As a manufacturer deeply rooted in the LCD industry, we understand the importance of display quality to customer experience. In the production and use of TFT-LCD displays, "liquid crystal bubbles" or "black blobs" are undesirable phenomena that may be encountered. Today, I will share my experience in the hope of helping you to better identify, analyze, and prevent such problems.
Why are black spots on TFT-LCDs more noticeable in grayscale images?
To understand the causes of black spots, we must first review the display mechanism of TFT-LCDs. Simply put, when power is applied, a voltage difference is generated between the pixel electrode (Pixel ITO) and the common electrode (COM ITO), driving the liquid crystal molecules to deflect. The magnitude of the voltage determines the deflection angle, thus affecting light transmittance and ultimately forming a black, gray, or white image.
Looking at the voltage-transmittance (V-T) curve, this curve is non-linear. In the low-voltage region (corresponding to low grayscale), the curve slope is obvious. Small voltage fluctuations are amplified into significant transmittance differences. This explains why black spots appear more severe at grayscale levels (such as L127 or L64)-minor non-uniformities are magnified here.
In addition, the characteristics of human vision also play a role. According to the Weber-Fechner law, the sensitivity of the human eye to changes in brightness is directly proportional to the background brightness. Under a bright white screen, a large brightness difference is required to detect defects; while under a darker grayscale, even small differences are very prominent. This is why many LCD defects (such as bezel murmurs, flickering, or ghosting) are often inspected under grayscale. Through these principles, we can diagnose problems more accurately and avoid production delays.
Seven methods for monitoring and excitation of black spots in TFT-LCD
Black spots in LCD panels sometimes fade or disappear over time, making subsequent analysis difficult. In such cases, monitoring methods can assess the potential risks of defect-free panels, while excitation methods can help reproduce the problem and confirm whether it originates from a failure of the frame sealant. Note that black spots differ from other bubble types (such as air bubbles or vacuum bubbles), which are often related to venting or liquid crystal volume. Below are seven methods we have summarized, each tailored to a specific application scenario.
High Temperature Environment Testing
High temperatures (80-85℃) can effectively amplify the size of black blobs. If the sealant is poor, high temperatures will cause internal air bubbles to expand, compressing the liquid crystal area and leading to enlargement of the black blobs. Simultaneously, the viscosity of the liquid crystal decreases, allowing contaminants (such as moisture) to diffuse more quickly, thus exacerbating the contamination area. It is recommended to use a high-temperature test chamber to ensure uniform heating; if conditions are limited, a hot plate can be used as a substitute, but the uniformity will be slightly inferior.
Core Mechanism: High temperatures promote air bubble expansion and accelerate the diffusion of contaminants, helping to quickly verify sealing issues.
High Temperature and High Humidity Environment Testing
Under conditions of 80-85℃ and 90-95%RH, the black spots worsen. Increased humidity allows more moisture to penetrate poorly sealed sealant, causing air bubbles to expand and the black spot area to increase. This is more severe than high temperature alone, making it suitable for accelerating reproduction.
Core Mechanism: Combining the dual pressures of heat and humidity to simulate extreme usage environments.
Boiling or PCT pressure cooker test
Similar to high temperature and high humidity, but with more extreme conditions (such as high-pressure cooking), this shortens the activation time, causing the black mass to enlarge rapidly. The mechanism is similar to the former, but more efficient, making it suitable for emergency fault analysis.
In practice, we have found that PCT testing can reduce the problem exposure time from days to hours, greatly improving diagnostic efficiency.
Red Ink Vacuum Test
After a vacuum test, the red ink in defective LCD black blobs will penetrate into the LCD cell. When the sealant around the LCD cell deteriorates, the red ink will seep into the cell, and the black blobs will also increase in size.
Microscopic Observation of Sealant Punctures
When unsure whether the LCD black patch defect is caused by a sealing problem with the LCD sealant, a microscope can be used to observe whether there are punctures in the sealant.
If the sealant is punctured by the liquid crystal, the LCD black patch defect can generally be attributed to a sealing problem with the LCD sealant.
It should be noted that sealant punctures are unavoidable in the ODF process; the width of the punctures can only be reduced by optimizing the LCD display panel manufacturing process.
Generally, if the width of the puncture is ≤1/3 of the sealant width, it can be considered OK; if the width of the puncture is >1/3 of the sealant width, it is considered NG (Not Acceptable).
LCD Sealant Peeling Off Test
Damage to the LCD sealant, inappropriate sealant selection, or improper matching of the sealant with the PI alignment film and liquid crystal can all lead to decreased peel strength between the TFT glass and CF glass of the LCD panel. The end result is that the LCD sealant fails the peeling off test.
Generally, the industry standard for LCD sealant peeling off testing is ≥10 kgf, with more stringent requirements at ≥15 kgf. Typically, five test points are used for the peeling off test.
Low-Temperature Environment Testing
In contrast to high temperatures, low temperatures (-20~-40℃) cause the black patches to shrink. Bubbles contract, liquid crystal viscosity increases, and contaminant diffusion slows. After removal from the high-temperature environment, the black patches gradually fade as the temperature returns to room temperature. This test provides reverse verification of sealing issues, helping to distinguish between temporary and permanent defects.
Core Mechanism: Utilizing the principle of thermal expansion and contraction to observe dynamic changes.
These methods are not isolated and can be combined, such as first stimulating at high temperatures and then observing at low temperatures, to obtain comprehensive data.
Conclusion
As a professional commercial display manufacturer, we are committed to improving product reliability and display consistency. We hope this article will provide valuable reference for your selection, use, and maintenance of TFT-LCDs.
For further questions or customized needs, please feel free to contact us.
