In a direct drive (or segment) display, a display panel typically is sandwiched between a common electrode layer and a backplane. The common electrode layer is a single electrode layer which covers the entire display area. The backplane comprises a substrate layer on which a desired graphic pattern is formed with a conductive material. The display panel may be an electrophoretic display, a liquid crystal display or other types of display, such as that prepared by the Gyricon technology.
An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably a dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between them. At least one of the electrodes, typically on the viewing side, is transparent. An electrophoretic dispersion composed of a dielectric solvent and charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the charged pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, may be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures were reported previously, for example, the partition-type EPD [M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026 and US applications, Ser. No. 60/443,893, filed Jan. 30, 2003 and Ser. No. 10/766,757, filed on Jan. 27, 2004) and the total internal reflection (TIR) type of EPD using microprisms or microgrooves as disclosed in M. A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311 (2001); and SID'02 Digest, pp. 522 (2002).
An improved EPD technology was disclosed in U.S. Pat. Nos. 6,930,818, 6,672,921 and 6,933,098, the contents of all of which are incorporated herein by reference in their entirety. The improved EPD comprises isolated cells formed from microcups and filled with charged pigment particles dispersed in a dielectric solvent. To confine and isolate the electrophoretic dispersion in the cells, the filled cells are top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from the group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
In an electrophoretic segment display, the charged pigment particles in the display panel in the area of the desired graphic pattern may migrate to either the side of the common electrode layer or the side of the backplane, depending on the voltage difference between the common electrode layer and the conductive pattern.
A liquid crystal display comprising display cells prepared by the microcup technology and filled with a liquid crystal composition optionally comprising a dichroic dye was disclosed in U.S. Pat. Nos. 6,795,138 and 6,784,953.
A display panel may also be prepared by the Gyricon technology (as disclosed in U.S. Pat. No. 6,588,131 assigned to Gyricon Media, Inc. and U.S. Pat. Nos. 6,497,942, and 5,754,332 assigned to Xerox). A Gyricon sheet is a thin layer of transparent plastic in which millions of small beads, somewhat like toner particles, are randomly dispersed. The beads, each contained in an oil-filled cavity, are free to rotate within those cavities. The beads are “bichromal” with hemispheres of two contrasting colors (e.g., black and white, red and white), and charged so they exhibit an electrical dipole. When a voltage is applied to the surface of the sheet, the beads rotate to present one colored side to the viewer. Voltages can be applied to the surface to create images such as text and pictures. The image will persist until new voltage patterns are applied.
The desired graphic pattern in a segment display may be alphabet letters, numerical displays (such as those utilizing the well-known 7 or 14 segment electrodes), logos, signs or other graphic designs.
The backplane is usually formed of a flexible or rigid printed circuit board. The conventional process for the manufacture of a printed circuit board involves multiple steps. The segment electrodes and conductive lines are first fabricated on the opposite sides of a non-conductive substrate layer laminated or coated with a conductive metal, and electrically connected to each other through copper-plated via holes. During formation of the via holes, steps of drilling, electroless plating, electroplating and plugging the copper-plated via holes with a non-conductive resin are employed. Since the entire area on each segment electrode must be electrically conductive and flat, steps of brushing and polishing are then needed to remove any protrusions associated with the copper-plated via holes plugged with the non-conductive resin, before the subsequent plating and segment patterning steps. In addition, the gaps between segment electrodes formed in the conventional process usually have a depth in the range of about 30 to about 60 um or even higher. Gaps of such a significant depth could be detrimental to the display panel laminated over the backplane, especially the display panel formed from the microcup technology. Therefore, additional gap-plugging steps such as solder mask coating, photolithography and brushing are required to planarize the gaps. The entire process therefore is not only costly and complex but also of a low yield.
It is noted that the whole content of each document referred to in this application is incorporated by reference into this application in its entirety.