Schematic cut through a KID substrate. In grey, the high-resistance silicon wafer, while in black, the aluminium films are represented. The green arrows illustrate the direction of the electric field. Left: the co-planar waveguide (CPW) transmission line without across-the-line bondings, associated to strongly non-uniform performance of the detector array. Centre: CPW with across-the-line bondings, configuration adopted in NIKA. Right: the microstrip (MS) configuration adopted in NIKA2, ensuring single-mode propagation and easiest implementation of very large arrays.