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May 2015 • The PCB Magazine 27 Chemistry Development To develop a new DC-based plating bath that can achieve good hole filling performance with minimal voiding, development efforts were directed towards the interaction of bright- ener and leveler additives. Levelers are prefer- entially adsorbed at areas of higher curvature such as the entrance of the hole, where mass transfer is enhanced, and thus filling behavior is strongly influenced by convection. The competitive adsorption between bright- eners and levelers result in a concentration gradient along the hole wall, with the leveler- rich corner effectively inhibited relative to the brightener rich center, resulting in the non- uniform plating rate required for through-hole ELECTrOPLATING THrOUGH-HOLES wITH DIFFErENT GEOMETry continues FEaturE Figure 5: Double-sided and one-sided laser drilled holes. Figure 6: Double-sided laser drilled through-hole in 100 μm (left) and 200 μm (right) thick panels. filling. Even with the use of a strong leveler, the leveler concentration has a huge impact on hole-filling capabilities and on the deposit ap- pearance and surface morphology (roughness and resistance to formation of nodules). For instance, a low leveler concentration will lead to a lack of inhibition at the entrance of the holes, with the result being excessive plating rate at the knees, leading to formation of seam voids. An excessive leveler concentra- tion will negate the ability to selectively inhibit only those areas with high curvature, resulting in a conformal deposit and increased risk of nodule formation. Electrochemical studies confirm that the adsorption of leveler on the hole openings and