Editor’s Note: As additive manufacturing techniques evolve, these processes offer significant advantages and benefits to the electronics industry. Microwave Journal investigates some of the aspects of this emerging area with a two-part article. This first part introduces the dielectric measurement concepts and some of the challenges. It also includes a portion of a panel discussion where RF dielectric measurement experts from the industry examine some of the most important topics in that area. The second part of this feature will continue the discussion with the panel of experts.
Recent advances in low dielectric constant (Dk) and low loss (Df) resins capable of being used in additive manufacturing (AM) applications have opened the door to creating dielectric lattice structures with tunable dielectric parameters. The ratio of the volume of air to the volume of dielectric lattice can be used to design complex dielectric properties. This technique can be an enabling factor in the realization of metamaterials, complex dielectric lattice structures and artificial dielectrics with performance that, historically, has not been viable with traditionally-fabricated dielectric materials. Increasing the design degrees of freedom brings the challenge of accurately determining the dielectric properties of the material early in the calculation, simulation and measurement process.
This article discusses this challenge and presents a variety of methods used to test dielectric performance, with many only suited to a narrow range of applications. The article also summarizes excerpts from interviews with some of the leading experts in the field. Their insights help frame the applications and benefits of these materials, along with some of the manufacturing challenges.
There have been extensive studies into dielectric materials and methods of characterization, measurement and prediction/simulation, with additional studies of more complex materials over the past 40 to 50 years.1-10 Methods for measuring simple materials, those with non-magnetic, linear, homogeneous and isotropic dielectric properties, are very well established. The emergence of new methods of dielectric fabrication over the past several years has warranted the exploration of new methods for characterizing, measuring and predicting the behavior of more complex dielectric structures. AM adds a new level of complexity to the design and testing of complex dielectrics since this process can also change the properties of the material. This means that the 2D and 3D structures may exhibit dielectric behavior that cannot readily be predicted or modeled using as-labeled dielectric material properties.
Considerations for Dielectric Structure/Media Testing
This article explores the concepts of measuring and modeling AM dielectric structures. There are a variety of methods for measuring dielectrics. The suitability of a given measurement method is determined by the properties, structure and state of the dielectric material, as well as the testing goals of the dielectric material. With modern, complex AM dielectric structures, there are additional considerations that compound the process of determining the most appropriate test method.
Historically, the type of dielectric material was categorized as either bulk or thin. This described the structure of the dielectric material as either a bulk dielectric that could be measured as a single monolithic structure or a thin dielectric sheet or coating, which requires a different set of testing considerations. In addition to these two categories, a lattice structure or complex dielectric structure must be added. These structures create additional measurement considerations beyond simple bulk dielectric material testing.
Other measurement considerations include the frequency range, the measurement accuracy and the permittivity/permeability range. The intended use of a dielectric generally determines the frequency range. However, in the case of complex dielectric structures, the structure may dictate the operational frequency range and this may deviate from the typical frequency range for the dielectric materials. Measurement accuracy relies on using the best test method, but it may also establish boundaries for the properties and geometry of a given sample. This may be a limiting factor for complex dielectric structures as these structures often have frequency-dependent design features that require certain geometries for optimal performance. Lastly, the permittivity/permeability range of a complex dielectric structure may be difficult to assess initially. Using a dielectric measurement method that is not well-suited to the actual permittivity/permeability range of the dielectric may result in unforeseen errors that are difficult to detect.
Brief on Linearity, Isotropy and Homogeneity of Dielectric Structures/Media
A material that is linear, isotropic and homogeneous will present the same response to a stimulating field regardless of the applied field strength, orientation or how the material’s constitutive parameters are positioned. However, a material may be nonlinear, anisotropic and/or inhomogeneous. These terms mean that the response of a material to a stimulus may depend on the field strength or the orientation of the stimulating field and it may have different profiles based on the constitutive parameter configurations. These factors must be taken into consideration when characterizing, designing or measuring dielectric media or structures. This is because small variations in the material makeup, manufacturing process or assembly can change the material properties, which may then impact the dielectric material features.
To some degree, all materials present some non-idealities that contribute to the overall uncertainty of a given measurement. It can often be difficult to determine the extent of this uncertainty and the appropriateness of various measurement techniques that may be sensitive to these non-idealities. In some cases, it may be appropriate to ignore these non-idealities to some degree and simply use a measurement method that best represents the real-world use of a dielectric structure/media. In other cases, it may be necessary to characterize the dielectric structure or media as completely as possible for the sake of modeling and simulation accuracy.
Common Methods of Dielectric Constant and Loss Tangent Measurement
A wide variety of dielectric measurement methods have been developed to account for the diversity of dielectric materials. The advent of AM and more complex dielectric structures has driven the need to develop measurement processes that best account for capturing the nuances of lattice structures. Due to the relative size and dimensions of lattice structures at various frequencies, there are only a few measurement methods that can be functionally used to make dielectric measurements on dielectric lattices.
Common Dielectric Measurement Methods
- Impedance analyzers/LCR meters
- Parallel plate capacitor or three terminal method (ASTM D150)
- Open-ended coaxial probe
- Dielectric loaded waveguide (filled waveguide)
- Coaxial transmission line (filled transmission line)
- Planar transmission lines
- Focused microwave or mmWave beam (free space)
- Resonant cavity or resonant surface structure
- Split cylinder resonator
- Split post dielectric resonator
- Fabry-Perot open resonator
- Cavity perturbation (ASTM D2520)
- Inductance measurement method.
Most of the common dielectric test methods are not suitable for measuring lattice structures because many of these structures are likely to be based on unit cells. This means that a number of these cells must be arranged to faithfully represent the lattice structure and replicate the desired dielectric performance. In addition, these structures likely will not exhibit the same dielectric behavior if they are improperly arranged into a flat disc or another sample shape that does not recreate the behavior of the dielectric structure. Given that the “bulk” performance of the dielectric lattice is likely what is being tested, this factor alone rules out most dielectric test methods. A unique case of this guideline can occur if the dielectric lattice structure is used as a planar substrate. In this case, the dielectric lattice is likely designed specifically for that purpose and testing using planar transmission line, parallel plate capacitor, split post dielectric resonator, Fabry-Perot open resonator or other methods that can leverage a planar/laminar sample may be appropriate.