LCD面板切割刀輪之刀尖形狀作用機制與創新輔助切割方法

Abstract

劃線與裂片製程的面板分割技術目前廣泛應用在LCD面板產業，有時後裂片後的面板彎曲強度很重要。本論文研究面板切割刀輪的刀尖形狀對切割後玻璃面板彎曲強度的影響，並提出一個體積裂紋作用係數來估計刀尖形狀的影響。論文結果指出體積裂紋作用係數與彎曲強度在微崩落產生之前相關性很大，因此以體積裂紋作用係數能夠判斷刀尖形狀對彎曲強度的影響。具有大角度、很小的鼻端半徑與光滑的表面粗度將會是得到彎曲強度最好的刀輪，然而，實際上很難製作同時具有這些條件的刀輪。 論文還提出了震動輔助切割LCD面板的方法，設法降低刀尖形狀對面板彎曲強度的敏感度。亦有一併提出該方法的基本原理，增加快速變動的負載不會使塑性變形區變大，反而增加彈性成分，使得增加的彈性成分會使中央裂紋增加。實驗結果顯示，0.5mm玻璃在某些頻率中央裂紋會明顯增加並達到沒有震動輔助切割時的兩倍，因此，震動輔助切割能夠得到目前只有有齒型刀輪切割才會得到的自動裂片的效果，並且彎曲強度是有齒型刀輪的兩倍。0.21mm玻璃振動輔助切割時，在所有實驗頻率得到自動裂片效果以外邊緣強度可增加原來的65%。 根據不同厚度玻璃切割實驗，震動頻率對不同厚度的玻璃有不同的效應， 0.5mm玻璃是因為增加的彈性成分造成中央裂紋增加而得到自動裂片效果，但是0.21mm玻璃是因為震動使玻璃表面產生的裂紋的拉應力造成自動裂片的效果，0.21mm玻璃能夠貢獻的彈性成分比0.5mm玻璃少很多，所以加上震動輔助增加的彈性成分相較之下少很多。根據實驗結果，0.7mm玻璃切割後無法自動裂片的原因為切割後之玻璃的邊緣強度未達到破壞力矩。論文最後有提出一個估計最佳震動頻率的方法和準則，此準則是以根據0.5mm玻璃震動輔助切割得到的經驗法則，以800Hz頻率的中央裂紋產生的飽和面積率為基準，並且會使中央裂紋增加的起始頻率估計方法，推算出能夠使該頻率發揮到最佳值使得中央裂紋最深的效果。

Scribing followed by a breaking method has been applied to separate the individual Liquid Crystal Display (LCD) glass panels from the mother cells. In some special occasions, especially for compact mobile devices, the bending strength of the glass substrates after breakage is the most important concern. In this study, the effect of the geometrical characteristics of the scribing wheel on the bending strength of LCD glass substrates was investigated. A volume-crack effective coefficient was defined to estimate the effect of different geometrical characteristics of the scribing wheels. It is found that there is a strong correlation between the bending strength and the coefficient before chipping appears as the scribing load is increased. Thus, the coefficient could be used for the assessment of the geometrical effect of scribing wheels on the bending strength of glass substrates. According to the experiment result, the bending strength is strongly related to the geometrical conditions of scribing wheel tip. A scribing wheel with a large tip angle, a small nose radius and a good surface condition would be an optimum choice for good bending strength. Nevertheless, in reality it is hardly able to manufacture an optimum scribing wheel with capability of large tip angle, small nose radius and good surface polishing at the same time. A technique of vibration assisted scribing of LCD glass substrate was developed to reduce the sensitiveness of wheel tip to the bending strength. A fundamental theory of vibration assisted scribing process was manifested. It was proposed that the rapidly varying periodical scribing load does not lead to as much an increase of the plastic deformation as that due to the servo controlled constant scribing load. Instead it mainly results in an increase of the elastic deformation. The induced elastic component would increase the median crack depth of the glass substrate. To verify the proposed theory, an additional piezoelectric actuator was implemented on the servo controlled loading mechanism of the scribing machine, and a periodically varying load superposed on the constant load was provided to the standard scribing wheel. Experimental results show that under a proper vibration frequency condition the median crack depth is significantly increased to about twice of that without vibration assisted scribing. Thus, the automatic separation of the glass without the need of breaking process similar to the case with the use of toothed wheel (Penett) could take place. On the other hand, while the bending strength of the glass panel after scribing by toothed wheel is merely about half of that resulting from scribing by standard wheel, it remains almost unchanged by the developed technique. The difference of the effect of the frequencies in vibration assisted scribing process for different thickness of LCD glass panels is explained. Median crack depth is increased and scribed glass panels are automatically separable in vibration assisted scribing of 0.5mm glass for some frequencies by elastic component. For 0.7mm glass it is not automatically separable because the value failure moment is not small enough. But the separable condition for 0.21mm glass is due to the tensile stress inside induced by the additional imposed vibration. The most noteworthy fact is the significant increment of bending strength which is better than in scribing of 0.5mm thick glass substrate. The largest bending force increment in vibration assisted scribing process is about 65% of normal scribing process. Experiment results show that the contribution of elastic component of 0.21mm glass becomes less and median crack depth does not increase as much as 0.5mm glass with increasing frequency. A simple way to estimate the optimal frequency based on a known experiment value as a criterion was also proposed. The criterion is based on the condition of crack growth of 0.5mm glass at frequency of 800Hz as an area saturation rate. A method to calculate the initial frequency which would be effective to increase median crack depth was proposed. Knowing the initial frequency, experiment could be conducted with a higher frequency to obtain the larger median crack depth.