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How To Design Conductive Inks For Printed Electronics

Printing books and newspapers can be seen as much easier to be manufactured than microelectronic components, but the difference becomes much narrower with time. Thanks to technological developments, it’s possible to produce advanced microelectronic devices with relatively similar scale, simplicity, and speed as when printing a New York Times bestseller or a popular newspaper.

The inks used in printed electronics are to some extent similar to the ones used for graphics printing, but they have a much greater impact on the final product. After all, normal ink only has to look good and withstand the test of time without fading away. Ink for printed electronics has to conduct electricity and be durable in a way that enables devices and systems to work properly and efficiently on end.

How do you go from printing newspapers to printing electronics? Let’s find out!

 

Step 1: To make conductive ink, you need special components

To become conductive, ink needs to be enriched with conductive components. For this purpose, in addition to the basic ink constituents such as solvents and additives, a variety of organic and inorganic materials must also be applied, such as:

  • Metal nanoparticles,
  • Carbon nanomaterials,
  • Conductive polymers,
  • Quantum dots.

These components, readily or later on, introduce electric charges which can move freely within the material. This ease of electron flow conducts electricity through the printed structures. Different “recipes” of employing these components can result in various additional properties of the final product and impact its end-use performance.

Choosing conductive materials and other ingredients is the first step of designing a conductive ink. Once we understand what type of ink will be best for our specific use case, next we need to take into account other vital aspects to successfully deposit high quality conductive paths that our electronics demand.

 

Step 2: Understanding the relationship between ink and printed technology

Choosing materials is one thing, but you need to know exactly what happens to your ink when you start printing it and during operation. Essentially, you’re adding nanomaterial ingredients to your ink, and you have to do it in just the right way to make a specific technology work.

For successful printing, it’s necessary to understand:

  • the physicochemical parameters of your ink and any material in it,
  • the substrate surface that you’ll be printing on,
  • the requirements of the tech you’re printing.

Image showing the relationships between the most important factors in ink design optimisation

Moving on, each printing method demands different ink properties, so your ink characteristics should be carefully tuned to the requirements of the chosen printing technique.

Next, the ink formulation has to be compatible with the surface on which it’s printed, with precisely engineered wettability and adhesion.

Lastly, you need to adjust the process parameters and sintering conditions, or you might also need to prepare the substrate surface in a special way.

To put the puzzle together and enable efficient printing, all these aspects should be considered when optimizing your ink formulation.

 

Step 3: Processability and printability of conductive inks

Conductive ink must be processable and printable to make the printing process work. Achieving this comes down to optimizing multiple characteristics of the conductive ink, including:

  • Size of functional particles,
  • Ink rheology,
  • Evaporation rate,
  • Surface wetting.

 

Ink rheology

Rheology describes how a fluid responds to applied force. During printing and when it’s in operation, an ink goes through different types of stress which influence its properties.

The two key rheology aspects in this case are viscosity and surface tension.

 

Viscosity

This tells us about the resistance of a fluid to flow, which is related to the internal friction of a moving fluid. When temperature goes up, viscosity goes down. Each printing technique requires a specific range of ink viscosity. Ultimately, optimizing viscosity comes down to a compromise between ink processability, printing resolution, and precision. Too low viscosity, and the ink might cause overspraying. Too high, and your ink may clog the printing nozzle or be hardly printable.

At the moment, printing tech is more powerful than ever and high-performance conductive features are in large demand, so higher ink viscosity is usually preferred. Plus, ink with high viscosity leaves a more solid print after the solvent evaporates, and it’s less sensitive to surface properties of a used substrate.

 

Surface tension

This is the effect of cohesive forces of liquid molecules. These forces push together molecules on the surface of a liquid, minimizing the occupied area. A liquid with a low surface tension spreads over a substrate surface more than a liquid with a higher surface tension.

Generally, higher surface tension of the ink is beneficial because it results in higher printing resolution and fewer satellite droplets outside of the desired printing area. If the print pattern has to stick and stay on the product, the surface tension of the ink has to be lower than the surface energy of the substrate.

 

Evaporation rate

Ink that dries quickly can be good for shorter production times, but it can also clog printing nozzles, which is a fundamental problem that’s best avoided. If the ink dries out too quickly or too slowly, the printed material might end up unevenly distributed—in other words, it might have non-uniform morphology.

What’s important, a non-uniform morphology of printed features might cause bad performance of the end device.

 

Surface wetting

This is another big factor that influences the morphology of the printed film, the adhesion to the substrate surface, and the printing resolution.

For the ink to behave properly, the wetting conditions need to be optimized—meaning that the surface tension of the ink must match the surface energy of the substrate. This is what makes the lines consistent, and determines minimum line width, as well as resolution between adjacent lines.

 

Result: High-quality printed conductive patterns

The key steps to design a conductive ink that meets your needs are:

  • Selecting conductive materials,
  • Understanding the relationship between ink properties, chosen printing method, and final electronics performance,
  • Optimizing key characteristics for processability and printability.

Proper ink design plays a huge role in printing high-quality conductive patterns. Optimizing ink for a specific use case (and specific printing technology) leads to efficient performance in the end product.

 


 

Thank you for reading this article! If you’re interested in nanoprinting for microelectronics, here’s what you can do next:

 

Written by:

Daria Więcławska, Business Development Specialist at XTPL
Ludovic Schneider, R&D Manager at XTPL

 

References:

S. M. F. Cruz, L. A. Rocha, and J. C. Viana, “Printing Technologies on Flexible Substrates for Printed Electronics,” Flex. Electron., 2018.
M. Zenou and L. Grainger, Additive manufacturing of metallic materials. Elsevier Inc., 2018.
C. Cano-Raya, Z. Z. Denchev, S. F. Cruz, and J. C. Viana, “Chemistry of solid metal-based inks and pastes for printed electronics – A review,” Appl. Mater. Today, vol. 15, pp. 416–430, 2019.
K. Kwon, K. Rahman, T. H. Phung, S. D. Hoath, and S. Jeong, “Review of digital printing technologies for electronic materials,” Flex. Print. Electron., vol. 5, no. 043003, 2020.

 

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