Opto-electric hybrid board and method of manufacturing same

An opto-electric hybrid board includes: an electric circuit board including an insulation layer and electrical interconnect lines formed on the front surface of the insulation layer; and an optical waveguide provided on the back surface side of the insulation layer of the electric circuit board, with a metal layer therebetween. At least one opening is formed by removing at least part of a region of the metal layer which is overlaid on the contour of an end portion of the optical waveguide. The optical waveguide is formed, with part of the optical waveguide extending into the opening. The opto-electric hybrid board is favorably usable over a prolonged period because the end portion of the optical waveguide provided on the back surface side of the electric circuit board does not peel off the metal layer.

TECHNICAL FIELD

The present invention relates to an opto-electric hybrid board including an electric circuit board and an optical wave guide which are stacked together, and a method of manufacturing the same.

BACKGROUND ART

With the increase in the amount of transmission information, optical interconnect lines in addition to electrical interconnect lines have been used in recent electronic devices and the like. A large number of opto-electric hybrid boards capable of transmitting electrical signals and optical signals at the same time have been used. As shown inFIG. 13, a known example of such opto-electric hybrid boards has a structure in which an electric circuit board E includes an insulation layer1made of polyimide and the like and serving as a substrate, and electrical interconnect lines2having an electrically conductive pattern and provided on the front surface of the insulation layer1, and in which an optical waveguide W is provided on the back surface side of the insulation layer1, with a metal layer9for reinforcement provided therebetween (see PTL 1, for example). The front surface of the electric circuit board E is insulated and protected by a coverlay3. The metal layer9is provided with through holes5and5′ for optical coupling between the optical waveguide W and an optical element (not shown) to be mounted on the front surface side of the electric circuit board E. The optical waveguide W includes three layers: an under cladding layer6; a core7serving as an optical path; and an over cladding layer8.

There is a difference in coefficient of linear expansion between the insulation layer1and the optical waveguide W provided on the back surface side thereof. If the insulation layer1and the optical waveguide W are directly stacked together, the difference in coefficient of linear expansion therebetween causes stresses and slight bending in the optical waveguide W due to ambient temperature, resulting in increased light propagation losses. The metal layer9is provided to avoid such increased light propagation losses. In accordance with trends toward a decrease in the size of electronic devices and an increase in the degree of integration thereof, the opto-electric hybrid boards have been often required to have flexibility in recent years for use in small spaces and in movable sections such as hinges. For the increase in flexibility of an opto-electric hybrid board in which the metal layer9is interposed as described above for the provision of the optical waveguide W, it has been proposed to partially remove the metal layer9itself to cause the cladding layers of the optical waveguide W to enter the sites where the metal layer9is removed, thereby increasing the flexibility (see PTL 2, for example).

RELATED ART DOCUMENT

Patent Document

SUMMARY OF INVENTION

For a further increase in flexibility thereof, opto-electric hybrid boards in which the electric circuit board E has an increased width only in its opposite end portions serving as optical coupling portions or portions for connection to connectors and reinforced with metal layers9and9′ while having a decreased width in its intermediate portion have been often used in recent years, as shown inFIG. 14Awhich is a view of such an opto-electric hybrid board as seen from the optical waveguide W side.

Unfortunately, such an opto-electric hybrid board having a high degree of flexibility is often greatly pulled or twisted. This generates different stresses between the metal layers9and9′ and the optical waveguide W which are made of different materials. It has turned out that the difference in stresses is concentrated in corner portions P (portions enclosed within small circles inFIG. 14A) on opposite ends of the optical waveguide W to appear in the form of distortions or warpage, thereby resulting in a problem such that peeling off in these portions is prone to occur.

As shown inFIG. 14B, an opto-electric hybrid board of the type having no metal layers9and9′ is configured such that the opposite ends of the optical waveguide W are directly disposed on the back surface of the insulation layer1made of polyimide and the like. In such a case, the optical waveguide W and the insulation layer1are also made of different materials although the two types of resins are joined together. It has hence turned out that the optical waveguide W tends to peel off in the corner portions P thereof due to the difference in stresses.

In view of the foregoing, it is therefore an object of the present invention to provide an opto-electric hybrid board favorably usable over a prolonged period while preventing end portions of an optical waveguide disposed in overlaid relation with a metal layer or an insulation layer on the back surface side of an electric circuit board from peeling off the metal layer or the insulation layer, and a method of manufacturing the same.

To accomplish the aforementioned object, a first aspect of the present invention is intended for an opto-electric hybrid board comprising: an electric circuit board including an insulation layer having front and back surfaces, and an electrical interconnect line formed on the front surface of the insulation layer; and an optical waveguide provided on the back surface side of the insulation layer of the electric circuit board, with a metal layer therebetween, the metal layer is overlaid on at least one end portion of the optical waveguide in such a configuration that the contour of the end portion of the optical waveguide is disposed inside the contour of the metal layer, wherein at least one opening is formed in at least part of a region of the metal layer which is overlaid on the contour of the end portion of the optical waveguide, and wherein part of the optical waveguide extends into the at least one opening. In particular, a second aspect of the present invention is intended for the opto-electric hybrid board wherein the at least one opening in the metal layer includes a plurality of openings, and the openings are formed discontinuously along the contour of the end portion of the optical waveguide.

A third aspect of the present invention is intended for an opto-electric hybrid board comprising: an electric circuit board including an insulation layer having front and back surfaces, and an electrical interconnect line formed on the front surface of the insulation layer; and an optical waveguide provided on the back surface side of the insulation layer of the electric circuit board, with a metal layer therebetween, wherein the metal layer is overlaid on at least one end portion of the optical waveguide in such a configuration that the contour of the end portion of the optical waveguide coincides with the contour of the metal layer or is disposed outside the contour of the metal layer, wherein at least one opening is formed in at least part of a region of the metal layer in which the contour of the metal layer is overlaid on the end portion of the optical waveguide, and wherein part of the optical waveguide extends into the at least one opening. In particular, a fourth aspect of the present invention is intended for the opto-electric hybrid board wherein the at least one opening in the metal layer includes a plurality of openings, and the openings are formed discontinuously along the contour of the metal layer itself.

A fifth aspect of the present invention is intended for an opto-electric hybrid board comprising: an electric circuit board including an insulation layer having front and back surfaces, and an electrical interconnect line formed on the front surface of the insulation layer; and an optical waveguide provided directly on the back surface side of the insulation layer of the electric circuit board, wherein the insulation layer is overlaid on at least one end portion of the optical waveguide in such a configuration that the contour of the end portion of the optical waveguide is disposed inside the contour of the insulation layer, wherein at least one recess is formed in at least part of a region of the insulation layer which is overlaid on the contour of the end portion of the optical waveguide, and wherein part of the optical waveguide extends into the at least one recess. In particular, a sixth aspect of the present invention is intended for the opto-electric hybrid board wherein the at least one recess in the insulation layer includes a plurality of recesses, and the recesses are formed discontinuously along the contour of the end portion of the optical waveguide.

A seventh aspect of the present invention is intended for an opto-electric hybrid board comprising: an electric circuit board including an insulation layer having front and back surfaces, and an electrical interconnect line formed on the front surface of the insulation layer; and an optical waveguide provided directly on the back surface side of the insulation layer of the electric circuit board, wherein the insulation layer is overlaid on at least one end portion of the optical waveguide in such a configuration that the contour of the end portion of the optical waveguide coincides with the contour of the insulation layer or is disposed outside the contour of the insulation layer, wherein at least one recess is formed in at least part of a region of the insulation layer in which the contour of the insulation layer is overlaid on the end portion of the optical waveguide, and wherein part of the optical waveguide extends into the at least one recess. In particular, an eighth aspect of the present invention is intended for the opto-electric hybrid board wherein the at least one recess in the insulation layer includes a plurality of recesses, and the recesses are formed discontinuously along the contour of the insulation layer itself.

A ninth aspect of the present invention is intended for a method of manufacturing an opto-electric hybrid board. The method comprises the steps of: preparing an electric circuit board including an insulation layer having front and back surfaces and an electrical interconnect line formed on the front surface of the insulation layer, a metal layer being formed on the back surface of the insulation layer; and forming an optical waveguide on the metal layer so that the contour of at least one end portion of the optical waveguide is disposed inside the contour of the metal layer, wherein the preparing the electric circuit board includes forming at least one opening by removing at least part of a region of the metal layer which is to be overlaid on the contour of the end portion of the optical waveguide, and wherein the forming the optical waveguide includes forming the optical waveguide such that part of the optical waveguide extends into the at least one opening in the metal layer. In particular, a tenth aspect of the present invention is intended for the method of manufacturing an opto-electric hybrid board, wherein the at least one opening in the metal layer includes a plurality of openings, and the step of preparing the electric circuit board includes forming the openings discontinuously along the contour of the end portion of the optical waveguide.

An eleventh aspect of the present invention is intended for a method of manufacturing an opto-electric hybrid board. The method comprises the steps of: preparing an electric circuit board including an insulation layer having front and back surfaces and an electrical interconnect line formed on the front surface of the insulation layer, a metal layer being formed on the back surface of the insulation layer; and forming an optical waveguide on the metal layer so that the contour of at least one end portion of the optical waveguide coincides with the contour of the metal layer or is disposed outside the contour of the metal layer, wherein the preparing the electric circuit board includes forming at least one opening by removing at least part of a region of the metal layer in which the contour of the metal layer is to be overlaid on the end portion of the optical waveguide, and wherein forming the optical waveguide includes forming the optical waveguide such that part of the optical waveguide extending into the at least one opening in the metal layer. In particular, a twelfth aspect of the present invention is intended for the method of manufacturing an opto-electric hybrid board, wherein the at least one opening in the metal layer includes a plurality of openings, and the step of preparing the electric circuit board includes forming the openings discontinuously along the contour of the metal layer itself.

A thirteenth aspect of the present invention is intended for a method of manufacturing an opto-electric hybrid board. The method comprises the steps of: preparing an electric circuit board including an insulation layer having front and back surfaces and an electrical interconnect line formed on the front surface of the insulation layer; and forming an optical waveguide on the insulation layer so that the contour of at least one end portion of the optical waveguide is disposed inside the contour of the insulation layer, wherein the preparing the electric circuit board includes forming at least one recess in at least part of a region of the insulation layer which is to be overlaid on the contour of the end portion of the optical waveguide, and wherein the forming the optical waveguide includes forming the optical waveguide such that part of the optical waveguide extends into the at least one recess in the insulation layer. In particular, a fourteenth aspect of the present invention is intended for the method of manufacturing an opto-electric hybrid board, wherein the at least one recess in the insulation layer includes a plurality of recesses, and the recesses are formed discontinuously along the contour of the end portion of the optical waveguide.

A fifteenth aspect of the present invention is intended for a method of manufacturing an opto-electric hybrid board. The method comprises the steps of: preparing an electric circuit board including an insulation layer having front and back surfaces and an electrical interconnect line formed on the front surface of the insulation layer; and forming an optical waveguide on the insulation layer so that the contour of at least one end portion of the optical waveguide coincides with the contour of the insulation layer or is disposed outside the contour of the insulation layer, wherein the preparing the electric circuit board includes forming at least one recess in at least part of a region of the insulation layer in which the contour of the insulation layer is to be overlaid on the end portion of the optical waveguide, and wherein the forming the optical waveguide includes forming the optical waveguide such that part of the optical waveguide extends into the recess in the insulation layer. In particular, a sixteenth aspect of the present invention is intended for the method of manufacturing an opto-electric hybrid board, wherein the at least one recess in the insulation layer includes a plurality of recesses, and the recesses are formed discontinuously along the contour of the insulation layer itself.

In the opto-electric hybrid board according to the present invention, the recess (or the opening for the metal layer by removing the metal layer) is formed by partially removing the region in which the metal layer or the insulation layer overlaid on the end portion of the optical waveguide is overlaid on the contour of the end portion of the optical waveguide or the region in which the contour of the metal layer or the insulation layer is overlaid on the end portion of the optical waveguide on the back surface side of the electric circuit board, and part of the optical waveguide extends into the recess.

In this configuration, part of the optical waveguide extends into either the opening provided in the metal layer or the recess provided in the insulation layer on the contour of the end portion of the optical waveguide overlaid on the back surface of the metal layer or the insulation layer and is less prone to peel off because of the concentration of stresses. Thus, the part of the optical waveguide extending into either the opening or the recess produces what is called an anchoring effect to make the optical waveguide less prone to peel off, as compared with a configuration in which flat surfaces are joined to each other.

In particular, the metal layer and the optical waveguide have a low peel strength at the laminated interface thereof. Thus, part of the optical waveguide extending into the opening provided in the metal layer is directly joined to the insulation layer to dramatically increase the peel strength therebetween. For this reason, if either the metal layer and the optical waveguide or the insulation layer and the optical waveguide differ from each other in internal stresses gene rated by external loads or heat in the laminate portion thereof, warpage and distortions based on the difference in stresses do not exert influences on the end portion of the optical waveguide.

Thus, the optical waveguide does not peel off in its end portions in the manufacturing steps including mounting an optical element and the like, in the step of incorporating the opto-electric hybrid board into an electronic device and the like and during the actual use thereof. This allows the opto-electric hybrid board to be used favorably over a prolonged period.

In particular, when the openings provided in the metal layer or the recesses provided in the insulation layer are formed discontinuously along the contour of the end portion of the optical waveguide or along the contour of the metal layer or the insulation layer itself, the present invention is excellent in the effect of preventing the optical waveguide from peeling off, which in turn is preferable.

The method of manufacturing an opto-electric hybrid board according to the present invention is capable of efficiently providing the opto-electric hybrid board of the present invention.

DESCRIPTION OF EMBODIMENTS

Next, embodiments according to the present invention will now be described in detail with reference to the drawings. It should be noted that the present invention is not limited to the embodiments.

FIG. 1Ais a schematic partial vertical sectional view of an opto-electric hybrid board according to one embodiment of the present invention.FIG. 1Bis a view as seen in the direction of the arrows A-A′ ofFIG. 1A.FIG. 1Cis a sectional view taken along the line B-B′ ofFIG. 1B.FIG. 2is a sectional view taken along the line C-C′ ofFIG. 1B. This opto-electric hybrid board10includes: an electric circuit board E including an insulation layer1and electrical interconnect lines2provided on the front surface of the insulation layer1; and an optical waveguide W provided on the back surface side of the insulation layer1.

In the electric circuit board E, the electrical interconnect lines2including optical element mounting pads2a, a connector mounting pad2b, other pads for mounting variable elements, grounding electrodes and the like (not shown) are formed on the front surface of the insulation layer1made of polyimide and the like. The electrical interconnect lines2except the pads2aand the like are insulated and protected by a coverlay3made of polyimide and the like. The front surface of the pads2aand the like not protected by the coverlay3is covered with an electroplated layer4made of gold, nickel and the like.

The optical waveguide W provided on the back surface side of the insulation layer1has a substantially rectangular shape elongated in a horizontal direction as seen in plan view, and includes an under cladding layer6, a core7formed in a predetermined pattern on the front surface (the lower surface as seen inFIG. 1A) of the under cladding layer6, and an over cladding layer8integral with the front surface of the under cladding layer6while covering the core7.

A portion of the core7corresponding to the optical element mounting pads2aof the electric circuit board E is in the form of an inclined surface at 45 degrees with respect to the direction in which the core7extends. The inclined surface serves as a light reflecting surface7a. The light reflecting surface7afunctions to change the direction of light propagated in the core7by 90 degrees to cause the light to enter a light-receiving portion of an optical element or to change the direction of light exiting from a light-emitting portion of an optical element by 90 degrees to cause the light to enter the core7.

A metal layer9for reinforcing the opto-electric hybrid board10is provided between the electric circuit board E and the optical waveguide W. The metal layer9is formed in opposite end portions (with reference toFIG. 14A) other than an intermediate portion where flexibility is required, and is patterned in such a configuration as to partially overlap opposite end portions of the optical waveguide W. The metal layer9is provided with a through hole5for ensuring an optical path between the core7of the optical waveguide W and the optical element. The under cladding layer6extends into the through hole5.

As shown inFIG. 1B, the metal layer9includes a total of four openings20of a rectangular plan configuration which are formed by partially removing two locations in each opposite side portion extending along the length of the optical waveguide W in a region of the metal layer9which is overlaid on the contour of the optical waveguide W. As shown inFIGS. 1C and 2, the under cladding layer6extends into the openings20, so that the insulation layer1is directly firmly joined to the under cladding layer6extending into the openings20. This is a striking feature of the present invention. InFIG. 1B, the through hole5is not shown, and an area where the metal layer9is formed is shaded by means of widely spaced diagonal lines extending from top left to bottom right (the same applies to the subsequent figures).

With reference toFIGS. 1A, 1B and 2, the right-hand portion of the opto-electric hybrid board10is identical in structure with the left-hand portion thereof shown in the figure except that the right-hand portion is a mirror image of the left-hand portion, and hence will not be described and shown.

Next, a method of manufacturing the opto-electric hybrid board10will be described (with reference toFIGS. 3A to 3DandFIGS. 4A to 4D).

First, the metal layer9of a planar configuration is prepared. A photosensitive insulating resin including polyimide and the like is applied to the front surface of the metal layer9to form the insulation layer1having a predetermined pattern by a photolithographic process (with reference toFIG. 3A). The insulation layer1has a thickness in the range of 3 to 50 μm, for example. Examples of a material for the formation of the metal layer9include stainless steel, copper, silver, aluminum, nickel, chromium, titanium, platinum and gold. In particular, stainless steel is preferable from the viewpoint of rigidity and the like. Although depending on the material of the metal layer9, the thickness of the metal layer9is in the range of 10 to 70 μm, for example, when stainless steel is used. When the thickness of the metal layer9is less than 10 μm, there is apprehension that a sufficient reinforcing effect is not provided. When the thickness of the metal layer9is greater than 70 μm, there is apprehension that an increase in the distance that light travels in the through hole5of the metal layer9results in an increase in optical losses.

Next, as shown inFIG. 3B, the electrical interconnect lines2(including the optical element mounting pads2a, the connector mounting pad2b, other pads, grounding electrodes and the like; the same shall apply hereinafter) are formed on the front surface of the insulation layer1by a semi-additive process, for example. This process is as follows. First, a metal film (not shown) made of copper, chromium and the like is formed on the front surface of the insulation layer1by sputtering, electroless plating or the like. This metal layer serves as a seed layer (a layer serving as a basis material for the formation of an electroplated layer) for a subsequent electroplating process. Then, a photosensitive resist (not shown) is laminated to the opposite surfaces of a laminate comprised of the metal layer9, the insulation layer1and the seed layer. Thereafter, a photolithographic process is performed to form holes having the pattern of the electrical interconnect lines2in the photosensitive resist on the side where the seed layer is formed, so that surface portions of the seed layer are uncovered at the bottoms of the holes. Next, electroplating is performed to form an electroplated layer made of copper and the like in a stacked manner on the surface portions of the seed layer uncovered at the bottoms of the holes. Then, the photosensitive resist is stripped away using an aqueous sodium hydroxide solution and the like. Thereafter, a portion of the seed layer on which the electroplated layer is not formed is removed by soft etching. Laminate portions comprised of the remaining seed layer and the electroplated layer become the electrical interconnect lines2.

Next, as shown inFIG. 3C, a photosensitive insulating resin including polyimide and the like is applied to portions of the electrical interconnect lines2other than the optical element mounting pads2a, part of the connector mounting pad2band the like to form the coverlay3by a photolithographic process.

Then, as shown inFIG. 3D, the electroplated layer4is formed on the front surfaces of the optical element mounting pads2a, the part of the connector mounting pad2band the like which are not covered with the coverlay3. In this manner, the electric circuit board E is formed.

Next, a photosensitive resist is laminated to the opposite surfaces of a laminate comprised of the metal layer9and the electric circuit board E. Thereafter, holes are formed by a photolithographic process in portions of the photosensitive resist on the back surface side (the surface opposite from the electric circuit board E) of the metal layer9which correspond to a portion not requiring the metal layer9, a portion (with reference toFIG. 1A) where the through hole5for the optical path is to be formed, and portions (with reference toFIG. 2) where the openings20are to be formed, so that the back surface of the metal layer9is partially uncovered.

Then, the uncovered portions of the metal layer9are removed by etching using an aqueous etching solution for the metal material of the metal layer9(for example, an aqueous ferric chloride solution is used as the aqueous etching solution when the metal layer9is a stainless steel layer), so that the insulation layer1is uncovered in the sites where the metal layer9is removed. Thereafter, the photosensitive resist is stripped away using an aqueous sodium hydroxide solution and the like. Thus, as shown inFIG. 4A, the metal layer9is formed only in a region where the reinforcement is required, and the through hole5(with reference toFIG. 1A) for the optical path and the openings20for entry of part of the optical waveguide W thereinto are formed at the same time.

For the formation of the optical waveguide W (with reference toFIG. 1A) on the back surface of the insulation layer1(back surface of the metal layer9in the portion where the metal layer9is formed), a photosensitive resin that is the material for the formation of the under cladding layer6is applied to the back surfaces (the lower surfaces as seen in the figure) of the insulation layer1and the metal layer9, as shown inFIG. 4B. Thereafter, the applied layer is exposed to irradiation light. This exposure cures the applied layer to form the under cladding layer6. The under cladding layer6is formed into a predetermined pattern by a photolithographic process. The under cladding layer6fills the through hole5for the optical path in the metal layer9(with reference toFIG. 1A). Also, part of the under cladding layer6extends into the openings20of the metal layer9, and is directly joined to the back surface of the insulation layer1. The under cladding layer6has a thickness (thickness as measured from the back surface of the insulation layer1) generally greater than the thickness of the metal layer9. A series of operations for the formation of the optical waveguide W are performed while the back surface of the insulation layer1on which the metal layer9is formed is oriented upward. However, the orientation is shown unchanged in the figure.

Next, as shown inFIG. 4C, the core7having a predetermined pattern is formed on the front surface (the lower surface as seen in the figure) of the under cladding layer6by a photolithographic process. The core7has a thickness in the range of 3 to 100 μm, for example, and a width in the range of 3 to 100 μm, for example. An example of the material for the formation of the core7includes a photosensitive resin similar to that for the under cladding layer6, and the material used herein has a refractive index higher than that of the material for the formation of the under cladding layer6and the over cladding layer8to be described below. The adjustment of the refractive indices may be made, for example, by adjusting the selection of the types of the materials for the formation of the under cladding layer6, the core7and the over cladding layer8, and the composition ratio thereof.

Next, as shown inFIG. 4D, the over cladding layer8is formed on the front surface (the lower surface as seen in the figure) of the under cladding layer6by a photolithographic process so as to cover the core7. In this manner, the optical waveguide W is formed. The over cladding layer8has a thickness (thickness as measured from the front surface of the under cladding layer6) not less than that of the core7and not greater than 300 μm, for example. An example of the material for the formation of the over cladding layer8includes a photosensitive resin similar to that for the under cladding layer6.

Specific composition examples of the materials for the formation of the optical waveguide W are as follows.

<Materials for Formation of Under Cladding Layer6and Over Cladding Layer8>

20 parts by weight of an epoxy resin containing an alicyclic skeleton (EHPE 3150 available from Daicel Chemical Industries, Ltd.)

80 parts by weight of a liquid long-chain bifunctional semi-aliphatic epoxy resin (EXA-4816 available from DIC Corporation)

2 parts by weight of a photo-acid generator (SP170 available from ADEKA Corporation)

40 parts by weight of ethyl lactate (available from Musashino Chemical Laboratory, Ltd.)

<Material for Formation of Core7>

50 parts by weight of bisphenoxyethanolfluorene diglycidyl ether (OGSOL EG available from Osaka Gas Chemicals Co., Ltd.)

1 part by weight of a photo-acid generator (SP170 available from ADEKA Corporation)

50 parts by weight of ethyl lactate (available from Musashino Chemical Laboratory, Ltd.)

An inclined surface inclined at 45 degrees with respect to the direction in which the core7extends is formed in a predetermined port ion of the optical waveguide W by laser beam machining, cutting and the like to provide the reflecting surface7a(with reference toFIG. 1A) for optical coupling to the optical element to be mounted on the front surface side of the electric circuit board E. Then, necessary members are mounted, for example, by mounting the optical element on the pads2aof the electrical interconnect lines2provided on the front surface side of the electric circuit board E.

In this manner, the opto-electric hybrid board10shown inFIGS. 1A to 1Cis provided. In the opto-electric hybrid board10, the four openings20are formed by partially removing the region of the metal layer9overlaid on each end portion of the optical waveguide W on the back surface side of the electric circuit board E and overlaid on the contour of the optical waveguide W. The under cladding layer6of the optical waveguide W extends into the openings20, and is directly joined to the insulation layer1.

If a force for peeling off the optical waveguide W is exerted because the metal layer9and the optical waveguide W differ from each other in internal stresses gene rated by external loads or heat in the laminate portion comprised of the metal layer9and the optical waveguide W, the aforementioned configuration in which the contour of each end portion of the optical waveguide W is partially directly joined to the insulation layer1achieves a high peel strength at the junction to thereby achieve a very high peel strength as a whole. Thus, the optical waveguide W does not peel off in its end portions in the manufacturing steps including mounting the optical element and the like, in the step of incorporating the opto-electric hybrid board10into an electronic device and the like and during the actual use thereof. This allows the opto-electric hybrid board10to be used favorably over a prolonged period.

Further, it is only necessary that, when the metal layer9is formed into a pattern, a predetermined portion thereof which is overlaid on the contour of each end portion of the optical waveguide W is patterned so that the openings20are formed. Thus, the opto-electric hybrid board10is easily attained without any special step. This is advantageous in high manufacturing efficiency.

A test to be described below was conducted for purposes of comparison between the peel strength obtained in the case where the metal layer9and the optical waveguide W were joined to each other and the peel strength obtained in the case where the insulation layer1and the optical waveguide W we re directly joined to each other.

A stainless steel plate (SUS 304 available from Nippon Steel Corporation and having a thickness of 0.02 mm) used widely as a metal layer for an opto-electric hybrid board was prepared. Then, using the materials for the formation of the optical waveguide W, a quasi-optical waveguide having a three-layer structure comprised of an under cladding layer with a thickness of 25 μm, a core with a thickness of 50 μm and an over cladding layer with a thickness of 25 μm was formed on the front surface of the stainless steel plate (Sample 1). Also, a polyimide (available from Nitto Denko Corporation and having a thickness of 0.01 mm) was formed on the aforementioned metal layer as an insulation layer for an opto-electric hybrid board, and the metal layer was etched. Then, a quasi-optical waveguide was formed on the front surface of the metal layer (Sample 2).

Then, 90-degree peel strengths in Samples 1 and 2 during the peeling off of the respective quasi-optical waveguides were measured pursuant to a testing method defined in JIS C 5016:1994. As a result, the peel strength in Sample 1 (stainless steel plate as a base) was 0.209 N/cm (21.3 g/cm), and the peel strength in Sample 2 (polyimide as a base) was 1.986 N/cm (202.6 g/cm).

It is hence found that a very high capability to prevent peeling off is imparted to the optical waveguide W by directly joining part of the optical waveguide W to the insulation layer1as illustrated in the aforementioned instance.

When the openings20are formed in the metal layer9in the aforementioned instance, it is preferable that 5 to 95% of the total length of the contour (indicated by zigzag lines inFIG. 1B) of the end portion of the optical waveguide W in overlaid relation with the metal layer9is in direct contact with the back surface of the insulation layer1because of the presence of the openings20. When the percentage of the contour in contact with the back surface of the insulation layer1because of the presence of the openings20is less than the aforementioned range, there is apprehension that the effect of preventing the optical waveguide W from peeling off is not sufficiently produced. When the percentage is greater than the aforementioned range, there is apprehension that the reinforcement achieved by the metal layer9is insufficient depending on the structure of the opto-electric hybrid board10, which in turn is not preferable.

The shape of the openings20is not limited to that of the aforementioned instance. For example, as shown inFIGS. 5A and 5B, strip-shaped openings20may be arranged in parallel, so that the openings20are discontinuously overlaid on the contour of the optical waveguide W.

In the instance shown inFIGS. 1A to 1C, the four openings20are provided in the metal layer9along the contour of the optical waveguide W. However, the openings20may be provided only in two locations overlaid on the two corners at the front end of the contour of the optical waveguide W, as shown inFIG. 6A, for example, for the following reason. When the insulation layer1and the optical waveguide W are directly joined to each other with a high peel strength in at least these two corner locations, these portions of the optical waveguide W are prevented from peeling off, so that the opto-electric hybrid board10is used favorably. Also, as shown inFIG. 6B, the provision of the openings20in three locations, i.e. two locations overlaid on the two corners at the front end and one intermediate location overlaid on an end edge of the optical waveguide W, increases the effect of preventing the end edge portion of the optical waveguide W from peeling off.

Alternatively, examples of the shape (shape as seen in plan view; the same shall apply hereinafter) of the opening(s)20provided in the metal layer9may be as follows: the shape of two strips extending along the opposite edges extending along the length of the optical waveguide W as shown inFIG. 6C; and the shape of one strip having two corners and extending along the contour of the end portion of the optical waveguide W as shown inFIG. 6D.

When the openings20are formed in locations overlaid on the two corners at the front end of an end portion of the optical waveguide W, examples of the shape of the two openings20may be a bent shape along the corners as shown inFIG. 6E, and a round shape as shown inFIG. 6F.

Further, examples of the shape of the aforementioned two openings20may be as follows: a rectangular shape with rounded corner portions as shown inFIG. 7A; the shape of strips extending obliquely with respect to the corners of the optical waveguide Was shown inFIG. 7B; and triangular shapes as shown inFIGS. 7C and 7D.

In this manner, the opening (s)20may have various shapes so as to produce the effect of preventing an end portion of the optical waveguide W from peeling off while consideration is given to a trade-off between a region to which rigidity is desired to be imparted by the metal layer9and a region to which flexibility is desired to be imparted.

In the aforementioned instance, the openings20are provided in the metal layer9, and part of the optical waveguide W extends into the openings20. In addition to the openings20formed by partially removing the metal layer9, recesses21may be formed by the process of denting the insulation layer1uncovered by the openings20, as shown inFIG. 8, to increase the step height, thereby further increasing the peel strength therebetween. Such a configuration makes the end portion of the optical waveguide W less prone to peel off to achieve greater durability.

The aforementioned process of denting the insulation layer1is performed, for example, in a manner to be described below. First, as shown inFIG. 9A, the electric circuit board E is formed in the same manner as in the aforementioned instance, and the openings20and the through hole5for optical coupling (with reference toFIGS. 1A to 1C) are formed in the metal layer9on the back surface side of the electric circuit board E. Then, as shown inFIG. 9B, alkaline etching is performed on portions of the insulation layer1uncovered in the openings20of the metal layer9while the remaining port ions are protected, so that the recesses21are formed. Then, as shown inFIG. 9C, the optical waveguide W is formed in the same manner as in the aforementioned instance. Thereafter, the processes of mounting an optical element and the like and forming the reflecting surface7aare performed, for example. This provides an intended opto-electric hybrid board.

It is preferable that the recesses21have a depth that is 5 to 70% of the thickness of the entire insulation layer1(for example, when the thickness of the entire insulation layer1is 10 μm, it is preferable to form the recesses21so that portions of the insulation layer1where the recesses21are formed by etching have a thickness in the range of 3 to 9.5 μm). When the recesses21are too shallow, no difference is found in the effect of preventing the optical waveguide W from peeling off in spite of the provision of the recesses21. When the recesses21are too deep, there is apprehension about a problem such as a break in the insulation layer1in the aforementioned portions, which in turn is not preferable.

The process for denting the insulation layer1is not limited to that described above. For example, in the stage of the configuration shown inFIG. 9A, a YAG laser or an excimer laser may be directed onto the portions of the insulation layer1uncovered in the openings20of the metal layer9to melt away a predetermined thickness of a predetermined region of the back surface (lower surface as seen inFIGS. 9A to 9C) of the insulation layer1, thereby forming the recesses21.

In the aforementioned instances, the end portions of the optical waveguide W are disposed in overlaid relation with the metal layer9having an increased width in the opposite end portions of the opto-electric hybrid board10, and the contour of each end portion of the optical waveguide W is inside the contour of the metal layer9. However, the present invention is applicable to the opto-electric hybrid board10of a strip-shaped configuration entirely having the same width. The present invention is also applicable to the opto-electric hybrid board10in which the contour of each end portion of the optical waveguide W coincides with the contour of the metal layer9or is outside the contour of the metal layer9. In these cases, the openings20may be formed by partially removing the metal layer9with an appropriate arrangement in an area where each end portion of the optical wave guide W coincides with each end portion of the metal layer9, whereby the effect of preventing the optical waveguide W from peeling off is also produced.

An example of the arrangement in which the contours of the metal layer9and an end portion of the optical waveguide W substantially coincide with each other is shown inFIG. 10A. In this example, the contour of the optical waveguide W is shown as placed slightly inside the contour of the metal layer9for the sake of clarity (the same applies toFIG. 10Band its subsequent figures). In this example, openings20′ are provided by cutting away two corner portions at the front end of the metal layer9disposed in such a configuration that the contour thereof substantially coincides with the contour of the end portion of the optical waveguide W. In this configuration, two corner portions at the front end of the optical waveguide W are directly joined to the insulation layer1. This effectively prevents the front end portion of the optical waveguide W from peeling off. Thus, the term “opening” formed in the metal layer9as used in the present invention is to be interpreted as including not only a closed opening surrounded on all sides but also a cutaway portion formed by cutting away an edge of the metal layer9.

In the case where the metal layer9and the end portion of the optical waveguide W are disposed in the same manner as mentioned above, an opening20′ may be provided by completely removing three sides, i.e. a front end edge and opposite side edges extending along the length of the optical waveguide W, of the contour of the metal layer9, as shown inFIG. 10B. Further, an opening20′ may be provided by removing only the front end edge of the contour of the metal layer9, as shown inFIG. 10C.

Alternatively, an opening20′ may be provided by removing the front end edge of the contour of the metal layer9, with the opposite end portions thereof left unremoved, as shown inFIG. 10D, rather than by completely removing the front end edge. This also sufficiently produces the effect of preventing the optical waveguide W from peeling off. That is, the degree of freedom of the optical waveguide W is limited also on its opposite ends where the opening20′ is not formed because the joint strength of the optical waveguide W is increased in the opening20′. As a result, the optical waveguide W is less prone to peel off even when the metal layer9is interposed on the opposite ends.

Likewise, openings20′ may be provided by removing the opposite side edges of the contour of the metal layer9which extend along the length of the optical waveguide W, with the opposite end portions of the side edges left unremoved, as shown inFIG. 10E. Alternatively, openings20′ may be provided by removing the opposite side edges of the contour of the metal layer9which extend along the length of the optical waveguide W, with the front end portions of the side edges left unremoved, as shown inFIG. 10F.

Depending on the opto-electric hybrid board10, there are cases in which the contour of the optical waveguide W is outside the contour of the electric circuit board E. In such cases, at least one opening20′ may be formed by cutting away a predetermined portion of the metal layer9, so that the end portion of the optical waveguide W is less prone to peel off in the same manner as in the aforementioned examples. For example, when the electric circuit board E and the optical waveguide W are disposed in overlaid relation in such a configuration that the contour of an end portion of the optical waveguide W is outside the contour of the electric circuit board E as shown inFIG. 11A, an opening20′ may be provided by completely removing three sides, i.e. a front end edge and opposite side edges extending along the length of the optical waveguide W, of the contour of the metal layer9inside the contour of the end portion of the optical waveguide W. This makes the end portion of the optical waveguide W less prone to peel off. In this example, the front end of the optical waveguide W is disposed outside the front end of the electric circuit board E. However, for example, when the front end of the optical waveguide W substantially coincides with the front end of the electric circuit board E or is disposed inside the front end of the electric circuit board E as shown inFIG. 11B, an opening20′ may be provided by completely removing the three sides of the contour of the metal layer9in the same manner as in the example ofFIG. 11A.

Also, when the front end of the optical waveguide W is disposed outside the front end of the electric circuit board E and the opposite longitudinal side edges of the optical waveguide W substantially coincide with those of the electric circuit board E or are disposed inside those of the electric circuit board E as shown inFIG. 11C, an opening20′ may be provided by completely removing the three sides of the contour of the metal layer9in the same manner as in the examples ofFIGS. 11A and 11B. This makes the end portion of the optical waveguide W less prone to peel off.

Further, as shown inFIG. 11D, openings20′ may be formed by cutting away two corner portions of the front end of the metal layer9, and part of the contour of the optical waveguide W may be directly joined to the insulation layer1in these openings20′, so that the contour of the optical waveguide W coinciding with the contour of the metal layer9is disposed outside the contour of the electric circuit board E.

Also, as shown inFIG. 11E, openings20′ may be formed by cutting away two corner portions of the front end of the metal layer9in the same manner as described above, and part of the contour of the optical waveguide W may be directly joined to the insulation layer1in these openings20′, so that the contour of the optical waveguide W coinciding with the contour of the insulation layer1is disposed outside the contour of the electric circuit board E.

In the examples of the aforementioned openings20′, strip-shaped openings20may be arranged in parallel, as in the examples shown inFIGS. 5A and 5B. Alternatively, openings20′ each having a small area may be spaced at predetermined intervals along the contour of the metal layer9.

When the metal layer9is overlaid on the end port ion of the optical waveguide W in such a configuration that the contour of the end portion of the optical waveguide W coincides with the contour of the metal layer9or is disposed outside the contour of the metal layer9as in the aforementioned examples, it is preferable that the metal layer9overlaid on the optical waveguide W is shaped to have rounded corners as seen in plan view. The rounded corner shape provides a stress relaxation effect at a boundary between a region where the metal layer9is present and a region where the metal layer9is absent.

The present invention is also applicable to the opto-electric hybrid board10in which the metal layer is not provided and in which the optical waveguide W is directly overlaid on the back surface of the insulation layer1, as shown inFIG. 12A. The insulation layer1and the optical waveguide W are made of different materials although the two types of resins are joined together in the laminate portion comprised of the insulation layer1and the optical waveguide W. Thus, warpage and distortions are prone to occur in the optical waveguide W due to the difference in stresses between the insulation layer1and the optical waveguide W, thereby making the optical waveguide W prone to peel off in some cases. To increase the peel strength of the insulation layer1and the optical waveguide W, recesses22may be formed in the back surface of the insulation layer1overlaid on the optical waveguide W, as shown inFIG. 12A, and part of the optical waveguide W may extend into the recesses22. The arrangement of the recesses22may be formed in conformity with the arrangement of the aforementioned openings20and20′.

In this configuration, part of the optical waveguide W extends into the recesses22to produce the effect of casting anchor in the insulation layer1. This further increases the peel strength, as compared with a configuration in which the insulation layer1and the optical waveguide W have flat joint surfaces, to further produce the effect of preventing the optical waveguide W from peeling off.

An example of the process of forming the recesses22in the insulation layer1is as follows. First, the electric circuit board E is formed in the same manner as in the aforementioned instance, as shown inFIG. 12B. Thereafter, alkaline etching is performed on the back surface of the insulation layer1while other than portions where the recesses22are to be formed are protected, so that the recesses22are formed. Then, as shown inFIG. 12C, the optical waveguide W is formed in the same manner as in the aforementioned instance. Thereafter, the processes of mounting an optical element and the like and forming the reflecting surface7aare performed, for example. This provides an intended opto-electric hybrid board. It is preferable that the recesses22have a depth that is 5 to 70% of the thickness of the insulation layer1for the same reason as in the formation of the recesses21.

Of course, the recesses22having a predetermined pattern may be formed by directing a YAG laser or an excimer laser onto the back surface of the insulation layer1, rather than by performing the alkaline etching.

In the aforementioned instance, the opto-electric hybrid board10has a bilaterally symmetric structure in which opto-electric coupling portions are provided in both left-hand and right-hand end portions thereof. However, an opto-electric coupling portion may be provided in one end portion, whereas the other end portion is merely for connection to a connector. In such a case, it is preferable that the configuration of the present invention is applied to an end portion of the optical waveguide W which is used for opto-electric coupling.

Also, in the aforementioned instances, the outside shape of the optical waveguide W is defined by both the under cladding layer6and the over cladding layer8. However, the outside shape of the optical waveguide W may be defined by only the over cladding layer8or the core7.

Although specific forms in the present invention have been described in the aforementioned embodiments, the aforementioned embodiments should be considered as merely illustrative and not restrictive. It is contemplated that various modifications evident to those skilled in the art could be made without departing from the scope of the present invention.

The present invention is applicable to an excellent opto-electric hybrid board usable with security over a prolonged period because an optical waveguide is less prone to peel off the back surface side of an electric circuit board portion.

REFERENCE SIGNS LIST