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84 The PCB Magazine • August 2017 Another crucial factor which determines the MD % is the mass transfer, which is directly pro- portional to the diffusion. While several factors influence diffusion, one significant factor is the current density at which the plating is done. At high cathodic current density, the abundance of electrons at the cathode accelerates the re- duction reaction of copper ions to copper met- al at the cathode. Due to this the copper ions in the diffusion layer will exhaust rapidly. If the copper ions in the diffusion layer continue to drop without replenishment from the bulk elec- trolyte, the deposit could show severe surface burning and cause poor distribution. On the other hand, at relatively low current density the plating rate will be low with the depletion of copper ions in the diffusion layer due to the re- duction reaction. Since the plating rate is low there will be enough time for the copper ions in the bulk electrolyte to replenish the diffusion layer. Owing to this equilibrium there will be an even distribution and no burning will occur. This phenomenon is clearly shown in Fig- ures 2, 3, and 4. As an example in Figure 2, at 10 ASF a 0.35 mm hole showed 99% MD where- as when the CD increased to 20 ASF the MD dropped to 95% for the same hole in the same board thickness and further, at 30 ASF the MD dropped to 87%. Surface, Structure, and Morphology All the plating conditions produced smooth, ductile, uniform, and mirror-bright surfaces. Ex- cellent leveling was seen inside the hole as shown in Figure 5. Further, Figure 5 shows no thin cop- per at the knee in the cross-sectional images for the 1.6 mm thick board plated at current den- sities 10, 20, and 30 ASF respectively. Uniform fine-grained copper layers throughout the hole is observed. After the microscopic evaluation, the sections were further evaluated using scanning electron microscopy (SEM). Figure 6 shows the results from the SEM analysis where three differ - ent areas were analyzed; inside the hole, corner, and the surface. Despite the current density dif- ference at the corner and inside the hole or at the surface, the morphology shows the same fine equiaxial grains. No preferred orientation like columnar grains were observed. Physical and Thermal Properties Final deposit plated under the influence of additives suppressor, grain refiner, and level- HIGH-THROW DC ACID COPPER FORMULATION FOR VERTICAL CONTINUOUS ELECTROPLATING PROCESSES Figure 2: Micro distribution for 1.6 mm panel aspect ratio: 8:1, 6.4:1, and 4.6:1. Figure 3: Micro distribution for 2.4 mm panel aspect ratio: 12:1, 9.6:1, and 6.9:1. Figure 4: Micro distribution for 3.2 mm panel as- pect ratio: 16:1, 12.8:1, and 9.1:1.