A film carrier with 48 peripheral-contacts, which is applicable to ultra-high speed GaAs digital integrated circuits (ICs) with a more than 10 Gbps operation, has been developed. The film carrier has been realized using the following newly developed techniques; (1) wave guides with a well-controlled characteristic impedance of 50 Ω, (2) precise vias of as small as 50 µm diameter conducting both sides of grounded metal planes on a polyimide film, and (3) a feed-through structure for high speed input signals with good impedance matching. The film carrier was molded by resin after ILB (inner lead bonding) to a chip with a copper plate heat spreader. As an application, the film carrier has been applied to a 3 Gbps operational 4-bit GaAs multiplexer IC, and has been proved to have excellent high-frequency characteristics.