Incremental printing of symbolic information – Thermal marking apparatus or processes – Specific resistance recording element type
Reexamination Certificate
2000-10-16
2001-12-18
Tran, Huan (Department: 2861)
Incremental printing of symbolic information
Thermal marking apparatus or processes
Specific resistance recording element type
Reexamination Certificate
active
06331868
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thermal printhead, or specifically a thick-film thermal printhead. It also relates to a method of making such a thermal printhead.
BACKGROUND ART
As is well known, a thick-film thermal printhead has a heating resistor and an electrode pattern (including a common electrode and individual electrodes) formed by printing and baking a conductive paste.
FIG. 11
of the accompanying drawings is a sectional view showing an example of prior art thermal printhead. The illustrated thermal printhead B includes a substrate
100
which is provided, entirely on the upper surface thereof, with a glaze layer
110
for heat retention. A common electrode
120
and a plurality of individual electrodes (not shown) are formed on the glaze layer
110
. The thermal printhead B further includes a heating resistor
130
electrically connected to the common electrode
120
and the individual electrodes.
An common electrode auxiliary layer
140
is formed on the common electrode
120
at a portion spaced from the heating resistor
130
. The common electrode auxiliary layer
140
is provided for preventing a voltage drop in the common electrode
120
.
The thermal printhead B has an overcoat layer
150
for covering the common electrode
120
, non-illustrated individual electrodes, the heating resistor
130
and the common electrode auxiliary layer
140
. Further, a protective layer
160
which is thinner than the overcoat layer
150
is formed on the overcoat layer
150
. The protective layer
160
is formed of a material which is less susceptible to wear and scratches than the material for the overcoat layer
150
. With such a structure, the common electrode
120
and other parts are prevented from directly contacting a recording paper S. As shown in
FIG. 11
, the protective layer
160
is formed not only on the upper surface of the overcoat layer
150
but also continuously on a side surface
100
s
of the substrate
100
.
As shown in
FIG. 11
, a platen roller C is provided on the thermal printhead B so as to contact the protective layer
160
. The platen roller C rotates in the direction of the arrow D
1
to transfer the recording paper S in the direction of the arrow D
2
in close contact with the protective layer
160
. At this time, the recording paper S transferred outwardly by the platen roller C warps downwardly by its own weight. The substrate
100
and the glaze layer
110
are chamfered in such a manner as to correspond to such a warp. As a result, the substrate
100
is formed with a first bevel portion
100
a
, whereas the glaze layer
110
is formed with a second bevel portion
110
a
. Accordingly, it is possible to prevent the recording paper S from being caught at a corner of the substrate
100
(or of the glaze layer
110
), and therefore, it is possible to transfer the recording paper S smoothly outwardly by the platen roller
While having the advantages described above, the prior art thermal printhead B has the following problems.
First, due to the difference in thermal expansion coefficient between the glaze layer
110
and the protective layer
160
, the protective layer
160
in the form of a thin film may break or may be released from the glaze layer
110
. Specifically, the protective layer
160
directly covers the glaze layer
110
at a portion of the upper surface and continuously at the inclined portion
110
a
. When the glaze layer
110
and the protective layer
160
are heated, they thermally expand to different degrees with each other. As a result, stress is concentrated on the ridge
160
a
of the protective layer
160
, resulting in the breakage of the protective layer
160
.
Secondly, with the structure of the prior art thermal printhead B, it is impossible to sufficiently urge the recording paper S toward the heating resistor
130
, which may cause improper printing. As shown in
FIG. 11
, the protective layer
160
has a first convex portion
160
b
(a portion above the heating resistor
130
) and a second convex portion
160
c
(a portion above the common electrode auxiliary layer
140
), with both of which convex portions the platen roller C engages. However, because of the existence of the common electrode auxiliary layer
140
, the second convex portion
160
c
is located considerably higher than the first convex portion
160
b
(See the sign “t” in the drawing). With such a structure, the pressing force by the platen roller C is mostly exerted on the second convex portion
160
c
, so that the recording paper S is not sufficiently pressed against the first convex portion
160
b
. As a result, heat from the heating resistor
130
is not sufficiently transmitted to the recording paper S, which may cause printing failure such as unclear printing results.
DISCLOSURE OF THE INVENTION
The present invention, which is conceived under the circumstances described above, aims to provide a thermal printhead which is capable of preventing a thin film protective layer on a bevel surface of a substrate from being peeled off or broken and which is capable of performing printing at suitable density.
Another object of the present invention is to provide a method of making such a thermal printhead.
In accordance with a first aspect of the present invention, there is provided a thermal printhead comprising: an insulating substrate having an upper surface and a side surface; a glaze layer for heat retention formed on the upper surface of the substrate; a heating resistor formed on the glaze layer; a common electrode having a plurality of teeth in connected to the heating resistor, and a connecting portion connecting the teeth with each other; a plurality of individual electrodes connected to the heating resistor; an electrode auxiliary layer formed on the connecting portion of the common electrode; an overcoat layer for covering the heating resistor and the electrode auxiliary layer; and a protective layer for covering the overcoat layer; wherein the connecting portion of the common electrode includes a first region contacting the glaze layer and a second region contacting the upper surface of the substrate.
Preferably, the electrode auxiliary layer contacts both the first region and the second region of the connecting portion.
Preferably, the electrode auxiliary layer includes a thinner portion contacting the first region of the connecting portion and a thicker portion contacting the second region of the connecting portion.
According to a preferred embodiment of the present invention, the protective layer includes a first protrusion positionally corresponding to the heating resistor and a second protrusion positionally corresponding to the thinner portion of the electrode auxiliary layer. The first and the second protrusions are substantially equal in height.
Preferably, the glaze layer includes an uneven portion contacting the first region of the connecting portion, and the uneven portion is tapered toward the side surface of the substrate.
According to a preferred embodiment of the present invention, the substrate has a bevel surface extending between the upper surface and the side surface of the substrate.
Preferably, the glaze layer is spaced from the bevel surface.
Preferably, the bevel surface is covered with a protective layer.
Preferably, the bevel surface is roughened.
In accordance with a second aspect of the present invention, there is provided a method of making a thermal printhead which comprises an insulating substrate having an upper surface and a second surface adjoining the upper surface; a heat retaining glaze layer formed on the upper surface of the substrate; a heating resistor formed on the glaze layer; an electrode pattern connected to the heating resistor; an electrode auxiliary layer formed on the electrode pattern; an overcoat layer for covering the heating resistor and the electrode auxiliary layer; and a protective layer formed on the overcoat layer. The method comprises the steps of: forming the glaze layer to be spaced from the second surface of a substrate; forming the electrode pattern to have a firs
Hayashi Hiroaki
Sako Teruhisa
Yamade Takumi
Bednarek Michael D.
Rohm & Co., Ltd.
Shaw Pittman LLP
Tran Huan
LandOfFree
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