1. A circuit board comprising:a base layer; and
a thermally expandable layer disposed on the base layer, the thermally expandable layer having a not-expanding region and an expanding region, the not-expanding region defining an insulating region of a circuit, the expanding region defining a conducting region of the circuit, and the thermally expandable layer including a plurality of microcapsules and a binder having an insulating property,
each of the plurality of microcapsules includes (i) a shell including a conducting component and (ii) a thermally expandable component contained in the shell and having a property of expanding by heating, and
the shell of a first microcapsule in the expanding region, from among the plurality of microcapsules, is deformable in accordance with expansion of the thermally expandable component of the first microcapsule by heating so as to come into contact with the shell of a second microcapsule of the plurality of microcapsules.