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14 The PCB Magazine • May 2015 fitting any of the plating cells. This author has had documented success with respect to im- proving throwing power and plating distribu- tion. More uniform plating distribution helps to reduce the plating cycle time (everyone likes higher productivity and efficient use of capital). Secondly, minimizing plating thickness on the surface of the board while enhancing throwing power in small diameter vias will provide less opportunity for circuit line width reduction due to etching. Fabricators should always keep this fact in mind especially as one migrates to 3 mil lines and spaces. Figure 4 shows an actual plat- ing cell (without chemistry) with eductors. Figure 5 shows a single eductor pod as a close-up. These eductors should be mounted on manifolds as shown in Figure 4. This type of agitation set-up requires a 2.5–4 HP pump for every 4000 liters of plating solution. Of course, solution movement (eductor ver- sus air agitation) cannot cure plating throwing power ills on its own. As mentioned above, there are other factors including the organic addition agents and the composition of the plating elec- trolyte itself. And many of these factors interact with each other. As the graph in Figure 6 below shows, when the organic addition agents (in this case the carrier component of the plating additives) are in the optimum concentrations, the throwing power is significantly better than when eductors are used to supply the solution movement. In this study, the circuit board test vehicle was 0.093 inches thick and the smallest via diameter was 0.008 inches. Cathode current density was set at 25 ASF (amps per square foot). One should not underestimate the im- portance of plating cell design and geometry when it comes to optimizing throwing power and overall plating distribution. Factors such as anode-to-cathode distances, anode length and placement and plating rack construction/design greatly affect plating uniformity. With respect to rack design, a picture frame rack (multiple rack points around the periphery of the board) will provide more uniform current distribution as opposed to a rack with a few attachment points at the top or side of the panel. In addition, anode length should be 3–4 inches shorter than the length of cathode. This will aid in mitigating excessive plating on edges of the panel. With primary distribution, the electron po- tential field depends solely on cell geometry in- cluding anode length and placement (Figure 7). It is possible to use floating shields in the plat- ing tank in order to divert current from those areas of the circuit board most prone to over- plating. OPTIMIzATION OF ACID COPPEr ELECTrODEPOSITION PrOCESSES continues FEaturE Figure 4: Photo of plating cell with bottom up educators. Figure 5: Close-up view of a single educator, also known as a pod.