Flexible circuit board

A printed wiring board comprising a main printed wiring board provided with a first wiring pattern, and an auxiliary flexible printed wiring board, provided with a second wiring pattern for modifying the first wiring pattern and bonded to the main printed wiring board so as to electrically connect the second wiring pattern to a desired part of the first wiring pattern. The auxiliary board may be affixed to the main board with a pressure-sensitive adhesive or thermosetting adhesive. The auxiliary board may be provided with marks for facilitating alignment, holes to provide access to the main board, a dummy pattern for preventing tears of the flexible auxiliary board, and a second dummy pattern to prevent warping from uneven stresses applied to the flexible auxiliary board by differences in thermal expansion of the auxiliary board and the pattern printed thereon. A method of making the board is also disclosed and claimed.

BACKGROUND OF THE INVENTION 
The present invention relates to a printed wiring board, and more 
particularly to a printed wiring board comprising a main printed wiring 
board of a single-layer or of multi layers, having a wiring pattern of a 
circuit for performing a desired function and a flexible auxiliary printed 
wiring board having a wiring pattern for effectively modifying the pattern 
on the main printed wiring board, and to a method for producing a printed 
circuit board comprising the above-described printed wiring board and 
electronic component parts mounted thereon. 
A wiring pattern of a printed wiring board (hereinafter referred to as a PW 
board) is designed and formed to serve a specific function, intended use 
or the like of the PW board. It sometimes occurs that need for 
reconstruction arises because of mistake in designing or design is 
modified. To cope with such a situation, jumper wiring is adopted. 
An example of conventional jumper wiring is shown in FIGS. 1 and 2. As 
illustrated, a wiring pattern and lands 5 are formed on the bottom surface 
of a substrate 1, and leads 3' of various component parts are made to 
penetrate through through-holes of the respective lands 6 and are soldered 
by solder 4 to the wiring pattern 5 and the lands 6. Jumper wires 2 are 
soldered to connect the lands 6 which are required to be connected 
together to modify the circuit. Such direct connection by jumper wires 
between the required points, different from the connection by the wiring 
pattern on the substrate 1 is called jumper wiring. 
The above-described jumper wiring has the following problems. That is, the 
conventional jumper wiring is conducted manually, one by one, for 
individual PW boards, and for that purpose wires have to be cut to the 
suitable length and the insulator of the wires has to be removed or 
peeled. Such work is time consuming and leads to miswiring. Also, jumper 
wiring requires skill. For these reasons, productivity is lowered. 
A solution to this problem is disclosed in Japanese Utility Model 
Application Publication No. 9592/1969 published on Apr. 18, 1969. The PW 
board disclosed therein comprises a main PW board having a wiring pattern 
and through-holes for mounting component parts, and an auxiliary PW board 
having through-holes in alignment with certain of the through-holes of the 
main PW board and jumper wiring formed to connect the through-holes, and 
pins extending through both of the through-holes of the two boards to 
achieve electrical connection as well as mechanical coupling between the 
two boards. 
Details of such a conventional PW board is shown in FIG. 3(a), FIG. 3(b) 
and FIG. 3(c), of which FIG. 3(b) shows a cross section along lines 
IIIb--IIIb in FIG. 3(a), and FIG. 3(c) shows a cross section along lines 
IIIc--IIIc in FIG. 3(a). 
As illustrated, the main PW board is provided with a printed wiring pattern 
5 for providing electrical connection between a land 6 having a 
through-hole for mounting an electronic component part and another land 6 
also having a through-hole. An auxiliary PW board 8 is laid on the main PW 
board 1, and is provided with lands 6' having through-holes in alignment 
with the through-holes of the lands 6 of the main PW board 1 and a wiring 
pattern 9 formed to provide connection to serve for the same purpose as 
the jumper wires 2 of FIGS. 1 and 2. 
Pins 17 shown in FIG. 3(c) provide electrical connection and mechanical 
coupling of the auxiliary PW board 8 to the main PW board 1. After the 
auxiliary PW board 8 is laid on the main PW board so that the 
through-holes of the lands 6' of the auxiliary PW board 8 are correctly in 
alignment with the through-holes of the lands 6 of the main PW board 1, 
the pins 17 are inserted in the through-holes of both boards 1 and 8 and 
soldering is conducted to secure the boards to each other. 
With this prior art PW board, manual wiring of the jumper wire 2 of FIGS. 1 
and 2 is no longer necessary, but an auxiliary PW board 8 with the 
required wiring pattern is joined to the main PW board by means of pins 
and solder, with the result that ommission of or error in jumper wiring 
can be prevented and the work for jumper wiring is much simplified. 
But the above-described PW board has the following disadvantages. First, 
joining the auxiliary PW board to the main PW board by means of pins and 
solder requires much work and time and hence increases the manufacturing 
cost. 
Secondly, the thickness of the auxiliary PW board, which is about the same 
as that of the main PW board makes it difficult to conduct a test after 
the electronic component parts are mounted and soldered. That is the lead 
pins of the electronic parts which are made to penetrate through the 
through-holes of the two boards are not exposed on the bottom side, 
because of the thickness of the auxiliary PW board, so that a test, such 
as conduction test is difficult. 
In addition, when it is desired to alter, e.g., cut, the wiring pattern of 
the main PW board after the auxiliary PW board is joined, it is necessary 
to remove the auxiliary PW board, with the work of the removal involving 
removal of the pins and the solder. Such work is also time consuming. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a PW board having a main PW board 
and an auxiliary PW board constituted by a flexible board having a jumper 
wiring pattern, and thereby to enable improvement of the productivity of 
the PW board. 
Another object of the invention is to enable production of a simplified 
multi-layer PW board with a flexible board on which a required wiring 
pattern is formed and which is joined to the main PW board and thereby to 
restrict increase of manufacturing cost due to increase of the number of 
the layers forming the PW board and time length required for designing a 
PW board. 
To attain the above-objects, the invention provides a PW board comprising a 
main PW board having a first wiring pattern and an auxiliary, flexible PW 
board having a second wiring pattern for modifying the first wiring 
pattern, with the auxiliary PW board being bonded to the main PW board so 
as to provide electrical connection between the second wiring pattern and 
corresponding parts of the first wiring pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 4 and 5 show an embodiment of the invention. As illustrated, a wiring 
pattern 5 and lands 6 are formed on the bottom surface (lower or rear 
side) of a main PW board 7, and leads 3' of various component parts 3 
mounted on the top surface (upper or front side) are inserted or made to 
penetrate through the through-holes of the lands 6 and are soldered to the 
wiring pattern 5 or the lands 6 by solder 4. An auxiliary PW board 8 is 
mounted or fixed to the desired portion of the main PW board 7, e.g., the 
portion where jumper wiring is to be made. 
The auxiliary PW board 8 is formed of a flexible board or sheet made of a 
material such as polyimid, polyester or the like and, as better 
illustrated in FIGS. 6 and 7, the auxiliary PW board 8 is provided with a 
wiring pattern 9 and lands 10 to provide the same electrical connection as 
the jumper wires 2 shown in FIGS. 1 and 2. A through-hole 11 is formed in 
each land 10. Moreover, a two-sided pressure-sensitive (tacky adhesive) 
sheet 15, having an adhesive character on both sides or surfaces, is 
attached to one side of the auxiliary PW board 8, with the lower side of 
the sheet 15 being bonded to the upper side of the auxiliary PW board 8. 
The auxiliary PW board 8 is produced in the following manner. First, a 
desired pattern of wiring 9 and lands 10 is formed on a flexible base 
material or substrate by means of known techniques such as etching. An 
adhesive sheet 15 is then attached to the flexible substrate 8. Finally, 
through-holes 11 are formed at the centers of the lands to penetrate 
through the flexible substrate 8 and the adhesive sheet 15 to produce a 
flexible PW board 8. 
The mounting of the auxiliary PW board 8 onto the main PW board 7 is 
achieved by bringing the auxiliary PW board 8 into engagement with the 
main PW board 7 in such a manner that the adhesive sheet 15 is interposed 
between the two boards 7 and 8, and that the through-holes 11 of the 
auxiliary PW board 8 are in alignment with the corresponding through-holes 
of the lands 6 of the main PW board 7 and applying a pressure on the 
auxiliary PW board 8 thereby bonding the auxiliary PW board 8 to the main 
PW board 7. 
The alignment is facilitated if one or more positioning marks 18 as shown 
in FIGS. 9(a), 9(b) and 9(c) are provided on the auxiliary PW board 8. In 
the example of FIG. 9(a), positioning patterns 18 in the form of a cross 
are formed on the auxiliary PW board 8 at positions corresponding to two 
selected lands 6' selected from the lands 6 in that region of the main PW 
board 7 which is covered by the auxiliary PW board 8. A perforation 19 is 
provided at the center of each cross pattern 18 to enable visual 
observation or confirmation of the land 6' through the perforation 19. 
Correct positioning is ensured by aligning the perforations 19 at the 
centers of the cross patterns 18 with the corresponding lands 6' when 
bonding the auxiliary PW board 8 to the main PW board 7. 
In the example of FIG. 9(b), rectangular patterns 18 are formed in place of 
the cross patterns. 
The example of FIG. 9(c) is usable where the auxiliary PW board 8 has a 
sufficient degree of transparency to permit the lands 6 or the 
through-holes in the lands, or the patterns 5 to be observed through the 
auxiliary PW board 8. In this example, the perforations 19 are not 
provided but only the cross patterns 18 are formed. During the bonding, 
the centers of the cross patterns 18 are aligned with the corresponding 
lands 6'. The positioning patterns may alternatively be superimposed on 
and aligned with part of the wiring patterns 5 formed on the main PW 
board. 
In each of the examples shown in FIGS. 9(a), 9(b) and 9(c), the positioning 
patterns 18 may be formed of the same material (e.g., copper foil) as the 
jumper wiring pattern 9, or of a paint. Choosing the same material as the 
jumper wiring pattern 9 is advantageous in that the positioning patterns 
18 can be formed simultaneously with the wiring pattern 9, and no 
additional production step is required. 
The use of the adhesive sheet 15 for bonding the auxiliary PW board 8 to 
the main PW board 7 is advantageous in that the auxiliary PW board 8 can 
be readily peeled or removed when need for removal arises, such as when 
the auxiliary PW board 8 is bonded at an incorrect position or when it is 
necessary to alter the jumper wiring formed on the auxiliary PW board 8. 
Electrical connection between the auxiliary PW board 8 and the main PW 
board 7 is accomplished by having, after the auxiliary PW board 8 is 
bonded to the main PW board 7, the leads 3' of the electronic component 
parts 3 penetrate through the through-holes 11 of the lands 10 of the 
auxiliary PW board 8, as well as the through-holes of the main PW board 7, 
as shown in FIG. 8, and bringing that side of the auxiliary PW board 8 
which is opposite to that bonded to the main PW board 7 into contact with 
molten solder to effect soldering. If the through-holes 11 of the 
auxiliary PW board 8 are made to be a little larger than the through-holes 
of the lands 6, the molten solder will flow through the through-holes 11 
and reach the lands 6 when the auxiliary PW board 8 bonded to the main PW 
board 7 is brought into contact with the molten solder. 
Mounting the electronic parts 3 can be conducted by the use of an automatic 
part inserting device (an IC insertor) since the auxiliary PW board 8 is 
formed of a flexible substrate and is hence thin, and accordingly the 
added or bonded auxiliary PW board 8 does not produce any considerable 
adverse effect on the use of the automatic part inserting device. 
Soldering can also be conducted by means of automatic soldering provided 
that the flexible substrate 8 and the adhesive sheet 15 are made of heat 
resistant materials. 
As an alternative to the pressure sensitive adhesive sheet 15, an adhesive 
material such as a thermo-setting synthetic resin may be interposed betwen 
the auxiliary PW board 8 and the main PW board 7, and a heat press may be 
applied on that side of the auxiliary PW board 8 which is opposite to that 
facing the main PW board 7 to effect adhesion between the two boards 7 and 
8. In this case, the auxiliary PW board 8 is produced in the following 
manner. First, a desired pattern of wiring 9 and lands 10 are fomed on a 
flexible substrate by means of etching and the like. Next, an adhesive 
material 15 such as a thermosetting resin is applied on the flexible 
substrate 8. Finally, through-holes 11 are formed at the centers of the 
lands 10 to penetrate through the flexible substrate 8 and the adhesive 
material layer to produce a flexible PW board 8. 
It has been described that the electronic component parts 3 are mounted 
after the auxiliary PW board 8 is bonded to the main PW board 7. But the 
auxiliary PW board 8 may be bonded to the main PW board 7 after the 
electronic component parts 3 are mounted and soldered to the main PW board 
7. More praticularly, the leads 3' may first be inserted in and soldered 
to the lands 6 of the main PW board, and the auxiliary PW board 8 is 
thereafter superimposed on the main PW board 7 in such a manner that the 
leads 3' are inserted in the lands 10, and the leads 3' an soldered to the 
lands 10. The through-holes 11 of the auxiliary PW board should preferably 
be a little larger to avoid the fillet 16 (indicated by a broken line in 
FIG. 8) of the solder produced as a result of soldering of the leads 3' to 
the main PW board 7. 
The lands 10 of the auxiliary PW board may be so formed as to have exposed 
portions on both sides, and the wall of the through-holes 11 may also be 
copper clad. This will facilitate flow of the solder and ensure firm 
electrical connection and fixing of the auxiliary PW board 7. 
In the above-described cases, the auxiliary PW board 8 is fixed to the main 
PW board 7 by means of soldering. The use of an adhesive for bonding can 
therefore be omitted. But if the two boards 7 and 8 are bonded by an 
adhesive in advance, the soldering is facilitated, so that use of an 
adhesive for bonding is preferrable. 
Where the electronic parts 3 are mounted to the main PW board 7 by the use 
of an automatic part inserting device the leads 3' are generally a little 
bent in the predetermined directions to prevent coming-off of the leads 
3'. If the through-holes 11 are circular, mounting the auxiliary PW board 
8 to the main PW board 7 having the electronic parts 3 already mounted 
thereto may be difficult because of the bent leads 3'. In such a case, it 
is preferrable that the through-holes 11 be elongated in the direction of 
the bent to facilitate the mounting of the auxiliary PW board 8. 
FIGS. 10 and 11 show another embodiment of the invention, which is 
generally identical to the embodiment of FIGS. 4 and 5. The difference is 
that the auxiliary PW board 8 is provided with perforations 12 at various 
positions corresponding to the positions at which the wiring pattern 5 is 
cut and/or at positions at which there is a certain degree of possibility 
that such cutting becomes necessary because of design modification or the 
like. 
The perforations 12 enables visual observation of the wiring pattern 5 to 
ascertain whether the wiring pattern is actually cut, and thereby enabling 
easy and quick inspection. Moreover, when the need arises for cutting of 
that portion of the wiring pattern 5 which lies under the auxiliary PW 
board 8, a cutter may be inserted through the perforation 12 and the 
wiring pattern 5 may be cut. This obviates the necessity of removing the 
auxiliary PW board 8 for the cutting. 
Instead of providing perforations 12, the auxiliary PW board 8 may be 
formed of a material having a certain degree of transparency (i.e., 
perfectly transparent or semi-transparent) to enable visual observation of 
the wiring pattern 5 on the main PW board 7. In this case, by forming the 
auxiliary PW board 8 of a material which is easy to cut, the wiring 
pattern 5 as well as that part of the auxiliary PW board 8 covering the 
wiring pattern 5 can be cut and removed. In this case too, the auxiliary 
PW board 8 need not be peeled for the cutting. 
Where a wiring pattern or lands are formed on that side of the auxiliary PW 
board 8 which faces tha main PW board 7, insulating means may be employed, 
e.g., an insulating layer may be formed to cover such wiring pattern or 
lands to prevent their contact with the wiring pattern on the main PW 
board 7. 
Where the wiring patterns are formed on both sides of the auxiliary PW 
board 8, the wiring density (amount of wiring per unit area) of the 
auxiliary PW board 8 is increased. Moreover, when a copper foil or layer 
is formed on the wall surfaces of the through-holes 11 of the auxiliary PW 
board 8, the flow of solder through the through-holes 11 is facilitated 
and it will be less likely that any failure should occur in electrical 
connection between the lands 6 and 10. 
Since the auxiliary PW board 8 is formed of a thin, flexible PW board, the 
work for mounting the auxiliary PW board 8 to the main PW board 7 is much 
improved since the auxiliary PW board 8 can for example be easily stuck. 
Moreover, the length of the projection of the bottom surface of the 
auxiliary PW board 8 from the bottom surface of the main PW board 7 is 
reduced, so that soldering, circuit testing conducted by having a 
contactor in engagement with the lands, and the like are facilitated. 
Furthermore, the cutting of the wiring pattern 5 is further facilitated. 
A problem associated with the use of a flexible substrate for the auxiliary 
PW board 8 is that it is relatively easy for the auxiliary PW board to 
tear or break, particularly at a position where the edge is inwardly 
curved. To solve this problem, a reinforcement dummy pattern 20 may be 
formed to extend along the edge, or periphery as shown in FIG. 12(a), and 
FIG. 12(b). In the example of FIG. 12(a), the dummy pattern 20 is formed 
to cover only the inwardly curved edge portions where a tear is most 
likely to occur. In the example of FIG. 12(b), the dummy pattern 20 is 
formed to extend all along the entire periphery. By providing the dummy 
pattern 20, any tear developing from the edge of the auxiliary PW board is 
prevented from further developing or penetrating into the area inside of 
the dummy pattern 20. In either case, the dummy pattern 20 is formed of 
the same material as the wiring pattern 9 to facilitate the production of 
the auxiliary PW board. 
Another problem associated with the use of a flexible PW board 8, is that 
warp or twist may occur when the wiring pattern 9 is not uniformly 
distributed over the surface of the PW board 8. This is due to the 
difference in thermal expansion coefficient between the conductor (e.g., 
copper) forming the wiring pattern and the substrate material (e.g., 
polyimid) of the auxiliary PW board 8, when warp or twist occurs in the 
auxiliary PW board 8, intimate contact between the main and the auxiliary 
PW boards 7 and 8 cannot be achieved, which lowers the efficiency of work 
on the boards and may cause failures or errors in the wiring. To solve 
this problem, an additional dummy pattern 21 for preventing warp of twist 
may be formed to cover the entire blank area where the wiring pattern 9 
required to provide the electrical circuit function is not formed. The 
dummy pattern 21 is not electrically connected to the wiring pattern 9, 
and is formed of the same material as the wiring pattern 9. The provision 
of the dummy pattern 21 will result in a better uniformity as regards the 
thermal expansion coefficient throughout the entire surface of the board 
8, and prevent warp and twist. The dummy pattern 21, which in the 
embodiment described above, is formed to cover the entire blank area where 
the wiring pattern 9 is not formed, may alternatively be formed to cover 
part only of the blank area where the wiring pattern 9 is not formed, in 
such a manner that the aggregation of the wiring pattern 9 and the dummy 
pattern 21 is distributed evenly throughout the auxiliary PW board 8. 
The various problems discussed above are encountered wherever a flexible 
board is used so that their solution can be applied to any electrical 
circuit device with a flexible board. 
The application of the auxiliary PW board is not limited to a situation 
when the wiring pattern formed on the auxiliary PW board is a jumper 
wiring pattern. For instance, where it is not necessary to add a whole 
wiring layer but is necessary to add part only of a wiring layer to meet 
the demand for increase in the amount of wiring, an auxiliary PW board as 
described above may be mounted to the main PW board. The cost for adding a 
whole wiring layer is about the same as the cost of a single-layer PW 
board. Substituting an auxiliary PW board as described above will 
substantially reduce the cost. 
As has been described, according to the invention, a wiring pattern such as 
a jumper wiring pattern is formed on an auxiliary PW board constituted by 
a flexible substrate, and the auxiliary PW board is mounted to the main PW 
board by means of adhesive or soldering. The efficiency of the work for 
wiring, such as jumper wiring, is therefore much improved. Moreover, 
because the auxiliary PW board is thin, its projection from the bottom 
surface of the main PW board is small, so that automatic processing made 
on the bottom side of the PW board such as soldering and testing is in no 
way obstructed. In addition, it is ensured that the ends of the leads of 
the electronic component parts which penetrate through the auxiliary PW 
board are exposed on the bottom side of the auxiliary PW board, so that 
circuit tests which are conducted by having a contactor contacted with the 
lands are facilitated. Furthermore, by providing that the part or parts of 
the wiring pattern on the main PW board be visible through the auxiliary 
PW board after its mounting, confirmation of the state of the wiring 
pattern on the main PW board and its cutting are facilitated.