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30 The PCB Magazine • May 2014 nology in smart phone production. It is one of the enabling production processes in the drive for miniaturization of consumer electronics fol- lowing the expectation of innovation and in- vention given by Moore's Law. New Challenges The main core thickness for IC substrates is thicker than 100 µm; today's cores have a thick- ness of up to 400 µm or more. These are ex- tremely difficult to fill without void formation and require a long plating time at a low plating current density. Up to 200 µm substrate thick- ness the industry now moved from using me- chanical drilling towards laser drilled through holes. Laser drilling causes new problems when it comes to drilling quality. Often the time for COPPER FILLING OF BLIND MICROVIAS AND THROUGH-HOLES continues the laser to vaporise the substrate is not suffi- cient to achieve smooth hole walls. Glass fibre protrusions occur, decreasing the diameter of the through hole but not always in the centre. During bridge plating the X-shape is made at the area where the diameter is minimum. When the gap is not closed at the centre, one of the two resulting BMVs will have a very high aspect ratio, increasing the risk of inclusions (Figure 8). Therefore a new test matrix was carried out in a small lab tool to determine which of the following parameters might have a significant influence; the results of these tests were then confirmed in a production tool: • Brightener (accelerator) • Leveller (inhibitor) concentration • Fe 3+ concentration • Cu concentration • Cl - concentration • Sulphuric acid concentration • Temperature • Electrolyte agitation • Current density and pulse parameters For this investigation, we stopped plating af- ter a certain time period and measured the thick- ness in the middle of the hole compared to the surface. Of course the organic additives have a significant influence on the filling performance, but for the improvement of the X-formation a change of leveller and brightener ratio did not significantly show a benefit. During our studies we found out that the effect of Fe 3+ concentra- tion is very strong. High ferric concentrations Figure 7: Filled stacked BmV, also called "any layer." Figure 8: challenge for filling of laser drilled through holes.