LCD Technology

Types of LCD are: Twisted Nematic (TN), High Twisted Nematic (HTN), Super Twisted Nematic (STN), Film Compensated Super Twisted Nematic (FSTN), Double Super Twisted Nematic (DSTN)

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 LCD´s have a 90 degree twist. 
 
 The basic Twisted Nematic (TN) LCD 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.

    On the outer surface of the glass plates, polarisers 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 polarisers are placed parallel to each other, and a light image on a dark background is formed.

    The TN technology comes in a single coloration; it is Black characters on a gray background. It is the least expensive, but has the lowest visual quality, primarily in viewing angle.

 

HTN (High Twisted Nematic) displays are based on a higher molecular twist (usually 110°) than TN (90°) and therefore offer wider viewing angles and improved contrast. In fact, these HTN products offer viewing characteristics close to those of STN technology.

    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.

    In this type of display, the LC material undergoes a greater than 90° twist from plate to plate; typical values range from 180 to 270°. The polarisers in this case are not mounted parallel to the LC at the surface but rather at some angle. The cell, therefore, does not work on a light "guiding" principle, as in Twisted Nematic LCDs, but instead on a birefringence principle. The position of the polarisers, the cell thickness, and the birefringence of the LC are carefully chosen to result in a particular colour in the "off" state. Usually, this is a yellow-green to maximize the contrast ratio. The LC in the cell is "super twisted" that will give it the ability to use a high multiplex rate. As the twist is increased, the LC molecules in the middle of the layer are aligned with the applied electric field by smaller changes in voltage. This gives rise to a very steep transmission vs. voltage curve, allowing up to 240-line multiplexing.

    The STN technology comes in two colorations, Green STN and Silver STN. The STN-Green has Dark Violet / Black characters on a Green background. The STN-Silver has Dark Blue / Black characters on a Silver background. It is in the middle of the road as far as cost, but has very good visual quality. The contrast is similar to TN technology.

 

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 colour 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 than the STN technology 

 

DSTN was the first commercial black and white conversion of the STN display. DSTN displays are actually two distinct STN filled glass cells glued together. The first is a LCD display, the second is a glass cell without electrodes or polarisers filled with LC material for use as a compensator which increases contrast and gives the black on white appearance.

    DSTN provides better contrast than STN and FSTN, and offers automatic contrast compensation with temperature. Its response time is significantly enhanced. DSTN reduces the tendency of a screen to be slightly red, green or blue. Since its polarizer mode is negative, DSTN LCDs need backlighting, this is provided by either LED or CCFL only. 

 

viewing angle is the maximum angle at which a display can be viewed with acceptable visual performance. The graphics may seem  poorly saturated, of poor contrast, blurry or too faint outside the stated viewing angle range, the exact viewing angle range depends on the type of the LCD.

Viewing direction is the direction from which we can see the graphic on LCD panel clearly. We name it after hours on a clock. Such as; 9:00, 12:00, 3:00 or 6:00

For example: If the viewing angle for TN LCD is 6:00 O'clock then the graphics would be seen blury from angle 9:00 andd 3:00 O'clock.

 

It the actual area of tha LCD display in use. Viewing area is always smaller than the actual area of the LCD.

the pixel pitch or dot pitch is the distance in millimeters from the center of a pixel to the center of the adjacent pixel. Since pixel pitch indicates the amount of space between two pixels, a smaller pixel pitch means there is less empty space between pixels. This equates to higher pixel density and improved screen resolution.Pixel pitch is important because it influences the optimal viewing distance for your display. An image achieves smoother borders and finer detail with lower pixel pitch values. This allows the viewer to stand closer to the screen and enjoy a clear image without the distraction of discerning individual pixels.

How to calculate pixel pitch?

Viewing area / Resolution = Pixel pitch

for example; if the size of the display is 285.7mm*214.3mm and resolution is 1024*768, so the pixel pitch is ;

285.7/1024 = 214.3/768 = 0.279mm

 

There are two types of Viewing modes for the LCD: Positive and negative mode

1. Positive Viewing Mode

A positive image on LCD display when the pixel "OFF" is transparent and pixel "ON" is opaque. This mode of operation is favored in the applications where ambient light is high and it will help in creating contrast on the display. 

2. negative Viewing Mode

A negative image on LCD is displayed when the "OFF" pixel is opaque and "ON" pixel is transparent. This mode is typically only used when there is backlight and ambient light is dim.

 

ITO glass and liquid crystal are transparent, the images we see on the LCD screen are 100% relied on the front and back polarizer films. If any one polarizer film is missing, we can see nothing.

Each LCD has two polarizers; front polarizer and rear polarizer. Front polarizer is always transmissive type of polarizer. Rear polarizer can be transmissive or reflective depending on the application and use of backlight.

There are three type of polarizers for LCD:

1. Transmissive Polarizer: 

Transmissive polarizer is clear polarizer. It is the most commonly used polarizer for LCDs. transmissive polarizer is used where backlight is to be used.

2. Reflective Polarizer: 

Reflective Polarizer is used where backlight is not being used. This type of polarizers are used when LCDs are operated in ambient lights. 

3. Transflective polarizer:

Transflective the the most expensive type of polarizer. Its a semireflective film. The light coming from front glass cant go through the back polarizer but light coming from backlight can be seen from the front polarizer.

 

 

Making passive matrix LCDs is multistage process. the cell forming process is as the following:

1. Cutting is done by diamond saw or scribe. Cutting is followed by polishig which done by the porcess called lapping and then substrates are washed. After drying the both glass substrates are coated with silicon dioxide (SiO2). 

2. To make transparent electrodes, surfaces of substrates are coated with thin ITO film (Indium Tin oxide). The mask is applied to make a desired pattern either by screen-printing or photolithography process. Then the unwanted ITO areas are etched away with chemical etching process.

3. After patterning of ITO is done on substrate glasses, the substrates are coated with polymer which allow the liquid crystals to align properly with the surface.

4. After coating is done, stroke the polymer coat in a single direction with soft material. This can result in small parallel grooves being etched into the polymer, or it may simply stretch the polymer coat. In any case, this process forces the liquid crystals to lie parallel to the direction of the stroke. The crystals may be aligned another way, by evaporating silicon oxide onto the glass surface at an oblique angle.

5. A sealing resin is next applied to the substrates, followed by plastic spacers that will give the liquid crystal cell the proper thickness. Next, the liquid crystal material is injected into the appropriate area between the two glass substrates.

6. To make LCDs more visible, polarizers are added. They are glued to the glass substrated with the polymer adhesives.

7. After polarizer film is attached, LCD cell is allowed to age. And at finaly stage, it is mounted on circuit boards which contains drive  and control electronics.

ITO stands for Indium Tin Oxide. The advatage of ITO film in LCD cell formation procedure is, it is highly transparent and well conductive. ITO film is applied on substrate glass of LCD (both front and back) and patterned to create conductive paths on substrate surface for driving the segments on LCD panel.

 

In LCD manufacturing, several printing and dispensing technologies are used in order to dispense various layers and seals. Dispensing is mainly used for low throughput and variable design displays. Printing technologies are used for high precision and high volume production cycles.

Screen printing is used for seal printing and thick layer printing. Flexo-printing is used for polyimide layer printing and printing of thin layers with high precision.

Screen printing is a technology, where the print material is pressed by a squeegee through a mesh that is fixed on a frame. The mesh carries a photoresist layer with openings defining the pattern to be printed.

What is Screen Frame?

The screen frame is the most fundamental element of the screen-printing process. The function of the screen frame is to providing a means for mounting the mesh, withstanding the force of the tensioned mesh, standing up to additional forces applied during printing, remaining flat and square and being light enough to handle easily.

Once tension is applied to the mesh and the mesh is affixed to the frame, the flatness of the frame becomes critical. The frame's job is to resist the forces of that tension. A suitable frame will remain flat and square for the time of use. Among other things, distortion of the frame causes mesh threads to follow the same, curved direction as the beams, resulting in mesh distortion at the edges of the screen. This drastically reduces the screen's "sweet spot" or maximum print area. It also leads to tension inconsistencies that further complicate your ability to achieve good registration from colour to colour.

 

2. What is mesh?

A coarse mesh will allow the printing material to pass through the screen easily. This will result in thick layers, fast prints and low resolution. A fine mesh will create difficulties in printing a bright design on dark-coloured substrates and it is difficult to press viscose material through the screen.For creating the image, the mesh is coated by a photosensitive emulsion, which is exposed using a mask, made from a glass or polymer film. The unexposed portion of the coating is washed off afterward. This portion let the seal material go through and you have its pattern on the substrate. 

 

3. What is squeegee?

The squeegee plays a major role in screen printing. It Forces the print material into the mesh - influenced by mesh opening size and viscosity. The possible speed of the squeegee depends again on the hardness of the squeegee and the viscosity of the printed material. It has to be adjusted well with the printing pressure.

 

 

This thin film forming technology is based on both flexographic printing technology and gravure printing technology. The doctor blade distributes a layer of the polyimide on the anilox roll. This roll carries a pattern of gravures which are filled with a volume of polyimide corresponding to a layer 3,5 times thicker than the desired printing thickness. This makes good for the transfer loss. Then the polyimde layer is transferred to the printing roll with the desired pattern. The printing roll is covered by a compressible polymer, called letterpress. From there the coating material is transferred to the glass plate. The resolution of this technology is better than screen printing. The printed layers are thin films with a thickness of less than a micron (40-100nm).

For small cells, the filling of displays with liquid crystals is done by sucking the liquid crystals into the cell after evacuation of the cell. For large LCDs this method is too slow and a new technology for integrated filling of LCDs in a vacuum assembly machine has been developed. This technology is called One Drop Filling ODF-Technology, because the exact volume of liquid crystals which is necessary to fill the cell is dispensed on the lower glass plate before cell assembly is done. This method requires extremely precise dispensing technology because too much liquid would cause an overflow of the cell during assembly and pressing, while too less liquid would create bubbles in the LCD. Vacuum assembly is necessary in order to avoid gas bubble enclosure.

 

1. Metal Pin Connection:

Metal pins are most commonly used for LCD connections. Metal pins can be soldered directly to the circuit boards.