Wiring board, electronic device, and electronic module

A wiring board (1) includes an insulating substrate (11) having a cutout (12) opened in a main surface and a side surface of the insulating substrate (11), and an inner electrode (13) formed on an inner surface of the cutout (12). The inner electrode (13) includes a plurality of metal layers. The inner electrode (13) includes, as an intermediate layer, at least one metal layer (17b) selected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as an outermost layer (17a). The metal layer (17b) is exposed at an outer edge portion of the inner electrode (13).

FIELD

The present invention relates to a wiring board, an electronic device, and an electronic module.

BACKGROUND

Wiring boards known in the art may have wiring conductors arranged either inside or on the surface of an insulating substrate, cutouts between side surfaces and the lower surface of the insulating substrate, and inner electrodes arranged on the inner surfaces of the cutouts and connected to the wiring conductors. When an electronic device including an electronic component and a wiring board is joined to, for example, a module substrate by soldering, the inner electrodes are joined to the module substrate with solder (refer to Japanese Unexamined Patent Application Publication No. 2002-158509).

BRIEF SUMMARY

Technical Problem

Wiring boards nowadays have higher circuit densities and use thin film deposition to form wiring conductors and other parts on the surface of an insulating substrate. However, inner electrodes formed by thin film deposition on the inner surfaces of cutouts can have lower adhesion than wiring conductors arranged on the surface of the insulating substrate. In joining the inner electrodes of the wiring board to the connection pads of the module substrate by soldering, the solder can be applied onto the outer edge portions of the inner electrodes. Such solder can transfer stress caused by the difference in thermal expansion between the wiring board and the module substrate to the outer edge portions of the inner electrodes. The inner electrodes can then come off the insulating substrate.

Solution to Problem

A wiring board according to a first aspect of the present invention includes an insulating substrate having a cutout opened in a main surface and a side surface of the insulating substrate, and an inner electrode arranged on an inner surface of the cutout. The inner electrode includes a plurality of metal layers. The inner electrode includes, as an intermediate layer, at least one metal layer selected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as an outermost layer. The at least one metal layer is exposed at an outer edge portion of the inner electrode.

An electronic device according to a second aspect of the present invention includes the wiring board with the above structure, and an electronic component mounted on the wiring board and electrically connected to the inner electrode.

An electronic module according to a third aspect of the present invention includes a module substrate including a connection pad on a main surface of the module substrate, and the electronic device having the above structure and including the inner electrode connected to the connection pad with solder.

Advantageous Effects

The wiring board according to the first aspect of the present invention includes an insulating substrate having a cutout opened in a main surface and a side surface of the insulating substrate, and an inner electrode arranged on an inner surface of the cutout. The inner electrode includes a plurality of metal layers. The inner electrode includes, as an intermediate layer, at least one metal layer selected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as an outermost layer. The at least one metal layer is exposed at an outer edge portion of the inner electrode. This structure prevents solder from being applied onto the outer edge portion of the inner electrode. In joining the inner electrode of the wiring board to the module substrate with solder, this structure prevents the solder from transferring stress caused by the difference in thermal expansion between the wiring board and the module substrate to the outer edge portion of the inner electrode. This reduces the possibility of the inner electrode coming off the insulating substrate. The wiring board can thus be small and have high circuit density and have highly reliable electrical connection to the module substrate over a long period of time.

The electronic device according to the second aspect of the present invention includes the wiring board with the above structure, and an electronic component mounted on the wiring board and electrically connected to the inner electrode. This electronic device has high electrical reliability.

The electronic module according to the third aspect of the present invention includes a module substrate including a connection pad on a main surface of the module substrate, and the electronic device having the above structure and including the inner electrode connected to the connection pad with solder. This electronic module has highly reliable electrical connection between the wiring board and the module substrate over a long period of time.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

First Embodiment

An electronic device according to a first embodiment of the present invention includes a wiring board1and an electronic component2mounted on the upper surface of the wiring board1as shown inFIGS. 1A to 4BandFIG. 7. As in the example shown inFIG. 7, the electronic device is joined to a module substrate5with solder6to form an electronic module.

The wiring board1includes an insulating substrate11having cutouts12opened in its main surface and side surfaces, and inner electrodes13each including multiple metal layers formed on the inner surfaces of the cutouts12. Each inner electrode13includes, as an intermediate layer, at least one metal layer17bselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as an outermost layer17a. The metal layer17bis exposed at an outer edge portion of the corresponding inner electrode. InFIGS. 1A to 4BandFIG. 7, the electronic device is mounted along an x-y plane in a virtual xyz space. InFIGS. 1A to 4BandFIG. 7, the upward direction refers to a positive direction of a virtual z-axis. The upward and downward directions herein are for descriptive purposes only and do not define the directions in actual use of the wiring board1or other parts.

The insulating substrate11, which includes either a single insulator layer11aor a plurality of insulator layers11a, has its upper surface defining a mounting area for the electronic component2. The insulating substrate11is a rectangular plate when viewed from above, or viewed from above in a direction perpendicular to its upper surface. The insulating substrate11functions as a support for the electronic component2. The insulating substrate11has the central mounting area on its upper surface, to which the electronic component2is adhered and fixed with a bonding member, such as a low-melting point brazing filler metal or a conductive resin.

The insulating substrate11may be, for example, a ceramic such as sintered aluminum oxide (alumina ceramic), sintered aluminum nitride, sintered mullite, or sintered glass ceramic.

For the insulating substrate11formed from, for example, sintered aluminum oxide, the powders of raw materials such as aluminum oxide, silicon oxide, magnesium oxide, and calcium oxide are mixed with an appropriate organic binder and a solvent to form slurry. The slurry is then shaped into a sheet using a doctor blade or by calendering to obtain a ceramic green sheet. The ceramic green sheet then undergoes punching. Where necessary, multiple ceramic green sheets prepared in this manner are laminated on one another as appropriate. The laminated sheets are then fired at high temperature (about 1600° C.) to complete the insulating substrate11. The cutouts12open in the main surface and the side surfaces of the insulating substrate11. In the examples shown inFIGS. 1A to 4BandFIG. 7, each cutout12opens across two surfaces, namely a lower main surface (lower surface) and one side surface of the insulating substrate11. Each cutout12may open across three surfaces, namely an upper main surface (upper surface), the lower main surface (lower surface), and the side surface of the insulating substrate11. In the examples shown inFIGS. 1A to 3, each cutout12is semielliptical as viewed from above, and has the shape of a partial ellipsoid and thus has a curved inner surface. Each cutout12may have the shape of a partial hemisphere, which is a semicircle or a semiellipse, as viewed from above.

These cutouts12are each formed by forming a hole to be the cutout12in the insulating substrate11by, for example, blasting. The inner surface of each cutout12is curved. Each cutout12may have the shape of a partial column or a partial square frustum that is a rectangle with rounded corners as viewed from above. The cutout12may also be semicircular, half elliptical, or semielliptical as viewed from above. The cutout12may also have the shape of a partial column or a partial frustum combining a plurality of cutouts12with different size profiles. These cutouts12are prepared by forming through-holes to be the cutouts12through some of the ceramic green sheets for the insulating substrate11by laser processing or by punching using a die.

The inner electrode13is formed on the inner surface of each cutout12, and wiring conductors14are formed inside and on the surface of the insulating substrate11. In the examples shown inFIGS. 1A to 3, the inner electrode13extends across the entire inner surface of the cutout12. As in the examples shown inFIGS. 1A to 3, the main surface opening the cutouts12further has main surface electrodes15, which are connected to the inner electrodes13. These inner electrodes13and the main surface electrodes15form external electrodes. The wiring conductors14and the main surface electrodes15are connected at the lower surface of the insulating substrate11. The inner electrodes13and the wiring conductors14are electrically connected through the main surface electrodes15.

The external electrodes including the inner electrodes13and the main surface electrodes15are used to join the wiring board1to the module substrate5. The inner electrodes13, the wiring conductors14, and the main surface electrodes15electrically connect the module substrate5to the electronic component2that is mounted on the wiring board1. The wiring conductors14include wiring conductors arranged inside or on the surface of the insulating substrate11, and feedthrough conductors extending through the insulator layers11aof the insulating substrate11to electrically connect the upper and lower wiring conductors.

The inner electrodes13or the main surface electrodes15each include multiple metal layers, namely, a thin-film layer16and a plating layer17. The thin-film layer16includes, for example, an adhesive metal layer and a barrier layer. The adhesive metal layer in the thin-film layer16extends on the main surface of the insulating substrate11and the inner surface of each cutout12. The adhesive metal layer contains, for example, tantalum nitride, nickel-chromium, nickel-chromium-silicon, tungsten-silicon, molybdenum-silicon, tungsten, molybdenum, titanium, or chromium. The adhesive metal layer is deposited on the surface of the insulating substrate11and the inner surface of each cutout12with a thin film deposition technique such as vapor deposition, ion plating, or sputtering. To form the adhesive metal layer by, for example, vacuum deposition, the insulating substrate11is first placed in a film deposition chamber of a vacuum evaporator, and a metal piece, which is to be the adhesive metal layer, is placed on a vapor deposition source in the film deposition chamber. The film deposition chamber is evacuated to create a vacuum (with a pressure of 10−2Pa or less), and the metal piece placed on the vapor deposition source is vaporized by heating. The molecules of the vaporized metal piece are then deposited onto the insulating substrate11to form a thin film metal layer, which is to be the adhesive metal layer. A resist pattern is subsequently formed on the insulating substrate11with the thin film metal layer by photolithography, and the remaining part of the thin film metal layer is removed by etching. This completes the adhesive metal layer. The barrier layer is deposited on the upper surface of the adhesive metal layer. The barrier layer, which has a high joining property and wettability with the adhesive metal layer and the plating layer, allows the adhesive metal layer and the plating layer to be firmly joined together and prevents mutual diffusion between the adhesive metal layer and the plating layer. The barrier layer contains, for example, nickel-chromium, platinum, palladium, nickel, or cobalt. The barrier layer is deposited on the surface of the adhesive metal layer with a thin film deposition technique such as vapor deposition, ion plating, or sputtering.

The adhesive metal layer may have a thickness of about 0.01 to 0.5 μm. An adhesive metal layer with a thickness less than 0.01 μm may not be firmly adhered to the insulating substrate11. An adhesive metal layer with a thickness more than 0.5 μm may easily come off because of its internal stress generated during formation. The barrier layer may have a thickness of about 0.05 to 1 μm. A barrier layer with a thickness less than 0.05 μm may have a defect such as a pinhole, and may degrade its function as a barrier layer. A barrier layer with a thickness more than 1 μm may easily come off because of its internal stress generated during formation.

The plating layer17is deposited on the surface of the thin-film layer16by electroplating or electroless plating. The plating layer17is formed from metals with high corrosion resistance and connectivity with a connection member, such as nickel, copper, gold, or silver. For example, a nickel plating layer with a thickness of about 0.5 to 5 μm and a gold plating layer with a thickness of about 0.1 to 3 μm are deposited on the surface one after the other. This effectively prevents corrosion of the inner electrode13and the main surface electrode15, and strengthens the joining of the inner electrode13and the main surface electrode15to a connection pad51formed on the module substrate5.

An additional layer of metal, such as copper (Cu) or gold (Au), may be placed on the barrier layer to allow intended formation of the plating layer17. This metal layer is formed in the same manner as the thin-film layer16.

The wiring conductors14may be formed from metal materials such as tungsten (W), molybdenum (Mo), manganese (Mn), silver (Ag), or copper (Cu). For an insulating substrate11formed from sintered aluminum oxide, for example, a conductor paste containing powdery refractory metal, such as W, Mo, or Mn, mixed with an appropriate organic binder and a solvent is preliminarily applied in a predetermined pattern by screen printing on a ceramic green sheet, which is to be the insulating substrate11. The metal, together with the ceramic green sheet to be the insulating substrate11, is fired to form the wiring conductors14applied at predetermined positions on the insulating substrate11. To form feedthrough conductors as the wiring conductors14, through-holes are formed in the green sheet by punching using a die or a punch or by laser processing. The through-holes are then filled with conductor paste for the wiring conductors14by printing.

The wiring conductors14each have an exposed surface on which a plating layer17is deposited by electroplating or electroless plating in the same manner as performed on the thin-film layer16of the inner electrode13and the thin-film layer16of the main surface electrode15. The plating layer17is formed from metals with high corrosion resistance and connectivity with a connection member, such as nickel, copper, gold, or silver. For example, a nickel plating layer with a thickness of about 0.5 to 5 μm and a gold plating layer with a thickness of about 0.1 to 3 μm, or a nickel plating layer with a thickness of about 1 to 10 μm and a silver plating layer with a thickness of about 0.1 to 1 μm are deposited on the surface one after the other. This effectively prevents corrosion of the wiring conductor14, and strengthens the adhesion between the wiring conductor14and the electronic component2and the joining of the wiring conductor14to a connection member3such as a bonding wire.

Each inner electrode13includes, as an intermediate layer, at least one metal layer17bselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as an outermost layer. The metal layer17bis exposed at an outer edge portion of the inner electrode13. The outer edge portion of the inner electrode13refers to its outer edge portion adjacent to one side surface of the insulating substrate11. In the examples shown inFIGS. 1A to 4B, the metal layer17bis exposed to a strip-like step part at the outer edge portion of the inner electrode13adjacent to the side surface of the insulating substrate11and along the opening of the cutout12at the side surface of the insulating substrate11. The metal layer17bis shaded inFIGS. 1B and 2B. When the exposed part of the metal layer17bat the outer edge portion of the inner electrode13has a width W in a range of about 0.01 to 0.2 mm inclusive as viewed from above, the wiring board1can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time. The wettability of the solder6on the outermost layer17a(metal layer), which is a gold layer, and on the metal layer17bselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer can be determined by applying a small amount of solder6to the outermost layer17a, which is a gold layer, and to the metal layer17bselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and determining the difference in the spreading of the solder6after a reflow process performed at about 200° C. In this case, the surface roughness Ra1of the outermost layer17aand the surface roughness Ra2of the metal layer17bmay be equivalent to each other, or one roughness may at least fall within ±30% of the other roughness.

The metal layer17b, which is formed from a metal with low wetting of the solder6, such as nickel (Ni), chromium (Cr), platinum (Pt), or titanium (Ti), is completed by removing a strip part of the outermost layer17a, which is formed from a metal with high wetting of the solder6, such as gold (Au), at the outer edge portion of the inner electrode13adjacent to the side surface of the insulating substrate11. For example, the intermediate metal layer17bmay be exposed by applying a laser beam to the outer edge portion of the inner electrode13including the thin-film layer16and the plating layer17adjacent to the side surface of the insulating substrate11and thus removing a strip part from the outermost layer17a. For example, when the plating layer17of the inner electrode13includes two layers, namely an Ni plating layer and an Au plating layer, the Ni plating layer, which is the intermediate metal layer17b, is exposed by applying a laser beam to the outer edge portion of the inner electrode13adjacent to the side surface of the insulating substrate11and thus removing a strip part from the Au plating layer, which is the outermost layer17a. In other words, the Ni layer, which has lower wetting of the solder6than the Au layer, is exposed on the surface to form the metal layer17bat the outer edge portion of the inner electrode13and also to allow the inner surface of the inner electrode13and the main surface electrode15to have higher wetting of the solder6.

As in the example shown inFIGS. 4A and 4B, a part of the metal layer17bmay also be removed in the thickness direction together with a part of the outermost layer17aat the outer edge portion of the inner electrode13adjacent to the side surface of the insulating substrate11. In this case, the part of the outermost layer17ais removed at the outer edge portion of the inner electrode13to expose the metal layer17bin a more reliable manner.

The plating layer17may not include the Ni/Au plating layers, but may be, for example, any other plating layer including Cu/Ni/Au plating layers or Ni/Pd/Au plating layers. When the plating layer includes three or more layers, strip parts may be removed from a plurality of metal layers near the surface including the outermost layer17aat the outer edge portion of the inner electrode13to expose the intermediate metal layer17b.

The solder6is formed from an alloy such as tin-copper (Sn—Cu), tin-silver-copper (Sn—Ag—Cu), or gold-tin (Au—Sn). For example, the solder6may be any alloy having high wettability on the outermost layer17a, which is formed from a metal such as gold (Au), including nickel (Ni), chromium (Cr), platinum (Pt), and titanium (Ti), and having lower wettability on the metal layer17b(lower wetting of the solder6) than the outermost layer17a.Such solder6connects the electronic device to the module substrate5.

In this manner, a laser beam is applied to parts of the wiring board1to prevent heat from transferring to the inner electrodes13, the wiring conductors14, and the main surface electrodes15, and to prevent the conductors from being altered by such heat transfer. The resultant wiring board1can strengthen the adhesion between the wiring conductors14and the electronic component2, the joining between the wiring conductors14and the connection member3such as a bonding wire, and the connection of the inner electrodes13and the main surface electrodes15to the connection pads51on the module substrate5.

Although the metal layer17bexposed at the outer edge portion of the inner electrode13may be spaced from the opening of the cutout12adjacent to the side surface of the insulating substrate11, the metal layer17bthat extends along the opening of the cutout12allows the outer edge portion of the inner electrode13to have no part with high wetting of the solder6and thus allows the inner electrode13to have a large inner space. The resultant wiring board1can have the inner electrodes13and the main surface electrodes15firmly joined to the connection pads51on the module substrate5.

The wiring board1includes the insulating substrate11having the cutouts12opened each in two surfaces, namely the main surface and the side surface, and the inner electrode13including multiple metal layers formed on the inner surface of each cutout12. Each inner electrode13includes, as the intermediate layer, at least one metal layer17bselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and includes a gold layer as the outermost layer17a. The metal layer17bis exposed at the outer edge portion of the inner electrode13. This structure prevents the solder6from being applied onto the outer edge portion of the inner electrode13. This prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and the module substrate5to the outer edge portion of the inner electrode13, and reduces the possibility of the inner electrode13coming off the insulating substrate11. The resultant wiring board1can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time.

In the examples shown inFIGS. 1A to 4B, the wiring conductors14and the main surface electrodes15are connected to each other at the main surface (lower surface) of the insulating substrate11. In the example shown inFIG. 5B, the wiring conductor14is connected to the inner electrode13at the inner surface of the cutout12opposite to the main surface (lower surface) of the insulating substrate11, and the wiring conductor14and the main surface electrode15are connected to each other with the inner electrode13. In the example shown inFIG. 5A, the inner electrode13and the wiring conductor14are connected to each other at the inner surface of the cutout12, and the wiring conductor14and the main surface electrode15are connected to each other with the inner electrode13extending from the inner surface of the cutout12directly at the main surface (lower surface) of the insulating substrate11. The wiring conductor14connected to the main surface electrode15allows the main surface electrode15to firmly adhere to the main surface of the insulating substrate11. This improves the electrical connection between the module substrate5and the wiring board1on which the electronic component2is mounted, in comparison with the structure including the inner electrode13connected to the wiring conductor14at the inner surface of each cutout12.

The wiring board1according to the first embodiment of the present invention may be manufactured with the method described below.

As in the example shown inFIG. 6A, an insulating mother substrate111is prepared to include multiple insulator layers111ahaving wiring conductors14placed inside as well as on its surface. The insulating mother substrate111includes multiple insulating substrates11arranged integrally, which are to be cut into individual pieces. The insulating mother substrate111has hemispherical recesses112, which are to be cutouts12, opened in the lower main surface. As described above, these recesses112are formed by, for example, blasting. As in the example shown inFIG. 6B, the inner electrode13including the thin-film layer16and the plating layer17is formed on the inner surface of each recess112, which is to be the cutout12in the insulating mother substrate111. The main surface electrode15including the thin-film layer16and the plating layer17is formed on the surface of the insulating mother substrate111. As in the example shown inFIG. 6C, a laser beam is applied to a predetermined area of the inner electrode13on the inner surface of each recess112to remove a strip part from the outermost layer17aalong the part to be the outer edge of the wiring board1. The metal layer17b, which is an intermediate layer, is exposed in the area from which the strip part has been removed. As in the example shown inFIG. 6D, the recesses112are then cut by slicing or another technique to complete the wiring board1having the inner electrodes13each having the metal layer17bis exposed at the outer edge portion of the inner electrode.

As in the example shown inFIG. 6A, each recess112has a width W2that is equal to or greater than the depth H2of the recess112(W2≥H2). This allows easier formation of the inner electrode13on the inner surface of the recess112as well as the metal layer17bexposed at the outer edge portion.

The metal layer17bformed on the inner surface of the recess112and exposed by removing a part of the outermost layer17amay be wider than the width of a blade for cutting the recess112by slicing. More specifically, the part of the metal layer17bto be exposed may have a width that is 110% or more of the blade width. This allows fabrication of the wiring board1including the inner electrodes13each having the metal layer17bexposed like a strip at the outer edge portion of the inner electrode13along the opening edge.

Multiple rows of metal layers17bmay be exposed like strips at the inner electrode13formed on the inner surface of each recess112. The recesses112may then be cut to complete the wiring board1including the inner electrodes13each having the metal layers17bexposed at the outer edge portion of the inner electrode13.

The above manufacturing method yields the wiring board1having good electrical connection to the electronic component2and to the module substrate5with high productivity.

The metal layer17bexposed at the outer edge portion of each inner electrode13prevents the solder from spreading to the exposed end of the thin-film layer16of the inner electrode13when the recesses112are cut and the thin-film layer16and the plating layer17of the inner electrode13are exposed at the side surface of the insulating substrate11as in the examples shown inFIGS. 1A to 4B.

In the structure including the inner electrode13connected to the wiring conductor14at the inner surface of the cutout12as in the examples shown inFIGS. 5A and 5B, the wiring conductor14may be formed to overlap the recess112in the insulating mother substrate111as viewed from above, and is exposed in the recess112by blasting or another technique. The inner electrode13may then be formed on the inner surface of the recess112and connected to the wiring conductor14.

After the recess112is cut, a part of the outermost layer17amay be removed by applying a laser beam to the outer edge portion of the inner electrode13formed on the inner surface of each cutout12to expose the intermediate metal layer17b. In this case, a laser beam may be applied to the outer edge portion of the inner electrode13in a direction lateral to the insulating substrate11after the cutting. When the cutout12(recess112) is either small or deep, the inner electrode13having the metal layer17bexposed at the outer edge portion of the inner electrode13is easily formed with high accuracy. The resultant wiring board1can have good electrical connection to a smaller electronic component2and to the module substrate5.

The electronic component2can be mounted on the upper surface of the wiring board1to obtain the electronic device. The electronic component2mounted on the wiring board1is, for example, a semiconductor device such as an integrated circuit (IC) chip or a large-scale integrated circuit (LSI) chip, a light-emitting device, a quartz oscillator, a piezoelectric element such as a piezoelectric vibrator, or one of various sensors. When, for example, the electronic component2is a semiconductor device to be connected by wire bonding, the semiconductor device is fixed to the wiring conductor14using a bonding member such as low-melting point brazing filler metal or a conductive resin, and then mounted on the wiring board1by electrically connecting the electrode of the semiconductor device and the wiring conductor14with the connection member3, such as a bonding wire. When, for example, the electronic component2is a semiconductor device to be connected by flip-chip, the semiconductor device is mounted on the wiring board1by electrically and mechanically connecting the electrode of the semiconductor device and the wiring conductor14with the connection member3, such as solder bumps, gold bumps, or a conductive resin (e.g., anisotropic conductive resin). The wiring board1can have a plurality of electronic components2or a small electronic component such as a resistance element or a capacitive element mounted as appropriate. The electronic component2is sealed with a sealant4, such as resin or glass, or may be sealed with a lid made of resin, glass, ceramic, or metal as appropriate.

As in the example shown inFIG. 7, the electronic device according to the present embodiment is connected to the connection pad51on the module substrate5with the solder6to complete the electronic module. The solder6is joined to the inner electrode13in the cutout12and to the main surface electrode15at the lower surface of the insulating substrate11. The solder6slopes and widens from the inner end of the inner electrode13excluding the metal layer17btoward the outer end of the connection pad51. The metal layer17bprevents the solder6from being applied onto the outer edge portion of the inner electrode13. This structure prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and the module substrate5to the outer edge portion of the inner electrode13, and prevents the inner electrode13from coming off the insulating substrate11. The electronic device is firmly connected to the module substrate5, and thus the resultant electronic module has higher connection reliability.

The wiring board1according to the present embodiment includes the insulating substrate11having the cutout opened12in the main surface and the side surface, and the inner electrode13including multiple metal layers formed on the inner surface of the cutout12. The inner electrode13includes, as the intermediate layer, at least one metal layer17bselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer, and a gold layer as the outermost layer17a, and the metal layer17bis exposed at an outer edge portion of the inner electrode. This structure prevents the solder6from being applied onto the outer edge portion of the inner electrode13. This prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and the module substrate5to the outer edge portion of the inner electrode13, and reduces the possibility of the inner electrode13coming off the insulating substrate11. The resultant wiring board1can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time.

The wiring board1according to the present embodiment may be used in a small-size and high-power electronic device, and improves the electrical connection of the wiring board1. For example, the wiring board1may be used as a small mount board for a light-emitting device, on which a high output light-emitting device as the electronic component2is mounted.

For the structure including the cutout12having a bottom, in comparison with a wiring board1according to a second embodiment (described later), an insulating substrate11with the cutout12may include a single insulator layer11a. This structure reduces the thickness of the wiring board1.

The structure with the cutout12having a curved inner surface allows easy application of a laser beam to the outer edge portion of an inner electrode13formed on the cutout12. This structure allows intended formation of the inner electrode13on the inner surface of the cutout12and the metal layer17bexposed at the outer edge portion of the inner electrode13. In addition, the solder6can easily spread across the entire inner area of the inner electrode13. The resultant wiring board1can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time.

The electronic device according to the present embodiment includes the wiring board1with the above structure, and thus has higher electrical reliability.

The electronic module according to another aspect of the present invention includes the module substrate5having the connection pad51on the main surface, and the electronic device having the above structure and including the inner electrode13connected to the connection pad51with the solder6. This electronic module has highly reliable electrical connection between the wiring board1and the module substrate5over a long period of time.

Second Embodiment

An electronic device according to a second embodiment of the present invention will now be described with reference toFIGS. 8A to 11.

The electronic device according to the second embodiment of the present invention differs from the electronic device according to the first embodiment in that each cutout12is a rectangle with rounded corners as viewed from above, and has a shape extending long along an outer edge of the insulating substrate11and corresponding to a portion obtained by dividing a partial square frustum, and a main surface electrode15is formed on the upper surface of the insulating substrate11as in the examples shown inFIGS. 8A to 11.

The wiring board according to the second embodiment of the present invention prevents solder6from being applied onto the outer edge portion of an inner electrode13in the same manner as in the first embodiment. This prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and a module substrate5to the outer edge portion of the inner electrode13, and reduces the possibility of the inner electrode13coming off the insulating substrate11. The resultant wiring board can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time.

The main surface electrode15formed on the upper surface of the insulating substrate11is used as a wiring for mounting an electronic component2or for connecting a connection member3. The main surface electrode15formed on the upper surface of the insulating substrate11allows the electronic component2to be mounted on the wiring board1with high accuracy. This allows, for example, accurate mounting of a light-emitting device as the electronic component2. The resultant light emitter can thus emit light with high accuracy.

The cutout12may have the shape corresponding to a portion obtained by dividing a partial column. However, the cutout12having the shape corresponding to a portion obtained by dividing a partial square frustum in which an opening width adjacent to the lower surface (main surface) of the insulating substrate11is greater than the bottom width of the cutout12as in the examples shown inFIGS. 8A to 11allows intended formation of the inner electrode13on the inner surface of the cutout12and the metal layer17bexposed at the outer edge portion of the inner electrode13.

As described above, the cutout12in the wiring board1according to the second embodiment is formed by forming a through-hole to be the cutout12through some of the ceramic green sheets for the insulating substrate11with laser processing or punching using a die.

The inner electrode13is formed on the inner-side and bottom faces of the cutout12as in the examples shown inFIGS. 8A to 11. The resultant wiring board1has high reliability of electrical connection to the module substrate5.

The wiring board1according to the second embodiment may be manufactured in the same manner as in the first embodiment except for the method for forming the cutout12.

Third Embodiment

An electronic device according to a third embodiment of the present invention will now be described with reference toFIGS. 12A to 13.

The electronic device according to the third embodiment of the present invention differs from the electronic device according to the first embodiment in having a cavity18in the upper surface of the insulating substrate11as in the examples shown inFIGS. 12A to 13.

The wiring board according to the third embodiment of the present invention prevents solder6from being applied onto the outer edge portion of an inner electrode13in the same manner as in the first embodiment. This prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and a module substrate5to the outer edge portion of the inner electrode13, and reduces the possibility of the inner electrode13coming off the insulating substrate11. The resultant wiring board can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time.

In the wiring board1according to the third embodiment, the cutout12having the shape of a partial hemisphere, which is a semiellipse as viewed from above, may have a depth smaller than a height from the bottom face of the cavity18(depth) as in the examples shown inFIGS. 12A to 13. This prevents the strength of the insulating substrate11from decreasing, and allows intended formation of the cutout12in the lower surface of the insulating substrate11.

As in the examples shown inFIGS. 12A to 13, the cutout12having the shape of a partial hemisphere, which is a semiellipse as viewed from above, may be formed not to overlap the cavity18. This prevents the strength of the insulating substrate11from decreasing, and allows intended formation of the cutout12in the lower surface of the insulating substrate11.

As in the examples shown inFIGS. 12A to 13, the insulating substrate11has the upper surface with the cavity18. The cavity18is formed by forming a through-hole that is to be the cavity18through ceramic green sheets by laser processing or punching using a die, and laminating these ceramic green sheets on other ceramic green sheets having no through-hole. When the insulating substrate11is thin, the ceramic green sheets may be first laminated and then a through-hole for the cavity18may be formed in the laminated sheets by laser processing or punching using a die. The through-hole has high accuracy in this case. As in the examples shown inFIGS. 12A to 13, the width of the cutout12is about 25 to 75% of that of the side wall of the cavity18.

When the cavity18is intended for containing a light-emitting device, the inner-side face and the bottom face of the cavity18form an obtuse angle θ, which may be 110 to 145 degrees. The angle θ set within this range allows stable and efficient formation of the inner surface of a through-hole to be the cavity18by punching. The light emitter including this wiring board1can thus easily be small. This light emitter can also radiate light emitted from its light-emitting device outwardly. The cavity18having an inner surface with this angle θ is formed by, for example, punching a ceramic green sheet using a punching die with a large clearance between the punch diameter and the die hole diameter. More specifically, when the punch is forced through the ceramic green sheet from one main surface to the other main surface with a large clearance between the die hole diameter and the punch diameter of the punching die, the green sheet is sheared from the edge of the punch contact surface toward the edge of the die hole contact surface, and the through-hole diameter widens from one main surface toward the other main surface. Setting the clearance between the punch diameter and the die hole diameter in accordance with, for example, the thickness of the ceramic green sheet adjusts the angle with the inner surface of the through-hole in the ceramic green sheet. This punching method can achieve an intended angle θ between the inner surface of the cavity18and the bottom of the cavity18without an additional process, and thus has high productivity.

The through-hole with an angle θ widening from one main surface toward the other main surface may be formed with a different method. After a through-hole with an angle θ of about 90 degrees is formed using a punching die with a smaller clearance between the punch diameter and the die hole diameter, a die having the shape of a truncated cone or a truncated pyramid is pressed against the inner surface of the through-hole. In this case, the angle θ formed by the inner-side face of the cavity18and the bottom face of the cavity18can be adjusted more accurately.

The wiring board1including the insulating substrate11having the cavity18on its upper surface for containing, for example, a light-emitting device may include a reflective layer on the interior wall of the cavity18for reflecting light emitted from the light-emitting device. The reflective layer includes, for example, a metal conductor layer formed on the interior wall of the cavity18and a plating layer deposited on the metal conductor layer. The metal conductor layer may be formed using the same material and the method as for the inner electrode13and the wiring conductor14or for the main surface electrode15.

When, for example, a light-emitting device is mounted on the wiring board1, a silver plating layer may be deposited on the outermost surface of the metal conductor layer, whereas a gold plating layer may be deposited on the outermost surfaces of the inner electrode13, the wiring conductor14, and the main surface electrode15. The gold plating layer has a higher joining property with the electronic component2, the connection member3, and the solder6than the silver plating layer, which has a higher light reflectance than the gold plating layer. A wiring in an area for receiving the light-emitting device and the outermost surface of the metal conductor layer may be an alloy plating layer of silver and gold, for example, a sliver-gold alloy plating layer of all-proportional solid solution.

The wiring board1according to the third embodiment may be used in a small-size and high-power electronic device as in the first embodiment, and improves the electrical connection of the wiring board1. For example, the wiring board1may be used as a small mount board for a light-emitting device, on which a high output light-emitting device as the electronic component2is mounted.

The wiring board1according to the third embodiment may be manufactured in the same manner as in the first embodiment.

Fourth Embodiment

An electronic device according to a fourth embodiment of the present invention will now be described with reference toFIGS. 14A and 14B.

The electronic device according to the fourth embodiment of the present invention differs from the electronic device according to the first embodiment in that each cutout12is opened in a side surface and the same main surface (upper surface) as the surface on which the electronic component2is mounted as in the example shown inFIGS. 14A and 14B.

The wiring board according to the fourth embodiment of the present invention prevents solder6from being applied onto the outer edge portion of an inner electrode13in the same manner as the wiring board in the first embodiment. This prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and a module substrate5to the outer edge portion of the inner electrode13, and reduces the possibility of the inner electrode13coming off the insulating substrate11. The resultant wiring board can be small and have high circuit density and have highly reliable electrical connection to the module substrate5over a long period of time.

This wiring board1can be joined at its upper surface to the module substrate5with the solder6. The wiring board1can thus have higher heat radiation when a member having a higher thermal conductivity than the insulating substrate11is joined to the entire lower surface of the wiring board1. When the insulating substrate11is formed from sintered aluminum oxide, materials having a higher thermal conductivity than the insulating substrate11include metal materials such as copper (Cu), copper-tungsten (Cu—W), or aluminum (Al), and insulators of sintered aluminum nitride. This wiring board1reduces heat transferred from the electronic component2mounted on the wiring board1to the cutout12. The resultant wiring board1has high heat radiation and reliability of electrical connection to the module substrate5over a long period of time.

The wiring board1according to the fourth embodiment may be used in a small-size and high-power electronic device in the same manner as in the first embodiment, and improves the electrical connection of the wiring board1. For example, the wiring board1may be used as a small mount board for a light-emitting device, on which a high output light-emitting device as the electronic component2is mounted.

The wiring board1according to the fourth embodiment may be manufactured in the same manner as in the second embodiment.

Fifth Embodiment

An electronic device according to a fifth embodiment of the present invention will now be described with reference toFIG. 15.

The electronic device according to the fifth embodiment of the present invention differs from the electronic device according to the first embodiment in that at least one metal layer15aselected from the group consisting of a nickel layer, a chromium layer, a platinum layer, and a titanium layer is exposed at an edge portion of a main surface electrode15as in the example shown inFIG. 15.

The wiring board according to the fifth embodiment of the present invention prevents solder6from being applied onto the edge portion of the main surface electrode15. This prevents the solder6from transferring stress caused by the difference in thermal expansion between the wiring board1and a module substrate5to the edge portion of the main surface electrode15, and reduces the possibility of the main surface electrode15coming off the insulating substrate11. This further prevents a short circuit on the main surface of the insulating substrate11in the structure including main surface electrodes15arranged at narrow intervals. The resultant wiring board can be small and have high circuit density and have highly reliable electrical connection to the module substrate5.

The metal layer15aon the main surface electrode15may be formed with the same method as the method for exposing the metal layer17bon the inner electrode13.

The wiring board1according to the fifth embodiment may be manufactured in the same manner as in the first embodiment.

The present invention is not limited to this embodiment, but may be modified variously. In the examples described above, one cutout12and one inner electrode13are formed on each of the two facing side surfaces of the insulating substrate11. However, the wiring board1may have one cutout12and one inner electrode13on each of all the four side surfaces of the insulating substrate11. The wiring board1may also have multiple cutouts12and inner electrodes13on each side surface. In the examples shown inFIGS. 1A to 15, the insulating substrate11includes two or three insulator layers11a. The insulating substrate11may include a single insulator layer11aor four or more insulating layers11a.

In the examples shown inFIGS. 1A to 15, each cutout12is opened in one of the two main surfaces and the side surface of the insulating substrate11. However, the cutout12may also be opened in the two main surfaces and the side surface of the insulating substrate11.

As in the examples shown inFIGS. 12A to 13, the wiring board1may include conductors other than wiring, or specifically may include an electronic component mount layer19and/or a central terminal layer20. When such conductors each include, for example, a thin-film layer16and a plating layer17, the conductors may be formed using the same material and the method as for the inner electrode13and the main surface electrode15. A conductor formed using the same method as for the wiring conductor14may have a metal plating layer17on its exposed surface. For example, the electronic component mount layer19is used for receiving the electronic component2, whereas the central terminal layer20is used for joining with the module substrate5in the same manner as the inner electrode13and the main surface electrode15. As in the example shown inFIGS. 12A and 12B, the central terminal layer20may also be connected to the inner electrode13on the inner surface of the cutout12.

The wiring board1according to each of the first to fifth embodiments may be flat or may have the cavity18. The wiring board1according to each of the first to fifth embodiments may also include the electronic component mount layer19and/or the central terminal layer20.

In the above example, the single electronic component2is mounted on the wiring board1, but a number of electronic components2may also be mounted on the wiring board1.

The wiring board1may also be fabricated as a mother substrate that is to be cut into multiple wiring boards.

REFERENCE SIGNS LIST