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34 The PCB Magazine • May 2014 COPPER FILLING OF BLIND MICROVIAS AND THROUGH-HOLES continues Tests run with 18g/l and 32g/l Cu on very thick panels (3.2 mm with 0.3 mm hole diameter) confirmed this result. With the 18 g/l we almost doubled the Cu thickness in the middle of the hole. For the Cl - concentration an increase up to 90 mg/l was beneficial, too. For the sulph- uric acid concentration we couldn't find a sig- nificant influence. The temperature influences the dimple performance in via filling which is depending on the adsorption/desorption ratio between accelerator and inhibitor. It turned out that the increase of temperature from 20°C to 40°C had a positive effect for the bridge plat- ing. Besides temperature and chemical param- eters the equipment has a major influence in improving the filling performance. Therefore the existing equipment was modified. We use an optimised, highly flexible rectifi- cation system, which is capable of running very complex and aggressive current pulses. By that we could proof that a strong reverse pulse (80 ms cathodic pulse at 5 ASD followed by a 4 ms anodic pulse at 50 ASD) for 15 minutes leads to reliable closure of the X-formation on 400 µm cores with 100 µm diameter. In our tests the electrolyte flow direction onto the panel was 90° angle towards the surface. Doubling the flow rate resulted in a consistent X-formation and centralised bridging (Figure 10). For this reason we increased the pump capacity for our production tool and could confirm this result also in production scale. Moreover a continu- ous flow control in combination with frequen- cy controlled pumps and automatic level con- trol grant a stable process performance. Summary and Outlook Copper via filling is a critical process for the production of HDI panels and IC packages. The number of BMVs found on a typical substrate is increasing as well as the dimensions of through holes, which pushes the limits for the produc- tion process. Reverse-pulse plating in combina- tion with the ferrous inpulse system offers a vi- able alternative to standard approaches as it is a consistent inclusion free technology for filling of mechanical as well as laser drilled through holes. Flexible pulse shapes empower the system to plate the X-profile first and then fill the cre- ated BMVs with very low copper deposition on the surface with the same type of equipment. Solution exchange in the holes has a big impact on the results of through hole filling, a high ex- change rate results in less inclusions. Available systems offer a supply of up to 20 m³/h at each anode and can be setup to the optimised vol- ume flow to meet complex requirements. Today it is possible to fill mechanical drilled through holes up to a substrate thickness of 300 µm and diameter of 100 µm inclusion free within 60 minutes. Laser drilled through-holes of 100 µm x 100 µm can be filled inclusion free in less than 60 minutes, 200 µm thick substrates still need >60 minutes plating time to guaran- tee void-free filling (Figure 11). Further devel- opment has to aim at inclusion free filling of thicker substrates (>300 µm) and to further re- duce plating time. PCB Henning Hübner is global product manager, panel/pattern plating at atotech deutschland GmbH in Berlin, Germany. ramona mertens is global assistant product manager panel/pattern plating at atotech deutschland GmbH in Berlin, Germany. dirk rüß (ruess) is sales manager for horizon- tal equipment at atotech deutschland GmbH in Feucht, Germany. Figure 11: laser drilled through-holes: (l) inclu- sion free 200 µm x 100 µm; (r) minor inclusions 400 µm x 100 µm.