## 3 Circuit Level Perspectives of JCRS RF Hardware
### Packaging Considerations  
Packaging of semiconductor circuits becomes critical with the increase in operational frequencies. The chip-to-board interconnect exhibits strong parasitics, affecting the system performance for mmWave carriers. Bondwires have been a popular choice for interconnects, which show frequency and length dependent inductance, making a bottleneck between the chip and carrier. Nevertheless, it has been extensively used in mmWave circuits with various parasitic compensation techniques, e.g., tight ground-signal-ground (GSG) configuration, half-wavelength, and planar lumped element matching [82; 83]. Matched bondwire interconnects have been used for operations above 100 GHz [84]. The alternative is the flipchip approach, offering very short length chip-to-carrier connections. The downsides of flipchip are complex placement, dielectric detuning, and restricted heat relief paths.  
Besides interconnects, array feed networks require customized passive RF blocks. Surface mount lumped components are typically not functional beyond the cmWave spectrum. Hence, distributed solutions are preferred for baluns, couplers, and bias-feeds, e.g., with antenna switches and phase shifters. PCT has been widely adopted in electronic packaging and antenna design. The first-class manufacturing facilities can offer multilayer packaging with design resolutions down to 70 \(\upmu\)m. The process tolerance of PCT is usually higher than thick and thin film processes. and becomes a point of concern as the wavelength shrinks. In contrast, advanced integration technologies, e.g., co-fired ceramics, quartz, and silicon interposers, can offer layout resolution of a few micrometers and an excellent accuracy, with the penalty of fabrication cost.  
The advancements in chip-to-carrier interconnect and planar patch antenna designs have enabled antenna-in-package (AiP) possibilities, easing interconnections and economical manufacturing. mmWave frequencies are favorable for AiP, offering compact form factors. A challenge in densely packed antenna packages is RF-level transmitter-to-receiver leakage, potentially saturating the receiver low noise amplifier (LNA). Various approaches are being research for transmitter-to-receiver isolation enhancement, namely DGS, EM bandgap assemblies (EBG), and interleaved antennas [51; 85]. Mitigating the transmitter leakage can enable JCRS implementation with antenna system sharing between in-band full-duplex communication and monostatic radars.  
At D-band, this becomes more challenging. The bondwire interconnects have frequency-dependent loss, and they are not preferable at D-band. Flipchips offer very short length interconnects and are still usable. Ref. [86] presented a prototype with a flipchip interconnect on an organic interposer containing the antenna, which is then bonded to PCB usingball grid arrays. Ref. [81] presented a packaging approach using EM coupled interconnects. A diced quartz with a printed antenna is attached to the top of chip. An on-chip feed line couples a 140 GHz signal to the patch antenna on the quartz superstrate. The chip-quartz packaging is then bonded to the PCB using bondwires for IF and LO ports. Other packaging options include fan-out wafer level packaging, used in [87], for hybrid beamforming in communication and localization.