Magnetic field distribution and mutual capacitance control for transmission lines are provided. A first circuit board is fabricated by attaching a reference plane layer to a dielectric material layer, and attaching a first trace to the second surface of the dielectric material. A surface profile of the reference plane layer is modified to decrease a resistance of a return current signal path through the reference plane layer, to reduce a magnetic field coupling between the first trace and a second trace. A second circuit board is fabricated by attaching a reference plane layer to a dielectric material layer, attaching a trace to the dielectric material, and forming a solder mask layer on the dielectric material layer over the trace. An effective dielectric constant of the solder mask layer is modified to reduce or increase a mutual capacitance between the first trace and a second trace on the dielectric material.