of which are drilled to form the grooves on each side of the respective copper line traces to simplify the discussion only the top surface layer in a multilayer structure is considered as per standard practice resin coated copper foil 21 or copper foil with prepreg is laminated onto a pwb backbone substrate 23 which may include the previously laminated layers of a multilayer board containing a ground plane 24 and a microstrip signal line 22 built on top of a layer of dielectric 25 a high speed laser is then used to drill a series of overlapping microvias along the entire length of the signal line on each side to form the grooves thereby exposing the ground metal all along the bottom of the channel if a yag yttrium aluminium garnet laser is available the microvias can be drilled directly from the top copper foil surface 21 through the dielectric 25 to the underlying ground plane 24 since this type of laser is capable of cutting through most materials including epoxy glass and copper after drilling a plasma etch or permanganate cleaning process is used to clean and smooth out the groove s standard metallization techniques e g electroless and electrolytic copper plating are then applied to the surface and groove s resulting in a complete metal shield around the copper signal line 22 if however only a co2 laser is available the desired groove pattern must be initially formed on.