A typical package core consists of copper clad dielectric which is drilled to produce the necessary through vias serving two functions either as electrical or as thermal conduits. The thermal vias are normally characterised by a low drill pitch and a localisation at the centre of the package core. The vias are plugged after metallisation with a resin material to give the core the required planar surface necessary to provide the basis for the subsequent build up layers. Use of electrolytic deposited copper has been introduced as a new method to fill such through vias, pure copper as a plugging material has obvious advantages due its higher thermal conductivity in comparison to any currently available plugging resins. Significant cost savings are also possible as a fully automated in line processing sequence is available for electrolytic copper deposition in comparison to the more labour intensive resin plugging methods. The package cores utilising this technology have however been restricted to a dielectric thickness in the range 60μm to 100μm and with through via diameter 75μm to 100μm due to limitations in the processing technology. With these dimensions a hole pitch of down to 250μm may be filled reliably and is currently in production, latest results from this technology are shown. Hole filling with higher aspect ratios and particularly with substrates thicker than 200μm has required improvements in processing to ensure uniform copper filling. This paper describes the optimised process for through hole filling and shows the results achieved with dielectric materials 200μm and up to 400μm thick. Current qualification results of substrate 400μm thick with through via 80μm diameter are shown together with the comparison of filling with via pitch variation between 1.0mm and 0.6mm

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