1. A planar reactor comprising:a core comprising:
an upper board;
a lower board; and
a pillar located between the upper board and the lower board, a winding space being located among the upper board, the lower board and the pillar;
a coil wound around the pillar and located in the winding space;
a heat conducting member disposed at the first end of the coil extending outside at least one of the upper board and the lower board and overlapping a part of the coil from an innermost ring to an outermost ring; and
a pouring sealant covering the coil and a plurality of surfaces of the heat conducting member;
wherein the pillar and at least one of the upper board and the lower board are coplanar at a first side of the planar reactor, the pillar is sunk into the winding space from a second side of the planar reactor, the first side is opposite to the second side, a first end of the coil is exposed from the first side of the planar reactor, a second end of the coil is hidden in the winding space partially or wholly at the second side of the planar reactor, the first end is opposite to the second end.