2025-10-26
Mura is a technical term in the LCD industry referring to various visual artifacts caused by uneven brightness in LCD/TFT screens. It degrades the viewing experience and diminishes the product’s premium feel. This is a high-frequency cosmetic defect in the manufacturing of medium-to-large automotive touch display modules.
Mura Detection: The simplest method involves switching to a black, white, or low-gray-scale image in a dark room. Visually inspect the edges of the AA zone for visible white, yellow, or other irregular colors. Observing from multiple angles reveals diverse Mura manifestations: stripes, patches, clusters, or random color patterns. Deliberately pressing the display area intensifies existing Mura, and the effect persists even after pressure is released.
From the working principle of liquid crystal displays, Mura is unavoidable whenever stress is applied to the LCD, varying only in severity. Fujifilm Corporation of Japan developed ND filters based on the principle of camera lens filters. These filters come in eight grades: 1%, 2%, 3%, 4%, 5%, 6%, 8%, and 10%. The industry classifies them by grade. Product specifications typically list 5% or 6%; other grades have not been encountered in actual work. Selection can be made according to specific needs.
Mura can originate from pre-production materials or processes affecting the display panel (solutions are unknown, so I won’t speculate), or from post-production assembly materials and techniques.
This article focuses solely on Mura caused by lamination stress after assembly. Assuming defects in cover glass, panel, polarizers, backlight, etc., are excluded, we address Mura resulting from liquid crystal cell deformation due to lamination stress.
As shown in the example below, common yellow spots indicate uneven display.
Detailed teardown analysis of the defective unit revealed no changes after removing the casing or backlight. However, the yellow spots disappeared after disassembling the CTP and TFT layers. Reapplying the CTP and leaving it undisturbed for several days caused the yellow spots to reappear.
This confirms that the Mura defect is related to the bonding materials or process.
Regardless of whether using OCA or water-based adhesive processes, the appearance of mura after curing or resting is significantly related to adhesive selection and curing procedures.
First, regarding the impact of optical adhesives: polymer-based adhesives tend to be relatively rigid. If the selected adhesive thickness is insufficient, it poorly absorbs strain forces. The display’s liquid crystal layer then experiences deformation stress, causing this deformation to propagate to the display surface. This results in poor uniformity of the liquid crystal cell gap, leading to uneven light transmission. Visually, this manifests as color unevenness, forming Mura.
Solutions:
1) Increasing the adhesive layer thickness allows it to absorb more deformation stress, but material costs will rise significantly—potentially doubling. Therefore, selecting 175μm or thinner OCA is not recommended.
2) Reducing the cover glass thickness minimizes deformation, but may compromise impact resistance.
3) Select soft adhesive (with lower elastic modulus), as overly soft adhesives are difficult to bond.
4) Opt for sandwich-structured adhesives, which combine the advantages of options 1 and 2. The NoseDispaly G93 series has been used in our projects, delivering the most optimal improvement.
Stress distribution diagram after bonding Stress distribution diagram after bonding
5) Appropriately reduce bonding pressure, prioritize rear-group backlighting whenever possible, and ensure the bonding platform remains as flat as possible to minimize damage to the LCD cell caused by excessive compression.
6) Improve the curing shrinkage rate of water-based adhesives by selecting materials with low shrinkage. In addition to changing adhesive types, adjusting curing process parameters—such as using low-dose, extended-duration UV curing or slow-temperature-rise curing—can mitigate Mura to a certain extent.
To learn more about MURA and the entire production process for automotive touchscreens, join the www.nosedisplay.com Technical Forum.