A superconductor on-chip microstrip line (2, 4) to off-chip microstrip line (7) transition of low characteristic impedance (15, 20, 22) is realized that obtains a bandwidth of 200 GHz for MCM application while employing solder bump (15, 17) technology to connect the chips (3, 5) to the off-chip microstrip and substrate (6). Circular openings (20, 22) through the respective ground plane layers (10 & 16) of the off-chip and on-chip microstrips are provided in positions respectively underlying and overlying the solder bump (15) for the signal. The openings may be sized to provide a desired ratio of inductance to capacitance, the larger the size, the greater the ratio value. This technique may be used to match characteristic impedance to give broad bandwidth low impedance interconnections needed for direct SFQ chip-to-chip communication on a passive MCM.