Flexible/Conformal Inkjet-Printed 3-D “Ramp” Interconnects for 5G/mmWave System-on-Package Designs and Wearable Applications

This article presents the first and most comprehensive design, fabrication, and reliability evaluation of the electrical and mechanical performance of fully inkjet-printed 3-D “ramp” interconnects. Inkjet-printed interconnects feature superior RF performance and better mechanical reliability for heterogeneous integration for 5G mmWave flexible electronics packaging. The packaged systems are required to survive various flexing conditions over a large number of cyclic bending for conformal applications. In this work, ramp interconnects are designed with excellent and reliable performance for flexible packaging over the 20–40-GHz frequency band. A test vehicle of a monolithic microwave integrated circuit (MMIC) attenuator die placed in the middle of two microstrip lines is used, where ramp interconnects are fabricated by inkjet printing SU8 dielectric ink to form ramp base and inkjet printing silver nanoparticle (SNP) ink for conductive interconnects to build the connection between the die and microstrip lines. The fabricated sample exhibits a superior $S_{21}$ performance with less than 1.16-dB insertion loss per interconnect throughout the whole operation range from 20 to 40 GHz. Monotonic bending tests are conducted over mandrels of various radii ranging from 50 to 10 mm, and the designed ramp interconnects are able to maintain robust transmission with a minimum variation of less than 0.15-dB insertion loss per interconnect. Moreover, the fabricated samples are able to survive over 20 000 times of cyclic bending tests over the mandrel of 10-mm radius, with less than 0.2-dB additional loss from each interconnect. The proposed design takes advantage of a low-cost, on-demand additive manufacturing method by selectively depositing SU8 ink layer by layer with varying dimensions to enable rugged ramp structures for curved mounting platforms. The results reported in this article could enable rapid production of high-performance and reliable flexible system-on-package (SoP) and multichip module (MCM) designs and build the foundation of next-generation 5G mmWave flexible hybrid electronics (FHE) technologies for various applications.