Circuit board provided with digging depth detection structure and transmission device with the same mounted

A circuit board provided with a plurality of conductive layers laminated in a first direction, a dielectric material which spaces the plurality of conductive layers each other, and a through hole which extends and exists in the first direction and electrically connects a pair of the plurality of conductive layers, wherein:

• the through hole is terminated with a digging aperture formed by digging the circuit board from one of main surfaces thereof crossing the first direction toward the first direction;
• an inside wall of the digging aperture which extends and exists in the first direction consists of the dielectric material;
• and
• one of the plurality of conductive layers other than the pair thereof is exposed from the inside wall of the digging aperture.

A transmission device provided with a signal wiring to transmit signals, conductive layers which are connected to a power supply or ground, a dielectric material to laminate the signal wiring and the conductive layers, and a through hole which extends in a direction of laminated layers of the signal wiring and the conductive layers and is for changing a lying position of the signal wiring in the direction of laminated layers, comprising:

• a circuit board which is provided with a digging depth check terminal which changes its conductive state with the through hole when the through hole reaches a desired length by being dug;
• and
• a plurality of optical modules which switch a transfer route of a transmission signal sent by an optical fiber, convert the transmission signal to an electrical signal, a plurality of optical fibers which transmit this optical signal, a plurality of switch ICs which determine the transfer route of the transmission signal, a plurality of switch boards which mount the optical module and the circuit board, a back board which is to transmit the signals between the plurality of switch boards, and a plurality of back-plane connectors which electrically connect the switch board and the back board;
• wherein the through hole is partly dug from a main surface of the circuit board toward the direction of laminated layers, and a digging of the through hole is stopped at the time point when the conductive state between the through hole and the digging depth check terminal has changed.