In the production and processing of resistive touch screens and liquid crystal displays, Newton’s rings (also referred to as rainbow patterns by some manufacturers) appear like a ghost haunting the workshop. If not careful, they can unexpectedly emerge during production and customer use, causing considerable distress for the process engineers managing the site. This is not because the rainbow is too beautiful, but rather because this beautiful quality killer can easily cause trouble in the current industry, allowing others to quickly identify defects.
In display modules, the areas where Newton’s rings appear are due to light interference, which causes color superposition, leading to incorrect final color representation. On the other hand, it also reduces the display contrast in that area, making it a critical defect.
1. Mechanism of Newton’s Ring Formation
We know that whether it is a resistive touch screen or a liquid crystal display, the supporting structure consists of two pieces of ITO glass or one piece of ITO glass and one piece of ITO film. If one side of the material deforms, the inner surface of the ITO material generates a curved surface with a radius of curvature, similar to the effect of a convex lens and a plane mirror discussed in classical optics that produces Newton’s rings. Newton’s rings also reflect the relationship between the path difference of reflected and incident light and the wavelength between the two surfaces. As the path difference increases, meaning the distance between the two surfaces increases, the spacing of the Newton’s rings will also increase.
In actual production, whether for resistive touch screens or liquid crystal displays, the gap at the external frame support is usually made slightly larger than that in the middle. If there is a slight deviation in the process parameters, this distance difference cannot be eliminated, leading to a certain inward concavity of the two surfaces. This results in different path differences of light between the two surfaces, and during the interference process of incident and reflected light, different wavelengths will be selected from different path difference regions, displaying corresponding colors.
2. Locations and Causes of Newton’s Rings in Actual Production
In liquid crystal display modules, there are three main areas where Newton’s rings are most likely to occur:
1. Internal rainbow formation in liquid crystal displays.
The thickness of the liquid crystal display box is generally below 10 microns. If the number of particles in the internal space is insufficient, unevenly distributed, or if the diameter of the particles in the external frame does not match the internal support design, rainbow defects will occur. Another major cause is contamination by external objects larger than the particle diameter during the box formation process, which is why strict cleanliness control is enforced in liquid crystal display manufacturing environments.
2. Water ripples (also called water patterns) between the liquid crystal display and the resistive touch screen.
This water ripple is also a type of Newton’s ring. Particularly when the resistive touch screen is activated, external pressure applied to the resistive touch screen causes deformation of its lower surface. As the force varies, the color radius of the Newton’s rings will shift, generate, or disappear, similar to the ripples created when a stone is thrown into water. Therefore, this type of Newton’s ring is also referred to as water ripples or water patterns, and it changes with the location and pressure of the action.
3. Internal Newton’s rings (also called rainbow patterns) in resistive touch screens.
During the production of resistive touch screens, to create a deformation in the touch area, the height of the external frame is generally much higher than that of the internal support points. If water-based adhesive is used for the external frame, the height is typically around 50-70 microns; if PET-based double-sided adhesive is used, it is generally over 50 microns, while the internal support points are usually less than 25 microns. If dots with diameters of 30-45 microns are used, the final height of the support points will only be around 15-25 microns.
Additionally, the operating surface of the resistive touch screen, which is typically made of flexible ITO film, is also designed to create a deformation in the touch area. If the process parameters are not controlled accurately during production, the ITO film in the central part of the product may sag and adhere to the support points, creating a curved deformation. This results in interference between the incident light and the reflected light from the two inner surfaces of the ITO, forming Newton’s rings.
3. Calculating Design Parameters to Prevent Newton’s Rings
In actual production, effectively preventing the formation of Newton’s rings can also be achieved by applying the optical principles of Newton’s rings.
At the beginning of product design, one can calculate the height values of the inner and outer frames based on the product dimensions, ensuring that the radius of the dark ring of the Newton’s rings falls outside the product dimensions. This allows for optimization of design parameters. (For calculation methods and principles, please refer to the attachment “Equal Thickness Interference Experiment”).
Of course, this method is only a theoretical verification. In practice, if we strictly follow the calculated data, we may not be able to meet customer requirements. However, this calculation can guide us in production to get as close to the theoretical values as possible.
Some may argue that the calculated results may not be accurate, and since we cannot avoid issues in production, is there no better solution? Yes, there is. We can refer to the methods in the attachment “Equal Thickness Interference Experiment” to measure how much the upper film deforms downwards before Newton’s rings appear. We can then calculate the minimum thickness of the internal space particles or support points when the Newton’s rings appear. If combined with a pressure gauge, we can also measure the pressure at which the product exhibits Newton’s rings. Therefore, the emergence of problems should first be validated through experimental methods, then verified through experimental data, and finally adjusted through process parameters.
4. Preventing the Formation of Newton’s Rings from a Process Perspective
1. Rainbows in liquid crystal displays.
The source of rainbow defects can be quickly identified using a microscope. If they are point-like, three situations can be observed under the microscope: insufficient number of space particles, aggregation of space particles, or foreign objects within the box. Such defects can be immediately addressed by adjusting the parameters of the particle distribution machine and maintaining cleanliness in the environment of the particle distribution and assembly sections.
If the entire liquid crystal display exhibits rainbow patterns, it may be due to incorrect matching of external frame and internal space particles, or insufficient quantity or absence of internal space particles. Such defects may require checking the process material parameters of the particle distribution and adjusting parameters based on detection data.
If rainbows appear near the external frame, it may be due to contamination of the external frame packaging material by foreign objects, which can be seen under a microscope, or uneven force during box formation, necessitating inspection and maintenance of the box formation equipment. Equipment failures during box formation or the presence of foreign objects can cause Newton’s rings to appear in the same position on adjacent products, indicating a pattern.
2. Water ripples between the resistive touch screen and the liquid crystal display.
To prevent the formation of water ripples, one approach is to increase the strength of the resistive touch screen. For example, replacing the original PC substrate with a stronger material or glass, or using a thicker substrate instead of a thinner one. This is one reason why resistive touch screen products exceeding a certain size require 2mm thick glass or tempered glass. All design parameter changes are justified and cannot be copied blindly from external sources. If the principles are not understood and copied blindly, it may lead to wasted materials and quality costs or unavoidable potential quality defects and product yield issues.
Additionally, under customer requirements, selecting a thicker double-sided adhesive for the assembly between the resistive touch screen and the liquid crystal display can move the dark ring of the Newton’s rings outside the product dimensions. Alternatively, support points can be printed on the assembly surface of the resistive touch screen and liquid crystal display to transfer external pressure through the support points to the liquid crystal display and mainboard casing, reducing the deformation of the resistive touch screen and preventing the formation of Newton’s rings, i.e., water ripples.
If the product’s display requirements can be met, and if acceptable display effects are considered, the polarizer on the liquid crystal display can be replaced with an anti-glare polarizer. Note that the near-view display effect of anti-glare polarizers is significantly worse than that of ordinary polarizers, especially for liquid crystal displays above QVGA. If replaced, the display pattern’s detail, response speed, and contrast will noticeably decrease, so final customer confirmation of the display effect is essential before making the change.
3. Newton’s rings in resistive touch screens.
Preventing Newton’s rings in resistive touch screens can be quite troublesome because most resistive touch screens use flexible ITO film as the operating surface.
To prevent Newton’s rings in resistive touch screens, it is crucial to select suitable tooling and equipment parameters during the conditioning process of the ITO film, ensuring that all parts of the ITO film shrink uniformly without causing unevenness due to non-uniform shrinkage or transferring unevenness from the loading fixture to the ITO film. Newton’s rings caused by improper conditioning of the ITO film have a distinct characteristic: if the product is assembled and no Newton’s rings are visible, they will appear approximately 48 hours after the product is separated into individual units.
Controlling the height of the support points is also a primary method to prevent Newton’s rings. As shown in the calculations above, if the height of the support points exceeds a certain value, it becomes difficult to form them, or they are spaced far apart and diluted. Therefore, we must ensure a certain height for the support points.
The height of the support points is certainly related to the area of adhesion of the support points. Under the same conditions, the higher the height, the larger the adhesion area. For liquid crystal displays with fine image quality, pixel sizes may only be 60 to 90 microns. If a support point exceeds half the size of a pixel, that pixel will exhibit display distortion, and the entire layout of the support points may become visible from a certain distance. To address this issue, Japanese manufacturers use molds or spacer combinations in small-sized products with narrow bezels to fill the product with air, artificially increasing the distance between the internal surfaces of the product, thus avoiding the appearance of Newton’s rings without the need for complex and difficult-to-manufacture vent channels in the external frame, while also allowing for the design of small adhesion area support points.
The defect of the air-filling method is that it cannot be used in environments with drastic temperature changes or in large-sized products. Especially for products using water-based adhesives, due to narrow bezel designs and operational difficulties, adhesion strength may be insufficient. After drastic temperature changes, it is easy to cause air leakage, and once air leaks, the prevention of Newton’s rings is compromised. Many manufacturers’ products may not exhibit issues when stored in their constant temperature warehouses before shipment, but may develop Newton’s rings after being placed in a customer’s ordinary warehouse for a week or simply after being transported by vehicle, which is the reason for this phenomenon. Therefore, air-filling is not a one-size-fits-all solution for Newton’s rings. If the product’s usage conditions and environment are not considered, along with other auxiliary processes, it will not be effective.
Newton’s rings in the sensitive area of resistive touch screens are also caused by the combination of various material thicknesses and inappropriate parameters of FPC hot pressing, which can generally be resolved by adjusting the thickness of the FPC material and hot pressing parameters.
As for large-sized display products viewed from a distance, since pixel sizes are also larger, support points of 50 to 60 microns can be used. Unless the support points are unevenly printed or the height of the support points collapses, Newton’s rings generally will not appear.
5. Conclusion
Newton’s rings are merely the result of interference between parallel incident and reflected light, which only manifests within a certain path difference. To avoid the formation of Newton’s rings, one method is to disrupt the direction of light propagation, such as using matte ITO film on resistive touch screens or anti-glare polarizers on liquid crystal displays. Another method is to ensure the parallelism of the inner surfaces, allowing the radius of curvature to be large enough to expand the dark ring area of the Newton’s rings outside the product dimensions. Additionally, increasing the path difference between the two surfaces can move the dark ring area of the Newton’s rings outside the product dimensions or increase the distance between adjacent rings to dilute the appearance of Newton’s rings. As long as the mechanism of Newton’s ring formation is controlled during production, they are not a quality ghost.
Source: (http://blog.sina.com.cn/s/blog_647159e60100gzu0.html) – Quality Defects of Newton’s Rings in Touch Screen Display Modules and Preventive Measures