Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-11-29
2003-05-06
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S130000
Reexamination Certificate
active
06559879
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an LED array head and a circuit board, and LED array chips that form the LED array head, whereby the LED array head is used as a light source for writing an electrostatic latent image on a photoconductive drum in an electrophotographic printer.
2. Description of the Related Art
With a conventional electrophotographic printer, a charging roller charges the surface of a photoconductive drum and an exposing unit such as an LED head writes an electrostatic latent image on the charged surface of the photoconductive drum. The LED array head emits light through a focusing rod lens array to illuminate the charged surface in accordance with print data. The electrostatic latent image is developed with toner into a toner image, which is subsequently transferred to a print medium. The toner image on the print medium is then fixed by a fixing unit.
FIG. 8
illustrates a general construction of an electrophotographic printer.
Referring to
FIG. 8
, there is provided a photoconductive drum
1
surrounded by a charging roller
2
, an LED array head
3
, a focusing rod lens
4
, a developing roller
5
, and a transfer roller
7
. Arrows indicate directions of rotation of the structural elements. The developing roller
5
applies toner to the electrostatic latent image on the photoconductive drum to develop the electrostatic latent image into a toner image. The transfer roller
7
transfers the toner image from the photoconductive drum
1
to a print medium
100
. A supply roller
6
rotates in contact with the developing roller
5
to supply toner to the developing roller
5
. A fixing device
101
fixes the toner image transferred to the print medium
100
.
The conventional LED array head
3
will be described in detail.
FIG. 9A
is a perspective view illustrating the general construction of the LED array head
3
when the driver chips
10
are arranged on one side of the row of the LED array chips
9
.
The circuit board
8
has conductive pattern i.e., wiring pattern formed thereon. Each of the LED array chips
9
has a plurality of light-emitting diodes fabricated therein. Driver chips
10
are connected to the corresponding LED array chips
9
through wires
11
, and drives the LED array chips
9
. The driver chips
10
are also connected to the circuit board
8
through wires
12
.
The driver chips
10
may be arranged on both sides of the row of the LED array chips
9
as shown in FIG.
9
B.
FIGS. 10A and 10B
illustrate the construction of the conventional LED array head
3
,
FIG. 10A
being a top view and
FIG. 10B
being a cross-sectional view taken along lines
10
B—
10
B.
FIG. 11
is a perspective view showing the structure of the conventional LED array chip
9
.
Referring to
FIGS. 10A
,
10
B, and
11
, the LED array chip
9
has light-emitting elements
13
and individual electrodes
14
a
formed thereon. The individual electrode pads
14
b
are formed on the LED array chip
9
by using the same material as the individual electrodes
14
a
. The individual electrode pads
14
b
and drive electrode pads
15
are provided for wire bonding. The LED array chip
9
has common electrode
23
formed on the underside thereof.
Each of driver chips
10
is formed with drive electrode pads
15
thereon. Signal inputting and outputting pads
16
are formed on the driver chips
10
for wire bonding. The LED array chips
9
and driver chips
10
are bonded on the circuit board
8
by an adhesive
28
that contains conductive particles therein. The circuit board is formed with conductive pattern
17
thereon.
The circuit board
8
takes the form of a glass epoxy board with copper conductive pattern
17
formed thereon. The driver chip
10
is formed on a silicon substrate, and the LED array chip
9
is formed on a compound semiconductor in which gallium arsenide phosphide is grown on a gallium arsenide substrate by epitaxy. The common electrode
23
formed on the bottom side of the LED array chip
9
is formed of gold or gold alloy. Wires
11
and
12
are gold.
The circuit board
8
carries as many LED array chips as there are driver chips
10
. The LED array chips
9
are connected to the driver chips
10
by using as many wires
11
as there are light-emitting elements. For example, if an image is to be printed with a resolution of 600 dpi on A4 size paper,
26
LED array chips
9
and
26
driver chips
10
are required to be mounted on the circuit board
8
. Each LED array chip
9
has 192 light-emitting elements aligned at intervals of 42.3 &mgr;m and 192 wires
11
are used to connect individual electrode pads on the LED array chip
9
to corresponding drive electrode pads on the driver chips
10
.
The common electrodes
23
on the LED array chips
9
are electrically connected to the conductive pattern
17
on the circuit board
8
by means of an adhesive
28
containing conductive particles. The adhesive
28
is a thermosetting epoxy resin type adhesive. Once the adhesive
28
sets, the conductive particles are sandwiched between the common electrode
23
and the conductive pattern
17
, thereby establishing electrical continuity therebetween.
The driver chips
10
are fixedly mounted on the circuit board
8
using an insulating epoxy resin type adhesive. The signal inputting and outputting pads
16
of the driver chips
10
are connected to corresponding conductive pattern (wiring pattern), not shown, on the circuit board
8
. The driver chips
10
receive electrical signal and drive the corresponding LED array chips
9
in accordance with the electrical signal, thereby selectively causing the light-emitting elements
13
to emit light.
The assembly operation of the LED array head
3
will be described.
First, the adhesive
28
is applied to the conductive pattern
17
formed on the circuit board and then the LED array chips
9
are mounted on the conductive pattern
17
using the die-bonding apparatus. Then, the insulating epoxy resin type adhesive is applied to predetermined areas of the circuit board
8
where the driver chips
10
are to be mounted, and then the driver chips
10
are mounted thereon.
The light-emitting elements are arranged at intervals of 42.3 &mgr;m for the resolution of 600 dpi. For good print results, adjacent LED array chips
9
must be positioned such that the distance L
2
between the endmost light-emitting elements of adjacent LED chips is exactly the same as the distance L
1
between adjacent light-emitting elements within the adjacent LED array chips. The distance L
1
is closely controlled to be 42.3 &mgr;m. Using the die-bonding apparatus, the LED array chips
9
are positioned with respect to the alignment patterns such that the distance L
2
is accurately 42.3 &mgr;m.
After having the LED array chips
9
and driver chips
10
bonded thereon, the circuit board
8
is placed in an oven. The circuit board is heated at 150° C. in the oven, so that the adhesive
28
sets to fix the LED array chips
9
and driver chips
10
in position. Then, the circuit board
8
is taken out of the oven and is cooled. The conductive particles contained in the adhesive
28
has a diameter of several microns. Heating the adhesive
28
causes the conductive particles to be coupled to one another, thereby making the electrical connection between the LED array chips
9
and the circuit board
8
.
After the die-bonding of the chips, individual electrode pads of the LED array chips
9
are wire-bonded to the corresponding drive electrode pads of the driver chips
10
. The signal inputting and outputting pads
16
of the driver chips
10
are wire-bonded to the conductive pattern
17
formed on the circuit board
8
. This completes the assembly of the LED array head
3
.
The thus assembled conventional LED array head
3
presents the following problem.
When the adhesive
28
is cooled down after the heating process, the dimension K of the LED array chip
9
in the direction in which the light-emitting elements are aligned becomes shorter. This is due to the fact that the adhesive
28
shrinks
Kobayashi Yu
Nakajima Norio
Shimizu Takatoku
Oki Data Corporation
Pham Hai
Rabin & Berdo P.C.
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