Growing Copper Bath Revisited
In the March 2007 Plating Clinic, I responded to a question regarding an increasing concentration of copper in a copper sulfate plating bath (Pitted Copper). One of our readers, Rob Berger of Univertical Corp., offered additional comments on this problem. The original question and my response are both given below, followed by Rob’s comments.
Q. My copper sulfate plating bath has grown in concentration. The copper concentration is now too high. Is there anything I can do to prevent this from happening? N. D.
A. The copper sulfate plating bath is a very efficient bath. Anode and cathode efficiency are both close to 100%. However, anode efficiency is slightly higher than cathode efficiency. For this reason, the amount of copper in the plating bath will increase over time.
In most situations, drag-out will remove this small amount of excess copper. If you have minimal dragout in your plating process the best way to correct this problem is to remove some of the plating solution and use dilution to bring the remainder back to the proper concentration.
Rob Berger’s Comments: N.D. may have a decreased anode surface area as a cause for the increasing copper concentration.
While the efficiency of an acid copper plating bath is nearly 100% at both the anode and cathode, current market conditions with the high price of copper can lead to the growth of copper in solution due to low surface area on the anode. Anode current density will grow if the amount of copper is decreased or if larger pieces of copper are used. The high current density “blows” copper away from the anode and allows it to dissolve chemically instead of electrolytically. Such an effect can increase the amount of dissolved copper in the bath faster than it can be dragged or plated out.
No experienced plating facility would readily allow the level of copper metal to drop in its baskets. But some will consider filling their baskets with large nuggets or balls because the basket will fill with a lower weight of copper (larger pieces have larger voids and pack less tightly than small pieces in the same basket). The basket is full, right? The operator may not realize the drastic drop in surface area that will occur when the larger pieces are used. The problem is that while the solution to using less metal was to increase the anode piece size, the final result was a higher consumption of copper metal with no increase in production.