By Ray Greenwood, Industrial Print Technology and Process Improvement Consultant
I want inkjet to succeed in the industrial/functional sector. My experience is in screen and flexo printing, but I am a “printer” and inkjet is a form of printing.
Inkjet technology is already well established in industrial printing, including graphic overlay in electronics, industrial inkjet in 3D bioprinting, micro-extrusion, and smart packaging, among others. On the hardcore functional side, we see DIW/micro-plotting/HPCaP, along with industrial and graphic inkjet.
These sectors are growing, but they represent only a fraction of the broader industrial printing market.
Increasing inkjet market share does not just require better chemistry, smaller functional particles, or better substrate treatments. There are functional particles whose scale is beyond the reach of current inkjet technology. While some of these functional materials may seem exotic, they are not uncommon. If inkjet can retool to jet particles larger than the nanoscale or sub-micron scale, it can eventually take on nearly any functional printing application
The Competition
Inkjet isn’t alone on the production floor. It competes with screen printing in all its forms — rotary, flat, and slot-die — alongside flexography, gravure, pad printing, etch-resist systems, and thermal ribbon technologies.
The photos above show some less mainstream but still very common examples.
The challenges holding inkjet back from deeper integration into functional industrial printing aren’t new. I’m simply adding some finer points that might help manufacturers find more effective ways forward.
Printed electronics and medical device printing clearly expose the challenges inkjet must overcome. Beyond simple circuitry, deposition printing is expanding into applications such as batteries, displays, sensors, and other multi-layer conductive and reactive devices.
Particle Size vs. Nozzle Size
The biggest challenge for inkjet is that the “graphic” inkjet head/nozzle industry has pursued high resolution and picolitre droplets with ever-smaller nozzles for decades.
To succeed in the functional arena, we need printhead nozzles with much larger openings. This may require a different fluid driver system than piezo (squeeze) or heated (push), or at least a larger scale to match larger particles and viscosity.
The two biggest factors at play on the functional side of the industrial market are particle sizes above nano- and sub-micron, and high particle loading. The result is leveraged by the solvents and binders being used, creating very high viscosity and altered rheology. In many electronic and electrochemical applications, we cannot reduce particle size or loading. Inkjet has to adapt to it.
Companies like Quantica and Fujifilm Dimatix have made progress with nozzle openings up to 70µm and larger, with inkjets optimized for functional applications. Fujifilm Dimatix’s models feature larger heads and apertures, proprietary internal anti-friction coatings for metallic and high-tack inks, monolithic silicon nozzle plates — which are smoother, longer-lasting, with tunable size openings.
They also feature energy dissipation nozzle technology that prevents the “ooze” or drip effect, which causes artifacts and clogs from super-thin, fast-drying, or highly viscous inks. These are the first steps, but the combined temperature range, viscosity, and volume are still not sufficient for many new applications. We need a change of scale.
Most nozzles designed for graphic or photographic printing about 10 to 15 years ago had nozzle diameters of 75µm-100µm, but they also used inks with larger particles. Graphic inkjet printers now have nozzles ranging from 20µm-24µm. Because these nozzles must be able to pass large quantities of particles in each micro-droplet to create visual density at ultra-thin depositions, the particle size for most graphic inkjet inks is nanoparticles or very close to it.
Particle Size and Solids Content
As a rule with conductive inks, the higher the metal content, the greater the conductivity and the less expensive metals (like silver) you need. However, there are some trade-offs that affect each printing platform. Generally, increasing the content of conductive solids increases viscosity.
However, this is not just from the absorbent effect of adding solids to a liquid. Electrolytically active inks, like zinc, tin, carbon, manganese, and graphene in salt form in the presence of moisture, can agglomerate or conglomerate into larger particles, and particle size and shape affect this as well.
For screen and stencil work: We see silver particle sizes in the 2µm-7µm range, give or take. Most PTF inks (e.g., flex circuits, membrane switches, etc.) have a silver solids content of 30%-50%. For more demanding work (e.g., solar, surface mount semiconductor, and some medical device sensors), we might have solids content upward of 80%. This changes the rheology from a shear-thinning semi-liquid ink (non-Newtonian) to an almost non-shearable paste (Newtonian).
Most of these inks use flake-shaped particles, but some are granular and have particle sizes near the bottom of the micron range. Typical single-pass deposition capability is 6µm-30µm.
For flexography/pad printing: Depending on the level of detail required, many flexo inks contain conductive solids in the 70%-85% range. However, in order for this to work in a much lower viscosity system, the particle size needs to range from 2µm-3µm maximum to a sub-micron size of 0.5µm-1.0µm. Common particle shapes are a mix of granular and flake shapes. While the higher solids content does mean higher conductivity, it also means a much thinner single-pass deposition capability of ~2µm-6µm maximum.
For inkjet: Most conductive ink consists of a mixture of sub-micron (0.5µm-1.0µm) and nanoparticle (<0.4µm). Some of it is granular shaped, but at the nanoscale we may see cubes, triangles, spheres, and asymmetrical particles. While the solids content maxes out at about 20% by weight, the conductivity per micron thickness of the dried-ink film is higher than that of thicker film inks with larger particles. However, the single-pass dried-ink deposition capability is ~2µm.
Thinner films limit the electrical current-carrying capacity in silver traces.
But can’t we just jet multiple passes for thicker deposition? Yes, we can. But depending on the application, no, we can’t. More on that next.
Continuous Film vs. Micro-Spots
Everything in printing is cumulative. Consider particle sizes, solids content, and viscosity, along with the fact that screen printing, flexo, and others are effectively “continuous ink films,” whereas inkjet uses “micro-spots” or an ink film formed by overlapping droplets. Without magnification, conductive inkjet silver “looks” superb. It offers high resolution and very fine texture when sub-micron and nanoparticle inks are used. However, from the electron point of view, it’s a film with areas of higher and lower conductivity.
This variation is not a problem in traces used for basic voltage and current applications. However, when “signal quality” or frequency response is involved, such as in an electrochemical sensor, defibrillator pad, EEG, or EKG medical electrode, it can cause signal distortion.
Can multiple print passes repair or mask this texture? In some applications, yes. However, there are other issues at play. Multiple passes tend to trap oxygen and moisture between ink layers, leading to oxidation, signal degradation, and lifespan issues.
We also see that, with inkjet, over printing the surface texture onto the printed part makes both the low and high spots thicker. While this softens the texture, it does not remove it.
All print platforms have this issue to some degree. It can be more significant in inkjet because it deposits very few microns, and there is no surface pressure like between the screen, plate, and anilox, which tends to deposit more in the valleys than on the peaks. Inkjet, being a series of spots, evenly coats the entire surface texture. On thicker traces and pads that require overprint, we sometimes see a visual banding effect from some conductive inkjets.
The Chemistry and Viscosity Issue
What is an “ink”?
Printing inks are not paints. Most printing platforms are a fluid film “transfer process” and all are based around specific limitations related to rheology, pH, polarity, and particle size. In a true, graphic printing “ink” we are allowed to modify the rheology factors using additives, such as various talcs, flow agents, slip agents, different particle grind sizes and shapes, solvents, and binders.
In many industrial applications, especially in printed electronics and medical devices, the “ink” used may serve a function beyond aesthetics, or it may be a reactive component in its own right. The performance modifiers in traditional printing inks become contaminants to inks with conductive, reactive, or biological functions and so cannot be used.
This makes some industrial and most functional inks difficult to transfer using otherwise normal plate and screen techniques. Screen, flexo, and gravure face other problems. This is why inkjet is such a sought-after solution in functional printing.
Most metals used in conductive inks (e.g., silver, gold, copper, silver chloride, zinc, tin, carbon, graphene, manganese, etc.) can be reduced to sub-micron or nanoparticle sizes and put through an inkjet printhead. However, that change in particle size also alters particle texture and shape. This affects how solvents wet the particles, which changes the structure of the ink deposition layer and ultimately affects how electrons flow from particle to particle. We argue that the smaller particles have fewer voids between them and are more conductive, but they can also usually be applied only in a thinner deposition thickness. Higher conductivity, but lower capability.
The very low viscosity solvents required by inkjets are more than just a viscosity issue. They are a “drying time” issue and sometimes a polarity issue. For electrolytically active conductives, solvent polarity influences particle agglomeration at the nozzle and connectivity within the ink film layer. With flake-type inks, it can literally affect whether the flakes lie down and overlap or stand on end and barely connect.
Inkjet conductive inks are already more complex due to the small size of the particles and nozzles, as well as the very low viscosity required. Typically, you have a low-viscosity additive like ethylene glycol, which dries more slowly to prevent nozzle clogs; a surfactant/emulsifier like Ethanolamine to prevent nanoparticle agglomeration; a dispersant to keep them suspended; and a primary solvent/vehicle, like water and ethanol.
Notice that the polarity of each solvent type is listed in the chart below. The only real significance is to note the few solvents in red and gold. They either interfere with or contaminate conductive particles, or have their own critical issues or uses.
While water is common in some flexo conductive and many inkjet conductive inks, it must be avoided in certain sensors, medical devices, and aerospace applications if there is a risk that the water cannot be completely driven off during the sintering/drying process. Moisture is a problem with some of the more reactive conductive particles (e.g., zinc, manganese, etc.). It creates a reaction. When glycol ethers are also present in the solvent blend, reactions increase because they are hygroscopic.
The Viscosity and Rheology Issue
While it is not a hard rule of rheology, it is very common that when the solids content of most functional inks rises above -70%, the ink tends to behave more like a Newtonian fluid. This means it is less able to shear thin and flow and is more pressure-driven or hydraulic in nature. At its worst, some of these exhibit dilatancy (shear thickening) under shear forces. These issues are very difficult to resolve in shear-based transfer printing platforms (e.g., screen, flexo, gravure, etc.).
Print accuracy: clean deposition and exact cartography
The viscosity, particle size, and loading affect the range of rheological variation that each printing process can be adapted to and still transfer ink at the correct deposition thickness, cartographic resolution (meaning point-to-point accuracy on the print), and topographic resolution (e.g., porosity, surface texture, profile, etc.).
This is a big part of the accuracy at speed that inkjet must overcome to be used successfully in any kind of electronics. Prints like the ZIF connector shown on page 40 must be correct in deposition thickness, have low porosity and texture, and very fine pitch spacing.
Economy of Scale
The ability to scale up and production speed are also related to deposition thickness because inkjet (with conductives) may have to make several passes to produce the required thickness that screen print, stenciling, or digital MPCaP/micro-plotting can produce, just to produce a single good sheet. The other “thick-film” methods can produce hundreds or thousands of complete, single-pass impressions per hour.
Multiple passes with conductives using inkjet technology can cause banding as the surface texture changes with overlap, trapping moisture and sealing the surface to prevent complete solvent evaporation.
Ink film structure and uniformity: agglomeration vs. conglomeration
Both terms refer to functional particles in the ink that become fewer but bigger. In screen and flexo printing, this can either cause problems with printing these particles out, or cause no problems with printing but lead to issues — such as texture, unequal particle dispersion, and poor thickness uniformity — on the printed surface. It is very hard to tell the difference between conglomeration and agglomeration once the ink has been printed on the part’s surface. It has to be observed on the screen or on the anilox roller.
In screen printing, we can see the “filtering” or clogging of the mesh. In flexo, we can see anilox cells loading up or clogging without apparent drying of the ink. It’s a particle size issue.
When it’s “conglomeration” within the liquid ink mass in the screen, or flexo ink circulation system, it is either an effect from particles being guided together into groups and crushed into larger particles by shear forces and physical contact with screen and ink system components, or by solvent evaporating or printing out, and often both.
When “agglomeration” of functional particles within the liquid ink mass occurs, it can happen without force even before printing begins. Changes in pH, electrolytic action, solvent polarity, and temperature change are at play and can be multiplied by shear forces once printing begins.
In inkjet technology, which uses specific chemicals to prevent agglomeration at the nanoparticle scale, when particle sizes increase in the ink supply upstream of the chamber and nozzle due to “agglomeration,” these particles may get filtered out by the system. In some applications, the anti-agglomeration chemistry can act as an electrical contaminant.
The Future of Functional Inkjet
The main point when discussing a deeper penetration of inkjet into the functional and industrial sectors is all about the heads and nozzles.
Given the current situation, it’s inaccurate to say that inkjet is not making progress in industrial and functional printing. But new ink chemistry and substrate treatments are not enough. There still isn’t enough speed or single-pass volume capability. Once they can utilize the existing multi-platform conductives, adhesive and elastomer inks that some screen and flexo can share (with proper preparation), it will be a game changer.
About the author:
Ray Greenwood is a process improvement, R&D and production technology consultant for the industrial, medical device, and printed electronics industry. His main focus is high-precision screen printing. Ray also has experience in flexography, offset lithographic printing, injection molding, process fixturing, packaging, material and ink technology, and converting.