Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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and thus each piece is electrically isolated from all of the other pieces. The rack, rather than being a large conductor, is now an insulator with individ- ual wires or embedded metal traces, to each piece. This is shown in the diagram in Figure 1. In this Figure, you can see the cut out area where 2 parts are located and the traces to each part from the top contact array shows in the detail drawing. One option for fabricating this type of rack is to Figure 2. Smart Rack for PCBs. utilize standard printed circuit board (PCB) tech- nology which incorporates thick copper traces within a fiber reinforced epoxy insulator. This structure can then be coated to protect it from the aggressive plating chemicals. An example of a PCB technology Smart Rack is shown in Figure 2. The second significant change is the presence of a new circuit, which can control the current to each individual piece at a preprogrammed value. (There will be a more detailed description of this circuit later in this paper.) This circuit acts like an individual programmable current supply for each piece. However, due to the availability of miniature micro-controllers, the overall size of the control circuit can be made very small, and even mounted on the rack itself. SMART RACK PLATING RESULTS Nickel Plating The first set of results described here are for Ni plating from a traditional nick- el sulfamate plating bath. Overall current density was 20 amps/sq. ft. with a bath temperature of 125°C. The pieces plated were copper substrates. The plating rack held 16 pieces and could be run in two different configura- tions. If all the plating locations were shorted together, the rack simulated a tra- ditional plating rack. If each conductor was controlled individually, this repre- sented the Smart Rack technology. Figure 3 shows the width of the plating thickness distribution for two different situations. The first plating configuration has all of the parts shorted together and a constant current run through the entire rack. This con- figuration is the traditional plating bath set-up. Note that the total distribution width was +/- 20%, which is typical for a Ni bath. In the second configuration, the individual traces to each part were isolated and a constant and equal current was run through each part. Said differently, the Smart Rack circuit forced the exact same cur- rent to flow through each part, irregardless of location on the plat- ing rack. Note that, in this case, the distribution width dropped to Figure 3. Comparison of plating thickness distribution in two scenarios. 137

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