Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 137 of 707

Figure 1a. Electrically isolated rack pieces. Figure 1b Macroscopic factors that affect these more microscopic reactions include: • Density of plating parts surrounding the individual, part being plated • Resistance drops across the plating rack • Location of the part on the rack • Plating rate • Uniformity of the electrochemical environment including variations in solution chemistry, temperature, agitation All of these variations lead to thickness variation in parts from one location of a plating rack to the next. Typically the thickest parts are located along the edge of the rack and the thinnest parts are located near the center. One reason for this trend is that the flow of metal ions to be plated is unimpeded near the edge of the rack, but near the center of the rack, surrounding parts "screen" the metal ion flow from their neighboring parts. This competition for plating ions results in a thinner deposited layer and a lower plating current density. DESCRIPTION OF SMART RACK CONCEPT The essential feature of the Smart Rack concept is to individually control the elec- trical current that flows to each piece during the plating process. This individ- ual current control allows active and direct control of the plating process, which can very significantly narrow the plating thickness distribution. This direct method is much more effective than indirect methods such as current thieves, and the range of adjustment is considerably higher. For example, if desired, the programmed current density of two parts on the same rack could vary by factors of 2 or more. In addition, this method is much easier to implement and control because changes in current densities can be effected by adjusting inputs to the plating program rather than making any physical changes to the plating bath or set up. To accomplish this requires two significant changes compared to the traditional plating approach. First, the plating rack must be constructed so that individual electrical current conductors are connected to each individual piece on the rack, 136

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