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NOVEMBER 2018 I PCB007 MAGAZINE 85 ion concentration and increasing the sulfuric acid amount in the bath. Results from plating in a bath with 200 g/L Cu 2 SO 4 ·5H 2 O and 100 g/L sulfuric acid are shown in Figure 13 (TP min = 78%, TP knee = 86.6% for 0.25-mm TH in a 1.6-mm panel, AR = 6.4:1). The throwing power was considerably en- hanced, TP min = 78% for AR=6.4:1. The small vias, three mils, are filled up with a dim- ple <15 μm. However, the via filling of four- mil vias was adversely affected, as shown in the cross-sections (Figure 13). Still, the thick- ness of copper plating on 5 x 4 via bottom was >150% of the surface copper thickness, which met the industry requirements in many cases. More work was done to further improve the via fill, keeping very good throwing pow- er for thicker panels. Tests with different wet- ter species were run, leading to good results by using a higher Molecular Weight (MW) organ- ic compound. TP min of about 76−78% and good via fill for three-mil as well as for four- mil vias were achieved when plating in Pro- cess A bath with basic electrolyte (VMS): 180– 200 g/L Cu 2 SO 4 ·5H 2 O, sulfuric acid = 80−100 g/L; Cl¯ = 45–50 ppm. The thickness variation across the cathode surface was improved. Soluble Anode Application In many cases, soluble anodes are the pre- ferred choice, especially if copper replenish- ment presents difficulties. The described pro- cess was modified for applications with copper phosphorous anodes. Tests were performed in a large-scale pilot tank. Figure 14 shows the process sequence and the plating tank. Educ- tor spray and/or air agitation were used; sparg- er pipes per side were eight with 20 nozzles on each pipe. Excellent via fill and TH plating were achieved, as shown in Figure 15 using Process B. The plating conditions were 16 ASF, 60 min- utes, and eductor flow 100 LPM/side. The sur- face copper thickness was 18–22 mm, TP min Figure 13: Plating with reduced copper ion concentration, 0.25-mm diameter TH in a 1.6-mm panel. Figure 14: Process sequence (650L tank, soluble anodes, air agitation, and side impingement/eductor flow).