Jack Kenny, Editor02.04.09
Kammann K61-OS screen press
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Of the several conventional printing processes in use today, it is likely that a few will emerge as the leaders in the manufacture of printed electronics. Offset lithography, in wide use for publication printing, probably will not take center stage. Letterpress is a disappearing process. Gravure, flexography and screen printing are the choices today, and most likely will remain in the forefront. These processes are joined by inkjet and vacuum deposition.
Printing electronic devices for mass consumption requires that manufacturing equipment be capable of producing endless lengths of roll-to-roll materials, using substrates that are challenging, and applying inks, coatings and other materials that continue to evolve in sophistication along with every other aspect of the industry.
Frost & Sullivan, the US-based research and consulting company, weighed in recently on manufacturing in printed electronics in one of its Insights reports: “In order to achieve widespread adoption, it is critical that the challenges involved in roll-to-roll processing or production of plastic electronic devices by the kilometer be addressed. Factors such as yield and resolution become important in the ability to drive down costs, which ultimately convinces end users into adoption.”
High volumes at high resolution will have the anticipated effect of eventually lowering the cost of printed electronics – significantly, perhaps.
Screen
Screen printing is already in fairly wide use for the production of printed electronics components, particularly in the medical device field. It is a highly versatile printing process, perhaps more so than others. It is used today to print on an amazing range of materials – fabrics both natural and synthetic, papers and paperboard, glass, metal, plastic – regardless of their size and caliper.
Perhaps the most significant advantage of screen printing is that it can apply a far greater thickness of ink to a substrate than any other printing process can. Screen is used to achieve tactile effects on labels and packages. For printed electronics the ability of a screen press to apply a wide range of ink thicknesses is critical, because complex products require densities that are rarely uniform.
It is also a fairly simple process, involving three basic components: Ink is pushed through a screen by a squeegee. (The term silkscreen is still used by some, even though silk has not been used in screen manufacture for a long time.) Screens are available in a wide range of meshes. The method by which the image to be printed is created on the screen is relatively simple and inexpensive.
There are two basic types of screen presses: flatbed and rotary. Flatbed is the original method, in which the screen is placed over the substrate, the squeegee moves across the screen to deposit the ink, and then the screen lifts and the substrate moves on. In a rotary screen unit the screen is cylindrical and rotates around a fixed squeegee.
Werner Kammann Maschinenfabrik GmbH has been manufacturing flatbed screen presses for decades. The German company, whose roll-to-roll presses are in use around the world, has been active in printed electronics since the 1990s.
“Our presses have been used to manufacture printed electronics components for quite a long time,” says Steve Gilbertson, vice president of sales and marketing for Kammann USA, the company’s North American arm. “In the medical industry alone our machines are used for printing of EKG sensors, defibrillator patches, and biotabs for diabetic applications – measuring glucose levels.”
Gilbertson says that two critical areas in the printing of electronic materials are control and drying. “We have invested heavily in how to control materials,” he says. Using a thinner material typically allows the user to have a less expensive carrier. Then you can also dry better. We have a tension controlled web through the machine, and we can run at relatively decent speeds with screen.” Decent, he adds, is about 20 to 25 meters per minute (65-80 feet per minute).
Drying is accomplished mostly by air and heat. “Some applications use infrared, some UV curing, but mostly it’s air,” Gilbertson says. “Controlling air across a web with a thin substrate is difficult. The tension has to be highly controlled, because you don’t want the web to grow or shrink too much. We typically want to heat it to 90° C and control it for a short period, without touching the printed surface.”
Kammann presses typically have two or three print stations. Gilbertson says that the presses are used to print “a lot of silver, dielectrics, truly organic materials. In the medical world, our presses print live enzymes for biotabs.” The machines Kammann makes to produce the biotabs are quite sophisticated, he adds. “There are up to 500 biotabs in one repeat.”
Kammann’s machines also are used to print small OLEDs. “OLEDs are hot,” says Gilbertson. “So are thin film batteries. Our presses have done all of these using flat screen, not rotary screen. With flat screen we can control the deposition better; we can control the line so that there is no stairstepping or broken images.”
Gilbertson adds that many companies engaged in printed electronics today are using clamshell type printing machines, “which are not high volume presses. They print a sheet, put it on a dryer rack, print the same sheet again and put it back on the dryer. There is a market for that, but it’s inefficient. As the industry moves from the introductory level to live production, that’s where we step in.”
One major need in the PE field today is education of product designers as to manufacturing technologies and capabilities, Gilbertson says. “The future of this industry is up to the creators. Our mission is to get to the electronics people, educate them as to what is possible. That’s the challenge. I don’t think the industry knows what is possible.”
Flexo
Flexography is a printing process that has moved from rough “rubber stamp” printing on corrugated to the production of the highest quality packaging products today. It utilizes a photopolymer plate with a raised image to transfer the ink to the substrate. Typical flexo presses today range from six or eight print stations up to 12 or 14.
One company that is investing in the PE market is Mark Andy, a Missouri-based manufacturer of narrow web flexographic presses. Mark Andy is grounded in the label printing industry, and is a global leader in that field. It’s also a company that knows the value of diversification, which is why its research and development manager, Kevin Manes, is spending a good amount of time with printed electronics.
“We recognize this as a fledgling market, but an unbelievable one,” says Manes. “It doesn’t just represent incremental growth, but explosive growth, as we see it. It offers so much for our converter base in terms of growth and diversification. We see this as an absolutely explosive potential, and also one that absolutely needs direction.”
Manes says that Mark Andy entered the PE arena about a decade ago as its customers were performing some rudimentary printed circuit applications, “like automotive seat heaters and rear window defrosters. It got converters involved with us, people thinking not just in terms of graphic ink but functional ink.”
The company became involved in RFID production in the early part of this decade, partnering with chip insert developers. Mark Andy, along with many others, thought that RFID would grow faster than it has, but Manes says that its involvement was a good step toward understanding printed electronics.
“The array of products and market space that printed electronics offers is vastly larger than what we were thinking about in terms simply of RFID,” Manes says. “We believe that what we are learning to develop in printed electronics – printing chips – we will get to the price points that will make RFID a go.”
Manes says that in his initial discussions with scientists about PE, they would invariably mention inkjet as the print process of choice. “Inkjet is the right format in the laboratory to deal with the very small quantities of material that they are using in R&D,” he says, “but not for production. As we started talking about having a liter of the stuff to pour into an ink pan, people would gasp at the thought of running a trial on a press. We have had to develop miniature ink pans, enclosed chambers on a small scale. The immediate concern is: How do we take the things that are done in the lab and begin to experiment with them on press? That’s where the dialog has been: how we establish this process.”
Flexographers are having to rethink some of their craft, Manes says, to be successful in PE. “We are used to printing dots, continuous tone in process images. That doesn’t work when you are trying to build a circuit. Flexo has a unique attribute in its ability to quickly image a line. That’s one of the chief advantages of flexo. The next layer has to do with feature size. We are driven to print finer and finer, to put down more components into a smaller area. The issue for us, for printed electronics, is frequency response. The holy grail is to print functional transistors.”
Manes says that the company is working on improving a limitation that flexography has in printing a fine gap, or tunnel, between the source and drain in a printed transistor.
“The finest gap we can print in flexo today is on the order of 50 microns. Any closer and the conductors flow together. That gap size, combined with the limited mobility of soluble organic semiconductors, limits us today to 100 Hz. With that we can print non-refresh displays on an active matrix, but not a streaming video.”
Mark Andy works closely with flexo plate manufacturers, aiming to improve the definition, surface energies and other plate attributes to produce a finer printed image. “Another issue is that semiconductive materials today are primarily being formulated with aromatic solvents, chlorinated solvents, which are damaging to photopolymer. That creates difficulty in the survivability of flexo plates,” Manes says. “There’s a lot of developmental work going on by the leaders in photopolymer plates to develop a solution for printed electronics work.”