Tutorial #114
George Riley
May 2011
Printed electronics applies well-established printing techniques to create a wide range of printed electronic devices.
Stencil printing of photoresists and other materials has long been used in silicon-based electronics. Inkjet printing of electronics materials continues to increase its capabilities.
Meanwhile, continuing developments combining conventional printing techniques with new media and inks are creating innovative active and passive printed electronic devices.
Printed electronics potentially offer lower costs and more versatile formats than present silicon-based electronics. However, while printed electronics have found high volume applications such as RFID tags and E-book readers, formidable obstacles are delaying their wider adoption.
TECHNOLOGIES
Inks Printing “inks” may be conductive or insulating fluids. Conductive printing inks are based upon dispersions of metallic oxide particles or nano-particles. Silver particles are preferred for high conductivity. Carbon inks are a lower cost, lower conductivity alternative. Carbon is also used for printed resistors and similar applications.
Printed polymers include semiconductors and dielectrics. Polymers may be doped to form diodes, transistors, or other active devices. Light-absorbing inks generate electricity in solar cells.
Printed inks combine with gels to form printed batteries. Printed conductors combine with electrophoretic materials to create the “printed” pages of some e-books.
Substrates Flexible printed electronics substrates broaden the range of applications. Unlike conventional silicon electronics, printed electronic devices need not remain flat. They may be curved in their final form or even be rolled up in target applications such as roll-up displays.
Flexible substrates can also allow reel-to-reel printing for high throughput at lower cost. Flexible polymeric substrate materials include Poly-Foil (PET), PEN, and polyimide (PI).
Printers The flatbed screen printers commonly used in silicon electronics are also suitable for flexible electronics. A screen, unique to each pattern, is first covered with a photosensitive emulsion. The emulsion is then exposed through an image of the desired circuitry, converting the circuitry pattern to openings in the screen.
The patterned screen is placed in the printer. The conductive or insulating ink is forced onto the substrate through the screen openings, replicating the pattern on the substrate.
Web-fed flexographic printers mount an image-patterned polymeric plate on a rotating printing plate cylinder. Other rotating cylinders transfer ink to the plate cylinder, which prints the image on the substrate. Web-fed roll to roll printing speeds may be hundreds of feet per minute.
Rotogravure printers have a large, etched gravure cylinder carrying controlled amounts of ink to an impression cylinder that presses the flexible substrate against the gravure cylinder, transferring the pattern. High speed and high costs limit it to mega-volume products.
Inkjet printing has proven effective for applying conductive epoxies and anisotropic conductive pastes for printed Radio-Frequency Identification (RFID) tag antenna assemblies. Printed RFID inlays incorporate operations that are already proven in semiconductor and electronics packaging.
Stationary jets dispense fluids onto the moving web without touching it. The jet cycles up to 200 times per second and “jets-on-the-fly”, delivering continuous dots onto the web in a fast and accurate process.
PRINTED ELECTRONICS LIMITATIONS
Electrical Performance. Printed conductors have relatively higher electrical resistance and lower charge mobility than present metal conductors. Thus they are not suitable for high current nor high speed electronics applications.
Lifetime Active organic electronics are subject to degradation from heat, moisture, and oxygen exposure, limiting their useful lifetime. Adding protective packaging increases size, weight, and cost, while limiting or eliminating flexibility.
Market Barriers Printed electronics is still in development. Like through-silicon via development, widespread application will require developing standards and design rules, design and test software, and an assured supply chain.
Volume production costs must low enough to make conversion to organics compelling.
The installed base for production of silicon electronics represents a large current investment that will not be easily given up to the newcomer. In addition, low performance organic electronics are not replacements for high performance silicon electronics.
Figure 1 compares the relative strengths and weaknesses of printed electronics and silicon-based electronics. As shown, they are complimentary, with low-cost low- performance printed electronics opening potential markets not available to high-cost high-performance silicon electronics.
Figure 1. Strengths and weaknesses of printed and silicon-based electronics. Source: Wikimedia Commons
CONCLUSION
If printed electronics develops manufacturing of marketable high volume low-cost products, it should both enlarge and capture that portion of the market from silicon.
FOR MORE INFORMATION
Bauer, Charles. E. & Neuhaus, Herbert J., “Reading the Fine Print: Challenges and Outlook for Printed Electronics,” Proceedings of SMTA International, November 2010, page 258.
Fenner, Daniel, “Applications Methods and Material Sets for Printed Electronics,” Proceedings of SMTA International, November 2010, page 334.
Baron, David, Bruening, Frank, & Taylor, Robin, “Organic and Printed Electronics: A View from the Traditional Market Place” Proceedings of SMTA International, November 2010, page 318.
Babiarz, Alec J., “Jetting Fluids in a Wide Variety of New Electronics and Semiconductor Non-Traditional Packaging & Assembly Applications,” Proceedings of SMTA Pan Pacific Symposium, January 2009.