The Root Cause And Countermeasures of Backlight Peeling Leakage in TFT Modules

The root cause and countermeasures of backlight peeling leakage in TFT modules

In TFT-LCD liquid crystal display modules, the "light leakage" problem is a very common phenomenon. The manifestations of light leakage and the components involved are also diverse. Different light leakage phenomena and those caused by different components have different causes, and they need to be analyzed specifically based on the specific phenomena. According to the manifestations of light leakage, there are usually: light leakage at the edge of the LCD display area, light leakage at the lamp socket of the LCD display area, light leakage at the blind hole of the LCD display area, and light leakage at the edge of the non-display area of the LCD, etc.

As the borders of TFT-LCD liquid crystal display modules become narrower and their thickness becomes thinner, the "edge light leakage problem in the non-display area of LCD" has become increasingly prominent. Among them, the "separation light leakage problem caused by the backlight adhesive" is the most common light leakage phenomenon in the edge of the LCD non-display area.

Question 1: The roughness of the plastic frame directly affects the bonding effect of the shading adhesive and the backlit adhesive frame, thereby influencing whether the shading adhesive and the BLU adhesive frame are prone to Peeling separation, and ultimately leading to the occurrence of light leakage.

Solution: Generally speaking, in order to ensure the bonding effect on the surface of the shading adhesive and the BLU adhesive frame, and to prevent the separation of the backlit adhesive frame and the shading adhesive from causing light leakage problems, it is recommended to control the roughness of the frame surface to be within the range of Ra ≤ 1.8 μm.

Question 2: The depth of the groove in the backlight adhesive film material is too small.

Solution: When the depth of the groove in the backlight adhesive film material is too small, under the condition that the thickness of each optical material remains constant, the gap between the optical material and the backlight adhesive film in the Z direction will decrease. After the reliability test, each optical material will expand and deform, thereby lifting the shading adhesive that is stuck to the backlight adhesive frame, resulting in separation and poor light leakage.

Generally speaking, for small-sized backlight adhesives and the gap in the Z direction between each optical material, it is recommended to control it according to Gap ≥ 0.03mm.

Question 3: The thickness difference of the backlight adhesive frame is too large.

Solution: At the positions where the backlight adhesive frame is thick, the corresponding shading adhesive can be fully activated after pressing; while at the positions where the backlight adhesive frame is thin, the corresponding shading adhesive has insufficient activation after pressing. At this time, peeling may occur, resulting in poor light leakage.

Generally speaking, there is no specific control standard for the thickness difference of the backlight adhesive frame. However, when analyzing defects, it is necessary to focus on testing the thickness difference of the adhesive frame at the Peeling leakage area and other positions.

Question 4: The bonding width of the backlight adhesive frame is too small.

Solution: The size of the draft angle of the mold during the integrated injection molding process and the size of the R-angle on the bonding surface of the adhesive frame will affect the effective bonding area between the adhesive frame and the shading adhesive. The larger the draft angle of the mold, the smaller the effective bonding width of the adhesive frame will be; the larger the R-angle of the bonding surface of the adhesive frame, the smaller the effective bonding width of the adhesive frame will be. Generally speaking, the draft angle of the mold for the backlight adhesive frame is recommended to be less than 2°, and the size of the R-angle of the bonding surface of the adhesive frame is recommended to be controlled within <0.05mm, thereby effectively ensuring the effective bonding width of the backlight adhesive frame.

Question 5: The warpage of the backlit adhesive iron exceeds the specification.

Solution: During the integrated molding process of the backlit adhesive iron, the adhesive iron will undergo plastic deformation at a temperature of 290°C in the plastic mold. When the temperature drops to room temperature, it will contract. The thermal expansion coefficient of plastic is 7 times that of steel (the thermal expansion coefficient of steel is: 1.2×10^-5/°C, and that of plastic is 8.4×10^-5/°C). During the cooling process, the contraction of plastic will be significant, and a 0.1mm-thick steel is too thin to have sufficient strength to support, thus causing the backlit adhesive iron that just comes out of the mold to have a warpage exceeding the specification. Generally speaking, for backlit adhesive iron less than 7.0 inches after shaping, the warpage is recommended to be controlled within the range of bowl warpage ≤ 0.3mm. Other types such as turtle warpage, inward bulge, S-shaped warpage and local deformation are not allowed.

Question 6: The rebound stress of the backlight adhesive is too high.

Solution: Generally speaking, the positions with the highest rebound stress of the adhesive are concentrated at the four R-angles of the backlight and the middle water droplet position. Therefore, these five positions are the frequent causes of separation and light leakage: ① Reduce the local height of the four R-frames to release the internal rebound stress of the frames. ② Make the pull-through holes at the four R-angles broken, reducing the internal rebound stress of the frames. ③ Reduce the local height of the water droplet frame, add pull-through holes at the middle position of the water droplet and make them broken, reducing the internal rebound stress of the frame at the water droplet position.