Answers for Frequent Asked Questions about LCD
What is a TFT-LCD?
TFT-LCD, which stands for thin-film-transistor liquid crystal display, consists of glass substrates on the bottom, a color filter substrate on the top and liquid crystal in between. The special properties of the liquid crystal are manipulated to create images. These technology intensive products represent the next generation in displays. They are flat and thin, easy to carry around and consume less power. Refer to the Technology section for details.
What is the future outlook for TFT-LCDs?
TFT-LCDs solve the problems of portability, bulkiness and electro-magnetic emissions that CRT displays present. The current brightness and response time problems with TFT-LCDs are being addressed with technology advancements. The main applications for TFT-LCD was notebook PCs and desktop monitors, where they replace CRT`s thick tubes. Importantly, industrial applications and value-added niche markets such as medical equipment and instrument panels are growing fast. In the future, TFT-LCDs will be found in most every facet of life.
What are the common types of LCDs used in the market?
There are two types of LCDs commonly used in the market. One is TN (twisted nematic); this type of LCD has a narrow viewing angle. The other is STN (super-twisted Nematic); this has a wider viewing angle.
How many types of display methods are available?
Basically there are three types of display methods:
Segment type (7segments, 14, segments, 16 segments)
Dot Matrix type.
What types of connectors are used in LCDs?
The connectors used for LCDs are elastomeric (rubber connector), pin and flexible cable.
How many types of viewing modes are used in LCD glass?
There are three types of basic modes:
"Reflective", "Trans-missive", and "Trans-flective". " Tran-missive" and "Trans-flective" are used with an LED, EL, or CCFL backlight.
Is temperature a matter of concern in LCDs?
Yes, since the LCDs are widely used, some of the applications will be in extreme weather conditions where a wide temperature range is required. Normally the wide temperature range for a TN type LCD is -30 ~ 75C (operation Temp.) and -30 ~ 70C (storage temp.)
What are the types of driving modes used in LCDs?
A. There are two types of driving modes. One is "Static" and the other is Multiplexed". "Static" is a method of activating an LCD where each segment has an independent connection, whereas the "Multiplexed" method is a method where the LCD is driven by different channels. This method requires additional information such as "Duty" and "Bias". For example, 1/2 duty, 1/4 Bias defines an 8 x 1 LCD display.
How do we define the viewing direction?
We use four different viewing directions for LCDs: 3 o'clock, 6 o'clock, 9 o'clock and 12 o'clock viewing means you look at the LCD from its left side, 6 o'clock means you look at the LCD from the bottom...and so on. 6'clock is the most common viewing direction. Examples of 6 o'clock viewing are calculators and notebooks.
How do I know what type of LCD to use for our product?
That will really depends on a number of factors including multiplex driving scheme, temperature, voltage/power, artwork design, contrast requirements and cost budget.
The main categories which we may supply by our own facility are: TN, HTN, STN, and also special product such as cholesteric bi-stable displays and single polarizer high contrast reflective TN/HTN display.
How does LCD gets driven?
LCD is driven by AC (alternating current), it is best to use standard LCD driver IC to generate as this will simplified your development time. Alternatively, for simple TN segment drive, most MCU may emulate the AC current but would need a lot of background know how. For example, simple TN segment drive would require as low as 40Hz to as high as 120 Hz. Depending on the operating environment such as temperature and would affect the contrast of the display. Some experiment may be need to find the optimum operating condition.
What is ESD and does that affect LCDs?
ESD stands for electro-static discharge, it is arc or discharge of electrons between two surface when potential difference is great enough for discharge to take place. Because LCD is made mainly by non-conductive material such as glass and plastic, ESD can happens in LCD. In most cases, this can be easily overcome by design of peripheral connection and mounting.
What is ITO?
ITO stands for Indium Tin Oxide, more precisely, it is a mixture of indium oxide and tin oxide. Two important characteristic about this materials are:
Optically transparent with refractive index of between 1.4 and 1.8 which is close to that of soda lime glass (commonly use as based supporting substrate of LCD)
It is conductive inorganic material. The conductivity of this layer depends on the thickness
What ITO resistance is available? For ITO, the term of resistance is based on ohms per square. Commonly, ITO resistance which we used are 200, 120, 100, 80 and 25 depending on the requirements. The choice could be based on artwork design (such as trace width and area), current requirements, etc.
What interconnection technology is available to connect LCD to peripherals?
Commonly available through us are pin connection (with pitch commonly at 2.54mm, 1.8mm standard)
heat seal connection
FPC (flexible PC)
zebra or sometimes know as elastomer interconnector
What test is done to LCD?
Standard tests and control includes
100% Electrical test (that test for operating voltage, logic of display)
Dimensional and cosmetic inspection including viewing angle
Sample inspection using thermal, thermal cycle, and humidity
What is viewing angle?
A. LCD utilize physical phenomena of liquid crystal know as birefrigerance (which describe its properties of having two different refractive index depending on orientation) to create the light blocking and passing effect. Because of this, normally, the best viewing cone is best at a certain direction which is know as viewing angle.
It is commonly specify by: 12:00, 6:00, 3:00 and 9:00 except in special situation such as automotive which may require nonstandard direction.
How does CCFL and EL get driven?
Inverter is used to generate the AC current needs for both device. However, due to the high voltage requirements by CCFL, a transformer is often need to increase the supply voltage.
Does UV light affect LCDs?
The answer really depends on material selection and configuration. Because liquid crystal is primarily organic compounds, most liquid crystal material are susceptible to UV disintegration. There are some liquid crystal which are specially design for outdoor application that is more UV resistance. When use with UV blocking polarizer, the LCD can be resistance to ultra violet damage.
What other factors may affect life of LCD?
Moisture - extreme moisture exposure will cause sodium migration in the glass which will contaminate the liquid crystal, often result in increase of current and effect extrusion of ITO patterns.
Heat - as the polarizer is made of plastic (often Mylar based material), heat will de-stress the polarizer which will eliminate the polarizing effect. Special iodine type polarizer may be necessary to use at cost of lower polarizing efficiency (thus darker and diminishing contrast). This is trade off one must take when specify the product. The cut off is normally at around +85 degree centigrade.
What is alignment layer and what is TOP coating?
As previously explained, liquid crystal has two distinctive refractive index known as birefrigerance, in classical view and text book, liquid crystal used in common display application are in what we call nematic phase which is more like a glass rod which will turn when influence by electromagnetic force. Alignment layer is used to anchor the liquid crystal molecule at the glass surface forming the boundary condition for the display to work.
coating is simply a material put under the alignment material which act as a barrier layer. The main purpose is to reduce chance of ionic contamination which will deteriorate the display performance and shorten its life time. Very often, silicon dioxide is used for this purpose.
Why does my LCD turns dark when left under the sun and is there any problems with the display?
For most cases, both TN, HTN and STN utilize the phase known as nematic for display purpose. Within this phase, the liquid crystal has a "rod shape" exists within the solution which has fast response and has excellent electro-optic properties. This phase, however, only exists within a limited temperature range. The higher end of this temperature range is known as clearing point, above which, the liquid crystal lost its birefrigerance properties and cannot bend the light path anymore. Thus the polarizer will then be the only factor which affect incoming and out coming light. When the LCD is cooled down to below its clearing point, the display should be working again. The temperature for the clearing point varies greatly from material to material and you should contact our engineers regarding what you have. Normally a safe margin should be used to avoid clearing point when designing the display.