Chapter 4 Results and Discussion
4.5 Effect of the Thickness of Glass Substrate, Properties of ACF and
4.5.2 Bonding Temperature, Modulus and CTE of ACF
The required temperature for mounting the Si chips on glass substrate was decided by the cure characteristic of ACF adhesive. Although the issue of pot life of ACF may arise when using the improvement curing agent, the ACF with lower bonding temperature was better for improving Mura defect due to the less degree of thermal gradient resulting in decreasing warpage and stress. Figure 4.15 shows the plot of the maximum localized warpage and maximum first principal stress as a function of the ACF bonding temperature using 0.3 mm glass substrate. The maximum localized warpage/stress in the case of 180 oC ACF bonding temperature was 7.64 μm/4.29 MPa and then decreased with the reduction of the ACF bonding temperature. The maximum localized warpage/stress was 5.08 μm/2.91 MPa and 4.12 μm/2.35 MPa in the case of 160 oC and 140 oC ACF bonding temperature, respectively.
The light leakage phenomenon was improved with decreasing the ACF bonding temperature as confirmed by the photographs of the LCD lighted-up experiment as shown in Figure 4.16. The severity level and region of light leakage were reduced with the lower ACF bonding temperature. Although there is a lack of experimental result for 140 oC case, the much improvement or elimination of light leakage can be prediceted by our simulation results.
140 160 180 4
6 8
localized warpage 1st principal stress
ACF bonding temperature (oC)
max. localized warpage (um)
2 3 4 5
max. 1st principal stress (MPa)
Figure 4.15 A plot of maximum localized warpage and first principal stress as a function of ACF bonding temperature using 0.3mm glass substrate
(a) ACF bonding temperature: 180oC
(b) ACF bonding temperature: 160oC
Figure 4.16 Photographs of lighted-up LCD panels with ACF bonding temperature at (a) 180 oC and (b) 160 oC using 0.3 mm glass substrate
In order to reduce the light leakage, improvement of the ACF properties was also an available method other than decreased the ACF bonding temperature. The two main mechanical properties of ACF adhesive, Young’s modulus (E) and coefficient of thermal expansion (CTE), were investigated to realize their effects on the localized warpage and light leakage phenomenon.
The ACF adhesive in COG assembly played the role of connection between Si chips and glass plates, and buffer layer which can absorb the thermal-induced stress of Si chips and glass substrate. Figure 4.17 shows the plot of maximum localized warpage and maximum first principal stress as a function of ACF modulus with conditions of 180 oC/0.3 mm. The original modulus of ACF was 1.6 GPa under room temperature. When the modulus of ACF increased to 2.9 GPa (80% increased), the warpage/stress increased from 7.64 μm/4.29 MPa to 8.34 μm/4.75 MPa. On the other hand, the warpage decreased to 6.25 μm/3.23 MPa when modulus of ACF decreased to 0.3 GPa (80% decreased). Based on the results, ACF adhesive with low modulus indicated that it was softer to be a better buffer layer, which could absorb more COG-induce stress. Thus, the warpage and stress were decreased with reduction of the ACF modulus. The photographs of lighted-up LCD panels with original modulus ACF and low modulus ACF were shown in Figure 4.18. The half-moon-shaped light leakage phenomenon with low modulus ACF adhesive became indistinguishable compared to panel using the original modulus ACF adhesive. The region of light leakage also shrank.
As for the CTE of the ACF, it had little effect on the warpage and stress since its variety of thermal-induced stress could be ignored when compared to the stress of the Si chips and the glass substrate. This was because of the small thickness ratio of ACF to the COG assembly (less than 5%) and its very low modulus compared to the Si chips and glass substrate. Thus, during the thermal bonding process, the
thermal-induced stress of ACF adhesive could be ignored as compared to the stress induced by Si chips and glass substrate. Changing the CTE of the ACF adhesive had no effect on the warpage and stress, indicating that the light leakage Mura defect can not be solved by this parameter.
0.0 0.5 1.0 1.5 2.0 2.5 3.0
6 8
localized warpage 1st principal stress
ACF modulus (GPa)
max. localized warpage (um)
3.0 3.5 4.0 4.5 5.0
max. 1st principal stress (MPa)
Figure 4.17 A plot of maximum localized warpage and maximum first principal stress as a function of ACF modulus with conditions of 180 oC / 0.3 mm
(a) Original modulus
(b)Low modulus
Figure 4.18 Photographs of lighted-up LCD panels with (a) original ACF modulus and (b) lower modulus under conditions of 180 oC/0.3 mm
So far, the effect of the COG packaging manufacturing parameters such as thickness of glass substrate, process temperature and the two mechanical properties of ACF on light leakage Mura has been studied. Next, the warpage reduction of these parameters was examined to realize which one had more effectiveness. Table 4.1 summarized the localized warpage data from the outside to central of the LCD panel by simulation. The maximum first principal stress data was omitted due to it had a positive correlation with the maximum localized warpage. Condition A represented the original case, and B, C, D meant the low ACF bonding temperature at 160 oC, low ACF modulus and low bonding temperature/modulus at the same time, respectively.
Conditions E-H turned a change of the thickness of glass substrate from 0.3 mm into 0.5 mm. The data in each case had the same trend, the localized warpage decreased from central to outside of the panel. Preliminary observation of these data showed that decreasing ACF bonding temperature was more effective than decreasing the ACF modulus on reduction of localized warpage.
Table 4.1 Localized warpage from outside to central of LCD panel in different sample conditions
Localized Warpage (μm) Sample Conditions
Outside → Central
A: 180oC / 0.3mm / 1.6GPa 3.12 4.54 6.17 7.64 B: 160oC / 0.3mm / 1.6GPa 2.82 3.39 4.12 5.08 C: 180oC / 0.3mm / 0.3 GPa 3.03 4.26 5.14 6.25 D: 160oC / 0.3mm / 0.3GPa 2.57 3.13 3.57 4.34 E: 180oC / 0.5mm / 1.6GPa 2.52 3.08 3.49 4.23 F: 160oC / 0.5mm / 1.6GPa 2.28 2.64 3.08 3.42 G: 180oC / 0.5mm / 0.3GPa 2.33 2.72 3.19 3.68 H: 160oC / 0.5mm / 0.3GPa 2.26 2.54 2.76 3.09
The maximum localized warpage in various conditions as shown in Figure 4.19.
The relationship between thickness of glass and ACF bonding temperature with fixed ACF modulus was first studied. When the ACF modulus was fixed at 1.6 GPa and the ACF bonding temperature decreased from the 180 oC to 160 oC with 0.3 mm glass substrate (sample A and B), the maximum localized warpage decreased 34% while only decreased 19% associated with 0.5mm glass substrate (sample E and F).
Similarly, when the ACF modulus fixed at 0.3 GPa, the maximum localized warpage decreased 30% (sample C and D) while only decreased 16% associated with 0.5 mm glass substrate (sample G and H). From the results, the effect of the ACF bonding temperature on the localized warpage was amplified when the thickness of glass substrate decreased.
Next, the relationships between thickness of glass substrate and the ACF modulus on maximum localized warpage with fixed ACF bonding temperature were examined. When the ACF bonding temperature was fixed at 180oC and the modulus of ACF decreased from 1.6 GPa to 0.3 GPa, the maximum localized warpage decrease 18% with 0.3 mm glass substrate (sample A and C) while decreased 13% associated with 0.5 mm glass substrate (sample E and G). When the ACF bonding temperature was fixed at 160oC, the maximum localized warpage decrease 15% with 0.3 mm glass substrate (sample B and D) while decreased 10% with 0.5 mm glass substrate (sample F and H). Again, the effect of the ACF modulus on localized warpage was amplified with decreasing thickness.
Although the effect of both ACF modulus and thickness of glass substrate was amplified when thickness of glass substrate was decreased, the effect of the ACF modulus was less than the ACF bonding temperature. Considering the LCD development that the glass substrate must become thinner, the ACF bonding temperature would be the one of the critical factors for improving the light leakage
defect.
On the other hand, compared to the original case A, a 33%, 18%, 45% reduction in warpage was found in condition B, C and E, respectively. Based on the powered, lighted-up TFT-LCD, the level of light leakage Mura defect of these cases could be visually seen from the photographs illustrated in Figure 4.14, 4.16 and 4.18. The Mura became severe when glass thickness was decreased from 0.5 mm to 0.3 mm. Mura defect can be effectively improved by decreasing ACF bonding temperature and modulus. In the condition D, a 43% decrease in localized warpage and a 41% drop in the stress level were found from simulation, implying that light leakage resulted from thinning glass substrate can be eliminated or improved.
Key results are sum up as follow,
1. Decreasing ACF bonding temperature and ACF modulus helped eliminating Mura which appeared when thickness of glass substrate was thinned down from 0.5 mm to 0.3 mm.
2. When the thickness of glass substrate was decreased from 0.5 mm to 0.3 mm, the effect of ACF bonding temperature and ACF modulus on localized was warpage amplified, especially the ACF bonding temperature.
3. ACF bonding temperature was the more important factor than modulus of ACF for improving Mura.
0
A: 180oC/0.3mm/1.6GPa B: 160oC/0.3mm/1.6GPa C: 180oC/0.3mm/0.3GPa D: 160oC/0.3mm/0.3GPa E: 180oC/0.5mm/1.6GPa F: 160oC/0.5mm/1.6GPa G: 180oC/0.5mm/0.3GPa H: 160oC/0.5mm/0.3GPa
A B C D E F G H
Figure 4. 19 The maximum localized warpage for different conditions