Demonstration and Charaterization of Fully3D-printed RF Structures

Chiara Mariotti∗, Manos M. Tentzeris†, Luca Roselli∗,∗Department of Engineering, University of Perugia, Perugia, Italy

Email: chiara.mariotti@studenti.unipg.it†School of Electrical and Computer Engineering (ECE), Georgia Institute of Technology, Atlanta, Georgia, USA

Abstract—In this paper an approach to fully 3D print Radio-Frequency (RF) components and devices, with commercial, low-cost platforms, is presented. The manufacturing methodology andmaterials are described, while ring resonators and microstriplines are used to characterize the materials up to 8 GHz. Thesubstrate used is Acrylonitrile Butadiene Styrene (ABS), whichhas been demonstrated to have a permittivity of 2.8 and losses of0.02, with the printing settings adopted in this work. The metalsare made with a low cost copper (Cu) paint, commonly employedin EM shielding; its conductivity is demonstrated to be 1e5 S/m.Results related to a fully 3D printed patch antenna working at10 GHz, are shown.

I. INTRODUCTION

Additive manufacturing is being recently investigated asa candidate for rapid prototyping of components and devicesin several areas of application. In particular, 3D printing hasattracted a huge interest due to its high customization, mechan-ical flexibility and materials variety [1]. For these reasons, themarket of 3D printing is dramatically growing in fields likebiomedical, aerospace, automotive, rapid lab prototyping andso forth.

In the Radio Frquency (RF) field and in electronics ingeneral, examples of 3D printed devices are microfluidicchannels for sensing/tunable wireless systems [2]–[7].

In the reported cases, only the plastic part of the device isactually extruded with the printer, while metals are depositedwith other methods. 3D printing of conductive traces repre-sents, so far, the main challenge for the next generation offully 3D printed electronic things, and a lot of effort is putby scientists in order solve this issue and be able to fabricateelectronic Front Ends (FE) and the hosting object all at once.

This will be a true manufacturing revolution with a hugeimpact on the new market of the Internet of Everything(IoE) [8], especially when an industrial process with massproduction and 3D printing features will be realized.

This work is a first proof-of-concept that intends to demon-strate that is possible to printing metals with a commercial low-cost 3D printer; in addition a characterization of the employedmaterials is provided along with an example of a fully 3Dprinted patch antenna.

II. TECHNOLOGY AND MATERIALS

The platform used for the prototyping is the low cost,compact, Hyrel 3D, provided by a company in Norcross,GA, USA [9]. This machine is based on the Fuse Deposition

Modeling (FDM) technology and allows to use various heads,based on the material viscosity and thickness. Two heads areused in the experiments: the MK1-250 (1.75mm filament at upto 250 ◦C) for plastics and the EMO-25, for metals.

The substrate and insulation layers are made with theAcrylonitrile Butadiene Styrene (ABS) [10], which is the mostcommon plastic in 3D printing. The metals are made of a thickaqueous solution of copper paint (i.e. CuPro-Cote), usuallyadopted in Electro-Magnetic (EM) shielding [11]. The Cu paintis cured at low temperature (i.e. 60 ◦C) in one hour, thusbeing suitable for deposition on plastics that usually exhibitlow fusion/deformation temperature, like ABS.

The software provided with the Hyrel is custom made bythe company that produces the printer and, as for now, it doesnot allow to simultaneously print with more than one head. Asa consequence, at the moment, dielectrics and metals can onlybe printed in separate steps.

In Fig.1(a) the printing platform is shown while in Fig. 1(b)a detail of the EMO-25 head in action during metal extrusion isdepicted. From the picture it can be seen that the EMO extruderhas been customized to be used with an internal syringewhere the copper paint is pumped before starting process. Thisvariation with respect to the head originally provided by thecompany, allows to improve the metal extrusion resolution byadopting 0.3mm diameter needles.

(a) (b)

Fig. 1. Hyrel 3D is the commercial, low-cost, 3D printing platform adoptedto fabricate the prototypes presented in this work.

III. MATERIALS CHARACTERIZATION

Rapid prototyping is revolutionizing the way of thinkingand designing RF devices, also introducing the possibility of

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fabricating circuits on non-conventional materials for electron-ics. Such an approach is motivated by the fact the electronicsis going to be more and more integrated with hosting ob-jects, thus producing electronic things with both their originalfunctions and some added electronic capabilities (for instance:sensing, energy harvesting, data transmission and so on).

It is in this scenario, usually referred to as Internetof Things (IoT) that materials like paper, plastics, wood,cork, and so forth, are being proposed for the first time asPCBs substrates thus making their electrical characterizationneeded [12]–[14]. This aspect has to be considered whenfabricating devices with 3D printers as well.

With 3D printing, the electrical properties of the adoptedmaterials are also dependent on the printing settings: slicingmethod, pattern density and resolution. As a consequence, itis hard to consider an unique and constant value for εr andtanδ of the substrates’ plastic and it is preferable to test themconsidering the specific brand, printer and process settings.

In order to characterized the ABS used in this work, testsbased on ring resonator and microstrip lines have been takenand a 3D full-wave model in CST has been used to fit themeasurements up to 8GHz.

The ring resonator geometry and fabricated prototype areshown in Fig. 2, where the line width is 1.8mm and the ringdiameter is 1 cm. The thickness of the printed substrate is setto 1.5mm. Note that for testing structures we create ABSwalls that follow the patterns in order to realize a path forthe Cu paint deposition. In this case, metals are manuallydeposited. Figure 3 shows the fitting of the measured S-

(a)

(b)

Fig. 2. Ring resonator design (a) and fabricated prototype (b), adopted forthe electrical characterization of the substrate up to 8GHz.

parameters with the simulated model. A very good agreementis obtained with ABS permittivity of 2.8 and losses of 0.02.Metal conductivity is 1e5S/m, that is lower than what we canobtain with other additive fabrication processes (i.e. adhesivelaminate or inkjet printing [13], [15]–[18]), but still very highconsidering that this is an early stage research work on fully3D printed RF components. Once the 3D model of the ringresonator has been extracted it has been used to compare

Fig. 3. Comparison between simulations and the experiments for the ringresonator S-parameters.

experiments and simulations of the microstrip line shown inFig. 4, where the thickness of the substrate and the printingsettings were the same of the ring resonator. Microstrip linesof 25mm and 35mm have been drawn, fabricated and tested.The S-parameters reported in Fig. 5 demonstrate a quite goodagreement of simulations and tests, especially considering thatthis is a laboratory level process.

Fig. 4. Microstrip lines design (a) and fabricated prototype (b), adopted forthe electrical characterization of the substrate up to 8GHz.

A summary of the materials properties extracted with thischaracterization is shown in Tab. I.

(a)

(b)

Fig. 5. Comparison between the simulations and the experiments for themicrostrip lines S-parameters, up to 8GHz.

IV. FULLY 3D PRINTED PATCH ANTENNA ON ABS

In this section the first, fully 3D printed example of RFcomponent, fabricated with the commercial, low-cost platformprovided by Hyrel, is described. The component is a microstrip

TABLE I. MATERIALS PROPERTIES SUMMARY.

Material εr tanδ σ (S/m) thickness

ABS 2.8 0.02 – 1.6 mm

Cu Paint – – 1e5 0.3 mm

patch antenna resonating at 10GHz. Such an antenna is still aplanar structure, however, it has been used to set up and testthe manufacturing process and it can be considered as a firstproof-of-concept of its kind. The design and the picture of theprototype are reported in Fig. 6(a) and Fig. 6(b), respectively.

Fig. 6. Microstrip patch antenna design (a) and fabricated prototype (b).

The patch antenna has been tested with an Anritsu VNAand it exhibits a resonant frequency of 10GHz, as expectedfrom simulations. The curves of the return loss, simulated andmeasured, are shown in Fig. 7, demonstrating a very goodmatching.

Fig. 7. Comparison between the simulations and the experiments for thepatch antenna return loss.

V. CONCLUSION

In summary, this work demonstrates that fully 3D printedRF geometries can be fabricated with low-cost, commercial3D printers.

A characterization of the dielectric and metal is providedup to 8GHz. The dielectric exhibits a εr of 2.8 and a tanδof 0.02 while the conductive copper-based paint has a σ of1e5S/m.

The model extracted with ring resonators and microstriplines methods are finally confirmed by the experimental resultsobtained for the fully 3D printed microstrip patch antenna onABS, resonating at 10GHz.

ACKNOWLEDGMENT

The authors would like to thank the COST Action IC1301on Wireless Power Transfer for funding the Short Term Sci-entific Mission (STSM) of Chiara Mariotti at Georgia Instituteof Technology. Such a STSM allowed her to work on theproposed 3D printing technologies.

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