A Review of the Opportunities and Processes for Printed Electronics (Part 2): Printing Technologies
In printed electronics, the selection of printing method to use is determined by requirements concerning printed layers and properties of printed materials, as well as economic and technical considerations for printed products.
Presently, printing technologies divide between sheet-based and roll-to-roll-based approaches. Sheet-based techniques, such as inkjet and screen printing, are best for low-volume, high-precision work. Gravure, offset and flexographic printing are more common for high-volume production, such as solar cells, reaching 10,000m²/h.
While offset and flexographic printing are mainly used for inorganic and organic conductors (the latter also for dielectrics), gravure printing is especially suitable for quality-sensitive layers like organic semiconductors and semiconductor/dielectric-interfaces in transistors, due to high layer image registration quality. Gravure printing is also suitable for inorganic and organic conductors in terms of high resolution. Organic field-effect transistors and integrated circuits can be prepared completely using mass-printing methods.
Screen printing
Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials that can be pressed through the mesh as a sharp-edged image onto a substrate. A roller or squeegee is moved across the screen stencil, forcing or pumping ink past the threads of the woven mesh in the open areas.
Screen printing is also a stencil method of printmaking in which a design is imposed on a screen of silk or other fine mesh with blank areas coated with an impermeable substance, and ink is forced through the mesh onto the printing surface. It is also known as silkscreen, seriography and serigraph.
Rotogravure printing
Rotogravure (roto or gravure for short) is a type of intaglio printing process, which means that it involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset and flexography, it uses a rotary printing press. The vast majority of gravure presses print on rolls (also known as webs) of paper, rather than sheets of paper. (Sheet-fed gravure is a small, specialty market.)
Rotary gravure presses are the fastest and widest presses in operation, printing everything from narrow labels to rolls of vinyl flooring 12ft (4m) wide. Additional operations may be conducted in line with a gravure press, such as saddle stitching facilities for magazine and brochure work. Once a staple of newspaper photo features, the rotogravure process is still used for commercial printing of magazines, postcards and corrugated (cardboard) product packaging.
Flexograph printing
Flexography (often abbreviated as flexo) utilizes a flexible relief plate. It is an updated version of the letterpress and can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging. It is also well-suited for printing large areas of solid color.
The greatest advances in flexographic printing have been in the area of photopolymer printing plates, including improvements to the plate material and the method of plate creation.
Digital direct-to-plate systems have been a good improvement in the industry recently. Companies like AV Flexologic, DuPont, MacDermid, Kodak and Esko have pioneered recent technologies with advances in fast washout and the latest screening technology.
Laser-etched ceramic anilox rolls also play a part in the improvement of print quality. Full color picture printing is now possible, and some of the finer presses available today, in combination with a skilled operator, allow quality that rivals the lithographic process. One ongoing improvement has been the increasing ability to reproduce highlight tonal values, thereby providing a workaround for the very high dot gain associated with flexographic printing.
There are five types of inks that can be used in flexography: solvent-based inks, water-based inks, electron beam (EB) curing inks, ultraviolet (UV) curing inks and two-part chemically-curing inks (usually based on polyurethane isocyanate reactions), although these are uncommon at the moment. Water-based flexo inks with particle sizes below 5μm may cause problems when de-inking recycled paper.
Ink control
In the flexographic printing process, ink is controlled by the inking unit. The inking unit can be either a fountain roll system or doctor blade system. The fountain roll system is a simple, old system. Yet if there is too much or too little ink, this system would not likely control ink in a good way. The doctor blade inside the anilox roller uses cell geometry and distribution. These blades ensure that the cells are filled with enough ink.
Offset printing
Offset printing is the most widely used printing technique for newspapers and magazines worldwide. Famous equipment, such as the Heidelberg and MAN-Roland presses, makes nearly two-thirds of the world’s newspapers and high-quality magazines. These include web and sheet-fed offset presses, as well as digital printing systems.
Offset has high resolution, but only a very thin ink film can be transferred, typically 1μm to 2μm on the substrate, but at a resolution of 50μm or less. This thickness is suitable for printing catalytic inks, allowing the conductor thickness to be subsequently increased by additive or semi-additive post-deposition processes.
From the simplest stenciling to the most sophisticated offset, traditional printing technologies are still the mainstay of printing today. For flexographic printing, the previous step was contact with an ink-coated roller where the ink thickness had been adjusted using a doctor blade. In the gravure printing example, the previous step would have been contact with a doctor blade to remove excess ink from the coated surface, which controls the depth of the cavities in the print roller. Photolithographic printing is another traditional technology. However, it has limited applicability, with the exception of the prospect of crossover of catalyst-coated materials that can be activated by light. This method has thus far been limited to molded interconnection devices.
Inkjet printing
An inkjet printer is a type of computer-controlled printing system that creates a digital image by propelling droplets of ink onto a substrate of paper or film. Inkjet printers are the most commonly used type of printer and range from small inexpensive consumer models to very large professional machines that can cost thousands of dollars.
The concept of inkjet printing has deep roots in the past, having originated in the 19th century. The technology was first extensively developed in the early 1950s. Starting in the late 1970s, inkjet printers that could reproduce digital images generated by computers were developed, mainly by Epson, Hewlett-Packard (HP) and Canon. In the worldwide consumer market, four manufacturers account for the majority of today’s inkjet printer sales: Canon, HP, Epson and Lexmark, a 1991 spin-off from IBM. Today, HP dominates the market for large format inkjet printers used for signs and printing.
There are a couple of different types of printheads. Thermal inkjet heads are a familiar type that uses a thin-film resistor to heat and volatilize a portion of the ink to produce a bubble that drives the ink out through its nozzle. Inkjet printers, especially models produced by Dimatix (now part of Fujifilm), JetXpert, Xennia Technology and Pixdro, are in wide use in many labs around the world for developing alternative deposition methods that reduce consumption of expensive, rare or problematic materials. These inkjet printers have been used in the printing of polymer, macromolecular, quantum dot, metallic nanoparticles, and carbon nanotubes. These printing materials are used for organic thin-film transistors, organic light-emitting diodes, organic solar cells, and sensors.
The emerging ink jet material deposition market uses alternative inkjet printing technologies, employing printheads that use piezoelectric crystals to deposit materials directly onto substrates. Piezo printheads are made by Xaar, Trident, Seiko-Epson, Kyocera, Konica-Minolta, Dimatrix and Ricoh.
Another inkjet printhead technology is one based on MEMS (micro electro mechanical systems) technology. Companies such as Dimatix, Orbotech, OCE and MicroCraft utilize both MEMS and piezo printhead technologies for printing circuit legend information, soldermask and etch resist ink on both PCBs and flexible printed circuits. Four models of Dimatix are available for PE printing. The new DMP-5005 joins the DMP-2800, DMP-3000 and DMP-5000 printers introduced in 2005, 2009, and 2010, respectively. All four printers are equipped with a PC and monitor, a drop watcher, a fiducial camera and a heated vacuum platen. They have automated printhead maintenance and cleaning and employ easy-to-use software. The equipment can be configured with multiple jets having either 1- or 10-pico liter MEMS-based printheads, and can be fitted with up to five different fluids for sequential production printing using a large 500x500mm printable area with positional accuracy and repeatability of ±5microns and ±1 micron.
The MetalJet 6000l is an inkjet printer for the PE industry that uses a catalyst ink on a special chemically-resistant paper to initiate copper metal deposition, as seen in Figure 1.
Figure 1: Thermal inkjet printing compared to piezo inkjet printing.
Imprinting/stamping
Imprinting is as old as Edison but also, the newest manufacturing method available. Imprinting was first used by Edison to make Victrola Music tubes/cylinders for his new invention, the phonograph. The time-honored technology is now used to manufacture the myriad of laser CDs, imprinting works off a master tool foil, a hard, nickel-plated tool that stamps or imprints its pattern onto various plastics. For CDs, the plastic is polycarbonate that will be later be coated with aluminum to reflect the laser.
Plasma printing
Plasma printing is a new development that comes from the Fraunhofer Institute in Braunschweig, Germany. Plasma printing and packaging technology (P3T) uses the principles of conventional rotogravure or lithography, with metal printer rollers used as printing masks. However, printing information is transferred to the substrate using a dielectric barrier discharge plasma rather than ink.
Reel-to-reel patterned plasma activation of the substrate, the polymer film, is carried out in the plasma printing process using cold atmospheric pressure microplasmas. The main components of the plasma printing setup include a high voltage (HV) electrode and the printer roller, which also represents the counter electrode. The HV electrode system has a curved shape with a curvature that corresponds to the radius of the printer roller. Printer rollers that have been used in research so far have consisted of a copper galvanized steel cylinder with a chromium top coat along with the structures to be plasma printed engraved using a high-resolution laser ablation technique called high-resolution laser stream. A gas shower positioned close to the HV electrode supplies the process gas.
During operation, the printer roller rotates and the polymer film is pulled through the gap between the electrodes to achieve area selective plasma activation of the polymer surface. The pressure of the HV electrode on the substrate ensures the formation of process gas-filled spaces and ignition of a plasma in these spaces in positions necessary for structures on the printer roller to be accurately reproduced on the substrate. Schematically, the plasma ignition occurs exclusively inside the ditches and depressions of the structures engraved on the printer roller during face-to-face contact between the printer roller surface and the polymer foil. Next, the plasma-activated areas of the substrate are selectively metallized by seeding with palladium chloride/hypophosphite or colloidal graphite solutions. Conductive paths are provided by electroless plating procedures, such as electroless copper or electroless nickel.
Typical operation parameters in this work included sinusoidal mid-frequency (15kHz) voltages estimated to be in the range of 10kV to 20kV, a gas flow of up to 70 standard liter per minute and reel-to-reel speeds from 0.5m/min to 3m/min.
Direct laser printing
There are three types of direct laser writing metallization: ablation, exposure, and thermal transfer material.
Ablation usually removes a thin tin or copper layer from the substrate. Exposure is the PCB technique of polymerizing a photoresist with laser energy. Thermal transfer is a thermal decomposition of coated film.
In direct laser printing, heat generated by the laser decomposes the precursor, which then generates and anneals the small metal particles, forming metal aggregates that are further embossed onto the surface of the substrate. Since the laser beam irradiates through a thin film layer instead of a liquid medium, the feature resolution problem is reduced. However, the minimum feature size is still limited, and the uniformity of shape of the deposit and its morphology still need to be improved.
There are several printing techniques based on the principle of thermal transfer. In the graphics world, these techniques are sometimes known as dye transfer, dye sublimation, thermal dye transfer or thermal imaging. These techniques use a laser to induce the transfer of material from a donor sheet to the substrate. The laser energy melts or vaporizes the surrounding organics, transferring them from the donor layer to the receiver. Unfortunately, the laser energy is sufficient to decompose many organic materials. The two flexible films, a multilayer donor and a receiver, are held together by vacuum. The laser beam is focused onto a thin absorbing layer that converts light into heat, an optional ejection layer placed directly underneath and the functional material to be transfer-coated on top. The heat generated at the metal interface decomposes the surrounding organics creating a gas bubble that transfers the conducting layer onto the receiver. After imaging is completed, the donor and receiver films are separated.
Curtain/roller coating
Curtain and roller coating are methods of applying a fluid material in a uniform thickness. In addition to printing, in which a patterned layer is produced, in many cases it may be sufficient and even desirable to coat the entire area of a substrate (or strips on the substrate) with a functional material for subsequent patterning or for creating an unpatterned state. There are a variety of coating methods leading to different costs, uniformities and film thicknesses, such as curtain, slot-die, wire bar or reverse nip gravure coating. Where patterning is not essential and material costs are not prohibitive, these methods of solvent jetting, laser ablation or embossing may prove to be more effective than printing.
Summary
Each printing method has certain attributes, advantages, limitations and requirements (Table 1). Screen and inkjet printing are the simplest methods, with gravure, lithographic and flexographic printing following in terms of complexity.
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In Part 3 of this five-part article series, Happy Holden writes about the different material and process developments for PE.
Editor's Note: This paper has been published in the proceedings of SMTA International.
