Prior work has demonstrated a new process utilizing room-temperature liquid metal, Galinstan, as an interconnect material for flip-chip bonding. This interconnect forms a flexible bond between chips and carriers, and, therefore, a flip-chip assembly using this technology is much less susceptible to thermomechanical stresses. This paper applies this concept to interconnect GaAs MMIC chips to 3-D Polystrata transmission-line structures. Passive assemblies are utilized to model, test, and verify liquid-metal interconnections, giving average losses per liquid-metal transition of about 0.11 dB out to 26.5 GHz, low parasitics per transition, and demonstrated reliability after temperature cycling. A prefabricated GaAs MMIC chip is postprocessed for liquid-metal assembly. Measured results show, over the MMIC's 4.9-8.5-GHz frequency range, the system's overall reduction in gain of the MMIC is 1.4 dB or 0.7 dB per RF transition as compared with direct probing of the MMIC chip.