Issue link: https://iconnect007.uberflip.com/i/306762
28 The PCB Magazine • May 2014 sions. It also takes more time (less productivity) and needs more copper on the surface as shown in Figure 3. The other approach is to use conformal plat- ing in the first step and then change to aggres- sive pulse plating in a second step in order to increase the amount of copper plated in the middle of the hole. The result is an X-shape in the hole built by two flat copper triangles on each hole wall. This X-profile can be seen as BMVs from both sides of the panel. These two BMVs are completely filled inclusion-free in the next step (Figure 4). Additionally, the formation of the X shape or so-called "bridge" offers the possibility to significantly reduce the Cu overburden. That is achieved by applying a combination of reverse- pulse plating and etching in one electrolyte. This electrolyte contains not only Cu but also Fe 2+ and Fe 3+ . This inpulse system is described for a better understanding schematically in Figure 5. Cu is deposited as usual on the panel (cath- ode) whereas the Fe 2+ ions are used to carry the current at the dimensionally stable anodes and thus prevent oxidation of organic additives. The oxidation product Fe 3+ is used to dissolve the consumed Cu chemically in a second cham- ber filled with pure Cu balls and reduced again to Fe 2+ . Moreover, the Fe 3+ cannot only be used to replenish Cu but also to reduce the plated Cu on the panel surface. The electrochemical on the side walls to fill the through holes. But similar to the conformal BMV filling method, this method tends to create a seam or inclu- COPPER FILLING OF BLIND MICROVIAS AND THROUGH-HOLES continues Figure 3: seam in through hole filled in dc mode. Figure 4: steps of through-hole filling by formation of "x."