Central Midori's Technology Blog

Functional Inks for Printed Electronics

A few years ago, confronted by a drop in sales, we had to reflect on our capabilities,  as defined by Clayton M. Christensen as Resources, Processes, and Priorities  and reexamine our strategy. 

Our choice was between continuing to service the same customers and adapting our technology or leveraging  our core competencies to open up  new markets and new applications. 

We came out of a three-day strategic planning session with the firm conviction that screenprinting was our core competency and that the Printed Electronic industry offers plenty of opportunities to fuel our growth across market segments such as medical, industrial, consumer electronics and defense.

In our company Printing is divided into Graphic and Functional. 

Graphic printing is considered inert or inactive as the role of the ink is merely visual. Unlike traditional CMYK (Cyan Magenta Yellow Black) printing, CMI mixes the colors prior to print.

Functional printing is a whole different story. The ink has a job to do and the printing process is essential to get the ink to do what it is supposed to. The inks can be electrically conductive, resistive, dielectric, semi conductive or contain other special characteristics such as

thermal conductivity, electroluminescent, light diffusing or piezoelectric. 

In this article we will review the various functional inks used at Central Midori and their applications.

To begin with, let’s discuss the different ingredients going into an ink formulation. 

Solvent based inks are composed of:

  • Fillers
  • Resin or binders
  • Solvent
  • Additives

In graphic ink, fillers are the pigments while in functional inks they are based on carbon particles, metal particles or polymers.

The curing/drying process will eliminate the solvent that was there in the first place to control the rheology. 

The binder is the vehicle that holds all the ingredients together, its role is also to ensure a good adhesion on the substrate.

Additives change the properties of the inks to meet the printing requirements.

UV ink contains no solvent. They are composed primarily of monomers, oligomers and photoinitiators. When the photoinitiators are exposed to UV energy at a certain wavelength, they absorb the light and produce free radicals that trigger the cross-linking process. 

  1. Conductive Ink

The resin is either thermoplastic (i.e the resin softens when exposed to temperature and hardens when cooled down) or thermoset (i.e polymers cross-link together during the curing process to form an irreversible chemical bond). Our most common conductive material is silver but carbon, copper, gold or nanosilver are also available. The Ag flakes come in a variety of sizes and shapes to maximize the points of contacts and provide higher conductivity.

  1. Resistive Ink

Resistive inks are a blend of a conductive element (generally carbon) and a dielectric component to adjust the resistivity. We use them for heating element and potentiometer applications.

  1. Dielectric Ink

Our dielectric inks are UV curable. They are particularly critical in crossover design where conductors are separated from each other at their intersection by dielectric material. As a rule we always print a minimum of two layers so that the second pass will fill in the pin holes in the first layer. The ink contains pigments typically blue or green but other colors are available per customer requirements.

  1. Transparent Electrode Ink

Based on conductive polymers the ink offers electrical conductivity and good optical transparency. Used as an ITO (Indium Tin Oxide) replacement in transparent electrodes it can be printed on PET for flexible applications.

  1. Force Sensing Ink

A Force Sensing Resistor (FSR) is a variable resistor in which resistance decreases when pressure is applied (more pressure = lower resistance). The ink can be blended with a composition of carbon and other components to change the resistance when force is applied and meet the customer requirements.

6- Light Diffusing Ink 

As you cannot place a discreet LED below each dome switch, light guide films are designed to provide a uniform illumination with a minimum number of LEDs. Central Midori has developed a unique technology which consists of a thin flexible material with printed dots arranged in a particular pattern. The base film provides high transparency along with high flexibility while the printed ink diffuses the light effectively. 

7- Still In Development

CMI continues to explore along with its customers and suppliers other ink formulations including Piezoelectric ink for pressure sensing or silver/silver chloride (Ag/AgCl) inks for reference electrodes. We have also tested inks designed to produce strain sensors, temperature sensors and moisture detection sensors.

Conclusion

Printed electronics is a vibrant industry with a tremendous amount of investment in Research and Development and functional inks represent a significant portion of it.