Low-Loss Aerosol-Jet Printed Wideband Interconnects for Embedded Devices

Abstract

This article presents a hybrid process to fabricate chip-to-board interconnects for microwave and millimeter-wave applications. It is a four-step process: 1) cavities are etched in a copper-backed organic substrate [liquid crystal polymer (LCP)]; 2) active and passive devices are mounted in the cavity using a soldering or adhesive material; 3) spacing between the chip and the cavity edge is filled using a UV curable epoxy; and 4) interconnects are formed between the chip pad and the substrate using aerosol-jet printed silver ink. Cavities in the LCP substrate are etched using a wet process, which is simple to implement and allows parallel processing of multiple cavities. Aerosol printing allows custom patterning of interconnects to achieve good impedance transition between the chip and the transmission lines on the LCP substrate. The filler material and the associated structures are first characterized up to 40 GHz. A 0-dB attenuator is utilized to characterize the interconnect process followed by the demonstration of an embedded amplifier using fully printed transmission lines, RF interconnects, and dc biasing. In addition, aerosol printing is utilized for the rework of damaged interconnects. The proposed embedded active process demonstrated is simple to implement and low-cost and provides an approach to design wideband interconnects. An existing shortcoming of the embedded process is the metal contact to the semiconductor substrate at the edge leading to the formation of a Ag/GaAs Schottky contact, which can be eliminated by depositing a thin dielectric film on the exposed semiconductor region.