The Use of Insoluble Anodes in Acid Copper Plating


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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.

anode_fig5.jpg

Figure 5: Image of MMO micro-structure courtesy of Umicore Galvanotechnic.

anode_fig6.jpg

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.

anode_fig7.jpg

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.

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