Flex PCB 3D Printing Processes and Materials

Printed electronics encompasses a huge range of products, some of which we don’t even notice. Components like sensors, displays, RFID tags, and even textiles are just a few categories of innovative devices that are fabricated on flexible materials and illustrate the design potential of integration into standard flex PCB materials. Now flex designers are taking 3D printing, an unrelated fabrication process and using it to build more intricate types of components.

3D printing uses an electrically functional ink to create electronic elements like capacitors, resistors, and even transistors on a flexible substrate. Aerosol or ink droplet deposition methods are being commercialized by 3D printer manufacturers to fabricate rigid PCBs in a production-level environment, but the same technology can be used to build flexible printed electronics. The printing technologies and materials used in the manufacturing of printed electronics are presented in this article.

Why Flexible Printed Electronics?

The printed electronics market is growing rapidly and market share is expected to increase by approximately $20 billion from 2020 to 2025. Flexible printed electronics have opened doors to countless new possibilities in a wide range of industrial applications including medical, automotive, aerospace, textile, industry 4.0, IoT and so on. Now as new fabrication techniques become more widely available, designers could have access to these tools for prototyping or small-run production purposes.

First, we’ll cover the broad class of materials used in 3D printed electronics. Materials for flexible printed electronics are categorized into two areas: insulating substrates and conductive inks.

Substrates

The substrate material determines the performance and desired properties of flexible printed electronics and PCBs. In addition to the obvious need for flexibility, the substrate must be capable of holding a conductive layer of material and act as an insulator. Polyimide is one common material that is already used in flex PCBs, but it can also be used as the substrate for 3D printed flex PCBs. The polyamide film does not soften when heated, yet it remains flexible after thermosetting.

In addition to polyimide, other materials that can be used as substrates for flexible printed electronics include:

• Polyesters
• PTFE (poly-tetrafluoro-ethylene)
• PEN (polyethylene naphthalate)
• Other polymers that can be dissolved in solvents, including semiconducting polymers

Printing Inks

The inks used in the printing process are typically composed of binders, solvents, additives, and functional elements. Silver is the most commonly used metallic functional material in printing inks, while semiconducting inks may include zinc oxide and titanium oxide. The inks used in 3D printers for electronic devices are specialized and are not universally compatible with every printer. Instead, the viscosity and aerosol jetting properties of these materials must be carefully engineered to ensure consistent deposition and device yield.

Flexible Electronics Printing Processes

3D printing processes must be tailored to specific materials and printing machines. There are two broad classes of 3D printing processes used in electronics fabrication: impact and non-impact printing.

Impact Printing

As the name suggests, impact printing requires a printing plate (or more generally, an “image carrier medium” as noted in the literature) that transfers the image to the substrate. Some examples of impact printing are:

Screen printing - A simple form of 3D printing, screen printers have a rubber squilgee, stencil, and a screen of the design template. The screen is made up of either plastic fibres, metal fibres or natural silk material. Screen printing can be used to print low-cost PCBs for electronic devices with little or no material waste, but only in standard 2D planar formats. The screen printing process requires highly viscous ink that has a significant concentration of solid functional materials.

Flexography - This is a roll-to-roll rotational indirect impact printing method with high throughput, essentially being a scaled version of screen printing. It is capable of providing a variety of ink thicknesses for the same resolution. However, flexography-compatible inks are not readily available and this is not a commonly used technique in printing flexible electronics at the industrial level. This technique is mostly in the R&D stage.

Gravure printing - Gravure printing produces high-quality prints using low-viscosity inks. The circuit pattern/image is engraved on a copper-coated steel gravure cylinder. This cylinder is submerged in an ink reservoir until it is properly coated with ink. A steel blade, known as the ‘doctor’ blade, removes any extra ink from the gravure cylinder to ensure accurate image transfer to the substrate material. The gravure cylinder transfers the ink onto the substrate at high pressure.

In all of the above, an off-the-shelf polyimide can be used without printing, or another substrate material could be printed directly from an appropriate 3D printer.

Non-impact Printing

These methods do not require a printing plate or an image carrier, and only the deposition material comes into contact with the substrate. This reduces the risk of contamination and damage to the substrate, and the pattern alignment is more precise. For flexible printed electronics, there are two dominant printing processes: ink-jet and aerosol jet printing.

Ink-jet printing - This technology does not use any physical image carriers to print the design. Instead, it uses a moving print head that sprays ink droplets onto the substrate. It uses a low-viscosity liquid ink. Ink-jet printing is best suited to applications that require organic semiconductor inks or nanoparticle inks. It is a slow process and throughput is very low, but when used with a dielectric ink for the substrate, an entire product can be printed with just about any structure.

Example inkjet 3D printer depositing active materials on a substrate.

Aerosol jet printing - Ink is aerosolized in an atomizer and then it is deposited onto the substrate via a controlled jet stream. It is a low-temperature process and can handle a wide range of materials and substrates. The process can also be scaled up to meet large-scale production requirements. Three-dimensional printing is also possible with this technique as the particle stream can be controlled and focused in the z-axis direction.

More on Additive Processes for PCBs

Additive processing in flexible printed electronics has been advancing significantly, and 3D printing is just one of the technologies used to build additively manufactured electronics. Other chemical processes, such as semi-additive processing, are used to fabricate rigid and flex PCBs with narrow line widths and high line densities. These processing options are currently available from some of the most advanced manufacturers, but they will eventually filter down to traditional fabrication providers and will be widely available to more designers.

• Tan, H. W., et al. "Metallic nanoparticle inks for 3D printing of electronics." Advanced Electronic Materials 5, no. 5 (2019): 1800831. [Link]
• Wiklund, J., et al. "A review on printed electronics: Fabrication methods, inks, substrates, applications and environmental impacts." Journal of Manufacturing and Materials Processing 5, no. 3 (2021): 89. [Link]

When you’re ready to start building flexible printed electronics, make sure you use the complete set of design features in OrCAD from Cadence to specify your design requirements and create your PCB layout. OrCAD includes the industry’s best PCB design and analysis software with flex PCB design tools. OrCAD 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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