The incoming 5G systems propose unprecedented communication capabilities that require a maximization of the channel and spectrum exploitation. Waveguide antenna arrays can play an important role in 5G environments due to their high performance in terms of efficiency and bandwidth. Moreover, higher frequencies appear necessary in order to achieve the requirements of channel capacity and spectrum bandwidth. At millimeter-wave frequencies, new challenges for the design and fabrication of the communication devices are encountered. The feasibility of traditional manufacturing techniques and conventional additive manufacturing processes severely decreases, since part tolerances, assembly, and alignment become critical for the antenna performance. In order to illustrate the possibilities of the diffusion bonding technique applied to waveguide antennas operating in the W-band of 5G systems, a circularly polarized cavity array has been successfully manufactured. This manufacturing process provides the advantages of both traditional and additive manufacturing. A 5.6 percent effective bandwidth at 90 GHz has been experimentally obtained with reflected power under 10 percent, axial ratio below 3 dB, and antenna efficiency over 50 percent.