An Introduction to Conductive Ink

Carlos Ospina (BF)

From writing on stones and papyrus to modern day photo paper and glass, ink has been used and improved for thousands of years. While today pine wood soot has been replaced by synthetic pigments, dyes, and metallic particles; new areas of development are appearing at an ever increasing rate.

Printing traces is a fundamental part of what BotFactory's Squink does. In order to print those trace, we use conductive ink, a special type of ink which allows electric current to flow through. There are several varieties of conductive inks on the market, and it is crucial to choose the right ink fitting your specific application.

The first question to ask yourself when selecting conductive ink is "How will I apply it?" The viscosity and particle size of the ink have a big influence on how you can apply it. Viscosity is the resistance of a fluid to flow; lower viscosity fluids (for example: water) flow more easily than higher viscosity fluids (for example: honey). Low viscosity inks are easier to print with an inkjet head while high viscosity inks may require extrusion, brushing, or silk-screening. Similarly, inks with small particles can be printed with an inkjet head while inks with large particles require extrusion, brushing, or silk-screening.

The second question you'll want to ask yourself is "How will I cure the ink?" Uncured ink is easy to wipe off the substrate and may not be conductive. The conductivity of the ink will increase as it cures. The ink we use is air-cured which means if left exposed to air it will dry and become conductive. Heat can be applied to accelerate the curing process but it is not always required. On the other extreme, certain inks can only be cured by a special ultra-violet light system.

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The final question (and arguably the most important) is "What kind of electrical properties do I want?" They are many electrical properties to consider but the most important one is the conductivity (or resistivity) of the ink. This describes how well (or poorly) the ink lets current flow. Capacitive sensors and traces for slow signals can be created with an ink that has high resistivity; however, high frequency signals should be created with inks that have low resistivity.

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This is only a brief introduction to selecting a conductive ink. In practice you will also need to consider price, supply, and several other factors. If you start by finding conductive inks based on the printing process, curing process, and electrical properties you will be very close to finding the ideal ink for your application.