A Review of the Opportunities and Processes for Printed Electronics (Part 3): Materials, Process Developments

Estimated reading time: 10 minutes

Table 3: Applications and substrates

Flexible materials are a key characteristic of PE. Many products traditionally utilize glass to protect the active layers. To replace glass, the flexible substrates need to be an effective barrier against oxygen and water vapor, be sufficiently strong not to rip or tear and, if a cover, transparent to visible light. Many plastics, such as such as Mylar, polyimide, PET and ORMOCER, have these characteristics. Substrates can even be papers and paper hybrids. Applications and substrates are summarized in Table 3.

The majority of the world's PET production is for synthetic fibers (in excess of 60%) with bottle production accounting for around 30% of global demand. In discussing textile applications, PET is generally referred to as simply polyester while PET is used most often to refer to packaging applications. The polyester industry makes up about 18% of world polymer production, followed by PE and polypropylene (PP).

PET consists of polymerized units of the monomer ethylene terephthalate, with repeating C10H8O4 units. PET is commonly recycled, and has the number 1 as its recycling symbol. PEN is a polyester with good barrier properties (even better than PET). Because it provides a good oxygen barrier, it is well-suited for bottling beverages, such as beer, that are susceptible to oxidation. It is also used in making high performance sailcloth. PPG’s Teslin is a polyolefin matrix with high silica filler.

Table 4: Characteristics of Substrates

DuPont’s Teijin Films are polyester films that are pre-treated on both surfaces to promote adhesion to most industrial coatings. Melinex, Mylar, Cronar and Teonex are registered trademarks of DuPont Teijin Films. Teslin and Teijin Films are designed for PE application like RFID, batteries, OLEDs and sensors.

PEEK is a semicrystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained at high temperatures. The Young's modulus is 3.6 GPa and its tensile strength is 90 to 100 MPa. PEEK has a glass transition temperature of around 143°C and melts at around 343°C (662°F). It is highly resistant to thermal degradation and attack by both organic and aqueous environments. At high temperatures, it is attacked by halogens and strong Bronsted and Lewis acids, as well as some halogenated compounds and aromatic hydrocarbons.

If it is very thin, glass can also be a flexible substrate. AGC and Corning have both developed ultra-thin glass (only 0.1mm thick) for OLEDs and other applications. Alkali-free glass is composed of silicon dioxide, boron oxide and aluminum oxide and is free of sodium and potassium, so it is used widely as a substrate for thin-film transistor liquid crystal display (TFT LCD) and OLED. Soda-lime glass is composed of silicon dioxide, sodium oxide and calcium oxide and is widely used in construction, automotives and many electronics devices.

Recently, a research group at the Hebrew University of Jerusalem has developed a nanotechnology ink that self-sinters at room temperature. The researchers have demonstrated that the new silver nanoparticle-based ink can be deposited and provides post-print conductive properties without a post-sintering process step. Instead, the properties of the nanomaterial are used to make the ink aggregate and self-sinter. The elimination of the temperature needs associated with normal sintering processes opens the door to processing cost reduction and the use of lower cost, temperature-limited substrates, including paper and lower melt-temperature plastics (Table 4).

Other companies are developing catalytic inks that can be electrolessly or electrolytically plated to achieve the necessary conductivity. One company of note—Conductive Inkjet Technology—has been developing a range of equipment for printing catalytic circuit patterns on material webs in a roll-to-roll fashion. When the catalytic ink is electrolessly plated, the company reports that the conductivity ranges from 50mΩ to 20mΩ per square. The company’s technology, which has an approach that is similar to a technology developed in Silicon Valley around 1990, can reliably produce typical feature sizes down to less than 250μm, which is finer than earlier technology that used a laser printer and catalytic toner. While some have performed its initial process and equipment development on an inkjet platform, the company's literature indicates that they can produce 50μm features using flexographic printing, which could prove to be an important departure.

Flexographic printing, as a follow-on technology in the same market space, opens the door to higher production rates and offers the potential to print both sides of a two-metal layer circuit in a single pass. With some creativity, it is possible that inkjet printing could be similarly adapted. One concern about this technology is that it will produce circuits that will build both vertically and laterally, reducing the space between circuits and potentially causing shorts.

Post plate-up technology shares many of the same concerns as laser printing technology in terms of managing feature dimensions in process. Metrics are usually predicated on three items:

• Resolution, or how many dots per unit length
• Drop size, or how small each dot is
• Print rate, or dots per second

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