An Additively Manufactured Flexible Millimeter- Wave Doppler Radar: Toward Fully Printed High-Frequency Multilayer Flexible Hybrid Electronics

Flexible hybrid electronics (FHE) is an emerging technology enabled through the integration of semiconductor devices and additive manufacturing technology. It unlocks tremendous market potential by realizing low-cost flexible circuits and systems that can be conformally integrated into various applications. However, the operating frequencies of most reported FHE systems are relatively low. It is also worth noting that reported FHE systems have been limited to simple design concepts since complex systems will impose challenges in aspects such as multilayer interconnections, printing materials, and notably, bonding layers. In this article, an additively manufactured flexible millimeter-wave Doppler radar is reported. Rogers RO4350B and flexible high-temperature filament TPC are chosen as the substrate. High-heat insulation epoxy is used as the bonding layer to increase the mechanical strength of the printing radar. Additionally, the RF properties of TPC and epoxy materials have been characterized from 20 to 50 GHz, paving the way toward fully additively manufactured radar systems. Mechanically drilled through vias and blind vias are employed to realize interconnections between each conductive layer. The realized gain of the unbent $\backslash $ bent printed millimeter-wave circuits was characterized, with a value of 12.80 and 9.355 dBi, matching well with expectations. The sensing performance and flexibility of the printed radar are also characterized and validated by general laboratory tests, field tests, and on-object tests. Our results demonstrate the feasibility of developing additively manufactured high-frequency multilayer FHE, which can be conformally and seamlessly integrated into irregular surfaces for applications such as vehicle radars.