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Today, the use of insoluble anodes in the electroplating industry is well known, especially in precious metals applications. However, it is less common in base metal applications such as the electrodeposition of copper in the printed circuit board industry, or the electrodeposition of tin in the electronics industry.

The main reason for this is essentially that, historically, the most cost-effective method of metal replenishment in large-scale industrial applications is by anodic dissolution of the metal. Use of an insoluble anode requires a new approach to metal replenishment, involving the use of a metal salt, concentrated metal solution or special separate diaphragm cell for electrolytic dissolution. All of these tend to add to the overall production cost. In addition, anodic oxidation reactions at the surface of insoluble anodes often require the matrix of the electrolyte to be adapted in order to ensure and maintain good deposit characteristics, especially if, for example, reverse pulse current techniques are used.

Over the years, various phenomena associated with the use of insoluble anodes have been observed. For instance, in copper-plating applications for printed circuits using pulse periodic reverse (PPR) technology, we know that the use of platinized titanium insoluble anodes is prohibited because the platinum coating peels off after a while in the presence of chloride in a sulfuric acid electrolyte. However, the use of an iridium or mixed metal oxide (MMO) coated titanium anode prevents the problem and allows the performance benefits of insoluble anode technology to be realized successfully in this application.

With this type of innovation and adaptation, the use of insoluble anode technology can be applied more broadly. In fact, the use of insoluble anodes has multiple advantages:

Anode geometry remains constant with time, allowing current distribution, and therefore metal distribution, to be optimized and maintained.

Insoluble anodes do not need to be filmed and are not subject to the filming problems associated with soluble anodes, such as build up of breakdown products, which can affect both anode and bath performance. This results in an increase of bath life.

Anode maintenance is minimized; there is no need to stop lines to clean and replenish anodes, change anode bags and refilm anodes, i.e. a productivity increase and a labor cost decrease; and

Insoluble anodes prevent potential problems with metal particulates associated with soluble anodes, which may cause rejects due to inclusions in the plating.

However, there still remain certain drawbacks with the use of “standard” insoluble anodes:

Initial cost is usually higher than soluble anodes.

Life of insoluble anodes is dependent on type and use.

Increased consumption of organic additives compared to that with soluble anodes.

Anodic oxidation of bath constituents due to nascent oxygen produced at the anode.

These negative aspects can sometimes present major obstacles when deciding whether or not to implement insoluble anode technology.

The R&D work reported in this article was designed to develop insoluble anodes that could overcome those negative points and so give the users a technical and economical alternative, so that they could benefit from the advantages of insoluble anodes.

The first objective was to focus on the additive consumption at the anodes and we started our work by the investigation of the acid copper electrolytes for PCB applications.

Actually, at the insoluble anode, we have the following reaction:

2H₂O → O₂^↑ + 4e^- + 4H⁺

This reaction, which generates oxygen (anodic oxidation), is largely responsible for the “overconsumption” of organics at the anode.

We carried out a large number of consumption tests using different types of insoluble anode designs and making additive analysis using our CVS instrument, the Titraplate® CP.

Our basic approach was to create a physical barrier between the anode and the working electrolyte. This was achieved by essentially placing a mesh on top of the anode. Acting like a membrane, this mesh limits the access of the organics on the active anode surface and therefore limits their destruction.

Using the above approach, a specific mesh/anode configuration was developed which significantly reduced additive consumption compared with the use of “standard” insoluble anodes.

This work also showed that this barrier is not only physical. An electrochemical effect has also been discovered by comparing plastic versus metallic mesh designs. Both configurations (plastic and metallic) reduce additive consumption. However, the metallic grid is much more efficient and allows for a much greater reduction of additive consumption (by a factor of 3).

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It is therefore obvious that an electrochemical phenomenon is taking place which causes the positively charged compounds to be repelled before they can reach the active surface of the anode.

It should be noted that this general concept has been developed and patented conjointly by METAKEM and MPC, and that the different types of material used have also been patented.

The following graph shows additive consumption plotted against amp-hours for different types of insoluble anode.

The very encouraging results with acid copper led us to consider those “modified” insoluble anodes (SIA®) for other applications such as tin electroplating.

As mentioned previously, with standard insoluble anodes we observed a strong oxidation of the tin ion at the anode:

Sn²⁺ → èSn⁴⁺ + 2 e^-

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The following table compares the formation of stannic tin in a methyl sulphonic acid-based pure tin bath when using standard insoluble anodes or the modified insoluble anodes.

The same results have been confirmed in a sulfuric acid system.

The principal reason for this behavior is a significant modification of the oxygen evolution we observed at the anode using the SIA® insoluble anode. Actually, it appears that the size of oxygen bubbles and their rate of generation at the anode are very different from those observed with the standard insoluble anode.

Although the phenomenon is difficult to quantify, it is obvious when we can observe it during plating using the two different types of anode.

Example of Gassing Evolution using insoluble anodes in an acid copper electrolyte.: Figure 1a. Standard Anode Insoluble. Figure 1b. SIA® Anode Insoluble.

In addition, other significant advantages have been reported with the use of the SIA® insoluble anodes in production:

A cathode efficiency increase of approximately 10% on the various installations of SIA® anodes in operation today.

An overall much lower oxidative degradation effect and therefore an improvement of plating bath stability with time.

Reduced problems with “bubble entrapment” on applications requiring horizontal positioning of the anode above or below the cathode (semiconductor/wafer or roller-gravure applications).

An actual increase of the anode lifetime.

Conclusion

This evolution towards a new generation of insoluble anodes is a very important step for the future and the evolution of the electroplating industry. Other new developments are in process in order to improve the performances of the SIA anodes by trying to further increase their lifetime as well as in evaluating them for other electroplating applications such as CrIII, Zinc and Zinc alloys (like Zn-Ni), etc.