A transformer includes a primary coil assembly and a secondary coil assembly juxtaposed in a stack and which are electromagnetically inductively coupled and substantially symmetrical to one another. The primary coil assembly includes a plurality of primary coils coplanar with first insulation layers, respectively, and symmetrical to each other, and a first via unit connecting adjacent ones of the primary coils to each other. The secondary coil assembly includes a plurality of secondary coils coplanar with a plurality of second insulation layers, respectively, and symmetrical to each other, and a second via unit connecting adjacent ones of the secondary coils to each other.