Developments in Conductive Inks

From Industrial Specialty Printing
by Sanjay Monie, Ph.d.

Printed electronics have come a long way from printed circuit boards (PCBs) that used copper containing thick-film pastes on rigid substrates, which were patterned using a two-step masking and etching process. This process resulted in a surfeit of waste and was expensive due to laborintensive processing of the final device.

By contrast, conductive inks are now available that use existing printing technologies such as screen, gravure, flexographic, offset, and inkjet to print conductive traces directly onto rigid and flexible substrates relatively cheaply. Much of the impetus for advancements in conductive inks has come from the field of organic electronics, where the promise of fully printed electronic devices and displays requires printable conductors for contacts and signal bus lines.

Printed electronics is experiencing explosive growth, as it provides the microelectronics industry a low-cost fabrication route compared to etched circuits for consumer electronics and similar applications. At the same time, it represents a highvalue opportunity for the printing industry during a period of decline in print media created by the advent of electronic media. In fact, it allows the printing industry to continue to play a role in these emerging technologies, such as electronic book readers, by printing functional components onto flexible substrates.

The process of printing conductive ink is used to produce active and passive components such as transistors, resistors, capacitors, diodes, and even complete circuits such as RFID tags, keypads, sensors, and electrodes, as well as backplanes of organic light-emitting diodes (OLEDs) and other electroluminescent displays. End-use applications for printed electronics include medical devices, photovoltaics, smart packaging, flexible displays, RFID labels, energy storage, and active clothing.

In general, printed electronic circuits have lower performance characteristics and lifetimes than etched circuits, but they also have a significantly lower cost.

However, they still remain too expensive for many applications. This is one of the main limitations of conductive inks, as the best performing inks based on silver nanoparticles are too expensive, while for many applications, lower cost alternatives are either insufficiently conductive (such as conductive polymeric inks), or lack in flexibility or handling characteristics (such as traditional carbon inks). One new entrant in the field of conductive inks is graphene, which could revolutionize the field of conductive inks by providing a high-performance, low-cost solution for many applications (Figure 1).

CONDUCTIVE-INK TECHNOLOGIES
The earliest printable conductive inks evolved from PCB thick-film pastes and used screen printing to print membrane switches and keypads. Although screen printing is still widely in use, high-speed printing processes such as gravure and flexography have also entered the path of common usage into print electronics.

The main ingredient of conductive inks and pastes is now silver in the form of pellets, flakes, and nanoparticles. Silver is not only one of the most highly conductive metals (it has a resistivity of 1.62 µ-cm), but it is also oxidatively stable and has the added advantage of an electrically conductive oxide. However, low-temperature curing presents a challenge with silver inks.

While some recent silver inks claim to be curable at temperatures below 302°F, most silver inks require higher temperatures and long times (on the order of minutes) to cure completely.

Achieving conductivities close to that of bulk silver with silver-based inks requires sintering at high temperatures (above 662ºF, and even up to 1292ºF), which severely limits the choice of substrates for certain applications. Sintering is typically a post-printing operation. It adds expense and affects throughput. In addition, burn-off of the binders and surface stabilizing agents in these inks can result in cracking of the film due to volume shrinkage, which can harm conductivity, especially at high film thicknesses. Silver is too expensive for many applications, and since silver prices tend to fluctuate, developing cost structures is difficult.

Carbon inks are used wherever possible as an alternative to expensive silver inks, but they’re far from an ideal solution. For instance, carbon does not have the required conductivity for many applications, and the inks tend to suffer from poor adhesion to substrates and poor cohesion within the ink film, resulting in poor flexibility and rub resistance, which can limit its use in applications that require a lot of handling.

Copper is a lower cost alternative to silver. Its resistivity, 1.67 μΩ-cm, is similar to that of silver; however, copper is prone to rapid pyrophoric oxidation and, unlike silver, the oxide is insulating, so the promised conductivity is difficult to achieve. To prevent oxide buildup, copper inks require sintering at high temperatures, which limits their use with thermally sensitive substrates, or expensive curing under an inert atmosphere, which has also inhibited widespread use. Conductive inks are formulated using nickel, iron, or aluminum, but they are all prone to oxidation and are inherently much less conductive than copper or silver.

In recent years, conductive polymers such as poly(ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS) and PANI (polyanilines) have been investigated as alternatives to particulate inks. However, these inks are unable to achieve the high conductivity levels required for many applications. Although the polymer conductivities can be increased in some cases by increasing the molecular weights, this tends to reduce their solubility in many of the solvents required to formulate a printable ink.

Carbon nanotubes (CNTs) have garnered a lot of attention as a highly conductive form of carbon, and attempts have been made to formulate them into printable inks, but success has remained elusive so far. The main drawbacks are poor dispersion stability and processability, and overall conductivity levels are unreliable because carbon nanotubes as produced are a mixture of conducting and semi-conducting forms.

Graphene
One of the most exciting entrants in the field is a class of inks based on graphene. Graphene is the most conductive form of carbon and is the thinnest material known, with unique electronic thermal and mechanical properties. It consists of a single sheet of carbon a single atom thick (which can be thought of as an unrolled carbon nanotube), through which electrons can travel with extremely high mobility. It represents a potential breakthrough in the field of printed electronics by enabling the production of conductive inks that provide very high performance at low cost compared to silver. In addition, the ability of graphene platelets to orient allows the deposition of thin films that are highly electrically conductive compared to other forms of carbon.

Regardless of the main functional in-gredient, one of the main challenges in formulating conductive inks is that the use of additives to improve printability and processability can interfere with the electrical characteristics, and thus the key functionality of the printed object.

In general, proper formulation includes a complex balance between the functional pigment dispersed in a suitable solvent system and the binders and other additives that ensure a printable, properly dispersed ink. In all cases, the ink should provide prints that have adequate film cohesion and adhesion to the substrate, in addition to possessing conductive characteristics.

High-performance, graphene-based conductive inks are formulated to work on a wide variety of screen, gravure, flex-ographic, and industrial inkjet printers, enabling a surface resistivity of 1 Ω/sq or below. This translates to bulk conductivities greater than 300 S/cm at 5 μm, depending on the print technology and substrate. Such inks are extremely flexible when printed (Figure 2) and allow only a minimal drop in conductivity after a multitude of folds over a mandrel. Another key property is excellent rub-resistance, demonstrated by a conductivity reduction of less than 10% after 10 rubs. These types of inks are able to achieve high conductivities at low film thicknesses with vastly improved flexibility and handling characteristics relative to carbon inks, thereby offering solutions that bridge the price-performance gap between silver and traditional carbon inks.

The future
While the biggest market for printed electronics remains consumer electronics, the ultimate goal is not to merely to replace PCBs, but rather to eventually replace silicon-based integrated circuits (ICs). For example, a low-cost field-effect transistor could be formed by printing a conductor layer forming source and drain electrodes, followed by a semiconductor layer, a dielectric layer, and another conductor layer as a gate electrode. By direct writing of multilayered devices directly onto flexible substrates using semi-conductive, conductive, and dielectric inks with low curing temperatures, one could avoid the expensive photolithographic patterning and etching steps of silicon-wafer-based fabrication, which wastes materials and has a negative environmental impact. Transistor devices can be printed directly onto inexpensive substrates using inkjet devices.

The next generation of electronics, or more demanding applications that require multilayer printing, will challenge printers to achieve higher resolutions, greater accuracy in print thicknesses, more stringent requirements for defect-free printing, and higher precision in layer-to-layer registration than what they can currently realize with presses developed for graphics printing. Nevertheless, future advances in printer technologies that are specifically targeted towards printing electronics, along with new generations of conductive inks such as graphene-based formulations, may make even an economical alternative to silicon-based fabrication achievable in the foreseeable future.

Sanjay Monie, Ph.D., Vorbeck Materials
Sanjay Monie is technology development manager at Jessup, MD-based Vorbeck Materials. He has actively researched and developed media for digital printing since 1996, and holds a Ph.D. in Materials Science from The Pennsylvania State University.