Introduction to Inkjet Technology

In the past, we’ve given you all of the information you need to learn more about conductive ink, the lifeblood of all additive electronics projects. Today, we're going to talk about how to choose the right dispensing technology for your application, and the tradeoffs you'll need to consider.

There are many different printing options out there for additive electronics: inkjet, screen printing, direct-write dispensing, flexographic, gravure, and aerosol, to name the most common. It can be challenging to pick a printing technology to start an additive electronics project because each technology has its own benefits and drawbacks. You’ll need to carefully consider which technology fits your use case as picking the wrong technology at the start of a project could end up costing you.

How?

Most technologies require significant investment upfront. Choosing the wrong one means sinking money into your hardware costs twice — an investment that is only getting higher over time. The other aspect is the massively extended timelines that printing technology switches entail.

We’re here to make sure you get it right the first time!

So let’s take a closer look.

What is inkjet?

Inkjet is a term most of us have heard at one point or another because it’s the common printing technology used in most home office printers! One of the main benefits of inkjet for prototyping printed electronics devices is that it is digital.

So what? Having a digital printing technology means that whenever you need to iterate on your design, all you need to do is upload the digital design file and press “print”. The new design can be printed on the spot without additional tooling! Not only can this save you on materials, but it results in drastically speeding up iteration time.

How does inkjet technology work?

Inkjet works by ejecting tiny droplets of your conductive ink from hundreds of tiny nozzles on a print head, in quick succession, to form any pattern on a substrate.

The way in which the droplets are created varies depending on the form of inkjet being used. Although all forms of inkjet are suitable for home use, not all forms of inkjet are suitable for printed electronics!

There are two main inkjet technologies — thermal inkjet and piezo inkjet.

Types of printing technology

Thermal inkjet

Thermal inkjet (sometimes referred to as bubble jet) uses resistive heating to quickly vaporize a small amount of conductive ink. The quickly expanding, vaporized conductive ink inside the ink channel forces a small amount of conductive ink to be ejected from the nozzle. However, using conductive inks with thermal inkjet is generally avoided for one main reason: heat! With every drop that’s ejected, a small amount of conductive ink is cured inside the nozzle, clogging it in short order.

Piezo inkjet

Piezo inkjet, on the other hand, does not expose the inks to heat but instead uses piezo elements to send a shockwave through the ink to eject a droplet.

Piezo elements are a type of ceramic that will deform when exposed to a voltage potential. Piezo inkjet printers leverage this property by sending voltage waveforms that have been tuned specifically for the geometry of the fluid path, the conductive ink being used, and the size of the droplets, which typically range from 1.5pl to 30pl.

Working with droplets that are so small requires a high magnification camera and strobe light system for waveform optimization. However, waveforms can only be adjusted on industrial or laboratory inkjet systems, whereas home office inkjet printers have fixed waveforms intended for regular color ink.

What About Materials and Performance?

The conductive inks suitable for use with inkjet technology have water-like consistencies, with viscosities in a similar range (~1cps). The compatible viscosity window for inkjet technology is much narrower than other printing technologies because high-resolution prints require small droplets and small droplets are easier to make if the fluid has a low viscosity. Inkjet can typically achieve resolutions from 300-1200 DPI (~20-30 µm). Minimum printed feature size will largely depend on ink properties, droplet size, as well as the substrate being used.

The low viscosity inks consist of 60% to 95% solvent content with the remainder being metallic particles. The purpose of the solvent is not only to provide a carrier fluid allowing the conductive particles to be printed, but it also includes additives to stabilize the ink under proper storage conditions. With such a high solvent content, most of the deposited material will evaporate during the curing process leaving behind a thin layer ranging from 1um to 5um thick. Since resistance is a function of cross-sectional area, a thinner layer thickness will result in a higher resistance limiting the amount of current a printed pattern can handle. The benefit of thin patterns is that less material is used lowering the overall material cost. The combination of these two factors makes inkjet a good candidate for low-power applications where low cost is a driving factor.

Conductive inks for inkjet tend to be more susceptible to improper storage conditions than the thicker conductive inks used in other printing technologies, like direct-write and screen printing. Due to the high solvent content and because solvents are very volatile and can start to evaporate over time under improper storage conditions, large agglomerates can form which can clog nozzles. Most inkjet systems also require a filter to remove any large particles and contaminants from entering the print head.

Aside from viscosity and stability, the surface interactions between the conductive ink and the substrate material is a critical factor that can have a huge impact on the final output. Substrates used in inkjet applications are either non-porous, like PET and Polyimide, or porous like paper: both pose their own unique challenges.

For non-porous substrates: If the surface energies of the substrate and conductive ink are too dissimilar, the conductive ink will refuse to wet the surface — leading to poor resolution and adhesion to the substrate.

For porous substrates: The conductive ink may soak into the substrate and not form any conductive pathways. Luckily, most substrate manufacturers offer substrates with special coatings to help improve wetting characteristics as well as adhesion. These coatings can have a game-changing impact and they should not be overlooked!

Assembling inkjet printed boards

Attaching electrical components to printed features created with inkjet technology can be tricky. Soldering is generally unsuccessful because printed features are so thin that without a robust binder within the structure to keep the metal in place, the molten solder will very quickly dissolve the metallic particles, destroying the printed pattern in the process. Component attachment is more successful using solder alternatives such as conductive epoxy or anisotropic conductive adhesives.

How to get started with inkjet technology

It is possible to use conductive inks with a home office piezo-based inkjet printer but mileage will vary in terms of the quality of your output — and since we’ve done it, we can say with some authority, we really don’t recommend you try this.

But if you’re going to ignore that warning…

An easy way to identify the underlying technology of any inkjet printer is to examine the ink cartridge. If the nozzles are removable with the cartridge, the printer uses thermal inkjet technology. Otherwise, the printer is piezo. Thermal print heads are much cheaper than piezo print heads, so manufacturers build them into the ink cartridge and replace them each time the ink runs out to keep the nozzles as clean as possible. That being said, there are a few things to note before using a home office printer for printed electronics:

• The print head must be flushed after every use to avoid conductive inks drying out or destabilizing within the print head.
• Since nozzles cannot be easily replaced on piezo-based inkjet printers it's best to avoid clogged nozzles as much as possible.
• Avoid using the same cartridge location to dispense different conductive inks.
• When a cartridge is loaded onto the print head, it will supply ink to a subset of nozzles on the print head. If that cartridge is replaced with a new type of conductive ink, it will supply material to the same subset of nozzles. Mixing conductive inks inside the print head is a recipe for clogged nozzles.
• The printer’s droplet waveform won’t necessarily be optimal for conductive ink.
• The printer will likely be able to still eject droplets, however, those droplets may be inconsistent in size or be ejected in arbitrary directions leading to a fuzzy printed pattern.
• Home office printers do not have the ability to align a new print with existing printed patterns.
• The new pattern will be printed at an offset from the previous one. Office printer paper feed systems are only really designed to detect the leading edge of a piece of paper and do very little to ensure precise lateral alignment. If you want to build up the thickness to improve resistance or use multiple conductive inks you’re likely going to be left wanting more.
• Very thin or thick substrates could be problematic for the printer’s paper feeder system.
• Home office printers were intended to be used with paper. A very thin film could wrinkle in the inner working and jam the paper feed system. On the flip side, a thick substrate may not be able bend enough to be fed through the printer.

The limitations of a home office printer for printed electronics often push developers to quickly move on to pursue laboratory inkjet systems that address these limitations and give them access to a lot of the settings under the hood. These laboratory systems cost at least $25,000 and can cost as much as six figures — but they can be helpful in improving the transition from prototype to production for applications that require inkjet technology.

Are inkjet printed electronics scaleable?

According to IDtechEX, inkjet only made up less than 2% of all commercial additive electronics products in 2017 while screen printing made up the remaining 98%.

Why?

It’s likely the upfront costs associated with creating a production facility capable of using inkjet printing technology!

A common situation we have heard from our customers is that they pursued inkjet initially because the idea of being able to prototype in-house without delays is extremely attractive. When they invested in more sophisticated laboratory equipment, got further into development, and closer to scaling up their design, they realized that their application didn’t directly benefit from the core strengths that inkjet has to offer and that their direction was based solely on the tools available for prototyping. This realization led them to pivot the core materials and printing technology used in their design to not only improve it but make it easier to scale. However, this decision came at the expense of their precious timelines, along with the significant capital investments required to pivot their entire technology stack (and the money wasted on inkjet prototyping when the entire process needed redoing anyway). Not to mention the operating costs and the risk — what if you can’t transfer your inkjet learnings to a scalable solution like screen printing?

Conclusion

So, in summary, inkjet uses small droplets of conductive ink to create high-resolution patterns on flexible substrates such as plastics and paper. The small quantity of conductive ink involved produces thin conductive layers making inkjet ideal for low cost and low power applications. By investing in the right equipment, the digital nature of inkjet makes the transition from prototype to production easy.

Although not the most widely used printing technology for printed electronics, inkjet printing is a mature technology and is a good candidate if the application calls for it. It’s also a hotbed for innovation — research into new applications and improvements to the technology is rampant! However, since the project requirements addressed by inkjet's value are quite specific, it's important to compare and contrast the project requirements with the benefits and limitations of this printing technology and associated materials before committing to it to avoid unnecessary investments and unexpected delays.