Backlight Module Principle And Bad Analysis Method
Sep 07, 2021
Backlight module principle and bad analysis method
1. The structure of liquid crystal display (TFT-LCD):
a. LCD glass substrate
Mainly used to control the display of graphics or text
b. Color filter (Color Filter; CF)
Mainly provide three colors of red, basket and green
C. Driver IC
The main function is to control the conduction of each pixel electrode on the panel, so that the PC (or TV) image signal can be correctly displayed on the LCD panel.
d. Polarizing plate
The main function is to convert the non-polarized aurora into a polarized aurora, and the liquid crystal display uses this polarized aurora plus the twisting characteristics of the liquid crystal to control the passage of light and form brightness and darkness.
e. Surface light source
Because TFT-LCD itself does not emit light, an external surface light source is needed to provide a uniform, high-brightness, wide-viewing display effect
1-1. Types of liquid crystal displays:
a. Twisted Nematic (Twisted Nematic; TN)
b. Super Twisted Nematic (Super Twisted Nematic; STN)
c. Color film type (Thin Film Transistors; TFT)
1-2. Types of surface light sources:
a. Front light film group
Located in front of LCD glass chips and color filters, mainly used for reflective and semi-reflective panels
b. Backlight film group
Located behind the LCD glass chip and color filter, mainly used for transmissive panels
2. What is a Back Light Module
Since the LCD panel itself does not have light-emitting characteristics, a light-emitting source must be added to the LCD panel to achieve the display effect. The backlight module (Backlight) is an optical component that provides a back light source in LCD display products. , The backlight module is a key component of the LCD display. Therefore, its development is closely related to the downstream LCD display.
3. Backlight module design mode:
a.Side light type:
Usually only a single lamp tube is used as the light source, and a wedge-shaped light guide plate design (direct-down design often uses a "flat plate") to strengthen the control of the direction of light travel, which is relatively difficult; this design considers "lightness" and " "Thin", "power saving", "framed" and other user needs, so at this stage, it is mainly used for LCD panels for notebook computers (backlight modules below 15 inches).
b. Direct type:
Place the lamp directly under the module, and use two or more cold cathode lamps to increase the intensity of the light source, but the thickness and weight of the module are relatively increased. At the same time, multiple lamps also increase power consumption, which is more Suitable for LCD monitors or LCD TVs. It is a direct-down structure without a light guide plate. After the light source is reflected by the reflector, it is uniformly dispersed upward through the diffuser and then emitted from the front. However, its high power consumption (using cold cathode tubes), poor uniformity and causing problems such as LCD heating Still need to ask for improvement.
c. Hollow type:
It is suitable for super-large LCDs. These super-large LCDs are used as monitors and wall-mounted TVs. They not only require large screens, high brightness and light weight, but also require low thermal effects under high power in electrical applications. They have developed in recent years. The hollow structure of the backlight module uses a hot cathode tube as the light source.
4. Principle of backlight module:
It transforms the commonly used point or linear light source into a surface light source product with high brightness and uniform brightness through a simple and effective light mechanism. The general structure is to use the linear light source of the cold cathode tube to enter the light guide plate through the reflector to transform the linear light source into a uniformly bright surface light source, and then through the uniform light effect of the diffuser and the light collection effect of the diamond lens to improve the brightness and uniformity of the light source. Evenness.
5. Main parts and functions of the backlight module:
a. Aluminum frame/bottom metal plate
b. Reflective film (attached to the bottom and sides of the module)
C. Lamp group (lamp and lampshade)
d. Light Guide Panel or LGP
e. Diffusion membrane (Diffuser)
f. Prism film (or called optical film, brightness enhancement film)
5-1. Lamp group:
1. To provide sufficient brightness, it must have the characteristics of high brightness and long life.
2. Type of light source:
a.Cold Cathode Fluorescent Lamp (CCFL)
b. Hot cathode fluorescent lamp
c. LED
d. Electroluminescent sheet (EL)
Among them, large-size TFT LCD is still the mainstream of CCFL, and LED has a relatively uniform light transmittance of RGB three colors, which is not as obvious as traditional light-emitting sources. Therefore, high-power LEDs may be the focus of development in the future, but LEDs are now used in large-size LEDs. There are still technical difficulties. At present, large-scale laboratory products using LEDs as light-emitting sources must use about 60, which is much higher in cost. Subsequent research and development will reduce the number of particles used to be competitive.
5-1-1. Luminescence principle of cold cathode tube:
The electrons escaping from the electrodes collide with mercury and excite the mercury to release ultraviolet rays, which irradiate the phosphor coated on the inside of the glass pavilion into visible light.
5-2. Light guide plate (LGP):
The principle of light guide plate:
The light guide plate uses light from the edge of a specially processed acrylic plate (some people also call this plate a light guide plate). The light completely reflects the entire acrylic plate while moving forward. The reflected dots processed by the acrylic sheet change direction to form light with a component smaller than the total reflection angle and appear on the surface of the acrylic sheet.
5-2-1. Types of light guide plates (method of making reflective dots):
a.screen printing method:
After the acrylic flat substrate is ejected, the printing material containing the high divergence light source material is printed on the bottom of the light guide plate substrate, and then cured and fixed. This is a mature process currently used in China.
The reflective dot is the reflective ink after the fusion of titanium dioxide (Ti02) or sedimentary barium sulfate (BaS04), which does not have optical absorption, and has high reflectivity, and the acryl-based binder (binder), and is coated by screen printing. cloth.
Advantages: small volume production, high stability, mature technology
Disadvantages: not suitable for mass production, not suitable for small size, high cost, complicated process, large light source loss
b.injection method (no printing)
The designed light guide pattern is made on the mold, and it is made by direct injection molding or embossing, eliminating the need for screen printing and curing and fixing steps, simplifying the manufacturing process
Advantages: simplified process, mass production, low cost, high brightness, suitable for small size
Disadvantages: mold development is more expensive, initial failure rate is higher, and equipment cost is higher
5-2-2. The position of the reflective dot:
The reflective dot is not coated on the front of the acrylic plate, but is coated according to the gradient mode as shown in the figure below. In other words, the part of the cold cathode tube close to the light source becomes smaller, and the farther away from the light source, the larger it becomes. Control so that the entire surface can emit light evenly. In detail, the mode of a lamp is not that the reflected dot at the farthest part of the cold-cathode tube will become the largest, but at a distance of 2~30% from the end face will become the largest.
5-4. Prism Sheet:
The main function is to increase the brightness produced by the backlight. The principle is the same as the light-gathering effect of the light guide plate. It is mainly made of Polyester or Polycarbonate as raw materials, using injection molding or pasted with acrylic resin. Method, making a zigzag-shaped board surface, by pressing the upper and lower two pieces vertically and staggered between the two diffuser films, the light that has been diffused can be concentrated again, reducing the light loss rate, and thus increasing the brightness.
6. Cost structure of backlight module
Observing the cost structure of the backlight module, the material cost accounts for the highest proportion. The material cost for monitors accounts for 65-75%, and the material cost for notebook computers accounts for 62-70%. About 60% to 70% is the material cost. , And labor costs account for about 10%, which means that the industry’s mass production experience, employee proficiency (reducing loss rate), stable supply of key component suppliers and price concessions will be important ways to reduce costs.
6-1. Material cost structure of backlight module:
Observing the material cost structure of the backlight module, the top three materials are optical film, light guide plate, and lamp tube. These three materials account for 50 to 60%. Among them, the main supply sources of optical film and lamp tubes are concentrated in several In the hands of major manufacturers, it belongs to an oligopolistic market. In terms of optical modules, the current global supply mainly includes 3M, Nitto, and Mitsubishi. Among them, 3M can almost be said to be able to overcome most of the market; as for the lamp, there are currently many global manufacturers It is concentrated in Japan, including Stanley, Harison, West, and Sanken. Among them, Harison and Sanken account for 90% of the market share of CCFL lamps for notebook computers, and there are not many new manufacturers. After deducting these two materials, domestic backlight modules mostly choose to invest in the research and development of light guide plate technology.
