Why do quality problems occur? Explaining the structure and principles of LCDs!

Liquid crystal molecules have the property of changing their arrangement when a voltage is applied.
When voltage is applied, the liquid crystal molecules that were aligned along the grooves in the alignment film rise vertically and align in the same direction.
Transparent electrodes in the X and Y directions are arranged perpendicular to each other on the glass, and TFTs that act as SWITCHES are placed at the intersections.
Voltage is applied to the liquid crystal molecules by switching this TFT on and off.

Polarizing plates are one of the essential components of LCDs.
A polarizing plate is a component that acts like a shutter, allowing only light in a certain direction to pass through. The polarizing plate has a row of fine slits, and only light in a certain direction can pass through the slits, while light in other directions cannot pass through the polarizing plate.

Figure 4. Principle of polarizing plates

By applying the principles explained so far, the LCD switches on and off.
First, of the light that passes through the first polarizer, only light traveling in a certain direction reaches the liquid crystal layer. Because light passes through the gaps between the liquid crystal molecules, when the TFT is OFF, the light is twisted 90 degrees along the liquid crystal molecules as it passes through, reaching the human eye. On the other hand, when the TFT is ON, the liquid crystal molecules stand vertically and face in a certain direction, so the light that passed through the polarizer passes through the liquid crystal layer without changing direction and reaches the second polarizer. Because this light is traveling in the same direction as the absorption axis of the polarizer, it cannot pass through the polarizer and does not reach the human eye.

Figure 5. LCD operating principle

As explained in the previous section, LCDs control the transmission/blocking of light by applying voltage to the liquid crystal molecules to change their orientation.
However, displaying the same image for a long period of time causes an increase in parasitic capacitance within the liquid crystal layer, and the charged DC voltage interferes with controlling the orientation of the liquid crystal molecules. This prevents the liquid crystal molecules from adjusting to their correct angle, making it impossible to turn the display off, resulting in burn-in. Mild burn-in can sometimes be resolved by turning POWER SUPPLIES off for a while, but in severe cases, turning POWER SUPPLIES off may not resolve the issue.

For this reason, it is important for manufacturers to consider measures to prevent burn-in from the application design stage. Typical measures include asking users to turn off POWER SUPPLIES for a certain period of time after continuous use, or playing videos when the LCD is not in use. A familiar example is the animated screen saver on a computer, which also plays a role in preventing burn-in.

It is impossible to completely eliminate foreign matter and dust during the manufacturing process, so it is also difficult to completely eliminate bright and dark spots.
For this reason, LCD manufacturers have established inspection standards, and many of them adopt a policy such as "a product is deemed to be good if there are x number of bright/dark dots or less." Also, because bright/dark dots come in a variety of sizes, some manufacturers also set standards for size, and define very small dots as not to be counted.

The visibility of bright/dark dots will vary depending on the application used.
You may have seen electrocardiograms displayed on a black screen in medical dramas, and bright spots become more noticeable in applications where the UI is entirely black. For this reason, applications that often display black screens often require LCDs with as few bright spots as possible.

However, as mentioned above, it is impossible to eliminate bright and dark dots, so it is important for users to carefully check the inspection standards issued by the LCD manufacturer and confirm the definition of bright and dark dots.

The vertical lines (source lines) and horizontal lines (gate lines) are driven by source driver ICs and GATE DRIVER ICs, respectively.
Therefore, if horizontal lines appear, it is highly likely that there is some kind of abnormality in GATE DRIVER IC TERMINALS or the wiring connected to them.On the other hand, if vertical lines appear, the source driver IC or the wiring connected to it is often suspected.

Figure 14. Source driver IC/ GATE DRIVER IC

However, as mentioned at the beginning, even if the horizontal line is the same, the condition is different if it is a single line or if a thick band-like line is visible.If a thick band-like line is visible, it means that an abnormality has occurred across multiple gate lines, which may indicate that the gate IC was not properly crimped during manufacturing or that dirt has adhered across multiple TERMINALS.

Conversely, if only one horizontal line is visible, it is unlikely that dirt or other contaminants have adhered to a specific TERMINALS, taking into account TERMINALS pitch of the driver IC, etc. Therefore, it is possible that the wiring was broken during LCD manufacturing.