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60 PCB007 MAGAZINE I AUGUST 2019 dummy plated for 1 Ah/L, analyzed, adjust- ed to correct additive levels, and then the test panel was plated. Each test panel went through a pre-clean cycle of one-minute acid cleaner, one-minute rinse, and one-minute 10% sulfu- ric acid before the plating. Conditions and Bath Components Table 1 shows the operational conditions and optimum additive levels for the two for- mulations. Typically, via fill baths have high copper and low acid to achieve the desired bottom-up fill. Via Fill Mechanism The growth rate of copper inside the via and on the surface of the panel is controlled by the additives. Figure 3 shows a schematic representation of via copper growth. The dif - ferent role played by each additive is shown. Even though the adsorption is exaggerated and shown as highly localized, both selec- tive and non-selective adsorption occur dur- ing plating. Additive compositions must be controlled in the set range shown in Table 1 to achieve the desired "bottom-up filling." Common analytical tools used in the indus- try such as cyclic voltammetry striping (CVS) analysis and hull cell plating may be utilized for this. In Figure 3, the suppressor is shown in green, the leveler in red, and the brightener in yellow. Wetter molecules are mainly adsorbed on the surface suppressing the plating there, while the leveler adsorbs selectively on to the negatively charged areas, due to the positively charged quaternized N group. This prevents over plating at the edges and avoids prema - ture closure of the via, which could result in voiding in its center. The brightener, being a small, sulfur-containing molecule, diffuses faster into the via and accelerates the plating. As the geometry of the via changes continu - ously during the plating process, the bright- ener becomes concentrated inside the via causing rapid plating in the via. This is called the curvature enhanced accelerator coverage (CEAC) mechanism [6]. Table 1: Bath components and plating conditions. Figure 3: Schematic representation of CEAC mechanism (suppressor is shown in green, leveler in red, and brightener in yellow).