A method for forming a circuit board includes forming a first dielectric layer, a first circuit layer in the first dielectric layer, a second circuit layer on the first dielectric layer, and a plurality of conductive vias in the first dielectric layer and connecting the first circuit layer to the second circuit layer; forming a second dielectric layer on the first dielectric layer and the second circuit layer; forming a plurality of openings in the second dielectric layer to expose a plurality of parts of the second circuit layer; forming a seed layer on the exposed parts of the second circuit layer and sidewalls of the openings; and forming a plurality of bonding layers on the seed layer, wherein the bonding layers and the seed layer are made of copper, and the bonding layers are porous.