(8) Make a circuit layer in the groove to obtain a circuit board unit, the circuit layer includes a connection pad located in the concave portion, the connection pad is shaped as a conductive protrusion which surrounds and is electrically connected to the conductor;

(9) Repeat step (1) to step (8) at least once; and

(10) Laminate the at least two of the circuit board units.

Further, in step (5), the side wall and bottom wall of the groove are processed by a plasma surface treatment machine.

Further, in step (7), the seed layer is formed by chemical vapor deposition or physical vapor deposition.

Further, after step (8) and before step (9), the method further includes the steps of: forming a metallization layer on the surface of the circuit layer.

Further, in step (1), one side of the substrate is connected to a carrier board through a separable film.

A circuit board includes at least two stacked circuit board units, and each of the circuit board units includes a substrate and a circuit layer. The substrate is provided with a conductive hole, and a conductor is provided in the conductive hole. A groove is provided on one side of the substrate. The groove includes a concave portion. The concave portion is located at the conductive hole, and the diameter of the concave portion is larger than the diameter of the conductive hole to expose a portion of the conductor. The circuit layer includes a connection pad located in the concave portion, and the connection pad is shaped as a conductive protrusion which surrounds and is electrically connected to the conductor. The circuit layer is located in the groove, and the conductive hole electrically connects the circuit layers of two adjacent circuit board units.