Optical circuit board and mounting structure

Abstract

This optical circuit board includes: a wiring substrate having a first surface; an optical waveguide positioned on the first surface; and a solder resist positioned on the first surface. The optical waveguide includes: a lower cladding; a core; and an upper cladding. The lower cladding is positioned on the first surface, and has a second surface located on the first surface side and a third surface located on the opposite side of the second surface. The core extends to the third surface and has a first end surface and a second end surface which are positioned in the extension direction of the core. The upper cladding is positioned on the third surface, and covers the core so that the first end surface and the second end surface are exposed. The optical waveguide has a first side surface including the first end surface and a second side surface including the second end surface. The solder resist is positioned apart from the optical waveguide.

Description

Optical circuit board and mounting structure

[0001] The present invention relates to an optical circuit board and a mounting structure using the optical circuit board.

[0002] In recent years, optical fibers capable of transmitting large volumes of data at high speeds have come to be used in information communications. Optical signals are transmitted and received between the optical fibers and optical components. Such optical components are mounted on optical circuit boards equipped with optical waveguides, as described in, for example, Patent Document 1. Optical signals are transmitted and received via these optical waveguides.

[0003] Japanese Patent Application Laid-Open No. 2003-161853

[0004] The optical circuit board according to the present disclosure includes a wiring board having a first surface, an optical waveguide located on the first surface, and a solder resist located on the first surface. The optical waveguide includes a lower clad, a core, and an upper clad. The lower clad is located on the first surface and has a second surface located on the first surface side and a third surface located opposite the second surface. The core extends to the third surface and has a first end face and a second end face located in the extension direction of the core. The upper clad is located on the third surface and covers the core so that the first end face and the second end face are exposed. The optical waveguide has a first side face including the first end face and a second side face including the second end face. The solder resist is located away from the optical waveguide.

[0005] A mounting structure according to the present disclosure includes the above-described optical circuit board, an optical component mounted on the optical circuit board, and an optical connector connected to the first side surface.

[0006] 1 is a plan view showing a mounting structure in which optical components and electronic components are mounted on an optical circuit board according to an embodiment of the present disclosure. It is an enlarged explanatory diagram illustrating a cross section of region X shown in FIG. 1. It is an enlarged plan view showing one embodiment of the optical waveguide portion in FIG. 2 (excluding the upper clad). It is an explanatory diagram illustrating a cross section when cut along line A-A shown in FIG. 3. It is an explanatory diagram illustrating another embodiment of the lower clad included in the optical waveguide. It is an enlarged plan view showing another embodiment of the optical waveguide portion in FIG. 2 (excluding the upper clad). It is an explanatory diagram showing a state in which an optical connector is connected to the optical waveguide portion shown in FIG. 6. FIGS. 8A to 8E are explanatory diagrams illustrating one embodiment of a method for forming an optical waveguide. FIGS. 9A to 9C are explanatory diagrams illustrating another embodiment of a method for forming an optical waveguide.

[0007] Conventionally, solder resist is positioned on the surface of a wiring substrate included in an optical circuit board to protect the wiring and maintain insulation. When the solder resist is in contact with an optical waveguide, i.e., when the solder resist is in contact with a lower clad included in the optical waveguide, adhesion between the solder resist and the lower clad is low. As a result, the lower clad may peel off from the solder resist. When the lower clad (optical waveguide) peels off from the solder resist, stress is applied from the solder resist to the lower clad, i.e., the optical waveguide, which may cause peeling between the optical waveguide and the wiring board. Therefore, there is a demand for an optical circuit board in which the optical waveguide does not peel off from the solder resist.

[0008] The optical circuit board according to the present disclosure has the configuration described in the section on means for solving the above problems, so that the optical waveguide does not peel off from the solder resist.

[0009] An optical circuit board according to an embodiment of the present disclosure will be described with reference to Figures 1 to 7. Figure 1 is a plan view showing a mounting structure 10 in which an optical component 4 and an electronic component 6 are mounted on an optical circuit board 1 according to an embodiment of the present disclosure.

[0010] An optical circuit board 1 according to an embodiment of the present disclosure includes a wiring board 2 and an optical waveguide 3. Examples of the wiring board 2 included in the optical circuit board 1 according to an embodiment include wiring boards that are generally used for optical circuit boards.

[0011] Although not specifically illustrated, the wiring board 2 includes, for example, a core layer and build-up layers laminated on both sides of the core layer. The core layer includes a core insulating layer and a core conductor layer. The core insulating layer is not particularly limited as long as it is made of an insulating material. Examples of insulating materials include resins such as epoxy resin, bismaleimide-triazine resin, polyimide resin, and polyphenylene ether resin. These resins may be used alone or in combination of two or more.

[0012] The core insulating layer may contain a reinforcing material. Examples of reinforcing materials include insulating fabric materials such as glass fiber, glass nonwoven fabric, aramid nonwoven fabric, aramid fiber, and polyester fiber. Only one type of reinforcing material may be used, or two or more types may be used in combination. Furthermore, the core insulating layer may contain an inorganic insulating filler. Examples of the inorganic insulating filler include silica, barium sulfate, talc, clay, glass, calcium carbonate, and titanium oxide, which may be dispersed in the inorganic insulating filler. Only one type of inorganic insulating filler may be used, or two or more types may be used in combination.

[0013] The core conductor layer is located on the surface of the core insulating layer. The core conductor layer is not particularly limited as long as it is made of a conductive material. Examples of conductive materials include metals such as copper.

[0014] A through-hole conductor is located in the core insulating layer to electrically connect the upper and lower surfaces of the core insulating layer. The through-hole conductor is located in a through-hole that penetrates the upper and lower surfaces of the core insulating layer. The through-hole conductor is formed of a metal such as copper. The through-hole conductor may be formed only on the inner wall surface, or may fill the through-hole. The through-hole conductor is connected to the core conductor layer on the surface of the core insulating layer.

[0015] The build-up layer is located on one or both sides of the core layer, and has a structure in which at least one build-up insulating layer and at least one build-up conductor layer are laminated.

[0016] The build-up insulating layer is not particularly limited as long as it is made of an insulating material. Examples of insulating materials include resins such as epoxy resin, bismaleimide-triazine resin, polyimide resin, and polyphenylene ether resin. These resins may be used alone or in combination of two or more.

[0017] When two or more build-up insulation layers are present, the build-up insulation layers may be made of the same resin or different resins. The build-up insulation layers and the core insulation layer may be made of the same resin or different resins. Furthermore, the build-up insulation layers may have the same thickness or different thicknesses.

[0018] The build-up insulating layer may contain a reinforcing material. Examples of reinforcing materials include insulating fabric materials such as glass fiber, glass nonwoven fabric, aramid nonwoven fabric, aramid fiber, and polyester fiber. Only one type of reinforcing material may be used, or two or more types may be used in combination. Furthermore, the build-up insulating layer may contain an inorganic insulating filler. Examples of the inorganic insulating filler include silica, barium sulfate, talc, clay, glass, calcium carbonate, and titanium oxide, which may be dispersed in the inorganic insulating filler. Only one type of inorganic insulating filler may be used, or two or more types may be used in combination.

[0019] The conductor layer for build-up is not particularly limited as long as it is made of a conductive material, for example, a metal such as copper.

[0020] Via-hole conductors are located in the build-up insulating layer to electrically connect the upper and lower surfaces of the build-up insulating layer. The via-hole conductors are located in via holes that penetrate the upper and lower surfaces of the build-up insulating layer. The via-hole conductors are formed, for example, by metal plating such as copper plating. The via-hole conductors are connected to build-up conductor layers located on both sides of the build-up insulating layer. The via-hole conductors may fill the via holes, or may be located only on the inner surfaces of the via holes.

[0021] As shown in Fig. 2, the optical waveguide 3 included in the optical circuit board 1 according to one embodiment is located on the surface of a metal layer 21a present on the first surface 21 of the wiring board 2. Fig. 2 is an enlarged explanatory view for illustrating a cross section of region X shown in Fig. 1. The optical waveguide 3 has a structure in which a lower clad 31, a core 32, and an upper clad 33 are laminated in this order from the metal layer 21a side.

[0022] The lower cladding 31 included in the optical waveguide 3 is located on the first surface 21 of the wiring board 2, specifically on the surface of the metal layer 21a present in the optical waveguide formation region of the wiring board 2. The material forming the lower cladding 31 is not limited, and examples thereof include resins such as epoxy resin and silicone resin. As shown in FIG. 2 , the lower cladding 31 has a second surface 312 located on the first surface 21 side of the wiring board 2 and a third surface 313 located on the opposite side of the second surface 312. The metal layer 21a is an optional component and may or may not be used. In other words, the wiring board 2 does not need to include the metal layer 21a.

[0023] The core 32 included in the optical waveguide 3 is a portion through which light that has entered the optical waveguide 3 propagates. The core 32 extends to the third surface 313 of the lower cladding 31, and has a first end face 321 and a second end face 322 located in the extending direction of the core 32. In other words, the first end face 321 and the second end face 322 are two end faces located in the extending direction of the core 32. In the optical circuit board 1 according to one embodiment, for convenience, the first end face 321 of the core 32 is the end face on the optical connector 5a side, and the second end face 322 of the core 32 is the end face on the optical component 4 side.

[0024] Specifically, an end face of an optical transmission path (Si waveguide) 41 included in an optical component 4 mounted in the mounting area of ​​the wiring board 2 is positioned opposite a second end face 322 of the core 32 of the optical waveguide 3. With this configuration, optical signals are transmitted and received between the core 32 and the optical transmission path 41. The material forming the core 32 is not limited and is appropriately selected taking into consideration, for example, the light transmittance and the wavelength characteristics of the propagating light. Examples of the material include resins such as epoxy resin and silicone resin. The core 32 has a thickness of, for example, 3 μm or more and 50 μm or less.

[0025] The upper clad 33 included in the optical waveguide 3 is located on the third surface 313 of the lower clad 31 and covers the core 32 so that the first end face 321 and the second end face 322 of the core 32 are exposed. The upper clad 33 is also formed of a resin such as epoxy resin or silicone resin. The lower clad 31 and the upper clad 33 may be made of the same material or different materials. Furthermore, the lower clad 31 and the upper clad 33 may have the same thickness or different thicknesses. The lower clad 31 and the upper clad 33 each have a thickness of, for example, 3 μm or more and 150 μm or less.

[0026] The optical waveguide 3 has a first side surface 3a including a first end surface 321 of the core 32 and a second side surface 3b including a second end surface 322 of the core 32. In other words, the first side surface 3a refers to a surface including an end surface of the upper cladding 33 and an end surface of the lower cladding 31 that are adjacent to the first end surface 321 of the core 32. The second side surface 3b refers to a surface including an end surface of the upper cladding 33 and an end surface of the lower cladding 31 that are adjacent to the second end surface 322 of the core 32.

[0027] A solder resist 22 is located on the first surface 21 of the wiring board 2, specifically on the metal layer 21a. Specifically, the solder resist 22 is located so as to cover the metal layer 21a. If the metal layer 21a is not located on the first surface 21, the solder resist 22 is located so as to cover the first surface 21 of the wiring board 2. The solder resist 22 is formed from a resin, and examples of the resin include an acrylic-modified epoxy resin.

[0028] Although not shown, the solder resist 22 has openings at desired positions. The openings expose the first surface 21 or the metal layer 21 a of the wiring board 2. For example, the optical components 4 and the electronic components 6 are mounted in the openings.

[0029] In the optical circuit board 1 according to one embodiment, the solder resist 22 is located away from the optical waveguide 3, as shown in Fig. 3. Fig. 3 is an enlarged plan view showing one embodiment of the optical waveguide 3 portion in Fig. 2 (excluding the upper clad 33). Because the solder resist 22 is located away from the optical waveguide 3 (lower clad 31), the optical waveguide 3 (lower clad 31) and the solder resist 22 do not peel off. As a result, the optical waveguide 3 is not subjected to stress from the solder resist 22, and adverse effects such as peeling of the optical waveguide 3 from the wiring board 2 do not occur.

[0030] The thickness of the solder resist 22 is not limited. The solder resist 22 may have a thickness of, for example, 8 μm or more and 22 μm or less. As shown in FIG. 4, when viewed in cross section, the thickness of the lower clad 31 may be greater than the thickness of the solder resist 22. In other words, the solder resist 22 may be thinner than the lower clad 31. FIG. 4 is an explanatory diagram for describing a cross section taken along line A-A shown in FIG.

[0031] If the thickness of the lower clad 31 is greater than the thickness of the solder resist 22, for example, the optical component 4 is less likely to be affected by the solder resist 22 when being mounted on the optical waveguide 3. As a result, the optical component 4 can be easily mounted on the optical waveguide 3.

[0032] 5, the third surface 313 of the lower cladding 31 may have an inclined portion 314 whose inclination gradually increases from the region where the core 32 and the upper cladding 33 are located toward the periphery located in a direction perpendicular to the extending direction of the core 32. FIG. 5 is an explanatory diagram showing another embodiment of the lower cladding 31 included in the optical waveguide 3.

[0033] The lower clad 31 has such an inclined portion 314, which makes it difficult for the adhesive that connects the optical connector 5a to flow along the lower clad 31 into the solder resist 22. As a result, misalignment of the optical connector 5a caused by the difference in thermal expansion coefficient between the solder resist 22 and the adhesive is reduced.

[0034] The thickness of the lower cladding 31 at the inclined portion 314 is not limited as long as it is the thickest in the lower cladding 31. When the thickness of the thinnest portion of the lower cladding 31 is defined as 1, the thickness of the lower cladding 31 at the inclined portion 314 may be, for example, 1.02 to 1.10. That is, it is sufficient that the thickness gradually increases from the thinnest portion of the lower cladding 31 to the inclined portion 314 so that the thickness of the inclined portion 314 is 1.02 to 1.10 times the thickness of the thinnest portion. For example, when the thickness of the thinnest portion of the lower cladding 31 is 100 μm, the thickness of the lower cladding 31 at the inclined portion 314 may be 102 μm to 110 μm.

[0035] In the optical circuit board 1 according to one embodiment, the solder resist 22 is not limited as long as it is positioned away from the optical waveguide 3. For example, in Fig. 3, the width between the solder resist 22 and the optical waveguide 3 is approximately constant. However, the width between the solder resist 22 and the optical waveguide 3 does not necessarily have to be constant, and there may be wide and narrow portions.

[0036] For example, as shown in Fig. 6, in a plan view, in a direction perpendicular to the extension direction of the core 32, the solder resist 22 and the first side surface 3a are located apart by a first width W1, and the solder resist 22 and the second side surface 3b are located apart by a second width W2. The first width W1 may be greater than the second width W2. The first side surface 3a includes an optical connector connection portion for connecting the optical connector 5a. Fig. 6 is an enlarged plan view showing another embodiment of the optical waveguide 3 portion in Fig. 2 (excluding the upper clad 33).

[0037] In the width between the solder resist 22 and the optical waveguide 3, the region having the first width W1 means, for example, a region from the first side surface 3a to 35% of the length of the core 32 in the extension direction of the core 32. When a general optical connector 5a is connected, the length from the first side surface 3a to 35% of the length of the core 32 is about twice the length from the first side surface 3a to the end face of the optical connector 5a.

[0038] When the first width W1 is larger than the second width W2, the solder resist 22, which has a large thermal expansion coefficient, is less likely to be connected to the optical connector 5a via the adhesive, as shown in Fig. 7. As a result, misalignment of the optical connector 5a is reduced. Fig. 7 is an explanatory diagram showing the state in which the optical connector 5a is connected to the optical waveguide 3 portion shown in Fig. 6.

[0039] There are no limitations on the method for forming the optical waveguide 3 on the first surface 21 of the wiring substrate 2. One embodiment of a method for forming the optical waveguide 3 will be described with reference to Figures 8A to 8E. Figures 8A to 8E are explanatory diagrams for explaining one embodiment of a method for forming the optical waveguide 3.

[0040] First, as shown in Fig. 8A, solder resist 22 is formed on the surface of metal layer 21a located on first surface 21 of wiring board 2. Solder resist 22 is not positioned in the region where optical waveguide 3 is to be formed. When forming a region having first width W1 as shown in Fig. 6, the mask used to form solder resist 22 may be changed to a desired shape.

[0041] 8B, a lower clad material 31a is formed so as to cover the metal layer 21a and the solder resist 22. Next, a mask 35 is formed on the surface of the lower clad material 31a except for the portion where the lower clad 31 is to be formed.

[0042] After forming the mask 35, the surface of the underclad material 31a is exposed and developed to form the underclad 31 as shown in Fig. 8C. As shown in Fig. 8B, the mask 35 is formed wider than the portion that overlaps with the solder resist 22, so that the underclad 31 is formed in a position that does not contact the solder resist 22 as shown in Fig. 8C. The material forming the underclad 31 is as described above, and a detailed description thereof will be omitted.

[0043] Next, as shown in FIG. 8D , the core 32 is formed on the third surface 313 of the lower cladding 31. The core 32 is formed, for example, as follows: First, a core material is formed on the third surface 313 of the lower cladding 31, and a mask is formed on the surface of the core material except for the portion where the core 32 is to be formed. After the mask is formed, exposure and post-exposure baking (PEB) are performed. Then, development, water washing, and baking are performed to form the core 32. The material forming the core 32 is as described above, and a detailed description thereof will be omitted.

[0044] Next, as shown in Fig. 8E, the upper clad 33 is formed so as to cover the third surface 313 of the lower clad 31 and the core 32. The upper clad 33 is formed, for example, as follows. First, an upper clad material is formed so as to cover the third surface 313 of the lower clad 31 and the core 32, and a mask is formed on the surface of the upper clad material except for the portion where the upper clad 33 will be formed. After the mask is formed, exposure and development are performed to form the upper clad 33 as shown in Fig. 8E. The upper clad 33 is formed so as to expose the first end face 321 and the second end face 322 of the core 32. The material forming the upper clad 33 is as described above, and a detailed description thereof will be omitted.

[0045] The optical waveguide 3 in which the third surface 313 of the lower cladding 31 has such an inclined portion 314 is formed, for example, by the procedure shown in Figures 9A to 9C. Figures 9A to 9C are explanatory diagrams for explaining another embodiment of the method for forming the optical waveguide 3.

[0046] 8A shows the procedure for forming solder resist 22 on the surface of metal layer 21a located on first surface 21 of wiring board 2, and detailed description thereof will be omitted. Next, as shown in Fig. 9A, a lower clad material 31a is formed so as to cover metal layer 21a and solder resist 22. Next, a mask 35 is formed on the surface of lower clad material 31a except for the portion where lower clad 31 is to be formed.

[0047] After forming the mask 35, the surface of the lower cladding material 31a is exposed to light. During exposure, the intensity of ultraviolet (UV) irradiation is increased compared to the case shown in FIG. 8B. By increasing the intensity of UV irradiation, a portion (periphery) of the region where the mask 35 is formed is also exposed. By developing after this exposure, a portion of the region where the mask 35 is formed remains as a semi-cured region, as shown in FIG. 9B.

[0048] Next, the core 32 is formed on the third surface 313 of the lower clad 31. Due to baking when forming the core 32, the semi-cured region shrinks significantly when cured, and an inclined portion 314 is formed as shown in FIG.

[0049] Next, a mounting structure according to the present disclosure will be described. As shown in FIG. 1 , a mounting structure 10 according to an embodiment of the present disclosure has a structure in which an optical component 4 and an electronic component 6 are mounted on an optical circuit board 1 according to an embodiment. The optical component 4 mounted on the mounting structure 10 according to an embodiment includes an optical transmission path 41. Examples of the optical component 4 including such an optical transmission path 41 include a silicon photonics device. Examples of the electronic component 6 include an ASIC (Application Specific Integrated Circuit) and a driver IC.

[0050] 2 , the optical component 4 is electrically connected to the wiring board 2. Specifically, the optical component 4 is electrically connected to a pad 21b located in a mounting area (an area for mounting the optical component 4) of the wiring board 2 via solder 7. The pad 21b is part of a conductor layer located on the upper surface of the wiring board 2.

[0051] A silicon photonics device will be described as an example of the optical component 4. The silicon photonics device has, for example, a silicon (Si) core and silicon dioxide (SiO ２ The silicon photonics device is a type of optical component having an optical transmission line 41 with a cladding of Si. The silicon photonics device includes a Si waveguide as the optical transmission line 41, and further includes a passivation film, a light source unit, a light detection unit, and the like, which are not shown. As described above, the optical transmission line 41 (Si waveguide 41) is located on the second side surface 3 b of the optical waveguide 3 so as to face the core 32 (second end surface 322) included in the optical waveguide 3.

[0052] For example, an electrical signal from the wiring board 2 is transmitted to a light source unit included in the optical component 4 (silicon photonics device) via the solder 7. The light source unit receives the transmitted electrical signal and emits light. The emitted optical signal is transmitted via the optical transmission path 41 (Si waveguide 41) and the core 32 to the optical fiber 5 connected via the optical connector 5a.

[0053] The embodiments of the present disclosure have been described above. However, the invention according to the present disclosure is not limited to the above-described embodiments, and various modifications and improvements are possible within the scope of the present disclosure as shown in (1) to (6) below.

[0054] (1) An optical circuit board according to the present disclosure includes a wiring board having a first surface, an optical waveguide located on the first surface, and a solder resist located on the first surface. The optical waveguide includes a lower clad, a core, and an upper clad. The lower clad is located on the first surface and has a second surface located on the first surface side and a third surface located opposite the second surface. The core extends to the third surface and has first and second end faces located in the extension direction of the core. The upper clad is located on the third surface and covers the core so that the first and second end faces are exposed. The optical waveguide has a first side face including the first end face and a second side face including the second end face. The solder resist is located away from the optical waveguide. (2) In the optical circuit board described in (1) above, the wiring board further includes a metal layer on the first surface, and the lower clad and the solder resist are located on the upper surface of the metal layer. (3) In the optical circuit board described in (1) or (2) above, the thickness of the lower clad is greater than the thickness of the solder resist when viewed in cross section. (4) In the optical circuit board described in any of (1) to (3) above, the third surface of the lower clad has a slope that gradually increases in slope from the region where the core and upper clad are located toward the periphery located in a direction perpendicular to the extension direction of the core. (5) In the optical circuit board described in any of (1) to (4) above, the first side surface includes an optical connector connection portion. In plan view, in a direction perpendicular to the extension direction of the core, the solder resist and the first side surface are located a first width apart, and the solder resist and the second side surface are located a second width apart. The first width is greater than the second width. (6) A mounting structure according to the present disclosure includes the optical circuit board described in any of (1) to (5) above, an optical component mounted on the optical circuit board, and an optical connector connected to the first side surface.

[0055] REFERENCE SIGNS LIST 1 Optical circuit board 2 Wiring board 21 First surface 21a Metal layer 21b Pad 22 Solder resist 3 Optical waveguide 31 Lower clad 31a Lower clad material 312 Second surface 313 Third surface 314 Inclined portion 32 Core 321 First end surface 322 Second end surface 33 Upper clad 3a First side surface 3b Second side surface 34 Adhesive 35 Mask 4 Optical component 41 Optical transmission path (silicon waveguide (Si waveguide)) 5 Optical fiber 5a Optical connector 6 Electronic component 7 Solder 10 Mounting structure

Claims

1. An optical circuit board comprising: a wiring board having a first surface; an optical waveguide located on the first surface; and a solder resist located on the first surface, wherein the optical waveguide includes a lower clad, a core, and an upper clad; the lower clad is located on the first surface and has a second surface located on the first surface side and a third surface located opposite the second surface; the core extends to the third surface and has first and second end faces located in the direction of extension of the core; the upper clad is located on the third surface and covers the core so that the first and second end faces are exposed; the optical waveguide has a first side face including the first end face and a second side face including the second end face; and the solder resist is located away from the optical waveguide.

2. The optical circuit board according to claim 1, wherein the wiring board further includes a metal layer on the first surface, and the underclad and the solder resist are located on an upper surface of the metal layer.

3. The optical circuit board according to claim 1 or 2, wherein the thickness of said lower clad is greater than the thickness of said solder resist when viewed in cross section.

4. An optical circuit board according to any one of claims 1 to 3, wherein the third surface of the lower cladding has an inclined portion that gradually increases in slope from the region where the core and the upper cladding are located toward the periphery located in a direction perpendicular to the extension direction of the core.

5. An optical circuit board according to any one of claims 1 to 4, wherein the first side includes an optical connector connection portion, and when viewed in a plane, the solder resist and the first side are located a first width apart in a direction perpendicular to the extension direction of the core, and the solder resist and the second side are located a second width apart, and the first width is larger than the second width.

6. A mounting structure comprising: an optical circuit board according to any one of claims 1 to 5; an optical component mounted on the optical circuit board; and an optical connector connected to the first side surface.

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