THICK-FILM THERMAL PRINT HEAD

Abstract: A thick-film thermal printhead comprises: an oblong rectangular substrate (1) having at least one longitudinal edge (1a); a partial glaze layer provided on the substrate along the longitudinal edge; a linear heating resistor (11) formed on the partial glaze layer; a common electrode (12) formed on the substrate and electrically connected to the heating resistor; and a plurality of individual electrode (13) formed on the substrate and electrically connected to the heating resistor. The common electrode includes a plurality of comb-like teeth (12A). Each of the comb-like teeth includes a tip portion (12c) having a smaller width and a base portion (12d) having a larger width. Each of the individual electrodes includes a tip portion (13d) having a smaller width and an intermediate portion (13e) having a larger width.

Description: TECHNICAL FIELD The present invention relates to a thick-film thermal printhead. BACKGROUND ART An example of a conventional thick-film thermal printhead is shown in Fig. 5 and Fig. 6. Each of these conventional thermal printheads (indicated by reference code P) comprises a rectangular head substrate 1' and a print substrate 2'. As shown in Fig. 5, the head substrate 1' has a first longitudinal edge 1a' and a second longitudinal edge 1b' extending in parallel to each other. Further, the head substrate 1' has a first end 1c' and a second end 1d' extending between the first and the second longitudinal edges. Likewise, the print substrate 2' has two longitudinal edges and two ends. The head substrate 1' has an upper surface entirely covered by a glaze layer 10' (Fig. 6) made of amorphous glass. On an upper surface of the glaze layer 10' , a linear heating resistor 11' extending along the first longitudinal edge 1a' is formed. The head substrate 1' is further formed with a common electrode 12' and a plurality of individual electrodes 13' . As shown in Fig. 5, the common electrode 12' extends along the first end 1c', the first edge 1a', and the second end 1d'. Further, the common electrode 12' has a plurality of comb-like teeth 12A' extending in parallel to each other. Each of the comb-like teeth 12A' has a tip potion 12a' contacting the heating resistor 11'. Each of the individual electrodes 13' has a first end portion 13a' and a second end portion 13b' away therefrom. The first end portion 13a' contacts the heating resistor 11' and extends between two adjacent comb-like teeth 12A'. On the other hand, the second end portion 13b' is formed with a bonding pad 13c'. The bonding pad 13c' is electrically connected to a drive IC 14' via a connecting wire W'. With the above constitution, the heating resistor 11' is divided into a plurality of regions 15' by the comb-like teeth 12A'. (Fig. 5 shows only one region 15'.) In each of the regions 15', electric current is passed selectively via the drive IC 14' , to heat the selected region 15', making each of the regions 15' function as a heating dot. The prior-art thick-film thermal printhead P as described above has a following disadvantage: Specifically, the thermal printhead P can provide a good printing result if the printing is performed at a speed of about 2 inches per second (2 ips). However, if the printing speed is increased to about 6 ips for example, printed image can be partially blurred, or an unintended whisker-like projection (feathering) can be printed on a printing sheet. Another thick-film thermal printed is disclosed in JP-A- 5092593. In this document there is described a thermal head having lead pattern layers provided on a partial glaze layer formed on a substrate. A thick-film heating element is formed having a width larger than the width of the partial glaze layer. As a result, part of the heating element contacts the substrate through the lead pattern layers or directly. Accordingly, heat from the heating element is dissipated easily. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a thick-film thermal printhead capable of eliminating or reducing the above-described problem in the prior art. According to the present invention there is provided a thick-film thermal printhead comprising: an oblong rectangular substrate having at least one longitudinal edge; a partial glaze layer provided on the substrate along the longitudinal edge; a linear heating resistor formed on the partial glaze layer, the linear heating resistor being narrower than the partial glaze layer and formed entirely within the width of the partial glaze layer; a common electrode formed on the substrate and electrically connected to the heating resistor, the common electrode having a plurality of comb-like teeth contacting the heating resistor,each of the comb-like teeth including a tip portion having a smaller width and a base portion having a larger width; and a plurality of individual electrodes formed on the substrate and electrically connected to the heating resistor, wherein the base portion of each comb-like tooth extends on both of the partial glaze layer and the substrate, each base portion extending onto the partial glaze layer only to a position spaced from the heating resistor so that only the tip portion of each comb-like tooth contacts the heating resistor. According to a preferred embodiment, the partial glaze layer has an arcuate cross section. Further, the partial glaze layer has a thickness of 10-25 µm and a width of 400-1000 µm. According to the preferred embodiment, each of the individual electrodes includes a tip portion having a smaller width and an intermediate portion having a larger width, the intermediate portion of each individual electrode extending on both of the partial glaze layer and the substrate, each intermediate portion extending on to the partial glaze layer only up to a position spaced from the heating resistor so that only the tip portion of each individual electrode contacts the heating resistor. Other objects characteristics and advantages of the present invention will become clearer from an embodiment to be described with reference to the attached drawings. BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS Fig. 1 is a plan view of a thick-film thermal printhead according to the present invention; Fig. 2 is a plan view of a primary portion of the thick-film thermal printhead in Fig. 1; Fig. 3 is a sectional view taken in lines III-III in Fig. 2; Fig. 4 is a graph showing a thermal response characteristic of a heating dot; Fig. 5 is a plan view of a prior art thick-film thermal printhead; and Fig. 6 is a sectional view taken in lines VI-VI in Fig. 5. BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a preferred embodiment of the present invention will be described with reference to Fig. 1 - Fig. 4. Fig. 1 is a plan view showing a thick-film thermal printhead X according to the present invention. As shown in the figure, the thick-film thermal printhead X comprises an oblong rectangular head substrate 1 and an oblong print substrate 2 mounted in adjacency thereto. The head substrate 1 is made of an electrically insulating material such as alumina ceramic whereas the print substrate 2 is made of an electrically insulating material such as glass epoxy resin. As shown in Fig. 1, the head substrate 1 has a first longitudinal edge 1a and a second longitudinal edge 1b extending in parallel to each other. Further, the head substrate 1 has a first end 1c and a second end 1d extending between the first and the second longitudinal edges. Likewise, the print substrate 2 has two longitudinal edges and two ends. The head substrate 1 has an upper surface formed with a partial, linear glaze layer 10 made of amorphous glass. The partial glaze layer 10 extends in parallel to the first longitudinal edge 1a (and the second longitudinal edge 1b), closer to the first longitudinal edge 1a than to the second longitudinal edge 1b. The partial glaze layer 10 has a thickness D1 (Fig. 3) of 10-25 µm, and a with D2 of 400-1000 µm. Advantages achieved from such an arrangement as this will be described later. The partial glaze layer 10 can be formed by applying an amorphous glass paste on the head substrate 1 and then baking the same. As shown in Fig. 3, the partial glaze layer 10 has a smooth arcuate upper surface. This is because the applied glass paste flows at the time of baking. Along a peak portion of the partial glaze layer 10, a linear heating resistor 11 is formed. The head substrate 1 is further formed with a common electrode 12 and a plurality of individual electrodes 13. As is clear from Fig. 1, the common electrode 12 extends along the first end 1c, the first edge 1a, and the second end 1d. Further, the common electrode 12 has a plurality of comb-like teeth 12A extending in parallel to each other. Each of the comb-like tooth 12A contacts the heating resistor 11. Each of the individual electrodes 13 has a first end portion 13a and a second end portion 13b away therefrom. The first end portion contacts the heating resistor 11 and extends between two adjacent comb-like teeth 12A. On the other hand, the second end portion is formed with a bonding pad 13c. The bonding pad 13c is electrically connected to a drive IC 14 via a connecting wire W. As shown in Fig. 2, each of the comb-like teeth 12A includes a tip portion 12c having a smaller width, and a base portion 12d having a larger width. The tip portion 12c is entirely formed on the partial glaze layer 10, and electrically contacted to the heating resistor 11. On the other hand, the base portion 12d is spaced from the heating resistor 11, and only a part of the base portion is formed on the partial glaze layer 10 . The other portion of the base portion 12d is formed on the head substrate 1. The width of the tip portion 12c is 20-25 µm for example, whereas the width of the base portion 12d is 80 µm for example. The tip portion 12c has a length of 400 µm for example. Likewise, the first end portion of each of the individual electrodes 13 includes a tip portion 13d having a smaller width, and an intermediate portion 13e having a larger width. The tip portion 13d is entirely formed on the partial glaze layer 10, and electrically contacted to the heating resistor 11. On the other hand, the intermediate potion 13e is spaced from the heating resistor 11, and only a part of the intermediate portion is formed on the partial glaze layer 10. The other portion of the intermediate portion 13e is formed on the head substrate 1 . The width of the tip portion 13d is 20-25 µm for example, whereas the width of the intermediate portion 13e is 80 µm for example. The tip portion 13d has a length of 400 µm for example. With the above structure, the heating resistor 11 is divided into a plurality of regions 15 by the comb-like teeth 12A. (Fig. 2 shows only one region 15.) In each of the regions 15, electric current is passed selectively via the drive IC 14, to heat the selected region 15, making each of the regions 15 function as a heating dot. The number of the heating dots is varied in accordance with conditions such as the size of recording paper to be used. For example, if printing is to be made to an A-4 size recording paper at a printing density of 200 dpi, 1728 heating dots are formed in a direction of secondary scanning. The common electrode 12 and each of the individual electrodes 13 can be formed by using the following method: Specifically, first, a paste containing an electrically conductive metal such as gold is prepared. Next, the paste is applied on the head substrate 1, and then baked. Then, finally, the baked material is etched by means of photolithography into a predetermined pattern. According to such a method as above, the common electrode 12 and the individual electrodes 13 can be formed simultaneously. The common electrode 12 and the individual electrodes 13 have a thickness of about 0.6 µm. The heating element 11 can be formed by first applying a resistor pate containing ruthenium oxide on the partial glaze layer 10, and then baking the applied paste. The heating resistor 11 has a thickness of about 9µm for example. As shown in Fig. 3, a protective coating 16 is formed to cover the heating resistor 11, the common electrode 12 and each of the individual electrodes 13. However, the bonding pads 13c of the individual electrodes 13 are not covered by the protective coating 16. The protective coating 16 can be formed by applying a glass paste on the head substrate 1 and then baking the glass paste. The protective coating 16 has a thickness of 4-8 µm for example. Alternatively, the protective coating 16 can be formed by an electrically conductive material such as Ti-sialon and SiC to a thickness of 4-8 µm. In this case, the formation of the protective coating 16 is performed by using such a technique as spattering and chemical vapor deposition (CVD) method. As has been described earlier, in the thick-film thermal printhead according to the present invention, the heating resistor 11 is formed on the partial glaze layer 10. Therefore, it becomes possible to make the heating resistor 11 appropriately contact the recording paper. The thickness D1 of the partial glaze layer 10 is 10-25 µm, whereas the width D2 is 400-1000 µm. By making the partial glaze layer 10 into the above given dimensions, thermal responsiveness of the heating resistor 11 can be improved over that of the prior art. This point will be described specifically hereafter. Generally, the thermal responsiveness of the heating resistor 11 decreases to deteriorate printing quality when the area of cross section of the partial glaze layer 10 increases. Conversly, if the area of cross section of the partial glaze layer 10 is too small, the heating resistor 11 does not properly contact the recording paper. It has been found that these problems can be eliminated by setting the thickness and the width of the partial glaze layer 10 to the values given above. Experiments were conducted with results shown in the table below. (The experiments were made with thermal printheads each having a printing density of 200 dpi, and printing was performed at a speed of 6 ips. The common electrode and the individual electrodes of each thermal printhead were formed by using gold to a thickness of 0.6 µm. The heating resistor was made from a resistor paste containing ruthenium oxide to a thickness of 9 µm.) Glaze Type Thickness [µm] Width [µm] Thermal Response Time (T:msec) Printing Quality Example 1 Partial Glaze 12 400 0.63 Good No blur No feathering Example 2 Partial Glaze 24 800 0.85 Good No blur No feathering Example 3 Partial Glaze 50 800 1.20 No good Some blur & Feathering Example 4 Entire Glaze 10 - 0.56 No Good Some blur & Feathering As understood from the Table, the thermal responsiveness of the heating resistor increases if the thickness of the partial glaze layer is 10-25 µm and the width thereof is 400-1000 µm, and as a result, good printing image is obtained. It should be noted here that, as shown in Fig. 4, the thermal responsiveness of the heating resistor is evaluated on the basis of a time T which is the time necessary for a surface temperature of the heating resistor to descend from 300°C to 100°C. Specifically, the shorter is the time T, better is the thermal responsiveness. The thick-film thermal printhead described has the following advantages: Specifically, as has been described with reference to Fig. 2, each of the comb-like teeth 12A and the individual electrodes 13 contacts the heating resistor 11 via the corresponding tip portion 12c or 13d which has the smaller width. According to such an arrangement as this, the area of each heating dot 15 can be increased compared with the prior art, without decreasing the density of the heating dots 15. Further, according to the present invention, rupture of each comb-like tooth 12A (or the individual electrode 13) can be effectively eliminated. Specifically, there is a step between the head substrate 1 and the partial glaze layer 10, and therefore the comb-like tooth 12A is formed as folded on the head substrate 1 and the partial glaze layer 10 (Fig. 3). Because stress concentrates onto such a folded portion as above, the folded portion is relatively easily ruptured. However, according to the present invention, the folded portion is the wider base portion 12d. Therefore, even with the stress concentration, the comb-like tooth 12A is not ruptured easily. In the described embodiment this also applies to each of the individual electrodes.

Claims: A thick-film thermal printhead comprising: an oblong rectangular substrate (1) having at least one longitudinal edge (1a); a partial glaze layer (10) provided on the substrate (1) along the longitudinal edge (1a); a linear heating resistor (11) formed on the partial glaze layer (10), the linear heating resistor (11) being narrower than the partial glaze layer (10) and formed entirely within the width of the partial glaze layer (10); a common electrode (12) formed on the substrate (1) and electrically connected to the heating resistor (11), the common electrode having a plurality of comb-like teeth (12A) contacting the heating resistor (11), each of the comb-like teeth (12A) including a tip portion (12c) having a smaller width and a base portion (12d) having a larger width, wherein the base portion (12d) of each comb-like tooth (12A) extends on both of the partial glaze layer (10) and the substrate (1), each base portion (12d) extending onto the partial glaze layer (10) only to a position spaced from the heating resistor (11) so that only the tip portion (12c) of each comb-like tooth (12A) contacts the heating resistor (11); and a plurality of individual electrodes (13) formed on the substrate (1) and electrically connected to the heating resistor (11). The thick-film thermal printhead according to Claim 1, wherein the partial glaze layer has an arcuate cross section. The thick-film thermal printhead according to claim 1 or claim 2, wherein the partial glaze layer has a thickness of 10-25 µm and a width of 400-1000 µm. A thick-film thermal printhead according to any one of claims 1 to 3, wherein each of the individual electrodes (13) includes a tip portion (13d) having a smaller width and an intermediate portion (13e) having a larger width, the intermediate portion (13e) of each individual electrode (13) extending on both of the partial glaze layer (10) and the substrate (1), each intermediate portion (13e) extending on to the partial glaze layer (10) only up to a position spaced from the heating resistor (11) so that only the tip portion (13d) of each individual electrode (13) contacts the heating resistor (11).

Inventors: OBATA SHINOBU, YOKOYAMA EIJI, OBATA, SHINOBU, YOKOYAMA, EIJI
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1999
Main classification: B41J2
IPC codes: B41J2