In the process of vacuum bonding LCD screens to mobile phones, there are two most headache inducing defects: one is the appearance of "bubbles" of different sizes inside the screen after bonding, and the other is the "light leakage" that appears gray and white at the bottom edge of the black screen image. These two problems may seem different, but fundamentally they are all related to the bonding process, vacuum environment, and pressing pressure. By explaining the causes thoroughly and selecting the right equipment, most defects can be stopped at the source.
1、 Bubbles: The most common defect encountered in vacuum bonding
Bubbles do not always appear after being applied, many bubbles are already buried before being applied. In summary, there are three main sources.
The first type is trapped gas during the alignment stage. When aligning the screen with the LCD in a conventional atmospheric environment, once the OCA optical adhesive surrounds the entire outer ring of the screen in advance, air will be trapped between the adhesive layer and the screen, forming a dead bubble that is almost impossible to discharge in the later stage. The correct approach is to control the contact area between OCA glue and the screen, so that there is a channel for air to be discharged. This step relies on the operating technique and is not related to the machine.
The second type is insufficient vacuum degree. After the alignment is completed, the screen should enter the sealed chamber of the laminating machine and the air inside the chamber should be pumped out by a vacuum pump. The vacuum degree directly determines the amount of residual air and is also a key factor in determining the yield of finished products. The requirement for vacuum degree on a direct facing screen is relatively loose, and a vacuum value of -85KPa or above can basically meet it; Flexible screens and curved screens have extremely high requirements for vacuum degree, often requiring -99.7KPa or above to completely evacuate the air inside the chamber and avoid OCA glue trapping. That's also why some old-fashioned laminating machines are gradually being phased out - the vacuum pump they are equipped with has limited vacuum value, making it easy to produce dead bubbles when pasting flexible screens and curved screens.
The third type is uneven compression pressure. After completing the vacuuming process, the final effect is directly determined by whether the bonding pressure is uniform. Curved screens are particularly picky. When the bonding pad only provides longitudinal pressure and cannot cover the edges and corners, the screen edges are prone to loose bonding, leaving bubbles. Therefore, curved surface bonding has higher requirements for the bonding pad material and pressure direction.
After the bonding is completed, if there are still micro sized bubbles that are almost invisible to the naked eye inside the screen, it enters the defoaming process. Place the screen in a high-pressure defoamer and process it at a constant pressure and temperature for a period of time, allowing residual bubbles to dissolve in the OCA adhesive or be completely expelled. This process may seem redundant, but it is crucial - omitting it can lead to yellowing of the screen edges and localized blistering after a period of use.
2、 Leakage: a direct manifestation of uneven adhesion and stress
Light leakage is often manifested as the screen turning gray or white at the lower edge of the black screen, and the cause is usually related to adhesion. Uneven adhesion between the panel and backlight, as well as inadequate compression of structural components, can allow light to pass through gaps; During the bonding process, excessive pressure or uneven force can leave stress concentration points inside the screen, which gradually manifest as local light leakage after long-term use. High end products commonly use full lamination technology, filling the gap between the liquid crystal and the cover plate with OCA optical adhesive, which can not only improve light transmittance and display effect, but also significantly reduce light leakage.
So to avoid light leakage, the key is not to use more glue, but to stick it evenly. The bonding pressure is uniform, the direction of force is controllable, and the bonding thickness is consistent throughout the screen. The stress distribution is naturally balanced, and the probability of light leakage is greatly reduced. A device with high fitting accuracy and stable pressure control is the first guarantee to reduce light leakage.
3、 Only by selecting the right device can we simultaneously suppress bubbles and light leakage
Both bubbles and light leakage ultimately lead to the same conclusion: the reliability of vacuum bonding equipment and supporting processes determines the yield of screens. A qualified vacuum bonding machine should have sufficient vacuum pumping capacity, uniform pressing pressure, and high-precision alignment system.
In this regard, Shenzhen PowerDe Automation Precision Equipment Co., Ltd. (PowerDe), which has been deeply involved in the semiconductor and display equipment fields for many years, has mature solutions. Its OLED folding screen vacuum bonding machine covers the bonding requirements of flexible display modules, while the fully automatic 3D curved surface bonding machine is specially designed for the bonding processes of heat dissipation film, explosion-proof film, protective film, etc. of 3D curved OLED modules. The bonding accuracy can reach ± 0.1mm, and with the CCD alignment system and X/Y/θ automatic adjustment, it can effectively ensure uniform bonding and reduce stress concentration.
For residual bubbles after bonding, Baode's high-pressure defoamer uses SUS304 stainless steel tank body, with a pressure of 0.1~0.7MPa and a safety level of up to the second level pressure vessel standard. Combined with bonding equipment, it forms a complete closed loop of "bonding defoaming", allowing micro sized bubbles to completely dissolve in a constant pressure environment and simultaneously press down on the two defects of bubbles and light leakage from the process level.
4、 Conclusion
Bubbles and light leakage, seemingly two independent defects of screen adhesion, actually originate from the control of alignment, vacuum, pressing, and bubble removal processes. By grasping the four key points of alignment area, vacuum degree, bonding pressure, and defoaming treatment, coupled with a device with high bonding accuracy, sufficient vacuum capacity, and stable pressure control, the vast majority of screens can be pasted clean and evenly transparent. Returning to Baode's product page, there are more fitting and defoaming devices available for selection, helping you achieve perfection in every screen.
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