PCB interconnection concepts for highly integrated Ka-band antenna for 5G and satcom front-ends

Future mobile communications, be they satellite or 5G, call for the use of active, electronically steerable, planar antenna arrays operating in the Ka-band. The demanding performance specifications of such systems are further complicated by the small size required. Therefore, it is often good practice to split the RF circuitry and the antenna aperture in two separate PCBs, in order to achieve vertical integration: (a) a multilayer (soft) substrate containing the RF Tx/Rx chains, which comprise densely packaged MMICs integrated in single-sided cavities, and (b) LTCC tiles (hard substrate) for the implementation of the passive antenna apertures. The key to the realization of the final assembly is the interconnection technology that implements the RF interface between the two boards. This vertical interconnect should combine high performance, high reliability, excellent reproducibility, manufacturability, and mechanical and thermal load capacity. In the AVISAT project, the use of small ball-grid-arrays (BGAs) for the connection between the soft- and hard-substrate boards is foreseen. Due to the large number of RF-chains and the high level of integration, miniaturization of the BGA footprint is of capital importance. This paper presents the results of the optimisation of such BGA interfaces, taking into account the different PCB material characteristics and the multilayer nature of both active and passive printed circuits.