Display device

A display device has a relatively large insulation substrate on which a metal layer is formed. On the metal layer on the insulation substrate, there are at least one pair of ceramic unit substrates which are disposed in close vicinity to each other, respectively having thereon metal layers bearing light emitting diodes. The metal layer on the insulation substrate is connected with the metal layers on the unit substrates at its end portions by means of, e.g., flexible lead frames.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
This invention relates to a display device using light emitting diodes 
(LED's), and, more particularly, to a large-sized display device including 
relatively small substances connected with one another. 
II. Description of the Prior Art 
Generally, in a display device for displaying desired patterns, such as 
letters, numerals, chinese characters, special symbols, illustrations, 
graphic patterns, etc., a plurality of light sources formed of LED's are 
closely disposed on a common substrate. A desired pattern is displayed by 
selectively supplying electric signals to the individual LED's. Since the 
electric signals are transmitted intermittently as short pulses, the 
individual LED's are driven only instantaneously. By repeatedly 
transmitting the electric signals at high speed, however, the pattern may 
be caused to look stationary with the aid of the afterimage effect of the 
human eye. 
Recently, display devices of the aforesaid type have come to show a 
tendency toward enlargement in size as the amount of information to be 
displayed has increased with the development of measuring instruments and 
electronic computers. There are proposed two measures for increasing the 
size of display devices: mounting of a plurality of LED's on a large-sized 
substrate and a method in which a plurality of relatively small 
substrtates each having LED's mounted thereon are electrically connected 
with one another. The former measure has not, however, been put to 
practical use because of its requiring complicated driving systems as well 
as its deteriorating yield of production. 
FIG. 1 shows a typical example of the latter measure. In this display 
device, as shown in FIG. 1, two substrates, for example, are connected by 
means of wires. More specifically, horizontally extending parallel wiring 
layers 12 are formed at regular intervals on a first substrate 11. Each 
wiring layer 12 has relatively wide regions 12a on which LED's 13 are 
fixed with their cathode side downward. LED's fixed on each two adjacent 
wiring layers are electrically connected by stitch bonding on the anode 
side with use of wires 14. 
A second substrate 15 has wiring layers 16 which are the same as the wiring 
layers 12 of the first substrate 11 and LED's 17 are connected with one 
another by means of wires 18. The second substrate 15 is disposed in close 
vicinity to the first substrate 11 so as to be symmetrical with respect to 
an axis A--A at right angles to the wiring layers 12 and 16. Bonding pads 
12b and 16b are provided respectively at the facing ends of the respective 
wiring layers of the substrates 11 and 15, and the wiring layers 12 and 16 
are connected at these bonding pads by means of gold wires 19. 
In such display device, the bonding pads 12b and 16b require a diameter of 
0.2 mm or more for wire bonding. In view of the performance of currently 
available bonding machines, moreover, a certain measure of space (usually 
0.5 mm) need be left between each bonding pad 12b or 16b and the LED 13 or 
17 nearest thereto. In consideration of the bonding accuracy of the 
adjoining portions of the substrates, furthermore, a space of 1.5 mm or 
more is required between the LED's 13 and 17. With such wide space, 
however, the portion of a display pattern corresponding to such space will 
be unclear to the human eye. 
Further, this second measure cannot be applied to the connection between 
substrates with double-layer matrix wiring structure. Conventionally, as 
shown in FIG. 2, one such substrate with double-layer wiring structure 
includes row wiring layers 22 formed on a ceramic substrate 21, an 
insulation layer 23 with openings formed on the row wiring layers, and a 
column wiring layer 24 on the insulation layer 23. LED's 25 are fixed on 
exposed portions of the row wiring layers, each LED being connected at its 
anode side with the column wiring layers 24 by means of wires 26. In the 
double-layer wiring substrate of such construction, as may be seen from 
FIG. 2, the row wiring layers 22 is embedded in the insulation layer 23 at 
the end portion. Accordingly, it is impossible to bond one such substrate 
to another by using wires in the manner shown in FIG. 1. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a display device 
without the above-mentioned drawbacks of the prior art devices. 
Another object of the invention is to provide a coupled-substrate display 
device with a reduced light source arrangement pitch. 
Still another object of the invention is to provide a coupled-substrate 
display device producing distinct display patterns. 
According to this invention, there is provided a display device comprising 
an insulation substrate, first conductive means formed on the insulation 
substrate and including at least one elongated metal layer, an insulation 
means formed on the first conductive means and including at least one pair 
of insulation layers facedly disposed near but spaced from each other 
along the longitudinal direction of the metal layer, second conductive 
means formed on the insulation means and including at least one pair of 
elongated metal layers formed respectively on the insulation layers in a 
position corresponding to the metal layer of the first conductive means, a 
light source formed on the second conductive means and including at least 
one light emitting diode, and connection means including at least one pair 
of connection members connecting the first and second conductive means at 
sides opposite to the facing sides of the pair of insulation layers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings of FIGS. 3 to 6, this invention will be 
described in detail. 
A display device shown in FIG. 3 includes two unit substrates 33 and 40 
coupled with each other. On a single large-sized insulation substrate such 
as a glass-epoxy substrate 31, as shown in FIG. 3, there are a plurality 
of elongated metal layers 32 formed of, e.g. solder, extending 
horizontally in parallel with one another substantially at regular 
intervals. On the metal layers 32, the ceramic substrates 33 and 40, e.g., 
alumina substrates, serving as insulation layers with light emitting 
elements are facedly disposed near but spaced from each other along the 
longitudinal direction of the metal layers 32. 
On the one alumina substrate 33, there are a plurality of elongated metal 
layers (wiring layers) 34 which, formed by printing and baking, e.g., hold 
paste at a temperature of 900.degree. C. to 1,100.degree. C., extend 
horizontally in parallel with one another substantially at regular 
intervals. The metal layers 34 lie in positions corresponding to the metal 
layers 32 on the insulation substrate 31. Further, an insulation layer 35 
is formed by printing and baking ceramic paste over the whole surface of 
each metal layer 34 except an end portion thereof on the opposite side to 
a portion where the substrate 33 adjoins the substrate 40 and 
predetermined portions on the metal layer 34. On the insulation layer 35, 
there are elongated metal layers (wiring layers) 36a, 36b, 36c and 36d 
formed of, e.g. gold, extending at right angles to the longitudinal 
direction of the metal layers 34. Thus, the lower metal layers 34 and the 
upper metal layers 36a, 36b, 36c and 36d constitute a double-layer matrix 
wiring structure. 
Cathodes of light emitting diodes (LED's) 37a, 37b and 37c constituting a 
light source are fixed to the predetermined of exposed portions of the 
metal layers 34 on the alumina substrate 33 by using low melting metal 
paste such as silver paste. These LED's may be manufactured by, for 
example, forming a p-type single crystal of gallium phosphide on an n-type 
single crystal substrate of gallium phosphide by liquid-phase growth 
method and dicing the p-type single crystal into pellets. A cathode and an 
anode are formed on n- and p-type layers, respectively. Each LED is caused 
to emit light by applying a positive voltage between the anode and 
cathode. 
The anodes of the LED's 37a, 37b and 37c are bonding-connected with their 
corresponding or adjacent metal layers 36a, 36b, 36c and 36d along the 
longitudinal direction of the metal layers 34 by means of gold wires 38 
with a diameter of, e.g., 30 .mu.m. 
The alumina substrate 40, like the alumina substrate 33, is provided with 
elongated metal layers 41 formed thereon, an insulation layer 42 formed on 
the metal layers 41, elongated metal layers 43a, 43b, 43c and 43d formed 
on the insulation layer 42, and LED's 44a, 44b and 44c formed on exposed 
portions of the metal layers 41 and having their anodes connected 
respectively with the metal layers 43a, 43b, 43c and 43d by means of gold 
wires 45. 
The insulation layer 35 exists at the end portion of the alumina substrate 
33 where it adjoins the substrate 40, while the LED 44a is disposed at the 
adjoining end of the substrate 40. Further, simultaneously with the 
formation of these metal layers, input-output terminals 39 and 46 of 
100-.mu.m thickness are formed respectively on the metal layers 34 and 41 
at end portions opposite to the facing sides of the substrates 33 and 40. 
The metal layers 34 substantially align with their corresponding metal 
layers 41. Moreover, as may be seen from the drawing, the metal layers 32 
on the insulation substrate 31 are exposed at both ends. 
One of the features of this invention resides in that the metal layers 34 
and 41, in the aforementioned structure, are connected with exposed 
portions of their corresponding metal layers 32 at the input-output 
terminals 39 and 46 by means of flexible lead frames 47 and 48 of, e.g., 
200-.mu.m width and 20-.mu.m thickness. Thus, the metal layers 34 and 41 
on the substrates 33 and 40 are made electrically common by means of the 
frames 47 and 48 and the metal layers 32 on the insulation substrate 31. 
In the above-mentioned display device of this invention, the substrates 33 
and 40 are not bonded at the facing ends thereof by means of wires, so 
that the problems of the prior art device shown in FIG. 1 may be 
eliminated. Actually, the arrangement pitch of the LED's could be reduced 
to approximately 0.8 mm, ensuring improved resolution or outward 
appearance of display patterns. Further, the junction of the substrates 31 
and 40 proved hardly noticeable. Moreover, according to the aforesaid 
display device of the invention, a plurality of substrates of multilayer 
wiring structure can be connected with one another, as is evident from the 
foregoing description. Also, yield may be improved since the substrates 
are not connected by means of the wires, unlike the case of the prior art 
device. 
Although only the two substrates 31 and 40 are used in the above-mentioned 
embodiment, a plurality of such pairs of substrates may be disposed at 
right angles to the longitudinal direction of the metal layers 32, thereby 
providing a further enlarged device. 
Moreover, the arrangement of the LED's on the substrates 31 and 40 is not 
limited to the aforesaid embodiment, and a multicolor display device may 
be obtained by closely arranging a plurality of LED's emitting different 
colors. In this case, two LED's constituting each unit light source should 
preferably be located in offset positions, as shown in FIG. 4. That is, as 
shown in FIG. 4, metal layers 51 equivalent to the metal layers 34 or 41 
of FIG. 3 are formed on a substrate equivalent to the substrate 33 or 40 
of FIG. 3. An insulation layer 52 equivalent to the insulation layer 35 of 
FIG. 3 covers the metal layers 51 and the substrate, and metal layers 53 
equivalent to the metal layers 36a, 36b, 36c and 36d, or 43a, 43b, 43c and 
43d of FIG. 3 are formed on the insulation layer 52. Further, an LED 
emitting red light (wavelength: approx. 6,500 A to 7,000 A), e.g., a 
gallium phosphide LED 55a with Zn and O at its luminous center, and an 
LED emitting green light (wavelength: approx. 5,550 A to 5,700 A) e.g., a 
gallium phosphide LED 55b with N as its luminous center, are disposed 
opposite and in parallel to each other in offset positions on each common 
rectangular metal layer 54 formed of, e.g., silver paste which is 
connected with each metal layer 51 at one end side. The LED's 55a and 55b 
are connected at their anodes with their respective nearest metal layers 
53 by means of gold wires 56. Offset width between each facing pair of 
LED's 55a and 55b is preferably approximately 10 to 110% of the length of 
the facing side of each LED. 
More specifically, distances B and A respectively between straight lines 
passing through the respective centers of the minor and major sides of one 
metal layer 54 and straight lines passing through the respective centers 
of the minor and major sides of each adjacent metal layer 54 may be equal, 
length of e.g., 1.27 mm, and the LED's 55a and 55b may be regular 
octahedrons with a side length of 0.3 mm. Further, distance C between the 
LED's 55a and 55b on each common metal layer 54 may be reduced to, e.g., 
20 to 50 .mu.m, and the LED 55b may be shifted with an offset width D of 
30 to 330 .mu.m toward a side 58 opposite to a side 57 of the LED 55a in 
parallel with the LED 55b as a datum line. 
In the display device with the aforementioned LED arrangement, light 
emitted from the LED's 55a and 55b on each common metal layer 54 
constituting a unit light source may be less attenuated by interception or 
adsorption by walls of the LED's, and interference between LED's adjoining 
along the vertical direction and emitting different colors may be reduced, 
so that the distinction or resolution of color tone may be improved 
without deteriorating the general brightness. 
In the construction shown in FIG. 4, furthermore, the metal layers 51 and 
54 may be integrated into such a configuration as shown in FIG. 5. In this 
manner, the LED's may easily be mounted on common metal layers 54' formed 
solidly with metal layers 51'.