A structure includes a first encapsulating layer, and a first coil in the first encapsulating layer. A top surface of the first encapsulating layer is coplanar with a top surface of the first coil, and a bottom surface of the first encapsulating layer is coplanar with a bottom surface of the first coil. A second encapsulating layer is over the first encapsulating layer. A conductive via is in the second encapsulating layer, and the first conductive via is electrically coupled to the first coil. A third encapsulating layer is over the second encapsulating layer. A second coil is in the third encapsulating layer. A top surface of the third encapsulating layer is coplanar with a top surface of the second coil, and a bottom surface of the third encapsulating layer is coplanar with a bottom surface of the second coil.