Quality Control of TFT LCDs During Manufacturing

A quality control worker inspects a finished product
A quality control worker inspects a finished product

Manufacturing TFT (Thin-Film Transistor) LCD (Liquid Crystal Display) screens involves a complex multi-step process that starts with creating the glass substrate. Then, a thin silicon layer etched into specific patterns is applied to form the transistors. 

Subsequent steps involve the crucial task of applying a layer of liquid crystal material between two glass panels. The insertion of electrodes manages the crystals’ alignment and re-alignment, thereby controlling the passage of light. 

The entire assembly is then enclosed and connected to a power source and a display controller. The controller manages the input signal and translates it into visual output on the screen.

Knowing the manufacturing process helps one understand LCD quality inspection standards and how companies perform them.

Why Perform Inspections and Testing?

At the heart of quality testing lies an imperative to foster trust and confidence in each product’s ability to perform its intended function effectively. Quality testing and inspection ensure our products meet consumer expectations and adhere to rigorous industry standards. This process thoroughly examines and evaluates the LCD’s various attributes to detect functionality and appearance defects. 

Whether the focus is on safe devices, sophisticated medical instruments, or critical components within industrial machinery, quality testing aims to reduce risks, curtail mistakes, and elevate the overall caliber of products in these high-stakes applications.

How Do Quality Control Inspectors Assess LCD Screens?

Before they reach the end-user, displays undergo several quality checks to ensure they meet the highest standards.

One of the initial inspections is called Product Appearance Testing and is performed using two 20W fluorescent lamps. The distance between the liquid crystal module (LCM) and fluorescent lamps should be 100 cm (about 4”) or more. Additionally, the distance between the LCM and inspector eyes should be 30 cm (1-3/16”) or more. The viewing direction for inspection is 45° from vertical against the LCM.

The inspector will be looking for major functional defects, including:

  • Backlight bleed occurs when light from the backlight leaks around the edges or corners of the screen. This is especially visible in a dark environment.
  • Flickering indicates that the screen does not consistently show brightness and contrast levels across its entire surface. Variations mean a substandard viewing experience for users.
  • Pixel defects refer to dead or stuck pixels not operating as intended. Depending on the size and classification of the screen, there are usually specific allowances for the number of pixel defects, which are classified as bright, dark, or sub-pixel defects.
  • Physical defects include cracks, broken corners, scratches, and other screen damage during manufacturing.
  • Color uniformity refers to consistent screen colors across the entire LCD. Color variations can indicate poor quality or potential issues with the screen’s backlight or color filters.
  • Image retention is sometimes called burn-in, or ghosting. This means assessing the risk and extent of static images burning into the screen, leaving a ghosted image.

In addition to these inspections, soldering work is checked. This process ensures it was completed while following the recommendations in the Inspection Standards Guide:

The Significance of Soldering Inspections

Soldering inspection on LCD panels is critical to ensuring the displays’ reliability and performance. It typically involves thoroughly examining the solder joints for defects that could compromise the electrical connections. 

Inspectors look for issues such as cold solder joints. Typically, these occur when the solder does not melt completely, creating a weak bond. Other defects to look for include:

  • Solder bridges are where excess solder connects points that should be separated.
  • Inadequate wetting indicates insufficient solder on joint surfaces.
  • Solder voids that can cause electrical failure. 

Inspectors can visually check the soldering, but for more precision, it may involve automated optical inspection (AOI) systems that use high-definition cameras and software to detect micro-level anomalies. Other times, an X-ray inspection will examine the integrity of solder joints beneath components or within the layers of a PCB. 

The soldering inspection process ensures all connections are correctly made and free from defects that could cause malfunctions in the LCD’s performance.

What are the Quality Control Standards for LCD Manufacturers?

Focus LCDs adheres to various quality standards that are typically international, national, or industry-specific:

The organization follows ISO 9001:2015, the latest iteration of the internationally recognized Quality Management Systems (QMS) standard, is published by the prestigious International Organization for Standardization (ISO). It provides a robust framework for organizations and industries to meet customer and other stakeholder needs within statutory and regulatory requirements related to a product or service. 

ISO 9001:2015 focuses on a process-oriented approach, emphasizing continuous improvement, risk-based thinking, and leadership and management commitment. It also integrates with other ISO management system standards. 

Obtaining the ISO 9001:2015 certification involves a rigorous external audit by an accredited certification body. However, the certification enhances an organization’s credibility and competitiveness by showing its commitment to quality.

The International Electrotechnical Commission (IEC) is another international standards organization that prepares and publishes standards for all electrical, electronic, and related technologies, including LCDs. 

LCD manufacturers must adhere to IEC standards to ensure LCD products’ safety, performance, reliability, and interchangeability. These standards can cover various issues, from testing methods for measuring display attributes like contrast ratio and response time to ensuring the safety of electronic display devices and environmental considerations in product design. 

By complying with these standards, LCD manufacturers enhance product quality and facilitate global trade by meeting internationally accepted specifications. Compliance with IEC standards often instills consumer confidence and can be a requirement for selling into certain markets or for specific clients.

In Summary

Maintaining stringent quality control measures throughout the LCD manufacturing process is crucial for ensuring the production of reliable and high-performing displays. Focus Display Solutions significantly reduces defects, lower costs, and enhances customer satisfaction by rigorously applying these procedures at every stage, from raw material selection to final product testing. 

Integrating advanced technologies and continuous improvement strategies further empowers the company to stay ahead in the competitive environment of display technology. Ultimately, a commitment to quality control is not just about adhering to standards; it’s about delivering excellence and innovation in every LCD panel that reaches the market.

If you have questions about our quality control process, contact us today!