Thermal print head

The lead wires extending from heating resistors to a driver IC for the heating resistors are arranged in the space under the driver IC. These lead wires are constructed in that they become narrower in wire width as approaching the driver IC so as to lie within the width of the driver IC, and the lead wires positioned under the driver IC are constructed in that they become broader in wire width as approaching the connecting terminals of the driver IC. Connection of the lead wires and the driver IC is carried out by the face-down bonding using Controlled Collapse Bonding. By providing the construction as mentioned above, a thermal print head of higher density and definition can be obtained.

BACKGROUND OF THE INVENTION 
The present invention relates to a thermal print head, particularly to 
packaging of the elements of a thermal print head. 
The thermal print system is widely adopted in facsimile and printers. 
Thermal print means a system in which Joule heat is given to a printing 
medium such as heat-sensitive paper and plain paper through ink sheet 
(transcription paper) by heating elements (heating resistors) to cause 
direct color development or fusion thereby to perform transcriptional 
print. 
A thermal print head usually comprises a plurality of heating elements 
disposed in a line on a ceramic substrate, and each heating element is 
heated by the electric current corresponding to a picture signal. 
A Japanese Patent Laid-Open No. 31778/1983 discloses an example of the 
construction of such a thermal print head. 
The thermal print head which has been known up to date, as shown in FIG. 2 
of the above Patent Laid-Open No. 31778/1983, has a construction that the 
driver IC's for the heating resistors (driver IC's) are packaged on a 
ceramic substrate by the so-called face-up packaging so that the face 
having the connecting terminals takes the upside position, and the 
connecting terminals provided on the lead wires from the heating resistors 
are wire-bonded to the connecting terminals of the driver IC's by gold or 
aluminum wires. 
In recent years, printed pictures of high definition having 8 lines/mm or 
more resolution have been required, and miniaturization of the print head 
itself has also been required. In this situation, it is necessary to 
densely package the electic elements constituting the print head, but 
there is a limit in the above-mentioned conventional construction. That 
is, to perform wire bonding, the distance between the connecting terminals 
needs to be at least 1 mm or so in view of the nature of the work, and 
even if the driver IC can be miniaturized, this space for wire bonding 
cannot be reduced. In addition, if the lead wires from the heating 
resistors are made narrower and thinner, electric resistance will increase 
and the fluctuation of resistance due to the differences in wire length 
cannot be ignored. Further, fall of the production yield due to pattern 
defects in the manufacture process is inevitable. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a thermal print head that 
can realize printed pictures of high definition. 
A further object of the present invention is to provide a miniaturized 
thermal print head. 
A still further object of the present invention is to provide a thermal 
print head in which the fluctuation of characteristics of the heating 
resistors and the driver circuit thereof mainly due to the fluctuation of 
electric resistance of the wires is eliminated. 
A still further object of the present invention is to provide a thermal 
print head of high production yield. 
In the present invention, the lead wires extending from the heating 
resistors to the driver IC for the heating resistors are not disposed 
around the driver IC, instead they are all disposed so as to be positioned 
under the driver IC. In addition, these lead wires are constructed in that 
they become narrower in wire width as approaching from the heating 
resistors to the driver IC so as to lie within the width of the driver IC, 
and the lead wires positioned under the driver IC are constructed in that 
they become broader in wire width as approaching the connecting terminals 
of the driver IC. The connection of the lead wires constructed in this way 
with the driver IC is achieved by the so-called face-down bonding, in 
which the connection is performed by the CCB (Controlled Collapse Bonding) 
method with the face of the driver IC having the connecting terminals 
being directed toward the substrate.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a perspective view of the driver IC mounting portion of a thermal 
print head for showing the method of connecting the lead wires from the 
heating resistors and the heating resistor driver IC according to the 
prior art. 
The lead wires 3 from the heating resistors provided on a ceramic substrate 
1 are drawn around a driver IC 4. Connection of the lead wires 3 with the 
driver IC 4 is achieved by wire bonding using wires 5. According to such 
prior art, it is necessary to ensure the space for wire bonding, and, 
thus, it is apparent that there is a limit in packaging driver IC's at a 
high density. 
FIG. 2 is a plan view showing the thermal print head according to the 
present invention. In the figure, except the heating resistors, the lead 
wires from the heating resistors and the driver IC mounting portions, 
various wires actually provided are omitted. 
Electric elements and wires such as heating resistors 2 and lead wires 3 
are provided on a substrate 1. The substrate 1 is usually made of ceramics 
having a thickness on the order of 1-1.5 mm. A large number of heating 
resistors 2 are arranged in a line so that they extend over the full width 
of a thermal print paper passing the thermal print head. For instance, 
when printing is carried out on a B4-size print paper (the paper width: 
256 mm) at a density of 8 lines/mm, 2048 heating resistors are needed if 
they are arranged at intervals of 125 .mu.m. Since the whole cannot be 
shown in the figure, only 64 of them are shown. These heating resistors 2 
can be constructed by a known material of the Cr-Si system, and they are 
patternized on a ceramic substrate by photolithography. 
The lead wires 3 are drawn from the respective heating resistors 2 at a 
same interval. The lead wires 3 are aluminum and are patternized by 
photolithography in a way similar to that for the heating resistors 2. The 
lead wires 3 may be made of a material other than aluminum, for instance, 
gold. 
The driving of the heating resistors 2 is carried out by a heating element 
driver circuit as shown in FIG. 3. Switching transistors 7 are 
respectively connected for each of the heating resistors 2, and, usually, 
as these switching transistors 7 and other driver circuits which are not 
shown, those which are integrated into an integrated circuit as a heating 
resistor driver IC are used. If this driver IC contains 32 switching 
transistors 7, it will have a width on the order of 1.8 mm. Since the 
switching transistors 7 respectively correspond to each of the heating 
resistors 2, a total of 64 driver IC's each containing 32 switching 
transistors as described above are needed in order to drive 2048 heating 
resistors for a B4-size print paper. In the present invention, the 
arrangement of these driver IC's 4 and the connection of the lead wires 3 
from the heating resistors 2 with the driver IC's 4 are constructed as 
follows. 
That is, a driver IC 4 is disposed on the ceramic substrate 1 in parallel 
with the heating resistors 2, as shown in FIG. 2. The lead wires 3 are 
narrowed as approaching the driver IC 4 so as to lie within the width of 
the driver IC. Namely, as shown in FIG. 2, the lead wires 3 arranged at 
intervals of 125 .mu.m are narrowed as approaching the driver IC till they 
are arranged at intervals of 55 .mu.m, all the lead wires 3 are positioned 
under the driver IC 4. The lead wires 3 narrowed in this way are radially 
arranged under the driver IC toward the connecting terminals provided 
around the driver IC. On this occasion, the lead wires 3 are constructed 
so as to become broader in wire width toward the end thereof as well as 
radially arranged at an appropriate angle from the linear portion thereof. 
By making the width of the wires broader toward the end of the wires in 
this way, the fluctuation of electric resistance of the wire conductors 
becomes small, whereby the voltage drop is compensated. Namely, the 
fluctuation of driving characteristic of each heating resistor can be 
eliminated. In addition, by broadening the wire width, breaking of wire or 
the like becomes difficult to occur. 
The end of the lead wires 3, or the portion to be connected to the driver 
IC 4 is provided with a pedestal for connecting the driver IC by 
Controlled Collapse Bonding (CCB) as described later. The pedestal may be 
constructed, for instance, by lamination of Cr, Cu and Au thin films. The 
driver IC 4 is applied with the so-called face-down, which means that the 
face having the connecting terminals opposes the substrate, and alignment 
is provided so that the connecting terminals are positioned above the 
above-mentioned pedestals. Then, they have only to be solder-welded in a 
furnace of N.sub.2 gas atmosphere to complete Controlled Collapse Bonding.