FLEXIBLE PRINTED WIRING BOARD AND ELECTRIC WIRING

Provided is a flexible printed wiring board including: a base film which is an insulating layer; a first conductor layer; a second conductor layer; and a through-hole, in which the first conductor layer is provided on one surface of the base film, the second conductor layer is provided on the other surface of the base film, and the through-hole is provided so as to penetrate the base film and electrically connect the first conductor layer and the second conductor layer to each other, and the second conductor layer has a solderable region.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Pat. Application No. 2022-073363 filed with the Japan Patent Office on Apr. 27, 2022, the entire content of which is hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a flexible printed wiring board and an electric wiring.

2. Related Art

Various devices include wirings for electrically connecting electric components to each other. For example, a flexible printed wiring board (hereinafter referred to as an FPC) integrally including many wirings has been suitably used (see Japanese Pat. No. 6975053 and Japanese Pat. No. 6636628, for example). The wiring (conductor layer) included in the FPC is electrically connected to a wiring of a rigid substrate or a terminal attached to a tip end of an electric wiring. The wirings of the FPCs are sometimes electrically connected to each other. For example, in a case where a wiring of an FPC is arranged between a stator and a rotor in a steering column of an automobile, a long wiring is required. For this reason, multiple FPCs are sometimes connected to form an electric wiring. For electrically connecting a wiring of an FPC and, e.g., a wiring of a rigid substrate to each other, connection by soldering has been broadly employed.

An FPC according to the prior art will be described with reference toFIGS.7A,7B, and8.FIGS.7A and7Bare schematic views of the FPC according to the prior art.FIG.7Ais a partial plan view of the FPC, andFIG.7Bis a sectional view along an EE line ofFIG.7A.FIG.8is a schematic sectional view showing the state of electric connection between wirings of the FPCs.

An FPC500includes a base film510which is an insulating layer, a conductor layer520provided on a surface of the base film510, and a cover film530which is an insulating layer provided on a surface on the opposite side of the conductor layer520from the base film510. The cover film530is bonded, with an adhesive layer540, to the conductor layer520and the base film510. Note that metal foil, such as copper foil, provided on the base film510is etched into a desired wiring (circuit). This wiring is equivalent to the conductor layer520. As shown inFIGS.7A and7B, part of the conductor layer520is exposed, and wirings or terminals of other members are electrically connected to such an exposed portion of the conductor layer520by soldering.

FIG.8shows the state of electric connection between the wirings of the FPCs. In the figure, the right FPC500and a left FPC500A have the same basic configuration. For example, in a state in which solder S in a paste form is applied to the exposed portion of the conductor layer520of the right FPC500, the exposed portion of the conductor layer520of the left FPC500A is bonded. Subsequently, the bonded portion is heated with sandwiched from both sides by a first heating tool610and a second heating tool620. For example, the first heating tool610is heated to 315° C., and the second heating tool620is heated to 400° C. Accordingly, the solder S in the paste form is melted, and thereafter, the first heating tool610and the second heating tool620are separated and the conductor layer520of the FPC500and the conductor layer520of the FPC500A are electrically connected to each other via the solder S.

During the above-described soldering process, the solder S portion is at a high temperature. Heat generated during the soldering process is transmitted from the solder S in a direction of separating from the solder S portion particularly via the conductor layer520having a high thermal conductivity. For this reason, air bubbles are caused inside the adhesive layer540or part of the cover film530is detached, leading to quality degradation.

SUMMARY

A flexible printed wiring board according to an embodiment of the present disclosure is configured to include: a base film which is an insulating layer; a first conductor layer; a second conductor layer; and a through-hole, in which the first conductor layer is provided on one surface of the base film, the second conductor layer is provided on the other surface of the base film, and the through-hole is provided so as to penetrate the base film and electrically connect the first conductor layer and the second conductor layer to each other, and the second conductor layer has a solderable region.

DETAILED DESCRIPTION

One object of the present disclosure is to provide a flexible printed wiring board and an electric wiring configured so that quality degradation due to soldering can be reduced.

The present disclosure employes the following technique in order to solve the above-described problem.

A flexible printed wiring board according to an aspect of the present disclosure includes: a base film which is an insulating layer; a first conductor layer; a second conductor layer; and a through-hole, in which the first conductor layer is provided on one surface of the base film, the second conductor layer is provided on the other surface of the base film, and the through-hole is provided so as to penetrate the base film and electrically connect the first conductor layer and the second conductor layer to each other, and the second conductor layer has a solderable region.

According to the present disclosure, in soldering, heat of the heated second conductor layer is transmitted to the first conductor layer via the through-hole, and therefore, the heat transmitted to the first conductor layer can be reduced.

It is preferred that a distance between an inner end portion of the solderable region in the flexible printed wiring board and an end portion of the through-hole closest to the end portion of the solderable region is 20 mm or less.

It is preferred that a width of a portion of the first conductor layer in a vicinity of a portion connected to the through-hole is partially narrower than those of other portions of the first conductor layer.

With this configuration, thermal conduction can be lowered at the narrow portion of the first conductor layer.

Further, an electric wiring according to the present disclosure includes a plurality of the flexible printed wiring boards described above, in which the second conductor layers of the flexible printed wiring boards are electrically connected to each other by soldering.

Note that the above-described configurations may be employed in combination to the extent possible.

As described above, according to the present disclosure, quality degradation due to soldering can be reduced.

Hereinafter, a mode for carrying out the present disclosure will be described in detail as an example with reference to the drawings based on an embodiment. Note that unless otherwise specified, the dimensions, materials, shapes, relative arrangement and the like of components described in this embodiment do not limit the scope of the disclosure.

Embodiment

A flexible printed wiring board (hereinafter referred to as an FPC) according to the embodiment of the present disclosure will be described with reference toFIGS.1A to6B. Note that in a plan view described below, part of a main internal configuration is shown as a see-through portion and is indicated by a dashed line.

The configuration of an FPC100according to the present embodiment will be described with reference toFIGS.1A and1B.FIGS.1A and1Bare schematic views of the FPC100according to the embodiment of the present disclosure.FIG.1Ais a partial plan view of the FPC100, andFIG.1Bis a sectional view along an AA line ofFIG.1A.

The FPC100includes a base film110which is an insulating layer, a first conductor layer111provided on one surface of the base film110, and a second conductor layer112provided on the other surface of the base film110. As shown inFIG.1B, in the present embodiment, the first conductor layer111is provided on the lower surface side of the base film110, and the second conductor layer112is provided on the upper surface side (front surface side) of the base film110. The first conductor layer111is utilized as a wiring, and the second conductor layer112is utilized as an electric connection portion.

A first cover film121which is an insulating layer is provided on the lower surface side of the base film110. The first cover film121is bonded, with an adhesive layer131, to the first conductor layer111and the base film110. Note that metal foil, such as copper foil, provided on the base film110is etched into a desired wiring (circuit). This wiring is equivalent to the first conductor layer111. As described above, the first conductor layer111is utilized as the wiring.

The FPC100according to the present embodiment further includes a through-hole113(via). The through-hole113is provided so as to penetrate the base film110and electrically connect the first conductor layer111and the second conductor layer112to each other. Moreover, in the FPC100according to the present embodiment, the second conductor layer112has a solderable region connectable to, e.g., wirings of other members by soldering. The solderable region is a region to be soldered, and can also be referred to as an expected soldering region.

As viewed from above, an end portion of the second conductor layer112is provided so as to overlap with an end portion of the first conductor layer111, and in the description below, such an overlap portion will be sometimes referred to as an overlap portion. The second conductor layer112is not present inside the overlap portion in the FPC100, and extends from the overlap portion toward the outside of the FPC100.

The through-hole113is provided in the overlap portion of the second conductor layer112and the first conductor layer111. An inner peripheral surface of the through-hole113, a surface of the first conductor layer111, and a surface of the second conductor layer112are applied with copper plating 113a, and therefore, are electrically connected to each other. In an example shown inFIG.1B, the through-hole113is provided so as to penetrate the base film110, the first conductor layer111, and the second conductor layer112, but the present disclosure is not limited to this example and the through-hole113may only be required to be provided so as to penetrate the base film110and electrically connect the first conductor layer111and the second conductor layer112to each other.

The FPC100according to the present embodiment may further include a second cover film122on the upper surface side of the base film110. The second cover film122is an insulating layer provided on the opposite side of the base film110from the first cover film121, and is bonded to part of the base film110and part of the second conductor layer112with an adhesive layer132.

In the FPC100according to the present embodiment, a portion of the second conductor layer112covered with no second cover film122is exposed. A portion of the second conductor layer112covered with the second cover film122which is the insulating layer is not solderable. On the other hand, the exposed portion of the second conductor layer112covered with no second cover film122is solderable. That is, the entirety of the exposed portion (region) of the second conductor layer112is equivalent to the solderable region. A distance H between the solderable region and the through-hole113is preferably 20 mm or less. As shown inFIG.1A, the distance H is a distance between an inner end portion of the solderable region in the FPC100and an end portion of the through-hole113closest to such an end portion of the solderable region.

Note that as the materials of the base film110, the first cover film121, and the second cover film122, e.g., polyimide, polyethylene naphthalate, or polyethylene terephthalate may be applied.

<Advantages of FPC According to Present Embodiment>

According to the FPC100of the present embodiment, in soldering, heat of the heated second conductor layer112is transmitted to the first conductor layer111via the through-hole113. Thermal resistance between the second conductor layer112and the first conductor layer111is the thermal resistance of the through-hole113and the thermal resistance of a portion of the base film110sandwiched between the first conductor layer111and the second conductor layer112. The thermal resistances of the through-hole113and the base film110are greater than that of the conductor layer, and therefore, heat transfer can be reduced as compared to a case where heat is transmitted only via a conductor layer as in a typical case. Thus, the heat transmitted to the first conductor layer111can be reduced. In the present embodiment, the distance H between the solderable region and the through-hole113is 20 mm or less. Thus, the heat of the second conductor layer112is not transmitted to deep in the FPC100, but is transmitted to the first conductor layer111via the through-hole113. Thus, heat transfer can be reduced. With the above-described configuration, air bubbles in the adhesive layers131,132and detachment of the first cover film121or the second cover film122can be reduced. Thus, quality degradation due to soldering can be reduced. Moreover, in soldering, a temperature does not need to be decreased, and therefore, soldering can be stably performed. Further, soldering can be performed with a typical facility by a typical production process, and a cost does not increase because special process and facility are not necessary.

<Application Examples of FPC100>

A first application example of the FPC100according to the present embodiment will be described with reference toFIGS.2A,2B, and3. Here, a case where multiple FPCs100form an electric wiring will be described.FIGS.2A and2Bare schematic views of the electric wiring according to the embodiment of the present disclosure.

Generally, in the case of requiring a long electric wiring, multiple FPCs100with a production limitation on a length are connected to each other, and in this manner, the long electric wiring can be obtained. That is, the multiple FPCs are connected to form the electric wiring according to the present embodiment.FIGS.2A and2Bshow the state of the vicinity of a connected portion of two FPCs100,100A.FIG.2Ais a plan view showing the state of the vicinity of the connected portion of the two FPCs100,100A, andFIG.2Bis a sectional view along a BB line ofFIG.2A. The configurations of the FPCs100,100A are as described above in the embodiment, and both the FPCs100,100A have the same basic configuration.

For example, in a state in which solder S in a paste form is applied to the exposed portion of the second conductor layer112of the FPC100, the exposed portion of the second conductor layer112of the FPC100A is bonded. Subsequently, the bonded portion is heated with sandwiched from both sides by two heating tools. Accordingly, the solder S in the paste form is melted, and thereafter, the two heating tools are separated and the second conductor layer112of the FPC100and the second conductor layer112of the FPC100A are electrically connected to each other via the solder S. In this manner, the second conductor layers112are electrically connected to each other by soldering. As described above, in the present embodiment, heat is less likely to be transmitted to the first conductor layer111of each of the FPCs100,100A in soldering, and therefore, quality degradation is reduced.

Note that inFIGS.2A and2B, a case where the two FPCs100,100A are connected perpendicularly to each other is shown as an example, but the multiple FPCs may be linearly connected to form the long electric wiring.

FIG.3shows a schematic configuration of a rotary connector10included in a steering column of an automobile. The rotary connector10includes a first case11and a second case12rotatably provided on the first case11. The first case11integrally includes a case body portion11aand a connector portion11b. The second case12integrally includes a case body portion12aand a connector portion12b. The FPC100is included inside the first case11and the second case12. Note that the FPC100is shown as a see-through portion and is indicated by a dashed line. The FPC100is connected to the connector portion11bat one end, and is connected to the connector portion12bat the other end. The FPC100is wound, for example, in a spiral shape with a sufficient clearance in the first case11and the second case12, and therefore, does not interfere with rotation of the second case12relative to the first case11. For the FPC used for the rotary connector10configured as described above, a long length is sometimes required. For this reason, the multiple FPCs are connected as described above, and in this manner, the long electric wiring can be obtained.

A second application example of the FPC100according to the present embodiment will be described with reference toFIGS.4A and4B. Here, a case where the FPC100and a rigid substrate are connected to each other will be described.FIGS.4A and4Bare schematic views showing a use example of the FPC according to the embodiment of the present disclosure.

The FPC100is sometimes connected to a rigid substrate200.FIGS.4A and4Bshow the state of the vicinity of a connected portion of the FPC100and the rigid substrate200.FIG.4Ais a plan view showing the state of the vicinity of the connected portion of the FPC100and the rigid substrate200, andFIG.4Bis a sectional view along a CC line ofFIG.4A. The configuration of the FPC100is as described above in the embodiment.

The rigid substrate200includes a substrate body210and a wiring220provided on a surface of the substrate body210. For example, in a state in which solder S in a paste form is applied to the exposed portion of the second conductor layer112of the FPC100, part of the wiring220of the rigid substrate200is bonded. Subsequently, the bonded portion is heated with sandwiched from both sides by two heating tools. In this manner, the solder S in the paste form is melted, and thereafter, the two heating tools are separated and the second conductor layer112of the FPC100and the wiring220of the rigid substrate200are electrically connected to each other via the solder S. As described above, in the present embodiment, heat is less likely to be transmitted to the first conductor layer111of the FPC100in soldering, and therefore, quality degradation is reduced.

A third application example of the FPC100according to the present embodiment will be described with reference toFIGS.5A and5B. Here, a case where the FPC100and a terminal-equipped electric wiring are connected to each other will be described.FIGS.5A and5Bare schematic views showing a use example of the FPC according to the embodiment of the present disclosure.

The FPC100is sometimes connected to the terminal-equipped electric wiring.FIGS.5A and5Bshow the state of the vicinity of a connected portion of the FPC100and a terminal-equipped electric wiring300.FIG.5Ais a plan view showing the state of the vicinity of the connected portion of the FPC100and the terminal-equipped electric wiring300, andFIG.5Bis a sectional view along a DD line ofFIG.5A. The configuration of the FPC100is as described above in the embodiment.

The terminal-equipped electric wiring300includes electric wirings310and terminals320attached to tip ends of the electric wirings310. For example, in a state in which solder S in a paste form is applied to the exposed portion of the second conductor layer112of the FPC100, part of the terminals320is bonded. Subsequently, the bonded portion is heated with sandwiched from both sides by two heating tools. In this manner, the solder S in the paste form is melted, and thereafter, the two heating tools are separated and the second conductor layer112of the FPC100and the terminals320are electrically connected to each other via the solder S. As described above, in the present embodiment, heat is less likely to be transmitted to the first conductor layer111of the FPC100in soldering, and therefore, quality degradation is reduced.

A modification of the FPC will be described with reference toFIGS.6A and6B.FIGS.6A and6Bare schematic views showing the modification of the FPC according to the embodiment of the present disclosure.FIGS.6A and6Bshow partial plan views of the FPC.

As shown inFIG.6A, a configuration may be employed, in which a thin wiring portion 111a which is a partially-narrow portion of the first conductor layer111in the vicinity of a portion connected to the through-hole113is provided. With this configuration, the thin wiring portion 111a of the first conductor layer111has a higher thermal resistance than those of other portions of the first conductor layer111, and therefore, the heat of the second conductor layer112is less likely to be transmitted at the thin wiring portion 111a. Thus, quality degradation can be further reduced.

In order to enhance reliability in electric connection, a configuration in which the first conductor layer111and the second conductor layer112are electrically connected to each other via multiple through-holes113may be employed as shown inFIG.6B. Note that in the case of employing this configuration, the distance H between the solderable region and the through-hole113closest to the solderable region may be at least 20 mm or less. Needless to say, the distance H is preferably 20 mm or less. Also, in the case of employing the configuration with the multiple through-holes113, the configuration of the thin wiring portion 111a as shown inFIG.6Amay be employed.