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May 2015 • The PCB Magazine 11 foil). Figure 1 illustrates the definition of throw- ing power. While in Figure 1 the throwing power is 77%, one would prefer throwing power of 100%. Electroplating of high-aspect ratio through-holes becomes increasingly more dif- ficult as via diameters decrease and board thick- nesses increase. Figure 1 depicts a condition aptly named "dog-boning" for its thinness in the center of the via. The following equation illustrates a model that provides insight into the difficulty factor encountered when aspect ratios increase. It is not as simple as it looks. As the model illustrates, board thickness has a much greater influence on degree of difficulty than does via diameter. Essentially, board thickness directly influences ohmic resistance through the via. Resistance is inversely proportional to plating propagation. Where: E = Voltage drop down hole (energy lost) J = Cathode current density K = Solution resistance d = Hole diameter L = Length of hole It is important to keep this in mind when designing plating processes for high-aspect ratio circuit boards. As circuit boards become more complex in their designs, more attention must be paid to plating cell design, plating chemistry, anodes and plating racks, electrical connections and solution agitation. This paper will also pres- ent various techniques on improving and en- hancing throwing power for high-aspect ratio plated through-holes, with a focus on direct current (DC) plating. Acid Copper Electroplating: Theoretical Aspects In order to enhance plating uniformity (defined not only as plating thickness across the PCB surface but in the via as well), en- gineers should understand that there are several factors that influence plating unifor- mity. The Fishbone diagram below details in concise fashion those parameters that influ- ence throwing power, plating distribution and overall quality of the finished printed cir- cuit board. OPTIMIzATION OF ACID COPPEr ELECTrODEPOSITION PrOCESSES continues FEaturE Table 2: aspect ratio, mechanically drilled hole via (max ratio).