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January 2017 • The PCB Magazine 49 ELECTROPLATED COPPER FILLING OF THROUGH-HOLES: INFLUENCE ON HOLE GEOMETRY separate steps utilizing two different plating solutions. The advantage is that each process can be optimized for its intended function. The combined process offers a much more robust bridge and fill system capable of filling a broader range of hole diameters and substrate thicknesses with copper while minimizing excessive plated surface copper. The two-step through-hole fill technology begins with either mechanically or laser-drilled through-holes processed through primary met- allization including plasma and/or perman- ganate desmear and made conductive either through electroless copper or the commonly available direct metallization processes such as graphite, carbon black, or organic polymer. A flash plate of copper can be used to ensure con- ductivity across the entire through-hole wall. The bridging of the center of the hole to form a double via utilizes a periodic pulse re- verse copper plating system optimized to pro- vide a cavity-free bridge with minimal surface copper. The filling of the resulting double vias utilizes via fill copper plating technology to provide accelerated filling of the vias while also minimizing surface copper. Bridge Step using PPR Plating PPR plating is widely used for the conformal plating of high aspect-ratio through-holes. New rectifier designs and software now offer greater flexibility in developing complex waveforms that can provide plating results previously unobtainable. One of the features of newer rectifiers is the ability to impress asynchronous waveforms into a plating panel (Figure 4). The use of asynchronous pulsed waveforms can accelerate the plating rate of the copper in the middle of the through-hole up to 5x that of conventional pulse waves (Figure 5). The electrolyte components of the bridge solution are typical of acid copper plating solu- tions: copper sulfate, sulfuric acid, chloride ion, and additives. The concentrations in this study are presented in Table 1. Filling Step Using Via Fill Plating Copper via filling technologies have been widely used in the manufacturing of HDI and IC packaging substrates. Copper via fill baths are DC plating systems that are specifically designed for filling vias. They provide preferential copper deposition within the via and inhibited deposition on the surface (Figure 6). This effect is accomplished by taking advan- tage of the difference in behavior of the additives in a via fill bath under different current density environments. The inside of the via is consid- ered a low current-density area versus the sur- face of the substrate. Suppressor additives in the via fill chemistry adsorb onto and inhibit copper deposition in high current-density areas. The Figure 5: Example of accelerated plating at hole center. Table 1: Bridge bath composition. Figure 4: Example of synchronous and asynchronous waveforms.