Solution

  Case Analysis   Solution for improving foreign objects in the backlight of TFT display modules   In the composition of a TFT module, the backlight is a very important component. It emits light through LEDs, and the light guide plate converts the point light source into a surface light source. In order to achieve better uniformity and higher brightness, films such as brightness enhancement films and reflective films are usually added.     In LCM module manufacturing plants, backlights have the highest defect rate and are the most difficult to improve. The most common problem in backlight assembly is foreign matter in the backlight. Common backlight foreign matter problems are divided into two types: foreign matter in the assembly of the backlight and the lower polarizer, and foreign matter inside the backlight film.       Solution for improving assembly foreign matter between backlight and lower polarizer:   Serial Number Risk level Improvement measures 1 High Each blister pack with backlighting needs to be covered with a large protective film to prevent dust. 2 Low Backlit incoming material inspection adds ion wind snake blowing to prevent dust falling. 3   Middle Daily cleaning of the backlight assembly machine: Wipe the conveyor belt with a lint-free cloth and pure water; clean the brushes under the skin with a vacuum cleaner; wipe the film-peeling platform and film-peeling rollers with a lint-free cloth and alcohol to remove residual adhesive; clean the dust from the air pipes and wiring surfaces inside the equipment with a vacuum cleaner; and wipe hard-to-reach corners with a lint-free cloth and alcohol. 4 High Humidification is added inside the backlight assembly machine to control the humidity at 70%. 5 High The nine-square grid analysis area is used to adjust the initial angle and area of ​​the ion wind snake according to each project to ensure that the entire backlight can be blown. FOG film should be cleaned of dirt and grime on small surfaces with alcohol before being uploaded. 6 Middle The viscosity of the backlight black adhesive is adjusted based on the dyne value of the lower polarizer surface. Too low a viscosity allows dust to easily enter, while too high a viscosity causes yellowing at the edges of the LCM display. For lenses with a dyne value ≤ 22, the backlight black adhesive viscosity is 1600; for lenses with a dyne value > 22, the corresponding backlight black adhesive viscosity is 850, and for the optical element, it is 500 for easy disassembly. 7 Middle The upper brightening material should be made of anti-small ball material to avoid scratches and white marks caused by foreign objects between it and the lower polarizer. 8 High The key point regarding the gap between the backlight and the housing of the LCM is to confirm the length, width, and height gaps between the components and the housing after the FPC is bonded to the backlight. The length, width, and height gaps should all be ≥0.5mm. Adjust the gaps in each direction appropriately based on the housing deformation and the offset dimensions of the FPC bonding.   The following are solutions to improve the problem of foreign matter inside the backlight film:   Serial Number Risk level Improvement measures 1 High Backlight film materials should be selected based on their compressive strength, resistance to microsphere brightening, and resistance to diffusion of soft particles in high-haze conditions. Silver has better compressive strength than white reflector. 2 High Cleanliness control of all components within the backlight supplier: Vibration of the glue frame for 30 minutes at 50Hz; ultrasonic cleaning of the glue frame for 12 minutes; using utility knife blades instead of round blades for slitting to ensure sharpness and prevent burrs; replacing the blade every 3 rolls; using black glue to clean the slitting material; applying dust pads or white tape to the sides before production, followed by freezing at -10℃ for 2 hours to reduce static electricity and identify components; humidifying the backlight lamination equipment to maintain 70% humidity; vacuum adsorption and luminescence rotation inspection; after backlight assembly, all components undergo continuous vibrating at 180-220 rpm for 2 hours; and a full vacuum adsorption and luminescence rotation inspection. 3 High Establish a comparative library of foreign matter for spectral analysis of different models, and identify the source of foreign matter by region, layer, spectrum, and composition. 4 High LCM manufacturers conduct a series of inspections on incoming backlight materials, including a running vibration test at 180-220 rpm for 2 hours, a vacuum adsorption and light-emitting rotation inspection, and an IQC (Independent Quality Control) spot check at 70°C for 2 hours to confirm whether the backlight film material is arching.   Improving LCM backlight foreign matter is a continuous and gradual process, and the root causes of problems will vary from project to project and batch to batch. Further investigation and confirmation must be carried out step by step according to the improvement plan above. Spectral analysis and comparison with the foreign matter library is a very useful tool and method for analyzing foreign matter white spot defects. Foreign matter lint defects in the touch film assembly can be analyzed in the same way as above.

  Case Analysis   Various reasons for short circuits in the silver paste circuit of the touch module.   In the production process of touch sensors, defects such as dark breaks and short circuits after laser etching of silver paste lines are very easy to occur. Moreover, the defect rate is unstable for each batch or each time period, and the functional yield fluctuates. The dark breaks and short circuits after laser etching of silver paste lines are more obvious when viewed from the back of the material.       Ⅰ.Direct Causes of Dark Fractures in Laser Etching of Silver Paste Circuits: After the printed silver paste circuits have dried, if certain areas are too thin or have holes, dark fractures are likely to occur after laser etching under the same conditions. These fractures can then lead to touch channel malfunctions during subsequent processes or transportation.   Ⅱ. Various Root Causes of Dark Fractures in Laser Etching of Silver Paste Circuits   Serial Number Risk level Improvement measures 1 Silver paste thickness <5um For a 350-mesh screen, the thickness of the photosensitive emulsion on the screen contacting the ITO film must be controlled to ≥15µm, and the thickness of the photosensitive emulsion on the screen contacting the squeegee surface must be controlled to 2-3µm. This is to avoid excessively thick photosensitive emulsion on the squeegee surface, which can lead to difficulty in ink application during silver paste printing and uneven pigment distribution within the silver paste lines. 2 Silver particle size <0.3um Replacing the silver particle size with 1/60th of the laser spot diameter, generally controlling the silver particle size to 0.5µm, such as Betterley’s 2880E silver paste, is quite suitable. 3 Laser etching parameters for fine line mismatch DOE verification was performed on the number of laser etching passes, etching energy, etching speed, and corner delay. The number of defects per large sheet (short circuits, open circuits, and burst points) was statistically analyzed. DOE Taguchi design was used to obtain optimal values. For traces with a Y-axis spacing greater than 0.08mm, the laser etching passes were generally 5. Burst points on the laser traces can cause dark breaks in the silver paste traces. When multiple laser lines are split in the same channel, a “multi-line” layer needs to be added and the laser etching parameters readjusted. Laser trace design graphics should avoid perpendicularity and require chamfering (C0.05). For different laser etching equipment, it was defined which equipment can be used for fine-line etching. A green light source was used for inspection after laser etching. 4 Silver paste line network version partial area blockage network Blockage in certain areas of the silver paste screen (FFU printing speed 950 rpm, dust measured on the printing press) can cause pinpoint air bubbles in these areas after silver paste factoring. After ink overflow, the silver paste thickness in these areas is thinner, making them prone to bursting and dark breaks after laser etching. This defect can be identified by comparing the location of the defect with the blocked areas on the silver paste screen (a fixed area). To prevent this, the screen should be thoroughly cleaned and dried with an air gun even after the workday ends. Before each screen setup, a check should be performed at the printing press for any screen blockages or punctures. For products with punctures, avoid wiping and manage the process carefully to prevent ITO damage or contamination.      

  Case Analysis   Framed project with watermark and heavy pressure reporting   In some low-end projects, a frame bonding process is used, which involves bonding the touch module and the display module together with foam adhesive. In addition to being prone to dust accumulation during use, this frame bonding structure can also cause watermarks when pressed and issues such as automatic touch point reporting after pressing.       For some 1.0mm narrow bezel display modules paired with narrow bezel touch modules, issues such as watermarks or pressure-based callouts are common. To address these issues, we recommend optimizing the following aspects:   Serial Number Risk level Improvement measures 1 High The overlap width between the foam adhesive and the polarizer on the display module on one side must be ≥0.5mm. The gap between the polarizer and the LCD outline should be controlled at 0.1±0.1mm and machine-applied. Oil sand or fine sand should be used to apply the polarizer to provide anti-adhesion capability. 2 Middle For foam adhesive, choose rigid foam with a thickness of 0.7mm before compression and 0.4-0.6mm after compression. Make beveled or Z-shaped openings on the side or bottom edge of the foam adhesive and conduct a dust test. For double-sided adhesive, choose Crown 7972TB black glue to avoid white edges in the 45° visible area (this is because transparent double-sided adhesive reflects light at the edges, so it was changed to high-adhesion black glue). 3 Middle A sufficiently large gap must be maintained between the backlight surface of the display module and the casing, and the conductive sponge covering the backlight surface must be replaced with a soft, fluffy material to prevent the backlight surface from being squeezed, which would reduce the gap between the LCM and TP, resulting in watermarks or pixelation. 4 High Misalignment between the display module and the touch module causes insufficient overlap between the foam adhesive and the polarizer on the display module, easily compressing the foam adhesive. To address this, fully automated lamination equipment can be used to eliminate the pressure-holding process, ensuring CCD alignment and improving the lamination accuracy between the display and touch modules. Foam adhesive application requires equipment or fixtures to guarantee an accuracy of ±0.1 mm. 5 Middle The casing uses a dispensing process. Excessive glue can cause it to seep into and compress the foam, leading to deformation. The width of the foam adhesive can be controlled to 0.8-0.85mm to increase the gap between the dispensing path and the foam adhesive. 6 Low For GF structure touch modules, the touch IC should preferably use the base 6540. The frame-mount process can be changed to a full lamination process. The thickness of the OCA in full lamination needs to be ≥0.175 to avoid interference from the display module in the GF structure touch module. However, full lamination is more expensive than frame-mount.  

  Display module ghosting and optimization solutions   When the LCD screen of the display module displays a single image for an extended period of time, an electric charge accumulates inside, causing a ghost image to appear due to the internal electric field.     Afterimages clustered together: Differences in LCD liquid crystal materials, pixel leakage, larger Cst design and smaller Feedthrough design in TFT circuits, moisture absorption of LCD PI, panel’s optimal VCOM drifting over time or temperature leading to severe image retention, image retention caused by special UI, and prolonged display of the same fixed image during manufacturing processes:   Afterimage Verification: The afterimage will disappear after baking at 80℃ for 4 hours. You can use a 5*5 checkerboard pattern to light up the image for the test time and then switch to 127 gray, or use the image of the customer where the afterimage appeared to light up the image for a long time and then switch to 127 gray to reproduce the image.     Chessboard grid lighting time Switch L127 time Judge Determination 30min 1min It is not allowed PASS 1hr 1min It is not allowed PASS 2hr 5min It is not allowed PASS   Image retention optimization:   Serial Number Improvement Effective measures to improve image retention 1 Screen manufacturers Optimize driver code and change the toggling method. 2 Screen manufacturers Optimize driver code and change scanning method 3 Screen manufacturers Optimize driver code and reduce VGL voltage. 4 Screen manufacturers Optimize driver code and adjust asymmetric GAMMA voltage to Black (+0.2V, -0.6V). 5 Screen manufacturers The forward and reverse VCOM values ​​in the overall machine driver code are masked, and a probe is used to perform single-chip OTP VCOM for each product. 6 Client For high-contrast interfaces, lower the contrast as much as possible. For example, change pure gray RGB=0*FFFFFF fonts or pure white RGB=0*FFFFFF icons to white RGB=0*F0F0F0 fonts and grayish-white RGB=0*F0F0F0 icons. 7 Client The grayscale level of the UI should be at least 127 pixels to avoid the previous screen leaving a residue on the grayscale. 8 Client The display refresh rate should ideally be ≥50Hz to minimize LCD pixel leakage time. 9 Client To avoid prolonged viewing on a single screen, based on our project experience, viewing at 50°C for more than 1 hour will result in ghosting. Therefore, the screen should be set to automatically hide or refresh if viewed for extended periods. 10 Screen manufacturers Drive LCD glass manufacturers to optimize liquid crystal material properties and TFT driving circuits

  The difference in the location of the display module initialization code within the motherboard driver   The initialization code for the display module is usually placed in the motherboard driver. Some products may be fully programmed using OTP, which eliminates the need to place the corresponding initialization code in the motherboard, allowing for direct plug-and-play functionality. However, it’s important to be aware of the “code-eating” phenomenon mentioned in previous articles.   The status of the parameter values ​​used by the driver IC is displayed when the initialization code is placed in different locations. Driver code architecture   OTP       Initialization 11   Initialization code 1129 Initialization   1129   29 Final parameter value state Depends on the parameter values ​​in the initialization code Depends on the programming parameters of the OTP Regardless of whether it has been programmed via OTP, the parameter values ​​still depend on the initialization code.   If the display module initialization code is placed before 1129, any registers that have been OTP’d will not be overwritten; otherwise, the value in the initialization code will be used. If the display module initialization code is placed between 11 and 29, the initialization code will be used regardless of whether OTP has been performed. This method is generally used to overwrite parameters that still have errors after OTP.   It is particularly important to note that the display module register OTP count is generally limited to 3 times, except for YuChuang’s display driver IC, which can implement unlimited OTP counts. Common OTP values ​​include ID (identity recognition), VCOM (flicker improvement), Gamma (color temperature improvement), and full code (plug-and-play).    

  Case Analysis   The phenomenon of screen distortion caused by the aggregation of conductive particles in the display module due to high current.   During the bonding process of the display module, an ACF containing conductive particles is used. When the conductive particles in the ACF are heated and compressed, they will burst and expose the metal balls inside. The metal balls are conductors. If they gather together, it will cause a short circuit between two PADs or between lines, resulting in abnormal situations such as large current.       This accumulation of conductive particles is a potential hazard. If no abnormalities are observed during the FOG’s lighting process, display anomalies or high current may appear when playing with the FPC or pressing and bonding the FPC. Generally, the accumulation of conductive particles after bonding occurs in the following two ways:   Ⅰ.If the FPC is too wide and covers the ITO traces of the LCD (after the LCD is cut into small pieces, the edge of the LCD will expose the ITO metal part), the conductive particles will accumulate at the boundary where the FPC and the LCD meet, causing a short circuit between the ITO metal parts on the side of the LCD. (The conventional design of the FPC will extend 2-5mm beyond the gold finger target on one side, forming a concave design in the neck bend area to prevent the bonding edge from lifting and falling off when the FPC is pulled.)       The test lines for some LCD glass are relatively close to the LCD1 target. After the LCD is cut into small pieces, ITO metal remains on the side. If there are no FPC gold fingers at this point when bonding with the FPC, conductive particles can easily accumulate at the junction of the LCD edge and the FPC, causing short circuits between the ITO lines.     For this type of situation, the following suggestions are made:   1. During structural design, the distance between the ITO test line and the LCD target should be confirmed using an actual LCD. The design of the FPC’s dimensions relative to the LCD target should also consider the bonding and FPC shape tolerances, while simultaneously incorporating the FPC’s neck bend and concave design to prevent edge pulling and separation. 2. The LCM finished product lighting test fixture should incorporate voltage and current testing to avoid voltage and current anomalies. Multiple voltage and current tests should be conducted to screen out such potential problems and prevent short circuits.   II. During bonding, if the FPC bonding gold fingers are too short, the cover film at the edge of the FPC bonding gold fingers may enter the LCD, creating a step difference that causes conductive particles to accumulate between the bonding gold fingers, resulting in a short circuit.     The root cause of this problem is that the length of the FPC bonding gold fingers does not exceed the shape of the LCD. If the bonding gold fingers exceed the shape of the LCD, the FPC is prone to breakage when bent. For this situation, it is recommended to: 1. Extend the FPC bonding gold fingers 0.2MM beyond the edge of the LCD, and after bonding, use a line of silicone to prevent bending the FPC from causing the entire edge of the LCD to break. The line of silicone can also prevent moisture from entering the ACF and causing the bonding FPC air bubbles to fall off.     2. The LCM finished product lighting test fixture after bending FPC should be equipped with voltage and current monitoring to avoid voltage and current abnormalities. Such hidden dangers should be screened out by the upper and lower limits of voltage and current to prevent them from flowing out.