Medical apparatus includes one or more magnetic field generators, which are configured to generate magnetic fields within a body of a patient. An invasive probe includes an insertion tube having a distal end, which is configured for insertion into the body, and a plurality of flexible splines configured to be deployed from the distal end of the insertion tube. Each spline includes a flexible, multilayer circuit board and a conductive trace that is formed in at least one layer of the circuit board and is configured to define one or more coils, which are disposed along a length of the spline and output electrical signals in response to the magnetic fields. A processor is coupled to receive and process the electrical signals output by the coils in order to derive respective positions of the flexible splines in the body.
