Normally the word "printed" in printed electronics refers to the circuit traces on a printed circuit board, even though these are not "printed" in the conventional sense. The field of printed electronics refers to devices that are printed from conductive nanoparticle inks and the field encompasses a huge range of products. Transparent electronics, wearables, textiles, EMI shielding coatings, and RF circuits can be fabricated using printing process, including on flexible substrates. These flex designs are being built with similar fabrication processes found in advanced 3D printing systems for planar devices.
The printing technologies and materials used in the manufacturing of printed electronics are presented in this article. Aerosol or ink droplet deposition methods are being commercialized by additive electronics manufacturers to fabricate rigid PCBs in a production-level environment, but the same technology can be used to build flexible printed electronics. The new paradigm that is coming on the horizon is the convergence between these printing technologies and conventional flex PCB materials.
Why Flexible Printed Electronics
Flexible printed electronics are quite literally printed; the conductive materials are printed in an additive process that provide comparable linewidths and spacings as in subtractive etching. Formerly this was reserved for components, like touchscreens or wearables, but now flexible printed electronics can be assembled in standard processing.
CCD sensor boards like this can be fabricated with a printing process or subtractive etching.
These systems gain their advantages from two possible areas:
- Substrate material
- Conductive material
There are already many components that are produced on flexible substrates and used in automobiles; displays are an excellent example. Additional device possibilities include printed circuits on flexible transparent materials, opening a new area of electronics that eliminates the need for etched transparent conductors like ITO or FTO.
The move from rigid to flex assemblies has lagged traditional rigid PCB production due to the lack of compatible processes and material systems. As new fabrication techniques and materials become more widely available, more designers could have access to the tools required for prototyping or small-run production designs on standard flex materials, or on rigid materials. The state-of-the-art attempts to print directly while being material agnostic, with plastics and polyester both being targeted as substrates for printed circuit assemblies.
Materials Used in Flexible Printed Electronics
Inks used in flexible printing processes are suspensions of conductive materials a binder, solvent, additives, and surfactants. The main conductive material used in conductive inks are typically metal nanoparticles, which then deposit and solidify on the flexible substrate to yield a printed circuit. Silver is the most commonly used metallic functional ink used in 3D printing processes and planar printing processes on flexible substrates. To reduce costs and improve compatibility with PCB surface plating materials, there is a lot of motivation to develop copper inks.
Flexible Substrate Materials
Polyimides, polyesters, and other polymer films such as PTFE (poly-tetrafluoro-ethylene) and PEN (polyethylene naphthalate) are useful as substrate materials for flexible printed electronics and circuits. Commercially, flex PCB designs overwhelmingly use polyimide materials, or Kapton. Polyamide film is highly useful as a substrate material in certain systems where reliability is needed. For a flexible printed PCBA, it offers a few important benefits:
- Vibration resistance - Flexible materials can dissipate vibration and mechanical shock because the board is more flexible. This prevents vibration from telegraphing to solder joints on components.
- Thermal stability - The base materials in flexible printed electronics will not soften when heated or thermally cycled. After thermosetting, these materials remain flexible.
- Dynamic folding - Because newer inks coalesce into more uniform conductive films, the resulting conductors are more elastic and the printed flex PCBA can be made to fold dynamically, just like subtractively formed flex PCBs.
Some of the properties that make polyimide an excellent choice include resistance to chemicals, high tensile strength, excellent electrical properties, and stability over a wide range of temperatures. Flexible PCBs and flexible printed electronics usually share common materials that are used as substrates. When a flexible device needs to be transparent at visible wavelengths, the material of choice would be transparent PEN. These systems were previously being used with etch FTO as conductors, but now more advanced functional inks with carbon nanotubes are providing an alternative flexible transparent conductor material.
Transparent flexible printed PCB.
Flexible Printed Electronics Processing
These processes make a direct impact with the substrate material through use of an image carrier medium, typically a printing plate. This medium transfers an image of the printed circuit elements to the substrate as a wet ink. These printing processes include screen printing, gravure printing, and flexography.
Screen printing - This simple process can produce low-cost PCBs for electronic devices with little material waste, even on flexible substrates. As long as features are not too fine, the screen printing process will have high yield. Note that screen printing requires highly viscous inks with significant concentration of solid functional elements.
Gravure printing - This process uses low-viscosity inks to print a circuit image on a substrate using an engraved copper-coated steel gravure cylinder. To transfer the image, the cylinder is submerged in an ink reservoir until it coated with ink, and a doctor blade is used to remove any extra ink from the gravure cylinder to ensure accurate image transfer to the substrate material at high pressure.
Flexography - Flexography is a roll-to-roll indirect impact printing technology with high throughput. It is capable of providing a variety of ink thicknesses for a given resolution. Flexography-compatible inks are not readily available and this is not a commonly used technique in flexible printed electronics.
These direct deposition methods are used in 3D printing with dielectric and conductive inks, so they are well-suited for use in flexible substrate materials. These printing processes do not require an image carrier medium and instead deposit printed elements directly to the substrate. The two most prominent non-impact printing techniques are aerosol jetting and inkjet printing.
Aerosol jetting - This process uses compressed air to form an aerosol ink suspension in an atomizer. The aerosol droplets are injected onto the substrate surface at low temperature. The process can be scaled to a production environment, including in 3D printed systems or on curved surfaces.
Inkjet printing - This process is similar to aerosol jetting, but an atomizer is not used. Instead, ink droplets are deposited onto the substrate from a low-viscosity ink suspension. Inkjet printing is currently being used with organic semiconductor inks, metal nanoparticle inks, and carbon nanotube inks.
The entire process for additively manufactured electronics is still being developed, with some notable companies focusing on building up the entire board from scratch with the advanced printing processes listed above. A further goal is to expand these systems such that they integrate with some reflow or selective soldering equipment. This would give a comprehensive process for developing printed electronics in a smart factory environment, where there is only major human input at the front and back end of the production process.
At the moment, inkjet printing is the major process being used for these purposes on both rigid and flexible substrates. The process is best suited for high-mix, low-volume production where something like an A-SAP/mSAP process would have low yield or throughput. These processes are also suitable for non-standard flexible materials like PET films. Building assemblies on PET films is a natural extension of flexible electronics, enabling products like transparent flexible PCB assemblies.
What about materials enabling flexible semiconductor devices? This is possible and has been researched in the past in academic settings. Some of these printable materials include:
- Semiconducting polymers that can be deposited directly as thin films (inkjet)
- Semiconductor nanoparticle suspensions or solutions (jetting)
While these types of systems will probably not be mainstream, they give designers a totally new way to approach complex flexible products. Expanding the available material sets for printable electronics gives new opportunities to build unique functionalities into flexible devices that are not available in integrated circuits.
Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.