JP2011160005A - Partially-multilayered flexible printed wiring board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a partially-multilayered flexible printed wiring board (partial multilayer FPC) in which only necessary parts are multilayered and on which high density surface mounting is possible. <P>SOLUTION: In the partially multilayered flexible printed wiring board, a first double-sided FPC 52 has a double-sided circuit 53 at one end, and a double-sided circuit 55 at the other end and the center is a signal line 54 being a single-sided circuit. A second double-sided FPC 60 is layered on the double-sided circuit 53 at one end by interposing a bonding sheet 2. Inner layer side via holes 16, 24 are formed in the double-sided circuit 53 of the first double-sided FPC 52 and the second double-sided FPC 60 layered by interposing the bonding sheet 2, respectively, thereby having a structure capable of preventing a surface mounting region on the outer layer side from being narrowed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a multilayer flexible printed wiring board used for a mobile phone or a small portable terminal.

  A flexible printed wiring board (hereinafter referred to as “FPC: Flexible Printed Circuit” in this section) is used in many electronic devices as a thin wiring board that can be bent. In particular, as electronic devices represented by mobile phones, small portable terminals, digital cameras, and the like have become smaller and higher in performance, the FPCs that are mounted are also required to have finer wiring, higher density, and lower thickness. There is a growing demand for aging. Therefore, recently, there is an increasing demand for a multi-layer FPC configured by overlapping a double-sided FPC and a double-sided FPC, a single-sided FPC and a single-sided FPC, or a combination of a single-sided FPC and a double-sided FPC.

  Patent Document 1 proposes a configuration in which a printed wiring board is laminated and bonded by a prepreg (sheet material) when forming a multilayer printed wiring board. In Patent Document 1, a through-hole is formed in a prepreg in advance and a conductive paste is filled, so that a printed wiring board is stacked with the prepreg interposed therebetween, whereby the inner via hole ( A technique capable of forming IVH (Inner Via Hole) has been proposed.

Patent Document 2 discloses an adhesive sheet that can be used to stack FPCs. This adhesive sheet comprises a base material which is a woven fabric or a non-woven fabric and a resin composition, and is an FPC.
It has a configuration suitable for stacking FPCs between them.
Patent Document 3 discloses a resin adhesive sheet used for stacking FPCs, and shows a structure in which a through hole is formed in the adhesive sheet and a conductive paste is filled in the through hole. Yes.

Patent Document 4 discloses a bonding sheet used for the configuration of a multilayer FPC. The bonding sheet disclosed in Patent Document 4 is a multilayer FPC bonding sheet comprising a prepreg in which a woven fabric substrate or a nonwoven fabric substrate is impregnated with an epoxy resin composition.
It has the effect of increasing the rigidity of C.
Patent Document 5 proposes providing a hollow portion in a part of an adhesive sheet (bonding sheet) for connecting an FPC in a multilayer FPC.

WO2007 / 46459 Japanese Patent No. 4237726 JP 2005-347414 A JP 2006-299209 A JP 2006-179679 A

A multilayer flexible printed wiring board (hereinafter referred to as “multilayer FPC” in this section)
As described in the background art, the demand for higher density and thinner thickness must be satisfied. To that end, when a connection sheet disclosed in Patent Document 1 or Patent Document 3, that is, a connection sheet having a through hole and filled with a conductive paste is used, the laminated multilayer F is used.
There is an advantage that the thickness of the PC can be reduced and the electrical connection between the wiring layers on both sides of the connection sheet can be satisfactorily achieved.

However, in the prior art, it cannot be said that the density of the multilayer FPC is sufficiently increased, and there is room for improvement. More specifically, since a multilayer FPC has electronic components and electronic elements mounted on a wiring layer exposed on the surface thereof, if the surface wiring layer is not formed as a fine circuit, a high density of electronic components, etc. There is a problem that mounting becomes difficult.
However, in the conventional technology, the surface wiring layer is formed with blind via holes (BVH: Blind Via Hole) and through via holes (TVH: Through Via Hole) from the surface side, and the surface wiring layer is formed together with the inner peripheral surface of BVH and TVH. Since plating is performed, the surface wiring layer becomes thick, and there is a problem that miniaturization of the surface wiring layer is hindered. In addition, since BVH and TVH are on the surface layer, that is, on a component mounting surface such as an electronic component, the component mounting area is narrowed, which hinders high-density mounting. Therefore, an improved multilayer FPC capable of high-density mounting has been desired.

  The present invention has been made based on such a background, and a main object thereof is to provide a partial multilayer FPC suitable for high-density mounting.

The first aspect of the invention is the first double-sided flexible printed wiring board (3) laminated on one side of the bonding sheet and the second laminated on the other side of the bonding sheet with the bonding sheet (2) interposed therebetween. A multilayer flexible printed wiring board including the double-sided flexible printed wiring board (4), wherein the bonding sheet (2) has a hole (7, penetrating from one side of the bonding sheet to the other side at a predetermined position). 8, 9, 10) and the hole is filled with a conductive paste (11), and the first double-sided flexible printed wiring board (3) and the second double-sided flexible print are filled with the conductive paste (11). The wiring board (4) is electrically connected, and the first double-sided flexible printed wiring board (3) and the second double-sided board are connected. Carboxymethyl Bull printed circuit board (4) are both insulating film and (13, 21), on both sides provided with an inner surface wiring layer of the insulating film (L2, L3)
And external wiring layers (L1, L4), and the external wiring layers (L1, L4) on the opposite side to the surface in contact with the bonding sheet (2) constitute wiring layers on which electronic elements can be mounted. The inner surface wiring layers (L2, L3) that are in contact with the bonding sheet (2) have openings at predetermined positions, communicate with the openings, pass through the insulating film, and face the outer surface wiring layers. , Inner layer side via holes (16, 24) for electrically connecting the inner wiring layer and the outer wiring layer.
) And a laminated region (53) including the bonding sheet (2), the first double-sided flexible printed wiring board (3) and the second double-sided flexible printed wiring board (4), and the bonding sheet (2) and the first double-sided flexible printed wiring board (4) or the first double-sided flexible printed wiring board (4) or the first double-sided flexible printed wiring board (4) or the second double-sided flexible printed wiring board (4) are not present. It is a partial multilayer flexible printed wiring board characterized by including the non-lamination area | region (54, 55) to which only the flexible printed wiring board (3) was extended.

In addition, the reference code | symbol attached | subjected to the component in the bracket | parenthesis shows the code | symbol in embodiment mentioned later. The same applies hereinafter.
The invention according to claim 2 is a partial multilayer flexible printed wiring board in which only necessary portions are laminated in a multilayer, and the first wiring layer (31) provided on both surfaces of one end side (53) of the insulating base film (13). L1) and a second wiring layer (L2) and a first double-sided flexible printed wiring board (52) having a first wiring layer (L1) and a second wiring layer (L2) provided on both sides of the other end side (55) The second wiring layer (L2) on the one end side (53) of the first double-sided flexible printed wiring board (52) has an inner layer side via hole (16) formed at a predetermined location, The first double-sided flexible printed wiring board (52) further includes a second double-sided flexible printed wiring board (60) laminated only on the second wiring layer (L2) on the one end side (53) of the first double-sided flexible printed wiring board (52). The second double-sided flexible printed wiring board (60) has a third wiring layer (L3) and a fourth wiring layer (L4) formed on both sides of the insulating base film (13), and the third wiring layer. An inner layer side via hole (24) is formed on the (L3) side, the third wiring layer (L3) and the fourth wiring layer (L4) are electrically connected, and the second double-sided flexible wiring board ( 60) and the second wiring layer (L2) on the one end side (53) of the first double-sided flexible wiring board (52), and a through-hole filled with a conductive paste (11) at a predetermined position Part having a hole and having a bonding sheet (2) for electrically connecting the second wiring layer (L2) and the third wiring layer (L3) with the conductive paste (11) Multilayer flexible printed wiring board A.

According to the present invention, a multilayer flexible printed wiring board having a partial multilayer structure can be provided as necessary, and it is incorporated well in a small space without taking unnecessary space in a built-in device. The multilayer flexible printed wiring board can be made.

FIG. 1 is a schematic cross-sectional view showing a configuration of a multilayer flexible printed wiring board according to a reference embodiment of the present invention (hereinafter, “flexible printed wiring board” is referred to as FPC in this section) divided into three block layers. It is. FIG. 2 is a schematic cross-sectional view of a multilayer FPC 1 according to a reference embodiment in which a first double-sided FPC 3 and a second double-sided FPC 4 are brought into contact with both surfaces of the bonding sheet 2 and heated and pressed to form a laminated structure. FIG. 3 is a schematic cross-sectional view of the finished multilayer FPC 1. 4A is a schematic cross-sectional view showing a configuration of a conventional multilayer FPC manufactured by a conventional process, and FIG. 4B is an illustration of a multilayer FPC 1 according to a reference embodiment of the present invention manufactured by the process of the present invention. FIG. FIG. 5A shows a manufacturing process of a conventional multilayer FPC, and FIG. 5B is a diagram showing a manufacturing process of the multilayer FPC 1 according to the reference embodiment of the present invention. FIG. 6 is a schematic sectional view showing the structure of the multilayer FPC 51 according to the embodiment of the present invention. FIG. 7 is a schematic cross-sectional view showing a configuration of a multilayer FPC 71 according to still another reference embodiment of the present invention. FIG. 8 is an illustrative view for explaining the difference in length between the outer FPC and the inner FPC during bending. FIG. 9 is a schematic cross-sectional view showing a configuration of a multilayer FPC 91 according to still another reference embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is an illustrative sectional view showing a configuration of a multilayer flexible printed wiring board (hereinafter referred to as “multilayer FPC” in this section) according to a reference embodiment of the present invention divided into three block layers as constituent elements. It is.
The multilayer FPC 1 according to this embodiment includes a bonding sheet 2 and a first double-sided FPC 3 stacked on one side of the bonding sheet 2 as a constituent block layer (component).
And a second double-sided FPC 4 stacked on the other side.

Bonding sheet 2 is a prepreg (epoxy sheet material impregnated with glass fibers), a core film (polyimide film, etc.) coated with an epoxy adhesive on both sides, or a thermosetting resin without a core. It is a sheet material made of a film.
Thus, by using the bonding sheet 2 as an insulating layer / adhesive layer, the multilayer FPC 1
Can be made thinner. Incidentally, in this embodiment, the bonding sheet 2 is
Its thickness is about 100 to 150 μm.

In the bonding sheet 2, holes 7, 8, 9, and 10 that penetrate from the one surface side 5 of the bonding sheet 2 to the other surface side 6 are formed at predetermined positions. Each of the holes 7 to 10 is filled with a conductive paste 11.
As the conductive paste, a conductive filler (silver, copper, silver-coated copper powder, etc.) made of an epoxy resin, for example, a conductive paste described in Japanese Patent No. 4109156 can be used.
Further, a conductive paste made of an epoxy resin using a low melting point metal and a high melting point metal as the conductor filler, for example, a conductive paste described in Japanese Patent No. 4191678 can be suitably used.

The holes 7 to 10 formed in the bonding sheet 2 and the conductive paste filled in the holes 7 to 10 function as via holes, as will be described later, and the inner layer circuit (second wiring layer L2) of the first double-sided FPC 3 and The inner layer circuit (third wiring layer L3) of the second double-sided FPC 4 is electrically connected.
In this embodiment, the bonding sheet 2 has a bonding sheet absence region 12 in the center in the left-right direction, and the region 12 where the bonding sheet 2 does not exist
As will be described later, the first double-sided FPC 3 and the second double-sided FPC 4 form a hollow region facing each other with a space therebetween.

In the first double-sided FPC 3, for example, a copper clad laminate (CCL: Copper Clad Laminated) in which copper foil is bonded to both sides of a polyimide insulating base film 13 is processed by a subtractive method, and the first wiring layer L1 is formed on the outer surface thereof. Those formed and having the second wiring layer L2 formed on the inner surface thereof can be used.
Alternatively, the first wiring layer L1 and the second wiring layer may be formed by a semi-additive method in which a seed layer having a thickness of about several thousand Å is formed on both surfaces of the insulating base film 13 and a wiring layer pattern is formed by electrolytic copper plating based on the seed layer. A layer in which the wiring layer L2 is formed can also be used.

A structural feature of the first double-sided FPC 3 is that an inner layer side via hole 16 is formed from the second wiring layer L2 that is an inner surface wiring layer toward the first wiring layer L1 that is an outer surface wiring layer. The first wiring layer L1 (outer wiring layer) and the second wiring layer L2 of the first double-sided FPC 3
(Inner wiring layer) is electrically connected.
More specifically, the inner layer side via hole 16 is formed from the second wiring layer L2 side which is the inner surface wiring layer, and is not a through via hole (TVH), but the first wiring layer L1 (outer surface wiring layer). It is not penetrating. The inner layer side via hole 16 is, for example, the second via hole.
In the wiring layer L2 (inner surface wiring layer), after removing the copper foil in the opening forming the opening by etching, only the insulating film 13 can be formed by a conformal method of opening with a laser.

In the first double-sided FPC 3, the inner layer side via hole 16 is formed from the second wiring layer L2 side which is the inner surface wiring layer, and the inner layer side via hole 16 does not penetrate the first wiring layer L1 which is the outer surface wiring layer. Accordingly, when the inner peripheral surface of the inner layer side via hole 16 is plated and the inner layer side via hole 16 is used as an interlayer conduction hole, the plating does not adhere to the first wiring layer L1 which is the outer surface wiring layer (the first wiring layer L1 is The first wiring layer L is finely processed in such a way that it is not plated)
1 can be thickened by plating so that fine circuit formation is not hindered.

Further, since the inner layer side via hole 16 does not appear in the first wiring layer L1, the inner layer side via hole 1
6 is advantageous in that the component mounting area of the first wiring layer L1 is not narrowed and high-density mounting is not hindered.
Thus, the first wiring layer L1 that is the outer wiring layer from the second wiring layer L2 side that is the inner wiring layer.
By forming the non-penetrating inner layer side via hole 16, the plating applied to the inner peripheral surface of the inner layer side via hole 16 does not affect the first wiring layer L1 that is the outer surface wiring layer at all. Since the component mounting area of the wiring layer L1 is not narrowed, the finely processed first wiring layer L
1, high-density mounting of electronic elements and electronic components can be performed satisfactorily.

The first double-sided FPC 3 has a second wiring layer L2 as an inner wiring layer on the inner surface facing the bonding sheet 2, and a first wiring layer L1 as an outer wiring layer on the corresponding outer surface. However, in the region facing the bonding sheet absence region 12, the inner surface 131 of the insulating film 13 faces the bonding sheet absence region 12, and the second wiring layer L
2 is not provided. A first wiring layer 14L as a signal line is provided on the outer surface side of the insulating film 13 in this region.

The outer surface of the signal line 14L is covered with an adhesive 19 and a cover layer 17 made of, for example, a polyimide film.
The configuration of the second double-sided FPC 4 is basically the same as the configuration of the first double-sided FPC 3 and is arranged symmetrically with the first double-sided FPC 3 with the bonding sheet 2 interposed therebetween. In the second double-sided FPC, 21 is an insulating film, L3 is a third wiring layer as an inner wiring layer, L4 is a fourth wiring layer as an outer wiring layer, 24 is an inner layer side via hole formed from the third wiring layer L3 side, 22L is a signal line in the fourth wiring layer L4, 20 is an adhesive, and 25 is a cover layer, which are respectively the insulating film 13, the second wiring layer L2, the first wiring layer L1 of the first double-sided FPC3.
, Corresponding to the inner layer side via hole 16, the signal line layer 14L, the adhesive 19 and the cover layer 17, and having the same configuration, a duplicate description is omitted.

Also in the second double-sided FPC 4, when plating is performed on the inner peripheral surface of the inner layer side via hole 24, the plating does not reach the fourth wiring layer L 4 that is the outer wiring layer. Therefore, the fourth wiring layer L4
It is possible to form a fine circuit. Further, since the inner layer side via hole 24 does not penetrate the fourth wiring layer L4, the inner layer side via hole 24 does not narrow the component mounting region in the fourth wiring layer L4.

FIG. 2 is a schematic view of a multilayer FPC 1 according to this embodiment in which a first double-sided FPC 3 and a second double-sided FPC 4 are brought into contact with both surfaces of the bonding sheet 2 described in FIG. It is sectional drawing. The bonding sheet 2, the first double-sided FPC 3, and the second double-sided FPC 4 are stacked by a batch stacking process, that is, a process of simultaneously heating and pressurizing the first double-sided FPC 3, the bonding sheet 2, and the second double-sided FPC 4 simultaneously.

As an example, the heating temperature for lamination is about 170 ° C., the pressure of pressurization is about 3 MPa,
The processing time is about 30 minutes.
By laminating the first double-sided FPC 3 and the second double-sided FPC 4 together with heating and pressurization with the bonding sheet 2 in between, each hole 7-10 of the bonding sheet 2 is obtained.
The conductive paste 11 filled in is electrically connected to the circuit of the second wiring layer L2 of the first double-sided FPC3 and is also electrically connected to the circuit of the third wiring layer L3 of the second double-sided FPC4. That is, through the lamination process, the holes 7 to 10 and the conductive paste 11 function as via holes, and the second wiring layer L2 that is the inner surface wiring layer of the first double-sided FPC 3 and the third wiring layer that is the inner surface wiring layer of the second double-sided FPC 4 An electrical connection with L3 is achieved.

Further, the resin component impregnated in the bonding sheet 2 is once dissolved and re-solidified by the laminating process by heating and pressurization, so that the first double-sided FPC 3 and the second double-sided FPC 4 are bonded by the bonding sheet 2. At that time, the second wiring layer L2 of the first double-sided FPC 3 and the third wiring layer L3 of the second double-sided FPC 4 are laminated so as to be buried inward from the surface of the bonding sheet 2. That is, the layer thicknesses of the second wiring layer L2 and the third wiring layer L3 are absorbed by being embedded in the bonding sheet 2, and the interlayer thickness of the first double-sided FPC3 and the second double-sided FPC4 of the multilayer FPC1 is approximately the thickness of the bonding sheet 2. Is equal to Therefore, it can be set as the multilayer FPC1 in which thickness reduction was achieved.

FIG. 3 is a schematic cross-sectional view of a finished product that has been subjected to a lower process and an inspection process after the outer surface of the multilayer FPC 1 shown in FIG. 2 is covered and surface-treated. As shown in FIG. 3, in the finished product of the multilayer FPC 1, a cover coat 27 is applied to the first wiring layer L 1 exposed on the upper surface except for the component mounting region, and the first wiring layer L 1 is applied by the cover coat 27. Is protected. Similarly, the fourth wiring layer L4 is covered and protected by the cover coat 28 except for the component mounting region.

4A and 4B show a comparative view of the cross-sectional structure of the conventional multilayer FPC and the cross-sectional structure of the multilayer FPC 1 according to the reference embodiment of the present invention.
4A and 4B, in the conventional structure shown in FIG. 4A, the surface of the second wiring layer L2 is covered with the polyimide coverlay film 31 bonded with the adhesive 36, and the third wiring layer. The surface of L3 is also covered with a cover lay film 32 adhered with an adhesive 38, and is connected by a bonding sheet 33 via cover lay films 31 and 32.

For this reason, the second wiring layer L2 and the third wiring layer L3 are not buried in the bonding sheet 33, and the interval between the second wiring layer L2 and the third wiring layer L3 is the cover lay film 31,
This is the sum of the thicknesses of the bonding sheet 33 and the coverlay film 32.
Further, the electrical connection between the first wiring layer L1 and the second wiring layer L2 is realized by a via hole from the outer layer to the inner layer, so-called blind via hole (BVH) 34. The electrical connection between the wiring layer L3 and the fourth wiring layer L4 is also achieved by the BVH 34. For this reason, in the first wiring layer L1 and the fourth wiring layer L4, the region where the BVH 34 is formed cannot be used as a component mounting region. Therefore, the component mounting regions in the first wiring layer L1 and the fourth wiring layer L4 are It is narrowed.

In addition, since the first wiring layer L1, the second wiring layer L2, the third wiring layer L3, and the fourth wiring layer L4 are electrically connected by a through via hole (TVH: Through Via Hole) 35, the presence of the TVH 35 exists. It can also be seen that the component mounting areas in the first wiring layer L1 and the fourth wiring layer L4 are narrowed.
On the other hand, in the multilayer FPC 1 according to the reference embodiment of the present invention shown in FIG. 4B, the second wiring layer L2 and the third wiring layer L3 are directly connected to the bonding sheet 2 and buried in the bonding sheet 2. Thus, it can be seen that the distance between the first double-sided FPC 3 and the second double-sided FPC 4 is substantially the thickness of the bonding sheet 2 and the thickness is reduced. Further, the second wiring layer L2 and the third wiring layer L3 are electrically connected by the conductive paste 11 filled in a hole formed in advance in the bonding sheet 2, and it is not necessary to form a through via hole (TVH). I understand that.

Further, since the inner layer side via holes 16 and 24 are formed from the inner surface wiring layers L2 and L3 for the first double-sided FPC3 and the second double-sided FPC4, the first wiring layer L1 and the fourth wiring layer L4.
The component mounting area is not narrowed, high-density mounting is possible, and the first wiring layer L
It can be seen that the miniaturized patterns of the first and fourth wiring layers L4 can be effectively used as they are.
5A and 5B are process comparison diagrams showing the manufacturing steps of the conventional multilayer FPC shown in FIGS. 4A and 4B and the multilayer FPC 1 according to the reference embodiment of the present invention in comparison.

The manufacturing process of the conventional multilayer FPC shown in FIG. 5A represents that the multilayer FPC is manufactured through 10 processes. On the other hand, the manufacturing process of the multilayer FPC 1 according to the reference embodiment of the present invention shown in FIG. 5B represents that the multilayer FPC 1 is manufactured through eight processes.
That is, the multilayer FPC 1 according to the reference embodiment of the present invention can be manufactured by two steps fewer processes than the conventional multilayer FPC, and the manufacturing time can be shortened.

More specifically, referring to FIG. 5A, in the conventional multilayer FPC, the first double-sided F
A PC board (flexible copper-clad laminate) and a second double-sided FPC board are prepared (step 1), each inner layer circuit is formed on these double-sided FPC boards (step 2), and the inner layer is covered with an inner layer cover layer (step 3). . At the same time, a bonding sheet is prepared (step 3).
Then, a first double-sided FPC (in this specification, a circuit formed on an FPC board is called an FPC) and a second double-sided FPC are stacked with a bonding sheet interposed therebetween (step 4). Subsequently, a blind via hole (BVH) is formed by laser processing from the outer layer side (step 5).
A through via hole (TVH) is formed by a drilling process (step 6). Then, copper plating is performed on the inner peripheral surface of each via hole (step 7), an outer layer circuit is formed (step 8), a cover coat is applied (step 9), and surface treatment such as gold plating is performed (step 10). The multi-layer FPC is completed.

On the other hand, the manufacturing process of the multilayer FPC 1 according to the reference embodiment of the present invention is as follows.
First, a first double-sided FPC board and a second double-sided FPC board are prepared (Step 1), and inner layer side via holes are formed in the second wiring layer L2 and the third wiring layer L3 by laser processing (Step 2). Then, copper plating is applied to the inner layer side via hole (step 3). Subsequently, inner and outer layer circuits are formed (step 4), and at the same time, a bonding sheet is prepared (step 4).
Then, a cover layer is applied to the outer layer (step 5). At the same time, after making a hole in the bonding sheet and filling the conductive paste, the first double-sided FPC and the second
A double-sided FPC is laminated (step 6), a cover coat is applied (step 7), and then a surface treatment such as gold plating is performed (step 8) to complete.

As is clear from the above comparison, the manufacturing process of the multilayer FPC 1 according to the embodiment of the present invention has two processes fewer than the manufacturing process of the conventional multilayer FPC, and shortens the manufacturing time. Can be realized.
FIG. 6 is a schematic sectional view showing the structure of the multilayer FPC 51 according to the embodiment of the present invention.

The multilayer FPC 51 shown in FIG. 6 is a partial multilayer FPC in which only necessary portions are stacked in multiple layers.
It is.
More specifically, the first double-sided FPC 52 has a first side on both sides of the insulating base film 13.
It has a wiring layer L1 and a second wiring layer L2. In the figure, a double-sided circuit unit 53 is provided on the left side, and a double-sided circuit unit 55 is provided on the right side through a signal line unit 54 that is a single-sided circuit unit. In the second wiring layer L2 of the first double-sided FPC 52, an inner layer side via hole 16 is formed at a predetermined location by, for example, laser processing. For this reason, for example, copper plating is applied to the inner peripheral surface of the inner layer side via hole 16, but the copper plating does not reach the first wiring layer L1, and the circuit in which the first wiring layer L1 is miniaturized is It is not adversely affected by plating.

In the first wiring layer L1, a mounting area for electronic components or the like is not narrowed by a via hole.
The second double-sided FPC 60 is laminated only on the second wiring layer L2 in the double-sided circuit portion 53 of the first double-sided FPC 52. The second double-sided FPC 60 is the insulating base film 1
The third wiring layer L3 and the fourth wiring layer L4 are formed on both surfaces of the third wiring layer L3.
An inner layer side via hole 24 is formed on the third side, and the third wiring layer L3 and the fourth wiring layer L4 are electrically connected.

The second double-sided FPC 60 and the double-sided circuit portion 53 of the first double-sided FPC 52 are laminated with the bonding sheet 2 interposed therebetween. Similar to the bonding sheet 2 described above, the bonding sheet 2 is formed with a through hole at a predetermined position, and the through hole is filled with the conductive paste 11. The conductive paste 11 allows the second wiring layer L 2 to be connected to the bonding sheet 2. The third wiring layer L3 is electrically connected.

Thus, it is good also as the partial multilayer FPC51 by which only the required part was laminated | stacked on the multilayer structure.
In FIG. 6, reference numerals 27 and 28 denote cover coats.
FIG. 7 is a schematic cross-sectional view showing a configuration of a multilayer FPC 71 according to still another reference embodiment of the present invention. The basic configuration of the multilayer FPC 71 shown in FIG. 7 is the same as that of the multilayer FPC 1 shown in FIG. 3, and the same or corresponding components are assigned the same numbers.

The feature of the multilayer FPC 71 shown in FIG. 7 is that the first double-sided FPC 3 is opposed to each other with the hollow region 12 therebetween.
The length of the signal line area 72 (length extending in the left-right direction in FIG. 7) and the length of the signal line area 73 of the second double-sided FPC 4 (length extending in the left-right direction in FIG. 7) are slightly longer. Well, it is different.
The reason is that when the multilayer FPC 71 is, for example, a wiring board built in a foldable mobile phone, the signal line regions 72 and 73 are bent so as to be bent by approximately 180 ° and are extended substantially linearly. And can be switched frequently. For this reason, it is required to have bending resistance such that both signal line regions 72 and 73 are curved while sandwiching the hollow portion 12 or extend straight, while maintaining a parallel state. Therefore, for example, when the signal line region 72 is bent so that the signal line region 72 is on the inner side and the signal line region 73 is on the outer side, the length of the signal line region 73 on the inner side is set to the length of the signal line region 73 on the outer side. By making the length shorter than this, each region is less likely to be subjected to compressive or tensile stress at the time of bending, and the flexibility at the time of bending can be improved.

For example, the length of the inner FPC 3 is M1, the length of the outer FPC 4 is M2, and the radius of curvature of the outer FPC 4 when bent is r, and the thickness of the hollow portion 12 is Δ, FPC 3,
If the thickness of the FPC 4 is t3 and t4, respectively,
M1 = M + π × (r−t4−Δ− (1/2) × t3)
M2 = M + π × (r− (1/2) × t4)
However, M is the basic length of FPC3 and FPC4, and the difference in length between M1 and M2 is
M2−M1 = π × ((1/2) × (t3 + t4) + Δ)
You can see that.

Table 1 shows bending data of the multilayer FPC whose length has been adjusted at the end of this section (form for carrying out the invention). According to Table 1, it is clear that the length adjusted is excellent in bending resistance.
FIG. 9 is a schematic cross-sectional view showing a configuration of a multilayer FPC 91 according to still another reference embodiment of the present invention.

A multilayer FPC 91 shown in FIG. 9 is a multilayer FPC having a six-layer wiring layer structure. That is, the first
The first wiring layer L1 and the second wiring layer L2 are formed by the double-sided FPC3, and the fifth wiring layer L5 and the sixth wiring layer L6 are formed by the second double-sided FPC4.
A multilayer structure 911 is laminated between the double-sided FPCs 3 and 4. The multilayer structure 911 has a structure in which one double-sided FPC, that is, a third double-sided FPC 92 is disposed between the two upper and lower bonding sheets 2, and the third wiring layer L 3 and the fourth wiring layer L 3 are formed by the third double-sided FPC 92. A wiring layer L4 is formed.

The configurations of the first double-sided FPC 3 and the second double-sided FPC 4 are the same as those described above with reference to FIG.
The configuration is the same as that of the double-sided FPC 3 and the second double-sided FPC.
On the other hand, the multilayer structure 911 is divided in the left and right in the drawing, with the central portion being interrupted, and separated by the hollow region 12. The left and right double-sided FPCs 92 have third
A via hole 93 for electrically connecting the wiring layer L3 and the fourth wiring layer L4 is formed. Also, between the first double-sided FPC 3 and the third double-sided FPC 92 and the third double-sided FPC.
A through-hole is formed between the second double-sided FPC 4 and the second double-sided FPC 4, and the bonding sheet 2 in which the conductive paste 11 is filled is disposed. The first double-sided FPC 3 with the bonding sheet 2 interposed therebetween, The third double-sided FPC 92 and the second double-sided FPC 4 are stacked.

As shown in FIG. 9, the multilayer structure 911 is made up of N bonding sheets (N is a natural number, N ≧ 2) and N−1 double-sided FPC between the first double-sided FPC 3 and the second double-sided FPC 4.
By adopting a structure in which the bonding sheets are alternately laminated so that the bonding sheets are located on the outer side (upper surface and lower surface in the drawing), a multilayer FPC 91 having a configuration of six or more wiring layers can be realized.

In the case of the configuration including the hollow region 12, a double-sided rigid printed wiring board or a combination of double-sided FPC and double-sided rigid wiring board may be used instead of the third double-sided FPC 92.
Also in this multilayer FPC 91, as in the first reference embodiment, it is possible to reduce the thickness and to prevent the miniaturization of the outer wiring layers L1 and L6, and to achieve high-density mounting without narrowing the component mounting area. A possible multilayer FPC can be provided.

In the present invention, a multi-layer FP using a first double-sided FPC, a second double-sided FPC, and a bonding sheet
Although the configuration of C has been described, for example, one of the double-sided FPCs may be a single-sided FPC (only the outer layer circuit). In this case, the base film is laser processed in advance to provide a connection terminal with the outer layer circuit, The connection terminal and the conductive paste filled in the bonding sheet may be connected to provide a multilayer FPC.

In addition, the present invention is not limited to the configuration described in the embodiment, and various modifications can be made within the scope of the claims.

1, 51, 71, 91 Multi-layer FPC
2 Bonding sheet 3 First double-sided FPC
4 Second-sided FPC
7, 8, 9, 10 hole 11 conductive paste 12 bonding sheet absent region (hollow region)
911 Multilayer structure L1 First wiring layer (outer wiring layer)
L2 Second wiring layer (inner wiring layer)
L3 Third wiring layer (inner wiring layer)
L4 Fourth wiring layer (outer wiring layer)

Claims (2)

A multilayer flexible printed wiring board including a first double-sided flexible printed wiring board laminated on one side of the bonding sheet and a second double-sided flexible printed wiring board laminated on the other side of the bonding sheet with the bonding sheet interposed therebetween There,
In the bonding sheet, a hole penetrating from one side of the bonding sheet to the other side is formed at a predetermined position, and a conductive paste is filled in the hole, and the first double-sided flexible print is formed by the conductive paste. The wiring board and the second double-sided flexible printed wiring board are electrically connected,
The first double-sided flexible printed wiring board and the second double-sided flexible printed wiring board both have an insulating film and an inner surface wiring layer and an outer surface wiring layer provided on both surfaces of the insulating film, respectively, and the bonding The outer wiring layer on the opposite side to the surface in contact with the sheet constitutes a wiring layer on which electronic elements can be mounted, and the inner wiring layer in contact with the bonding sheet has an opening formed at a predetermined position. An inner layer side via hole is formed to communicate with the opening, penetrate the insulating film, face the outer surface wiring layer, and electrically connect the inner surface wiring layer and the outer surface wiring layer, and the bonding sheet, A laminated region including one double-sided flexible printed wiring board and a second double-sided flexible printed wiring board, the bonding sheet, The first double-sided flexible printed wiring board or the second double-sided flexible printed wiring board does not exist, and only the second double-sided flexible printed wiring board or the first double-sided flexible printed wiring board is extended. A partially multilayer flexible printed wiring board characterized by comprising: It is a partial multilayer flexible printed wiring board in which only necessary parts are laminated in multiple layers,
A first double-sided flexible printed wiring board having a first wiring layer and a second wiring layer provided on both surfaces on one end side of an insulating base film, and a first wiring layer and a second wiring layer provided on both surfaces on the other end side. Prepared,
In the second wiring layer on the one end side of the first double-sided flexible printed wiring board, an inner layer side via hole is formed at a predetermined location,
A second double-sided flexible printed wiring board laminated only to the second wiring layer on the one end side of the first double-sided flexible printed wiring board;
The second double-sided flexible printed wiring board has a third wiring layer and a fourth wiring layer formed on both sides of the insulating base film, and an inner layer side via hole is formed on the third wiring layer side to form a third wiring layer. And the fourth wiring layer are electrically connected, and
Sandwiched between the second double-sided flexible wiring board and the second wiring layer on the one end side of the first double-sided flexible wiring board, and having a through hole filled with a conductive paste at a predetermined position; A partial multilayer flexible printed wiring board comprising a bonding sheet for electrically connecting the second wiring layer and the third wiring layer with a paste.

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