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50 The PCB Magazine • January 2017 brightener additives adsorb onto and accelerate copper deposition in low current density areas. At the beginning of the fill process, where the current density differences between the bottom of the via and top are the greatest, the differential in deposition rate is greatest result- ing in bottom-up filling. As the via fills, this dif- ferential in deposition rate decreases until the via is near filled, at which point the deposition rates in the via and on the general surface be- come equal due to the equalization of current densities. At this point, the copper deposits at an equal rate on both the surface and in the via. The electrolyte components of the via fill 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 2. Experimental One of the key steps in the through-hole bridge and fill process is the determination of the bridge cycle to form a via with a shape and dimensions that is optimal for filling. Generally, a via of approximately 7 mils or less in diameter with an aspect ratio of approximately 0.75–1.0 is preferred for optimal filling. In this paper, a constant hole diameter and substrate thickness were used to illustrate bridge optimization with respect to time. Plating was done for various times and via depth, aspect ratio, surface cop- per, and bridge thickness were recorded. Measurements of plated substrates of vari- ous thicknesses, hole diameters, and pitches were collected from various sources under opti- mized bridge and fill cycles. The data was ana- lyzed to demonstrate the effect of hole size and panel thickness on the resulting deposits. Results Bridge Optimization The results of the bridging cycle testing with respect to time are summarized in Figure 7. The results showed that the bridge thickness increased quickly and linearly during the early stages of the process where the copper deposit extends outward from the center of the hole to close it and then begins to thicken and grow Figure 6: Example of accelerated plating within via. Table 2. Via fill bath composition. ELECTROPLATED COPPER FILLING OF THROUGH-HOLES: INFLUENCE ON HOLE GEOMETRY