The Use of Insoluble Anodes in Acid Copper Plating
Introduction
Electrolytic acid copper is the process that builds the traces that carry the current throughout a PCB. How to optimize your electrolytic acid copper plating for today’s designs—aspect ratios >20:1 for through-holes and >1:1 aspect ratios for blind via fill—is the challenge. The use of insoluble (mixed metal oxide-coated, or MMO-coated, titanium mesh) produces a consistent and reproducible plated product, is environmentally friendly (eliminates waste), and eliminates anode maintenance, thus increasing the productivity of the plating line.
Vertical acid copper plating remains a very common way for plating PCBs. For best results, the equipment should be optimized with proper rectification and connectivity. The electrolyte and the additives used, along with the plating current density, all play a role in the copper thickness distribution on the plated panel. The anodes have a direct impact on copper thickness distribution. The anode shape, size, and location play a critical role in the thickness distribution of the plated copper in vertical plating of panels in a tank. The vertical plating tank is challenging in its own way, unlike horizontal conveyorized plating where all the panels are exposed to the identical set of anodes, as the part is conveyed through the plating module. In horizontal plating, if the anode setup is not optimum, the thickness distribution within the panel may vary; however, the variation from panel to panel is eliminated.
Soluble Anodes
Soluble anodes need to be filmed for proper dissolution. This is achieved by dummying a fresh copper anode at low current density for 2–3 hours. Once filmed, the film is renewed as the dissolution proceeds. As a byproduct of anode film formation, this film (copper oxide) will sluff off the anodic copper, and if left unattended, will create nodules on the surface of the plated panel. To keep the sluffed copper oxide—referred to as sludge—from contaminating the bath, the anodes are bagged. The bags should be replaced during anode maintenance.
In vertical plating tanks, panels are plated in different cells and various locations within the cell. To minimize variation in copper thickness distribution from the panel racked on the outside edge of the tank relative to the panel in the center of the flight bar and from cell to cell in the tank and from tank to tank requires a good understanding of the role of the anode.
Anode Placement
Proper placement relative to the cathode window of the anode baskets or slabs has a direct impact on the copper thickness distribution. In the case of panel plating, the copper thickness will always be higher toward the edges versus the center of the panel. The outside 2–3”, top, bottom, left, and right edges will exhibit a much higher thickness as compared to the inside area. The thickness increases as the measuring location goes further out away from the center. The increase could be >50%; as an example, the area away from the edges could average 1.0 mil. And as you move toward the outside 2–3 inches of edge, the thickness will gradually increase up to 1.5–2.0 mils at the extreme end of the edge (Figure 1).
Figure 1: Copper thickness distribution.
Ideally, the length of the anode should be 3–4 inches short of the bottom of the panel. This will minimize the increased thickness at the bottom edge of the panel. Butting the vertical edges (of panels) together eliminates the extra thickness along the vertical edges, virtually making the cathode one large panel with only the extreme outside edges needing special attention. The easiest way to reduce over-plating on the outside vertical edges is to tuck the anodes inside the cathode window by 3–4 inches. This leaves the top horizontal edge plated with thicker copper. The remedy here is a lot simpler. Rack the panels within 1 inch of solution level. This will cut off the lines of flux that would cause excessive plating at the top edge of the panel (Figure 2).
Figure 2: Ideal anode placement.
Anodes that are excessively short of the bottom of the panel will favor the top of the panel and plate less on the lower part (Figure 3). If the anode is too long, exceeding the length of the panel; this will favor plating on the lower half of the panel as compared to the top half (Figure 4).
Figure 3: Effect of a short anode on distribution.
Figure 4: Effect of a long anode on distribution.
In short, the length and placement of the anodes plays a very important role in the copper thickness distribution on the surface. If properly maintained this anode configuration relative to the cathode (panels) will yield good, consistent copper thickness distribution.
The biggest challenge is the continuous dissolution of the anode. If slab anodes are used, they lose thickness in both the vertical and the horizontal dimensions. One starts with a full volume anode copper slab and ends with what looks like a spear. The anode would lose ~70% of its effective area before it is replaced with a new slab. This will have a direct impact on the anode current density, which would start at its lowest and most efficient and steadily increase as the anode area diminishes.
On the other hand, if anode baskets full of copper balls are used, the basic shape and size of the effective anode are more stable. As the balls dissolve, the basket will pack down, and could—if not replenished—become shorter on the top. Anode baskets accumulate sludge in the bottom of the bag over time; this would change the effective length of the anode, making it shorter on the bottom.
The whole concept of copper dissolution at the anode over time will eventually increase the copper concentration in the bath. This source of variability is contained by periodic dilution of the copper in the bath. The excess copper is then dealt with at waste treatment adding to the cost of the process.
As long as the variability in copper thickness falls within the specified limits as indicated on the design drawing, all is fine. Of late, designs including controlled impedance and fine lines and spaces for high-density interconnects (HDI) have much tighter limits on copper thickness distribution. One of the solutions to meet these new tighter criteria is the use of insoluble anodes. Insoluble anodes have been used in PCB manufacturing for decades, mostly as platinized titanium anodes for gold plating. Platinized titanium anodes would not work in acid copper because of the very low pH or high acidity of the electrolyte. The answer here was the MMO-coated titanium anode.
The MMO Anode
MMO anodes are MMO-coated titanium mesh. The metal oxides used are iridium oxide, ruthenium oxide, tantalum oxide, and titanium oxide. The first two are conductive and the active ingredients in the mixture. The latter two are non-conductive and serve a binding or cementing function to keep the coating together. This coating is applied to a titanium mesh, which may come in different sizes. A common dimension for the mesh is 12.5 mm x 7 mm x 1 mm. The wire is 1 x 1 mm.
The coating is applied in layers by different techniques (Figures 5 and 6). Each layer is then baked at high temperature for 30–40 minutes. Figure 6 shows the layers of the coating. The thickness of the coating is expressed in g/m2. The thickness of the coating plays a part in determining the cost of the coating and the effective life of the anode.
Figure 5: Image of MMO micro-structure courtesy of Umicore Galvanotechnic.
Figure 6: Coating morphology and layering courtesy of Umicore Galvanotechnic.
The anodes are cut to order to fit the tank design. A titanium strip is welded to the anode to serve as the conduit between the anode bar and the MMO anode.
MMO Anode Life
The effective life of the anode varies with utilization. It is not uncommon to get two years of functionality out of the anodes. The anode coating may be monitored by doing an X-ray spectrum of the used anode, comparing it to the original spectrum of the new anode. Anodes are functional at 30% of the original effective coating. Another method to determine the functionality of the MMO anode is to monitor plating efficiency, which should always be greater than 90%.
The use of MMO anodes requires solutions to two attributes that are not found in the use of soluble anodes. The first is the continuous evolution of oxygen when the anode is electrolyzed, and the second is the depletion of copper from the electrolyte during plating.
Oxygen Containment
Oxygen evolution will create an oxidative environment in the electrolyte. This results in the oxidation of the additives, particularly the brightener. This will deplete the brightener in a matter of hours. A very effective solution for this problem is to isolate the oxygen gas from the bulk of the solution. One way to do that is to bag the anode. The bag keeps the oxygen contained and allows it to dissipate in the air above the electrolyte. Figure 7 shows a bagged anode for oxygen containment.
Figure 7: Bagged anodes.
Copper Replenishment
MMO anodes require continuous replenishment of copper during plating. One of the more common methods of achieving that is the continuous addition of copper oxide to the electrolyte during plating. There are commercially available dispensing systems designed for the controlled addition and dissolution of copper oxide. The addition is based on a feedback system that responds to amp-hours of plating. The system is capable of maintaining the copper concentration within 1–2% of the target. Copper oxide must be purchased within specific limits for metallic contaminants. Copper oxide is only 80% copper and costlier than the use of copper metal.
The most dramatic advantage of the insoluble MMO-coated titanium anode is the consistency of its shape. The MMO anode does not change its shape as it is being used. Once the anode is set in place and optimized for plated copper thickness distribution, the anode will continue to perform and reproduce the plating results of the initial setup for months on end.
The MMO anode behaves the same as the soluble anode as far as placement is concerned. This anode must be shorter than the panel that is being plated and must also be tucked in the outside edges of the cathode window. Setting up the MMO anode follows the same logic used for optimized soluble anodes. Figure 8 shows the design of the insoluble anode for optimum surface thickness distribution.
Figure 8: Optimum anode configuration.
The MMO anodes are lightweight and easy to move around up and down and left to right. This facilitates the setup of the anodes with respect to the cathode for optimum uniformity of the plated copper thickness. The MMO anode also never requires dummy plating to film the copper. This saves production time and eliminates copper waste (the copper plated on the dummy panels). More important is that insoluble anodes do not require any anode maintenance, eliminating the labor associated with anode maintenance and increases production time of the copper plating line.
Waste Elimination
Waste of anode copper that is associated with soluble anodes, such as thinned out copper slabs or very small balls accumulated in the bottom of the basket, is eliminated. Anodic copper is bought at a premium, and residual copper is sold as scrap metal. A soluble anode is a titanium basket full of copper balls. The titanium basket remains filled with copper balls throughout the life of the plating line. The cost of the original copper to fill the baskets is tied-up capital that is never retrieved.
The use of MMO anodes turns a wasteful electroplating process into an environmentally green process. The copper content of the electrolyte is always being depleted and must be constantly replenished. There should never be a time when the electrolyte must be diluted generating solution waste that requires waste treatment and disposal. The use of MMO insoluble anodes is environmentally beneficial as it eliminates both solution and copper metal waste.
Conclusion
Soluble anodes have been the staple of the industry for decades, but they require extensive maintenance and generate waste in both copper metal and electrolyte. As plated copper thickness uniformity requirements become more stringent, more and more time will be needed for anode maintenance to ensure the uniformity of the anodic setup.
A properly designed insoluble anode produces a very consistent, uniform copper thickness distribution within the panel—from panel to panel, plating cell to plating cell, and bath to bath. In addition, the MMO anode maximizes productivity time by eliminating anode maintenance. The MMO anode is environmentally friendly, as it eliminates electrolyte solution waste and anode copper waste.
George Milad is national accounts manager for technology at Uyemura International Corporation.
This technical article was originally published in the proceedings of SMTA International 2018.
