Nanoparticle inks and drop-on-demand ink jet printers offer a unique opportunity to generate fine-line additive circuits on flexible, three-dimensional substrates.

Ink jet printing has been gaining interest for use in electronics manufacture. This article is intended to provide a brief overview of ink jet printing; a review of some of the applications of high-frequency electronics from the literature; and, finally, some ideas as to possible further applications.

In a review of ink jet printing for graphics applications, Le defined the technology as follows; “Ink-jet is a non-impact dot-matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media to create an image¹.”

To facilitate this technology, there are two general designs of ink jet printers. The designs are continuous and drop-on-demand (DOD). As the names imply, these designs differ in the frequency of generation of droplets.

In continuous ink jet printers, droplets are generated continually with an electric charge imparted to them. As shown schematically in Figure 1, the charged droplets are ejected from a nozzle. Depending upon the nature of the imposed electric field, the charged droplets are either directed to the media for printing, or they are diverted to a recirculation system. Thus, while the droplets are generated continuously, they are directed to the media only when/where a dot is desired.

In DOD ink jet printers, droplets are generated only when they are needed. The droplets can be generated by heating the ink to boil off a droplet (so called thermal ink jet). Alternatively, the droplets can be ejected mechanically through the application of an acoustic pulse or electrically stimulating a piezoelectric to elicit a deformation, which will generate a droplet as shown in Figure 2.

An advantage of ink jet printing is its modularity. Individual printing nozzles are combined into a single print head. Multiple print heads can be combined within a single printer. This allows printing more than one ink at a time (e.g., different colors). The modular nature also allows for combining printheads laterally for use in large area printing. Currently, a major application for ink jet printing is in the production of billboards.

Inks for Printing Electronic Products

To apply ink jet printing technology to electronics, the ink to be printed is made up of electronic materials rather than the pigments used in graphics printing. As with other fluid-based printing methods, the ink used in ink jet printing is a complex formulation of solvents, plasticizers and surfactants in addition to the appropriate electronic materials. For the most part, DOD printers have been used in ink jet printing of electronics.

Ink jet printing has been used to produce conducting and semiconducting polymers², as well as displays³. The work discussed here will concentrate primarily on ink jet printing of conductors. Work has been reported on ink jet printing of copper⁴, silver⁵ and gold⁶. Typically, nanoparticles of the desired conductor are dispersed in the ink. However, due to the high surface area of copper nanoparticles, oxidation becomes a concern. This necessitates using inks containing a copper metal precursor, rather than copper nanoparticles. Commercially available silver or gold inks can be obtained from several companies such as Cabot (U.S.), Cima NanoTech (Israel/U.S.), Ulvac (Japan), Harima Chemical (Japan) and Advanced NanoProducts (Korea). These inks contain up to 60% by weight of metal.

The small particle size of the nanoparticles in electronic inks allows for lower firing temperatures to densify the printed ink. Silver nanoparticles fired as low as 260˚C have resulted in dense films with conductivity about 50% of bulk silver⁷. This low densification temperature allows for circuits on plastic substrates that could not otherwise handle the higher temperatures typically used to densify metallic particles.

Ink Jet Printing for Electronic Products

There are many advantages to employing ink jet printing for electronic circuits, several of which are listed below. The first four are currently being utilized. The others are potential advantages that, to the author’s knowledge, have not yet been exploited for electronic applications.

Digital process. Ink jet is a dot-matrix technology, a digital process. This means that patterning can be directly computer controlled. No masks or screens are necessary. Changes can be made quite easily by altering the computer-controlled pattern rather than awaiting a new mask or screen.

Additive process. Materials are applied only where desired. This can be important when expensive materials are being deposited. Additionally, the extra steps and environmental concerns of stripping are eliminated with ink jet printing.

Non-contact. The print nozzles do not contact the substrate, preserving delicate surfaces. They are typically positioned 0.5 to 1.0 mm above the substrate.

3-D printing. Through programming of the individual nozzles and repeated passes, it is possible to vary the thickness of the deposited ink with position. This has been used in forming 3D ceramic structures⁸ and to print spiral conductors⁹.

Different materials can be printed simultaneously. As with multicolor printing of graphics, it is possible to print different electronic materials (e.g., conductors, resistors, dielectrics) simultaneously. By ganging printheads containing the different materials, it would be possible to print them in a single pass. This could eliminate the need for pattern registration for subsequent depositions and increase throughput.

Large sizes. As mentioned above, ink jet printing is currently used in the graphics world to print billboards. The same print heads that are used in desktop printing of electronic materials can be ganged laterally to print onto significantly larger-sized substrates. It is important to note that this scale up is direct, that is the printheads are the same, so that the parameters found applicable at small sizes should be usable.

For many electronic applications, screen printing has been the preferred deposition method. The line widths and line spacing typical of ink jet printing are one-half those typical of screen printing: 30 µm vs. 60 µm¹⁰. Even finer geometries have been reported in developmental ink jet printing: 10 µm lines and spacing¹¹.

While there are advantages to ink jet printing of electronics, there are drawbacks. The physics of generating droplets from very narrow diameter nozzles constrains the allowable ink rheology. The ink typically needs to have a surface tension >35 mN/m and a viscosity of 1 to 10 cPoise¹². In part because of these rheological constraints and the present limited market, there are not many commercially available inks for electronics. However, it is anticipated that as the market grows, additional suppliers and ink compositions will become available.

High-Frequency Applications

Reports of ink jet printing used in high-frequency electronics are reviewed below to give a sense of the breadth of applications possible.

Radio-controlled watch module. Seiko Epson has reported fabricating 20 metal layer circuit boards of 200 µm thickness (not including the base polyimide substrate) by ink jet printing using a piezoelectric DOD printer¹³. The boards measured 20 x 20 mm. Silver conductor lines and vias were ink jet printed as were the interlayer dielectrics. The silver conductor lines were 4 µm thick and 50 µm wide with a minimum spacing of 110 µm. No details of firing conditions were reported.

By using this technology, the module size for radio-controlled watches was reduced by 65% over a chip-on-board (COB) process using a glass-epoxy substrate. The ink jet PCB contained all of the passive components except for the antenna, connector and crystal.

Co-planar waveguides in PCBs. Joint work carried out by Motorola and the University of Illinois (Chicago) studied the possibility of using ink jet printing to form PCBs. For one of the test vehicles, they used a DOD printer to fabricate coplanar waveguides consisting of silver transmission lines between copper etched ground planes and signal pads on polyimide¹⁴. The conductor line was densified by heating at 300˚C for 15 min. Their lines were 120 to 200 µm wide and 1 to 3 µm thick.

The resistivity of the conductor lines was 3.5 x 10-5 Ωcm.

They found good, repeatable electrical results between 0 to 4 GHz. They also found that test vehicles subjected to 85% humidity and 85°C for up to 172 hours showed an average increase of only 4% in the resistivity of ink jet printed silver conductors on polyimide.

RFID components. Researchers at the University of California–Berkeley have been active in fabricating RFID components via ink jet printing. They have printed nanoparticles of gold onto polyester, creating spiral inductors and conductors using a DOD printer¹⁵. To improve the uniformity of the printed conductors, they overlaid drops of ink with 5 to 15 µm spacing. They then printed several lines on top of each other. To further increase line uniformity, they printed with the substrate heated to 160˚ to 190˚C.

Using their print conditions, the best results were found for printing three layers or more. The resultant conductors were about 160 µm wide and had resistivity as low as 23 mΩ/n. They also used ink jet printing to deposit polyimide dielectrics for crossovers and capacitors. Pin-hole free dielectric films were formed as thin as 340 nm and as thick as 3 µm. Spiral inductors of 350 nH were formed with three printed layers of gold. The inductors had radii of 5,000 µm with line widths of 160 µm and line spacing of 100 µm.

Transmission lines for cell phones. Nokia has investigated the possibility of using ink jet printed conductors on polymers for possible use in cell phones and other electronic devices. They printed silver nanoparticles onto polymer substrates using a DOD printer¹⁶. They printed several different sized conducting traces and cured them in an infrared oven. For thin conductor lines (<10 µm), they found the optimal curing conditions to be 240˚C for 60 min.

To test the electrical parameters of the ink, they fabricated 50 Ω transmission lines on ceramic substrates. They compared transmission lines with ink jet printed silver conductors to etched copper lines. The line width and length were the same for the two conductors. The thickness of the ink jet printed conductor was 2 µm, while that of the etched copper was 17 µm. The insertion losses between 0 to 5 GHz were slightly larger for the ink jet printed silver conductor but were judged to be acceptable.

Future Applications

Hopefully, the applications described above demonstrate the applicability of ink jet printing for high-frequency electronic products.

Using a tabletop dedicated materials deposition system from Dimatix¹⁷, shown in Figure 3, product development on additional applications of ink jet printing is ongoing at Advanced Materials Solutions. The Dimatix system has a 200 x 300 mm printing area with a vacuum platen that is heatable to 60°C. The system has integrated fiducial and drop watcher cameras. It can be used to print Ag and AgCu conductors onto Mylar, ceramic and textile substrates.

The printer uses replaceable cartridges, which avoids any cross contamination of inks. The cartridges have silicon MEMS printheads containing 16 independently controllable nozzles. The printhead can be heated up to 70°C. The nozzles have 21 µm orifices and 254 µm spacing (100 dpi). The cartridges are user fillable. The following applications are under development.

Quickturn Antenna Prototypes

As has been described, ink jet printing is a digital process. No masking is necessary, only a digital file. This could lead to quick production of prototypes, such as antennas. It would allow for a quick turnaround for changes. We envision an iterative process in which a prototype can be printed from a digital file, tested and proposed changes to the pattern introduced digitally. This cycle would then be repeated until the desired characteristics were obtained. Delays associated with waiting for new masking or screens to be produced could be eliminated while optimizing the design.

Conformable Circuits

Ink jet printing can be performed on flexible substrates. These substrates could be bonded to a non-planar surface to permit conformable circuits. (It should be pointed out that the flexibility of the densified conductor traces has not yet been demonstrated.) Alternatively, with a change of fixturing, it may be possible to print directly onto non-planar surfaces.

Circuits on E-Textiles

E-Textiles are textiles into which metal wires are woven. There has been work done on using these for transmission lines¹⁸. By ink jet printing additional conductors and passive components, more advanced capabilities can be built-in with the integral conductors acting as interconnects. Potential applications include wearable electronics (e.g. GPS or antennas), physiological monitors and animal monitoring via RFID. Conductive traces have been successfully demonstrated on E-textiles via ink jet printing¹⁹.

Conclusion

This article was designed to provide an introduction to the applicability of ink jet printing in the electronics industry. The utility for several high-frequency applications has been demonstrated. It is hoped that this article will stimulate the reader to consider other possible applications of this exciting technique. PCD&F

John Blum is Principle, Advanced Materials Solutions LLC. He can be contacted at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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