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78 The PCB Magazine • February 2016 Solution agitation is initiated at the bottom of the tank; it may be air sparging or eductor agitation. There should be two manifolds, one in front of the panel and one in the back. The spacing between the top of the sparging system and the bottom of the panel should be at least six inches. It is intended to produce a uniform laminar flow across the surface of the panel. The flow should be vigorous to ensure sufficient leveler replenishment at the entrance of the via. Via filling will not occur if solution agitation is inadequate as demonstrated in Figure 2. Part agitation is not necessary in this setup. Rectification The rectifier should be sized to supply the desired output. For example if the platable area is limited, a smaller rectifier would be the right choice, it is not advised to use a 200 ampere rec- tifier to plate at 10–20 amperes per load. Ripple (% AC or alternating current) should be less than 5% to ensure the deposit is fine-grained, adherent and equiaxed. High ripple would have an adverse effect on the physical properties of the deposit. The rectifier control accuracy or resolution should be 1–2% to ensure consisten- cy of output. The ASF to be used in plating the blind via should be optimized for the via dimensions. More demanding vias (higher aspect ratio) will require lower ASF for a longer time. Tradition- ally there are twp approaches to the plating current density (CD). The first approach is a single CD for the duration of plating. The sec- ond approach is to step up the CD with time, for example, 10 ASF for 45 minutes followed by 20 ASF for 30 minutes and then 25 ASF for 15 minutes. Both methods work and the choice should be made at the initial installa- tion through experimentation and testing. Once established, this becomes the method. The method may be optimized again as the next level of difficulty (higher aspect ratio) is encountered. Figure 6: Variation in current density distribution in "dot" pattern plating. how to set up a suCCessful blind via hole fill dC plating proCess