Plating Q&A: Copper Buffing Prior to Nickel-Chrome Plating
What are some options to reduce plating time and increase my throughput?
Q. We are plating metal parts that get copper buffed prior to nickel-chrome plating. I have to reduce my amperage 10-15 amps per square foot so I don’t burn the “high current density” areas in my acid copper plating bath. We are also experiencing low thickness in some areas as well, which extends my immersion time to almost two hours. I have ample time in my nickel baths, which are currently running at 50% utilization with the long acid copper immersion times. What are some options to reduce this plating time and increase my throughput?
A. There are several factors to consider. Burning typically takes place when the solution temperature is cold, chloride levels are low, sulfuric acid levels are low, anode-to-cathode distance is short and part design has some difficult geometry. A typical acid copper plating solution is operated between 25 and 50 amps per square foot of surface area. Because you are running much lower than this range, I would suggest some potential resolutions: part shielding, agitation enhancement, placement of anodes and chemistry modifications.
Part shielding can be successful, but requires some trial and error. Distance from the cathode to the anode may be a restriction when using shields. Agitation enhancement (such as higher air, cathode agitation and eductors) can all help replenish the cathode film with ions and organic additives to reduce burning. There are limitations to this method with regard to hydraulic actions and tank size/design. The anode placement may be another option, if your tank can accommodate the added spacing. Typically, you will find that larger anode-to-cathode distance will help reduce variation in the metal distribution. However, this may also require additional voltage from your rectifier. Maybe the easiest thing to try first is a chemistry adjustment.
Inorganic Additives and Their Functions
- Copper sulfate provides the necessary copper 2+ ions at the cathode/part surface to be reduced to copper metal during the electroplating process.
- Sulfuric acid enhances solution conductivity and prevents the formation of cuprous oxide at the cathode surface, which causes roughness. Higher sulfuric acid concentrations enable higher current densities while plating.
- Chloride ion assists with anode dissolution and grain refinement. The cathode will “burn” if this level is too low. If the chloride level is too high, >150 mg/l, then the formation of cuprous chloride at the anode surface restricts the dissolution of the anode and increases the required voltage necessary to achieve amperage. This reduces the overall efficiency of the bath and can be easily seen as a white film on the copper anodes.
- Grain refiners typically improve the deposit with regard to overall finish.
- Wetting agents assist with preventing burning.
- Levelers assist with overall deposit leveling and low current brightness.
I have seen where certain types of dye-based acid copper solutions with higher sulfuric acid content are able to run at elevated temperatures. These two factors enable better current distribution and reduced burning, particularly in the high-current density areas. Additives that are designed to run at elevated temperatures will not deplete as quickly as those that are typically used in acid copper processes. This is a viable option with no capital investment to increase throughput. At one location, they were able to reduce the overall acid copper immersion time by 50%, while improving the deposit and eliminating burning.
Originally published in the January 2016 issue.
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