Light leakage Mura defect involving non-uniform brightness became severe for a 13” TFT-LCD using COG package with 8 IC drivers mounting on glass substrate directly, when the thickness of glass substrates decreased from 0.5 mm to 0.3 mm.
The CTE mismatch in components of TFT-LCD (such as glass plates and Si chips) resulted in the warpage and non-uniform stress in the TFT-LCD after the thermal bonding process. To investigate the relationship among Mura, warpage and stress, a 3-D finite element analysis (FEA) model coupled with transient thermal analysis using ANSYSTM has been established and validated by surface contour measurement tool using KOSAKA ET4000A.
Based on these results, excellent correlation between COG-induced Mura and localized warpage (or principal stress) has been found. The light leakage phenomenon was successfully related to the localized warpage and non-uniform maximum principle stress distribution. In addition, two possible root-causes for COG-induced Mura were proposed: (1) the director of liquid crystal was reorientated and (2) the polymer chains at the surface of alignment layer were redirected by stress.
The COG-induced warpage and stress were caused by the Si chips attachment process through ACF adhesion, due to the CTE mismatch between different materials.
Thus, reducing the stress and localized warpage induced by COG packaging was critical for improving light leakage. Decreasing bonding temperature and modulus of ACF showed a 20-30% reduction of the warpage (or stress) and eliminated the Mura which appeared when thickness of glass decreased from 0.5 mm to 0.3 mm. Besides, the CTE of ACF had little effect on localized warpage and stress, and the effect of the ACF modulus was less than the ACF bonding temperature. By considering the LCD
development for light-weight LCD, the ACF bonding temperature would be the most important parameter when the glass substrate became thinner. This warrants new material design and chemistry to enable a lower thermal bonding temperature ACF material.
To improve Mura, increasing thickness of glass substrate, decreasing ACF bonding temperature and using low modulus ACF can decrease the localized warpage and stress. However, increasing glass substrate was opposite to the trend of modern LCD development. Most of LCD manufacturers currently reduced warpage by adjusting the properties of ACF which can be applied to a lower bonding temperature and low modulus.
In this study, other methods such as changing arrangement and dimensions of Si chips were proposed for solving Mura defect. According to the results, shrinking the chip’s height/length could provide significantly reduction (60%-80% up) in localized warpage and stress from simulation. In addition, a novel method, dummification, was also proposed for improving Mura defect by placing an appropriate dummy material between two Si chips to constrain the thermal-induced warpage. Using glass as the dummy material, the localized warpage and stress can be reduced ~60%. As to other materials for dummification, materials with high modulus and proper CTE (close to the substrate’s CTE) would be the better choice.
To summarize the methods for improving Mura, Figure 5.1 showed the plot of maximum localized warpage in various approaches. The methods of decreasing bonding temperature and modulus of ACF can reduced 20%-30% warpage and stress, which can improve the light leakage Mura as the experimental pictures shown, the methods of changing dimensions of Si chips (especially height and length) and dummification which can reduce 60-80% warpage and stress, indicating its great promise and potential to eliminate the light leakage Mura.
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