There are two different structures for circuit boards: core structure and foil structure.

　　In the core structure, all conductive layers in the circuit board are laid on the core material; In the foil structure, only the internal conductive layer of the circuit board is applied to the core material, and the external conductive layer is made of a foil dielectric plate. All conductive layers are bonded together through a multi-layer lamination process using a medium.

　　Nuclear materials are double-sided foil plates used in factories. Because each core has two faces, when fully utilized, the number of conductive layers on the PCB is even. Why not use foil on one side and nuclear structure on the other? The main reason is the cost of PCB and the curvature of PCB.