**1. Introduction**

According to the American Society of Testing and Materials (ASTM), additive manufacturing (AM) is defined as "the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies." Often this term is substituted by 3D Printing (3D Printing is typically associated with people printing at home or in the community; additive manufacturing is typically associated with production technologies and supply chains, but they both produce parts by the addition of layers). AM technologies can be classified into seven categories, namely, binder jetting, material jetting, direct energy deposition, sheet laminations, material extrusion, powder bed fusion, and vat photopolymerization. Each category includes several processes that share the same principle used for layer modeling and different materials that can be processed (**Table 1**).

AM has been first applied for rapid prototyping of visualization models and tooling. Recently, the improvement in the process's accuracy and material properties of the manufactured objects have expanded the field of applications. Indeed, AM is currently used to manufacture personalized prostheses, replacement organs, and implants in the medical sector and produce complex lightweight components for the aerospace, automotive, and sports industries. Recently, AM has been applied by


#### **Table 1.**

*Seven categories of AM technologies [1].*

RF industries for the development of next generations of microwave and millimeterwave components for sensors, imaging systems, and satellite communication (SATCOM) [1].

A generic AM process starts with a model generated using a three-dimensional Computer-Aided Design (3D CAD) system. Then the model is converted into the STL file format that approximates the 3D model with a mesh of triangles. Then, this file is transferred to the AM machine to set the process parameters. Such settings are defined according to the geometry of the model (e.g., position and orientation of the components, design of support structures) and to the building process (e.g., energy source, material constraints, and layer thickness). At the end of the printing process, the part is removed from the building platform and prepared for the post-processing operations, for example, cleaning, sandblasting or shot-peening, thermal treatments, and plating [2].

The main advantage of AM process is manufacturing lightweight components with complex internal surfaces in a single part. Moreover, by eliminating tools, the design flexibility is increased. On the other hand, the main concerns are the manufacturing accuracy and the surface roughness that are worse than standard manufacturing processes and strongly depend on the material and process parameters.

In the microwave area, Selective Laser Melting (SLM)<sup>1</sup> , Stereolithography (SLA), and Fused Deposition Modeling (FDM™) are the most investigated technologies. The application of AM processes in manufacturing microwave and millimeter-wave components strictly depends on the accuracy, cost, and performance requirements. From this point of view, basic knowledge of the characteristics of a single process is necessary. This overview is reported in Sections 2–4. Section 5 is, instead, devoted to giving a survey of the principal results (in terms of realized components) actually achieved in the specialized literature. In this summary, all the important aspects for microwave

<sup>1</sup> The process Selective Laser Melting is, nowadays, sometimes called Laser Powder Bed Fusion (LPBF).

engineering are reported, i.e., operative band, measured results versus the expected ones, and an explanation of this difference. This section is split into subsections for reader convenience according to the component category.
