Flexible circuit card with laser-contoured VIAs and machined capacitors

A flexible circuit such as a membrane probe (10) is made by forming a trench (50, 60) in the upper surface (39) of a polyimide substrate (38) with a trench base (52, 62) spaced below the upper surface. The trench has an end wall (54, 64) ramped at an obtuse angle to the substrate upper surface and the trench base. A conductive layer deposited on the upper surface is patterned to form a line trace (44, 46) extending continuously over the substrate upper surface, down the ramped end wall and along the trench base, to contact a ground plane or form a distributed capacitance. An excimer laser is used, at a wavelength of 308 nm., an energy density less than 0.54 J./cm2 (preferably 0.18 to 0.35 J./cm2), and a pulse frequency of about 100 Hz., to ablate successive incremental thicknesses (80) of polyimide from the substrate in sweeps of depthwise decreasing length.