In the production and processing of resistive touch screens and liquid crystal displays, Newton’s rings (some manufacturers also call them rainbow patterns, or simply rainbows) act like a ghost drifting in the workshop. If not careful, they can appear from time to time during production and customer use, causing many process technicians on-site management to be perplexed. It’s not because the rainbow is too beautiful, but because this beautiful quality killer can easily cause trouble in the current industry, allowing others to quickly spot the defects.
In the display module, the area where Newton’s rings appear can cause color overlay due to light interference, leading to incorrect color representation, and on the other hand, it reduces the contrast of the display area, making it a major fatal defect.
Let’s take a look at a design that has no issues:
The Mechanism of Newton’s Rings Formation
We know that whether it is a resistive touch screen or a liquid crystal display, the supporting body 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 creates a curved surface with a certain radius of curvature, similar to the effects of a convex lens and a plane mirror discussed in regular physical optics that produce Newton’s rings. Newton’s rings also reflect the relationship between the difference in optical path of reflected and incident light on the two surfaces and the wavelength. As the optical path difference increases, meaning the distance between the two surfaces increases, the spacing of the Newton’s rings also increases.
In actual production, whether for resistive touch screens or liquid crystal displays, the gap distance at the supporting frame is generally made slightly larger than that in the middle. If the parameters in the process deviate slightly, this distance difference cannot be eliminated, causing a certain inward concave deformation between the two surfaces. This results in a different optical path difference between the incident and reflected light during their interference, selecting different wavelengths to show the corresponding colors.
Common Locations and Causes of Newton’s Rings in Actual Production
In liquid crystal display modules, there are three places where Newton’s rings are most likely to occur:
1. Internal rainbow produced inside the liquid crystal display.
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 design process, rainbow defects will occur. Another main cause is that during the boxing process, the inside is contaminated by external objects exceeding the particle diameter, which is why the liquid crystal display workshop is very strict about maintaining a clean environment.
2. Water ripples (also known as water patterns) between the liquid crystal display and the resistive touch screen.
These water ripples are also a type of Newton’s rings. Especially when the resistive touch screen is in action, external pressure applied to the resistive touch screen causes the lower surface to bend and deform, causing the color radius of the Newton’s rings to move, generate, or disappear with changes in force, similar to the ripples formed when a stone is thrown into water. Thus, this type of Newton’s rings is also referred to as water ripples, changing with the location and pressure of the action.
3. Newton’s rings (also known as rainbow patterns) inside the resistive touch screen.
In the production of resistive touch screens, to create a deformation amount in the touch area, the height of the outer frame is generally much higher than the internal support points. If water-based adhesive is used for the outer frame, the height is generally around 50 to 70 microns; if double-sided adhesive based on PET material is used, it is generally above 50 microns, while the height of the internal support points is generally less than 25 microns. If a DOT with a diameter of 30 to 45 microns is used, the final height of the support point is only around 15 to 25 microns.
Additionally, the operating surface of the resistive touch screen is also designed to create a deformation amount in the touch area, usually made of flexible ITO FILM. Therefore, if the process parameters are not precisely controlled during production, the ITO FILM in the middle part may collapse and adhere to the support point, creating a curved deformation. This causes interference between the incident light and the reflected light on the two inner surfaces of the ITO, forming Newton’s rings.
Let’s illustrate with a picture:
How to Calculate Design Parameters to Prevent the Formation of 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, based on the product dimensions, we can calculate the height of the inner and outer frames, ensuring that the radius of the dark ring of the Newton’s rings falls outside the product dimensions, thus optimizing our design parameters. (For how to calculate and the principle formula, please refer to the attachment “Equal Thickness Interference Experiment”).
Of course, the above method is only a theoretical verification. In reality, if we strictly follow the calculated data, we may not be able to produce products that meet customer requirements. However, this calculation can guide us in how to approach theoretical values during production.
Some may say that the calculations may not be accurate, and we cannot avoid this in production. Is there no better method? Yes, we can still refer to the method in the attachment “Equal Thickness Interference Experiment”. By using a microscope or micrometer, we can measure how much deformation occurs in the upper film before the Newton’s rings appear, and 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 the problem should first be verified through experimental methods, then validated with experimental data, and finally adjusted through process parameters.
How to Prevent the Formation of Newton’s Rings from a Process Perspective
1. Rainbow in liquid crystal displays
The source of rainbow defects can be quickly identified using a microscope. If it is a point defect, three situations can be observed under the microscope: insufficient number of particles, particle aggregation, or foreign objects within the box. Such defects can be immediately addressed by adjusting the parameters of the particle dispensing machine and maintaining the cleanliness of the dispensing and assembly environment.
If the entire liquid crystal display exhibits a rainbow, it is either due to incorrect matching of external frame and internal space particles or insufficient quantity of internal space particles. Such defects may require checking the process material parameters of the particle dispensing and adjusting parameters based on particle detection data.
If a rainbow appears near the external frame, it may be due to contamination of the packaging materials or uneven force during boxing. This requires checking the boxing equipment and maintaining it. Faults in the boxing equipment or foreign objects during the boxing process can cause Newton’s rings to appear in adjacent products at the same location, indicating a pattern.
2. Water ripples between the resistive touch screen and the liquid crystal display
To prevent the formation of water ripples, one can increase the strength of the resistive touch screen. For instance, switching from PC material to a stronger glass material or increasing the thickness of the material. This is one of the reasons why resistive touch screen products exceeding a certain size require 2MM thick glass or tempered glass. All changes in design parameters are logical and cannot be blindly copied; doing so without understanding the principles can either waste materials and quality costs or fail to avoid potential quality defects and product yield issues.
Additionally, under customer requirements, using thicker adhesive between the resistive touch screen and liquid crystal display can move the dark ring of the Newton’s rings outside the product dimensions. Alternatively, adding support points on the assembly surface can 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 or water ripples.
If it meets the display requirements, one can also replace the polarizing film on the liquid crystal display with an anti-glare polarizing film. Note that the close-up display effect of anti-glare polarizing films is significantly worse than that of regular polarizing films, especially for liquid crystal displays above QVGA. If replaced, the clarity, response speed, and contrast of the displayed patterns will noticeably decrease, so it must be confirmed with the final customer 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 operating surfaces of resistive touch screens use flexible ITO FILM materials.
To prevent Newton’s rings in resistive touch screens, it is crucial to select appropriate tooling and equipment parameters during the conditioning process of the ITO FILM, ensuring that all parts of the ITO FILM contract uniformly without causing unevenness that could be transferred to the ITO FILM. Newton’s rings caused by improper conditioning of the ITO FILM have a clear characteristic: if the product is assembled and then disassembled, no Newton’s rings are visible, but they will appear approximately 48 hours after the product is disassembled.
Controlling the height of the support points is also a primary method for preventing Newton’s rings. From the calculations above, it can be seen that exceeding a certain height for the support points will make it difficult for them to form or they will be spaced far apart, thus fading. Therefore, we must ensure a certain height for the support points.
The height of the support points is definitely related to the area of the support points. Under the same conditions, the higher the height, the larger the area of the support point. For liquid crystal displays with fine image quality, pixel sizes may only range from 60 to 90 microns. If a support point exceeds half the size of the pixel, that pixel will show distortion in display, and at a certain distance, the entire layout of the support points will become visible. To solve this problem, Japanese manufacturers use molds or spacers in the design of small-sized products with narrow edges to fill the air, artificially increasing the distance between the inner surfaces of the product, thus avoiding the occurrence of Newton’s rings without needing to design overly complex air passages, while also allowing for smaller support point areas.
The defect of the air-filling method is that it cannot be used in environments with significant temperature changes or in large-sized products. Especially for products using water-based adhesives, due to narrow edge designs and difficulty in operation, it results in insufficient bonding strength. After drastic temperature changes, it is easy to cause air leaks, and once air leaks occur, the effect of preventing Newton’s rings is lost. Many manufacturers’ products have no issues when placed in their constant temperature warehouses before delivery, but after being placed in a customer’s ordinary warehouse for a week or simply transported by car, the phenomenon of Newton’s rings appears, which is the reason. Therefore, air-filling is not a universal remedy for Newton’s rings; if product usage conditions and environments are not considered, along with other auxiliary processes, it will not be effective.
Newton’s rings in the sensitive areas of resistive touch screens are similarly caused by combinations of material thicknesses and inappropriate parameters of FPC heat pressing, which can generally be resolved by adjusting the thickness of the FPC materials and heat pressing parameters.
As for larger-sized display products viewed from a distance, since pixel sizes are also larger, support points of 50 to 60 microns can be used, and unless the support points are unevenly printed or the heights of the support points collapse, Newton’s rings generally should not occur.
After discussing so much, a picture is often better than words:
Conclusion
Newton’s rings are merely the result of interference between parallel incident light and reflected light, which only manifests under certain optical path differences. To avoid the formation of Newton’s rings, one method is to disrupt the propagation direction of light, for example, using frosted ITO FILM on resistive touch screens; on liquid crystal displays, using anti-glare polarizing films, etc.; another method is to ensure the parallelism of the inner surfaces, making the radius of curvature sufficiently large to expand the dark ring area of the Newton’s rings outside the product; or increasing the optical path difference between the two surfaces to ensure that the dark ring area falls outside the product dimensions or increasing the distance between adjacent rings to fade the Newton’s rings. As long as we control the production process based on the mechanism of Newton’s rings formation, they are not a quality ghost.
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