Apparatus for thermal printing

A laminated wiring conductor layer of a thermal printing head consists of common wiring connected to one end of a patterned heating resistor layer, and individual wirings connected to the other end of the patterned heating resistor layer. The individual wirings are separated from the common wiring by a predetermined interval. The laminated wiring conductor layer includes a plurality of conductor layers in which a first conductor layer exhibits excellent bonding to the heating resistor layer and cannot be readily soldered and hence prevents flow of a solder while a second conductor layer laminated on the first layer is easily soldered and less corrosive than aluminum.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a thin-film type thermal printing head for 
use in a printing portion of facsimiles, printers or the like, and a 
method of manufacturing the same. More particularly, the present invention 
pertains to the structure of wiring connected to heating resistors, the 
structure of solder connecting portions which provide an electrical 
connection to an external circuit by means of soldering, and a method of 
manufacturing such structures. 
2. Description of Prior Art 
The structure of a conventional thermal printing head will be described 
below with reference to FIGS. 1 to 3. 
FIG. 1 is a perspective view of the conventional thermal printing head. 
Driving ICs 110 and one end of a flexible printed board 120 are soldered 
to a thermal printing head base 100 bonded to a heat sink 80. Connectors 
130 are soldered to the other end of the flexible printed board 120. 
Signals for driving the head enter the thermal printing head through the 
connectors 130 from an external circuit, and control the driving ICs 110 
and thereby drive heating resistors (not shown). 
FIG. 2 is a plan view of the essential parts of the conventional thermal 
printing head base 100. Heating resistors 20 formed on a high-resistance 
substrate 10 are electrically connected to a common wiring 50 and 
individual wirings 30. The heating resistors 20 are also connected to an 
external circuit (not shown) through the flexible printed board 120 at 
solder connecting portions 61 connected to electrodes of the driving ICs 
110 and at solder connecting portions 62 connected to electrode terminals 
of the flexible printed board 120. 
FIG. 3 is a sectional view taken along the line A--A' of FIG. 2. A heating 
resistor layer 21 made of an alloy of chromium and silicon is formed on 
the high-resistance substrate 10 composed of a ceramic layer 11, a glaze 
layer 12 and tantalum pentaoxide layer 13 by sputtering, and a 0.1 .mu.m 
thick chromium layer 31 and a 0.8 .mu.m thick aluminum layer 34 are then 
formed on the heating resistor layer 21 in sequence by sputtering to form 
the wiring 30. Thereafter, an unnecessary portion of the wiring 30 and 
that of the heating resistor layer 21 are removed by the photolithographic 
process to form the heating resistors 20. 
Next, to protect the heating resistors 20 and the wiring 30, a protective 
layer 40 consisting of two layers is formed first by forming a silicon 
dioxide layer 41 to a thickness of 4.0 .mu.m by sputtering and then 
forming through-holes by the photolithographic process and then by forming 
a polyimide layer 42 to a thickness of 3.5 .mu.m and then forming 
through-holes by the photolithographic process. Subsequently, the common 
wiring 50 and the solder connecting portions 60, each composed of a 
chromium layer 51, a copper layer 52 and a gold layer 53, are formed at 
the same time using both the sputtering and the photolithographic process. 
Thereafter, an abrasion resistant protective layer 71 made of, for 
example, silicon nitride, is formed selectively on both the common wiring 
50 and the heating resistor 20 by the plasma CVD process. 
The thermal printing head of the above-described type may be employed in 
the thermal printing method. In that case, a thermal printing paper is 
moved, perpendicularly to the paper on which FIG. 3 is depicted, from the 
right to the left by a platen roller (not shown) while being pressed 
against the heating resistors. In consequence, lees 91 of the printing 
paper remain at the shoulder of the common wiring 50, deteriorating 
contact of the printing paper with the heat transmitting portion of the 
upper portions of the heating resistors. This necessitates cleaning of the 
head once a month in a case where the head is used at a normal frequency. 
In the above-described conventional thermal printing head, the protective 
layer 40 is made up of the silicon dioxide layer 41 and the polyimide 
layer 42 to attain reliability because the easily corrosive aluminum layer 
34 is used to form the wiring 30. The thickness of the silicon dioxide 
layer 41 is particularly important. That is, to prevent corrosion of the 
aluminum layer 34, the silicon dioxide layer 41 must have a thickness of 
4.0 .mu.m or above. The silicon dioxide layer 41 is formed on the heating 
resistor 20 also, and the thickness thereof thus greatly affects the 
printing characteristics. In the case where aluminum is used as a metal 
for wiring, a level of printing energy must therefore be enhanced because 
of the thickness of the silicon dioxide layer 41. Further, the polyimide 
layer 42 is used as a stress relieving film to prevent the glaze layer 12 
from being cracked by the stress applied thereto from the electrode 
connecting solder when the driving ICs are mounted. 
The use of the wiring made up of at least two layers, as in the case of the 
above-described conventional thermal printing head, e.g., the use of the 
wiring made up of, for example, a lower chromium layer and an upper 
aluminum layer, as disclosed in Japanese Patent Unexamined Publication No. 
61-43449, assures economic wiring substrate. However, this necessitates 
formation of another solder connecting metal layer on the aluminum layer 
because the normally employed solder does not alloy with aluminum. 
In the solder connecting portion 60 employed in the above conventional 
thermal printing head, the copper layer 52 is connected to a solder, the 
gold layer 53 has a function of preventing oxidation of the surface of the 
copper layer 52, and the chromium layer 51 has a function of bonding the 
solder connecting portion 60 to a layer disposed below it. 
Japanese Patent Unexamined Publication No. 63-28665 discloses a thermal 
printing head which employs copper as a wiring metal and an alloy of 
nickel and copper as a solder connecting metal. Although the alloy of 
nickel and copper ensures excellent solder connection, the number of metal 
layers in the thermal printing head is increased, making the manufacturing 
process complicated. Furthermore, no consideration is given to a change in 
the thickness of the protective layer caused by a change in the wiring 
metal. 
Thus, in the conventional thermal printing heads, the lees 91 of the 
printing paper easily remain at the shoulder of the common wiring. This 
makes frequent cleaning of the head necessary. Furthermore, in a case 
where aluminum is used as a wiring metal, the thickness of the protective 
layer must be increased. This prevents reduction in the power consumption 
of the thermal printing head. Also, in a case where aluminum is used as a 
wiring metal, since the electrical connection with an external circuit is 
achieved by the soldering process, a solder connecting metal other than 
that used in the wiring must be used. 
As stated above, the conventional thermal printing heads have disadvantages 
in that the thickness of the protective layer must be increased and the 
level of printing energy must therefore be enhanced because of the use of 
aluminum as the wiring metal, in that the use of different metals for the 
wiring and for the solder connecting portions and common wiring makes the 
overall configuration complicated, and in that frequent cleaning is 
required, making the operation of the head uneconomical. 
SUMMARY OF THE INVENTION 
In view of the aforementioned problems of the prior art, objects of the 
present invention are to provide a thermal printing head which requires 
less amount of printing energy, which ensures highly reliable connection, 
and which eliminates frequent cleaning and is hence economical, and to 
provide a method of manufacturing the thermal printing head. 
One of the above-described objects of the present invention is achieved by 
the provision of a thermal printing head in which common metals are used 
for wiring and solder connection and in which one of at least two layers 
constituting the wiring is made of a solder connecting metal which is less 
corrosive than aluminum while the other one layer is made of a metal which 
cannot be readily soldered and therefore prevents flow of a solder. 
According to one aspect of the present invention, there is provided a 
thermal printing head which comprises: a patterned layer of a plurality of 
heating resistors arranged in line on a high-resistance substrate; a 
laminated wiring conductor layer consisting of common wiring connected to 
one end of the patterned heating resistor layer and individual wirings 
connected to the other end of the patterned heating resistor layer, the 
individual wirings being separated from the common wiring by a 
predetermined interval; a heat-resistant insulating layer formed at least 
one ht laminated wiring conductor layer and on an exposed portion of the 
patterned heating resistor layer on which the wiring conductor layer is 
not formed; an abrasion-resistant protective layer provided at least above 
the exposed portion of the patterned heating resistor layer with the 
heat-resistant insulating layer being interposed therebetween; solder 
connecting portions formed by forming through-holes in a portion of the 
heat-resistant insulating layer placed on the individual wirings, the 
solder connecting portions being connected to driving ICs; and driving ICs 
soldered to the solder connecting portions. The laminated wiring conductor 
layer includes a plurality of conductor layers in which a first conductor 
layer exhibits excellent bonding to the heating resistor layer and cannot 
be readily soldered and hence prevents flow of a solder while a second 
conductor layer laminated on the first layer is easily soldered and less 
corrosive than aluminum. 
In one preferred form of the present invention, the laminated wiring 
conductor layer consists of the first and second layers, and a groove is 
formed around an exposed portion of the second conductor layer in each of 
the driving IC soldering portions formed by forming the through-holes in 
the portion of the heat-resistant insulating layer placed on the 
individual wirings to expose the first conductor layer. The exposed 
portion of the first conductor layer serves as a solder flow preventing 
portion during solder connection. 
In another preferred form of the present invention, the laminated wiring 
conductor layer includes three layers with a third layer being laminated 
on the second layer. A portion of the third layer located in each of the 
solder connecting portions formed by forming the through-holes in the 
portion of the heat-resistant insulating layer placed on the individual 
wirings is selectively removed to expose a portion of the second layer to 
make it serve as a solder connecting portion. 
In that case, the first and third layers may be made of the same metal. 
The first layer may be made of a simple metal selected from a group 
consisting of chromium, titanium, molybdenum and tungsten, or an alloy of 
the metals, and the second layer may be made of copper or a copper alloy. 
The heat-resistant insulating layer may be made of silicon dioxide, and the 
abrasion-resistant protective layer may be made of silicon nitride. 
According to another aspect of the present invention, there is provided a 
method of manufacturing a thermal printing head, which comprises the steps 
of: forming a patterned layer of a plurality of heating resistors arranged 
in line on a high-resistance substrate; forming a laminated wiring 
conductor layer consisting of at least first and second layers on the 
patterned heating resistor layer; forming heating resistors by conducting 
selective etching to form a wiring pattern in which a portion of the 
wiring conductor layer located on one end of the patterned heating 
resistor layer is left as a common wiring layer, in which a portion of the 
wiring conductor layer located on the other end of the patterned heating 
resistor layer is left as an individual wiring layer, and in which a main 
surface of the patterned heating resistor layer located between the wiring 
layers is exposed; forming a heat-resistant insulating layer at least on 
the wiring pattern and on a portion of the patterned heating resistor 
layer which is exposed by the selective etching of the wiring pattern; 
forming solder connecting portions connected to driving ICs, the solder 
connecting portions being formed by forming through-holes in a portion of 
the heat-resistant insulating layer located above the individual wiring 
layer; and forming an abrasion-resistant protective layer above the 
heating resistors with the heat-resistant insulating layer being 
interposed therebetween. The first layer of the wiring conductor layer is 
made of a simple metal selected from a group consisting of chromium, 
titanium, molybdenum and tungsten, or an alloy of the metals, and the 
second layer is made of copper or a copper alloy. 
According to still another aspect of the present invention, there is 
provided a method of manufacturing a thermal printing head, which 
comprises the steps of: forming a patterned layer of a plurality of 
heating resistors arranged in line on a high-resistance substrate; forming 
a laminated wiring conductor layer consisting of first, second and third 
layers on the patterned heating resistor layer; forming heating resistors 
by conducting selective etching to form a wiring pattern in which a 
portion of the wiring conductor layer located on one end of the patterned 
heating resistor layer is left as a common wiring layer, in which a 
portion of the wiring conductor layer located on the other end of the 
patterned heating resistor layer is left as an individual wiring layer, 
and in which a main surface of the patterned heating resistor layer 
located between the wiring layers is exposed; forming a heat-resistant 
insulating layer at least on the wiring pattern and on a portion of the 
patterned heating resistor layer which is exposed by the selective etching 
of the wiring pattern; forming solder connecting portions connected to 
driving ICs, the solder connecting portions being formed by forming 
through-holes in a portion of the heat-resistant insulating layer located 
above the individual wiring layer and then by conducting selective etching 
on the third wiring conductor layer to expose the second wiring conductor 
layer; and forming an abrasion-resistant protective layer above the 
heating resistors with the heat-resistant insulating layer being 
interposed therebetween. The first layer of the wiring conductor layer is 
made of a simple metal selected from a group consisting of chromium, 
titanium, molybdenum and tungsten, or an alloy of the metals, and the 
second layer is made of copper or a copper alloy. 
The first and second wiring conductor layers may be made of the same metal. 
The laminated wiring conductor layer may be continuously formed by 
sputtering. 
The heat-resistant insulating layer may be made of silicon dioxide and 
formed to a thickness of less than 4 .mu.m by sputtering, and the 
abrasion-resistant protective layer may be made of silicon nitride and 
formed by the plasma PVD process. 
In the present invention, reliability can be maintained even when common 
metals are used to form the wiring portion and solder connecting portions. 
In consequence, the manufacturing process can be simplified and economical 
manufacture of thermal printing heads is thus made possible. Furthermore, 
since the thickness of the protective layer for the wiring metals can be 
reduced, the level of printing energy supplied can be reduced. 
Furthermore, since the amount of lees of printing paper can be reduced, 
the frequency of cleaning the thermal printing head can be reduced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will now be described with reference 
to FIGS. 4 to 12. 
Embodiment 1 
FIG. 4 is a plan view of the essential parts of an embodiment of a thermal 
printing head according to the present invention, and FIG. 5 is a 
sectional view taken along the line B--B' of FIG. 4. 
In FIGS. 4 and 5, the same reference numerals are used to denote parts 
which are the same as those in FIGS. 2 and 3. As in the case of the 
configuration shown in FIG. 2, the heating resistors 20 formed on the 
high-resistance substrate 10 are electrically connected to the common 
wiring 50 and the individual wirings 30. The heating resistors 20 are also 
connected to an external circuit through the solder connecting portions 61 
connected to the electrodes of the driving ICs 110 and through the solder 
connecting portions 62 connected to the terminals of the flexible printed 
board 120. However, the common wiring 50 is formed concurrently with the 
formation of the solder connecting portions 60 in the case of the 
structure shown in FIG. 2, whereas in the present embodiment it is formed 
together with the individual wirings 30. 
This will be described below in detail with reference to FIG. 5. 
After a heating resistor layer 21, made of an alloy of chromium and 
silicon, has been formed to a thickness of 20 to 30 nm by sputtering on 
the high-resistance substrate 10 composed of the ceramic substrate 11, the 
glaze layer 12 and the tantalum pentaoxide layer 13, a chromium layer 31, 
which is the first layer of the wiring 30, a copper layer 32, which is the 
second layer of the wiring 30, and a chromium layer 33, which is the third 
layer thereof, are respectively formed by sputtering to thicknesses of 
0.15 .mu.m, 2.5 .mu.m and 0.03 .mu.m on the heating resistor layer 21. 
Next, an unnecessary portion of the chromium layer 33 is removed by the 
photolithographic process using a predetermined mask, and unnecessary 
portions of the copper layer 32, chromium layer 31 and heating resistor 
layer 21 are then removed to form the heating resistors 20 and the solder 
connecting portions 60. The heating resistors 20 are connected to the 
common wiring portion 50 constituted by the wiring 30 and to the 
individual wiring portion 30. The solder connecting portions 60 are 
through-holes formed in the chromium layer 33 of the individual wiring 
portion 30. Subsequently, the silicon dioxide layer 41 is formed by 
sputtering to a thickness of 2.0 .mu.m on the heating resistors 20 and on 
the wiring 30 as a protective layer. Then, the silicon nitride layer 71 is 
partially formed by the plasma CVD process to a thickness of 1.5 .mu.m as 
the abrasion-resistant protective layer on the silicon dioxide protective 
layer 41 below which the common wiring and the heating resistors 20 exist. 
Thereafter, through-holes are formed by the photolithographic process in 
the portions of the silicon dioxide layer 41, which correspond to the 
solder connecting portions 60. The through-holes 61 constitute the solder 
connecting portions 60 in the present embodiment. 
The thus-obtained thin-film type thermal printing head has a structure 
which permits continuous manufacture by a normally adopted carousel type 
sputtering apparatus except for the silicon dioxide layer 41. It is 
therefore possible to form the heating resistors, wiring and driving IC 
connecting metal layer in sequence within the same film forming apparatus. 
Furthermore, since the copper layer 32 is used as a wiring metal in place 
of the conventionally employed easily corrosive aluminum, high reliability 
can be assured with the protective film 41 composed of the silicon dioxide 
film having a thickness of only 2.0 .mu.m. Also, since the stress applied 
by the soldering conducted on the thermal printing head provided with the 
glaze layer 12 is relieved by the copper layer 32, the stress does not 
directly reach the glaze layer 12. It is therefore possible to maintain 
reliability. 
Since the protective silicon dioxide film 41 is formed also on the heating 
resistor layer 21, the thickness of the silicon dioxide film 41, which was 
at least 4.0 .mu.m in the conventional thermal printing head, can be 
reduced to 2.0 .mu.m, thereby making it possible to reduce by 2.0 .mu.m, 
the distance between the heating resistors 20 and the printing paper. This 
improves the heat conduction efficiency, which leads to reduction in the 
printing energy. 
FIG. 8 is a graph showing the printing characteristics of the thermal 
printing head according to the present embodiment. The abscissa axis 
represents power applied to the heating resistors, and the ordinate axis 
represents optical darkness of the printing paper. For comparison, a 
printing characteristic curve b of the conventional thermal printing head 
shown in FIGS. 2 and 3 is also shown. The curve b is on the right side of 
the printing characteristic curve a of the present embodiment, which means 
that the conventional thermal printing head requires a higher level of 
energy for printing. For example, the conventional thermal printing head 
requires 0.3 mJ of printing energy to achieve an optical darkness of 1.0, 
whereas the present embodiment requires only 0.24 mJ of energy to obtain 
the same optical darkness, which is about 20% reduction. 
FIG. 9 is a graph showing the results of measurements of the connection 
strength with which the heating resistor driving ICs are soldered to the 
IC connecting portions 60. The abscissa axis represents the number of 
times the driving ICs are repaired, and the ordinate axis represents the 
shearing strength. The number of times the driving ICs are repaired refers 
to the number of times the defective driving ICs are replaced with new 
ones. The thermal printing head generally employs a large number of 
driving ICs, and the technique of repairing the defective ICs is 
inevitable. In the graph shown in FIG. 9, 0 time means the initial stage 
of the use. To facilitate comparison, the solder connected area is made 
the same in both examples. As can be seen from FIG. 9, the initial 
connection strength is the same in both the comparative example shown in 
FIG. 3 and the present embodiment shown in FIG. 5. However, in the 
conventional example, as the number of times the driving ICs are repaired 
increases, the connection strength reduces, and the dispersion in the 
measurement of connection strength is wide, whereas in the present 
embodiment repair does not reduce the connection strength, and the 
dispersion in the measurement of connection strength is narrow. 
In the thermal printing head shown in FIGS. 4 and 5, the chromium layer 33 
which is the third layer of the wiring consisting of the three layers acts 
as a solder flow preventing layer. This makes the wiring a highly reliable 
and economical one. Furthermore, since the second metal layer in the 
wiring is made of 2.5 .mu.m thick copper which allows for soldering 
connection and which has a low specific resistance, the outlet side of the 
printing paper (not shown) can be made flat, thus reducing the frequency 
with which the paper lees are removed. Furthermore, since the protective 
film 41 for the copper layer 32 is made up of the silicon dioxide layer 
having a thickness of 2.0 .mu.m, the heat emanating from the heating 
resistors 20 reaches the thermal printing paper more efficiently. This 
makes it possible to reduce the level of printing energy. 
As stated above, it is possible by the thermal printing head of the present 
embodiment to reduce the level of printing energy and to increase the 
strength of the solder connected portions. Furthermore, since the wiring 
layer 30 constitutes both the common wiring and the individual wirings, 
the manufacturing process can be reduced and the manufacturing efficiency 
can thus be enhanced. Furthermore, in the conventional thermal printing 
head, the lees of the printing paper readily remain at the shoulder of the 
abrasion-resistant protective film 71 located near the outlet side of the 
printing paper, so cleaning is required at least once a month. However, in 
the thermal printing head of the present embodiment, there exists no 
shoulder, and the frequency with which cleaning is done can thus be 
reduced to about once a year. 
Embodiments of the method of manufacturing a thermal printing head 
according to the present invention will now be described with reference to 
FIGS. 10 to 12. 
FIGS. 10 to 12 mainly show the process of manufacturing the solder 
connecting portions 60. 
Embodiment 2 
FIGS. 10A-10D show an embodiment of the simplest manufacturing process. As 
shown in FIG. 10A, the chromium layer 31 which is the first layer of the 
wiring layer 30, the copper layer 32 which is the second layer thereof, 
and the chromium layer 33 which is the third layer thereof are formed in 
sequence by sputtering. Thereafter, a resist mask having a predetermined 
pattern is formed on each of the three layers and etching is conducted 
thereon one layer at a time, starting from the third layer, as shown in 
FIGS. 10B to 10D. That is, the third chromium layer 33 is selectively 
etched first to form the solder connecting portions 60, as shown in FIG. 
10B. Next, the second copper layer 32 is selectively etched, as shown in 
FIG. 10C, and the first chromium layer 31 is then selectively etched to 
partially expose the heating resistor layer 21, as shown in FIG. 10D. 
Thereafter, although not shown in the drawing, partial etching of the 
heating resistor layer 21, formation of the protective film 41 and 
abrasion-resistant protective layer 71, connection of the flexible printed 
board 120, mounting of the driving ICs 110, electrode connection and so on 
continue until manufacture of the thermal printing heads is completed. 
Embodiment 3 
FIGS. 11A-11C are views similar to FIGS. 10A-10D, showing another 
embodiment of the thermal printing head manufacturing method according to 
the present invention, in which the number of resist mask forming 
processes is reduced by one to reduce the amount of chemicals used and 
working hours. 
In the present embodiment, as shown in FIG. 11A, after the wiring layer 30 
has been formed in the same manner as shown in FIG. 10A, the third 
chromium layer 33 and the second copper layer 32 are successively and 
selectively removed by etching using the same photoresist mask, as shown 
in FIG. 11B, to make the first chromium layer 31 exposed. Thereafter, the 
resist mask is removed, and the unnecessary portions of the third chromium 
layer 33 and the unnecessary portions of the first chromium layer 31 are 
removed by etching at the same time, as shown in FIG. 11C. Removal of the 
unnecessary portions of the third chromium layer 33 forms the solder 
connecting portions 60. In this way, the number of resist pattern forming 
processes can be reduced by one, thereby reducing the amount of chemicals 
and the working hours. However, in a case where the third chromium layer 
33 and the second copper layer 32 are successively removed by etching, 
since a mixture solution of iodine and ammonium iodide, which is used for 
etching the copper layer 32, etches the side portions of the copper layer 
32 excessively, overhanging portions 33' of the third chromium layer 33 
may be generated, as shown in FIG. 11B. The overhanging portions 33' are 
separated from the chromium layer 33 to form separated portions 33" when 
the photoresist mask is formed for the first chromium layer 31. The 
separated portions 33" placed below the resist mask pattern may cause 
pattern defects, which in turn causes entry of foreign matter in the 
subsequent sputtering process. Therefore, formation of the separated 
portions 33" must be eliminated. This problem is solved by Embodiment 4 
described below. 
Embodiment 4 
FIGS. 12A-12D are views similar to FIGS. 10A-10D, showing an embodiment of 
the present invention in which, in comparison with the embodiment shown in 
FIGS. 11A-11C, the overhanging portions 33' of the third chromium layer 33 
are removed by etching before they are separated from the chromium layer 
33. 
In the present embodiment, as shown in FIG. 12A, after the wiring layer 30 
has been formed in the same manner as shown in FIG. 10A, the third 
chromium layer 33 and the second copper layer 32 are successively etched 
using a photoresist mask 81, as shown in FIG. 12B. Thereafter, etching is 
conducted on the chromium layer 33 again using potassium ferricyanide 
which is the selective etchant for the chromium layer 33, as shown in FIG. 
12C, to remove the overhanging portions 33' of the chromium layer 33. It 
is thus possible to reduce the number of resist mask forming processes by 
one without generating the wiring pattern defects and, hence, without 
increasing the amount of foreign matter in the sputtering process, thereby 
reducing the amount of chemicals used and working hours. It is noted that 
the third chromium layer 33 and the first chromium layer 31 are etched at 
the same time by the same etchant. However, it is possible to completely 
prevent the separated portions 33" from being formed and to restrict 
etching on the first chromium layer 31 to a light etching thus leaving the 
first chromium layer 31 over the entire surface of the substrate, by 
making the difference in thickness between the two layers 33 and 31 large 
and by conducting etching on both the overhanging portions 33' and the 
chromium layer 31 exposed by the etching of the copper layer 32 for a time 
sufficient to etch only the overhanging portions 33' after etching has 
been conducted on the copper layer 32. 
Thereafter, as shown in FIG. 12D, a predetermined photoresist mask is 
formed and selective etching is then conducted on both the third chromium 
layer 33 and the first chromium layer 31 in the same manner as shown in 
FIG. 11C to form the solder connecting portions 60 and the wiring pattern 
for the heating resistors 20 at the same time. 
In the process shown in FIG. 12C, the first chromium layer 31 is left over 
the entire surface of the substrate. In a case where only the portion of 
the first chromium layer 31 placed below the second copper layer 32 is to 
be left, however, it is not necessary to give consideration on the 
difference in the thickness between the third chromium layer 33 and the 
first chromium layer 31, and the first chromium layer 31 may be 
continuously etched after selective etching has been conducted on the 
copper layer 32. Conversely, in a case where the third and first layers of 
the wiring portion 30 are made of metals which permit selective etching, 
the overhanging portions can be removed completely without the metal which 
forms the first layer being lightly etched. 
In the thermal printing head manufacturing methods shown in FIGS. 10 to 12, 
the third chromium layer 33 acts as a metal layer which prevents flow of 
solder. This makes the methods highly reliable and economical. 
Embodiment 5 
FIG. 6 is a plan view of the essential parts of an embodiment of the 
thermal printing head according to the present invention in which the 
wiring 30 is composed of the first layer made of chromium and the second 
layer made of copper, and FIG. 7 is a sectional view taken along the line 
C--C' of FIG. 6. 
The configuration of the present embodiment is basically the same as that 
of the Embodiment 1 shown in FIGS. 4 and 5. The same reference numerals 
are thus used to denote parts which are the same as those of the 
Embodiment 1. 
The differences between the present embodiment and the Embodiment 1 are 
that the wiring 30 in the present embodiment consists of the first layer 
made of 0.1 .mu.m thick chromium layer 31 and the second layer made of 2.5 
.mu.m copper layer 32, and that a solder is connected to the second copper 
layer 32 while the chromium layer 31 acts as a solder flow preventing 
layer. 
With the thin film structure of the present embodiment, the heating 
resistors 20, the wiring 30 and the driving IC connecting electrodes 60 
can be continuously formed within the same film forming apparatus, like 
that of the Embodiment 1. Furthermore, since conventionally used, easily 
corrosive aluminum is not used as the wiring metal, a protective silicon 
dioxide layer 41 having a thickness of 2.0 .mu.m is enough to achieve 
reliability of the wiring. Furthermore, as compared with the Embodiment 1, 
the cost of materials and the working hours can be reduced because of 
absence of the third chromium layer 33. 
When solder connection is conducted on the thermal printing head of the 
present embodiment for mounting the driving ICs, however, a solder may 
flow around the side face of the copper layer 32 of the connecting 
portions 61. In that case, since the chromium layer 31 is unable to 
relieve stress applied by the solder, stress may be transmitted to the 
glaze layer 12, generating cracks therein. Hence, the use of a 
high-resistance substrate 10 which does not employ a glass or glaze layer 
is desired. 
In the present embodiment, a groove is provided around the copper layer 32 
which forms the solder connecting portion 61, i.e., between the connecting 
portion 61 and the protective layer 41, to expose the chromium layer 31 
and thereby enable it to act as a solder flow preventing layer. 
In the thermal printing head having the structure shown in FIGS. 6 and 7, 
since a solder is connected to the copper layer 32 in the wiring portion 
30 while the chromium layer 31 acts as a solder flow preventing layer, 
formation of the wiring is made economical. Furthermore, since the second 
layer of the wiring portion is made of 2.5 .mu.m thick copper which has a 
low specific resistance and which exhibits excellent solder connection, 
the outlet portions of the heating resistors 20 from which the printing 
paper leaves the heating resistors 20 can be made flat. This allows the 
frequency with which the lees of the paper are removed to be reduced. 
Furthermore, since the protective film 41 for the copper layer 32 is made 
of 2.0 .mu.m thick silicon dioxide, the efficiency with which the heat 
emanating from the heating resistors 20 reaches the thermal printing paper 
can be increased. This allows the level of printing energy to be reduced. 
In the above-described typical embodiments of the present invention, 
chromium which is readily bonded to the patterned heating resistor layer 
21 and which is not readily connected to a solder is used to form the 
first and third layers of the wiring layer 30. However, titanium, 
molybdenum, tungsten or an alloy of these metals may be employed to form 
these layers. Also, copper used to form the solder connecting second layer 
which is less oxidized (corrosive) than the aluminum layer may be 
replaced, for example, by NiCu, CrCu or a copper alloy. Furthermore, the 
first and third layers may not be formed of the same metal. Metals may be 
adequately selected in accordance with the pattern forming process. 
Furthermore, the wiring layer 30 may consist of four or more layers when 
necessary. However, a wiring layer consisting of two or three layers is 
practical. 
As will be understood from the foregoing description, the thin film 
structure for the wiring of the thermal printing head according to the 
present invention consists of two or more layers, wherein at least one 
layer is made of copper or a copper alloy which exhibits excellent solder 
connection and the other at least one layer is made of chromium, titanium, 
molybdenum, tungsten or an alloy of these metals which exhibits poor 
solder connection and which therefore enables the layer to act as a solder 
flow preventing layer. In consequence, the structure of the thin film can 
be simplified, and the wiring portions and the solder connecting portions 
can be formed successively using the same equipment. 
Furthermore, since the thermal printing head according to the present 
invention does not employ easily corrosive aluminum as the wiring metal, 
the wiring protecting film can be simplified, and the thickness of the 
protecting film made of, for example, silicon dioxide, can be greatly 
reduced, thereby reducing the level of printing energy required. 
Furthermore, in the thermal printing head according to the present 
invention, copper or a copper alloy which forms the wiring portion is used 
to form the common wiring also. In consequence, the outlet portions of the 
heating resistors from which the printing paper leaves the heating 
resistors can be made flat to achieve reduction in the amount of lees of 
the printing paper. 
According to the thermal printing head manufacturing method according to 
the present invention, the multi-layers which constitute the wiring can be 
successively formed by the thin-film forming technique, i.e., sputtering. 
Furthermore, the common wiring, individual wirings and solder connecting 
portions on the individual wirings can be easily formed utilizing the 
known fine pattern forming lithographic technique.