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12 The PCB Magazine • May 2015 the board thickness increases, ohmic resistance dominates and addition agents have less of an influence [2] . In this case, mass transfer of the plating solution into the vias is critical. How- ever, with the higher-aspect ratio holes, mass transfer gives way to ohmic resistance. In or- der to compensate, one needs to reduce the ca- thodic current density and also reduce plating solution resistances. The latter is accomplished by increasing electrolyte conductivity, reducing resistances in plating cables and other cell de- sign enhancements that will be mentioned in the next section. Since it becomes increasingly difficult to achieve near 100% throwing power in aspect ratio holes of 10:1 and above, the plat- ing electrolyte must be set-up to maximize plat- ing solution conductivity. This is accomplished by increasing the sulfuric acid concentration and reducing the amount of available copper Acid Copper Electroplating for High-Aspect ratio PCBs: Practical Aspects For any process engineer, getting a handle on the key elements involved in the process will provide the insight and understanding required to achieve optimum plating results. The first part of this equation is to recognize the rela- tionship and interactions of the plating chem- istry (inorganic and organic). Influence of the Organic Additives and Plating Electrolyte (inorganic) The organic addition agents utilized in the plating solution have an influence on plating distribution and throwing power under certain conditions. The additives distinctively influence this behavior when charge transfer and ohmic resistance dominate the plating system behav- ior. However, as the PTH diameter increases and OPTIMIzATION OF ACID COPPEr ELECTrODEPOSITION PrOCESSES continues FEaturE Figure 2: These factors and their influence on plating distribution will be presented throughout this paper.