1. Field of the Invention
The present invention relates to flexible printed circuits and display devices using the same, and particularly to the structure of terminals of a flexible printed circuit that are connected to electrode terminals of a display panel.
2. Description of the Background Art
A conventional liquid-crystal display device includes two rectangular glass substrates, each having two longer sides and two shorter sides, with liquid crystal sandwiched therebetween, driver circuitry connected to interconnections on the glass substrates, and an illumination device placed on the back of the glass substrates. Particularly, in a liquid-crystal display device using thin film transistors (TFTs), the TFTs are arranged in a matrix on a first glass substrate, and an opposing electrode is formed on the second glass substrate. Also, electrode terminals are provided in the peripheral area of the first glass substrate in order to connect interconnections extending from the TFTs to the driver circuitry. Accordingly, the first glass substrate is larger than the second glass substrate by the area for the formation of the electrode terminals, and the periphery of the first glass substrate projects over the second glass substrate.
Each TFT is connected to the corresponding pixel and turns on/off to control an image signal sent to the pixel. The image signal is supplied from a source line connected to the source electrode of the TFT. The source lines are placed in parallel with the shorter sides of the glass substrate and connected to the electrode terminals provided in a peripheral area along a longer side of the glass substrate. On the other hand, control signals for controlling the TFTs are supplied from gate lines connected to the gate electrodes of the TFTs. The gate lines are placed in parallel with the longer sides of the glass substrate and connected to the electrode terminals provided in a peripheral area along a shorter side of the glass substrate.
The electrode terminals are connected to the driver circuitry through a flexible printed circuit (FPC). The FPC includes an FPC film made of an insulative film having a thickness of about 30 pm to 70 pm, FPC interconnections made of copper foil having a thickness of about 8 pm to 25 pm, and a polyimide-based solder resist covering the FPC interconnections. The FPC film is formed of a material that can be bent freely. Also, FPC terminals are formed at ends of the FPC interconnections, and the solder resist is absent on this area.
The FPC terminals are connected to the electrode terminals on the glass substrate through an anisotropic conductive film (ACF). The FPC terminals and the electrode terminals are thus electrically connected to each other through conductive particles contained in the ACF, but adjacent ones of the FPC terminals, and adjacent ones of the electrode terminals, are not electrically connected to each other because of the presence of insulative epoxy resin around the conductive particles.
In conventional liquid-crystal display devices, an insulative coating material is applied with a dispenser to an area extending from a sealing material of the liquid-crystal panel to the electrode terminals, in order to prevent corrosion of the interconnections in this area. Such techniques for preventing corrosion of electrode terminals are disclosed as an example of a liquid-crystal display device in Japanese Patent Application Laid-Open No. 2003-195336 (hereinafter referred to as Patent Document 1), and as an example of a plasma display device in Japanese Patent Application Laid-Open No. 2004-93860 (hereinafter referred to as Patent Document 2).
Patent Document 1 discloses an example of a liquid-crystal display device in which an insulative coating material coats an area from the electrode terminals connected to the FPC terminals to the sealing material of the liquid-crystal panel. Patent Document 2 discloses an example of a plasma display device in which an insulative coating material coats an area from the electrode terminals connected to the FPC terminals to the sealing material of the panel. Thus, conventional display devices have some means to prevent corrosion of the electrode terminals.
However, when such a display device undergoes a reliability test in a severe environment, e.g., in a hot and humid environment, water may penetrate the insulative coating material to invade the ACF. When water invades the ACF, higher-potential electrode terminals may corrode away as minus ions in areas where the higher-potential electrode terminals are placed adjacent to lower-potential electrode terminals, which results in disconnection of the electrode terminals (the phenomenon called erosion or corrosion). The disconnection of electrode terminals stops supply of desired signals to pixels and then the display device suffers from display defects.