or activity), the effect of all additives in the production solution is compared to the effect of the additive of interest present in known concentration in a standard solution. Traditionally, the effective concentration of electroplating additives was obtained by using a Hull Cell test. The interpretation of the results of a Hull Cell test is subjective and requires extensive personnel training. Since the Hull Cell test is a qualitative, overall performance test, it is not suitable for the separate determination of individual components of additives.
In 1979 Rockwell International patented the application of Cyclic Voltammetric Stripping (CVS) for the determination of electroplating additives. (2) CVS, as patented by Rockwell, utilizes the effect that organic additives exert on the rate of metal deposition. Indeed, the grain size and throwing power are functions of the electrode overpotential () and the faradaic resistance (), respectively. Therefore, regardless of the specific mechanistic effects of the organic additives, they modify the electroplating properties of metals (3,4) and these changes can be observed on the. CVS voltammogram.
CVS measurement is performed in a three electrode cell with a rotating platinum working electrode. During the experiment, the potential of the electrode is swept at a constant rate between two potentials and the resulting current is measured as a function of the applied voltage. An example of CVS voltammogram is shown in Figure 1.
Figure 1. Typical Voltammogram of Non-Cyanide Alkaline Zinc Electrolyte
The voltammogram can be divided into several distinct regions:
- 1-2-3 Plating of metal onto the platinum electrode
- 3---4: Stripping of deposited metal
- 4-5-6: Cleaning of platinum electrode surface
- 6---1: Adsorption of organic additives
The major analytical signal of CVS is the stripping peak area, which is directly proportional to the amount of metal deposited onto the platinum electrode. At fixed experimental conditions, this area also represents the average plating rate in the potential range 1 -2-3.
Since 1979, CVS has become the de-facto standard technique for monitoring copper electroplating baths.(5) Besides, it was used for monitoring of Tin, Tin-Lead, Zinc, and Nickel electroplating solutions, as well as Cleaners, and Conditioners. This publication summarizes our development in control of Zinc and Nickel electroplating solutions.
Most of the electrochemical measurements were performed with a Qualiplate QP-4000™ Plating Bath Analyzer. Some measurements were done with a commercially available generic potentiostat. Both potentiostats employed the QPMS-4 measuring stand of the QP-4000, with a rotating disk platinum electrode, 4 mm diameter, 316 Stainless Steel counter electrode, and double junction Ag/ AgCl reference electrode. All non-proprietary chemicals were of Reagent Grade. Proprietary additives and samples of actual plating solutions were supplied by customers (plating facilities) and by commercial suppliers of plating solutions.
All analyses were performed at room temperature unless otherwise noted. Electrode preparation prior to the analysis was done by conditioning electrodes for 5-25 cycles in zinc plating solutions usually without organic additives and/ or chemical cleaning in 10% HCL or 50% ethyl alcohol for 1-3 min.
Each data point in CVS analysis was taken after a reproducible reading of peak area (within 2%) was obtained after 2-5 cycles.
Some experimental results were normalized by the following procedures:
- Normalized Concentration was calculated as the ratio of the actual additive concentration to the optimum concentration in the plating solution.
- Normalized Peak Area, Ar/Ar(O) was calculated as the ratio of the peak area in solution with additive (Ar) to peak area in a blank solution without this additive (Ar(O)).
Stripping Efficiency was calculated as a ratio of Stripping Peak Area to Plating Peak Area.
RESULTS AND DISCUSSIONS
Reproducibility of the CVS Measurement
To demonstrate the reproducibility of the CVS technique, we monitored the results of30 successive cycles in a Zinc electrolyte for a new (unconditioned) platinum electrode (Figure 2). Experimental results indicate excellent reproducibility of results with relative deviation less than 1%.
Figure 2. Reproducibility of Analytical Signal
Analysis of Zinc Electrolytes
Despite differences in operation of non- cyanide alkaline, chloride, or sulfate electrolyte, the CVS behavior of proprietary additives used in these processes is similar.
While some suppliers still use premixed ail-in-one organic additives, there is a definite trend in industry toward splitting additive packages in separate Carrier, Brightener, and sometimes Purifier additives.
Figures 3,4 illustrates effect of Carrier and Brightener on voltammogram of Non-Cyanide Alkaline Electrolyte of Supplier X. The optimum concentration of organic additives in this electrolyte is:
Carrier: 6 ml/1
Brightener: 0.3 ml/1
Figure 3. Effect of Carrier on Voltammogram of Zinc Electrolyte
Figure 4. Effect of Brightener on Stripping part (top) and Plating part (bottom) of Voltammogram of Zinc Solution with 6ml/l of Carrier
Experimental results indicate that Carrier is a Primary Suppressor. The increase of Carrier concentration escalates the polarization of the cathode, thus providing grain-refining effect.
Brightener has minor suppression effect on plating part ofvoltammogram. At the same time it increases the anodic polarization of stripping process that indicates the formation of more energetically stable coating in the presence of Brightener.
Figure 5 illustrates the marginal suppression effect of separate Carrier and Brightener components. It allows accurate determination of Carrier in presence of other components by Electrochemical Titration.(3,6,7)
Figure 5. Effect of Additives on Analytical Signal
Electrochemical Titration utilizes the dramatic suppression effect of Carrier (Primary Suppressor) in the concentration range 0-1 mill. To collect data in this region, one titrates a supporting electrolyte with the production solution. The peak area is measured prior to analysis and after each addition (see Figure 6).
Figure 6. Analysis of Carrier in Production Solution by Electrochemical Titration
The endpoint of this titration is defined by an empirical suppression level (e.g., 50% suppression). Normalization of the peak area minimizes the effects of temperature variation and "memory effects" of the solid platinum electrode.
Prior to the analysis of production solutions, a calibration factor is determined by titrating the supporting electrolyte with a standard solution containing a known amount of Carrier. The calibration factor is used for calculation of Carrier concentration in the production bath. The Electrochemical Titration procedure is completely automated and typically requires 10-30 minutes with 1-5 minutes of operator time.
Validation of analytical procedures was performed by repeated analysis of standard and unknown solutions. We also tested the recovery of dilution and spikes of Carrier and Brightener in production solutions. Some of the results are shown in Figure 6. As one can see, titration curve follows the change in concentration of Carrier while stay unaffected by "spike" of test solution with Brightener. The results, summarized in Table 1, indicate that CVS provides accurate (better than 15%) determination of the effective concentration of Carrier in the presence of the other components of the plating solution.
Determination of Brightener exploits Synergistic effect of Carrier and Brightener. Brightener has very little effect on the stripping peak area when present alone (see Figure 5). At the same time, Brightener significantly enhances the suppression effect of Carrier (Figure 7). Therefore, the effect of Brightener becomes measurable if the supporting electrolyte contains Carrier (Figure 8).
Figure 7. Effect of Brightener on Response of Carrier
Figure 8. Response of Brightener in Support Electrolyte
Once Carrier concentration is determined, the brightener concentration becomes the only unknown variable and can be measured by Calibration Curve. In this case one should first measure Carrier and prepare a supporting electrolyte with identical concentration of Carrier.
Sometimes, it becomes possible to provide truly independent procedure for determination of Brightener. One of the analytical approach is to "saturate" supporting electrolyte with Carrier. As on can see from Figure 4 the effect of Carrier in VMS reaches saturation at 2 ml/1. At concentration of Carrier above 4 ml/1, the response of Brightener becomes almost independent to variations of Carrier concentrations. By optimizing operation parameters we were able to decrease required addition of solution down to 10% (see Figure 8). It allows to use Electrochemical Titration procedure utilizing universal supporting electrolyte of fixed Carrier concentration 5 ml/1 for analysis of Brightener in any production solution. Indeed, if concentration of Carrier in production bah can be 5 ml/1 ±50%, the resulted concentration of Carrier in test solution will be 5 ml/1 ± 5%.
Thus, both Carrier and Brightener can be measured independently by the same analytical procedure and the same experimental set-up. The only difference is the composition of the supporting electrolyte.
Purifier component usually contains oxidizable component (e.g. thiourea), which can be directly measured through the oxidation current (Figure 9).
Depending on specific formulation of Purifier additive, oxidation current is proportional either to square root of concentration or directly to concentration of Additive (Figure 10).
Figure 10. Response of two different Purifier Additives
The same analytical approaches were used for analysis of various Alkaline Non-Cyanide, Chloride, and Sulfate Zinc proprietary electrolytes, listed in Table 2. All analytical procedures were validated by reproducibility, "spike" recovery, and "dilution" recovery tests.
Analysis of Nickel Electrolytes
CVS procedures for control of Nickel electrolytes stand aside from all other applications. Indeed, the major analytical signal in CVS technique is a stripping peak area, and Nickel is well known for its irreversible behavior and passivation at anodic polarization.
Complete stripping of nickel from a platinum electrode during CVS cycle can be achieved by introducing of chloride or bromide ions. Still, the observed peak cannot be used for analytical purposes because it represents the total charge of nickel stripping and chloride oxidation (See Figure 11 ).
Figure 11. Voltammogram of Nickel Electrolyte without Organic Additives
The further corrosion promotion is provided by organic additives, namely Stress Reducers. These sulfur-containing substances (e.g., sodium saccharin) form Ni-S deposit, which has significantly lower corrosion resistance. Experimental results shown in Figure 12 indicate that addition of Stress Reducer into electrolyte causes the shift of nickel stripping to less positive potential. Finally at some critical concentration of Stress Reducer, the nickel stripping is completely transferred into the left anodic peak.
Figure 12. Effect of Stress Reducer on Voltammogram of Nickel Electrolyte
The procedure for determination of Stress Suppressor exploits this shift of nickel stripping. The utilized procedure is very similar to Electrochemical Titration used for analysis of Zinc Electrolytes. But in this case, the analytical signal is the ratio ofleft stripping peak area to plating peak area.
At the beginning of titration the left peak is very small, and the analytical signal is close to zero. As stripping of nickel is shifted into left peak, the value of analytical signal reaches the current efficiency (ratio of total stripping charge to total plating charge). The end point of calibration is defined as transition point of the curve.
For most stress reducers the transition is very sharp even at room temperature, while some additives require analysis to be done in the jacketed beaker at 50°C (operating temperature of that electrolyte) for a sharp transition (see Figure 13)
Figure 13. Effect of Temperature on Response of Stress Reducer
Figures 14-15 illustrate excellent day-to-day reproducibility of Stress Reducer analysis, as well as recovery of spike and dilution. In all experiments the accuracy was better than 15% (See Table 3).
Figure 14. Reproducibility of Analysis of Stress Reducer
Figure 15. Determination of Stress Reducer in Production Solutions
Stress Reducer is the only organic component of electrolytes for plating of matte nickel coating. Electrolytes for bright plating contain an additional Brightener component.
Brightener component alone does not have noticeable effect on stripping peak area in support electrolyte without other additives. Once support electrolyte is "saturated" with Stress Reducer, Brightener behaves as Suppressor and can be analyzed by Electrochemical Titration Technique. (Figure 16).
Figure 16. Response of Brightener
Analytical procedures have been validated. According to experimental results Stress Reducer and Brightener do not interfere with determination of each other. In all experiments the accuracy was better than 15%.
Some suppliers are used premixed ail-in-one package containing both Stress Suppressor and Brightener. Even in this case, CVS provides accurate determination of sub-components. Figure 17 shows the effect of such mixture on both plating and stripping area. Since plating peak area is not affected by Stress Reducer component - the observed suppression of plating peak is the effect of brightener component alone and can be used for its analytical determination. Besides, brightener can be measured by Electrochemical Titration procedure in support electrolyte "saturated" with sodium saccharin (a common stress reducer) as described above.
Figure 17. Response of Premixed Additive
Both Stress Reducer and Brightener affect stripping peak area that is reflected in dual effect of premixed additive on stripping peak area ( see Figure 17). The shift of stripping peak due to effect of Stress Reducer causes the initial raise of Stripping. Once transition of nickel stripping is complete, the change of stripping peak is governed by Brightener. The effect of Brightener component is easily discriminated by the normalization (Stripping Peak Area over Plating Peak Area), as shown in Figure 18.
Figure 18. Normalized Response of Premixed Additive
One special group of organic additives used sometimes in nickel plating is Sodium Allyl Sulfonate. These substances have both functional groups essential for Stress Reducing and Brightening effect on deposit. From CVS point of view, SAS is equivalent to premixed additive containing both Stress Reducer and Brightener.
Utilizing the same analytical approaches as we described above for premixed additive, CVS can measure separately the effective concentration of both functional groups in electrolytes.
Table 4 lists tested proprietary electrolytes. All procedures were validated and accuracy better than 15% was observed.
Incoming Outgoing Inspection of Plating Additives
The synthesis of the organic additives must be controlled to provide predictable activity of the product. Generally employed spectrophotometric or chromatographic techniques measure an absolute concentration of each subcomponent rather than activity of the product. The Hull Cell test, used sometimes for this purposes, is capable of a qualitative estimate only.
As of today, most suppliers of additives for Acid Copper, Tin, and Tin-Lead electroplating are using CVS for quantitative SPC control of additives based on their electrochemical activity.
Most organic additives have a limited shelf-life and specific storage requirements. That is why out-going inspection at the supplier site should be supplemented by incoming inspection at electroplating facility. Indeed, an additive with a different activity threatens underdosing or, even worse, overdosing of the electrolyte. Loss of analytical control of the organic additives leads to plating rejects and, sometimes, the necessity to carbon treat the electrolyte to remove excess organics.
Effectiveness of Carbon Treatment Procedures
As shown above, CVS is capable of detecting organic additives in Zinc and Nickel plating solutions in quantities less than 1% of operation concentration. This makes CVS especially attractive to evaluate improvements of carbon treatment procedures and monitoring of their effectiveness.
CVS as a Research Tool
In addition to process control applications, CVS is indispensable as a research tool for optimization of existing plating processes and development of new formulations.
Valuable information on the activity ("effective concentration") of the individual components, allow the study of:
- the effect of various organic components on electroplating
- the effectiveness of organic additives at various operating conditions (temperature, current densities)
- the transformation and breakdown of additives during plating
- consumption rate of individual components during plating
- decay rate of individual components during shut-downs
- effectiveness of removal of individual components during carbon treatment by various types of carbon materials and at different operation parameters
SUMMARY AND CONCLUSIONS
Applications of CVS for control of Zinc and Nickel solutions have been reviewed and summarized. The available applications include most of the commercial Zinc and Nickel electroplating formulations.
Validation of the analytical procedure was performed by repeated analysis of standard and production solutions. We also tested the recovery of spikes of additives into production solutions, as well as recovery of dilution of production solution. In all tests the results were better than 15%.
Using examples of proprietary additives and actual production solutions, it has been shown that CVS is a reliable tool for:
- Analysis and control of organic additives and their components in plating solutions
- Incoming inspection of electroplating additives at plating facilities as well as outgoing inspection of additives at supplier sites
- Improvement of carbon treatment procedures and monitoring of their effectiveness
- Study of the consumption rate of individual components
- Optimization of existing processes and development of new formulations of tin and tin/lead plating
The analytical procedures described in this paper are available in automated Plating Bath Analyzers that provide accurate and rapid analysis with operator time limited to 1-5 minutes.
This publication is based on cooperative research ofECI Technology and other organizations and persons during the last fifteen years. We would like to especially acknowledge the contribution of William Ward (Pavco Inc.), Dennis Cupolo (Avon), and Michael Igumnov (Moscow Institute of Fine Chemical Technology) to the development of CVS analytical applications. We also acknowledge the cooperation of all suppliers and electroplating facilities in releasing technical information and samples of proprietary organic additives and electroplating baths.
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2. Tench, D. and C. Ogden., US patent 4,132,605 (1979)
3. Freitag, W.O. et al. ''Detennination of the Individual Components in Acid Copper Plating Baths," Plating and Surface Finishing; 70 (10) :55; 1983
4. McCaskie, J. E. "Electroanalytical Methods for Process Control of Printed Circuit Plating" Proceedings of the AESF Technical Conference. p. M-3, 1987
5. Hamilton Jr., A.C. "Acid Sulfate and Pyrophosphate Plating." Plating and Surface Finishing, 82 (8): 48; 1995
6. Bratin, P. et al. "Control of Tin and Tin /Lead Electroplating Solutions by Cyclic Voltammetric Stripping" Proceeding of the IPC Technical Conference, p. S 16-2, March, 1996
7. Bratin, P ''New Developments in the Use of Cyclic Voltammetric Stripping Analysis of Plating Solutions" Proceedings of the AESF Analytical Method Symposium, p. N1, March. 1985
8. Bratin, P. et al. "Application ofCVS to the Analysis of Organic Additives in Nickel Plating Baths", Proceeding of the AESF Technical Conference, June, 1985