1. A method for constructing an external circuit structure, applied to an inner circuit substrate, which comprises: a core board, both surfaces of which are coated with copper circuits; at least one via-hole, drilled through the core board and filled with conductive medium, wherein, the method comprises:laminating a copper foil and a prepreg on the inner circuit substrate; wherein, the prepreg is laminated between the copper foil and the inner circuit substrate;
drilling at least one blind via from the copper foil through the prepreg to reach one of the copper circuits coated on the core board;
desmearing the at least one blind via;
after the desmearing step, corroding off the copper foil;
after the corroding step, electroless copper plating on the prepreg to form an electroless plating copper layer on the prepreg; wherein, during the electroless copper plating process, swelling a surface of the prepreg with alkaline swelling liquid, thereby increasing a distance between molecules of the surface of the prepreg and superficial area of a micro structure of the prepreg; and neutralizing the alkaline swelling liquid left on the surface of the prepreg.