WIRING BOARD

A wiring board of the present invention includes a build-up layer having a plurality of insulating layers laminated one upon another, a groove formed on a major surface of each of the insulating layers, and a wiring conductor formed in the groove. A surface of the wiring conductor lies lower than the major surface of each of the insulating layers which is formed in the wiring conductor in the groove.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wiring board having high-density fine wiring.

2. Description of the Related Art

FIG. 3shows a schematic sectional view of a conventional wiring board C having high-density fine wiring. The wiring board C is obtained by laminating a build-up portion42on upper and lower surfaces of a core substrate41. The core substrate41includes a core insulating plate43and a core wiring conductor44. The build-up portion42is formed of build-up insulating layers45ato45d, a build-up wiring conductor46, and a protective solder resist layer47. At the center of the upper surface of the wiring board C, there is formed amounting portion48on which a semiconductor element is mounted.

The core insulating plate43has a plurality of through holes49penetrating from an upper surface to a lower surface of the insulating plate43. The core wiring conductor44is deposited on the upper and lower surfaces of the insulating plate43and in the through holes49.

The build-up insulating layers45ato45dare laminated in pairs on both surfaces of the core substrate41. Each of the insulating layers45ato45dalso has a plurality of via holes50penetrating from an upper surface to a lower surface thereof. Each of the insulating layers45ato45dhas a plurality of grooves51in a surface thereof. The build-up wiring conductor46is deposited in the via holes50and the grooves51in each of the insulating layers45ato45d. The wiring conductor46is filled with the grooves51so as to be flush with the surface of each of the insulating layers45ato45d.

A part of the wiring conductor46deposited on the surface of the insulating layer45bthat is the outermost layer on an upper surface side forms a semiconductor element connection pad52to be connected to a semiconductor element. Apart of the wiring conductor46deposited on the surface of the insulating layer45dthat is the outermost layer on a lower surface side forms an external connection pad53to be connected to an external circuit board.

The solder resist layer47is formed on the surface of each of the outermost insulating layers45band45d. The solder resist layer47on the upper surface side has an opening47ato expose the semiconductor element connection pad52. The solder resist layer47on the lower surface side has an opening47bto expose the external connection pad53.

By connecting an electrode of the semiconductor element to the semiconductor element connection pad52, and connecting the external connection pad53to the wiring conductor of the external electric circuit board, the semiconductor element is electrically connected to the external electric circuit board. This conventional wiring board is described in, for example, Japanese Unexamined Patent Publication No. 2006-41029.

Meanwhile, with the advance of downsizing and higher function of electronic devices represented by portable communication devices and music players, there is also a demand for downsizing and higher function of wiring boards mounted on these electronic devices. Hence, in the build-up wiring conductors on the wiring boards, fine wiring conductors whose width and intervals are respectively, for example, 5 μm or less are to be formed at high density.

However, in the conventional wiring board C, the build-up wiring conductor46is formed so as to be flush with the surface of each of the insulating layers45ato45d. Consequently, the surface of the wiring conductor46has a height identical with that of an interface between the insulating layer45aor45cand the insulating layer45bor45d, and an interface between the insulating layer45bor45dand the solder resist layer47. These interfaces are physically and chemically weak, thus making it easier for metal ions to move along these interfaces. This leads to the problem that electrical insulation reliability between the wiring conductors adjacent to each other may be poor particularly in the fine high-density wiring as described above.

SUMMARY OF THE INVENTION

An embodiment of the present invention has an object to provide a high-density wiring board having excellent insulation reliability.

The wiring board according to the embodiment of the present invention includes a build-up layer having a plurality of insulating layers laminated one upon another, a groove formed on a major surface of each of the insulating layers, and a wiring conductor formed in the groove. A surface of the wiring conductor lies lower than the major surface of each of the insulating layers which is formed in the wiring conductor in the groove.

With the wiring board according to the embodiment of the present invention, the wiring conductor is formed in the groove formed on the major surface of each of the insulating layers. Further, the surface of the wiring conductor formed in the groove lies at the location lower than the major surface of each of the insulating layers in which the wiring conductor is partially formed. Even though spacing between the wiring conductors adjacent to each other remains unchanged, it is possible to increase an interface distance that connects the wiring conductors along the interface between the insulating layers. This ensures insulation properties between the wiring conductors adjacent to each other. Furthermore, the interface is formed of a horizontal interface and a vertical interface, and metal ion movement is effectively inhibited by the vertical interface. It is consequently possible to provide the wiring board having high-density wiring with excellent insulation reliability.

DESCRIPTION OF THE EMBODIMENTS

A wiring board according to one embodiment is described with reference toFIG. 1. The wiring board A shown inFIG. 1is obtained by laminating a build-up portion2on upper and lower surfaces of a core substrate1. The core substrate1includes a core insulating plate3and a core wiring conductor4. The build-up portion2is formed of build-up insulating layers5ato5d, a build-up wiring conductor6, and a protective solder resist layer7. At the center of the upper surface of the wiring board A, there is formed a mounting portion8on which a semiconductor element is mounted.

The core insulating plate3has a plurality of through holes9penetrating from an upper surface to a lower surface thereof. The core wiring conductor4is deposited on the upper and lower surfaces of the insulating plate3and in the through holes9. The wiring conductor4in the through holes9establishes continuity between the wiring conductors4formed on the upper and lower surfaces of the insulating plate3. The insulating plate3is formed of an insulating material obtained by, for example, impregnating a glass cloth with epoxy resin, bismaleimide triazine resin, or the like, followed by thermosetting. The through holes9are formed by, for example, drilling, laser processing, or blast processing.

The build-up insulating layers5ato5dare laminated in pairs on both surfaces of the core substrate1. Each of the insulating layers5ato5dalso has a plurality of via holes10penetrating from an upper surface to a lower surface thereof. Each of the insulating layers5ato5dhas a groove11on a surface thereof that is the side opposite the core substrate1. The build-up wiring conductor6is formed in the via holes10and the groove11. The wiring conductor6in the via holes10establishes continuity between the wiring conductors6located above and below with the insulating layers5ato5dinterposed therebetween, or between the wiring conductor6and the wiring conductor4.

The insulating layers5ato5dare formed of an insulating material obtained by thermosetting, for example, bismaleimidetriazine resin, polyimide resin, or the like. The via holes10or the groove11is formed by, for example, laser processing.

The wiring conductor6is formed in the via holes10and the groove11of the insulating layers5ato5das described above. A part of the wiring conductor6deposited on the surface of the outermost insulating layer5bon the upper surface side functions as a semiconductor element connection pad12to be connected to the semiconductor element. A part of the wiring conductor6deposited on the surface of the outermost insulating layer5don the lower surface side functions as an external connection pad13to be connected to an external circuit board. The wiring conductors4and6are formed of a satisfactorily conductive metal, such as copper foil and copper plating, and are formed by well-known subtractive method, semi-additive method, or the like.

The solder resist layer7is formed on the surface of each of the outermost insulating layers5band5d. The solder resist layer7is formed of a thermosetting resin, such as polyimide resin. The solder resist layer7on the upper surface side has an opening7ato expose the semiconductor element connection pad12. The solder resist layer7on the lower surface side has an opening7bto expose the external connection pad13.

By connecting an electrode of the semiconductor element to the semiconductor element connection pad12, and connecting the external connection pad13to the wiring conductor of the external electric circuit board, the semiconductor element is electrically connected to the external electric circuit board.

In the wiring board A shown inFIG. 1, a surface of the wiring conductor6in the via holes10and the groove11of each of the insulating layers5ato5dis located closer to the core substrate1than an interface between the laminated insulating layers5aand5b, an interface between the insulating layers5cand5d, an interface between the insulating layer5band the solder resist layer7, and an interface between the insulating layer5dand the solder resist layer7. That is, the surface of the wiring conductor6is not flush with the major surface of each of the insulating layers5ato5din which the wiring conductor6is partially formed, and lies lower than the major surface of each of the insulating layers5ato5d. Depending on a thickness of the insulating layers5ato5dand spacing between the wiring conductors6, the surface of the wiring conductor6preferably lies approximately 0.5-5 μm lower than the major surface of each of the insulating layers5ato5d.

For example, the surface of the wiring conductor6lies closer to the core substrate1by approximately 2 μm from a boundary surface.

That is, for example, when spacing between the wiring conductors6disposed adjacent to each other is 3 μm, an interface distance that connects the wiring conductors6adjacent to each other along an interface having weak insulating properties can be increased up to approximately 7 μm by lowering the surface of the wiring conductor6by approximately 2 μm. This ensures insulating properties between the wiring conductors6adjacent to each other. Furthermore, the interface is formed of a horizontal interface and a vertical interface, and metal ion movement is effectively prevented by the vertical interface. It is consequently possible to provide the wiring board having high-density wiring with excellent insulation reliability.

No particular limitation is imposed on a method with which the surface of the wiring conductor6in the via holes10and the groove11of each of the insulating layers5ato5dis located closer to the core substrate1than the interface between the laminated insulating layers5aand5b, the interface between the insulating layers5cand5d, the interface between the insulating layer5band the solder resist layer7, and the interface between the insulating layer5dand the solder resist layer7. For example, an employable method includes forming a satisfactorily conductive metal, such as copper plating, into the via holes10and the groove11, and then reducing the thickness of the conductive metal by etching.

The present invention is not limited to the one embodiment as described above, and various modifications are possible as long as they are within the scope of the claims. For example, with the above embodiment, each of the insulating layers5ato5dhas a single-layer structure. Alternatively, each of the insulating layers5ato5dmay have a two-layer structure as shown inFIG. 2. In this case, a portion of each of the insulating layers which forms the groove11is made into a layer containing less or no inorganic filler, and a portion of each of the insulating layers which does not form the groove11is made into a layer containing a large amount of the inorganic filler.

Owing to the two-layer structure, the layer in which the fine high-density wiring conductor6is disposed in the groove11can be made into a layer that is less apt to cause a gap between the inorganic filler and an insulating resin, thus having more excellent insulation properties. With the layer including no groove11, physical property values, such as coefficient of thermal expansion, is controllable by adjusting the kind and amount of the inorganic filler. Thus, the wiring board having two advantages is obtained by employing the two-layer structure.

The foregoing embodiment exemplifies the wiring board A including the core substrate1. The foregoing embodiment maybe applied to a coreless substrate without the core substrate1.