WIRING BOARD AND ELECTRONIC DEVICE

To sufficiently reduce transmission loss and noise of a signal. A signal wire is located on a first surface of an insulation substrate, a first ground and a second ground are located on the first surface of the insulation substrate with the signal wire interposed therebetween, a via conductor electrically connecting the first ground and the lower ground is located at least one of on a second surface of the insulation substrate or inside the insulation substrate, and a side-surface metal layer electrically connecting the second ground and the lower ground is located on a third surface of the insulation substrate.

TECHNICAL FIELD

The present disclosure relates to a wiring board and an electronic device.

BACKGROUND OF INVENTION

Communication devices used for information communication systems are becoming more compatible with high frequencies. Also in a wiring board used for a communication device, transmission loss and noise of a signal (high-frequency signal) are reduced by, for example, the following configuration.

In a wiring board described in Patent Document 1, a first ground and a second ground are located on an upper surface that is a first surface of an insulation substrate with a signal wire interposed therebetween. A lower ground is located on a lower surface that is a second surface of the insulation substrate, and the lower ground faces the signal wire, the first ground, and the second ground with the insulation substrate interposed therebetween. A first via electrically connecting the first ground and the lower ground is located inside the insulation substrate. A second via electrically connecting the second ground and the lower ground is located inside the insulation substrate.

CITATION LIST

Patent Literature

Patent Document 1: JP 2000-340700 A

SUMMARY

A wiring board according to the present disclosure includes: an insulation substrate comprising a first surface, a second surface located opposite to the first surface, and a third surface connected to each of the first surface and the second surface; a signal wire located on the first surface and extending in a first direction; a first ground located farther away from the third surface than the signal wire is and a second ground located closer to the third surface than the signal wire is, the first ground and the second ground being located on the first surface with the signal wire interposed between the first ground and the second ground; a lower ground located at least one of on the second surface of the insulation substrate or inside the insulation substrate; a via conductor located inside the insulation substrate and electrically connecting the first ground and the lower ground; and a side-surface metal layer located on the third surface. In the wiring board, the side-surface metal layer is located on the third surface and electrically connects the second ground and the lower ground.

An electronic device according to the present disclosure includes: the wiring board described above; an electronic component mounted on the wiring board; a bonding material located on the second surface of the insulation substrate; and a mounting substrate bonded to the wiring board via the bonding material.

DESCRIPTION OF EMBODIMENTS

A wiring board and an electronic device according to embodiments of the present disclosure will be described below in detail with reference to the drawings. However, each of the figures, which will be referred to below, is a simplified representation of only components necessary for description of the embodiments, for convenience of description. Accordingly, the wiring board and the electronic device according to the embodiments of the present disclosure may be provided with an optional component that is not illustrated in the referenced drawings. In addition, the dimensions of the components in the drawings do not faithfully represent the actual dimensions of the components, the dimension ratios of the members, and the like.

In the present disclosure, a “first direction” is a left-right direction in the plan view illustrated inFIG.1. In addition, a second direction is a vertical direction in the plan view illustrated inFIG.1, in other words, a direction orthogonal to the first direction on a first surface. A third direction is a thickness direction of an insulation substrate and is a direction orthogonal to the first direction and the second direction. The third direction is the vertical direction in a side view. In the side view and the cross-sectional view of the present disclosure, a first surface of the insulation substrate is located on an upper side, and a second surface of the insulation substrate is located on a lower side. In addition, in the drawings of the present disclosure, “LB” indicates the first direction, “WB” indicates the second direction, “TB” indicates the third direction, “U” indicates the upper side, and “D” indicates the lower side. In the drawings of the present disclosure, the cross-sectional portions are hatched and the surfaces of the conductor portions are dotted.

First Embodiment

A first embodiment of the present disclosure will be described with reference toFIG.1toFIG.6.FIG.1is a schematic plan view of an electronic device according to a first embodiment.FIG.2is a schematic side view of the electronic device illustrated inFIG.1as viewed from a second direction.FIG.3is a schematic plan view of a wiring board according to the first embodiment.FIG.4is a schematic side view of the wiring board illustrated inFIG.3as viewed from the second direction.FIG.5is a schematic cross-sectional view taken along a line V-V inFIG.3.FIG.6is schematic bottom view of the wiring board illustrated inFIG.3.

(Outline of Electronic Device100)

An electronic device100according to the first embodiment illustrated inFIGS.1and2is used for an optical transceiver (not illustrated). The electronic device100includes a wiring board1, an electronic component5mounted on the wiring board1, a bonding material7located on a second surface of an insulation substrate, and a mounting substrate6bonded to the wiring board1via the bonding material7. In the present disclosure, the electronic component5may be, for example, a laser diode that converts an electrical signal into an optical signal. Although not illustrated, the optical transceiver may include an optical fiber that transmits an optical signal and a lens located between the laser diode and an incident end of the optical fiber. The lens causes the optical signal converted by the laser diode to exit toward the incident end of the optical fiber. The mounting substrate6may be an insulation substrate or the like made of a ceramic or a resin. The electronic device100may also be used for a communication device other than the optical transceiver.

As in the example illustrated inFIGS.3to6, the wiring board1may include an insulation substrate11having a rectangular plate shape. The insulation substrate11may be made of, for example, a ceramic, such as an aluminum oxide sintered body (alumina ceramic), an aluminum nitride sintered body, a mullite sintered body, or a glass ceramic sintered body. The insulation substrate11may be composed of one or more insulation layers. The insulation substrate11may have a first surface11u,a second surface11dlocated opposite to the first surface11u,and a third surface11sconnected to each of the first surface11uand the second surface11d.In each drawing, an example in which an angle formed between the first surface11uand the third surface11sof the insulation substrate11is 90° and an angle formed between the second surface11dand the third surface11sof the insulation substrate11is 90° is illustrated, but the present disclosure is not limited thereto. Note that the side surfaces other than the third surface11s(the other surfaces where the first surface11uand the second surface11dare connected) are not particularly limited.

A signal wire12that transmits a signal (high-frequency signal) for driving the electronic component5may be located on the first surface11uof the insulation substrate11. The signal wire12may be made of a metal material containing, for example, titanium (Ti), platinum (Pt), and gold (Au). The signal wire12may extend in the first direction on the first surface11uof the insulation substrate11. The signal wire12may be disposed from the vicinity of one end portion of the insulation substrate11in the first direction to the vicinity of the other end portion thereof. The signal wire12may be bent in a plan view. As illustrated in the example of the side view ofFIG.2, since the third surface11shas a rectangular shape whose long side extends in the first direction, it can be said that the signal wire12extends in a direction along the third surface11s.

The signal wire12may be located close to the third surface11swith respect to a center position11c,in the second direction, of the insulation substrate11. In other words, the signal wire12may deviate toward the third surface11swith respect to the center position11c,in the second direction, of the insulation substrate11. A proportion of deviation of the signal wire12with respect to the center position11c,in the second direction, of the insulation substrate11may be 25% or more. The proportion of deviation of the signal wire12refers to a ratio of an amount of deviation of a center position of the signal wire12with respect to the center position11c,in the second direction, of the insulation substrate11to a half-length, in the second direction, of the insulation substrate11. The case where the signal wire12is located close to the third surface11swith respect to the center position11c,in the second direction, of the insulation substrate11means that the signal wire12is located close to a side-surface metal layer17. With such a configuration, it is possible to further reduce the transmission loss of the signal transmitted by the signal wire12. In addition, on the first surface11u,the laser diode that is the electronic component5can be mounted at a central portion, in the second direction, of the wiring board1or at a position close to the central portion. Therefore, when the electronic device100is used for, for example, an optical transceiver, it is possible to reduce a positional deviation in the second direction between the lens of the optical transceiver and the wiring board1and to reduce a size of the optical transceiver.

On the first surface11uof the insulation substrate11, a first ground13and a second ground14defining a reference potential may be located with the signal wire12interposed therebetween. The first ground13may be located farther away from the third surface11sof the insulation substrate11than the signal wire12is. The second ground14may be located closer to the third surface11sof the insulation substrate11than the signal wire12is. One end portion of the second ground14is connected to the first ground13. Each of the first ground13and the second ground14may be made of a metal material containing, for example, titanium (Ti), platinum (Pt), and gold (Au). The insulation substrate11may be exposed at a portion between the signal wire12and the first ground13on the first surface11uof the insulation substrate11. The insulation substrate11may be exposed at a portion between the signal wire12and the second ground14on the first surface11uof the insulation substrate11.

The first ground13and the signal wire12may be insulated therebetween by the insulation substrate11. A distance between the first ground13and the signal wire12need not be constant along the signal wire12. In addition, the second ground14and the signal wire12are insulated therebetween by the insulation substrate11. A distance between the second ground14and the signal wire12need not be constant along the signal wire12.

A lower ground15that defines a reference potential may be located on the second surface11dof the insulation substrate11. The lower ground15may face the signal wire12, the first ground13, and the second ground14with the insulation substrate11interposed between the lower ground15and the signal wire12. In other words, the lower ground15may overlap the signal wire12, the first ground13, and the second ground14when the wiring board1is viewed in a plane perspective. The lower ground15may be made of a metal material containing, for example, titanium (Ti), platinum (Pt), gold (Au), and the like. The lower ground15and the signal wire12may be insulated therebetween by the insulation substrate11.

As illustrated inFIG.5, a via conductor16electrically connecting the first ground13and the lower ground15may be located inside the insulation substrate11. The via conductor16may be made by metalizing metal powder containing tungsten (W), molybdenum (Mo), manganese (Mn), silver (Ag), copper (Cu) or the like as a component. The via conductor16is formed by embedding a metalized paste for the via conductor in a through-hole, which has been formed in a ceramic green sheet by a processing method such as punching processing or laser processing, by a screen printing method, a press-fitting method, or the like, and firing the paste together with a powder compact formed by layering one or more ceramic green sheets.

A plurality of via conductors16may be provided. In the example illustrated inFIG.5, an upper end portion of each via conductor16may be connected to the first ground13, and a lower end portion of each via conductor16may be connected to the lower ground15. The plurality of via conductors16may be arranged at equal intervals along the signal wire12. The plurality of via conductors16may be arranged at unequal intervals. When the wiring board1is viewed in a plane perspective, a distance between each via conductor16and the signal wire12may be the same or different. When the plurality of via conductors16are arranged, the shielding property of the signal wire12can be enhanced by the plurality of via conductors16. Furthermore, when the plurality of via conductors16are arranged along the signal wire12, the shielding property of the signal wire12can be further enhanced. Therefore, according to the first embodiment of the present disclosure, it is possible to further reduce the transmission loss of the signal transmitted by the signal wire12.

The side-surface metal layer17electrically connecting the second ground14and the lower ground15may be located on the third surface11sof the insulation substrate11. The side-surface metal layer17may be made of a metal material such as Ti (titanium). A top edge portion of the side-surface metal layer17may be connected to an edge portion (an edge portion on the third surface11sside) of the second ground14. The top edge portion of the side-surface metal layer17may overlap the edge portion of the second ground14. In the example illustrated inFIG.1, a bottom edge portion of the side-surface metal layer17may be connected to an edge portion (an edge portion on the third surface11sside) of the lower ground15. The bottom edge portion of the side-surface metal layer17may overlap the edge portion of the lower ground15.

When the side-surface metal layer17electrically connects the second ground14and the lower ground15each other, the shielding property of the signal wire12can be enhanced, as compared with a case where a via conductor electrically connecting the second ground14and the lower ground15is provided inside the insulation substrate11. Therefore, it is possible to sufficiently reduce the transmission loss and noise of the signal (high-frequency signal) transmitted by the signal wire12. In addition, a length of the insulation substrate11in the second direction can be reduced, as compared with a case where a via conductor electrically connecting the second ground14and the lower ground15is provided inside the insulation substrate11. Therefore, miniaturization of the wiring board1, in other words, miniaturization of the electronic device100can be achieved.

As in the example illustrated inFIGS.2and4, the side-surface metal layer17may be disposed on the entire third surface11sof the insulation substrate11so as to extend along the entire signal wire12, and may be connected to the second ground14and the lower ground15. The side-surface metal layer17may be disposed in a partial region of the third surface11sof the insulation substrate11so as to extend along a half region (half length) or more of the signal wire12and may be connected to the second ground14and the lower ground15.

The wettability of the third surface11sof the insulation substrate11with respect to the bonding material7that bonds the insulation substrate11to the mounting substrate6may be lower than the wettability of the second surface11dof the insulation substrate11with respect to the bonding material7. For the insulation substrate11, ceramics such as an aluminum oxide (Al2O3) sintered body, an aluminum nitride (AlN) sintered body, a silicon carbide (SiC) sintered body, a glass ceramic, and a silicon nitride (Si3N4) sintered body and insulation materials such as an epoxy resin, a polyimide resin, and a polyimide siloxane resin may be exemplified. The bonding material7contains a solder material (for example, gold tin or the like), a brazing material, or the like. The wettability of the third surface11sof the insulation substrate11with respect to the bonding material7refers to the ease of wetting and spreading of the bonding material7on the side-surface metal layer17located on the third surface11sof the insulation substrate11. The wettability of the second surface11dof the insulation substrate11with respect to the bonding material7refers to the ease of wetting and spreading of the bonding material7on the lower ground15located on the second surface11dof the insulation substrate11.

When the wettability of the third surface11sof the insulation substrate11with respect to the bonding material7is lower than the wettability of the second surface11dof the insulation substrate11with respect to the bonding material7, it is possible to reduce creeping up of the bonding material7to the side-surface metal layer17and the second ground14when the insulation substrate11is bonded to the mounting substrate6. Therefore, when the insulation substrate11is bonded to the mounting substrate6, a difference in thermal expansion between both ends, in the second direction, of the insulation substrate11can be reduced to reduce an occurrence of cracking, chipping, or the like of the insulation substrate11. In addition, when the insulation substrate11is bonded to the mounting substrate6, it is possible to reduce floating of one side, in the second direction, of the insulation substrate11. When the electronic device100(wiring board1) is used for, for example, an optical transceiver, it is possible to reduce an inclination or the like of the electronic device100with respect to an installation position of the electronic device100, improving the reliability of the optical transceiver.

Here, the signal wire12, the first ground13, and the second ground14form a coplanar line. The signal wire12and the lower ground15form a microstrip line. The first ground13is electrically connected to the lower ground15by the via conductor16and is maintained at the same potential as the lower ground15. The second ground14is electrically connected to the lower ground15by the side-surface metal layer17and is maintained at the same potential as the lower ground15.

(Manufacturing Process of Wiring Board1)

When the insulation substrate11is made of, for example, an aluminum nitride sintered body, a slurry is fabricated by adding and mixing a suitable organic binder, a solvent, and the like to raw material powder of aluminum nitride (AlN), erbium oxide (Er2O3), yttrium oxide (Y2O3), or the like. A ceramic green sheet is fabricated by forming the slurry into a sheet shape by a doctor blade method, a calender roll method, or the like. Next, suitable punching processing is performed on the ceramic green sheet, and one or more ceramic green sheets are layered to fabricate a powder compact. The powder compact is fired at a high temperature (about 1800° C.) to fabricate a sintered body from which a plurality of insulation substrates11are to be obtained. Then, after a surface of the sintered body is polished, the sintered body is sliced to take out a plurality of insulation substrates11from the sintered body.

The via conductor16is made by metalizing metal powder containing tungsten (W), molybdenum (Mo), manganese (Mn), silver (Ag), copper (Cu), or the like as a component. The via conductor16is formed by embedding a conductive paste for the via conductor in a through-hole, which has been formed in a ceramic green sheet by a processing method such as punching processing or laser processing, by a screen printing method, a press-fitting method, or the like, and firing the paste together with a powder compact formed by layering one or more ceramic green sheets. The signal wire12, the first ground13, and the second ground14are metal layers containing titanium (Ti), platinum (Pt), gold (Au), and the like as components, and are formed on the first surface11u(upper surface) of the sintered body whose surface has been polished, by a thin film forming method such as a vapor deposition method, an ion-plating method, or a sputtering method. The lower ground15is a metal layer containing titanium (Ti), platinum (Pt), gold (Au), and the like as components, and is formed on the second surface11d(lower face) of the sintered body whose surface has been polished, by a thin film forming method such as a vapor deposition method, an ion-plating method, or a sputtering method.

The side-surface metal layer17is a metal layer containing titanium (Ti) or the like, for example, and is formed on the third surface11sof the sliced insulation substrate11by a thin film forming method such as a vapor deposition method, an ion-plating method, or a sputtering method. The side-surface metal layer17may be a metal layer having the same configuration as those of the signal wire12, the first ground13, the second ground14, and the lower ground15, or may be a metal layer having a different configuration. For example, when the lower ground15is a metal layer of Ti/Pt/Au and the side-surface metal layer17is a metal layer of Ti, the wettability of the third surface11sof the insulation substrate11can be made lower than the wettability of the second surface11dof the insulation substrate11with respect to the bonding material7.

According to the first embodiment of the present disclosure, by using the electronic device100(wiring board1) for, for example, an optical transceiver, it is possible to obtain an optical transceiver that transmits a good optical signal while reducing the size of the optical transceiver, and to improve the reliability of the optical transceiver.

Another Aspect of First Embodiment

Another aspect of the first embodiment will be described with reference toFIG.7.

FIG.7is a schematic partial side view of a wiring board according to another aspect of the first embodiment as viewed from a second direction.

As in the example illustrated inFIG.7, in a wiring board1A according to another aspect of the first embodiment, the side-surface metal layer17may be formed in, for example, a netlike shape or a perforated shape, and may have a plurality of openings17a.The netlike shape includes a mesh shape, a lattice shape, a honeycomb shape, and the like. An opening area fraction of the side-surface metal layer17may be, for example, 80% or less.

When the side-surface metal layer17has a plurality of openings17a,it is possible to reduce creeping up of the bonding material7to the side-surface metal layer17and the second ground14in bonding of the insulation substrate11to the mounting substrate6. Therefore, according to another aspect of the first embodiment, when the insulation substrate11is bonded to the mounting substrate6, a difference in thermal expansion between both ends, in the second direction, of the insulation substrate11can be reduced to reduce an occurrence of cracking, chipping, or the like of the insulation substrate11. Furthermore, according to another aspect of the first embodiment, when the lower ground15is bonded to the mounting substrate6, it is possible to reduce floating of one side, in the second direction, of the insulation substrate11. According to another aspect of the first embodiment, when the electronic device100(wiring board1A) is used for, for example, an optical transceiver, it is possible to reduce an inclination or the like of the electronic device100with respect to an installation position of the electronic device100, improving the reliability of the optical transceiver.

In addition, according to another aspect of the first embodiment, the same operational effects as those of the first embodiment described above are obtained.

Second Embodiment

A second embodiment will be described with reference toFIGS.8and9.FIG.8is a schematic cross-sectional view of a wiring board according to a second embodiment.FIG.9is a schematic cross-sectional view of a wiring board according to another aspect of the second embodiment.

As in the examples illustrated inFIGS.8and9, a wiring board2according to the second embodiment has the same configuration as the wiring board1according to the first embodiment except for a part of the configuration. Among the configurations of the wiring board2, a configuration different from that of the wiring board1according to the first embodiment will be described. For convenience of description, a member having the same function as a member described in the first embodiment is denoted by the same reference sign.

As illustrated inFIGS.8and9, the wiring board2includes a first lower ground25, as the lower ground. The first lower ground25is an example of the lower ground. In the wiring board2according to the second embodiment, instead of the lower ground15being located on the second surface11dof the insulation substrate11, the first lower ground25may be located inside the insulation substrate11. As for the first lower ground25, a part of the insulation substrate11may be interposed between the first lower ground25and the signal wire12. In addition, the first lower ground25may face the signal wire12, the first ground13, and the second ground14with a part of the insulation substrate11interposed therebetween. In other words, the first lower ground25may overlap the signal wire12, the first ground13, and the second ground14when the wiring board1is viewed in a plane perspective. The first lower ground25and the signal wire12may be insulated therebetween by the insulation substrate11. As illustrated inFIGS.8and9, the first lower ground25and the side-surface metal layer17may be connected at the third surface11sof the insulation substrate11.

The first lower ground25may be located closer to the first surface11uthan to the second surface11dof the insulation substrate11. In other words, a distance between the first lower ground25and the first surface11uof the insulation substrate11may be shorter than 50% of a thickness dimension of the insulation substrate11. The case where the first lower ground25is located closer to the first surface11uthan to the second surface11dof the insulation substrate11means that the first lower ground25is located close to the signal wire12. With such a configuration, it is possible to further reduce the transmission loss of the signal transmitted by the signal wire12, and to enhance the heat dissipation property of the electronic component5(seeFIG.1) mounted on the wiring board2. When the wiring board2is used for, for example, an optical transceiver, it is possible to transmit a good optical signal, improving the reliability of the optical transceiver.

The via conductor16may electrically connect the first ground13and the first lower ground25. When a plurality of via conductors16are provided, an upper end portion of each via conductor16may be connected to the first ground13, and a lower portion of each via conductor16may be connected to the first lower ground25.

As in the example illustrated inFIG.8, each via conductor16may penetrate through the first lower ground25along the third direction. In this case, a lower end portion of each via conductor16may be exposed from the second surface11dof the insulation substrate11. In addition, as in the example illustrated inFIG.9, each via conductor16need not penetrate through the first lower ground25along the third direction.

The side-surface metal layer17may electrically connect the second ground14and the first lower ground25. A top edge portion of the side-surface metal layer17is connected to an edge portion (an edge portion closer to the third surface11s) of the second ground14. The top edge portion of the side-surface metal layer17may overlap the edge portion of the second ground14. As in the example illustrated inFIGS.8and9, the side-surface metal layer17may be connected to the edge portion of the first lower ground25exposed on the third surface11s.An end portion of the first lower ground25may be exposed in a band shape on the third surface11salong the first direction and may be connected to the side-surface metal layer17in a band shape.

Here, the signal wire12, the first ground13, and the second ground14form a coplanar line. The signal wire12and the first lower ground25form a microstrip line. The first ground13is electrically connected to the first lower ground25by the via conductor16, and is maintained at the same potential as the first lower ground25. The second ground14is electrically connected to the first lower ground25by the side-surface metal layer17, and is maintained at the same potential as the first lower ground25.

The signal wire12, the first ground13, the second ground14, the via conductor16, and the side-surface metal layer17are manufactured using the same material and method as those in the manufacturing process of the wiring board1of the first embodiment.

In addition, the first lower ground25of the wiring board2according to the second embodiment is made by metalizing metal powder containing tungsten (W), molybdenum (Mo), manganese (Mn), silver (Ag), copper (Cu), or the like as a component. The first lower ground25is formed by printing and applying a metalized paste for the first lower ground25on a ceramic green sheet by a printing method such as a screen printing method, and firing the paste together with a powder compact formed by layering more than one ceramic green sheet, for example.

In addition, according to the second embodiment, the same operational effects as those of the first embodiment described above are obtained.

Another Aspect of Second Embodiment

Another aspect of the second embodiment will be described with reference toFIG.10.FIG.10is a schematic partial side view of a wiring board according to another aspect of the second embodiment as viewed from a second direction.

As in the example illustrated inFIG.10, in a wiring board2A according to another aspect of the second embodiment, the side-surface metal layer17may be divided into a first side-surface metal layer17ulocated on a first surface11uside (upper side) of the third surface11sof the insulation substrate11and a second side-surface metal layer17dlocated on a second surface11dside (lower side) of the third surface11sof the insulation substrate11. A division point of the side-surface metal layer17is a tangent point or a tangent between the first lower ground25and the side-surface metal layer17. The second side-surface metal layer17dmay be formed in, for example, a netlike shape or a perforated shape, and may have a plurality of openings17a.An opening area fraction of the second side-surface metal layer17dmay be, for example, 80% or less. When the side-surface metal layer17has a plurality of openings17a,it is possible to reduce creeping up of the bonding material7to the side-surface metal layer17and the second ground14in bonding of the insulation substrate11to the mounting substrate6. Therefore, according to another aspect of the second embodiment, when the insulation substrate11is bonded to the mounting substrate6, a difference in thermal expansion between both ends, in the second direction, of the insulation substrate11can be reduced to reduce an occurrence of cracking, chipping, or the like of the insulation substrate11. In addition, according to another aspect of the second embodiment, when the lower ground15is bonded to the mounting substrate6, it is possible to reduce floating of one side, in the second direction, of the insulation substrate11. According to another aspect of the second embodiment, when the electronic device100(wiring board2A) is used for, for example, an optical transceiver, it is possible to reduce an inclination or the like of the electronic device100with respect to an installation position of the electronic device100, improving the reliability of the optical transceiver.

In addition, according to another aspect of the second embodiment, the same operational effects as those of the second embodiment described above are obtained.

Third Embodiment

A third embodiment will be described with reference toFIGS.11and12.FIG.11is a schematic cross-sectional view of a wiring board according to a third embodiment.FIG.12is a schematic cross-sectional view of a wiring board according to another aspect of the third embodiment.

As in the examples illustrated inFIGS.11and12, a wiring board3according to the third embodiment has the same configuration as the wiring board2according to the second embodiment except for a part of the configuration. Among the configurations of the wiring board3, a configuration different from that of the wiring board2will be described. For convenience of description, a member having the same function as a member described in the second embodiment is denoted by the same reference sign.

As illustrated inFIGS.11and12, the wiring board3includes a first lower ground25and a second lower ground35, as the lower ground. The second lower ground35is an example of the lower ground. In the wiring board3, in addition to the first lower ground25being located inside the insulation substrate11, the second lower ground35may be located on the second surface11dof the insulation substrate11. A part of the insulation substrate11is interposed between the signal wire12and the first lower ground25, and a part of the insulation substrate11is interposed between the first lower ground25and the second lower ground35. The second lower ground35may face the signal wire12, the first ground13, and the second ground14with the insulation substrate11interposed between the second lower ground35and the signal wire12. In other words, the second lower ground35may overlap the signal wire12, the first ground13, the second ground14, and the first lower ground25when the wiring board1is viewed in a plane perspective. The second lower ground35and the first lower ground25may be insulated therebetween by the insulation substrate11. Therefore, according to the third embodiment, it is possible to strengthen the ground of the wiring board3, further reducing the transmission loss of the signal transmitted by the signal wire12.

As in the example illustrated inFIG.11, each via conductor16may penetrate through the insulation substrate11along the third direction. In this case, each via conductor16may be connected to the first lower ground25, and the lower end portion of each via conductor16may be connected to the second lower ground35. In addition, as illustrated inFIG.12, each via conductor16need not penetrate through the insulation substrate11along the third direction. In this case, each via conductor16may be connected to the first lower ground25, and the lower end portion of each via conductor16need not be connected to the second lower ground35.

The side-surface metal layer17may be located on the third surface11sof the insulation substrate11. The side-surface metal layer17may electrically connect the second ground14, the first lower ground25, and the second lower ground35. A top edge portion of the side-surface metal layer17is connected to an edge portion (an edge portion closer to the third surface11s) of the second ground14. As in the example illustrated inFIGS.11and12, the side-surface metal layer17may be connected to the edge portion of the first lower ground25exposed on the third surface11s.A bottom edge portion of the side-surface metal layer17may be connected to an edge portion (an edge portion closer to the third surface11s) of the second lower ground35. The bottom edge portion of the side-surface metal layer17may overlap the edge portion of the second lower ground35.

The wettability of the third surface11sof the insulation substrate11with respect to the bonding material7that bonds the insulation substrate11to the mounting substrate6may be lower than the wettability of the second surface11dof the insulation substrate11with respect to the bonding material7. In this case, when the insulation substrate11is bonded to the mounting substrate6, it is possible to reduce creeping up of the bonding material7to the side-surface metal layer17and the second ground14. Therefore, when the insulation substrate11is bonded to the mounting substrate6, a difference in thermal expansion between both ends, in the second direction, of the insulation substrate11can be reduced to reduce an occurrence of cracking, chipping, or the like of the insulation substrate11. In addition, when the insulation substrate11is bonded to the mounting substrate6, it is possible to reduce floating of one side, in the second direction, of the insulation substrate11. When the electronic device100(wiring board3) is used for, for example, an optical transceiver, it is possible to reduce an inclination or the like of the electronic device100with respect to an installation position of the electronic device100, improving the reliability of the optical transceiver. In addition, when the side-surface metal layer17has the same configuration as the side-surface metal layer17of the wiring board1A according to another aspect of the first embodiment, the same operational effects as those of the another aspect of the first embodiment are obtained. In the third embodiment of the present disclosure, the wettability of the second surface11dof the insulation substrate11with respect to the bonding material7refers to the ease of wetting and spreading of the bonding material7on the second lower ground35located on the second surface11dof the insulation substrate11. In addition, the side-surface metal layer17may have the same configuration as the side-surface metal layer17of the wiring board1A according to another aspect of the first embodiment.

Here, the second lower ground35is a metal layer containing titanium (Ti), platinum (Pt), gold (Au), or the like as a component, and is formed on the second surface (lower surface) of the sintered body whose surface has been polished, by a thin film forming method such as a vapor deposition method, an ion-plating method, or a sputtering method.

In addition, according to the third embodiment, the same operational effects as those of the second embodiment described above are obtained.

Another Aspect of Third Embodiment

Another aspect of the third embodiment will be described with reference toFIG.13toFIG.15.FIG.13is a schematic cross-sectional view of a wiring board according to another aspect of the third embodiment, corresponding toFIG.5.FIG.14is a schematic cross-sectional view of a wiring board according to another aspect of the third embodiment.FIG.15is a schematic enlarged side view of the wiring board illustrated inFIG.13.

As in the examples illustrated inFIGS.13to15, a wiring board3A according to another aspect of the third embodiment has the same configuration as the wiring board3according to the third embodiment except for a part of the configuration. Among the configurations of the wiring board3A according to another aspect of the third embodiment, a configuration different from that of the wiring board3according to the third embodiment will be described. For convenience of description, a member having the same function as a member described in the third embodiment is denoted by the same reference sign.

In the examples illustrated inFIGS.13to15, the side-surface metal layer17may be located only on an upper portion of the third surface11sof the insulation substrate11. A top edge portion of the side-surface metal layer17may be connected to an edge portion (an edge portion closer to the third surface11s) of the second ground14. The top edge portion of the side-surface metal layer17may overlap the edge portion of the second ground14. As in the examples illustrated inFIGS.13to15, the side-surface metal layer17may be connected to the edge portion of the first lower ground25exposed on the third surface11s.In addition, as in the examples illustrated inFIGS.13to15, a lower end portion of the side-surface metal layer17may be separated from an edge portion (an edge portion closer to the third surface11s) of the second lower ground35. The insulation substrate11may be exposed at a part (lower part) of the third surface11s.

As in the example illustrated inFIG.13, each via conductor16may penetrate through the insulation substrate11along the third direction. In addition, as in the example illustrated inFIG.14, each via conductor16need not penetrate through the insulation substrate11along the third direction.

When the lower end portion of the side-surface metal layer17is separated from the edge portion of the second lower ground35and the insulation substrate11is exposed from a part of the third surface11s,it is possible to reduce creeping up of the bonding material7to the side-surface metal layer17and the second ground14when the insulation substrate11is bonded to the mounting substrate6because the wettability of the insulation substrate11is low. Therefore, according to another aspect of the third embodiment, when the insulation substrate11is bonded to the mounting substrate6, a difference in thermal expansion between both ends, in the second direction, of the insulation substrate11can be reduced to reduce an occurrence of cracking, chipping, or the like of the insulation substrate11. In addition, according to the present variation, when the insulation substrate11is bonded to the mounting substrate6, it is possible to reduce floating of one side, in the second direction, of the insulation substrate11. According to another aspect of the third embodiment of the present disclosure, when the electronic device100(wiring board3A) is used for, for example, an optical transceiver, it is possible to reduce an inclination or the like of the electronic device100with respect to an installation position of the electronic device100, improving the reliability of the optical transceiver.

In addition, according to another aspect of the third embodiment, the same operational effects as those of the third embodiment described above are obtained.

The invention according to the present disclosure has been described above based on the drawings and the examples. However, the invention according to the present disclosure is not limited to the above-described embodiments. That is, the invention according to the present disclosure may be varied in diverse manners within the scope described in the present disclosure, and an embodiment obtained by appropriately combining technical means disclosed in different embodiments is also included in the technical scope of the invention according to the present disclosure. That is, note that one skilled in the art can easily make various variations or modifications based on the present disclosure. Also note that these variations or modifications are included in the scope of the present disclosure.

REFERENCE SIGNS

100Electronic device1Wiring board (wiring board according to first embodiment of the present disclosure)5Electronic component6Mounting substrate7Bonding material11Insulation substrate11uFirst surface11dSecond surface11sThird surface11cCenter position12Signal wire13First ground14Second ground15Lower ground16Via conductor17Side-surface metal layer17aOpening1A wiring board (wiring board according to another aspect of the first embodiment of the present disclosure)17uFirst side-surface metal layer17dSecond side-surface metal layer2Wiring board (wiring board according to second embodiment of the present disclosure)25First lower ground3Wiring board (wiring board according to third embodiment of the present disclosure)35Second lower ground3A Wiring board (wiring board according to another aspect of third embodiment of the present disclosure)