With the rapid development of automotive intelligence, the in car touch display industry chain is actively laying out, and the automotive cockpit is upgrading from an "intelligent cockpit" to an "intelligent cabin", with more emphasis on designing the interaction between "people" and vehicles. The development trend of in car displays is towards large screens, multi screens, and multi forms, and touch control is redefining the human-computer interaction experience in cars. In this process, the emergence of full fit has provided new possibilities for the development of in car display technology. Faced with high and low temperatures, vibrations, strong light, etc. in the environment, the process requirements are higher, so the selection of adhesive is also more demanding. According to Omida's forecast, the global shipment of automotive display screens will reach 207 million pieces by 2025, further estimating that the automotive display market is expected to reach around 19.1 billion US dollars by 2025. According to DataBridge data, the global OCA optical adhesive market size in 2021 was $2.07 billion, which is expected to increase to $4.74 billion in 2029, with a compound annual growth rate of 10.9% from 2021 to 2029.
1、 Optical Full Fit Technology
From the structure of the display screen, it can be mainly divided into three parts: protective glass, touch screen, and display screen from top to bottom. These three parts form a touch screen through bonding, which generally requires two bonding processes, namely the bonding between the protective glass and the touch screen, and the bonding between the display screen and the touch screen.
Optical full fit technology, also known as non air gap technology, is a technique that uses OCA optical film or OCR liquid optical adhesive to fit and fill the middle air layer.
Compared to traditional frame stickers, full fit has the following advantages:
(I) Enhance the display effect. Full fit technology replaces the air layer with an OCA/OCR layer with a refractive index closer to the upper and lower layers. The interlayer reflection is lower, which can reduce the interference of ambient light reflection, increase the light transmittance of the display screen, and enhance image contrast.
(II) The screen isolates dust and moisture, ensuring screen cleanliness and improving screen environmental adaptability.
(III) Reduce screen thickness, lower overall thickness, and increase embedded touch sensitivity.
(IV) Reduce signal noise interference, improve touch operation accuracy and sensitivity.
2、 Optical Full Fit Process&Features
1. Classification of Optical Full Fit Processes
① Process
The optical full lamination process classification is mainly divided into 1st LAM (TP lamination) and 2nd LAM (LCD lamination) according to the lamination process classification.
② Optical materials
According to the classification of optical bonding materials, optical full bonding processes are mainly divided into four categories: OCA (Optically Clear Adhesive), UV-OCA, LOCA (Liquid Optical Clear Adhesive), and SLOCA (Slit Liquid Optical Clear Adhesive).
（Ⅰ）OCA
OCA optical adhesive is a double-sided adhesive tape that uses optical organic silicon or acrylic materials as substrates, and then adheres a release film on the upper and lower layers to obtain a substrate material. It is mainly used for combination of touch screen panels or glass cover plates with displays.
（Ⅱ）UV-OCA
All UV OCA optical adhesive is a single component UV curable adhesive that is 100% curable. It can be cured by irradiation at a wavelength of 365nm without the need for baking.
（Ⅲ）OCR/LOCA（Liquid Optical Clear Adhesive）
OCR (OpticalClear Resin) liquid optical transparent adhesive, also known as LOCA (Liquid OpticalClear Adhesive), is a special adhesive mainly used for bonding transparent optical components.
（Ⅳ）SLOCA（Slit Liquid Optical Clear Adhesive）
SLOCA (Slit Liquid Optical Clear Adhesive) refers to a scraping type liquid optical adhesive that is transformed into a state similar to OCA optical tape through "scraping+UV pre fixing", effectively combining the characteristics of OCA and LOCA.
2. Analysis of Optical Full Fit Technology
① OCA bonding process
(I) OCA bonding process
The display screen is mainly composed of protective glass, touch screen, and display screen, which are fully adhered to the commonly used CTP structures of OCA: On/In cell, OGS, GF/GFF. In the OCA bonding process, the FPC bonded sensor and cover glass are bonded together, requiring secondary bonding, namely, Sense glass and PSA bonding and Sense glass and Cover glass bonding.
(1) Apply OCA film onto the sensor, commonly known as soft and hard. Manually place the sensor on the device countertop, manually remove the isolation paper on the upper layer of OCA (a small piece of tape can be used to stick it off, which is more convenient), and the device will automatically align and complete the attachment.
(2) Attach the sensor that has been coated with OCA film to the cover glass, commonly known as hard adhesive. Manually place the cover glass and sensor glass that has been attached to OCA onto the corresponding countertop of the equipment, and after CCD automatic alignment is completed, pressurize and adhere them in the vacuum chamber.
(II) Disadvantages of OCA bonding process circle
(1) OCA process dots
1) Reduce glare, reduce the loss of light emitted by LCD, increase LCD brightness and provide high transmittance, and reduce energy consumption;
2) Increase contrast, especially under strong light irradiation;
3) Face connections have higher strength;
4) Avoid Newton's rings;
5) The surface of the product is smoother;
6) Boundless, expanding the visible area;
7) The thickness gap control is precise, and the gap thickness is the thickness of the OCA adhesive film, with a small amount of adhesive used;
8) The investment in bonding equipment is relatively small, and the price of OCA bonding equipment is one order of magnitude lower than that of OCR bonding equipment;
9) The process is relatively simple, with high repeatability and efficiency;
10) Translucency>92-99%.
(2) Disadvantages of OCA process
1) The surface of OCA adhesive film is sticky, and it is easy to leave marks on the surface when peeling off the release film, and bubbles are easily generated during adhesion. Easy to adsorb dust and impurities, causing secondary pollution.
2) During the process of OCA film and FILM bonding, uneven manual pressure can easily cause wrinkles and produce bubbles. For G+G bonding, a vertical pressure combination machine is used, and the air under heating and pressure is difficult to remove, which is very easy to produce bubbles. Moreover, the function of the defoaming machine is not significant.
3) OCA has poor flow performance, making it difficult to fill the gaps or ink gaps in the ITO circuit during sensor or cover glass bonding.
4) The adhesive performance of OCA is not strong, and products that adhere well have a risk of rebound.
5) OCA is not conducive to large-sized bonding, resulting in low production efficiency and high labor costs.
6) It is difficult to perform OCA fitting on G+G for medium sizes (around 10 inches), and for large sizes (such as 15.6 inches, 48 inches, and 72 inches), and the difficulty increases as the size increases. Low production efficiency and low yield rate.
7) OCA bonding cannot effectively increase the strength and explosion-proof ability of the screen. Especially for OGS, explosion-proof film adhesion is required. After fitting, the explosion-proof effect of the screen is poor, not resistant to falling, and the screen is prone to damage.
② UV-OCA process
The car mounted screen with an iron frame requires control of the thickness of the surrounding adhesive layer, often using a full UV OCA "dam+surface adhesive" full bonding scheme. The process mainly includes: applying sealant/dam sealing adhesive - UV pre fixing - applying surface adhesive - bonding and leveling - UV curing - obtaining the finished product. Due to its ability to absorb large segment differences and without the need for adhesive, this solution has unique advantages in adhering to screens with iron frames or other uneven shapes.
③ OCR/LOCA（Liquid Optical Clear Adhesive）
(I) OCR/LOCA bonding process
Organic silicon OCR leverages the advantages of traditional organic silicon, such as strong cold and hot stability, good UV stability, low shrinkage rate, fast leveling rate, and convenient repair, to meet the stringent requirements of ultra-high standards for display screens in vehicles, ships, and military industries. The car display screen OCR provides suitable solutions for different bonding materials, such as G+P, G+F, G+G, etc. In addition, corresponding bonding schemes are also provided for different bonding processes, such as dispensing, coating, and injection. The process flow mainly includes two types:
(1) Full adhesion coating process
It is recommended to use a coating process for the flat and smooth surface of the product's LCD+TP bonding, which involves coating, hot pre bonding, bonding, defoaming, and hot bonding. The slit coating has high coating efficiency, good uniformity, and wide coating thickness range. OCR is extruded from the slit cutter head under a certain pressure, transferred to the substrate to form a shaped adhesive film, and converted into gel after low temperature pre curing. Most of the shrinkage stress has been released after pre curing. The shrinkage stress of full curing after bonding is lower, reducing the risk of macular. At the same time, the pre cured OCR can fix the upper and lower bonding surfaces and is easy to separate. After defoaming, defects and defective parts can be reworked.
(2) The fully integrated scheme of "dam enclosure+grouting" process
The process of dam enclosure and glue injection scheme is to first point the structural glue around (leaving drainage holes) to align the two products, then inject glue into the middle of the two products and perform thermal curing. This scheme is suitable for the bonding of irregular screens.
(II) Disadvantages of OCR/LOCA bonding process circle
(1) OCR/LOCA fitting process dots
1) Strong applicability, one adhesive can correspond to multiple adhesive products;
2) Liquid glue has strong fluidity, is not sensitive to ink thickness, and has good gap filling properties;
3) Suitable for large-sized lamination, with no limit on the size of the laminating product;
4) Suitable for surface fitting or complex structure fitting;
5) High bonding yield and good rework performance;
6) Chemical bonding, bonding is less affected by temperature and has strong bonding stability.
(2) Disadvantages of OCR/LOCA bonding process
1) When LOCA is cured, it is difficult to completely cure the framework and FPC, so side irradiation curing is necessary;
2) LOCA has the problem of glue overflow, so precise control of the dispensing pattern is required, and higher process control requirements are required;
3) LOCA is prone to producing bubbles during the process of dispensing, leveling, and covering the sensor or cover glass;
4) During the LOCA leveling process, the adhesive thickness per unit area may vary;
5) During the solidification process, the movement of LOCA on the conveyor belt may cause displacement. The larger the area of the sensor or cover glass, the more likely it is to occur.
④ SLOCA full bonding process
SLOCA (Slit Liquid Optical Clear Adhesive) refers to a scraping type liquid optical adhesive. The SLOCA full bonding process is a bonding scheme that, under a certain pressure, presses SLOCA out along the mold gap and transfers it to the substrate, and converts it into a state similar to OCA optical tape through "scraping+UV pre fixing".
It is recommended to use SLOCA full bonding process for LCD+TP bonding with smooth and flat product surfaces. The process is: coating - UV pre bonding - bonding - defoaming - bonding. The slit coating has high coating efficiency, good uniformity and wide coating thickness range. OCR is extruded from the slit cutter head under a certain pressure, transferred to the substrate to form a shaped adhesive film, and converted into gel after pre curing. Most of the shrinkage stress has been released after pre curing. The shrinkage stress of full curing after bonding is lower, reducing the risk of macular. At the same time, the pre cured OCR can fix the upper and lower bonding surfaces and is easy to separate. After defoaming, defective and defective parts can be reworked by heating and separating.
3、 Optical Full Fit Screen Technology
1. Optical Full Fit Technology
In full fit technology, it can be further divided into GFF and OGS、Oncell、Incell。
①GFF
The GFF process refers to placing touch sensors on transparent and flexible substrates, and then attaching them to CG (Cover Glass). Simply put, the full bonding process changes the touch layer of the intermediate glass substrate in the non full bonding process to a thin film substrate, and then applies a conductive coating (IOT Film, indium tin oxide film) on the upper and lower sides of the thin film substrate, reducing the thickness. The process flow mainly includes: (I) large pieces of glass ->cutting ->front and back film application ->strong acid corrosion edge sawtooth ->chemical strengthening ->cover glass. (II) Flim yellow light process/laser (making touch circuit on PET material) -->film sensor. (III) Cover glass+adhesive+film sensor -->Apply OCA (optical adhesive) -->Apply LCM -->Complete.
②OGS
OGS technology refers to directly attaching touch sensors to the cover glass, with a relatively thin overall thickness, high touch sensitivity, and good optical effects. However, due to the fact that large pieces of glass in the production process need to be cut after the touch circuit is made, the cutting process will produce many serrated edges and corners, and the product may have edge collapse problems, resulting in low strength. The process flow mainly includes: large pieces of glass ->chemical strengthening (to make the glass structure full) ->yellow light process (to make touch circuits) ->cutting ->front and back film application ->strong acid corrosion edge sawtooth ->chemical strengthening ->OCA application ->LCM application.
③Oncell
The Oncell process refers to embedding touch sensors between the polarizer and filter of the display screen. Installing touch sensors on the LCD panel reduces the difficulty of In Cell technology significantly. Samsung, Hitachi, LG and other manufacturers have made rapid progress in On Cell structured touch screens. Currently, On Cell is mostly used in Samsung AMOLED panel products, but the technology has not yet overcome the problems of thinness and uneven color when touched.
④Incell
The Incell process refers to embedding touch sensors into the LCD display layer of a display screen. Embedding touch sensor function inside the display screen can make the screen lighter and thinner. At the same time, the In Cell screen also needs to be embedded with a matching touch IC, otherwise it is easy to cause incorrect touch sensing signals or excessive noise. Therefore, for any display panel manufacturer, the threshold for entering the In Cell/On Cell touch screen technology is indeed quite high, and the difficulty of low yield rate still needs to be overcome. Because once the In Cell is damaged, not only the touch screen is lost, but also the display screen will be scrapped together, manufacturers have higher requirements for the In Cell yield.
2. Differences and Similarities in Optical Full Fit Technology
Strictly speaking, GFF full lamination is not a true full lamination technology. The true full lamination technology involves fusing the touch layer of the middle layer upwards or downwards, while GFF uses optical glue to lamination the ITO Film touch layer upwards, which is more advanced than non full lamination, but in fact, it is not a strict "fusion". True full fit technology such as OGS touch unit up and protection layer fusion, such as Oncell, Incell touch unit down and LCM fusion. Based on process integration, touch panel manufacturers mainly promote GFF and OGS technologies, while display panel manufacturers mainly promote Oncell and Incell technologies.
4、 Common issues with optical full fit
1. Bubble generation
From the figure below, we can see that bubbles mainly exist at the edge of the TP window, and the size of the bubbles is about 1mm, which is a very serious bubble. We see that bubbles in this phenomenon mainly exist in the window, which can be eliminated by high temperature and pressure. However, after being eliminated for a long time, the bubbles will rebound and gradually expand.
① The principle of bubble generation
The individual surfaces of LCM and TP are not completely flat and have no tolerance. In addition, there is a difference in ink segment on TP, so residual air film is inevitable at the edges of ink and TP VA area. In addition, a small amount of bubbles will also be generated in G+G full bonding. The mechanism of defoaming mainly involves increasing the fluidity and moisture of the adhesive and generating appropriate compression pressure, catalyzing the phenomenon of air dissolution to remove the air quality. Air quality will not be eliminated or disappear, but will only diffuse to the surface of OCA and integrate into OCA. Using appropriate defoaming temperature, pressure, and time can prevent the edges of the adhesive from absorbing too much air.
② Bubble type
We know that after using a vacuum laminating machine for lamination, bubbles are easily left on the bonding surface, and most of them can be removed through defoaming. However, there is a 20% chance that small single bubbles will be left. Usually, we refer to these small bubbles as poor defoaming and Delay Bubble.
(I) Poor defoaming
Poor defoaming refers to the difficulty of removing small bubbles left after a single defoaming, as the bubbles shrink and the OCA under the relative area increases, forming a wall effect. This means that pressure cannot be effectively transmitted to small bubbles, resulting in incomplete defoaming. Single point pressure defoaming can be used to solve this problem.
（Ⅱ）Delay Bubble
The definition of a Delay bubble is a bubble that relapses immediately or after a certain period of time after the completion of defoaming, and its causes can be summarized into two characteristics: a rigid type of re bubble and an internal stress type of re bubble.
(1) Rigidity type recurrent bubbles
After the G+G bonding is applied, pressure is generated on the TP ink segment difference, and the stiffness of the TP material will not disappear. Therefore, there will be rigid re bubbles at the edge of the ink, which can be eliminated by single point pressure defoaming. However, TP stiffness always exists, which has the possibility of re emergence. We effectively reduce the imbalance between TP stiffness stress and OCA stress recovery by using the method of "slowly releasing pressure through defoaming".
(2) Stress type recurrent bubbles
This type of Delay Bubble is the most troublesome type, and it is caused by particles (impurities) in the OCA and OCA/TP/LCM interlayer. However, not all particles will produce this type of Rebubble, and it is also independent of the size of the particles. It cannot be prevented and controlled solely by measurement screening. The main key point is the three-dimensional shape of the particles, and generally three-dimensional particles are prone to producing bubbles.
2. Mura generation
The main display principle of LCD is controlled by a thin film transistor (TFT) to rotate the angle of the liquid crystal (LC), and then the light source (B/L) penetrates through the liquid crystal arranged at different angles through the color filter (CF) to display different color combinations. Simply put, mura refers to the phenomenon where the brightness of a monitor is uneven, causing various marks. The main reason for the emergence of MURA is the visual perception of color differences due to different frequency responses to the perceived light source.
① Mura classification
② Mura generation principle
(I) CF process
(1) Film thickness: Uneven coating of film thickness can lead to uneven color distribution.
(2) Coating: If there is blockage in the nozzle during coating, it will cause Vertical Mura.
(II) TFT process
Over Lap: The main composition of TFT is the overlap between layers. If a certain layer undergoes displacement during the production process, it may produce anomalies, which may affect the characteristics of TFT and cause Mura.
(III) LCD process
(1) Cell Gap: Uneven Cell Gap can cause Mura phenomena with higher or lower orders.
(2) Spacer: The uniformity of Spacer dispersion and the uniformity of Spacer's raw materials may both affect the generation of Mura.
(3) Roller: Due to excessive pressure on the roller of the machine or foreign objects adhering to the wheels, the panel may produce pellets or strips of Mura after passing through the rollers.
(4) Stage: Due to excessive suction on the stage of the machine, a Stage Mura may be formed after adsorption, or the flatness of the Stage may be poor. When passing through the process machine, there may also be related Mura.
(IV) LCM process
(1) Roller: Due to excessive pressure on the roller of the machine or foreign objects adhering to the rollers, the panel may produce pellets or strips of Mura after passing through the rollers.
(2) Vacuum: StageMura is formed after adsorption due to excessive Stage suction.
(3) Black Light: Poor film material or other abnormalities caused by incoming materials can also lead to Mura phenomenon in B/L. Usually, cross validation and other methods are used to confirm whether there are any abnormal phenomena in the backlight.
(4) Polarizing film: The polarizing film itself has poor incoming materials, causing Mura to form after being biased. Usually, after replacing the polarizing plate, it is OK.
(5) Human factors: Due to improper handling of the panel, the force of fingers pressing on the panel is too strong, which can easily create a pill like Mura.
③ Mura Analysis of Vehicle Display
After the full fit (OCR/OCA) process, the onboard display module will experience edge linear or point like macular defects, which are commonly known as Mura yellowing and are common on the four sides and corners of the display screen. On the premise of excluding defects in the LCD itself, the generation of mura requires more consideration of material and process defects.
(I) Material properties
The car mounted display screen requires excellent optical performance to ensure the clarity of the final image. Therefore, if the performance of the material itself changes, it is easy to lead to the final Mura problem of the product. This yellowing may be caused by prolonged exposure to high temperatures during the manufacturing process or environmental testing process: the glue turns yellow, such as the yellowing or deformation of membranes such as filters, brighteners, and light diffusion films.
Material yellowing is easy to determine: by disassembling the module, yellowing or deformation can be visually detected. To address this issue, it is necessary to replace it with a product with better performance.
(II) Technological process
If the above issues are excluded, the main source of defects caused by the manufacturing process is the fully bonded processing process. The main cause is that the liquid crystal layer of LCD is subjected to stress stretching. The liquid crystal CF layer is affected by stress, resulting in the formation of cell gaps.
(1) Mura stress source
OCR has a process from liquid to solid, and after solidification, its uneven volume shrinkage leads to stress on the liquid crystal; The warping of the LCM/cover plate itself; Abnormal OCR film thickness, etc; During the curing process, it is subjected to external stress.
(2) Solution
Control the uniformity of adhesive film thickness; HybridOCR is pre cured or surface cured to form a pure heat cured material before bonding; Choose adhesive materials with low modulus, low hardness, and low curing shrinkage rate; Control CG&OGS/LCM warping.
5、 Introduction to Zhongda Optical Full Fit Materials
1. OCR optical bonding material
① TP fitting
(I) G+G dot gluing process
In response to the G+G spot adhesive process in TP bonding process scenarios, Xiaoda is promoting a GG303 acrylic series liquid type UV cured OCR, which has strong adhesion to substrates, strong QUV and yellowing resistance, environmental aging resistance, and long-term reliability.
(II) G+P dispensing process
For the G+P dispensing process in TP bonding process scenarios, Xuda mainly promotes three OCRs: GP301, GP201, and GP102.
(1) Zuoda GP301 is a acrylic based liquid type UV cured OCR with strong adhesion to substrates, strong QUV and yellowing resistance, environmental aging resistance, and long-term reliability.
(2) Zuoda GP201 is a silicone based liquid type UV delay cured OCR with strong adhesion to substrates, low curing shrinkage, strong QUV and yellowing resistance, high temperature aging resistance, and long-term reliability.
(3) Zuoda GP102 is an organic silicon based two liquid type heat cured OCR with strong adhesion to substrates, low curing shrinkage, strong QUV and yellowing resistance, high temperature aging resistance, long-term reliability, and easy repair.
(III) G+G slit coating
For the G+G narrow seam coating process in TP bonding process scenarios, Xuda mainly recommends two OCRs: GG305 and GP201.
(1) Zuoda GP305 is an acrylic based liquid type UV cured OCR with strong adhesion to substrates, strong QUV and yellowing resistance, environmental aging resistance, and long-term reliability.
(2) Zuoda GP201 is a silicone based liquid type UV delay cured OCR with strong adhesion to substrates, low curing shrinkage, strong QUV and yellowing resistance, high temperature aging resistance, and long-term reliability.
② Optical full fit
(I) TP+LCM dispensing process
For the TP+LCM dispensing process in optical full lamination process scenarios, Xuda mainly promotes three OCRs: TL201, TL101, and TL301.
(1) Zuoda TL201 is a silicone based liquid type UV delay cured OCR with strong adhesion to substrates, low curing shrinkage, strong QUV and yellowing resistance, high temperature aging resistance, and long-term reliability.
(2) Zuoda TL101 is an organic silicon based two liquid type heat cured OCR with strong adhesion to substrates, low curing shrinkage, strong QUV and yellowing resistance, high temperature aging resistance, long-term reliability, and easy repair.
(3) Zuoda TL301 is an acrylic based liquid type UV cured OCR with strong adhesion to substrates, strong QUV and yellowing resistance, environmental aging resistance, and long-term reliability.
(II) TP+LCM slit coating
For the TP+LCM narrow seam coating process in optical full lamination process scenarios, Xiaoda mainly promotes two OCRs: TL305 and TL201.
(1) Zuoda TL305 is an acrylic based liquid type UV cured OCR with strong adhesion to substrates, strong resistance to QUV and yellowing, environmental aging resistance, and long-term reliability.
(2) Zuoda TL201 is a silicone based liquid type UV delay cured OCR with strong adhesion to substrates, low curing shrinkage, strong QUV and yellowing resistance, high temperature aging resistance, and long-term reliability.
(III) TP+LCM steel screen printing
For TP+LCM steel screen printing technology in optical full bonding process scenarios, we are promoting a TL245 organic silicon one liquid UV delay curing OCR, which has strong adhesion to the substrate, low curing shrinkage rate, strong QUV and yellowing resistance, high temperature aging resistance, and long-term reliability.
