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32 The PCB Magazine • May 2014 lead to very slow X-formation. Consequently the control of ferric ions is extremely important for handling the process. Therefore the online control unit between the plating and the dis- solving tank is mandatory to keep the ferric ion concentration within ±0.1 g/l (Figure 9) and as- sure bridge plating. Another interesting result is the effect of copper concentration on the X-formation. In contrast to the BMV filling process, a lower Cu concentration turned out to be beneficial. Its influence is significant for the X-formation. Figure 9: effect of ferric ions on the x-formation: left 1.0 g/l and right 0.5 g/l ferric. Figure 10: Bridging improvement by higher flow in advanced equipment. COPPER FILLING OF BLIND MICROVIAS AND THROUGH-HOLES continues