Opto-electric hybrid board

An inventive opto-electric hybrid board includes: opto-electric module portions respectively defined on opposite end portions of an elongated insulation layer, and each including a first electric wiring of an electrically conductive pattern and an optical element provided on a front surface of the insulation layer; and an interconnection portion defined on a portion of the insulation layer extending from the opto-electric module portions, and including an optical waveguide optically coupled with the optical elements. A metal reinforcement layer provided on a back surface of the insulation layer as extending over the opto-electric module portions into the interconnection portion. A portion of the metal reinforcement layer present the interconnection portion has a smaller width than the opto-electric module portions, and the smaller width portion has rounded proximal corners.

TECHNICAL FIELD

The present invention relates to an opto-electric hybrid board including an opto-electric module portion and an interconnection portion.

BACKGROUND ART

In recent electronic devices and the like, optical wirings are employed in addition to electric wirings to cope with increase in information transmission amount. With a trend toward size reduction of the electronic devices and the like, there is a demand for a wiring board which has a smaller size and a higher integration density so as to be mounted in a limited space. An opto-electric hybrid board as shown inFIG. 8A, for example, is proposed as such a wiring board, in which an opto-electric module portion E including an electric wiring13of an electrically conductive pattern and an optical element10mounted on pads13ais provided on each (or one) of opposite end portions of a front surface of an insulation layer12such as of a polyimide, and an optical waveguide W including an under-cladding layer20, a core21and an over-cladding layer22is provided on a back surface of the insulation layer12(see, for example, PTL 1).

In the opto-electric hybrid board, an optical signal transmitted through the core21of the optical waveguide W as indicated by a one-dot-and-dash line P inFIG. 8Ais converted into an electric signal by the optical element10of the opto-electric module portion E for electrical control. Further, an electric signal transmitted through the electric wiring13is converted into an optical signal by the optical element10. The optical signal is transmitted through the optical waveguide W to another opto-electric module portion (not shown) provided on an opposite side, and is taken out as an electric signal again.

In the opto-electric hybrid board, the insulation layer (such as of the polyimide)12contacts the optical waveguide W (such as made of an epoxy resin). Therefore, the optical waveguide W is liable to be stressed or slightly warped due to a difference in linear expansion coefficient between the insulation layer12and the optical waveguide W by an ambient temperature. Problematically, this increases the light transmission loss of the optical waveguide W. When the optical element for the optical-to-electric signal conversion and the electric-to-optical signal conversion and an IC for driving the optical element are to be mounted on the opto-electric module portion E, a mount surface of the opto-electric module portion E is liable to be unstable without provision of a reinforcement layer. Therefore, it will be impossible to properly mount the optical element and the IC on the opto-electric module portion E or, if possible, the opto-electric module portion E will fail to establish a reliable connection.

To cope with this, it is proposed to provide a metal reinforcement layer11such as of stainless steel on the back surface of the insulation layer12to impart the opto-electric module portion E with a certain level of rigidity, whereby the stress and the slight warpage of the optical waveguide W are prevented to suppress the increase in light transmission loss. Without provision of the metal reinforcement layer11in an interconnection portion of the opto-electric hybrid board other than the opto-electric module portion E, it is possible to ensure the flexibility of the optical waveguide W, so that the opto-electric hybrid board can be mounted in a smaller space to establish optical and electrical connections in a complicated positional relationship.

RELATED ART DOCUMENT

Patent Document

SUMMARY OF INVENTION

In an opto-electric hybrid board in which opto-electric module portions E, E′ each reinforced with a metal reinforcement layer11are connected to an interconnection portion B including a flexible optical waveguide W, as schematically shown inFIG. 8B, the optical waveguide W (indicated by rough hatching) extends from portions of the opto-electric hybrid board each provided with the metal reinforcement layer11(regions indicated by fine hatching) to a portion of the opto-electric hybrid board not provided with the metal reinforcement layer11. Therefore, the optical waveguide W is liable to be stretched and twisted by the highly rigid metal reinforcement layers11at boundaries X, X′ between the opto-electric module portions and the interconnection portion whenever the optical waveguide W is moved. Thus, the optical waveguide W is liable to be broken or folded at the boundaries.

It is recently proposed to reduce the width of an interconnection portion B as shown inFIG. 8Cin order to increase the flexibility of an opto-electric hybrid board. Further, it is contemplated that metal reinforcement layers11are configured such as to partly project into the interconnection portion in order to increase the strength of boundaries between the opto-electric module portions E, E′ and the interconnection portion B. Even if the metal reinforcement layers each have this configuration, the optical waveguide W is liable to be torn or folded along boundaries Y, Y′ between metal reinforcement layer present regions and a metal reinforcement layer absent region as in the aforementioned case. In addition, the optical waveguide W is liable to be stressed or damaged at width reduction corners Z, Z′ by the warpage and the twisting of the metal reinforcement layers11.

Therefore, it is strongly desirable to configure the flexible opto-electric hybrid board so that the optical waveguide W is not badly stressed at the boundaries between the metal reinforcement layer present regions and the metal reinforcement layer absent region.

In view of the foregoing, it is an object of the present invention to provide an excellent opto-electric hybrid board which includes a sufficiently flexible interconnection portion including an optical waveguide protected from bending and twisting of the interconnection portion and is substantially free from increase in light transmission loss.

According to a first inventive aspect to achieve the aforementioned object, there is provided an opto-electric hybrid board, which includes: an elongated insulation layer; an opto-electric module portion defined on at least one end portion of the insulation layer; an interconnection portion defined on a portion of the insulation layer extending from the opto-electric module portion; and a metal reinforcement layer provided on a back surface of the insulation layer as extending over the opto-electric module portion into a portion of the interconnection portion; wherein a first electric wiring of an electrically conductive pattern and an optical element are provided on a front side of the opto-electric module portion, wherein an elongated optical waveguide is provided on a back side of the interconnection portion and optically coupled with the optical element provided on the opto-electric module portion, and wherein a portion of the metal reinforcement layer present in the interconnection portion has a smaller width than a greater width portion of the metal reinforcement layer present in the opto-electric module portion, and a boundary between the smaller width portion and the greater width portion of the metal reinforcement layer is rounded so as to include a rounded proximal corner.

According to a second inventive aspect, a second electric wiring is further provided in the interconnection portion in the opto-electric hybrid board. According to a third inventive aspect, the rounded proximal corner has a curvature radius R of 0.3 to 5 mm in the opto-electric hybrid board.

According to a fourth inventive aspect, the metal reinforcement layer is configured such as to extend longitudinally partway of the interconnection portion, and a distal end port ion of the metal reinforcement layer in the interconnection port ion is rounded so as to include a rounded distal corner in the opto-electric hybrid board according to any one of the first to third inventive aspects. According to a fifth inventive aspect, the rounded distal corner has a curvature radius R′ of 0.1 to 5 mm in the opto-electric hybrid board.

According to a sixth inventive aspect, a portion of the metal reinforcement layer extends along the entire length of the interconnection portion in the opto-electric hybrid board according to any one of the first to third inventive aspects.

In the present invention, the term “width” refers to a dimension measured in a transverse direction perpendicular to a longitudinal direction of the opto-electric hybrid board including the elongated insulation layer as a base, as viewed in plan.

In the inventive opto-electric hybrid board, the metal reinforcement layer is provided on the back surface of the insulation layer (serving as the base) as extending over the opto-electric module portion into the interconnection portion. The portion of the metal reinforcement layer present in the interconnection portion has a smaller width, and the proximal corner between the smaller width portion and the greater width portion is rounded (or has an arcuate contour). With this configuration, the smaller width portion of the metal reinforcement layer has the rounded proximal corner and, therefore, even if the interconnection portion is bent or twisted, the interconnection portion is less liable to be stretched by the opto-electric module portion imparted with higher rigidity by the provision of the metal reinforcement layer. Thus, a stress exerted on the rounded portion is distributed along the rounded portion to be alleviated without concentrating on a flexible portion of the interconnection portion. Therefore, the interconnection portion is maintained intact during prolonged use without a certain portion thereof being torn, badly folded or broken. Since the interconnection portion is not locally stressed, a core of the optical waveguide provided along the interconnection portion is free from the slight warpage and the like, thereby suppressing the increase in the light transmission loss of the optical waveguide.

Particularly, where the second electric wiring is further provided in the interconnection portion in the present invention, a greater amount of information can be transmitted in the form of optical signals as well as in the form of electric signals and, therefore, this arrangement is advantageous. Where the rounded proximal corner has a curvature radius R of 0.3 to 5 mm, the stress exerted on this portion can be more effectively and advantageously distributed along this portion.

Particularly, where the metal reinforcement layer extends longitudinally partway of into the interconnection portion and the distal end of the metal reinforcement layer has the rounded distal corner in the present invention, a portion of the elongated interconnection portion extending partway can be reinforced with the metal reinforcement layer. Further, a stress is distributed along the rounded distal corner without concentrating on the rounded distal corner. This more advantageously suppresses the adverse influence on the interconnect ion portion and the optical waveguide.

Where the rounded distal corner has a curvature radius R′ of 0.1 to 5 mm, the stress exerted on this portion can be more effectively and advantageously distributed along this portion.

Where the portion of the metal reinforcement layer reinforcing the interconnection portion extends along the entire length of the interconnection portion in the present invention, the interconnection portion can be longitudinally entirely reinforced with the metal reinforcement layer. This arrangement is advantageous because the interconnection portion is flexible and less liable to be folded or twisted.

DESCRIPTION OF EMBODIMENT

An embodiment of the present invention will hereinafter be described in detail based on the drawings,

FIG. 1Ais a plan view schematically illustrating an opto-electric hybrid board according to one embodiment of the present invention, andFIG. 1Bis an explanatory diagram schematically illustrating a major portion of the opto-electric hybrid board in section on an enlarged scale.

The opto-electric hybrid board includes a pair of left and right opto-electric module portions A, A′ each having a generally square plan shape and an interconnection portion B provided between the opto-electric module portions A, A′ and having an elongated shape as a whole. In the present invention, a component extending longitudinally is regarded as having an elongated shape even if it has a widthwise projection. More specifically, a unitary elongated insulation layer (in this embodiment, a transparent polyimide layer)1is employed as a base, and the opto-electric module portions A, A′ are respectively provided on front surfaces of left and right wider end portions of the insulation layer1and each include an optical element10,10′ and a first electric wiring2of an electrically conductive pattern. In this embodiment, the optical element10of the opto-electric module portion A serves as a light receiving element which receives an optical signal and converts the optical signal to an electric signal. The optical element10′ of the opto-electric module portion A′ serves as a light emitting element which receives an electric signal and converts the electric signal to an optical signal.

An optical waveguide W is provided on a back side of a smaller width portion of the insulation layer1between the left and right opto-electric module portions A and A′, and this portion serves as the interconnection portion B for transmit ting optical signals. The opto-electric module portions A, A′ may each further include an IC, an active element and the like for driving the optical element10,10′ as required. In this embodiment, illustration and description of these elements will be omitted. The opto-electric module portions A, A′ may each further include a connector for connection to another electric circuit board or the like. Since the opto-electric module portions A, A′ basically have symmetrical structures, only the opto-electric module portion A will be described and the description of the opto-electric module portion A′ will hereinafter be omitted.

In the opto-electric module portion A, the first electric wiring2is provided as having the predetermined electrically conductive pattern, which includes pads2afor mounting the optical element10and ground electrodes2b. The pads2aeach have a surface coated with a gold plating layer4for increasing the electrical conductivity thereof. A portion of the first electric wiring2other than the pads2ais covered with a cover lay3to be protected for insulation (inFIG. 1A, the cover lay3is not shown).

A metal reinforcement layer (in this embodiment, a stainless steel layer)6is provided on the back surface of the insulation layer1as extending over the opto-electric module portion A into the interconnection portion B so as to stably maintain the planarity of this portion. A reference numeral5designates through-holes through which the optical element10is optically coupled with the optical waveguide W.

The metal reinforcement layer6will be described in greater detail. As shown inFIG. 2which is a diagram of the opto-electric module portion A of the opto-electric hybrid board seen from the back side, the metal reinforcement layer6includes a greater width portion6ahaving an outer shape generally conformal to the outer shape of the opto-electric module portion A, and a smaller width portion6bextending from one end of the greater width portion6ainto the interconnection portion B and having a smaller width like the width of the interconnection portion B. The reference numeral5designates the through-holes for the optical coupling (seeFIG. 1B). The optical waveguide W is provided below the metal reinforcement layer6though not shown (only the contour of the optical waveguide W is shown by a one-dot-and-dash line).

The smaller width portion6bhas rounded proximal corners30. The smaller width portion6bextends partway of into the interconnection portion B, and has rounded distal comers31. With the provision of the rounded portions, stresses exerted on the rounded proximal corners30and the rounded distal corners31are distributed along the rounded proximal corners30and the rounded distal corners31to be alleviated without concentrating on a flexible portion of the interconnection portion B, even if the interconnection portion B is bent or twisted to be stretched by the opto-electric module portion A imparted with higher rigidity by the metal reinforcement layer6. Therefore, the interconnection portion B is maintained intact during prolonged use without being partly torn, badly folded or broken. This is a major feature of the present invention.

On the back side of the insulation layer1(referring back toFIG. 1B), on the other hand, the optical waveguide W extends from the interconnection portion B, and an end portion of the optical waveguide W is optically coupled with the optical element10provided on the front side of the insulation layer1via the through-holes5of the metal reinforcement layer6. More specifically, the optical waveguide W includes an under-cladding layer7, a core8including a plurality of core portions arranged parallel to each other below the under-cladding layer7, and an over-cladding layer9covering the core8. The under-cladding layer7, the core8and the over-cladding layer9are provided in this order downward from the back surface of the insulation layer1. The under-cladding layer7partly enters the through-holes5and is in contact with the metal reinforcement layer6.

Therefore, the opto-electric hybrid board is freely bendable with excellent flexibility. In addition, even if the opto-electric module portion A, A′ and the interconnection portion B are brought into a significantly bent positional relationship to be stretched or twisted, the stresses occurring due to the stretching or the twisting can be uniformly distributed along the rounded portions (the rounded proximal corners30and the rounded distal corners31of the smaller width portion6b) of the metal reinforcement layer6connected to the interconnection portion B to be thereby alleviated. Therefore, as previously described, the interconnection portion B is maintained intact during prolonged use without any damage and breakage thereof. Since the interconnection portion B is not locally stressed, the core8of the optical waveguide W provided along the interconnection portion B is free from the slight warpage or the like, thereby suppressing the increase in the light transmission loss of the optical waveguide W.

The opto-electric hybrid board may be produced, for example, in the following manner.

First, as shown inFIG. 3A, a flat elongated metal reinforcement layer6is prepared. Exemplary materials for the metal reinforcement layer6include stainless steel, copper, silver, aluminum, nickel, chromium, titanium, platinum and gold, among which stainless steel is preferred for strength and bendability. The metal reinforcement layer6preferably has a thickness in a range of 10 to 70 μm. If the metal reinforcement layer6is excessively thin, it will be impossible to sufficiently provide the opto-electric hybrid board reinforcing effect. If the metal reinforcement layer6is excessively thick, on the other hand, the metal reinforcement layer is liable to have an excessively high rigidity. Therefore, the opto-electric hybrid board is liable to have poorer bendability, greater bulkiness, and poorer handleability with an excessively great overall thickness.

Then, a photosensitive insulative resin such as a resin containing a polyimide resin is applied onto a surface of the metal reinforcement layer6, and formed into an insulation layer1of a predetermined pattern by a photolithography process. In this embodiment, holes la through which the surface of the metal reinforcement layer6is partly exposed are formed at predetermined positions in the insulation layer1for formation of ground electrodes2bin contact with the metal reinforcement layer6. The insulation layer1preferably has a thickness in a range of 3 to 50 μm.

In turn, as shown inFIG. 3B, a first electric wiring2(including pads2afor mounting an optical element10, and the ground electrodes2b) for an opto-electric module portion A is simultaneously formed, for example, by a semi-additive method. In this method, a metal film (not shown) such as of copper is formed on a surface of the insulation layer1by sputtering or electroless plating. The metal film serves as a seed layer (a base layer for formation of an electro-plating layer) in the subsequent electro-plating step. Subsequently, a photosensitive resist (not shown) is applied to both surfaces of a stack including the metal reinforcement layer6, the insulation layer1and the seed layer, and then holes for an electrically conductive pattern of the first electric wiring2are formed in a photosensitive resist layer present on the seed layer by a photolithography process. Thus, surface portions of the seed layer are exposed in bottoms of the holes.

In turn, an electro-plating layer of an electrically conductive material such as copper is formed on the surface portions of the seed layer exposed in the bottoms of the holes by electro-plating. Then, the photosensitive resist is lifted off with a sodium hydroxide aqueous solution. Thereafter, a portion of the seed layer not formed with the electro-plating layer is removed by soft etching. Remaining portions of a stack of the seed layer and the electro-plating layer serve as the first electric wiring2. Preferred examples of the electrically conductive material include highly electrically conductive and highly ductile metal materials such as chromium, aluminum, gold and tantalum in addition to copper. Other preferred examples of the electrically conductive material include alloys containing at least one of these metals. The first electric wiring2preferably has a thickness in a range of 3 to 30 μm. If the thickness of the electric wiring is smaller than the aforementioned range, the electric wiring is liable to have poorer characteristic properties. If the thickness of the electric wiring is greater than the aforementioned range, on the other hand, the opto-electric module portion A is liable to have an excessively great overall thickness including the thickness of the metal reinforcement layer6provided on the back side and, hence, have greater bulkiness.

Subsequently, as shown inFIG. 4A, an electroless plating layer (not shown) such as of nickel is formed on a surface of the first electric wiring2for the opto-electric module portion A, and then a photosensitive insulative resin such as a resin containing a polyimide resin is applied and patterned by a photolithography process to form a cover lay3on a portion of the first electric wiring2other than the optical element mounting pads2a. The cover lay3preferably has a thickness in a range of 1 to 20 μm. Where the thickness of the cover lay3fails within this range, the cover lay3can effectively protect and reinforce the first electric wiring2.

In turn, as shown inFIG. 4B, parts of the electroless plating layer (not shown) present on the pads2aof the first electric wiring2are removed by etching, and then an electro-plating layer such as of gold or nickel (in this embodiment, a gold-plating layer)4is formed on the pads2afrom which the electroless plating layer has been removed.

Subsequently, a photosensitive resist (not shown) is applied to both surfaces of a stack of the metal reinforcement layer6and the insulation layer1, and then holes are formed in a photosensitive resist layer present on the back surface of the metal reinforcement layer6(opposite from that formed with the first electric wiring2) as corresponding to parts of the metal reinforcement layer6to be removed (for an interconnection portion B and light path through-holes) by a photolithography process, so that parts of the back surface of the metal reinforcement layer6are exposed in the holes.

Then, as shown inFIG. 4C, the parts of the metal reinforcement layer6exposed in the holes are removed by etching with the use of an etching aqueous solution suitable for the material for the metal reinforcement layer6(with the use of a ferric chloride aqueous solution, for example, where the metal reinforcement layer6is a stainless steel layer), whereby the insulation layer1is exposed from the removed parts, Thereafter, the photosensitive resist is lifted off with the use of a sodium hydroxide aqueous solution. Thus, the metal reinforcement layer6is configured to have two left and right, portions which respectively extend from the opto-electric module portions A, A′ to opposite end portions of the interconnection portion B on the back side of the opto-electric module portions A, A′ as shown inFIGS. 1A and 1B.

Subsequently, an optical waveguide W (seeFIG. 1B) is fabricated on the back surfaces of the insulation layer1and the metal reinforcement layer6. More specifically, as shown inFIG. 5A, a photosensitive resin as a material for an under-cladding layer7is first applied on the back surfaces (lower surfaces inFIG. 5A) of the insulation layer1and the metal reinforcement layer6, and then the resulting layer is cured by exposure to radiation. Thus, the under-cladding layer7is formed. The under-cladding layer7preferably has a thickness in a range of 3 to 50 μm (as measured from the back surface of the metal reinforcement layer6). The under-cladding layer7may be patterned in a predetermined pattern by a photolithography process.

Then, as shown inFIG. 5B, a core8is formed in a predetermined pattern on a surface (a lower surface inFIG. 5B) of the under-cladding layer7by a photolithography process. The core8preferably has a thickness in a range of 20 to 100 μm. The core8preferably has a width in a range of 10 to 100 μm. An exemplary material for the core8is the same type of photosensitive resin as the under-cladding layer7, but has a higher refractive index than the materials for the under-cladding layer7and an over-cladding layer9to be described later (seeFIG. 5C). The refractive index may be controlled in consideration of the selection of the types of the materials and the formulations of the materials for the under-cladding layer7, the core8and the over-cladding layer9.

Subsequently, as shown inFIG. 5C, the over-cladding layer9is formed over a surface (a lower surface inFIG. 5C) of the under-cladding layer7by a photolithography process to cover the core8. The over-cladding layer9has a greater thickness than the core8, i.e., preferably has a thickness of not greater than 300 μm (as measured from the surface of the under-cladding layer7). An exemplary material for the over-cladding layer9is the same type of photosensitive resin as the under-cladding layer7. For the formation of the over-cladding layer9, the photosensitive resin may be patterned into a predetermined pattern by a photolithography process.

Then, as shown inFIG. 5D, a light reflecting surface8ainclined at 45 degrees with respect to a longitudinal axis of the core2is formed in a portion of the optical waveguide W (each end portion of the optical waveguide W as seen inFIG. 1B) corresponding to the pads2aprovided on the front surface of the insulation layer1by a laser processing method, a cutting method or the like. Then, an optical element10is mounted on the pads2a. Thus, an intended opto-electric hybrid board is provided.

In the production method described above, the pads2aof the first electric wiring2for mounting the optical element10are covered with the gold plating layer4, but the coverage with the plating layer is not necessarily required depending on the material for the first electric wiring2and the required characteristic properties of the first electric wiring2.

In the embodiment described above, the opto-electric module portions A, A′ are provided integrally on left and right sides of the interconnection portion B. However, it is not necessarily required to provide the left and right opto-electric module portions A, A′ in pair, but only one of the opto-electric module portions may be provided. In this case, a distal end of the interconnection portion B may be connected to another opto-electric module portion via a connector or the like.

In the embodiment described above, the rounded proximal corners30of the smaller width portion6b(seeFIG. 2) of the metal reinforcement layer6preferably each have a curvature radius R of 0.3 to 5 mm, particularly preferably 0.5 to 5 mm. Where the curvature radius R of the rounded proximal corners30falls within this range, a stress exerted on the rounded proximal corners can be particularly effectively distributed along the rounded proximal corners and, therefore, the interconnection portion B can be maintained intact. Similarly, the rounded distal corners31of the smaller width portion6bpreferably each have a curvature radius R′ of 0.1 to 5 mm, particularly preferably 0.3 to 5 mm.

Only the proximal corners30may be rounded. That is, the distal corners31are not necessarily required to be rounded. The rounded proximal corners30can provide the intended effect to uniformly distribute the stress exerted on the interconnection portion B to some extent.

In the embodiment described above, the ratio (T:T′) of the width (T inFIG. 2) of the greater width portion6ato the width (T′ inFIG. 2) of the smaller width portion6bof the metal reinforcement layer6is preferably about 1:0.98 to about 1:0.05. If a difference between the width of the greater width portion6aand the width of the smaller width portion6bis small, the interconnection portion B is liable to have insufficient flexibility. If the smaller width portion6bhas an excessively small width, on the other hand, the interconnection portion B is liable to have insufficient strength.

The shape of the metal reinforcement layer6is not limited to that of the aforementioned embodiment, but the metal reinforcement layer6may have different-patterns. For example, as shown inFIG. 6A, the smaller width port ion6bmay be configured such as to include two elongated portions41,42extending longitudinally and a slit40provided between the two elongated portions41,42(the through-holes5for the optical coupling are not shown inFIG. 6Aand so on). In this case, the elongated portions41,42preferably each have rounded distal corners43, and the slit40preferably has a rounded proximal corner44. This configuration imparts the interconnection portion B with higher flexibility than that shown inFIG. 2without reduction in reinforcement effect.

Further, as shown inFIG. 6B, the smaller width portion6bof the metal reinforcement layer6may be configured such as to have an elongated oval slit45extending longitudinally. This configuration imparts the interconnection portion B with higher flexibility while providing the interconnection portion reinforcing effect.

As shown inFIG. 6C, the smaller width portion6bmay be configured such that the two elongated portions41,42shown inFIG. 6Aextend along the entire length of the interconnection portion B to the opposite opto-electric module portion A′. With this configuration, the interconnection portion B is imparted with flexibility, and is less liable to be irregularly twisted. Therefore, the opto-electric hybrid board can be advantageously mounted in a smaller space.

As shown inFIG. 7A, the smaller width portion6bmay be configured such as to have a very small width and extend a long the entire length of the interconnection portion B to the opposite opto-electric module portion A′. This configuration imparts the interconnection portion B with higher flexibility and, therefore, is advantageously employed for an application in which the interconnection portion B is required to be relatively freely bendable in various directions.

Further, as shown inFIG. 7B, the smaller width portion6bmay be configured such as to include two elongated portions41,42extending longitudinally on the interconnection portion B and an auxiliary elongated portion50extending longitudinally in a middle portion of the interconnection portion B. The auxiliary elongated portion50overlaps two left elongated portions41,42and two right elongated portions41,42, whereby the interconnection portion B is easily bendable in a region S enclosed by a broken line inFIG. 7Band is less liable to be folded. Therefore, the opto-electric hybrid board is advantageously mounted in a smaller space.

In contrast to the aforementioned configuration, as shown inFIG. 7C, the smaller width portion6bmay be configured such as to include a single smaller width portion extending partway of into the interconnection portion B and two auxiliary elongated portions51provided in a middle portion of the interconnection portion B as respectively extending longitudinally along opposite edges of the interconnection portion B. Thus, opposite left and right end portions of the interconnection portion B are easily bendable and less liable to be folded.

In the aforementioned embodiments, the opto-electric hybrid board is configured such that the interconnection portion B provided between the left and right opto-electric module portions A, A′ has a smaller width than the opto-electric module portions A, A′, and the metal reinforcement layer6is configured according to this configuration to include the greater width portions6arespectively provided on the back side of the left and right opto-electric module portions A, A′ and the smaller width portion6bprovided on the back side of the narrower interconnection portion B. Alternatively, as indicated by a one-dot-and-dash line inFIG. 7C, the opto-electric hybrid board may entirely have an elongated shape having a constant width, and only a portion of the metal reinforcement layer6present on the back side of the interconnection portion B may serve as the smaller width portion6b. In this case, it is possible to provide the same effects as in the aforementioned embodiments.

In the aforementioned embodiments, the interconnection portion B is configured to include only the optical waveguide W, but may be configured to further include a second electric wiring.

Next, an inventive example will be described in conjunction with a comparative example. It is noted that the invention be not limited to the following inventive example.

EXAMPLES

The opto-electric hybrid board shown inFIGS. 1A and 1Bwas produced in the aforementioned manner. The interconnection portion B had a length of 20 cm. A 20-μm thick stainless steel layer was provided as the metal reinforcement layer. The rounded proximal corners of the smaller width portion of the metal reinforcement layer each had a curvature radius R of 1.5 mm, and the rounded distal corners of the smaller width portion projecting to the interconnection portion each had a curvature radius R′ of 0.8 mm. The ratio (T:T′) between the thickness T of the greater width portion and the thickness T′ of the smaller width portion was 1:0.3. The insulation layer had a thickness of 5 μm, and the under-cladding layer had a thickness of 10 μm (as measured from the back surface of the insulation layer). The core had a thickness of 50 μm and a width of 50 μm. The over-cladding layer had a thickness of 70 μm (as measured from the surface of the under-cladding layer). The first electric wiring had a thickness of 5 μm.

Comparative Example 1

An opto-electric hybrid board was produced in substantially the same manner as in Example 1, except that the metal reinforcement layer was configured at shown inFIG. 8Bwithout the provision of the rounded portions.

[Measurement of Light Input Loss]

The same types of light emitting element and light receiving element as those used in Example 1 and Comparative Example 1 were prepared. The light emitting element, was ULM850-10-TT-C0104U available from ULM Photonics GmbH, and the light receiving element was PDCA04-70-GS available from Albis Optoelectronics AG. The amount Ioof light emitted from the light emitting element and directly received by the light, receiving element was measured. Then, the opto-electric hybrid boards of Example 1 and Comparative Example 1 were each twisted widthwise once and, in this state, laterally stretched with a force of 0.5 N and fixed. Light emitted from the light emitting element provided in the opto-electric module portion A′ was received by the light receiving element provided in the opto-electric module portion A via the core of the optical waveguide W. The amount I of the light thus received was measured. Then, a light input loss (−10×log(I/Io)) was calculated based on these values. As a result, the light input loss of the opto-electric hybrid board of Example 1 was 2.3 dB. In contrast, the light input loss of the opto-electric hybrid board of Comparative Example 1 was 2.8 dB. The light input loss was suppressed in the opto-electric hybrid board of Example 1.

As in the measurement of the light input loss, the opto-electric hybrid boards of Example 1 and Comparative Example 1 were each twisted widthwise once and, in this state, laterally stretched. Then, the stretching load was increased, and a stretching load (breaking strength) was measured when the interconnection portion B was broken. As a result, the opto-electric hybrid board of Example 1 had a breaking strength of 12 N, and the opto-electric hybrid board of Comparative Example 1 had a breaking strength of 6 N. Thus, it was confirmed that the opto-electric hybrid board of Example 1 had a much higher breaking strength than the opto-electric hybrid board of Comparative Example 1.

While a specific form of the embodiments of the present invention has been shown in the aforementioned inventive example, the inventive example is merely illustrative of the invention but not limitative of the invention. It is contemplated that various modifications apparent to those skilled in the art could be made within the scope of the invention.

The inventive opto-electric hybrid board can be widely used for a variety of electronic devices required to have flexibility, particularly for image display devices and mobile communication devices for consumer use, and for inspection apparatuses for industrial and medical use which are each required to have a smaller size and a higher information processing capability.

REFERENCE SIGNS LIST