3D-MID-LDS

When developing RF devices field simulation software is typically used to optimize the RF characteristics. Since the exactness of this modeling increases the accuracy of the simulated results the efficiency of the development process can be significantly influenced by knowledge of these RF properties. This is the reason why one aspect in our research activity is the characterization of RF properties.

Currently our main activities are in field of Laser Direct Structuring (LDS), a fabrication method for MIDs that is already common for large scale productions of antennas in consumer devices such as smartphones, tablets and laptops. Injection molded plastic parts combined with laser activated surfaces that allow a selective metallization opens up almost unlimited design scope. This includes the 3D design scope in the development process itself, as well as the adaptability of a design during an ongoing manufacturing process. The typical LDS process is based on an injection molded thermoplastic substrate material and the subsequent metallization of the laser structured surfaces. 

The evaluation of the RF properties is carried out for the substrate materials as well as the applied metallization. Different available LDS substrate polymers are evaluated regarding their dielectric properties in the frequency range from 100 MHz up to 80 GHz. Due to the fact that these LDS thermoplastics are in some cases filled with glass or carbon fibers that results in an anisotropic behavior the characterization is done according to the direction of the applied electrical field. 

Secondly, the RF conductor characteristics for different LDS metallization compounds are evaluated based on measurement and simulation. This also includes a detailed mechanical evaluation different metallized surfaces to characterize the surface structure and its influences on the RF characteristics.

At the institute an FDTD (Finite Difference Time Domain) based simulation software is used. The FDTD uses cartesian discretization for numerical solving of Maxwell’s equations. For fine layered 3D modelling of curvy structures this cartesian mesh might not be the most efficient variant of discretization, but this inefficiency is equalized by a very efficient algorithm of the software in solving the equations, so that using the well-known software is the method of choice. One focus of research is to improve CAD usability and simulation performance of the existing software for 3D-LDS-MID structures without changing discretization type, which is a project done together with der software developing company.

Main aspects of this project are the improvement in CAD of conformal line and antenna structures, evaluation of effects of conformal projection on RF properties of typical used RF lines and antennas, evaluation of the influence of LDS-MID-specific material (e.g. dielectric anisotropy) and process (e.g. roughness) parameters on RF performance and efficient modelling of these effects and evaluation of and decreasing the influence of cartesian meshing of these structures on simulated RF performance.

Using the design scope of 3D manufacturing methods seems to be especially interesting in the field of antenna design. The 3D shaping can be used to modulate surfaces and reduce geometric dimensions with remaining electrical length. Doing so is not new but especially for large scale productions the possibilities for an antenna design is often limited by the available fabrication methods. As an example, the widely used circuit boards reduces the antenna design scope to only two dimensions. The continuous reduction of installation spaces requires somehow other, more flexible solutions. Functionalizing plastic parts that are in most of the cases used anyway as mechanical part or as housing seems to be an attractive solution to integrate antennas. Both aspects the volume efficiency by integrating antennas on given plastic surfaces and the intended antenna optimization by shaping a plastic surface as needed for optimal antenna performance are also an interesting topic in the field of antenna development. The Institute of Microwave and Wireless Systems works together with partners in automotive and mobile communication sector developing new antenna solutions that meet current and future requirements.

The research activities include antenna concepts for automotive RF systems that communicate on the basis of different standards like mobile communication (LTE, GSM), WiFi, car-to-car communication (ETIS ITS-G5) and satellite navigation (e.g. GPS). These antenna concepts are verified with a various number of LDS fabricated prototype antennas. 

Another research field are the development of antenna concepts using possibilities of 3D MID for future RF systems operating in millimeter wave range, like it is indicated for 5G mobile communication or already used for broadband data transfer with IEEE 802.11ad Standard (WiGig) at 60 GHz. The antenna concept consists of the approach to use the given plastic part to integrate waveguide fed antenna elements directly into it. This seems advantageous due to the fact that antennas can be designed that directly radiate out of a small part of the plastic housing reducing reflections on material boundaries.

A RF system typically consists of a radiating antenna element and a data processing circuit. The possibilities of 3D MID can not only be used to design antennas but also to integrate the circuitry needed directly on or into a plastic part. Our activities in this includes a high miniaturized DC controlling circuitry in context of a 24 GHz RFID sensor transponder directly integrable in metallic parts developed as a part of the Collaborative Research Center 653 (CRC). Besides the integration of DC circuitry especially for future RF systems in millimeter wave range the integration of RF transceiver circuitry directly on the antenna plastic part seems to be a promising solution. Doing so reduces the lengths of the transmission lines and allows to build transitions between antenna and circuitry without using possibly expensive connectors.

A promising field for mmWave LDS-MIDs is the upcoming 5G mobile communication. Especially the wireless backhaul will be based on mmWave communication links, but also handheld devices will use mmWave communication to get higher bandwidth and data rate. To overcome high pathloss and high dense and so high interfering small cell communications complex antenna systems consisting of mmWave ICs, line and filter structures and antenna arrays for adaptive beamforming will be implemented. The low space integration scenarios (handhelds, small cell base stations) require 3D manufacturing. Besides, there is a need to build conformal antenna arrays. Both criteria can be handled using LDS fabrication process. The actual research at the institute focuses on testing LDS materials and process on mmWave capability, but also demonstrator concepts are developed and will be manufactured together with our project partners. Fig. 1 shows a concept for a 28 GHz beamforming system integrated into a smartphone using a beamforming IC on an LDS-MID as core component. The MID contains the beamforming IC and microstrip lines for coupling of the four slot antennas in the smartphone chassis, but can contain as well power and signal processing circuitry and thermal management blocks.