A method of fabricating a substrate core structure, and a substrate core structure formed according to the method. The method includes: laser drilling a first set of via openings through a starting insulating layer; filling the first set of via openings with a conductive material to provide a first set of conductive vias; providing first and second patterned conductive layers on opposite sides of the starting insulating layer; providing a supplemental insulating layer onto the first patterned conductive layer; laser drilling a second set of via openings through the supplemental insulating layer; filling the second set of via openings with a conductive material to provide a second set of conductive vias; and providing a supplemental patterned conductive layer onto an exposed side of the supplemental insulating layer, the second set of conductive vias contacting the first patterned conductive layer and the supplemental patterned conductive layer at opposite sides thereof.