Mechanism of Wheel Tip Geometry in Scribing LCD Glass Substrates and an Innovative Assisted Scribing Method
Date Issued
2010
Date
2010
Author(s)
Saw Chi, Mya
Abstract
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.
Subjects
vibration assisted
scribing
LCD glass substrate
scribing wheel
breaking process
Type
thesis
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