A method of fabricating a substrate core structure comprises: providing first and second patterned conductive layers defining openings therein on each side of a starting insulating layer; providing a first and a second supplemental insulating layers onto respective ones of a first and a second patterned conductive layer; laser drilling a set of via openings extending through at least some of the conductive layer openings of the first and second patterned conductive layers; filling the set of via openings with a conductive material to provide a set of conductive vias; and providing a first and a second supplemental patterned conductive layer onto respective ones of the first and the second supplemental insulating layers, the set of conductive vias contacting the first supplemental patterned conductive layer at one side thereof, and the second supplemental patterned conductive layer at another side thereof.