In this article, we describe the advantages of using, the key design requirements and the critical properties of insoluble titanium mixed metal oxide anodes in the manufacture of printed circuit boards.
Insoluble vs. soluble anodes in the electroplating of printed circuit boards
In the production of printed circuit boards, copper plating is a key process to create the conductive patterns and vias between the different layers. Traditionally, copper plating consists of an initial electroless copper plating step, followed by electrolytic copper plating. Copper electroplating is carried out in an electrolytic cell containing an acidic electrolyte, with the PCB forming the cathode and a soluble copper plate or an insoluble metal serving as the anode. Many PCB electroplating processes use insoluble titanium mixed metal oxide anodes as they have several advantages over soluble anodes (Milad, 2019):
- They provide constant anode conditions as they are stable and do not change their geometry
- They provide an even distribution of the copper layer, resulting in better quality PCBs
- They are lighter and therefore easier to install
- No dummy plating required to "activate" the anode (copper oxide film formation)
- They require little maintenance and there is no formation of anode sludge (an environmental benefit)
- They allow operation at higher current densities and therefore higher PCB lead throughputs
However, the use of insoluble anodes also has disadvantages:
- An external copper source and a system for supplying copper to the electrolyte are required
- Under oxygen development conditions, high anode potentials lead to additive consumption (e.g. brighteners and levelers)
Anodes based on mixed metal oxide are used in vertical and horizontal PCB electroplating processes.
Use of titanium mixed metal oxide anodes in vertical copper plating (VCP)
In the vertical copper plating process (VCP), a number of PCBs are plated simultaneously in a vertical arrangement, with the anodes on opposite sides of the PCB. Most VCP processes are designed for acid copper plating, i.e. the electrolyte consists of a solution of copper sulphate (CuSO4) and sulphuric acid (H2SO4). The reaction at the insoluble anode is the oxygen evolution reaction (OER). In view of the strongly acidic and oxidizing potential, a stable OER anode is required. The insoluble anode support therefore consists of a titanium mesh or a perforated plate. The electrical conductivity, ductility, high corrosion resistance and acceptable cost of titanium make it a suitable anode support (Bewer, Debrodt, & Herbst, 1982). The titanium support is coated with a catalytic coating, typically iridium mixed metal oxide (Ir-MMO) (Hayfield, 1998). This type of coating has proven successful in many industrial processes that require a stable OER anode, such as steel strip plating, electrowinning of metals and cathodic corrosion protection. In addition, PCB electroplating requires a low consumption of electrolyte additives compared to these applications. These additives improve the copper layer. Therefore, special Ir-MMO coatings have been developed that contain selective barrier layers. These selective barrier layers enable the oxygen evolution reaction (OER) on the Ir-MMO catalyst, but reduce the consumption of additives by preventing the additives from reaching the oxidizing conditions on the catalyst layer through size and/or charge exclusion. In the laboratory, the additive consumption of an Ir-MMO anode is measured in a Haring cell and using cyclic voltammetric stripping (CVS) (ECI Technology, 2021). High additive consumption leads to high operating costs of PCB electroplating as additives are expensive and need to be replenished frequently.
Use of titanium mixed metal anodes for horizontal copper plating
In the HCP (Horizontal Copper Plating) process, PCBs are produced in a continuous mode (e.g. plate to plate or reel to reel). Atotech's Uniplate system (Atotech, 2024) uses the iron/iron ion (Fe2+/Fe3+) redox system at the anode. Fe2+ is oxidized to Fe3+ at the anode. To replenish the Fe2+, the Fe3+ formed is converted back into Fe2+ in an external module with copper metal. The advantages of this technology are the low anode potential, which prevents the development of oxygen and thus the accumulation of bubbles and reduces additive consumption (Barthelmes, 2000). Anodes for HCP also consist of Ti mesh coated with an Ir-MMO electrocatalyst. The composition of the Ir-MMO coating is optimized for the different anode potentials of the Fe2+/Fe3+ redox system.
Fig. 2: A Haring cell (left) is used to test the additive consumption of an Ir-MMO anode during electrolysis. The additive concentration is measured using cyclic voltammetric stripping (CVS; right) at different time intervals of the electrolysis. The additive concentration is determined by integrating the area under the peak of the cyclic voltammogram
Pictures: Magneto special anodes
Technical challenges
In addition to low additive consumption, the durability of the anode is crucial for low operating costs. The service life is determined by the design of the anode and its use and is typically in the range of 1-2 years. The lifetime of Ir-MMO anodes increases with the loading of the electrocatalyst. However, at only 7-8 tons per year, Ir is scarce (Johnson Matthey, 2024) and expensive (>150 €/g [Umicore, 2024]). It is therefore crucial to reduce its loading and at the same time ensure a sufficiently long and reproducible lifetime of the anode. Depending on the use of the electrode, the service life is influenced by various mechanisms that can lead to anode failure. Ideally, the electrocatalyst wears out at a low and predictable rate. However, passivation or even corrosion of the titanium-Ir MMO interface can lead to an inactive anode with high residual Ir loadings. Alternatively, the barrier layer may fail, resulting in high additive consumption, while the electrocatalyst may still be active. The conditions of use of the anode determine the service life to a large extent. For example, electrolytes containing corrosive substances such as fluorides lead to rapid corrosion of the titanium substrate (Wang, 2014) and consequently to delamination of the Ir-MMO coating and anode failure. Therefore, the electrolyte composition should be kept free of fluorides. Copper electroplating in PCB manufacturing is typically operated at currents of 1-10 A/dm2. A higher current density enables a higher throughput, but reduces the service life of anodes. The current is applied as direct current (DC) or 'pulse plating' current. Pulse plating is used, for example, for flash plating or plating with high aspect ratios, such as vias. With reverse pulse plating (RPP), the current is alternated between positive and negative currents (or potentials). This RPP current signal has a significant impact on the lifetime of the Ir-MMO anode, as the passive and protective oxide layer of the titanium substrate and the mixed metal oxides of the coating are degraded at an increased rate. Ir-MMO coatings have therefore been developed that are especially suitable for RPP conditions. The anode must lead to the application of a homogeneous copper layer on the entire PCB. The mixed metal oxide coating must therefore be very homogeneous. To validate the performance of the anode, the production line operator carries out a plating test and measures the surface distribution at various positions. A lack or excess of deposited copper is an indication of inhomogeneous anode performance, e.g. caused by areas that have less or no electrocatalytic coating or areas with different conductivity (e.g. caused by insufficient current connectors). Therefore, a thorough understanding of the use of the anodes as well as the operating conditions is crucial to optimize the anode design. In addition, an operator should consider that a change in operating conditions, e.g. a change from direct current (DC) to RPP or a change in electrolyte composition, can affect the lifetime of the anodes.
Future prospects
Production lines for printed circuit boards are constantly evolving. Therefore, the anodes must meet the exact specifications in terms of achieving the required throughput, PCB quality, low additive consumption and anode life. Based on their own findings from research and development, the authors foresee a number of short-term improvements in the application of Ir-MMO anodes. With the focus on the energy transition, a new technology for copper-plating photovoltaic (PV) modules has emerged from PCB production. The scarcity and high prices of some of the raw materials used in PVs are pushing PV manufacturers to develop more cost-effective technologies. Currently, the electrical contacts in PVs are made from silver paste, where the expensive silver can be replaced by using electroplated copper (PV Magazine, 2022). Various companies are bringing this new technology to the market and require titanium MMO anodes in their coating processes. The technical challenges for anode technology are comparable to PCB manufacturing as DC and RPP processes are being developed with different chemicals added.
References
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