The present invention relates to a thermal printhead for performing printing on a recording medium such as a thermal paper. The present invention also relates to a method for manufacturing a thermal printhead.
In the thermal printhead X, the heating portion 93a is located at a position retreated from a recording medium relative to the electrodes 94A and 94B. A structure like this is also disclosed in FIG. 1 of Patent Document 1 identified below. With this structure, however, of the protective film 95, the portion covering the heating portion 93a is pressed against a recording medium with a relatively small pressure, which hinders an increase in the printing speed.
Moreover, as the printing speed increases, the heating cycle of the heating portion 93a shortens. Since the heating portion 93a is directly formed on the glaze 92, the cycle in which the glaze 92 receives heat also shortens. Thus, during the printing operation by the thermal printhead X, the glaze 92 is kept at a relatively high temperature while repeating temperature rise and temperature drop. As a result, excessive thermal stress is applied on the glaze 92, which may cause cracking of the glaze 92.
Patent Document 1: JP-A-2001-246770
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a thermal printhead which is capable of increasing the printing speed.
According to a first aspect of the present invention, there is provided a thermal printhead comprising a substrate, a glaze formed on the substrate and extending in the primary scanning direction, a first and a second electrodes overlapping the glaze and spaced from each other in the secondary scanning direction, a heating resistor overlapping the first and the second electrodes and including a heat generating portion spaced apart from the electrodes, and an insulating film intervening between at least part of the heating portion of the heating resistor and the glaze. At least an end of each of the electrodes is embedded in the glaze. The insulating film has a hardness which is higher than the hardness of the glaze and lower than the hardness of the heating resistor and a thermal conductivity which is higher than the thermal conductivity of the glaze and lower than the thermal conductivity of the heating resistor.
With this structure, the level difference between each of the electrodes and the glaze is reduced. Further, owing to the existence of the insulating film, the heating portion is positioned close to a recording medium. As a result, the portion of the protective film which covers the heating portion is pressed against a recording medium with a high pressure. Thus, the printing speed can be increased. Moreover, the heat from the heating portion is transferred to the glaze via the insulating film. Thus, the amount of heat which the glaze receives is prevented from suddenly increasing or decreasing. As a result, the thermal stress generated in the glaze reduces, whereby the glaze is prevented from cracking. Further, the insulating film functions as a buffer between the glaze and the heating resistor. Thus, the thermal expansion or contraction of the glaze is prevented from being suppressed or promoted by the heating resistor. This is suitable for preventing the glaze from cracking.
Preferably, the insulating film is formed to bridge the first and the second electrodes. With this arrangement, the region of the glaze which is positioned between the first and the second electrodes is completely covered. Thus, the heating resistor and the glaze do not come into contact with each other at all, which is preferable for preventing the glaze from cracking.
Preferably, the insulating film is made of either one of Ta2O5 and SiO2. This arrangement is suitable for achieving the above-described hardness and thermal conductivity of the insulating film.
According to a second aspect of the present invention, there is provided a method for manufacturing a thermal printhead. The method comprises the steps of forming a glaze extending in the primary scanning direction on a substrate and forming a first and a second electrodes spaced from each other in the secondary scanning direction on the glaze, sinking at least an end of each of the electrodes into the glaze by softening at least part of the glaze by heating, forming an insulating film to cover at least part of a region of the glaze which is sandwiched between the first and the second electrodes, and forming a heating resistor to overlap the glaze and the first and the second electrodes in such a manner as to bridge the first and the second electrodes. The insulating film is formed to have a hardness which is higher than the hardness of the glaze and lower than the hardness of the heating resistor and a thermal conductivity which is higher than the thermal conductivity of the glaze and lower than the thermal conductivity of the heating resistor. With this method, the level difference between each of the electrodes and the glaze is easily reduced. Further, owing to the existence of the insulating film, the heating portion is positioned close to a recording medium. As a result, the printing speed can be increased. Moreover, the insulating film suppresses the temperature variation in the glaze and prevents the thermal expansion or contraction of the glaze from being suppressed or promoted by the heating resistor. Thus, the glaze is prevented from cracking.
Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.
Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.
The substrate 1 is a flat insulating plate which is in the form of an elongated rectangle extending in the primary scanning direction as viewed in plan and made of e.g. alumina ceramic material. The glaze 2 is formed on the substrate 1 by printing and baking amorphous glass paste, for example. The glaze 2 serves to retain heat and provide a smooth surface on which the electrodes 3A and 3B are to be formed. The glaze 2 is elongated in the primary scanning direction and bulges in the thickness direction of the substrate 1 in cross section. Due to this shape, the glaze 2 enhances the contact pressure between a recording medium such as thermal paper and the portion of the protective film 6 which covers the heating portion 5a, which will be described later. The maximum thickness of the glaze 2 is about 50 μm.
The first and the second electrodes 3A and 3B are formed in pairs on the substrate 1 and the glaze 2 by e.g. printing and baking gold resinate paste. As shown in
The insulating films 4 are made of e.g. Ta2O5 and formed by baking Ta films formed by sputtering, for example. The insulating films 4 cover the regions of the glaze 2 which are sandwiched between the paired electrodes 3A and 3B. In this embodiment, each of the insulating films 4 partially overlaps the ends 31A and 31B of the paired electrodes. That is, each insulating film 4 extends in such a manner as to bridge the paired electrodes 3A and 3B. The insulating film 4 has a thickness of about 0.1 to 0.2 μm. The hardness of the insulating film 4 is higher than that of the glaze 2 and lower than that of the heating resistors 5. The thermal conductivity of the insulating film 4 is higher than that of the glaze 2 and lower than that of the heating resistors 5. The insulating film 4 may be made of SiO2 instead of Ta2O5.
The heating resistors 5 are formed on the insulating films 4 in such a manner as to bridge the electrodes 3A and 3B. The heating resistors 5 are made of e.g. TaSiO2. Of each heating resistor 5, the portion which is not in direct contact with the electrodes 3A and 3B provides a heating portion 5a (see the diagonally shaded portion in
The protective film 6 covers the glaze 2, the electrodes 3A, 3B, the insulating films 4 and the heating resistors 5. The protective film 6 is formed by e.g. sputtering using SiC or SiAlON. The protective film 6 prevents the electrodes 3A, 3B and the heating resistors 5 from coming into direct contact with a recording medium or being affected chemically or electrically. The protective film 6 also serves to provide a smooth surface. The thickness of the protective film 6 is about 4.0 μm.
A method for manufacturing the thermal printhead A1 will be described below with reference to
First, as shown in
Then, as shown in
As shown in
Then, as shown in
Thereafter, heating resistors 5 covering the insulating films 4 and bridging the electrodes 3A and 3B are made by forming e.g. a TaSiO2 film having a thickness of about 0.05 μm and then subjecting the film to dry etching. Thereafter, by sputtering using e.g. SiC or SiAlON, a protective film 6 is formed to cover the electrodes 3A, 3B and the heating resistors 5. Thus, the thermal printhead A1 as shown in
The advantages of the thermal printhead A1 will be described below.
According to the foregoing embodiment, a stepped portion is not provided between each of the electrodes 3A, 3B and the glaze 2. Further, owing to the existence of the insulating films 4, the heating portions 5a are positioned close to a recording medium. As a result, the portions of the protective film 6 which cover the heating portions 5a are pressed against a recording medium with a high pressure. Thus, the printing speed of the thermal printhead A1 can be increased.
Further, the heat from the heating portions 5a is transferred to the glaze 2 via the insulating films 4. Thus, the degree of increase or decrease of the heat the glaze 2 receives can be smaller. As a result, the thermal stress generated in the glaze 2 reduces, whereby the glaze 2 is prevented from cracking. Further, the insulating films 4 function as a mechanical buffer between the glaze 2 and the heating resistors 5. Thus, the thermal expansion or contraction of the glaze 2 (which has a relatively low hardness) is prevented from being suppressed or promoted by the heating resistors 5 (which have a relatively high hardness). This is suitable for preventing the glaze 2 from cracking.
To make the insulating films 4 have a hardness of a level between the hardness of the glaze 2 and that of the heating resistors 5 and a thermal conductivity of a level between the thermal conductivity of the glaze 2 and that of the heating resistors 5, it is preferable to form the insulating films 4 using Ta2O5. In this embodiment, the region of the glaze 2 which is located between the electrodes 3A and 3B is entirely covered with the insulating film 4. With this structure, the heating resistor 5 and the glaze 2 do not come into contact with each other, which is advantageous for preventing the glaze 2 from cracking.
With this arrangement, the heat from the heating portions 5a is transferred well to the glaze 2 via the dummy pattern 3C. As a result, the variation in amount of heat transferred from the heating portions 5a to a recording medium is alleviated. When the amount of heat applied to the thermal paper varies largely, sticking of the thermal paper to the protective film 6 is liable to occur. According to this embodiment, such sticking phenomenon is prevented.
According to this embodiment, a level difference is hardly defined between each of the electrodes 3A, 3B and the insulating film 4. Thus, the heating resistor 5 is formed on a relatively smooth surface. By forming the heating resistor 5 on a smooth surface free from a level difference, the heating resistor 5 is prevented from having a non-uniform thickness and breaking due to e.g. the pressure applied in the printing process.
Number | Date | Country | Kind |
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2006-230495 | Aug 2006 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2007/066529 | 8/27/2007 | WO | 00 | 2/25/2009 |
Publishing Document | Publishing Date | Country | Kind |
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WO2008/026533 | 3/6/2008 | WO | A |
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