WIRING BOARD

A wiring board according to the present disclosure includes a first insulation layer including a first surface, a land located on the first insulation layer and including a second surface, a second insulation layer located at the first surface of the first insulation layer and including a via hole extending over the second surface of the land, and a via-hole electrical conductor located in the via hole. The land includes a plurality of recessed portions on the second surface, and at least one recessed portion selected from the plurality of recessed portions includes a buffer body containing resin. The via-hole electrical conductor is in contact with the second surface of the land and the buffer body.

TECHNICAL FIELD

The present invention relates to a wiring board.

BACKGROUND OF INVENTION

A wiring board includes a plurality of insulation layers and wiring conductor layers formed on the surface of the insulation layers, as illustrated in Patent Document 1, for example. The insulation layers include via-hole electrical conductors, and via the via-hole electrical conductors, electrical connection between wiring conductor layers located in the different insulation layers, that is, electrical connection in the thickness direction of the wiring board is performed.

Each of the via-hole electrical conductors is usually connected to a land formed on a different insulation layer. For example, at a contact surface between the via-hole electrical conductor and the land, when there is a relatively large difference between the linear expansion coefficient and the Young's modulus of a conductor included in the via-hole electrical conductor and the linear expansion coefficient and the Young's modulus of a conductor included in the land, stress may be generated at the time of thermal expansion and contraction, and the connection reliability between the via-hole electrical conductor and the land may be reduced. For example, at a contact surface between the via-hole electrical conductor and the land, when one is made of copper and the other is made of a metal other than copper (for example, nichrome (NiCr) or the like), the connection reliability between the via-hole electrical conductor and the land is likely to be reduced.

CITATION LIST

Patent Literature

Patent Document 1: JP 2015-216344 A

SUMMARY

Solution to Problem

A wiring board according to the present disclosure includes a first insulation layer including a first surface, a land located on the first insulation layer and including a second surface, a second insulation layer located at the first surface of the first insulation layer and including a via hole extending over the second surface of the land, and a via-hole electrical conductor located in the via hole. The land includes a plurality of recessed portions on the second surface, and at least one recessed portion selected from the plurality of recessed portions includes a buffer body containing resin. The via-hole electrical conductor is in contact with the second surface of the land and the buffer body.

DESCRIPTION OF EMBODIMENTS

As described above, in a known wiring board, at the contact surface between the via-hole electrical conductor and the land, when one is made of copper and the other is made of a metal other than copper (for example, nichrome (NiCr) or the like), the connection reliability between the via-hole electrical conductor and the land is likely to be reduced. There is thus a need for a wiring board with improved connection reliability between the via-hole electrical conductor and the land without affecting the electrical reliability between the via-hole electrical conductor and the land.

In a wiring board according to the embodiment of the present disclosure, the land includes a plurality of recessed portions on a surface of the land, and a buffer body containing resin is located in each of the recessed portions. Thus according to the present disclosure, a wiring board with improved connection reliability between the via-hole electrical conductor and the land without affecting the electrical reliability between the via-hole electrical conductor and the land can be provided.

A wiring board according to one embodiment of the present disclosure will be described with reference toFIGS.1and2.FIG.1is a schematic view illustrating a wiring board1according to one embodiment of the present disclosure.

The wiring board1according to one embodiment includes a plurality of insulation layers2and a wiring conductor layer4located on a surface of each of the plurality of insulation layers2. The wiring conductor layer4includes a land41.

The insulation layers2are not particularly limited as long as they are made out of a material having an insulating property. Examples of the material having an insulating property include resins such as epoxy resin, bismaleimide-triazine resin, polyimide resin, polyphenylene ether resin, and liquid crystal polymer. These resins may be used alone or in a combination of two or more. As illustrated inFIG.2A, insulating particles23may be dispersed in the insulation layers2. Examples of the insulating particles23are not limited, and include inorganic insulating fillers such as silica, alumina, barium sulfate, talc, clay, glass, calcium carbonate, and titanium oxide.

In the wiring board1according to the embodiment, one of the plurality of insulation layers2is a core layer21and the remaining insulation layers2are build-up layers22. The core layer21has a thickness of, for example, from 0.04 mm to 3.0 mm.

The core layer21includes a through-hole conductor3for electrically connecting the wiring conductor layers4on the upper and lower surfaces of the core layer21to each other. The through-hole conductor3is located in through holes penetrating the upper and lower surfaces of the core layer21. The through-hole conductor3is made of a conductor made of metal plating such as copper plating, for example. The through-hole conductor3is connected to the wiring conductor layers4on both surfaces of the core layer21. The through-hole conductor3may be formed only on an inner wall surface of the through hole or the through hole may be filled with the through-hole conductor3.

The build-up layers22have a thickness from 5 μm to 100 μm, for example. The build-up layers22may be formed of the same resin or different resins. The above-described insulation layer2includes a first surface S1on which the land41is located.

The wiring conductor layer4is located on the surface of the insulation layer2, that is, on the surface of the core layer21and on the surface of the build-up layers22. The wiring conductor layer4is formed of a conductor such as copper, for example, copper foil or copper plating. The thickness of the wiring conductor layer4is not particularly limited and is, for example, from 2 μm to 50 μm. In a case where there are a plurality of the wiring conductor layers4, the wiring conductor layers4may be composed of the same conductor, or may be composed of different conductors. Some of the wiring conductor layers4are used as the land41for connecting a via-hole electrical conductor5, which will be described later. The lands41such as those described above include a second surface S2excluding a portion in contact with the first surface S1of the insulation layer2.

Each of the build-up layers22includes the via-hole electrical conductor5for electrically connecting the wiring conductor layers4located above and below via the build-up layer22to each other. The via-hole electrical conductor5is obtained by depositing, for example, copper plating, in a via hole penetrating the upper and lower surfaces of the build-up layer22. The via hole penetrating the upper and lower surfaces of the build-up layer22has, for example, an inner diameter from 2 μm to 100 μm at the bottom, and is formed through, for example, a laser machining process such as one using CO2laser, UV-YAG laser, or excimer laser. The via-hole electrical conductor5may be positioned to fill the via hole, or the via-hole electrical conductor5may be adhered to the inside surface of the via hole which is then filled with resin in portions where the via-hole electrical conductor5is not provided.

FIG.2Ais an enlarged explanatory view for illustrating a region X inFIG.1. As illustrated inFIG.2A, the land41is located on the surface (first surface S1) of a first insulation layer22a. In the present specification, the terms “first insulation layer” and “second insulation layer” described later merely define one insulation layer as the “first insulation layer” and the other insulation layer as the “second insulation layer” for convenience, and are not limited to a layered structure in which two insulation layers are layered.

To be specific, regardless of the number of insulation layers layered, in any two insulation layers in which the land and the via-hole electrical conductors are in contact with each other, an insulation layer in which the land is located on the surface (first surface S1) of the insulation layer is defined as the “first insulation layer”, and an insulation layer including the via-hole electrical conductor in contact with the land is defined as the “second insulation layer”. Thus, when a land is formed on the surface of the insulation layer including the via-hole electrical conductor and an insulation layer including the via-hole electrical conductor in contact with the land is further layered, the former insulation layer is defined as the “first insulation layer” and the latter insulation layer is defined as the “second insulation layer”.

The land41located on the surface (first surface S1) of the first insulation layer22ais formed of a first metal layer41a, a second metal layer41b, a third metal layer41c, and a fourth metal layer41d. The first metal layer41ais formed of, for example, nichrome (NiCr) and has a thickness from 1 nm to 100 nm. The second metal layer41bis formed so as to cover the surface of the first metal layer41a. The second metal layer41bis formed of, for example, copper and has a thickness from 100 nm to 1000 nm. The third metal layer41cis formed so as to cover a surface of the second metal layer41b. The third metal layer41cis formed of, for example, copper and has a thickness from 1 μm to 60 μm.

The fourth metal layer41dis formed so as to cover a surface of the third metal layer41c, a side surface of the first metal layer41a, a side surface of the second metal layer41b, and a side surface of the third metal layer41c. The fourth metal layer41dis formed of, for example, a conductor containing tin and has a thickness from 0.1 nm to 10 nm. The upper surface and a part of the side surfaces of the fourth metal layer41dcontain an alloy with copper. When the fourth metal layer41dis formed of the alloy of tin and copper, because tin has excellent compatibility with a silane coupling agent described later, the adhesiveness between the land41and the second insulation layer22bcan be further improved.

As illustrated inFIGS.2A and2B, a plurality of recessed portions42are present on the surface of the land41.FIG.2Bis an enlarged explanatory view for illustrating a region Y illustrated inFIG.2A.

A buffer body24is located in an inner portion of the recessed portions42. The buffer body24contains resin. Examples of the resin include, but are not limited to, resins such as epoxy resin, bismaleimide-triazine resin, polyimide resin, polyphenylene ether resin, cycloolefin resin, and liquid crystal polymer. These resins may be used alone or in a combination of two or more. In particular, these resins may be the same resin as the resin forming the second insulation layer22b.

Insulating particles24′ may be included in the buffer body24. Examples of such insulating particles24′ are not limited, and include inorganic insulating fillers such as silica, alumina, barium sulfate, talc, clay, glass, calcium carbonate, and titanium oxide. The insulating particles24′ may be, for example, the same as the insulating particles23included in the second insulation layer22b.

The via-hole electrical conductor5is formed of a first metal layer5a, a second metal layer5b, and a third metal layer5c. The first metal layer5ais located on an inner peripheral surface of the via hole included in the second insulation layer22band on a peripheral edge portion of the via hole. The first metal layer5ais formed of, for example, nichrome and has a thickness from 1 nm to 100 nm. The second metal layer5bis located so as to cover the surface of the first metal layer5a. The second metal layer5bis formed of, for example, copper and has a thickness from 100 nm to 1000 nm. The inside of the via hole is filled with the third metal layer5cso as to cover a surface of the second metal layer5b. The third metal layer5cis formed of, for example, copper.

Even when there is a relatively large difference between the linear expansion coefficient and the Young's modulus of the conductor (in particular, the fourth metal layer41d) included in the land41and the linear expansion coefficient and the Young's modulus of the conductor (in particular, the first metal layer5a) included in the via-hole electrical conductor5, the presence of the buffer body24on an upper surface of the land41can alleviate, for example, a stress generated during thermal expansion and contraction. As a result, the connection reliability between the via-hole electrical conductor5and the land41is improved. The buffer body24is present only in the recessed portion formed on the upper surface of the land41, and does not affect the electrical reliability between the land41and the via-hole electrical conductor5.

In order to further improve the connection reliability without reducing the electrical reliability between the via-hole electrical conductor5and the land41, a ratio of a surface area occupied by the buffer body24to a surface area of the bottom portion of the via hole in plan view may be, for example, from 1% to 20%. The surface area occupied by the buffer body24may be determined from an electron micrograph. Specifically, it may be determined by the following procedure.

After the via hole is formed in the second insulation layer22b, the surface of the land41serving as the bottom portion of the via hole is observed with a field emission scanning electron microscope (FE-SEM), and a reflected electron micrograph of the surface of the land41is captured. The captured reflected electron micrograph is binarized by image processing software “Image J”. The buffer body24portion appears black, and thus a portion having a degree of blackness of 70% or more is recognized as black, and the surface area occupied by the buffer body24is obtained. From this surface area and the surface area of the bottom portion of the via hole, a ratio of the surface area occupied by the buffer body24to the contact surface area between the land41and the via-hole electrical conductor5may be calculated.

A silane coupling agent is present in at least a part of a non-contact portion with the via-hole electrical conductor5in the land41. Specifically, the non-contact portion with the via-hole electrical conductor5is the side surface of the land41, the peripheral edge of the via-hole on the upper surface of the land41, and the inner surface of the recessed portion. The adhesiveness between the land41made of metal and the second insulation layer22bmade of resin may be poor. When the silane coupling agent is present between the land41and the second insulation layer22b, the adhesiveness between the land41and the second insulation layer22bcan be further improved. The silane coupling agent is a compound including, in the molecule, a functional group that reacts with an inorganic material and a functional group that reacts with an organic material. Thus, the metal (land41) which is an inorganic material and the resin (second insulation layer22b) which is an organic material are bonded to each other via the silane coupling agent, and the adhesiveness between the land41and the second insulation layer22bis further improved. The presence of such a silane coupling agent can be confirmed, for example, by analyzing the above functional group structure using Fourier transform infrared spectroscopy (FTIR) or by performing mass analysis using time-of-flight secondary ion mass spectrometry (TOF-SIMS).

In the wiring board1according to the embodiment, the method of forming the land41and the via-hole electrical conductor5is not limited, and the land41and the via-hole electrical conductor5are formed by, for example, the following method.

The first metal layer41ais formed on the surface of the first insulation layer22a. The first insulation layer22ais as described above, and detailed description thereof will be omitted. The first metal layer41ais made of nichrome by, for example, sputtering. The thickness of the first metal layer41ais as described above. Next, the second metal layer41bis formed so as to cover the surface of the first metal layer41a. The second metal layer41bis made of copper by, for example, sputtering. The thickness of the second metal layer41bis as described above.

Next, in order to form the third metal layer41c, a plating resist including openings is formed on the surface of the second metal layer41b. Thereafter, the second metal layer41bis subjected to an electrolytic copper plating process and copper is deposited in the openings. Next, the plating resist is peeled off, and the second metal layer41bin a portion covered with the plating resist is removed by, for example, an acid (a mixed solution of sulfuric acid and hydrogen peroxide water). Next, the first metal layer41ais removed by, for example, an acid (such as a mixed solution of hydrochloric acid and sulfuric acid). At this time, the surface of the copper deposited by the electrolytic copper plating is also eroded by the acid to form depressions each having a diameter and a depth, for example, from about 10 nm to about 200 nm.

Next, the surface of the copper on which the depressions are formed is subjected to a soft etching process. The soft etching is performed using, for example, a sulfuric acid-hydrogen peroxide-based chemical solution. By performing the soft etching process, the diameter and the depth of the depression are set to, for example, from about 10 nm to about 100 nm. The soft etching process is performed in order to adjust the size of the buffer body24which is finally formed in the recessed portions42. When the soft etching process is performed for a long time, the copper deposited by the electrolytic copper plating is also eroded in the width direction. Thus, when the surface of the copper is eroded and the diameter and the depth of the depression are within the above-described range, the soft etching process is terminated. In this way, a layer which finally becomes the third metal layer41cis formed.

Next, the surfaces of the layered metal layers (copper) are subjected to a substitution tin plating process. The surface of the copper is substituted with tin by the substitution tin plating. At this time, the copper is substituted with tin, and tin is deposited even in the depressions formed by the soft etching process. Thereafter, the surface of the tin is treated with nitric acid to remove the part of the tin in the depressions, thereby forming the recessed portions42in which tin is deposited on an inner wall surface. Next, the silane coupling agent is adhered to the surface of the tin. For example, the recessed portions42such as those described above each have a diameter and a depth, for example, from about 10 nm to about 500 nm. When the above-described soft etching process is not performed, for example, the depth of the recessed portions42may become excessively deep.

Next, the second insulation layer22bis formed so as to cover the first insulation layer22aand the third metal layer41cwith tin deposited on the surface thereof. The second insulation layer22bis as described above, and detailed description thereof will be omitted. At this time, some of the resin forming the second insulation layer22bis also embedded in the recessed portions42formed in the surfaces of the layered metal layers. When the insulating particles23are contained in the resin constituting the second insulation layer22b, some of the insulating particles23are also embedded in the recessed portions42. In this way, the buffer body24(including the insulating particles24′ if necessary) is formed in the recessed portions42.

Subsequently, via holes are formed in the second insulation layer22b. The via holes are formed at positions where a part of the layered metal layers serves as a bottom portion by, for example, the above-described laser machining process. Thereafter, the first metal layer5ais formed so as to cover the second insulation layer22band the via holes. The first metal layer5ais made of nichrome by, for example, sputtering. The thickness of the first metal layer5ais as described above. Next, the second metal layer5bis formed so as to cover the surface of the first metal layer5a. The second metal layer5bis made of copper by, for example, sputtering. The thickness of the second metal layer5bis as described above. Due to heat generated during the sputtering, an alloy of tin and copper is formed from tin formed on the surface of the layered metal layers and copper contained in the third metal layer41c. Thus, the land41is formed. In the first metal layer5a, although nichrome is described as an example, in addition to this, transition metals of Group 4, Group 5, or Group 6 of the periodic table such as titanium, chromium, nickel, tantalum, molybdenum, tungsten, and palladium may be formed using sputtering or vapor deposition other than sputtering.

Next, in order to form the third metal layer5cin the via hole, a plating resist is formed on the surface of the second metal layer5bother than the via hole and the peripheral edge portion of the via hole. Thereafter, the second metal layer5bis subjected to an electrolytic copper plating process and copper is deposited on a plating resist unprocessed portion. Next, the plating resist is peeled off, and the first metal layer5aand the second metal layer5bin the portion covered with the plating resist are removed by, for example, the above-described acid. In this way, the via-hole electrical conductor5connected to the land41is formed.

When an insulation layer is further layered as the build-up layer, the same or a similar procedure may be repeated from the above-described soft etching process. That is, the procedure after the soft etching process for adjusting the diameter and the depth of the depression formed in the surface of the via-hole electrical conductors5(third metal layer5c) may be repeated.

Needless to say, the core layer21may be regarded as the “first insulation layer” and the land41may be formed on the surface of the core layer21by the above-described procedure, and the first insulation layer22amay be regarded as the “second insulation layer” and the via-hole electrical conductors5may be formed by the above-described procedure.

In this way, the wiring board1can be obtained in which the via-hole electrical conductor5is connected to the surface of the land41and the buffer body24located on the surface of the land41. A part of the via-hole electrical conductor5is connected to the buffer body24, and thus the connection reliability between the via-hole electrical conductor and the land can be improved without affecting the electrical reliability between the via-hole electrical conductor and the land.

The wiring board of the present disclosure is not limited to the above-described embodiments.FIG.2Bdescribed above illustrates an embodiment in which the buffer body24is located in the entirety of the recessed portions42. However, as illustrated inFIG.3, the buffer body24may be located only on a part of an inner wall of the recessed portions42. In this case, the contact surface area between the first metal layer5aof the via-hole electrical conductor5and the fourth metal layer41din the recessed portions42is increased. Thus, the adhesiveness between the land41and the first metal layer5a(via-hole electrical conductor5) can be further improved. When the buffer body24contains air or moisture, there is an advantage in that the air or moisture can be removed more easily as compared with when the buffer body24is located in the entirety of the recessed portions42.

A metal oxide film may be located on at least a part of the surface of the buffer body24. Examples of the metal oxide film include, for example, a nickel oxide film. Nickel oxide has a smaller Young's modulus than, for example, nickel used as the first metal layer5a. Thus, when the metal oxide film is located between the buffer body24and the first metal layer5a, there is an advantage in that a stress-relieving effect is improved as compared with when the buffer body24and the first metal layer5aare in direct contact with each other.

REFERENCE SIGNS