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Printed Electronics World
Posted on October 16, 2008 by  & 

Aerosol Jet printing of copper inks

Copper has always been the conducting metal of choice for the printed circuit board industry. However, today silver is the leading metallic ink printed in a large number of current products such as UHF antennas for RFIDs, membrane keyboards, battery testers, flexible displays and is the leading metallic ink for any other printed electronics applications. Unfortunately, it is predicted that the cost of silver will increase exponentially over the years and as a result of this cost escalation, low cost but high quality metallic inks are required.
In principle copper can replace silver in many of these applications because copper has excellent electrical conductivity properties; however inking copper presented a lot of challenges, such as purity, oxidation, particles' size, dispersion quality that needed to be overcome.
As part of their strategic partnership, Applied Nanotech Inc. (ANI) is developing metallic inks that are suitable for printing with Optomec's Aerosol Jet print solution. The Optomec Aerosol Jet system is ideally suited for printing ANI's metallic inks onto flexible and solid substrates. The broad viscosity range available with the Aerosol Jet printing system allows for a wide range of vehicles, dispersants, additives and copper loadings to be used. ANI's metallic inks coupled with a simple laser sintering system were demonstrated by Optomec and ANI to achieve excellent copper metallic traces on a variety of substrates. These metallic inks have excellent compatibility with flexible substrates, and their ease of flow allows them to be conveniently used in the Optomec system. The handling of the inks in air without the need of exotic inert gases, the fact that drying temperature of these inks is less than 100 degrees C and the demonstrated stability of the final metallic conducting traces make the ANI/Optomec solution very attractive for customers seeking a complete metallic ink printing solution.
Copper nanoparticle inks used for the initial tests were chosen due to the ANI progress in achieving wide range of loadings, and excellent dispersion qualities of the copper nanoparticle in the inks. These breakthroughs were achieved by using proprietary approaches to produce adequate copper nanoparticles with predesigned properties and careful choice of vehicles and dispersants for the inks' formulations. One important advantage of these nanoparticle copper inks is the fact that they can be rapidly sintered by electro-magnetic irradiation, for example using a laser, in air at room temperature to consistently achieve metallic conducting traces with low resistivity in the micro-ohm-cm range.
These conductors also have good adhesion to flexible substrates. This additive process involving Aerosol Jet printing of the inks followed by simple sintering offers manufacturers a rapid and inexpensive method for printing electronic circuits onto a wide range of substrates. Both organic solvent based and water based conductive metallic inks have been formulated and printed with the Optomec system.
The Optomec system delivers between 1 and 5 micron size aerosol droplets at a dispersing rate of 0.25 micro liters per second, and can print line widths of between 10 and 150 microns (Figure 1). Using the copper inks from ANI, Figure 2 shows a 30 micron wide copper line achieved on a glass substrate.
Figure 1. Optomec Aerosol Jet Delivery System
Figure 2. ANI Copper Inks Aerosol Jetted and cured on Glass
The Optomec system and ANI inks were also applied on Kapton (polyimide) substrates. Sintered copper lines that are 60 micron wide and 8 micron thick were demonstrated (Figure 3).
Figure 3. ANI Copper Inks Aerosol Jetted and cured on Kapton
Excellent adhesion to the Kapton substrate was obtained. The broad viscosity range of the Optomec system (0.7 to 2500 centipoise) allows for a wide range of metallic inks and pastes to be rapidly printed and sintered for a wide variety of applications for the printed electronic industry in the future.
By Lauren Johnson, Applied Nanotech
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