1. Field of the Invention
The present invention relates to a sheet switch module which is installed in switch panels of mobile phones, mobile terminal devices, or the like.
2. Description of Related Art
A sheet switch 10 as shown in FIG. 7 has conventionally been used in a key input part of various kinds of electronic equipment, such as mobile phones, mobile terminal devices, or the like. Each of a number of sheet switches has an illumination function including key switches 8 and light sources or light emitting elements 6 each of which is disposed about each of the key switches 8.
On the other hand, FIG. 8 illustrates a sectional structure of a conventional sheet switch 1 including one or more light emitting elements 6 and a flat plate-shaped light guiding plate 5 (for reference, see Japanese Patent Application Publication No. 2004-69751, FIG. 9). The sheet switch 1 further includes a circuit board 2 provided with fixed contacts 3 and wiring patterns, movable contacts 4 each of which is disposed on the circuit board 2 to cover each of the fixed contacts 3 and includes a tact spring, and key tops 7 to press the movable contacts 4. The light guiding plate 5 is disposed over the circuit board 2. The light emitting elements 6 illuminate one side surface of the light guiding plate 5. Each of the key tops 7 has a lower portion which passes through the light guiding plate 5 and faces the corresponding movable contact 4. In the sheet switch 1, the entirety of the key tops 7 can be illuminated by guiding light emitted from the light emitting elements 6 to the lower portion of each of the key tops 7 passing through the light guiding plate 5.
Also known is an even thinner sheet switch together with an illumination function which has been a response to the recent trend towards thinning of electronic instruments (for reference, see Japanese Patent Application Publication No. 2004-69751, FIGS. 6 and 9). FIG. 9 illustrates a sectional structure of the sheet switch 11 mentioned in Japanese Patent Application Publication No. 2004-69751. The sheet switch 11 includes a circuit board 12, a sheet switch part 18 which is provided on the circuit board 12 and comprises a plurality of fixed contacts 13 and a plurality of movable contacts 14 each of which is disposed to cover each of the fixed contacts, a light guiding plate 15 disposed over the sheet switch part 18, key tops 17 disposed above the light guiding plate 15 to face the movable contacts 14 and one or more light emitting elements 16 disposed on one end portion of the circuit board 12. In the sheet switch 11, light emitted from the light emitting elements 16 enters a side surface of the light guiding plate 15, thereby the entire light guiding plate 15 is lighted up to illuminate the key tops 17 from below.
However, in the sheet switch 10 in which the light emitting elements 6 are disposed in the vicinity of the key switches 8, as shown in FIG. 7, because additional light emitting elements 6 must be provided as the number of key switches 8 increases, there arises a problem that both power consumption and the overall size of the sheet switch 10 are increased.
Moreover, in the sheet switch 1 as shown in FIG. 8, because the light emitted from the light emitting elements 6 is directed to the key tops 7 through the light guiding plate 5, it is sufficient to use a small number of light emitting elements 6, but a light guiding plate 5 having a greater than usual thickness must be used to guide light emitted from the light emitting elements 6 efficiently without leaking. Using a thick light guiding plate 5 causes an increase in the thickness of the entire sheet switch 1, therefore making it difficult to achieve a thin sheet switch.
On the other hand, in the sheet switch 11 as shown in FIG. 9, because the light emitted from the light emitting elements 16 enters the light guiding plate 15 by passing through one end thereof and is guided throughout the entire light guiding plate 16, a brightness of the key tops 17 remote from the light emitting elements 16 is reduced, giving rise to a problem that overall variations in brightness stand out. Because the light guiding plate 15 is formed by acrylic resin or the bike in a planate state and merely placed on the sheet switch part 18, a large clearance tends to occur between the light guiding plate 15 and the sheet switch part 18, or even between the light guiding plate 15 and each of the light emitting elements 16. The large clearance in the vicinity of the light guiding plate 15 causes a light leak through the large clearance, and problems occur that an amount of the light guided to the key tops 17 is deficient or that there are variations in brightness.
Also, in the sheet switch 11, when the key tops 17 are clicked, because the movable contacts 14 are moved through the light guiding plate 15, if the light guiding plate 15 is formed by a hard resinous material, the clicking sensation is lost, and the switching characteristic is adversely affected.