Three-dimensional (3D) printing has captured the imagination of mechanical design engineers everywhere for its creative physical shape forming. Although design engineers may still be curious of the potential of 3D printing for electronic components, Rogers Corp. has little doubt that the right materials and printing processes may hold the future to affordable millimeter-wave circuits and components, including antennas for Fifth Generation (5G) and Sixth Generation (6G) wireless networks. But starting with a 3D-printable dielectric resin such as Radix™ 3D-Printable RF material from Rogers Corp., detailed scalable components with fine features for millimeter-wave frequencies and even with gradient index structures can be manufactured quickly and repeatedly from computer models.
As explained in the earlier Rog Blog, “3D-Print Antennas with Dielectric Resin,” Radix material works with 3D printers that use digital light processing (DLP) and stereolithographic additive (SLA) manufacturing techniques to transform computer models into physical parts. Partnering with 3D printing innovator Fortify and their DLP printers, Rogers Corp. has fabricated a variety of high frequency components, including a millimeter-wave Luneberg lens antenna described in that earlier blog. With the resolution possible with 3D printing, many of the components needed at millimeter-wave frequencies, such as antenna radomes, conformal transmission lines, graded index (GRIN) antennas, and waveguide, can be formed by SLA manufacturing on DLP 3D printers. In a GRIN architecture, the permittivity or dielectric constant (Dk) is adjusted in such a way to create a continuous gradient Dk between two points and a reflectionless signal path for higher, millimeter-wave signal frequencies.
One of the keys to achieving high performance of these components is the consistency and stability of the printing material’s permittivity or dielectric constant (Dk). The dielectric resin must maintain its consistency during the 3D printing process and then when subjected to its operating conditions. GRIN lenses, for example, consist of a detailed lattice with effective Dk that varies across the lens (see figure). To achieve the tightly controlled gradient Dk values, the dielectric printing material must undergo the printing process without exhibiting any change in Dk behavior so that it can be precisely shaped into the physical forms and effective Dk values required across the full antenna. The Dk should change as little as possible over a wide temperature range, a trait characterized by the material’s temperature coefficient of dielectric constant (TCDk).
Radix Printable Dielectrics are thermoset materials produced from resin-based composites; they are curable by ultraviolet (UV) light. Unlike traditional solid dielectric materials, they start in liquid form; structures are not created by subtracting material from a base but by adding layers through 3D printing. By processing Radix materials through FLUX Series DLP printers from Fortify, 3D dielectric structures with micrometer dimensional resolution and fine surfaces can be formed with the dimensions and tolerances needed for RF/microwave and millimeter-wave circuits and components.
With a long history in high-performance materials and characterizing them, Rogers Corp. performs extensive testing on new materials and Radix material is no different. Although starting as a fluid, once printed into a solid panel Radix material can be evaluated as any dielectric material for electronic applications, with parameters such as Dk, moisture absorption, and dissipation factor (Df). With its long history in high-performance materials, Rogers Corp. explores the behaviors of its latest materials as fully as possible, applying calibrated test instruments according to industry-standard test methods.
In solid form, the Dk through the z-axis or thickness of Radix material is 2.8 when measured at either 10 or 24 GHz. Compared to standard solid dielectric circuit materials, this is a very low-loss material, with a Df value of 0.0043 at 10 GHz and 0.0046 at 24 GHz, both measured in the z-axis of the material according to the IPC-TM-650 188.8.131.52 test method. In terms of thermal behavior, Radix material has a coefficient of thermal expansion (CTE) of 76 ppm/°C in the x-y plane of the material and 75 ppm/°C in the z-axis from -50 to +50°C. At higher temperatures, from +50 to +250°C, the CTE is 123 ppm/°C in the x-y plane and 120 ppm/°C through the z-axis of the material. The thermal conductivity is 0.3 W/(m-K) and decomposition temperature (Td) is +313°C.
Printing the Proof
How well does Radix material behave when printing fine-featured components? When it was printed into a 30-GHz Luneberg lens antenna, the dielectric material achieved about 3-dB higher gain than a similar 3D-printed GRIN Luneberg lens antenna 3D-printed with standard DLP material. The key difference between the materials was a Df of 0.0044 at 10 GHz for the low-loss Radix material compared to a Df of ~ 0.0390 for the standard DLP material. The Dk for the standard DLP material was slightly higher, about 2.9 at 10 GHz, than the 2.8 at 10 GHz of the Radix dielectric material. Measurements on both antennas were made in two positions, with 0° and 90° rotation. The measured results matched closely with the computer simulation of both antennas at both test positions, with higher gain for the Radix material-based antenna.
Since the dielectric material performed well for a millimeter-wave antenna, it made sense to fabricate a companion component, an antenna radome. Radomes provide environmental protection, such as from rain and ice, for antennas but they must be invisible to an antenna and its reliant systems. Radomes can be made in many shapes but must have the lowest loss possible for the protected antenna to perform properly.
Several millimeter-wave radome shapes were 3D printed with Radix 3D-printable RF material, including a rectangular radome and a radome with a lens effect. The radomes were evaluated with a test antenna, a microstrip-fed patch fabricated on 5-mil-thick RO3003G2™ laminate from Rogers Corp. The test antenna featured a 1.5-GHz bandwidth centered at 76.9 GHz. For comparison with the S11 magnitude data for the antenna radome simulation models, return loss measurements were made of the 3D-printed radomes, with close matches between modeled and measured responses. With optimization as needed, the responses of the different radomes could be made even more consistent across the full 1.5-GHz bandwidth of the antenna. The close match between simulated performance and the actual performance of 3D printed parts only begins to hint at the diversity of components that could be fabricated for higher frequency applications, using the combination of the Radix 3D-printable RF material and the FLUX Series 3D printers from Fortify.
For those attending “Microwave Week” and the 2023 IEEE International Microwave Symposium (IMS) in San Diego, CA, June 11-16, 2023, and wishing to learn more about 3D printing with the Radix material, the author will be making two short but informative presentations on Rogers’s materials at the show. The first, on Radix, “3D Printed RF Structures Open the Potential to Think Out of the Box,” is scheduled for Wednesday, June 14, 2023 from 4:30 to 4:45 PM. Immediately following this, the author will explain the importance of thermal management especially at millimeter-wave frequencies, in “Thermal Stability Consistency is Even More Important at Millimeter-Wave Frequencies,” scheduled for Wednesday, June 14, 2023, from 4:45 to 5:00 PM. Learn more about 3D printing as well as how to “beat the heat” when working with traditional circuit materials. And get a chance to meet the author at the RF/microwave industry’s biggest and best attended annual event.
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