Metal Finishing Guide Book


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irregardless of location on the plating rack. Note that, in this case, the distribution width dropped to +/- 8.5%, over a factor of 2 improved. This result demonstrates the dramatic reduction in the distribution width that can be accomplished by forcing the same current density through each individual piece during plating. Because of the significant flexibility inherent in the Smart Rack approach, further improvements can be made by individually adjusting 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 thickness. The result, show as configuration 3, had a distribution of +/3%. This is shown along with the previous results in Figure 4. Figure 4. Figure 5. Silver Plating Figure 5 right shows the results for Ag plating. DISCUSSION The results above illustrate that individual current control can have a dramatic impact in reducing the thickness distribution during electroplating, even for materials 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 attributable 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 265

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