Printed circuit board for use with an image forming apparatus

A circuit board for mounting a CPU for controlling an image processing apparatus, comprising a multilayered circuit board for mounting the CPU, and a shield layer for shielding the multilayered circuit board without connecting the shield layer to the circuits of the board. The circuits of the multilayered circuit board are produced by a printing or an electrolytic plating method or an electroless plating method.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a multi-layer printed circuit board 
provided with a means for absorbing noise in an image forming apparatus. 
2. Description of the Related Art 
The performance of image forming apparatus, such as copiers or the like 
have recently been enhanced and they have been made more compact, 
resulting in more complicated electronic circuits used in the apparatus. 
Therefore, the apparatus employs a printed circuit board which is 
compactly composed and is multi-layered. 
In image forming apparatus, however, there are noise-generating sources 
such as plural high voltage power units, motors, plungers, relays and 
others are locate therein. Therefore, when a CPU (Central Processing Unit) 
controls a sequence of image forming, the control is disturbed by the 
noise of the noise-generating sources, resulting in possible problems. In 
particular, the influence of noise is more serious on printed circuit 
boards in a photoreceptor drum which tends to be located in the vicinity 
of a high voltage noise source. Therefore, noise-preventing items such as 
capacitors, coils and EMI filters are connected to the electronic circuit 
related to the noise source. 
As stated above, the effect of noise prevention by means such as 
capacitors, filters and others depends practically on the configuration of 
a printed circuit board and on the kinds of parts to be used. Accordingly, 
many parts are connected on a trial and error basis on each occasion, 
resulting in a cost increase. 
Further, a multi-layer printed circuit board makes it difficult to 
incorporate additional parts later, and the printed circuits mutual 
interference is not only two dimensional but three dimensional, resulting 
generally in a drastic fall in the level of noise resistance. For example, 
in the case of a normal printed circuit board having circuits on both 
sides, the distance between both circuits is about 1.6 mm, while on a 
multi-layer printed circuit board, the distance is about 50 .mu.m which is 
less than one hundredth of the normal circuit distance. Therefore, it has 
been confirmed that anti-noise property falls drastically from 5 locations 
on a double-sided printed circuit board down to "locations on a 
multi-layer printed circuit board, when they are compared in terms of the 
number of location points where an error is caused by a given prescribed 
noise signal at 150 locations. Further, in a comparison of discharge noise 
resistance, measurements of arcing in the vicinity of a printed circuit 
board have proved that a double-sided printed circuit board can withstand 
up to as much as 4 KV, while a multi-layered printed circuit board can 
withstand up to 2 KV at the most. Thus, a conventional multi-layered 
printed circuit board has many problems. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a printed circuit board for an 
image forming apparatus wherein existing problems have been solved by 
providing a shield layer on a multi-layered board to avoid errors in an 
image forming apparatus wherein a CPU is used for controlling the sequence 
of image forming, the errors being caused by noise from high voltage power 
units, motors, solenoids and others. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be learned by practice of the invention. The 
objects and advantages of the invention will be realized and obtained by 
means of the elements and combinations particularly pointed out in the 
appended claims. 
The aforesaid object of the invention is attained by a printed circuit 
board for an image forming apparatus wherein a board, having a CPU mounted 
thereon and is incorporated in a photoreceptor drum unit, is 
multi-layered, the circuit layers are formed by a printing method, and a 
layer which is not connected to any points of a circuit is formed entirely 
on aforesaid board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention will be explained as follows with an example of a printed 
circuit board used in an ordinary copying machine. 
In the example of FIG. 1, base material 1 is a paper phenol of about 1.6 mm 
in thickness or formed of glass, epoxy or the like, and thereon, the, 
first circuit layer Z is formed to be about 35 .mu.m through a printing 
process or an electrolytic plating method. On the surface of the first 
circuit layer, namely on the surface that is opposite to the contact face 
between base material 1 and first circuit layer 2, epoxy type insulating 
materials are coated through a plurality of processes to form the first 
insulation layer 3. This insulation layer 3 is about 50 .mu.m in thickness 
and has a dielectric strength of a printing process of about 1000 V. 
Further, the second circuit layer 4 is provided, by means of an 
electroless plating method, on the surface of the first insulation layer 
3, namely on the surface that is opposite to the contact face between 
first insulation layer 3 and first circuit layer 2. In addition to that, 
the second insulation layer (insulating protective layer) 5 is formed on 
the surface of second circuit layer 4, namely on the surface opposite to 
the contact face between first insulation layer 3 and second circuit layer 
4, for the purpose of protecting the circuits. The multi-layered printed 
circuit boards mentioned above are generally conventional in structure. In 
the invention, shield 6 is provided on the entire surface of base material 
1 of aforesaid multi-layered printed circuit board, namely on the surface 
opposite to the contact face between base material 1 and the first circuit 
layer. Shield 6 is formed to incorporate isolated holes 7, so that CPU 8 
can be connected by connector terminals 9. The shield 6, through a ground 
is basically an electromagnetic and electrostatic shield. Therefore, 
noises of various types generated in an image forming apparatus are 
absorbed by this shield 6, resulting in a great improvement in anti-noise 
properties. An example of anti-noise properties is shown in FIG. 2, 
wherein there is indicated the number of erroneous actions obtained, in a 
method wherein signals of DC 260 V and 50 ns width are connected through 
the coupling connection and a prescribed noise is inputted at 150 
locations that represent all input and output ports of the sequence 
control board of an image forming apparatus in which the sequences are 
controlled by a CPU. For the purpose of comparison, the results of 
measurements for three types of printed circuit boards are shown in FIG. 
2. A shows the number of points for erroneous actions for a double-sided 
printed circuit board of a conventional type, B shows those for a 
multi-layered printed circuit board of a conventional type and C shows 
that for a multi-layered printed circuit board of the invention. The 
figure shown on the top of each bar of the bar graph is the number of 
points of erroneous actions, and the smaller the figure is, the higher the 
anti-noise property is. From this, it is clear that the multi-layered 
printed circuit board of the invention is superior to conventional ones in 
anti-noise properties. 
The aforesaid description is for a 2-layer printed circuit board. The 
invention can also be applied in the same way to printed circuit boards 
having a plurality of layers. It is possible to take actions which are 
more effective for reducing the influence of noise by providing, on each 
interface, a shield suitable for the circuit as a boundary layer. 
FIG. 3 shows an example wherein shield 6 is provided between base material 
1 and the second base material 11, while FIG. 4 shows an example wherein 
shield 6 is provided between the first insulation layer 3 and the second 
insulation layer 5. FIG. 5 shows an example wherein shield 6 is provided 
on the outer surface of the second insulation layer 5, namely on the 
surface opposite to the contact face between the second insulation layer 5 
and the second circuit layer 4. When the invention is applied to a printed 
circuit board having no CPU thereon, and incorporated in a photoreceptor 
unit that is subject to the influence of high voltage noise in an image 
forming apparatus, the noise is absorbed directly by the printed circuit 
board. 
As stated above, in an image forming apparatus wherein its sequence is 
controlled by a CPU, high voltage power supplies motors, plungers and 
others have caused the CPU to generate erroneous actions, resulting in an 
occurrence of serious problems happening as a chain reaction. Owing to the 
present invention, however, it has become possible, by providing shields 
on a multi-layered printed circuit board, to eliminate the aforesaid 
problems and thereby to provide a multi-layered printed circuit board for 
an image forming apparatus which drives electronic circuits normally and 
safely. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only. Therefore, the invention in its broader 
aspects is not limited to the specific details, representative devices, 
and illustrative examples shown and described. Accordingly, departures may 
be made from such details without departing from the spirit or scope of 
the general inventive concept as defined by the appended claims and their 
equivalents.