JP3592440B2 - Thermal print head - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thermal print head for printing on a recording medium by a thermal transfer method or a thermal recording method.
[0002]
[Prior art]
A general configuration of a conventional thermal print head will be described below with reference to FIGS. 6 to 8 by taking a so-called thick film type as an example. On a top surface of a substrate 11 made of an insulating material such as alumina ceramic, a heating element 12 and a plurality of drive ICs 13 for controlling and driving the heating element 12 are mounted. In the case of a thick film type thermal print head, the heating element 12 is formed in a narrow band shape along one side edge on an insulating substrate by a thick film printing method. As shown in detail in FIG. 8 (a), a comb-shaped common electrode 14 and a comb-shaped individual electrode 15, each tip of which is inserted between the common electrodes 14, are formed below the heating element 12. Is arranged. Each common electrode 14 is commonly connected to a common line 16. Each individual electrode 15 extends to the other side edge on the head substrate 11 and is connected to an output pad of the drive IC 13 by wire bonding. The drive IC 13 is also provided with power supply system and signal system terminal pads, which are connected to a predetermined wiring pattern formed on the head substrate 11 by wire bonding. The wire bonding portion for connecting the drive IC 13 and the upper surface pad of the drive IC 13 to the wiring pattern (individual electrode, power supply system and signal system wiring pattern) on the substrate is made of a thermosetting resin such as an epoxy resin. Is sealed by the protective coat 17.
[0003]
When the selected individual electrode 15 is turned on by the drive IC 13, the current flows to the area (the area shown by oblique lines in FIG. 8A) of the strip-shaped heating element 12 between the common electrodes 14, which sandwich the individual electrode 15. Flows, and this region generates heat. That is, the area defined by each of the common electrodes 14 forms a heating dot 18.
[0004]
For example, when the thermal print head is configured to be able to record on a recording paper having an A4 size width at a print density of 8 dots / mm, the common electrodes 14 are formed at equal intervals at a pitch of 125 μm. become. The resistance value of the heating dot 18 defined by the area between the adjacent common electrodes is usually adjusted to a predetermined range by, for example, pulse trimming.
[0005]
[Problems to be solved by the invention]
The conventional thermal printhead having the above configuration has the following problems.
[0006]
That is, first, the surface temperature distribution at the time of driving the dots becomes as shown in FIG. 8 (b) due to the heat radiation effect of the common electrode 14 and the individual electrode 15 which are in contact with the belt-shaped heating element 12, and the printing in the main scanning direction is performed. The connection of dots tends to be poor, and this tendency has been a factor in lowering print quality when performing multi-tone printing as described later.
[0007]
Second, when printing is easily performed for a plurality of gradations, the number of gradations tends to be limited. That is, when printing with a plurality of gradations is performed without changing the output of each heating dot, it is necessary to form dither using, for example, a 2 × 2 or 3 × 3 dot matrix. This is described for a 2 × 2 dot matrix. As shown in FIG. 9, when all the dots are off (a), when one of the dots is on (b), two of the dots are on In the case of (c), when any three dots are on (c), when all the dots are on (d), five levels of density expression can be simply made. However, even when any two dots are on or when any three dots are on, the density as a whole differs slightly depending on which two or three dots are on. However, basically, the above five levels of density expression are possible.
[0008]
However, for example, when performing color printing by performing overprinting using the ink ribbons of Y (yellow), M (magenta), and C (cyan), the above-described five-level density expression requires various methods. Color expression cannot be performed. To solve such a problem, it is preferable to increase the number of dots in the matrix. However, doing so causes another problem that the resolution of the image is deteriorated.
[0009]
The present invention has been conceived in view of such circumstances, and has improved a thermal print head suitable for multi-tone printing while improving the connection of print dots on a recording medium. The task is to provide by change.
[0010]
DISCLOSURE OF THE INVENTION
In order to solve the above problems, the present invention employs the following technical means.
[0011]
That is, the thermal print head according to the present invention has a comb-shaped common electrode commonly connected to a common line and a comb-shaped individual electrode having a tip inserted between the common electrodes on a head substrate. By forming a band-shaped heating element on the common electrode and the individual electrode, a thermal print head is formed in which a large number of heating dots are formed in rows by dividing the heating element between the common electrodes. The heat generating dots are characterized in that the intervals between the common electrodes defining the heat generating dots are set so as to be alternately different, so that those having different resistance values are arranged adjacent to each other. Can be
[0012]
In a preferred embodiment, the heating dots are configured such that adjacent ones are given different resistance values by pulse trimming.
[0015]
All heating dots are electrically arranged in parallel. Therefore, a heating dot having a small resistance value causes more current to flow than a heating dot having a large resistance value, and generates Joule heat. As a result, the printing dots formed by the heating dots having a small resistance value are larger than the printing dots formed by the heating dots having a large resistance value. Therefore, when a print dot formed by a heating dot having a small resistance value and a print dot formed by a heat generation dot having a large resistance value are adjacent to each other, the connection between these print dots is further improved, which contributes to an improvement in print quality.
[0016]
Then, for example, the resistance values of the heating dots are set to two types, heating dots having large and small resistance values are alternately arranged, and two heating dots in the main scanning direction and two heating dots in the sub-scanning direction, for a total of four heating dots. When dithering by a matrix of dots, if all dots are off, if one dot with large resistance is on, if two dots with large resistance are on, if one dot with small resistance is on, When two dots with a small resistance value are on, one dot with a large resistance value and one dot with a small resistance value are on, and when two large dots with a resistance value and one dot with a small resistance value are on, the resistance value Nine levels of density expression are possible when one small dot and two large resistance dots are on, and when all dots are on. Therefore, as compared with a conventional thermal print head of this type in which the resistance of each heating dot is uniform, a wider variety of density expressions can be achieved without increasing the number of dots in the matrix.
[0017]
In the case of a thick-film type thermal print head, as described above, the resistance value of the heating dot can be easily changed by changing the dimension between the comb-shaped common electrodes or by changing the degree of pulse trimming. It can be performed, and hardly causes an increase in cost in implementing the present invention.
[0018]
Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the drawings.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a preferred embodiment of the thermal print head of the present invention will be described in detail with reference to FIGS. In each of the embodiments shown in the drawings, the present invention is applied to a so-called thick film type thermal print head. 1 to 5, the same or equivalent members as those in the conventional example shown in FIGS. 6 to 8 are denoted by the same reference numerals.
[0020]
1 to 3 show a first embodiment of a thermal print head 10 according to the present invention. On an upper surface of a substrate 11 made of an insulating material such as alumina ceramic, a band-like heating element 12 is formed along one side edge 11a, and the heating element 12 is formed by partitioning the heating element 12 in the longitudinal direction. A plurality of drive ICs 13 for individually driving the heating dots 18 to generate heat are mounted. In the lower layer of the heating element 12, a comb-shaped common electrode 14 and a comb-shaped individual electrode 15 whose tip enters between the common electrodes 14 are arranged. 16, each individual electrode 15 extends toward the other side edge 11 b of the head substrate 11, and is connected to an output pad of each drive IC 13 by wire bonding. The drive IC 13 is also provided with power supply system and signal system terminal pads, which are connected to a predetermined wiring pattern formed on the head substrate 11 by wire bonding.
[0021]
The selected individual electrode 15 is turned on by the drive IC 13, and a current flows in a region of the belt-shaped heating element 12 between the common electrodes 14 sandwiching the individual electrode 15, and this region generates heat. That is, the areas between the adjacent common electrodes 14 and 14 respectively constitute the heating dots 18.
[0022]
The common electrodes 14 and the individual electrodes 15 are formed at a fine pitch corresponding to the printing density required for the thermal print head, and are therefore extremely fine patterns. A conductive film such as gold is formed on the glass glaze layer 20 (FIG. 3) formed on the head substrate 11, and this is formed by performing a photolithography process or the like.
[0023]
The belt-shaped heating element 12 is formed, for example, by a thick film printing method using a resistor paste containing a resistance material such as ruthenium oxide as a main component. In addition, the surface of the head substrate 11 on which the wirings and electrodes are formed and the heating element 12 is formed as described above, except for a portion to be exposed on the surface, such as a terminal portion, and a protective coating 21 (made of glass). 3). Then, the drive IC 13 or the wire bonding portion is sealed with a protective coat 17 made of a thermosetting resin such as an epoxy resin.
[0024]
Now, in the present invention, it is assumed that a large number of the heating dots 18 formed in a row as described above are periodically arranged with different resistance values. As shown in FIG. 2A, in the present embodiment, two kinds of resistance values, namely, a heating dot 181 having a first resistance value and a heating dot 181 having a second resistance value larger than that. The dots 182 are alternately arranged.
[0025]
In this embodiment, in order to make the resistance values of the heat generating dots 181 and 182 different, the dimension between the common electrodes 14 is alternately set to be long and short. The resistance value of the heating dot 182 formed of the region between the common electrodes having a long interval is relatively large, and the resistance value of the heating dot 181 formed of the region between the common electrodes having a short interval is relatively small.
[0026]
Since the common electrodes 14 are commonly connected to the common line 16, the heating dots 181 and 182 are electrically parallel. Therefore, in the energized state, more current flows through the heating dots 181 having a small resistance value, and Joule heat generated in a certain period of time becomes larger. When printing is performed using the two types of heating dots 181 and 182 having different heating values, the printing dots 191 formed by the heating dots 181 having a large heating value are larger than the printing dots 192 formed by the heating dots 182 having a small heating value.
[0027]
When the print dots 191 formed by the heating dots 181 having a large heat value and the print dots 192 formed by the heat dots 182 having a relatively small heat value are adjacent to each other, as shown schematically in FIG. The connection of the printing dots is improved, which leads to the improvement of the printing quality.
[0028]
As described above, the resistance values of the heating dots are set to two types, the heating dots having large and small resistance values are alternately arranged, and two in the main scanning direction and two in the sub-scanning direction. When a dither is formed by a matrix of heat generating dots, as shown schematically in FIG. 4, when all dots are off (FIG. 4A), when one dot having a large resistance value is on (FIG. b)), when two dots with a large resistance value are on (FIG. 4C), when one dot with a small resistance value is on (FIG. 4D), and when two dots with a small resistance value are on (FIG. 4 (e)), when one dot having a large resistance value and one dot having a small resistance value are ON (FIG. 4 (f)), two dots having a large resistance value and one dot having a small resistance value are ON. In the case (FIG. 4 (g)), one dot having a large resistance value and two dots having a small resistance value are on. In this case (FIG. 4 (h)) and when all dots are on (FIG. 4 (i)), nine levels of density expression are possible. Therefore, as compared with a conventional thermal print head of this type in which the resistance of each heating dot is uniform, a wider variety of density expressions can be achieved without increasing the number of dots in the matrix.
[0029]
FIG. 5 is a schematic plan view showing a main part of the thermal print head 10 according to the second embodiment of the present invention.
[0030]
In the thermal print head 10 according to this embodiment, heating dots 181 and 182 having two different resistance values, large and small, having different resistance values are alternately arranged by the same method as in the first embodiment. The second embodiment differs from the first embodiment in that the individual electrodes 15 of two adjacent heating dots 181 and 182 are commonly connected. In this case, each two adjacent heating dots 181 and 182 form a pair to form substantially one heating unit. However, since a dot having a predetermined heat value and a dot having a larger heat value are adjacent to each other, if printing is performed in such a heat generation unit, as shown in FIG. Since the small print dots 192 and the relatively large print dots 191 appear on the recording paper with an overlap, it is expected that the print quality is improved by improving the connection of these print dots.
[0031]
Of course, the scope of the present invention is not limited to the above embodiment. For example, in the above embodiment, the dimension between the common electrodes is changed. However, even when the dimension between the common electrodes is fixed, pulse trimming is performed for each heating dot, and two types of resistance values are applied to the heating dot. It is also possible to give
[0032]
In the above embodiment, the heating dots having two kinds of resistance values are alternately arranged. However, the heating dots having three or more kinds of resistance values can be arranged periodically.
[0033]
Further, although the above embodiment is a thick-film type thermal print head, it goes without saying that the present invention can be similarly applied to a so-called thin-film type thermal print head.
[Brief description of the drawings]
FIG. 1 is an overall perspective view of a thermal print head according to a first embodiment of the present invention.
FIG. 2A is an enlarged plan view of a main part of a thermal print head according to a first embodiment of the present invention. (B) is an operation explanatory view.
FIG. 3 is a sectional view taken along the line III-III in FIG.
FIG. 4 is a schematic diagram for explaining the operation of the thermal print head according to the first embodiment.
FIG. 5 is an enlarged plan view of a main part of a thermal print head according to a second embodiment of the present invention.
FIG. 6 is an overall perspective view of a thermal print head according to a conventional example.
FIG. 7 is a sectional view taken along the line VII-VII in FIG. 6;
FIG. 8 is an enlarged plan view of a main part of a thermal print head according to a conventional example.
FIG. 9 is a schematic diagram for explaining the operation of a thermal print head according to a conventional example.
[Explanation of symbols]
Reference Signs List 10 thermal print head 11 head substrate 12 heating element 13 drive IC
14 Common electrode 15 Individual electrode 18 Heating dots 191, 192 Printing dots

Claims (2)

A comb-shaped common electrode commonly connected to a common line on the head substrate, and a comb-shaped individual electrode having a tip inserted between the common electrodes are formed, and a strip-shaped common electrode is formed on the common electrode and the individual electrode. By forming the heating element, in the thermal print head in which a large number of heating dots are formed in a row by partitioning the heating element between the common electrodes ,
The heating dots, by spacing dimension between the common electrodes defining this is set to be different alternately, characterized in that those having different resistance values are arranged next to each other , Thermal print head. 2. The thermal print head according to claim 1, wherein the heating dots are further configured so that different resistance values are given to adjacent ones by pulse trimming .

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