Circuit board, circuit board connection structure, and method of manufacturing circuit board connection structure

A circuit board comprising:

• a multilayer body defined by laminated thermoplastic resin layers, the multilayer body including a first main surface on one end in a lamination direction and a second main surface on another end in the lamination direction;
• a mounted conductor on the first main surface;
• and
• a planar conductor located closer to the second main surface than the mounted conductor and other conductors provided in the multilayer body;
• wherein
• the multilayer body includes a connection portion joined to an external board via a conductive joint material through use of the mounted conductor, and a circuit portion other than the connection portion;
• a first region includes a first opening hole and a second region includes a second opening hole, the first region being a region of the circuit portion of the planar conductor, the second region being a region of the connection portion of the planar conductor;
• and
• a ratio of an opening area of the second opening hole to an area of the second region is larger than a ratio of an opening area of the first opening hole to an area of the first region.