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3D printing has been around for many years and, as time has progressed, so has this technology. However, one aspect of 3D printing seems to have not been developed as much until recently. That aspect is related to the materials which are used for 3D printing and, specifically, materials which are compatible with RF or high-speed digital (HSD) applications. There are many materials that can be used for 3D printing, but until recently, none of these had properties which were conducive to printing circuits with good RF or HSD performance. Rogers recently released the first 3D printable material for digital light processing (DLP) and stereolithography (SLA) 3D printing, which has good properties for RF and HSD circuit considerations.
There are several different types of 3D printing technology and depending on the technology, there are different types of materials which can be used for printing. We teamed up with Fortify, a leading DLP printing platform for filled and reinforced photopolymer materials, to enable the manufacture of the new, high-performance material. For photopolymer 3D printing processes like DLP and SLA printing, the materials that have been historically used have a high dissipation factor (Df), a dielectric constant (Dk) that is not well controlled, and some of these materials have a high moisture absorption property. Table 1 has information that can be used for comparison between the traditional 3D printed materials and these new high performance 3D printable materials.
As shown in Table 1, most of these materials have very similar Dk values when tested at 10 GHz. What is not obvious is how consistent these Dk values are for potential Dk variation within a small area or Dk variations within a batch or variations within batch-to-batch. Rogers formulated low loss, high-performance circuit materials over the past five decades and now extends this expertise for consistent RF performance in DLP and SLA 3D printing processes.
No matter how well the electrical properties are controlled, if the moisture absorption property is not low, the RF benefits of controlled Dk and Df can be negated. If the moisture absorption for a circuit material is high, that will allow the circuit to absorb moisture from the atmosphere as related to humidity. When there are significant humidity changes in the atmosphere, that can also cause changes in the amount of absorbed moisture in the circuit.
The absorbed moisture, water vapor, has become embedded in the circuit material and will increase the Dk and the Df of the material. The new 3D printable dielectric material has an extremely low moisture absorption rate at 0.08% by volume. Generally, good moisture absorption for a high-frequency circuit material is 0.3% by volume or less and moisture absorption below 0.1% considered excellent.
Having a 3D-printable material with good high frequency or HSD properties will allow the engineer to make and evaluate prototype structures much faster and cheaper than before. This could certainly open tremendous possibilities to evaluate structures that are 3D and were previously very difficult to make with PCB technology. Additionally, 3D printed technology with good RF properties can be used to make planar circuits which are similar to PCBs but also combine 2D circuit features with 3D areas, such as cavities, as an example.
We have investigated many different RF structures that can be achieved with using the RF printable 3D technology and one very interesting example is a Luneburg lens. The Luneburg lens is often used as an antenna lens and it uses different layers of slightly different Dk. This can be done easily with the 3D-printable RF material technology, where specific layers are printed to have a specific density lattice structure, while other layers have a higher or lower density than the previous layer. The difference in density will cause a difference in the effective Dk for that layer. An example of this technology is shown in Figure 1.
Historically, prototype circuitry that are built to prove-out concepts can be expensive for time and resources. Having the ability to 3D print an RF structure, or multiples of the same RF structure with slight differences, has the potential to significantly speed the development of RF and HSD projects.
This column originally appeared in the December 2021 issue of Design007 Magazine.