Electric circuit

Abstract: An electric circuit wherein an insulating material (8) having a coefficient of linear expansion approximately equal to that of glass is bonded to a part where an electrode pattern (3) formed on a glass substrate (2) is connected to a conductive pattern (6) formed on a substrate made of an insulating material such as polyimide or the like, thereby restricting the thermal stress to be small at the connecting part between the conductive pattern (6) and electrode pattern (3).

Description: This invention relates to an electric circuit according to the preamble of claim 1 for use for instance in a liquid crystal display, in which an electrode pattern arranged on a glass substrate is connected to a conductive pattern on a flexible insulating film. Fig. 1 is a perspective view of a conventional liquid crystal display using a flexible printed circuit (FPC) disclosed, for example, in the Japanese Patent Application Laid-Open No. 1-253791 (253791/1989). A liquid crystal panel 1 is constituted of two glass substrates 2 facing each other and a liquid crystal sealed between the glass substrates 2. The glass substrates 2 are provided in such a manner that the electrode patterns arranged in stripes thereon intersect each other. On the three sides of the liquid crystal display panel 1, plural FPC substrates 21 of insulating films with IC chips thereon for supplying signals to the electrodes are provided. On the fourth side of the liquid crystal display panel 1, there is provided an FPC substrate 22 of an insulating film for connecting the gate bus lines formed on the upper and lower halves of the liquid crystal display panel 1. Fig. 2 is a plane view of a conventional insulating film with a conductive pattern formed thereon by the tape automated bonding (TAB) method disclosed in the Japanese Patent Application Laid-Open No. 2-74922 (74922/1990), for instance. One ends of input terminals 25 and output terminals 26 formed on a base film 23 are connected to an IC chip 24, with the other ends thereof being connected to an electrode pattern formed on a glass substrate (not shown). In this case, the output terminals 26 are connected to the electrode pattern on the glass substrate by means of an anisotropic conductive film as depicted, e.g., in the Japanese Patent Application Laid-Open Nos. 2-22694 (22694/1990), 2-46426 (46426/1990) and the Japanese Utility Model Application Laid-Open No.2-33032 (33032/1990). Fig. 3 is a plane view of another insulating film having a conductive pattern formed thereon through TAB, disclosed, e.g., in the Japanese Utility Model Application Laid-Open No. 2-51322 (51322/1990). Similarly to the above example of Fig. 2, one ends of input terminals 25 and output terminals 26 formed on a base film 23 are connected to an IC chip 24, and the other ends thereof are connected to an electrode pattern on a glass substrate (not shown). The output terminals 26 and electrode pattern on the glass substrate are connected with each other through soldering as described, for example, in the Japanese Utility Model Application Laid-Open No. 2-119150 (119150/1990). The operation of the conventional electric circuit as aforesaid will be explained hereinafter. In the liquid crystal display of Fig. 1, the displaying data generated by the IC chip 24 on the FPC substrate 21 are fed to source bus lines, so that the voltage impressed to the liquid crystals changes the transmittance or color of the liquid crystal panel 1, thereby obtaining the aimed images. Meanwhile, as indicated in Fig. 2, the IC chip 24 on the base film 23 is connected to the input and output terminals 25 and 26 via electrodes, and the IC chip 24 outputs an image signal to the output terminal 26 on the basis of an input signal from the input terminal 25. Also in Fig. 3, the IC chip 24 mounted on the base film 23 supplies the data inputted to the input terminal 25 to the terminal 26 as an image signal for each pixel. A very similar connecting method and packaging structure, showing an electric circuit according to the preamble of claim 1, is known from EP-A-0 289 026. In the aforementioned structures of the electric circuit employed hitherto in the liquid crystal display, in the case where the FPC substrates 21, 22 and base film 23 are polyimide, although the nominal coefficient of linear expansion of polyimide is 20×10 -6 and, thus, is larger than that of glass (9×10 -6 ), the difference of the coefficients of linear expansion is no problem when the output terminal of the FPC substrate or TAB film, namely, a connected part to the electrode pattern on the glass substrate 2 is relatively short. However, when an increase in the number of output signals from each IC chip 24 or a plurality of IC chips 24 mounted to a single FPC substrate or TAB film (multi-chip TAB) elongates the part connected to the electrode pattern on the glass substrate 2, this becomes a problem. In other words, when the connected part is exposed to the heat cycle generated by the power of the apparatus being turned ON or OFF, the bond strength at the connected part reduces due to the thermal stress resulting from the difference of the coefficients of linear expansion, thereby increasing the resistance against connection, or in extreme case, leading to a disconnection. Accordingly, the object of this invention is to improve an electric circuit according to the preamble of claim 1 in such a way that the thermal stress brought about at the connecting part is restricted, thereby preventing an increase of the resistance against connection in the connected part or a disconnection due to the deterioration of the bond strength. This object, according to the present invention, is solved by the advantageous features indicated in the characterizing part of claim 1. In US-A-4 643 526, there is disclosed a liquid crystal display device in which a chip disposed at the peripheral part of the display main substrate is completely covered by a molding material; this molding material further covers a peripheral portion of a further substrate including wires by means of which signals are applied to the chip. In order to prevent any deformation of this molding material, there is further provided a glass plate which covers the molding material. Therefore, the further substrate is kind of mechanically clamped to the main substrate so that the a.m. mismatch of the coefficients of linear expansion does not impose any problems. The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings, in which: Fig. 1 is a perspective view of a liquid crystal display employing a conventional electric circuit; Fig. 2 is a plane view of a conventional insulating film having a conductive pattern formed thereon; Fig. 3 is a plane view of another conventional insulating film having a conductive pattern formed thereon; Fig. 4 is a cross sectional view of a connecting part of an electric circuit according to this invention; Fig. 5 is a perspective view of a liquid crystal display device employing the electric circuit of this invention; Fig. 6 is a perspective view of a modified liquid crystal display device employing the electric circuit of this invention; and Fig. 7 is a perspective view of a further modified liquid crystal display device employing the electric circuit of this invention. A preferred embodiment of this invention will be discussed hereinafter with reference to the accompanying drawings. In a liquid crystal display panel 1 shown in Fig. 4, an electrode pattern 3 is arranged in stripes on a glass substrate 2 forming a main substrate. An end of the electrode pattern 3 is electrically connected via an anisotropic conductive film 7 to an end of a conductive pattern 6 formed on a base film 4 of an insulating material forming a further substrate. A ceramic plate 8, as an insulating material having approximately the same coefficient of linear expansion as glass, is bonded by an adhesive 9 to the part of the base film 4 where the conductive pattern 6 and electrode pattern 3 are connected to each other. The other end of the conductive pattern 6 is connected to an IC chip 5 mounted on the base film 4. As described earlier with reference to the conventional examples, polyimide is generally used as the insulating film of an FPC substrate or a TAB where a conductive pattern is formed. The coefficient of linear expansion of polyimide is nominally 20×10 -6 , but greatly varies due to the manufacturing lots. In some cases, 40×10 -6 measured. When the ceramic plate 8 having the coefficient of linear expansion of 5.3×10 -6 is bonded by the thermosetting adhesive 9 while the polyimide having the above-mentioned property is used as the base film 4, the coefficient of linear expansion of the bonded part becomes 7.5×10 -6 , which is almost equal to 9×10 -6 of the coefficient of linear expansion of the glass substrate 2. Since the difference of the coefficients of linear expansion between the base film 4 and glass substrate 2, therefore, is very small, the thermal stress brought about at the connecting part between the conductive pattern 6 on the FPC or TAB and the electrode pattern 3 on the glass substrate 2 decreases to a large extent. Fig. 5 is a perspective view of a liquid crystal display device using the electric circuit of Fig. 4. The ceramic plate 8 is provided for every insulating film 11 of TAB having the conductive pattern thereon. Fig. 6 is a perspective view of a modification of a liquid crystal display device using the electric circuit of Fig. 4. A long ceramic plate 8a is bonded onto a FPC insulating film 12 onto which a plurality of IC chips 5 are mounted. In Fig. 7, the glass substrate 2 is electrically connected by an FPC using an insulating film 14 to a glass epoxy printed circuit board 13 having plural IC chips thereon. To be exact, the electrode pattern 3 is connected to the conductive pattern on the printed circuit board 13 via a conductive pattern on the insulating film 14. Each connecting part of the insulating film 14 to the glass substrate 2 and to the printed circuit board 13 is bonded by a long ceramic plate 8b. Although the ceramic plate is used as an insulating material having approximately the same coefficient of linear expansion as glass in the foregoing embodiments, any other insulating material may be used as long as the coefficient of linear expansion thereof is approximately equal to that of the glass substrate 2. Moreover, although the above description is related to the electric circuit of a liquid crystal display device , this invention is applicable to an electric circuit composed of plural circuit patterns formed on a plurality of substrates, respectively, of different coefficients of linear expansion.

Claims: Electric circuit, comprising: [a] a main substrate (2) having a first coefficient of linear expansion and comprising a circuit pattern, said circuit pattern being arranged on one surface of said main substrate (2) and comprising connection electrodes (3) at a peripheral portion of said main substrate (2); [b] at least one further substrate (4, 11, 12, 14) having a second coefficient of linear expansion being different from said first coefficient, and comprising a circuit pattern, said circuit pattern being arranged on one surface of said further substrate (4, 11, 12, 14) and comprising connection electrodes (6) at a peripheral portion of said further substrate (4, 11, 12, 14); [c] the connection electrodes (3) of said main substrate (2) being connected to the connection electrodes (6) of said further substrate (4, 11, 12, 14) at said peripheral portions, said substrates (2; 4) therefore being overlappingly and opposingly linked at said peripheral portions; characterized by [d] an insulating plate (8) which is bonded by an adhesive (9) exclusively to the second surface of said further substrate (4, 11, 12, 14) in such a way that it exends over the whole peripheral portion of said further substrate (4, 11, 12, 14) and over a portion in the immediate neighborhood thereof and whose coefficient of linear expansion is selected such that the effective coefficient of linear expansion of the ended structure consisting of said further substrate (4, 11, 12, 14) and said plate (8) is substantially the same as that of said main substrate (2). Electric circuit according to claim 1, characterized in that the coefficient of linear expansion of said main substrate (2) is smaller than the coefficient of linear expansion of said further substrate (4, 11, 12, 14). Electric circuit according to claim 1 or 2, characterized in that said insulating plate (8) is made of ceramics. Electric circuit according to one of claims 1 through 3, characterized in that said further substrate (4, 11, 12, 14) is made of polyimide. Electric circuit according to one of claims 1 through 4, characterized in that said main substrate (2) is made of glass. Electric circuit according to one of claims 1 through 5, characterized in that said main substrate (2) constitutes the substrate of a liquid crystal display (1). Electric circuit according to one of claims 1 through 6, characterized in that said further substrate (4, 11, 12, 14) carries an integrated circuit (5) which provides control signals for the circuit pattern of said main substrate (2).

Inventors: YOSHIOKA KAZUO, YOSHIOKA, KAZUO, Yoshioka, Kazuo, c/o Nagasaki Seisakusho
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L23
IPC codes: H05K1, H05K3, H01L23, G02F1, H01L21, G09F9