Articles

Evaluating Finishes Using SERA

Over the years, the hot air solder leveling (HASL) process has been established as a primary surface finish for circuit boards. It still remains the most solderable, and arguably the most durable, surface finish. Recent developments in PCB manufacturing have highlighted some problems with these finishes, creating an urgent

Solder has been used for a long time as the preferred coating to protect the solderability of circuit boards and components, and HASL has become a favorite process for the application of solder to PCBs. In general, it has been a reliable coating process for retaining the solderability of printed circuits; however, it is far from being a trouble-free process.

For years, the industry has been searching for a viable method to evaluate the solderability of solder finishes. Tench et al. has evaluated the use of electroanalytical techniques for this purpose, and proposed the technique known as SERA for the evaluation of tin and solder finishes.^1,2

The SERA technique involves isolation of a surface pad or a plated through-hole using a rubber gasket with an accurately defined area. Then the hole, or covered surface pad, is filled with borate electrolyte, and a small, but accurately defined, current is applied between the tested surface and the auxiliary electrode placed within the cell above the board. The electrode potential of the tested surface is then measured as a function of time against a reference electrode, which is placed in the electrolyte’s storage vessel. (Figure 1)

The recorded potential-time curve consists of a series of plateaus with characteristic shapes depending on the type and quality of the surface. For example, Figure 2 shows one such curve obtained for a tin-lead surface.

It shows the presence of oxidized Cu/Sn intermetallic compounds, SnO and SnO₂ on the surface of the solder coating. The final plateau on the curve indicates hydrogen evolution due to reduction of the electrolyte, thus providing the highest cathodic value achievable with the given electrolyte. Other species that need to be reduced at a more cathodic potential cannot be detected in the above manner.

A number of publications^1-5 have described the use of SERA for evaluation of surface species and solderability of solder finishes on printed circuits, as well as for process control during the various stages of the PCB manufacturing process. The objective of this article is to concentrate on the evaluation of alternate finishes, currently being proposed to replace HASL and electroplated tinlead coatings.

Despite its wide-spread usage, the HASL coating is not trouble-free or universally applicable. Presently, when circuit board features are being miniaturized and environmental pressures are mounting, the search continues for the best candidate to replace the HASL process. Features, advantages, disadvantages, and costs of alternative coatings are being considered.

In spite of the many papers presented and published highlighting or evaluating various alternative coatings, no single coating has been established as a single viable process to replace HASL. All seem to have some advantages as well as disadvantages. The SERA technique can be used as a tool for monitoring quality and consistency of alternative coatings. In addition, it is our feeling that all of these coatings will be used to a certain extent, and more will emerge in the near future as “the only viable process.”

SOLDER COATINGS

SERA is presently used for evaluating the solderability of solder finishes, as well as a process control tool during its preparation. It became clear from the initial work at Rockwell¹ that an increase in the tin oxide reduction potential of the solder surface can be correlated with an increase in the PCB defect rate during the assembly

process. Tench and others have shown that as the potential of the reduction process shifts to more cathodic (negative) values, the defect rate increases for a given flux. This provides a basis for evaluation of surfaces, fluxes, and soldering conditions during PCB manufacturing, as well as prior to assembly. Since higher cathodic potential means that the reduction of the oxide becomes more difficult, flux requirements can be selected prior to the assembly, based on the quality of the solder surface. This means that the higher the amount of SnO₂ on the solder surface, the more aggressive the flux condition must be to assure proper soldering.

In addition to the value of the reduction potential, other SERA curve features are being evaluated as potential process control indicators. These features include:

presence and amount of the intermetallic compounds on the surface, open circuit potential, and hydrogen evolution potential. All of these parameters have been observed to vary during comparison of “good” v. “bad” boards. Currently, only the presence of intermetallic compounds has been clearly correlated with solderability problems. When a significant amount of intermetallic compounds are detected on the board’s surface, solderability issues arise. Figure 3 shows one such “bad” board with the presence of intermetallic compounds.

HASL ALTERNATIVES

A number of finishes have emerged as potential alternatives to HASL. They include thin and thick OSPs, immersion silver, immersion tin, and immersion gold over nickel. As these finishes are examined, we will discuss how the SERA method can be used as a tool for process control, and for surface evaluation of alternative coatings.

ORGANIC SOLDERABILITY PRESERVATIVES

Unlike the HASL process, where soldering is done by melting the existing solder coating, soldering with OSPs requires the removal of the OSP coating with heat and acid (flux). This must be done so that the attachment can take place on the copper surface under the OSP coating. Hence, OSPs must keep the Cu surface solderable prior and during the soldering process.

During storage, unprotected Cu surfaces are oxidized, leading to the formation of Cu₂O, and later, to a mixture of Cu₂O and CuO. If the copper surface has not been protected from oxidation, it might indicate the presence of three SERA detectible species. The nature of these species and reduction voltages are given in Table 1.

Coating copper surfaces with OSP protects the surface by formation of a complex between the OSP (imidazole, benzotriazole or substituted benzimidazole) and the copper surface.8 Figure 4 shows SERA curves of protected and unprotected Cu surfaces after one year of natural storage at ambient conditions.

Clearly, the imidazole-protected surface prevented further oxidation of copper. This observation can be used as a basis for detection of OSP on the surface. Figure 5 shows SERA curves for surfaces with and without thin OSP coating after a short heating cycle.

The protected surface is clearly distinguishable. Such a procedure is currently used by some PCB manufacturers to detect problems with thin OSP coatings soon after the manufacturing process. Since thick OSPs shift the reduction potential of the Cu-OSP complex, depending on their thickness, their presence is easily detected even without application of the heat. Thus, SERA offers a simple method for detecting the presence of OSPs on the PCB surface. Also, variations in the curve can be used as a useful process control indicator.

Moreover, thin OSPs (25-100Å) are generally nearmonolayer coatings, and their application is quite uniform across the surface. However, the coating thickness is too thin to be measured electrochemically. On the other hand, thick OSPs (500-6,000Å) generally vary in thickness depending on the type of OSP, the control of the chemical composition of the applied solution, and the type and quality of the application process. By using different electrolytes, SERA curves can be used to measure the thicknesses of thick OSP coatings. As an OSP thickness increases, the amount of the reducible material increases, and the time required to “drill” through the OSP coating is then correlated to the thickness of the OSP. Because SERA can be used on very small surfaces (1.6 mm diameter or less), it can be used to measure the thickness distribution of OSPs directly on the PCB, thus becoming a very powerful process control tool. Measurements of the relative
distribution of the OSP on the surface allows one to improve and control the process itself. A number of papers describing SERA testing of OSPs have been published.6-8

The ideal goal is to correlate SERA measurements of OSP coatings to their solderability. Work performed at NPL9 revealed that SERA curves, obtained with yet another electrolyte, showed an increase in copper oxide formation and a decrease in OSP thickness, as the boards undergo consecutive simulated reflow cycles.

The NPL study involves a number of U.K.-based PCB shops, using different types of OSP coatings. Another, more thorough study has been focused on a single OSP coating with different thicknesses and different storage conditions.10 During this work, the results of SERA measurements were correlated to the results of wave soldering, which was performed by the counting of filled holes in specially prepared solderability test vehicles. It was found that the SERA technique can be used for monitoring boards coated with a “proprietary” OSP coating.

Summarizing the work with OSPs, the SERA method can be used as a process control tool to evaluate the presence, quality and thickness of the OSPs, as well as to show some correlations to solderability after simulated reflow cycles.

IMMERSION SILVER

Silver is considered to be a “precious” metal, but its chemical properties distinguish it from other precious metals such as gold and palladium.

The silver surface is quite reactive, and atmospheric corrosion of the silver surfaces occurs almost immediately upon contact with air. Corrosion, as expected, proceeds even faster in the production environment than in clean air. Presence of free sulphur or sulphur compounds in the environment causes rapid formation of the silver sulfide (Ag2S). Higher humidity increases the rate of silver corrosion in the presence of sulphur compounds.

The main components of tarnish films on the silver surface are silver oxide (Ag2O), silver sulfide (Ag2S), silver sulphate (Ag2SO4) and silver chloride (AgCl). The
presence of sulphur compounds on the surface causes solderability problems.11, 12

Since the immersion silver coating is relatively thin (0.1 μm) and sensitive to tarnishing, the surface of silver must be protected to maintain its solderability
characteristics. The corrosion of silver can be inhibited by placement of thin organic or inorganic films on the surface.12 Commercially available immersion silver coatings are coupled with thin organic films which are supposed to provide the required protection from tarnishing; however, this compound is fragile, especially at elevated temperatures and under humid conditions. The solder wettability of fresh silver coatings is good, but degrades rapidly with the formation of a tarnish film.

Therefore, it follows that the presence of a protective film on the surface of silver is extremely important for the protection of silver from tarnishing. Also of importance is a sufficient thickness of the silver layer itself. The presence of proper analytical techniques to monitor conditions and thicknesses of protective layers, tarnishing layers, and the thickness of silver itself is of utmost importance.

When a constant current is applied to a silver surface (SERA test), the resulting potential-time curve reveals a number of plateaus that correspond to the sequential reduction of compounds in the tarnishing film. SERA curves obtained from silver surfaces are similar in shape to those obtained from copper surfaces.

Fundamental work has recently been performed to identify and quantify the species of tarnishing films.13 It was found that components of tarnishing film reveal the following reduction potentials: Ag2O +0.28 V, AgCl +0.12 V, Ag2S -0.62 and Ag2SO4 -0.92 V.

Since SERA has detected the presence of both organic inhibitor and tarnish iflm, SERA can consequently be used as a process control tool for immersion silver coating. In addition, using an anodic current, the silver coating can be stripped coulometrically. This allows for an accurate (but destructive) measurement of the thickness of the silver coating.

IMMERSION TIN

Immersion tin is another of the emerging alternative coatings that can be evaluated using the SERA technique. For this process about a 1-μm-(40μ inches)-thick
immersion tin layer is placed on top of a copper substrate. Some of the coatings might contain organic or inorganic materials that act as diffusion barriers between copper and tin, or protect the tin surface from excessive oxidation.

We have studied several immersion tin processes. Figure 7 shows SERA curves for a thin tin coating (0.26 μm, 10μ inch), before and after heat treatment (reflow).

As can be seen from the curves, before the treatment, only SnO is visible on the surface. This does not pose any solderability problems. After the heat treatment, only oxidized Cu-Sn intermetallics are present on the surface. This indicates that all of the tin has diffused into the copper substrate. Hence, this surface would present serious solderability problems.

The above observation was confirmed by the destructive oxidation of the surface. Before the heat treatment, tin is clearly present on the surface, followed by a thicker layer of Cu5Sn6 intermetallic, and a very thin layer of Cu3Sn intermetallic. After heat treatment, no pure tin can be observed on the surface, the Cu5Sn6 intermetallic layer is slightly thinner, while the thickness of the Cu3Sn material has increased.

The presence of SnO2 and oxidized intermetallic compounds can be correlated to solderability problems on tin coatings. Thus, SERA can be used for QA process
control of this coating. The destructive oxidation of the coating can be used for precise thickness determination of the tin coating, as well as for determining intermetallic thicknesses.14 The solderability of heated immersion tin was shown to diminish as the thickness of the Cu3Sn layer increased.

ELECTROLESS PALLADIUM

The promise of electroless palladium coating lies in its capability to be applied directly over copper. Previously published results10 showed that between 6 to 9
microinches (0.15 to 0.23 μm) thick palladium coating are needed to completely cover a copper surface. Even thicker coatings are required to prevent copper migration through the palladium layer during heating. Thus, the presence of copper on the surface or in the pores of a palladium coating is easily detected, and it can be used for evaluation of electroless palladium coatings on a copper substrate.

IMMERSION GOLD

The last coating to be discussed is immersion gold over a nickel barrier. This coating is being tested extensively as a promising alternative coating that can yield solderable and wirebondable surfaces. Some preliminary results have been presented in an earlier paper.10

One possible cause for solderability problems on immersion gold coatings is the porosity of the gold, which allows oxidation of the exposed nickel barrier through the gold layer pores. As shown in Figure 8, the reduction of nickel oxides takes place at potentials more negative (cathodic) than hydrogen evolution on the gold surface. This makes it impossible to detect the reduction of nickel oxides in the presence of an exposed gold surface.

Tench1 has described a technique similar to SERA, named potentiomentric evaluation of substrate oxidation (PESO). The PESO technique uses an acidic electrolyte such as NH4Cl (pH=4.0), and the corrosion potential of the gold surface is measured as a function of time. The presence of plateaus can be related to the presence (and thicknesses) of the nickel oxides, and the corrosion potential can be related to the porosity of the gold surface.

A second solderability problem arises due to the presence of organic materials on the gold surface. These materials can come from various sources, such as solder mask, oils, or contaminated ovens. Their presence on the surface of gold can be observed by a shift in the rest potential of the gold surface in the borate buffer (pH=8.4) electrolyte. Both the potential shift, as well as the signal “noise” disappear when the surface is cleaned with a strong acid and the solderability is restored.

The combination of SERA and PESO techniques can be used to evaluate immersion gold coating surfaces for organic contamination, porosity, presence of oxidized nickel in the pores of the gold, and presence of copper oxides on the gold surface.

These can lead to a loss of both solderability and wirebondability for this alternative coating. Further tests on this surface are being conducted.

CONCLUSION

In conclusion, it has been shown that the SERA technique can be a powerful tool for evaluating the surfaces of alternative coatings. More work needs to be done and additional studies are being performed which will be reported in the near future. Despite the work that still remains, SERA has already been established as a valuable process control tool for PCB manufacturing.

AUTHORED BY

Peter Bratin, Michael Pavlov and Eugene Shalyt

REFERENCES

1. D.M. Tench, “Solderability Assessment via SERA,”Journal of Applied Electrochemistry, No. 24, pp. 46-50, December, 1994.
2. D.M. Tench, D.P. Anderson, Method of Assessing Solderability, U.S. Patent 5,262,022, Nov. 19, 1993.
3. J. Reed, J. Cheng, “Sequential Electrochemical Reduction Analysis (SERA) Application on Process Characterization and Troubleshooting for Printed CircuitBoard Fabrication,” Proceedings of IPC Technical Conference, pp. 12-22, April 1995.
4. D.M. Tench, M.W. Kendig, D.P. Anderson, D.D. Hillman, G.K. Lucey, and T.J. Gher, “Production Validation of SERA Solderability Test Method,”Soldering & Surface MountTechnology,No. 13, 46, pp. 18-29, February, 1993.
5. M. Pavlov, P. Bratin, E. Shalyt and R. Gluzman, “Solderability Assessment by SERA,” Proceedings of AESF SUR/FIN ’94 Technical Conference, June 1994.
6. P. Bratin, M. Pavlov and E. Shalyt, “New Applications of SERA Method – assessment of the Protective Effectiveness of Organic Solderability Preservatives,” Proceedings of AESF SUR/FIN ’95 Technical Conference, pp. 538-589, Indianapolis, June 1995.
7. S. Gutierrez, P. Tune, “Organic Coatings and The Challenge No Clean Presents,” Proceedings of IEEC/ECTC ’95 Technical Conference, May 1995.
8. G. Wenger, U. Ray, “Four OSPs: Quality Assessment,” SMT magazine, May, 1996.
9. C. Hunt, “A Model of Solderability Degradation,” Proceedings of the SMI Technical Conference, pp. 650-660, September, 1996.
10.P. Bratin, M. Pavlov and E. Shalyt, K. Wengenroth, J. Fudala, A. DelGobbo, “Studying the Solderability of Organic Solderability Preservatives Using SERA,” Proceedings of National Conference on Solderability and Alternative Finishes, Vol. 2, pp. 86-91, September, 1998.
11. D. Hillman, P. Bratin, M. Pavlov, “Demonstrating the Relationship Between Wirebondability and Solderability of Various Metallic Finishes for Use in Printed Circuit Assembly,” Proceedings of SMI Technical Conference, pp. 687-693, September, 1996.
12. K. Wlassink, Soldering in Electronics, 2d ed; 1989, Electrochemical Publications, Scotland.
13.P. Bratin, M. Pavlov and E. Shalyt, “Surface Evaluation of the Silver Finishes via SERA,” Circuit World, Vol. 25, October, 1998.
14. R. Edgar, “Immersion White Tin,” Printed Circuit Fabrication, December, 1998.

Control of Plating Solutions in the Automotive Industry by Stripping Voltammetry

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.

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.

EXPERIMENTAL

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%.

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 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)

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).

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).

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).

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 ).

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.

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).

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).

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).

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.

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.

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.

ACKNOWLEDGMENTS

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.

REFERENCES

The Fundamentals of Cyclic Voltammetric Stripping

CVS provides more objective information on the plating bath than a Hull Cell. A Hull Cell provides a qualitative indication of the bath condition. Using CVS, the plater can precisely replenish the additives to keep the bath operating properly.

CVS ANALYSIS

Cyclic Voltammetric Stripping is an electrochemical technique used for the measurement of organic additives in plating baths. It is based on the effect that

the additives have on the rate of electroplating. Regardless of the specific type of organic additive (brightener, leveler, grain refiner, etc.), its activity is reflected in a change in the plating rate.

The analysis is performed in an electrochemical cell using a three-electrode system, one of which is a platinum rotating disk electrode. During measurement, the potential of the platinum electrode is controlled by the instrument. The potential is scanned at a constant rate back and forth between negative and positive voltage limits. A small amount of metal from the plating bath is alternatively plated onto and stripped off the working electrode as the potential is changed. During the scan, the current at the working electrode is measured as a function of potential.

The activity of the additive will affect the plating rate of the metal onto the electrode. The plating rate is determined by calculating the charge required to strip the metal off the working electrode. The relationship between the stripping charge and the activity of the additives is used to quantitatively measure the additives and their components. Fresh additive as supplied by the manufacturer is used as a standard. The activity of the additive in the production bath is expressed as concentration (mL/L) of the fresh additive.

An instrument such as the QUALILAB QL-5® Plating Bath Analyzer from ECI Technology performs the patented CVS analysis automatically. The QL-5 is software-controlled and designed especially for the analysis of plating baths. The user is prompted through the analytical procedures while the QL-5 makes all of the calculations and reports the active concentrations of additives in the bath.

USING CVS FOR PLATING BATH ANALYSIS

An example of a voltammogram (plot of the measured current vs. applied potential) for an acid copper plating bath is shown in Figure 1.

Figure 1 is an actual display from the QUALILAB Plating Bath Analyzer. The potential determines the electrochemical reaction that occurs. The potential is displayed on the X-axis with potentials becoming more positive from left to right. A positive-going potential is becoming more strongly oxidizing, while a negative going potential generates a more strongly reducing condition. Current is shown on the Y-axis. A positive current corresponds to an oxidation while a negative current is due to a reduction. The voltammogram has several regions of interest:

Plating region: Points 1 to 2 and 2 to 3. The current is negative in this region as the platinum electrode is plated with metal. Since the plating time is well-defined by the scan rate, the plating rate can be accurately estimated as the amount of metal that is plated in the plating region.

Stripping Region: Points 3 to 4. As the current of the electrode becomes positive, the metal is stripped off the electrode. The area of the stripping peak represents the charge required to strip the deposited metal. The instrument integrates the current to measure the area beneath the stripping peak. The area is reported in milli-Coulombs (mC) and is shown in the box at the lower right of Figure 1. Since the quantity of stripped metal is the same as quantity of plated metal, the stripping peak area is proportional to the amount of metal plated. It is more accurate to measure the amount of metal plated by integrating the current in the Stripping Region rather that in the Plating Region. Electrochemical processes other than metal deposition, such as hydrogen reduction, may also be taking place in the Plating Region.

Cleaning Region: Points 4 to 5 and back from 5 to 6. A positive potential on the electrode is used to desorb organics and clean the surface of the platinum electrode. This cleaning segment is critical for accurate and reproducible results with CVS. This region is also used to monitor oxidizable organic contaminants and chloride levels in acid copper baths.

Adsorption Region: Points 5 to 6. In this region the organic additives are adsorbed onto the electrode surface, even though no metal is plated. The adsorption that takes place in this region enhances the sensitivity of CVS.

The influence of several organic additives on the plating rate is shown in Figure 2.

In general, additives such as levelers or wetters act as suppressors and decrease the plating rate while brighteners increase the rate of deposition. Use caution, however, since there is no standard terminology for additives within the industry.

During electroplating, the organic additives undergo various chemical transformations such as oxidation, reduction, fragmentation, and polymerization. Most of the products of such transformations also affect the plating process. CVS provides a measure of the effective concentration of additive (the activity of the additive in terms of the fresh additive) which is more important than the concentration of a specific substance. Most analytical techniques (spectroscopy, HPLC) measure absolute concentration. The transformed additive will still be electroactive, but not responsive to spectroscopy, or vice versa. Because CVS is based on the plating process, it is the only technique that provides a true measure of the activity of the organic additives.

DETERMINATION OF ADDITIVES AND THEIR COMPONENTS

The quantitative determination of organic additives is based on the CVS response of the production bath before and after the addition of a known amount of a standard. The fresh additive as supplied by the manufacturer, which is used for replenishment of the bath, is used as the standard. Since the fresh additive is used as the standard, the concentration of additive in the production bath is expressed as the equivalent activity of the fresh additive. In this way, replenishment of the bath to the proper additive level is very convenient.

It is usually possible to quantify all of the additives in the plating bath. An example of an acid copper bath is shown in Figure 3.

The carrier acts as a suppressor and inhibits the rate of the plating process. This can be seen from the reduction in peak area as the concentration of the carrier is increased. Notice that the activity of the carrier is very high, i.e., a small increase in concentration has a large effect on the rate of plating. The fresh carrier as supplied by the manufacturer was used to spike the solution.

The plating rate changes dramatically as the carrier is increased from 0-1 mL/L. As the carrier concentration is increased beyond 1 mL/L, there is little additional effect on the plating rate.

The brightener has a different effect. In the presence of carrier, higher brightener concentration causes an increase in plating rate. The activity of the brightener is lower than the carrier. The difference in activity between the carrier and the brightener can be exploited to quantify both components.

Some manufacturers offer a pre-mixed solution containing both the carrier and the brightener. These components can be clearly distinguished in Figure 3. CVS can be employed to insure that the carrier and the brightener are in balance in the production bath.

The analysis of plating baths using CVS benefits from experience. ECI Technology has invested over 15 years in developing CVS methods for a wide variety of plating baths. In addition to manufacturing CVS instrumentation, ECI Technology is the leading source of CVS applications information. CVS consultation is available to every user of ECI Technology products.

OUTGOING AND INCOMING INSPECTION OF ADDITIVES

CVS is a powerful tool for the outgoing inspection of additives by suppliers as well as incoming inspection at users’ sites. Figure 4 illustrates the variations between several lots of the same brightener.

By performing a fast, easy assay, the user avoided under dosing or overdosing the brightener and losing control of the bath.

THE INDUSTRY STANDARD

Since its initial development in the late 70’s, CVS has experienced tremendous growth. The contributions of CVS to both suppliers and users of plating chemistries has been significant. This patented technology is currently used by all major chemistry suppliers for quality control or routine analysis of customers’ baths. ECI Technology is the major driving force behind CVS. The development of both CVS instrumentation and analytical procedures by ECI Technology has been a critical contribution to the growth of CVS.

Users of CVS instrumentation from ECI Technology include the most trusted manufacturers of semiconductors and printed wiring boards such as Motorola, IBM, Intel, Texas Instruments, Northrop, Tyco, Photocircuits, Sanmina, and others. CVS may be found in plating laboratories in the Americas, Europe, and Asia. In addition the innovative engineering of ECI Technology was recognized by an R&D 100 Award.

APPLICATIONS OF CVS

Prevention of plating problems by quantitative measurement of the organic additives in plating solutions keeps the chemistry in balance.

• Quantitative determination of additives and their components
• Individual fingerprints of plating solutions
• Incoming/outgoing inspection of plating additives
• Monitoring the level of plating solution contamination
• Scheduling baths for carbon treatment and optimizing of the treatment

Study, develop, and optimize new plating technology

• Study the effect of various parameters of the plating process (concentration of additives, current density, mass transport, temperature, etc.) on the performance of plating solutions
• Study the consumption and transformation of additives during the plating process
• Study the effect of various additives on the plating process

1Cyclic Volatmmetric Stripping (CVS) is a patented technology. QUALILAB QL-5 is a trademark of ECI
Technology, Inc. No part of this material may be copied, used or distributed in whole or in part without
permission from ECI Technology, Inc.