LCD screens are ubiquitous in our daily lives, found in devices such as landline telephones, mobile phones, and smart
home systems. The era of LCD screens continues to rise, but what is the manufacturing process behind them?
Liquid crystals possess the light refraction properties of solid-state crystals while
maintaining the fluid characteristics of liquids. When driven by electrodes, they can align according to the main
controller's instructions, regulating the intensity of transmitted light. By using red, green, and blue color
filters, each pixel is finely tuned to create a complete image. LCD panels can be categorized into two parts: the
liquid crystal panel and the backlight system. The manufacturing process involves multiple complex steps, mainly
divided into three stages: the front-end Array process, the mid-stage Cell process, and the back-end Module assembly.
The detailed workflow is as follows:
I. Front-End Array Process (Thin Film/Photolithography/Etching/Stripping)
Glass Substrate Processing
High-purity glass substrates are selected and processed through cleaning, photoresist
coating, exposure, and development to form a pixel array (TFT matrix). Using photolithography, circuit patterns are
transferred onto the glass substrate, and etching removes unwanted areas.
Thin Film Deposition and Processing
The movement and alignment of liquid crystal molecules require electronic control. TFT
glass, the carrier of liquid crystals, must include conductive components to regulate molecular motion. This is
achieved using ITO (Indium Tin Oxide), a transparent conductive metal, allowing the backlight to pass through.
To precisely control liquid crystal molecules, ITO film undergoes special processing,
similar to printing circuits on a PCB board. Conductive traces are drawn on the entire LCD panel. The ITO film is
deposited onto the TFT glass and washed with ionized water to ensure a smooth and uniform layer.
Photolithography Process (Coating, Exposure, and Development)
Photoresist Coating:
A uniform layer of photoresist is applied over the ITO film. After baking, solvent
components evaporate, enhancing adhesion between the photoresist and ITO glass.
Exposure:
A UV light source projects pre-designed electrode patterns onto the photoresist layer
through a photomask.
Development:
The exposed photoresist is washed away with a developer, leaving behind only unexposed
sections.
Etching & Stripping
Etching:
Acidic etchants remove ITO film in areas unprotected by photoresist, forming conductive
electrode patterns.
Stripping:
The remaining photoresist is dissolved using an alkaline solution (e.g., NaOH), leaving
behind the final ITO pattern.
This photolithography process is repeated multiple times to create complex and precise
electrode structures required for controlling liquid crystal molecules. At this stage, the front-end Array process is
complete.
II. Mid-Stage Cell Process (TFT Glass and Color Filter Bonding)
Liquid Crystal Coating
The LCD structure resembles a sandwich:
the lower TFT glass and upper color filter encapsulate the liquid crystal layer. The TFT
glass is first cleaned with ionized water, ensuring an impurity-free surface.
Alignment Layer and Spacers
A polymer alignment layer is applied to ensure uniform molecular arrangement.
A rubbing process orients the alignment layer, facilitating precise liquid crystal
alignment.
Spacer particles are distributed on the color filter to maintain a controlled gap between
the layers.
Glass Bonding and Liquid Crystal Injection
Sealant is applied around the TFT glass panel for structural integrity.
Liquid crystal material is injected into the cell under vacuum conditions.
A conductive sealant is added to ensure external electronic connectivity.
High-temperature curing stabilizes the assembly.
Finally, the LCD panel is cut into predefined sizes, and polarizers are affixed on both
sides. Polarizers selectively allow light to pass through, enabling LCD functionality.
III. Back-End Module Assembly
Frame and Circuit Installation
The back-end process involves integrating the LCD panel with driver ICs and printed
circuit boards (PCBs). The driver IC receives display signals and controls liquid crystal molecule orientation. The
process includes:
Anisotropic Conductive Film (ACF) Bonding:
Connects the driver IC to the LCD panel.
Source Driver IC:
Controls pixel data input.
Gate Driver IC:
Regulates liquid crystal rotation speed.
Flexible Printed Circuit (FPC) Bonding:
Bridges the external PCB with the LCD panel.
Backlight System Integration
Since liquid crystals do not emit light, an additional backlight system is required. The
main components include:
Light Source:
CCFL (Cold Cathode Fluorescent Lamps) or LED backlighting.
Light Guide Plate (LGP):
Converts point/line light sources into uniform surface illumination.
Diffuser and Prism Films:
Enhance brightness uniformity.
CCFL backlights have historically been used, but LED backlighting is now the industry
standard due to energy efficiency. Edge-lit LED designs reduce cost by minimizing LED usage while maintaining
brightness uniformity.
IV. Key Process Considerations
Environmental Control
Manufacturing must occur in a cleanroom with temperature maintained at 23°C
±1°C and humidity at 55% ±5% RH.
Material Selection
High-purity glass and liquid crystal materials ensure optimal optical performance.
Quality Control
Multiple inspection points throughout the process detect and rectify defects.
Optical Testing:
Automated Optical Inspection (AOI) scans for pixel defects and uniformity issues.
Aging Test:
Simulates extended operation to identify potential failures.
V. Applications and Future Trends
Current Technologies:
LCDs continue to dominate the TV and monitor markets, with Mini-LED enhancing contrast
and brightness.
Innovations:
Flexible LCDs (e.g., automotive curved displays) and low-blue-light solutions are driving
industry advancements.
This complex process involves over 300 technical steps and requires highly specialized
equipment and expertise. The entire production cycle is extensive but crucial for producing high-quality LCD panels.
