Twisted Nematic (TN), Super Twisted Nematic (STN), Film Compensated STN (FSTN), and TFT are the terms used to describe four types of Liquid Crystal Displays, each twisting the orientation of the light passing through the Liquid Crystal Display structure differently to effect contrast and coloration.
TN displays have a twist ( the rotation of the molecules from one plane of the display to the other) of 90 degrees or less. All passive direct drive , active matrix, and most passive low level (x2 to x32) multiplexed XRD’s have a 90 degree twist.
The basic Twisted Nematic (TN) XRD consists of a layer of liquid crystal material supported by two glass plates. The liquid crystal material is a mixture of long, cylindrically shaped molecules with different electrical and optical properties, depending on direction
On the inner surfaces of the glass plates are transparent electrodes, which are patterned to form the desired visual image. The inner surfaces are coated with a polymer, which is rubbed so that the liquid crystal material at one surface lies perpendicular to the other. Across the film of liquid crystal, the molecules form a 90° twist.
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On the outer surface of the glass plates, polarizers are placed so they are parallel to the liquid crystal orientation and perpendicular to each other. in the "off" state, light entering the first polarizer is guided by the liquid crystal layer twist to the second polarizer, through which it is transmitted. When the cell is energized, the LC material is aligned with the electric field; light transmitted through the first polarizer is blocked by the second polarizer, forming a dark image. The effect may be reversed if the polarizers are placed parallel to each other, and a light image on a dark background is formed |
FEATURES:
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
SUPER TWISTED NEMATIC (STN) LCDs
Super Twisted Nematic LCD’s have a twist that is greater than 90 but less than 360 degrees. Currently most STN displays are made with a twist between 180 and 270 degrees. The higher twist angles cause steeper threshold curves which put the on and off voltages closer together. The steeper thresholds allow multiplex rates greater than 32 to be achieved.
FEATURES:
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
Film Compensated Super Twisted Nematic (FSTN) LCDs
The most recent advance has been the introduction of Film compensated Super Twisted Nematic (FSTN) displays. This adds a retardation film to the STN display that compensates for the color added by the birefringence effect. This allows a black and white display to be produced and provides for a higher contrast and wider viewing angle.
The FSTN technology comes in a single coloration, Black characters on a White / Gray background. Of the three technologies listed here, it is the most expensive, but it has better viewing angles and contrast that the STN technology listed above.
FEATURES:
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)
SMT
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
COB
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 XRD standard Character LCD modules are of the Chip-On-Board design.
Advantages:
Very compact
Space savings over Surface Mount Technology assembly.
Cost savvy comparatively with SMT, since there is no plastic package.
COG
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 XRD with the regular mass production delivery time.
Advantages:
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.
Disadvantages:
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.
TAB
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 XRD.
Advantages:
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.
Disadvantages:
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.
