The groove 15 includes a concave portion 151. The concave portion 151 is located at the conductive hole 12, and the diameter of the concave portion 151 is larger than the diameter of the conductive hole 12.

In a fifth step, a surface treatment is performed on a side wall and a bottom wall of each of the two grooves 15 to increase roughness.

In a sixth step, the two peelable films 13 are removed.

In a seventh step, a seed layer is formed on the side wall and bottom wall of each of the two grooves 15.

In an eighth step, referring to FIG. 13, a circuit layer 20 is formed in each of the two grooves 15 to obtain a circuit board 300.

Each of the circuit layers 20 includes a connection pad 21 located in the concave portion 151. The shape of the connection pad 21 is a conductive protrusion, and the two circuit layers 20 are electrically connected through the conductor 111.

FIG. 10 is a third embodiment of the present disclosure of the circuit board 200. The circuit board 200 includes at least two stacked circuit board units 100. Each of the circuit board units 100 includes a substrate 10 and a circuit layer 20.

The substrate 10 has a conductive hole 12 penetrating therethrough. A conductor 111 is provided in the conductive hole 12.