# Waveguide Manufacturing Technologies for Next-Generation Millimeter-Wave Antennas
## 4 Stereolithography (SLA) and Post-Metallization Process
Nevertheless, the cross-slot inclusion in the cylindrical waveguide was incompatible with accurate manufacturing, although enquiries were made to several suppliers. In addition, this DMLS process is sig  
Figure 11: Spray metallization scheme developed by JetMetal [47]. nificantly more expensive than plastic SLA 3D printing. This is an important consideration, in addition to the considerable weight reduction, especially in a cellular system such as the next 5G mobile communication generation, in which massive deployment is required and the cost increase is a significant aspect. The manufacturing processes involved in the different parts of the antenna presented in [52] were described. In the first instance, the cylindrical waveguide (including all slots) was manufactured with Accura Xtreme resin material and a 100 \(\upmu\)m thick layer resolution, using a SLA 3D printing process performed by ProtolabS [26]. Second, the 3D printed prototype was plated by the JetMetal sprayed metallization process [47], as shown in Figure 12b. In this context, the optimal metal thickness was significant. In both of the previously detailed processes (chemical Swissto12 or sprayed JetMetal coatings), the metal thickness could be controlled with the exposure time, set beyond the E-field penetration depth at the operation frequency band. At first sight, a silver coating thickness of just a few micrometers may seem brittle and prone to easy removal. In fact, the standard ECSS-Q-ST-70-71C Rev.1 (2019), followed by the European Space Agency (ESA), states that plated layers of less than 1 \(\upmu\)m thickness tend to be porous, which may lead to corrosion. On the other hand, the same ESA standard reminds us that thick coatings can generate mechanical stresses and fairly high temperatures during their cure. In addition, thicker coatings require larger amounts of material, thus increasing costs. For this reason, coating thickness should be carefully chosen, taking into account mechanical, thermal, and electrical aspects. Coating thicknesses in the order of microns are common in the space industry. As the abovementioned ESA standard states, aluminum surfaces are frequently treated with alodine, a conductive coating with a thickness of less than 1 \(\upmu\)m. Other protective coatings to prevent electrostatic discharges and provide thermal relief can also have thicknesses lower than 1 \(\upmu\)m [54]. Regarding the silver plating, which is usually employed to maximize electrical conductivity, thicknesses around these values are also often considered. The  
Figure 12: Photographs of the conformal array antenna manufactured by SLA + spray metallization: (a) whole antenna prototype; (b) slotted cylindrical waveguide detail; and (c) dual-mode feeder. reflector of the Kepler Space Telescope is coated with a 0.5 micrometer silver layer [55], and some Ka-band waveguide devices have been post-processed with a 1-2 micrometer silver coating [56]. The latter have successfully undergone environmental tests, being exposed to severe humidity and thermal conditions or mechanical stress. Other surface treatments can be applied to prevent corrosion and provide thermal relief. These treatments are usually in the form of paints, examples of which can be found in the standard ECSS-Q-ST-70-71C Rev.1, with thicknesses in the order of tens of microns. Thus, the 3D antenna prototype printed in plastic material was plated with a 2 um silver coat by the spray metallization process developed by JetMetal. Finally, some parts were mechanized in aluminum, following a traditional methodology based on computer numerical control (CNC). For example, the dual-mode feeder shown in Figure 11(c), as well as the metallic short-circuit of the cylindrical waveguide, shown in Figure 11(a), were both manufactured by CNC techniques. The only critical part in the assembly process of the 3D printed antenna prototype can be the screws fixing to the contact parts of the metallized device. The JetMetal process significantly reduces this risk, due to film absorption.