Issue link: https://iconnect007.uberflip.com/i/507417
18 The PCB Magazine • May 2015 timized in addition to manipulating second- ary current distribution.) The potential is de- termined by Ohm's Law, which is the solution resistance between the anodes and cathodes. Additional resistances arrive through the volt- age required to corrode the anode and the resis- tance required to reduce the metal ions to metal at the cathode. In order to improve throwing power and plating uniformity, one can increase the con- ductivity of the plating electrolyte and increase the polarization. Polarization and conductiv- ity are both dependent on solution operating temperature, solution agitation and cathode current density. The main consideration is rec- ognizing that plating distribution across the surface of a printed wiring board panel and from the surface of the panel through the holes will vary due to resistances. By mitigating the effects of these resistances, plating distribution is improved. The Importance of Quality Anodes-Care and Maintenance It is surprising how little attention is paid to the quality and maintenance of the copper anodes. The grain structure is the most critical characteristic of copper anodes. This structure is predicated on the manufacturing process of the anodes, which in turn determines the grain structure of the copper. Fine grained size of the copper is the preferred orientation. The unifor- mity of grain size and the dispersement of the phosphorous along the grain boundaries will aid in the orderly dissolution of the copper. Large grained copper anodes dissolve uneven- ly, leading to heavy sludging and potential for impurities and particulate matter to enter the plating solution. An example of different anode quality is shown in Figure 8. There are three different anodes from three different suppliers, each claiming quality an- odes for printed circuit fabrication. Note that each anode looks much like the other. Each anode ball was cut with a diamond saw to reveal structure. Figure 9 shows an SEM of each of those. Clearly there is a distinctive dif- ference among the grain structures and phos- phorous (shown as the dark areas). To read more about anode manufacturing and quality, see Reference 3. Each anode ball was cut with a diamond saw and given a light etch to reveal. Figure 9 shows an SEM of each of those. Clearly there is a dis- tinctive difference among the grain structures OPTIMIzATION OF ACID COPPEr ELECTrODEPOSITION PrOCESSES continues FEaturE Figure 8: external view of three anodes from three different manufacturers. Figure 9: Manufacturer 1 (left), Manufacturer 2 (middle) and Manufacturer 3 (right).