The manufacturing process for substrate cores requires production of a planar surface which is subsequently used as the starting point for the high-density build-up layers. The core may be of varying thickness and is normally mechanically drilled to produce the required through-via connections which are then metalized and plugged with a thermally cured resin material. There is a current tendency for reduced core thickness and this has implications for yield, quality and ultimately for cost for product from the complete process. The plugging process itself is relatively labor-intensive and requires, as part of the sequence, a mechanical abrading or brushing process after resin cure, which can cause problems of dimensional stability, particularly for substrate cores less than 100µm thick. The plugging resin itself has disadvantages in that it is a high-solid content material, which has a different coefficient of thermal expansion (CTE) to that of the surrounding material, including the copper metal in the via and also to that of the dielectric of the core itself, typically a glass-reinforced resin material.

The disadvantages of the existing plugging process can be eliminated by using pure copper to produce the planar surface. For the next-generation process, copper is deposited into the through vias as an integral part of the metallization process. The drilled dielectric is made conductive and a thin layer of copper metal is deposited to give a seed layer for the copper deposition process. The through-vias are then completely filled by a modified electrolytic copper deposition process, which can be accomplished in a single, fully automatic continuous processing line.

The use of pure copper has obvious advantages in that its thermal characteristics are significantly better than any type of resin material available.  This fact can give more design options to utilize the improved thermal transfer capability of vias in a substrate than are currently available.  The CTE of the copper-filled core is dependant only on the copper metal and the glass-supported resin of the drilled dielectric.  The copper structures in the subsequent layers may be positioned directly above the copper-filled through-vias with no reliability implications. In fact, the conductive path within the substrate may be designed to utilize the more direct and parallel connection from one side of the substrate to the other.