A Review of the Opportunities and Processes for Printed Electronics (Part 2): Printing Technologies

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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.



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