Nikkei Electronics Asia -- January 2007
Tech Columns
Flexible Displays Challenge Materials, Processes

Jan 2, 2007 16:47 Nikkei Electronics Asia

Flexible displays are one of the newest uses for flex circuits and associated technologies. They are also one of the most challenging, since they demand greater performance and thermal stability than more traditional flex circuit applications. Although much attention has been focused on paper replacements, especially newspapers, the potential uses of flexible displays are quite broad. These include displays for cell phones, point-of-purchase displays, flexible display panels and electronic displays, as well as displays in various types of handheld system. All of these applications are still in R&D, but some manufacturing infrastructure in the form of pilot facilities is now beginning to be put into place.

However, making the leap from developing and perfecting these new technologies to incorporating them into products has been slow, in part because of the lack of materials supply chains. In addition, a greater emphasis is now being placed on the development of backplanes specifically for flexible displays. Yields are also an issue for some providers.

Substrate Materials
A wide range of materials and manufacturing processes are being used for the various parts that make up a display. Substrates, for example, which must have the optimum thermal, optical, mechanical and chemical characteristics, may be composed of thin glass, plastic or metal foil. The rigidity and fracturability of glass thin enough to be bent make it an undesirable material for flexible displays.

Like glass, metal foils can handle high processing temperatures and are impervious to air and water. They are also relatively low in cost, especially the stainless steel usually employed for this purpose. However, they are only usable for displays such as OLEDs and electrophoretic types, which are non-transmissive.

Plastics remain the most commonly used substrate material for flexible displays. The biggest issue with polymers is their relatively high coefficient of thermal expansion (CTE), or the ability to retain dimensional stability, after processing at high temperatures. While the CTEs of glass and metal are quite low, at approx 5ppm/¡ëC, the CTEs of polymers used for flexible displays are an order of magnitude or more higher, at 50 to 100ppm/¡ëC. Heat stabilization can reduce this to about 25ppm/¡ëC.

Unlike metal or glass, plastics are not impervious to air and water and, in addition, are not as transparent as glass. They are also susceptible to damage by solvents used at various stages in the manufacturing process, which means that they are likely to require encapsulation.

Display Types, Options
Several different options exist for the display itself, including traditional and more expensive types such as OLEDs and LCDs, as well as electrochromic or electrophoretic displays. Electrophoretic is the technology that has seen the greatest gains in terms of high volumes for producing flexible displays on plastic, in part because it requires less encapsulation and fewer additional components. Its applications to date have been primarily point-of-purchase signage, although other types of signage, electronic display cards and electronic readers are expected to increase. LCDs and OLEDs involve challenges such as viewing angle and visual distortions for LCDs and water sensitivity and easily damaged thin-film layers for OLEDs. That implies that OLEDs require encapsulation, but this has not yet been achieved.

Displays formed using electrophoretic processes have slow switching speeds and some are limited to only one or two colors, making them unlikely candidates for video. Those produced with electrochromic methods have low switching voltages, but require an additional current for maintaining stability and also suffer from slow switching speeds.

Backplanes can be based on either a passive matrix or a simple direct drive, on one hand, or an active matrix with either organic or inorganic transistors, on the other. Flexible panels based on simple direct-drive or passive matrix backplanes are already available.

Electronic readers and rollable electronic "paper" have been much discussed as highly desirable features of flexible display panels for applications such as newspapers. However many applications do not require continual flexibility during operation and a majority of uses for flexible displays is likely to be small and/or simple displays such as point-of-purchase signage, electronic display cards and electronic shelf labels.

by Ann R. Thryft

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