Circuit board structure

A circuit board structure comprising:

• two circuit board units, each of the circuit board units comprising:
• a substrate having an opening penetrating the first dielectric layer, and the substrate comprising:
• a first dielectric layer having a first surface and a second surface opposite to each other;
• a second dielectric layer filling the opening, the second dielectric layer having a third surface and a fourth surface opposite to each other;
• a first inner circuit layer disposed on the first surface;
• a second inner circuit layer disposed on the second surface;
• and
• a conductive connection layer covering an inner wall of the opening, the conductive connection layer being connected to the first inner circuit layer and the second inner circuit layer;
• a third dielectric layer covering the first inner circuit layer and the third surface;
• a fourth dielectric layer covering the second inner circuit layer and the fourth surface;
• a first external circuit layer disposed on the third dielectric layer, the first external circuit layer comprising a first signal circuit and a first ground circuit;
• a second external circuit layer disposed on the fourth dielectric layer, the second external circuit layer comprising a second signal circuit and a second ground circuit;
• a conductive through hole penetrating the third dielectric layer, the second dielectric layer, and the fourth dielectric layer, the conductive through hole being electrically connected to the first external circuit layer and the second external circuit layer;
• a first annular retaining wall disposed in the third dielectric layer, surrounding the conductive through hole, and being electrically connected to the first external circuit layer and the first inner circuit layer, wherein the first ground circuit, the first annular retaining wall, and the first inner circuit layer define a first ground path, and the first ground path surrounds the first signal circuit;
• and
• a second annular retaining wall disposed in the fourth dielectric layer, surrounding the conductive through hole, and being electrically connected to the second external circuit layer and the second inner circuit layer, wherein the second ground circuit, the second annular retaining wall, and the second inner circuit layer define a second ground path, and the second ground path surrounds the second signal circuit;
• and
• a connection structure layer disposed between the circuit board units, the connection structure layer being electrically and structurally connected to the first external circuit layers of the circuit board units to butt the circuit board units together, wherein the first ground path of each of the circuit board units is connected through the connection structure layer and defines a third ground path.