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May 2014 • The PCB Magazine 29 Figure 6: surface thickness reduction during through-hole filling development. Figure 5: Inpulse plating system containing cu and Fe. reactions at the cathode in the Inpulse system with corresponding potential are given below (I) and (II) as measured against standard hydro- gen electrode. Cu 2+ +2e - → Cu E = 0.340 mV (I) 2 Fe 3+ + 2e - → 2 Fe 2+ E = 0.771 mV (II) Electrochemically reaction (II) is favoured against reaction (I) and thus the amount of Fe 3+ reduces and determines the amount of cop- per deposited onto the cathode (Panel). A Fe 3+ - control unit between the plating and dissolv- ing tank regulates the exchange between both compartments and controls the amount of Fe 3+ within a range of ±0.1 g/l. This online control is a precondition for a good process control and has been proven in production scale. The solu- tion exchange on the surface is much stronger than in a hole so the etching effect becomes ef- fective mainly on the surface and not in vias and especially not in blind microvias. With this technology we were able to re- duce the amount of Cu overburden for a 100 µm thick core from 50 µm down to now 13–15 µm (Figure 6). COPPER FILLING OF BLIND MICROVIAS AND THROUGH-HOLES continues Without this reduction, actual production would not have been possible as the surface plated copper thickness determines in the end the achievable lines and spaces. The reduction of surface thickness for filled BMVs allows the production of thinner stacked filled structures. As mentioned before, typically 10 layers of stacked copper BMV filling is the preferred tech-