Introduction
The analysis of organic additives in electroplating baths by cyclic voltametric stripping (CVS) is a robust method and widely used in the electroplating industry. To ensure that continuously reliable results are obtained standardized daily operation procedures are of highest importance. They all contribute to keep the measuring system at best performance.
This document acts as a guideline through the working day. Procedures are described, which should be carried out when starting up the system, during the working day and when the system is shut down. Further general recommendations and procedures, which do not have to be carried out on a daily basis are described.
The recommendations and descriptions in the following refer to the analysis of acid copper plating baths. For the analysis of other plating baths, e.g. tin, tin/lead or electroless copper plating baths some recommendations may not apply.
Solutions
The solutions for CVS should be stored appropriately to address their individual stabilities.
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VMS
There is no special requirement for the storage of the virgin make-up solution (VMS). The VMS consists of CuSO4, H2SO4 and chloride in case of acid Cu baths. This solution is very stable.
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Additives
Store the organic additives in a gas-tight vessel in a dark, cool place, e.g. a cupboard. The organic additives, especially brighteners, are sensitive to light, high temperature and oxygen.
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Calibration and Check Standard Solutions
Prepare calibration standards and check standard solutions freshly before they are used. These solutions have a limited stability. Especially diluted brightener solutions should not be used longer than one day. The stability in VMS can change between different brighteners. For some brighteners it might also be necessary to prepare a new check standard solution after a few hours.
Calibration Curves and Intercept Values
Values which are used for multiple determinations, such as the calibration factor for a suppressor determination, intercept value for a brightener determination by LAT or the response curve for a leveler determination, should be recorded regularly. Exact numbers cannot be given since the measurement interval strongly depends on the additive.
The validity of these values can be checked by the determination of a check standard solution.
- The recovery for a suppressor determination should be (100 ± 10)%.
- The recovery for a brightener or leveler determination should be (100 ± 20)%.
Maintenance of Dosing Units
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Actions
Carry out a regular maintenance of the dosing units, e.g. every 2-4 weeks.
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Comments
Dosing units are very useful devices which allow highly precise automatic addition of solutions, even down to very small volumes. This high precision requires proper handling and taking care of the dosing units.
The solutions used in CVS are often highly concentrated solutions of CuSO4 or organic substances, which can crystallize out and block the dosing unit. To avoid this, we recommend a regular maintenance of the dosing units.
In this procedure they are disassembled completely and thoroughly cleaned. After complete drying the necessary parts are greased and the dosing units are reassembled.
When greasing the parts indicated below only very little grease should be used to avoid that excess grease is transported into the measuring solution, where it can interfere with the measurement.
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Procedure
- The dosing unit is emptied and disassembled completely (see also «Manual 807 Dosing Unit», 8.807.8002, chapter 3.7).
- All parts are rinsed with plenty of deionized water.
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The valve and distributor disk are cleaned with ethanol:
- All parts are left to dry completely.
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The centering tube, dosing piston and the housing are greased as indicated.
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Those parts that are in contact with solution should only be greased slightly. The valve disk is greased slightly. Grease the valve disk and remove excess grease with a soft tissue. Excess grease would be introduced into the dosing channel and is eventually transported into the measuring vessel, which has to be avoided. When a finger is put on the valve disk no fingerprint should be visible.
- The dosing unit is reassembled (see also «Manual 807 Dosing Unit», 8.807.8002, chapter 3.8).
Working Electrode - WE (Pt RDE)
The standard and recommended platinum working electrode is 6.1204.610 with an electrode disk of 2mm in a glass shaft. Some of the advantages of this electrode are:
- Very good chemical resistance, since only the inert materials platinum and glass get in contact with the measuring solution.
- Longer lifetime compared to a platinum electrode with a PEEK shaft, since no glue is used at the intersection between glass and platinum. Glue slowly decomposes when in contact with aggressive media.
- Faster conditioning and more reproducible measurements compared to a platinum electrode with a PEEK shaft, due to an ideal intersection platinum – glass.
- Applications can smoothly be transferred from a 3 mm PEEK electrode to the 2 mm glass electrode.
The working electrode is a consumable. Even with proper handling as described before it will only last for a certain time. To assess the performance of the electrode it is recommended to monitor the charge obtained during conditioning of the electrode and, in case of doubts, run a cyclic voltammetric test.
Monitor Charge of Electrode Conditioning
As a measure for the performance of the electrode the charge obtained during conditioning should be monitored over time. A variation in the charge in the range of ±20% is acceptable. If suddenly the area increases or decreases drastically, it can be an indication that the end of the lifetime has been reached.
Cyclic Voltammetric Test of Platinum RDEs
In case of doubts about the state of the electrode a cyclic voltammetric test should be carried out to assess the wear of the electrode. This can be done by means of the preinstalled methods, which can be found in the installation directory of the viva software (%ProgramFiles(x86)%\Metrohm\viva\examples \methods\CVS) or in the program data path of the 797 VA Computrace (Windows 7, 32 bit: %ProgramData%\Metrohm\797 VA Computrace\Method\CVS …). The resulting cyclic voltammogram should look like the example below. Assessment criteria are:
- The difference between anodic and cathodic current at +0.1 V should be as small as possible.
- The progression of the voltammogram should be approximately horizontal.
- The hydrogen adsorption and desorption peaks between -0.2 V and +0.1 V should be clearly visible. Note that the absence of these peaks does not necessarily mean that the electrode is defective. The peaks can also disappear if the electrode is contaminated.
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More details on the cyclovoltammetric test can be found in the Application Work AW VA CH4-0453 122006.
Driving Axle
The driving axles do not require maintenance. Keep in mind that this item is a consumable. The standard driving axle (6.1204.210/6.1204.510) has a lifetime of approx. 3 – 6 months. The driving axle with mercury contact (6.1204.220/6.1204.520) has a lifetime of approx. 1 year.
With both driving axles care has to be taken that no solution enters the gap between the driving axle and the RDE, as discussed below.
Fill Level in the Measuring Vessel
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Actions
The fill level in the measuring vessel should be only so high, that no solution can enter the gap between the RDE and the titanium axle. The maximum level should be kept approx. 1 cm below the gap between driving axle and RDE.
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Comments
This also has to be considered when the measuring vessel is rinsed.
The rotating part of the driving axle is made of titanium and the black part of PTFE. Both parts have excellent chemical resistance. Nevertheless it should be avoided that measuring solution enters the gap between the titanium axle and the PTFE part. Otherwise crystallization can occur, which leads to mechanical abrasion of the PTFE. Both crystals and abrasion particles can lead to a blockage of the driving axle. In addition, the particles can fall into the measuring vessel. However, this does not have an influence on the application.
If the application requires a higher volume of the measuring solution, a bigger measuring vessel should be used. Various sizes are available from Metrohm.
Dosino Configuration
For dosing units, it is recommended to use the following setup, where port 3 is used for the preparation.
In addition to the aspiration tubing at port 2, where the additive or VMS is filled into the dosing unit and port 1, where the dosing is carried out port 3 should be used for preparation. An FEP tubing with an inner diameter of 2 mm is connected there, which leads to a waste container.
The use of port 3 for preparation needs to be defined in the software.
In the viva software this setting is found under the properties of the respective dosing unit. Note that the length of the waste tubing should be specified as 0 cm to avoid that this tubing will also be completely filled during preparation.
In the VA Computrace software the setting is found under «Settings/General settings».
Additional Support
If you encounter any issues or need additional help, please submit a support ticket through Metrohm Technical Support.
For additional training on any of these topics, explore our Metrohm Custom Training options.