Patent Publication Number: US-6043971-A

Title: Electrostatic discharge protection device for liquid crystal display using a COG package

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display and, more particularly, to an electrostatic discharge protection device for a liquid crystal display using a chip on glass (COG) package, which protects the input pads of the display from electrostatic charge. 
     2. Discussion of Related Art 
     In a thin film transistor-liquid crystal display (TFT-LCD), packaging techniques for connecting a driving IC, which provides a driving signal to the LCD to the TFT-LCD panel, include wire bonding (WB), tape automated bonding (TAB) and chip on glass (COG) methods. In the WB method, the driving ICs are connected to the panel electrodes with Au wires. The TAB method mounts a package, in which the driving IC is connected to a film carrier, on the panel. Finally, the COG method forms a bump on a bare chip and mounts it on the panel in which inner and outer lead pads are formed. 
     During a TFT-LCD fabricating process, static electricity may be applied to the TFT-LCD, destroying its inner elements, such as a TFT. To prevent the damaging effect of the static electricity after forming the TFT-LCD array, a shorting bar is used to facilitate the inspection of the TFT-LCD array. The shorting bar structure consists of one shorting bar connected to each gate line of the TFT-LCD and another shorting bar connected to each data line. To check if the gate line is short-circuited during the array inspection, the power is applied to the shorting bar connected to each gate line and the opposite end of the gate line is checked. The data line is checked using the shorting bar in the same manner as the gate line. 
     The shorting bar is not used in actual driving of the TFT-LCD but used only for electrostatic discharge protection and array inspection. The shorting bar is then disconnected from the TFT-LCD array. That is, after the TFT-LCD array is fabricated and its inspection is completed, the upper and lower plates of the TFT-LCD are attached to each other and a scribe process and a grinding process are sequentially carried out. Thereafter, the shorting bar is disconnected. Meanwhile, it is difficult to form the shorting bar according to the packaging methods since the configurations of the gate lines and data lines depend on the packaging methods. In other words, with the TAB method, the carrier film is connected to both sides of the bare chip so that a space on the TFT-LCD panel can be secured. It allows the shorting bars to be formed on one side of the gate line pad and one side of the data line pad which are also the areas reserved for placing the driving ICs. 
     On the other hand, with the COG method, a space on the TFT-LCD panel is difficult to secure because the inner and outer leads must be formed on the driving IC mounting areas of the panel. Thus, the shorting bar cannot be formed on the driving IC mounting areas. To solve this problem, there has been developed a technique for forming the shorting bar on the TFT-LCD in COG package. This technique is explained below with reference to the attached drawings. 
     FIG. 1 is a layout of a conventional liquid crystal display in a COG package having a shorting bar. FIG. 2 is a layout of a conventional liquid crystal display in a COG package having an electrostatic discharge protecting circuit. Referring to FIG. 1, a plurality of gate lines 1 are arranged in one direction on a substrate at regular intervals, and a plurality of data lines 2 are arranged at regular intervals perpendicular to the gate lines 1. A gate line pad 3 and a data line pad 4 are formed at one end of each gate lines 1 and one end of each data line 2, respectively. A plurality of gate line input pads 7 are formed at regular intervals opposite to gate line pads 3, and a plurality of data line input pads 8 are arranged at regular intervals opposite to data line pads 4. The gate driving IC mounting area corresponds to the region of the gate line input pads 7, and data driving IC mounting area corresponds to the region of the data input pads 8. 
     First and second shorting bars 5 and 6 are formed on the gate driving IC mounting area 9 and the data driving IC mounting area 10, respectively. That is, a first shorting bar 5 is formed between the gate line pads 3 and gate input pads 7 and connected to each of the gate line pads 3. A second shorting bar 6 is formed between the data line pads 4 and the data input pads 8 and connected to each of the data line pads 4. Reference numeral 11 denotes a laser cutting line. After the TFT-LCD is constructed as above, which includes TFTs and pixel electrodes, an inspection of the array is performed, followed by the attachment of the upper and lower plates with respect to each other. Then, portions between shorting bars 5 and 6 and gate line pads 3 and data line pads 4 are cut using a laser cutting apparatus. 
     Referring to FIG. 2, the conventional TFT-LCD using a COG package, having an electrostatic discharge protection circuit, is constructed in such a manner that a plurality of gate lines 1 are arranged in one direction on a substrate at regular intervals, a plurality of data lines 2 are arranged at regular intervals perpendicular to the gate lines 1, and a common electrode 12 is formed around the TFT-LCD. A gate line pad 3 and data line pad 4 are formed at one end of each gate lines 1 and one end of each data line 2, respectively. A plurality of gate line input pads 7 are formed at regular intervals opposite to gate line pads 3, and a plurality of data input pads 8 are arranged at regular intervals opposite to the data line pads 4. Both ends of the gate line 1 and the data line 2 are connected to the common electrode 12 through an electrostatic discharge protection circuit 13. The gate driving IC mounting area corresponds to the region of the gate line input pads 7, and the data driving IC mounting area corresponds to the region of the data input pads 8. The common electrode 12 is connected to a common electrode (not shown) of the upper plate of the display using an Ag dot. 
     The conventional TFT-LCD in a COG package has the following problems. While the inner circuit of the TFT-LCD is protected from electrostatic discharge because either the shorting bar or the electrostatic discharge protection circuit is provided to the outer lead pad placed on the TFT-LCD array side, the input pads are all floating so that there is no path through which static electricity is discharged. Thus, the input pads are susceptible to and can be damaged by the electrostatic discharge. Furthermore, after ICs are attached to the input/output pads, static electricity generated from the ICs remains in the input pad, damaging the ICs with the electrostatic discharge. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an electrostatic discharge protection device for an LCD using a COG package that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an electrostatic discharge protection device for an LCD using a COG package, in which a common electrode line is formed on an IC mounting area and each input pad is connected to the common electrode through an electrostatic discharge protection circuit to protect the input pad and IC from the electrostatic discharge. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the electrostatic discharge protection device for a liquid crystal display using a COG package, includes: a plurality of gate lines and data lines each of which has an output pad at its end; a plurality of gate line input pads and data line input pads formed opposite to the output pads of the gate lines and data lines, respectively; a common electrode formed between the plurality of gate line input pads and output pads and between the data line input pads and output pads, the common electrode being formed in one body; and a plurality of electrostatic discharge protection circuits connected between the input pads and common electrodes. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     FIG. 1 is a layout of a conventional liquid crystal display using a COG package and having a shorting bar; 
     FIG. 2 is a layout of a conventional liquid crystal display using a COG package and having an electrostatic discharge protection circuit; 
     FIG. 3 is a layout of an electrostatic discharge protection device for a liquid crystal display in a COG package according to a first embodiment of the present invention; 
     FIG. 4 is an electrostatic discharge protection circuit according to the present invention; and 
     FIG. 5 is a layout of an electrostatic discharge protection device of a liquid crystal display in a COG package according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 3 is a layout of an electrostatic discharge protection device of a TFT-LCD in a COG package according to a first embodiment of the present invention. Referring to FIG. 3, the TFT-LCD is constructed in such a manner that a plurality of gate lines 1 are arranged in one direction on a substrate at regular intervals, a plurality of data lines 2 are arranged at regular intervals perpendicular to the gate lines 1, and a common electrode 12 is formed around the TFT-LCD. A gate line output pad 3 and data line output pad 4 are formed at one end of each gate lines 1 and one end of each data line 2, respectively. A plurality of gate line input pads 7 are formed at regular intervals opposite to gate line input pads 3, and a plurality of data line input pads 8 are arranged at regular intervals opposite to data line output pads 4. 
     A gate driving IC mounting area 9 corresponds to the region for the gate line input pads 7, and the data driving IC mounting area 10 corresponds to the region for the data input pads 8. A common electrode 12 is formed on the gate driving IC mounting area 9 and the data driving IC mounting area 10. The common electrode 12, gate line input pads 7 and data line input pads 8 are connected to a plurality of electrostatic discharge protection circuits 13. Each one of the gate lines 1 and the data lines 2 is connected to the common electrode 12 placed around the TFT-LCD through the plurality of the electrostatic discharge protection circuits 13. 
     Referring to FIG. 4, each electrostatic discharge protection circuit has first and second transistors Q1 and Q2 serially connected between a gate line 1, a data line 2 or input cads 7 or 8 and the common electrode 12. The gate of a third transistor Q3 is connected to the contact point of the first and second transistors Q1 and Q2. The source and drain electrodes of the third transistor Q3 are connected to the gates of the first and second transistors Q1 and Q2, respectively. Each of the transistors constructing the electrostatic discharge protection circuit has the same structure as the TFT (not shown) of the TFT-LCD array, and fabricated by the same process as that of the TFT. Each one of the transistors in the electrostatic discharge circuit is preferably an NMOS-type transistor. 
     FIG. 5 is a layout of an electrostatic discharge protection device for a liquid crystal display using a COG package according to a second embodiment of the present invention. Referring to FIG. 5, the TFT-LCD is constructed in such a manner that a plurality of gate lines 1 are arranged in one direction on a substrate at regular intervals, a plurality of data lines 2 are arranged at regular intervals perpendicular to the gate lines 1, and a common electrode 12 is formed around the TFT-LCD. A gate line output pad 3 and data line output pad 4 are formed at one end of each gate lines 1 and one end of each data line 2, respectively. A plurality of gate line input pads 7 are formed at regular intervals opposite to the gate line input pads 3, and a plurality of the data line input pads 8 are arranged at regular intervals opposite to data line output pads 4. 
     A gate driving IC mounting area 9 corresponds to the region for the gate line input pads 7, and the data driving IC mounting area 10 corresponds to the region for the data input loads 8. A first common electrode 12a is formed on the gate driving IC mounting area 9 and intersects gate lines 1 placed in the peripheral region of the TFT-LCD. A second common electrode 12b is formed on the data driving IC mounting area 10 and intersects the data lines 2 placed in the peripheral region of the TFT-LCD. A plurality of electrostatic discharge protection circuits 13 are connected between the first common electrode 12a and the gate line input pads 7 and the gate lines 1. Another plurality of electrostatic discharge protection circuits 13 are connected between the second common electrode 12b and the data input pads 8 and the data lines 2. First and second common electrodes 12a and 12b are connected to each other through an electrostatic discharge protection circuit 13. The electrostatic discharge protection circuit shown in FIG. 5 is identical to that of FIG. 4. 
     According to the present invention, the common electrode is formed on the IC mounting areas and connected to each input pad through the electrostatic discharge protection circuit, protecting the input pad of the TFT-LCD from electrostatic discharge. Moreover, a discharge path for discharging static electricity generated from the IC is also formed. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the electrostatic discharge protection device for a TFT-LCD using a COG package of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.