BACKGROUND Three-dimensional (3D) printing, known more formally as additive manufacturing, has become the focus of media and public attention in recent years as the decades-old technology has at last approached the performance necessary for direct production of end-use devices. The most popular forms of standard 3D printing include vat photopolymerization, powder bed fusion, material extrusion, sheet lamination, directed energy deposition, material jetting, and binder jetting, each creating parts layer by layer and offering different options in terms of cost, feature detail, and materials. Whereas traditional manufacturing technologies, such as casting, forging, machining, and injection molding, are well suited for mass production of identical commodity items, 3D printing allows for the creation of complex geometric shapes that can be mass-customized, because no die or mold is required and design concepts are translated into products through direct digital manufacturing. Furthermore, the additively layered approach enables the merging of multiple components into a single piece, which removes the requirement for subsequent assembly operations. Recently, the patents for the original 3D printing processes have begun to expire, which is resulting in a burgeoning number of low-cost desktop systems that provide increased accessibility to society at large. Industry has recognized the manufacturing advantages of these technologies and is investing in production systems to make complex components for jet engines, customized bodies for cars, and even pharmaceuticals. Although standard 3D printing technologies have advanced so that it is now possible to print in a wide range of materials including metals, ceramics, and polymers, the resulting structures are generally limited to a single material, or, at best, a limited number of compatible materials. ADVANCES For the technology to become more widely adopted in mainstream manufacturing, 3D printing must provide end-use products by fabricating more than just simple structures with sufficient mechanical strength to retain shape. Recently, research has resulted in the capability to use new materials with commercial 3D printers, and customized printers have been enhanced with complementary traditional manufacturing processes, an approach known as multiprocess or hybrid 3D printing. Collectively, these advancements are leading to fabrications that are not only geometrically complex, but functionally complex as well. By introducing the robotic placement of components, micromachining for intricate detail, embedding of wires, and dispensing of functional inks, complex structures can be constructed with additional electronic, electromagnetic, optical, thermodynamic, chemical, and electromechanical content. OUTLOOK Multiprocess 3D printing is a nascent area of research in which basic 3D printing is augmented to fabricate structures with multifunctionality. Progress will lead to local manufacturing with customized 3D spatial control of material, geometry, and placement of subcomponents. This next generation of printers will allow for the fabrication of arbitrarily shaped end-use devices, leading to direct and distributed manufacturing of products ranging from human organs to satellites. The ramifications are substantial, given that 3D printing will enable the fabrication of customer-specific products locally and on demand, improving personalization and reducing shipping costs and delays. Examples could include replacement components for grain-milling equipment in a remote village in the developing world, biomedical devices created specifically for a patient in a hospital before surgery, and satellite components printed in orbit, thus avoiding the delays and costs associated with launch operations. The automotive, aerospace, defense, pharmaceutical, biomedical, and consumer industries, among others, will benefit from the new design and manufacturing freedom made possible by multiprocess 3D printing.

http://science.sciencemag.org/content/sci/353/6307/aaf2093.full.pdf

Multiprocess 3D printing for increasing component functionality.

3D printed microfluidics and microelectronics

3-D additive manufacturing (AM) offers unprecedented flexibility in the realization of complicated 3-D structures. Polymer jetting is one of the promising 3-D AM techniques that utilizes photosensitive polymers as the build material and is capable of precisely printing electromagnetic (EM) components up into the THz range. In this paper, important design and implementation aspects of polymer-jetting-based 3-D-printed EM components are discussed. A number of 3-D-printable polymer materials and their broadband EM characterization from GHz to THz are introduced. Design methodologies specific for 3-D-printed antennas and other EM components are presented. As examples, various 3-D-printed devices operating from GHz to THz frequency, including electromagnetic crystals (EMXT), waveguide, horn antenna, gradient index (GRIN) lenses, as well as 3-D AM-enabled new designs, such as millimeter wave (mmW)/THz, reflect array antennas, computer-generated THz holograms, and so on are reviewed. Moreover, current limitations and possible future improvements of the polymer jetting technique for EM applications are discussed. This type of 3-D AM technique is likely to enable many novel antenna and circuit architectures as well as various interesting 3-D metamaterial structures.

3-D-Printed Microwave and THz Devices Using Polymer Jetting Techniques

This paper reports on the design, fabrication and characterization of a 3-D printed RF front end for a 2.45 GHz phased array unit cell. The printed unit cell, which includes a circularly-polarized dipole antenna, a miniaturized capacitive-loaded open-loop resonator filter and a 4-bit phase shifter, is fabricated using a direct digital manufacturing (DDM) approach that integrates fused deposition of thermoplastic substrates with micro-dispensing for deposition of conductive traces. The individual components are combined in a passive phased array antenna unit cell comprised of seven stacked substrate layers with seven conductor layers. The measured return loss of the unit cell is ${>}12$ dB across the 2.45 GHz ISM band and the measured gain is ${-}11$ dBi including all components. Experimental and simulation-based characterization is performed to investigate electrical properties of as-printed materials, in particular the inhomogeneity of printed thick-film conductors and substrate surface roughness. The results demonstrate the strong potential for fully-printed RF front ends for light weight, low cost, conformal and readily customized applications.

A 2.45 GHz Phased Array Antenna Unit Cell Fabricated Using 3-D Multi-Layer Direct Digital Manufacturing

The performance of additive manufactured (AM) RF circuits and antennas is continuously improving, and in some cases these AM components are comparable to state-of-the-art circuits made with traditional manufacturing techniques. Medium to high-power waveguides made with AM methods such as copper-plated plastics, selective laser melting (SLM), and copper additive manufacturing (3-D CAM) have shown good performance up to terahertz frequencies. In this paper, binder jetting (BJ) metal printing is characterized using electron beam microscopy [scanning electron microscopy (SEM)] and energy dispersive spectroscopy. The RF performance of the 3-D-printed circuits is benchmarked with Ka-band cavity resonators, waveguide sections, and a filter. An unloaded resonator $Q$ of 616 is achieved, and the average attenuation of the WR-28 waveguide section is 4.3 dB/m. The BJ technology is tested with a meshed parabolic reflector antenna, where the illuminating horn, waveguide feed, and a filter are printed in a single piece. The antenna shows a peak gain of 24.56 dBi at 35 GHz.

Ka-Band Characterization of Binder Jetting for 3-D Printing of Metallic Rectangular Waveguide Circuits and Antennas

This paper presents high frequency characterization data of the commonly used 3D-printing material Acrynolitrile Butadiene Styrene (ABS). A cavity resonator was used to characterize the material around 30 GHz. The extrapolated data was used to design a microstrip patch antenna around 25 GHz. The antenna was fabricated using a combination of fused deposition modeling (FDM) and direct print additive manufacturing (DPAM). Good agreement was found between simulated and measured results. These are the first known results of ABS characterization and a digitally-manufactured antenna to be reported in this frequency range.

Ka-band characterization and RF design of Acrynolitrile Butadiene Styrene (ABS)

3D multi-material additive manufacturing has promising potential to improve the performance and form factor of microwave and electromagnetic components. The ability to design and produce truly 3D form factors can lead to smaller volume and better performing antennas, allow the combining of different materials to optimize and reduce the weight of distributed circuit designs, and cost effectively produce structural electronic systems. Along with these new capabilities come increased needs for material choices, innovative methods for materials characterization, and changes in the CAD tools that are used for design. These challenges, along with examples of 3D RF design with additive manufacturing are given in this paper.

Miniaturization of microwave components and antennas using 3D manufacturing

This paper presents a near-field microwave microscope (NFMM) capable of simultaneous non-contact imaging of electrical conductivity (σ) and topography across the surface of microwave circuits without the need of a distance sensor. The microscope monitors the resonant frequency of a dielectric resonator-based microwave probe to acquire the surface topography, and the quality factor to determine the electrical conductivity. Conductivity and topography images of copper foil reveal an average conductivity of about 4e7 S/m and an arithmetic roughness of 0.2µm, respectively. Measured average roughness and conductivity of CB028 silver paste are 0.7e6 S/m and 1.1µm, respectively. The NFMM data reveal significant and correlated variation in surface features and conductivity across the surface of the printed CB028 films. The topography and conductivity images obtained demonstrate that the NFMM can be employed for localized characterization of smooth and rough conductive materials used in microwave devices.

Simultaneous RF electrical conductivity and topography mapping of smooth and rough conductive traces using microwave microscopy to identify localized variations

A dielectric resonator-based microwave probe for low loss and lossy liquids characterization is presented in this paper. When the probe is operated under in-situ conditions, improvement of the probe sensitivity to variations in the permittivity of the liquid was observed. Measurements of the quality factor of the resonator reveal that the probe is able to resolve variations around 0.04 in e' for low loss liquids. The spatial resolution of the probe is 10 µm, and was experimentally verified by scanning a MMIC detector covered with a 30 µm thick layer of mineral oil, at a standoff distance of 10 µm. Scanning results confirm that the presence of the mineral oil layer on top of the circuit improves the sensitivity of the probe without degrading the spatial resolution. Simulated data using a lumped-element model of the probe and sample support the measured results.

Maria F. Cordoba-Erazo

Papers

A 2.45 GHz Phased Array Antenna Unit Cell Fabricated Using 3-D Multi-Layer Direct Digital Manufacturing

Ka-band characterization and RF design of Acrynolitrile Butadiene Styrene (ABS)

Miniaturization of microwave components and antennas using 3D manufacturing

Simultaneous RF electrical conductivity and topography mapping of smooth and rough conductive traces using microwave microscopy to identify localized variations

Liquids characterization using a dielectric resonator-based microwave probe