Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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+/- 8.5%, over a factor of 2 improved. This result demonstrates the dramatic reduction in the dis- tribution width that can be accom- plished by forcing the same cur- rent density through each individual piece during plating. Because of the significant flexi- bility inherent in the Smart Rack approach, further improvements can be made by individually adjust- ing the current density at each point to minimize the distribution width. To do this, small increases in current were programmed into parts with slightly lower than mean thickness, and small decreases in current were programmed into parts with higher than mean thick- ness. The result, show as configu- ration 3, had a distribution of +/- 3%. This is shown along with the previous results in Figure 4. Silver Plating for Ag plating. Figure 5 right shows the results DISCUSSION The results above illustrate that individual current control can have a dramatic impact in reducing the thickness distribution during electroplating, even for mate- rials like Ni that are typically very difficult to control. Another interesting aspect of this technology is the comparison between Configuration 2 and Configuration 3. At first pass, it would appear that setting a fixed and equal constant current at each plating site should yield the minimum distribution width. Since a constant electrical current is flowing through each part, the difference in plated layer thicknesses across the rack in Configuration 2 must be attribut- able to differences in plating versus non-plating electrochemical reactions at the part surface. By increasing the current density slightly for the lower thickness parts, as is done in Configuration 3, it is clear that the rate of plating reaction can be brought closer to plating rate for the mean thickness parts. Thus, differences in the plating reaction rates from one area on the rack to the next can be minimized by relative adjustments of current density as is shown for Configuration 3. This idea is illustrated schematically in Figure 6. DESCRIPTION OF THE SMART RACK CIRCUIT There are three major portions of the control circuitry. A computer (which could also be a micro-controller), a master control circuit and an individual 138 Figure 4. Figure 5.

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