Full-3D Printed Electronics Fabrication of Radiofrequency Circuits and Passive Components

Abstract

This doctoral thesis raises the idea that 3D printing can change the paradigm of radio- frequency electronics, which has been traditionally developed mainly conceiving planar topologies. A review on additive manufacturing and the different existing technologies is reported. To focus on the concerning topic, several applications of 3D-printed electronics in the RF field are collected to elaborate the State-of-the-Art. The main objectives of this project is to develop a 3D manufacturing technology for RF electronics passive components and circuits and to generate innovative research about the possibilities of AM in this area.

Once the context is exposed, the manufacturing process for 3D-printed electronics developed within the frame of this project is described and characterized. This technology consists of three different steps. First of all, the 3D model of the prototype is designed using a CAD environment with electromagnetic simulation features, hence size parameters are adjusted to fit the specifications. Hereon, the 3D polymer substrate is printed by using either stereolithography or material jetting techniques. Stereolithography is found to be a cheaper AM technology while material jetting offers a better printing resolution and softer surface endings. Finally the object is partially metallized to obtain the conductive layer of the component or circuit using an electrolytic process, such as electroless plating or electroplating. The characterization includes the electromagnetic specifications of the dielectric substrates (i.e. the dielectric constant and the loss tangent) and the quality of the metallization (i.e. the resistivity and the layer thickness). The results of the plating resitivity are found to be competitive compared to the SoA.

In order to demonstrate the possibilities of the developed technology, several devices are designed and tested. The key factor of these prototypes is that they would be very difficult, costly or impossible to manufacture using conventional technologies. As a preliminary demonstration, a hello-world circuit to turn on a LED proves that almost any kind of shape can be plated, including vias; both through hole and SMD components can be soldered and that mechanical stress such as USB plugging is resisted by the metal layer. In addition, a study on conical inductors is carried out showing the advantages of these components for broadband applications with compact devices. They offer a larger bandwidth cylindrical solenoids and are more compact than planar coils. As an application example, they are used in the manufacturing of 3D passive filters. The prototypes present agreement with simulations and the ideal response. Slight discrepancies are caused by the manufacturing tolerances. Moreover, 3D filters are also designed as one single-printed part, a new technique for 3D discrete component integration. That permits to reduce the number of components to assembly so that it does not increase with the order of the filter. These single 3D-printed prototypes present improvement in performance and compactness as well.

In addition to the lumped circuits, a whole chapter is dedicated to distributed-element devices. A study on helical-microstrip transmission lines is carried out showing an important enhancement for line segment miniaturization. Hereon, they are implemented on the design of impedance transformers, which also benefit from bandwidth broadening. Another proposed device is the hybrid branch-line coupler, which, besides the implementation of helical lines, it has been designed conceiving a capacitively loaded folded structure. This coupler gives very interesting results in compactness improvement, without significant reduction of the bandwidth. The prototypes have been compared to the conventional topology as well as to other designs found within the SoA. Finally, helical-microstrip coupled-line couplers have also been designed, fabricated and studied. They offer a good enhancement in terms of compactness though it goes in slight detriment of the coupling factor due to the manufacturing tolerances.

Aquesta tesi doctoral proposa la idea que la impressió 3D pot canviar el paradigma de l’electrònica de radiofreqüència. S’hi anomenen i expliquen les tecnologies de manufactura additiva existents. Per centrar-se en el principal tema d’interès, s’exposa un compendi d’aplicacions d’electrònica impresa en 3D en el camp de la RF amb el qual s’ha confeccionat l’estat de la qüestió.

Un cop exposat el context, el procés de manufactura per a electrònica impresa en 3D que s’ha desenvolupat en el marc d’aquest projecte és descrit i caracteritzat. Aquesta tecnologia consisteix en la impressió en 3D d’un substrat de polímer utilitzant tècniques basades, o bé en estereolitografia, o bé en material jetting. Posteriorment, el component o circuit es metal·litza parcialment mitjançant un procés electrolític ja sigui electroless plating o electroplating. La caracterització inclou les especificacions electromagnètiques del substrat dielèctric i la qualitat de metal·lització, que s’han resultat ser competitives relació amb l’estat de la qüestió.

Amb l’objectiu de demostrar les possibilitats de la tecnologia desenvolupada, s’han dissenyat i testejat dispositius electrònics de RF, concebent-los en l’espai tridimensional. El punt clau és que els dispositius dissenyats serien molt difícils, costosos o directament impossibles de fabricar usant tecnologies convencionals. A remarcar, s’ha dut a terme un estudi sobre inductors cònics, mostrant els avantatges d’aquests components per a aplicacions de banda ampla amb dispositius compactes. Aquests inductors shan fet servir per a la fabricació de filtres passius en 3D. A més, a més, s’han dissenyat filtres 3D per ser impresos en una sola part, una tècnica nova que per produir circuits 3D amb components discrets integrats.

A part dels circuits d’elements discrets, s’ha dedicat un capítol sencer als dispositius d’elements distribuïts. S’ha dut a terme un estudi sobre línies de transmissió microstrip helicoidals, les quals aporten una millora important de miniaturització dels segments de línia. Partint d’aquí, aquestes línies s’han implementat en el disseny de transformadors d’impedància, que també milloren en termes d’ample de banda, acobladors híbrids de tipus branch-line i acobladors basats en línies acoblades. Aquests dispositius han resultat tenir millores importants de compacitat respecte els dissenys convencionals fabricats en estructures planars.