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Liquid Crystal Technologies


TN Twisted Nematic. A type of liquid crystal whereas the alignment surface and therefore the LC molecules are oriented 90 from each surface of glass. Produces images in two modes: Positive and Negative. Positive Mode provides white background with black segments. Negative Mode provides black background and white segments.
When two polarizing filters are arranged along perpendicular axes, as in the first illustration, light passes through the lead filter and follows the helix arrangement of the liquid crystal molecules. The light is twisted 90 degrees, thus allowing it to pass through the lower filter. When voltage is applied, however, the liquid crystal molecules straighten out of their helix pattern. Light is blocked by lower filter and the screen appears black because of there being no twisting effect.
The multiplex rate is equal to the number of rows that can be displayed simultaneously. For example, a display with a multiplex rate of 16 can display 16 rows of information simultaneously.

  Backgrounds: Black, White
  Viewing Angle: 45 degrees (typical)
  Multiplex Ratio: up to 16:1
  Most inexpensive
  Response Time: 150 msec at 4.7 V and 64 Hz


STN Super Twisted Nematic. A high information content LCD that uses adjustable interference of two optical modes to achieve a large number of multiplexed lines, higher contrast and higher level of gray scale. It results in the birefringence mode. An improved twisted nematic fluid ( twist or greater) which has better contrast and optimum viewing range than standard twisted nematic.
The graph shows a comparison of a voltage versus transmitted light curve of a typical TN and a supertwist nematic (STN) display. (Generally, a greater twist angle equates to higher multiplexibility.) The V90 and V10 points on the graph indicate the voltages that yield 90 percent and 10 percent light transmission, respectively.
As the figure shows, STN displays have a steeper curve than TN displays, which allows higher multiplex levels for STN displays. (STNs were developed primarily to overcome the difficulties encountered in multiplexing TN displays.)
The multiplex rate is equal to the number of rows that can be displayed simultaneously. For example, a display with a multiplex rate of 400 can display 400 rows of information simultaneously.

  Backgrounds: Yellow-Green, Gray, Blue
  Viewing Angle: 75 (typical), 90 (maximum) degrees
  Multiplex Ratio: up to 480:1
  Response Time: 250 msec at 4.5V (lower than TN)
  Suitable for Graphic applications 


FSTN  Film Compensation Super Twisted Nematic. LCD with an extra film added to the outside of the cell to compensate the color shift of blue on green to black on white. The film is made of a polymer with double refraction to remove the interference of colors. It results in retardation compensation.
The film (the top layer in the figure) is placed in the display, either under or over the top polarizer. Some film compensation systems use two films, one on the rear that serves as a collimator, and one on the front that serves as a dispersion film to broaden the viewing cone. Film compensation improves the viewing angle, but does not affect switching time. FSTN is all standard STN displays with a polymer film applied to the glass as a compensation layer instead of the second cell as in the case of the DSTN. This simpler and more importantly cost effective method provides the preferred black on white image for this display technology. 

  Backgrounds: Black, White
  Viewing Angle: 80 degrees (typical)
  Multiplex Ratio: up to 480:1 (segments / commons)
  Response Time: 250 msec at 4.5V (lower than TN)

IC Mounting Technologies


Surface Mounting Technology (SMT) using quad flat packages on printed circuit boards was the most popular at the early years of liquid crystal display industry, and is still available for mass production.
Plastic Quad Flat Package (QFP) represents itself as a flat rectangular integrated circuit package with its leads projecting from all four sides of the package without radius. Used with surface mounting method. Made of black epoxy resin. Very moisture absorbent


Chip-on-Board (COB) is a popular IC mounting method that provides wire bonding as the direct attachment of bare die to laminated printed circuit boards. The LCD driver is formatted into an area on the PCB. Electrical connections are made by micro diameter gold wires. The entire area is then covered with epoxy. All of DLC standard Character LCD modules are of the Chip-On-Board design.
  Very compact
  Space savings over Surface Mount Technology assembly.
  Cost savvy comparatively with SMT, since there is no plastic package.


Chip-On-Glass is one of the high-tech mounting methods that uses Gold Bump or Flip Chip IC, and implemented in most compact applications. Chip-On-Glass integrated circuits were first introduced by Epson. In flip-chip mounting, the IC chip is not packaged but is mounted directly onto the PCB as a bare chip. Because there is no package, the mounted footprint of the IC can be minimized, along with the required size of the PCB. This technology reduces a mounting area and is better suited to handling high-speed or high-frequency signals. Currently, there are 12 standard C.O.G. LCD modules available at DLC with the regular mass production delivery time.
1. Very space economical. Chip-On-Glass LCD modules can be as thin as 2 mm.
2. Cost effective over COB, especially in graphic LCD modules, because much less IC"s are required.
3. More reliable than TAB because of the weakness in the bond area of TAB.
1. COG can only be used at a certain resolution level where the lines are not too fine. At very fine pitches COG becomes difficult to test, and TAB is the preferred approach.
2. It may be more cost-effective to use TAB or COB, if a designer has to integrate a keypad or indicator around the display.
3. The active area is not centered within the outline but offset, because of the area where the circuits are.
Since the Chip-On-Glass integrated circuit has been invented by Epson, COG technology became very popular due to the demand for more compact applications. In the near future we will see this IC mounting method finding its applications in many other equipment than cellular phones, PDA"s, computer network servers, satellite receivers, etc.


Tape Automated Bonding (TAB) LCD driver or controller electronics are encapsulated in a thin, hard bubble package, of which the drive leads extend from the bubble package on a thin plastic substrate. The adhesive along the edges is used to attach the TAB to the LCD glass and/or PCB.
Tape Automated Bonding IC mounting method uses the same type of integrated circuits as Chip-On-Glass technology - Gold Bumped Flip Chips. After this type IC chip has been produced, we a gold bump is placed on the IC chip and then sealed onto the polymide board. (This procedure is called ILB or Inner Lead Bonding) and is how the TCP IC is produced. TAB LCD modules are always custom made from DLC.
  Offers compactness (IC and its interfacing circuitry can be bent behind the LCD glass panel).
  Some times more cost-effective than Chip-On-Glass, if a designer has to integrate a keypad or indicator around the display.
  The active area is centered (differently from COG).
  Can provide interfacing at very fine pitches.
  The bonding area is weak. Less reliable than COG.
  More expensive than COG. Even though TAB LCD modules use the same type of IC as COG, tape automated bonding requires a package.



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