Printed Electronics Enable New RF Designs

The printed electronics devices were first fabricated over 100 years ago with stamping of copper foils on paraffin paper. Fast forward to today, and printed electronics is a budding industry with many commercial opportunities. Some standard printing processes like screen printing and gravure printing are scalable up to high volume with resolutions that are competitive with standard etching processes. This has helped bring printed electronics into the mainstream.

Now printed electronics is expanding into new areas with production equipment resembling typical 3D printing processes. Some of these areas involve lower volume, higher mix, and non-standard device fabrication, as well as rapid prototyping with an intent to scale to high volume production.

In this article, we’ll give readers a short overview of some design ideas for printed RF electronics. With materials advances and lower cost printing processes, innovators can build compact RF systems on unique substrates, flexible substrates, and even curved substrates. Let’s jump in and look at some innovative RF designs that can be fabricated with additive printing processes.

Three Ideas for Printed RF Electronics

Printed electronics has seen significant development over the past several years, with much of the innovation focusing on decreasing the costs involved in rapid prototyping and iterative innovation. Printed electronics refers to additive manufacturing of electronic devices, but not necessarily with 3D printing processes. Instead, this involves deposition and curing of conductors in a circuit, followed by standard assembly.

To date, the biggest application of RF printed electronics has been in smartphones, where it has been used for fabricating the primary antenna and the GPS antenna. These are currently fabricated on standard rigid and flexible substrate materials. New design opportunities can be found beyond the standard material sets, with current opportunities in biodegradable, non-planar, and transparent electronics.

Biodegradable Electronics

Recycling of consumer electronics that use RF devices is a persistent concern as the materials used in circuit boards and semiconductors do not easily degrade back into the environment. Biodegradable materials used in PCBAs allows these products to become more sustainable, either by allowing throwaway products to degrade quickly or by enabling a recycling process for retrieving raw materials from expended devices.

Biodegradable printed circuits demonstrated by researchers at Stanford. [Source]

The advantage of using a printing process is in deposition of a biodegradable conductor for RF circuits or a biodegradable active RF material, specifically conductive polymers or semiconducting polymers. Another opportunity is deposition of copper RF circuits and nonlinear RF circuits on biodegradable substrates, where the recycling process involves stripping and retrieval of remnant materials from the PCB. The materials used in biodegradable electronics also offer design opportunities in medical or implantable devices, which could include an RF section for data transmission.

Non-Planar Electronics

Look at any printed circuit board, and any integrated circuit: everything is manufactured with purely planar processes. Even flexible circuit boards are manufactured using planar processing. The standard fabrication processes, particularly photoresist exposure, demand planar substrates to ensure high quality exposure and etching. Fabrication of non-planar electronics with printed elements demands an additive process that can directly deposit conductors on curved or irregular surfaces.

Among the fabrication methods used for non-planar electronics, several examples have been published:

• Direct deposition on a curved substrate from solution
• Additive deposition, such as aerosol jetting or inkjet
• Deposition on a film, followed by coating onto the substrate
• Deposition through a micromachined or printed mask

In the RF domain, this enables fully integrated RF systems on curved surfaces, including antennas, filters, couplers, and power dividers.

Patch antenna fabricated on rigid conical PTFE-based substrate, produced by Rogers and Averatek.

Non-planar electronics on opaque surfaces is already interesting as these substrates can be fabricated on ceramics, PTFE-based materials, and other rigid or flexible materials. There is another class of materials where RF printed circuits can be fabricated: transparent materials.

Transparent RF Electronics

Some commercially available printing systems can now deposit transparent conductive films directly onto transparent surfaces, including flexible or curved surfaces. This opens another opportunity for fabrication in transparent electronics, where conductors are deposited on a film and the film is coated onto the target substrate, such as a window or a polyethylene (PET) film.

In the past, transparent conductors would be deposited directly from doped tin oxide films, but the resistance of these transparent conductors is too high for use in RF designs. Inkjet printing can be used to fabricate RF circuits from thin silver mesh films and other conductive metal oxides or polymers. These films can provide lower resistance than traditional transparent conductors like doped tin oxides, which is sorely needed in RF circuits.

Printed Electronics Design Rules

The design rules in printed electronics are very similar to the rules that must be implemented in standard electronics. The principle production constraint that dominates printed electronics production is linewidth/spacing values. With inkjet printing, sub-mm feature sizes are possible, with some manufacturers offering 50 micron (2 mil) feature sizes. Before jumping into printed electronics, make sure you understand the limits of your fabrication process, both during prototyping and when scaling to production.

Engineers building advanced electronic systems can design for 3D printing processes with the complete set of design tools in Allegro PCB Designer from Cadence. Allegro is the industry’s best PCB design and analysis software, offering a range of product design features with a complete set of management and version control capabilities. Allegro users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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