Patent Application
20140124250
The present invention relates to a wiring board including wirings which are formed on both surfaces of an electric insulating base material and coupled to each other by via-hole conductors, and a method for manufacturing the wiring board.
A wiring board including wirings which are formed on both sides of an electric insulating base material and coupled to each other by via-hole conductors formed by filling conductive paste into holes formed in the electric insulating base material is known. Furthermore, a via-hole conductor, in which metal particles containing copper (Cu) instead of the conductive paste are filled and the metal particles are fixed together by an intermetallic compound, is known. Specifically, a via-hole conductor is known, in which conductive paste including tin (Sn)-bismuth (Bi) metal particles and copper particles is heated at a predetermined temperature, thereby forming a tin (Sn)-copper (Cu) alloy in the vicinity of the copper particles.
Via-hole conductor 2 is brought into contact with wiring 1 formed on a surface of the wiring board. Via-hole conductor 2 includes metal portion 11 and resin portion 12. Metal portion 11 has first metal region 8 including a plurality of copper (Cu)-containing particles 3, second metal region 9 including a tin (Sn)-copper (Cu) alloy, or the like, and third metal region 10 mainly composed of bismuth (Bi). Note here that prior art literatures related to this invention include, for example, Patent Literature 1.
A wiring board of the present invention includes an electric insulating base material including an incompressible member and a thermosetting member; a first wiring and a second wiring formed with the electric insulating base material interposed therebetween; and a via-hole conductor that penetrates the electric insulating base material and electrically connects the first wiring and the second wiring. The via-hole conductor includes a resin portion and a metal portion. The metal portion has a first metal region mainly composed of copper (Cu), a second metal region mainly composed of a tin (Sn)-copper (Cu) alloy, and a third metal region mainly composed of bismuth (Bi). The second metal region is larger than the first metal region, and larger than the third metal region.
When conventional via-hole conductor 2 undergoes thermal shock in, for example, reflow treatment, Cu diffuses into Sn—Bi metal particles to generate intermetallic compounds such as Cu3Sn and Cu6Sn5. At this time, as shown in
Furthermore, in conventional via-hole conductor 2, a volume fraction of resin portion 12 in via-hole conductor 2 is large and a volume fraction of metal portion 11 is small. Therefore, via resistance (a resistance value of entire via-hole conductor 2) may be high.
Hereinafter, a structure of a multilayer wiring board in accordance with the present exemplary embodiment is described.
Electric insulating base material 130 includes incompressible member 220 such as a heat-resistant film, and thermosetting adhesive layers (thermosetting members) 210 formed on both surfaces of incompressible member 220. First wiring 120a and second wiring 120b formed by patterning metal foil 150 such as copper foil into a predetermined shape are adhesively bonded to incompressible member 220 by way of thermosetting adhesive layer 210. Note here that thermosetting adhesive layer 210 may be formed on only one surface of incompressible member 220.
Via-hole conductor 140 includes metal portion 190 and resin portion 200. Metal portion 190 has first metal region 160 mainly composed of copper, second metal region 170 mainly composed of a tin-copper alloy, and third metal region 180 mainly composed of bismuth. Second metal region 170 is larger than first metal region 160, and larger than third metal region 180.
Resin portion 200 is, for example, epoxy resin. Epoxy resin has excellent reliability. Resin portion 200 is a cured product mainly of resin added into via paste, but a part of thermosetting resin constituting thermosetting adhesive layer 210 may be mixed.
The size (or volume fraction or weight fraction) of second metal region 170 is larger than that of first metal region 160. Furthermore, the size (or volume fraction or weight fraction) of second metal region 170 is larger than that of third metal region 180.
When the size of second metal region 170 is made to be larger than that of first metal region 160 and larger than that of third metal region 180, a plurality of wirings 120 can be electrically coupled to each other mainly by second metal region 170. Furthermore, first metal regions 160 and third metal regions 180 can be scattered (scattered in a state of isolated small islands) in such a manner that they are not brought into contact with each other in second metal region 170.
Furthermore, second metal region 170 includes intermetallic compounds Cu6Sn5 and Cu3Sn, and the ratio of Cu6Sn5/Cu3Sn is 0.001 or more and 0.100 or less. By reducing the amount of Cu6Sn5, it is possible to prevent Cu6Sn5 remaining in multilayer wiring board 110 from being changed into Cu3Sn in a heat treatment process such as solder reflow. As a result, generation of Kirkendall voids or the like can be suppressed.
Note here that the ratio of Cu6Sn5/Cu3Sn is desirably 0.100 or less, and more desirably 0.001 or more and 0.100 or less. A reaction time is limited and it is practical that the reaction time is within 10 hours at most. Therefore, it is not likely that the ratio of Cu6Sn5/Cu3Sn is completely 0 within such a limited reaction time. Also, it becomes difficult to quantitatively analyze Cu6Sn5 that may remain in only a small amount.
When a usual measuring device is used as mentioned above, it is thought that Cu6Sn5 may not be detected (for example, a detected amount becomes 0 in relation to the detection limit of a measuring device). Therefore, when the usual measuring device is used, the ratio of Cu6Sn5/Cu3Sn is 0 or more and 0.100 or less (note here that 0 includes a case where a detected amount is not more than the detection limit that is measurable by the measuring device, or a case where a detection cannot be carried out by a measuring device). Note here that when the measurement accuracy of a measuring device is sufficiently high, the ratio of Cu6Sn5/Cu3Sn may be 0.001 or more and 0.100 or less.
Note here that it is desirable that the ratio of Cu6Sn5/Cu3Sn is 0.001 or more and 0.100 or less as a result of evaluation using an XRD (X-ray diffraction device). However, it is difficult to take out only a minute via portion (or a via paste portion) constituting an actual wiring board, and to analyze the portion by the XRD device. Therefore, a general evaluation device, for example, an elemental analyzer (for example, XMA, EPMA, and the like) using fluorescence X-ray, which is attached to a SEM device, may be used as a measuring device. Furthermore, even if such an elemental analyzer (for example, XMA, EPMA, and the like) is used, the ratio of Cu6Sn5/Cu3Sn is desirably 0.001 or more and 0.100 or less. The XRD carries out a kind of mass spectrometric analysis, and the EPMA carries out a kind of cross-sectional analysis, but there is no substantial difference between them. As mentioned above, in measuring the ratio of Cu6Sn5/Cu3Sn of a minute via portion (or via paste portion), evaluation may be carried out by selecting one of appropriate devices from XRD, XMA, EPMA, or other devices similar to these devices.
Electric insulating base material 130 includes incompressible member 220 such as a heat-resistant film, and thermosetting adhesive layer 210 formed on at least one surface of incompressible member 220.
Note here that it is practical that the definitions of compressibility and incompressibility in the present exemplary embodiment are given based on a configuration of a core material. That is to say, a member has compressibility when it uses, as the core material, woven fabric or non-woven fabric in which a plurality of fibers, regardless of whether the fibers are glass fibers or resin fibers, are entangled with each other. The reason thereof is as follows. The core material using woven fabric or non-woven fabric is provided with through-holes, and the through-holes are filled with conductive paste. When pressure is applied thereto, the through-holes are deformed or widened because they are pushed by metal particles included in the conductive paste.
On the other hand, since a member using a film as the core material does not have a space inside thereof, the member has incompressibility. The reason thereof is as follows. The core material using a film is provided with through-holes, and the through-holes are filled with conductive paste. When pressure is applied thereto, the diameter of the through-hole is not substantially changed.
When woven fabric or non-woven fabric using glass fibers is used as the core material and when through-holes are formed by laser or the like, tip ends of the woven fabric or non-woven fabric made of glass fibers in the periphery of the holes may be melted and solidified. Also in this case, however, the core material has compressibility. The reason thereof is as follows. Glass fibers melted to be integrated with each other by laser or the like are present only in the periphery of the holes, and glass fibers in other parts (that is to say, a part that is little apart from the thorough holes formed by laser) are just entangled with each other. This is also because all of the glass fibers exposed to the periphery of the holes are not melted to be integrated with each other.
Furthermore, in a case of non-woven fabric using glass fibers, a portion in which fibers are entangled with each other may be fixed. Also in this case, however, a member including the non-woven fabric as the core material has compressibility.
Since incompressible member 220 does not have air bubble portions or the like for expressing compressibility inside thereof, it has excellent incompressibility.
When the incompressible member is used, via paste can be compressed with high pressure. As a result, via-hole conductor 140 including 74.0 vol % or more and 99.5 vol % or less of metal portion 190, and 0.5 vol % or more and 26.0 vol % or less of resin portion 200 can be produced.
By reducing the volume fraction (vol %) of resin portion 200 that is an insulating component in via-hole conductor 140, the volume fraction (vol %) of metal portion 190 is increased, and via resistance is reduced. The via resistance herein denotes a resistance value of entire via-hole conductor 140. Also, in order to enhance mechanical strength of a via portion, it is preferable to increase the volume fraction of metal portion 190 in via-hole conductor 140.
Furthermore, by increasing a contact area between wiring 120 and via-hole conductor 140, connection resistance between wiring 120 and via-hole conductor 140 is reduced. Therefore, it is preferable to reduce the volume fraction of resin portion 200 in an interface portion between wiring 120 and via-hole conductor 140.
Since the configuration of the present exemplary embodiment allows the specific resistance of via-hole conductor 140 to be 1.00×10−7 Ω·m or more and 5.00×10−7 Ω·m or less, the via resistance is stabilized.
Furthermore, in the present exemplary embodiment, an alloying reaction between copper and tin is almost perfectly completed.
Note here that resin portion 200 constituting via-hole conductor 140 is made of a cured product of curable resin. The curable resin is not particularly limited, but specifically, it is preferable to use, for example, a cured product of epoxy resin having an excellent heat-resistant property and a low coefficient of linear expansion.
One example of a method for manufacturing wiring board 600 and multilayer wiring board 111 is described.
Firstly, as shown in
Examples of incompressible member 220 include a polyimide film, a liquid crystal polymer film, a polyether ether ketone film, and the like. Particularly preferable among them is the polyimide film. However, incompressible member 220 is not particularly limited as long as it is a resin sheet that is resistant to soldering temperatures.
Examples of thermosetting adhesive layer 210 include an uncured adhesive layer made of, for example, epoxy resin. Furthermore, in order to thin the multilayer wiring board, the thickness of the thermosetting adhesive layer per one surface is preferably 1 μm or more and 30 μm or less, and further preferably 5 μm or more and 10 μm or less.
Examples of the protective film include resin films of PET (polyethylene terephthalate), PEN (polyethylene naphthalate), and the like. The thickness of the resin film is preferably 0.5 μm or more and 50 μm or less, and further preferably 1 μm or more and 30 μm or less. When the resin film has such a thickness, protruding portions made of via paste and having a sufficient height are allowed to protrude by peeling off the protective films as mentioned below.
Examples of a method for attaching protective film 240 onto uncured base material 230 include a method of directly attaching the film by using surface tackiness (or bonding force) of uncured base material 230 or thermosetting adhesive layer 210 on the surface of uncured base material 230.
Next, as shown in
Next, as shown in
A method for filling via paste 260 is not particularly limited. Examples of the method include a screen printing method.
Next, as shown in
Next, as shown in
Since incompressible member 220 is used as a part of uncured base material 230, at the time when pressure is applied as shown by arrows 280 (furthermore, at the time when heating is carried out), the diameter of through-hole 250 is not widened, so that strong pressure is applied to via paste 260. As a result, intervals between copper particles and Sn—Bi particles included in via paste 260 are narrowed, and the particles are brought into close contact with each other. Consequently, the ratio of the resin portion in via paste 260 is reduced. In other words, the ratio of the metal portion in via paste 260 is increased.
Then, when heat is applied in a state in which compression state is kept, an alloying reaction occurs and then, metal portion 190 and resin portion 200 (see
In this alloying reaction, the size (or volume % or weight %) of second metal region 170 is made to be larger than that of first metal region 160. Furthermore, the size (or volume % or weight %) of second metal region 170 is made to be larger than that of third metal region 180. As a result, reliability of via-hole conductor 140 is enhanced and the strength thereof is increased.
Furthermore, when first metal region 160 and third metal region 180 are scattered in a state in which they are not brought into contact with each other in second metal region 170, the reliability of via-hole conductor 140 can be enhanced.
Furthermore, second metal region 170 includes intermetallic compounds Cu6Sn5 and Cu3Sn, and the ratio of Cu6Sn5/Cu3Sn is made to be 0.001 or more and 0.100 or less. Thereby, the reliability of via-hole conductor 140 can be enhanced.
Pressurizing conditions are not particularly limited, but it is preferable that a die temperature is set at temperatures from an ordinary temperature (20° C.) to a temperature lower than the melting point of Sn—Bi solder particle. Furthermore, in this pressurization process, in order to allow curing of thermosetting adhesive layer 210 to proceed, a hot press that has been heated to a temperature necessary for allowing the curing to proceed may be used.
Next, a photoresist film is formed on the surface of metal foil 150. Then, the photoresist film is exposed to light via a photomask. Thereafter, development and rinsing are carried out, and the photoresist film is selectively formed on the surface of metal foil 150. Then, metal foil 150 that is not covered with the photoresist film is removed by etching. Thereafter, the photoresist film is removed. In this way, wiring 120a (first wiring) and wiring 120b (second wiring) are formed. Thus, wiring board 600 is obtained. For formation of the photoresist film, liquid resist may be used or a dry film may be used.
As shown in
According to the above-mentioned processes, multilayer wiring board 111 in which wiring 121a on the upper layer and wiring 121b on the lower layer are coupled to each other by way of via-hole conductor 140 is obtained. By making multilayer wiring boards 111 be more multilayered, multilayer wiring board 110 as shown in
Next, with reference to
The average particle diameter of copper particle 290 is preferably 0.1 μm or more and 20 μm or less, and further preferably 1 μm or more and 10 μm or less. When the average particle diameter of copper particle 290 is too small, the tap density (JIS X 2512) of copper particles 290 is reduced. Therefore, through-hole 250 (see
Examples of the particle shape of copper particle 290 include a spherical shape, a flat shape, a polygonal shape, a scale shape, a flake shape, or a shape having protrusions on the surface, but the particle shape is not necessarily limited to these shapes. Furthermore, the particles may be primary particles or secondary particles.
Sn—Bi solder particle 300 denotes solder particle 300 containing Sn and Bi.
Furthermore, wettability, flowability, or the like, may be improved by adding indium (In), silver (Ag), zinc (Zn), or the like, into solder particle 300. The content rate of Bi in Sn—Bi solder particle 300 is preferably 10% or more and 58% or less, and further preferably, 20% or more and 58% or less. Furthermore, a melting point (eutectic point) is preferably 75° C. or higher and 160° C. or lower, and further preferably, 135° C. or higher and 150° C. or lower. As Sn—Bi solder particle 300, combination of two types or more of different particles may be used. Particularly preferable among them is Sn-58Bi solder particle 300, which is lead-free solder having a eutectic point that is low as 138° C., from the environmental viewpoint.
The average particle diameter of Sn—Bi solder particle 300 is preferably 0.1 μm or more and 20 μm or less, and further preferably 2 μm or more and 15 μm or less. When the average particle diameter of the Sn—Bi solder particle is too small, the specific surface area becomes larger and the ratio of an oxide film on the surface is increased, and therefore melting does not easily occur. On the other hand, when the average particle diameter of Sn—Bi solder particle is too large, via paste 260 cannot be easily filled into through-holes 250.
Examples of thermosetting resin component 310 include glycidylether-type epoxy resin, alicyclic epoxy resin, glycidyl amine type epoxy resin, glycidyl ester type epoxy resin, other modified epoxy resin, or the like.
Furthermore, thermosetting resin component 310 may include a curing agent. Types of the curing agent are not particularly limited, but it is preferable to use a curing agent containing an amine compound having at least one or more hydroxyl groups in a molecule. Such a curing agent acts as a curing catalyst of epoxy resin, and reduces the oxide film that is present on the surface of copper particles and Sn—Bi solder particles 300, thereby lowering contact resistance at the time of bonding. An amine compound having a boiling point that is higher than the melting point of the Sn—Bi solder particle is particularly preferable because it lowers the contact resistance at the time of bonding.
Examples of such amine compounds include 2-2-methylaminoethanol, N, N-diethylethanolamine, N, N-dibutylethanolamine, N-methylethanolamine, N-methyldiethanolamine, N-ethylethanolamine, N-butylethanolamine, diisopropanolamine, N,N-diethylisopropanolamine, 2,2′-dimethylaminoethanol, triethanolamine, and the like.
Via paste 260 is obtained by mixing copper particles 290, Sn—Bi solder particles 300 containing Sn and Bi, and thermosetting resin component 310 such as epoxy resin. Specifically, for example, via paste 260 is obtained by adding copper particles and Sn—Bi solder particles into resin varnish containing epoxy resin, a curing agent and a predetermined amount of an organic solvent, and mixing the obtained product by using, for example, a planetary mixer.
The ratio of thermosetting resin component 310 in via paste 260 is preferably 0.3 mass % or more and 30 mass % or less, and further preferably 3 mass % or more and 20 mass % or less from the viewpoint of obtaining a low resistance value and securing sufficient workability.
Furthermore, as a blending ratio of copper particles 290 and Sn—Bi solder particles 300 in via paste 260, it is preferable that copper particles 290 and Sn—Bi solder particles 300 are contained such that the weight ratio of Cu, Sn and Bi is in a range of a region surrounded by a quadrangle having apexes A, B, C, and D in a ternary diagram as shown in
As shown in
Incompressible member 220 is used so that through-hole 250 (see
With reference to
Since compressible member 340 has air bubbles (or voids), or the like, inside thereof, when it is pressed, the thickness thereof is compressed by about 10% to 30%.
Through-hole acting as a via is formed in compressible member 340 and filled with via paste to provide a protruding portion. Then, when the pressure is applied thereto, a diameter (or a sectional area) of the through-hole after pressure is applied becomes larger by about 10% to 20% as compared with the diameter before pressure is applied.
This is because a part of glass fibers is cut when the through-hole is formed. That is to say, when prepreg including woven fabric or non-woven fabric is used as the core material, sufficient pressurization and compression cannot be carried out in some cases.
In
When compressible member 340 as shown in
Note here that a volume fraction in the case where spherical bodies are randomly packed in a container is known to be about 64% at maximum as “random close packing” (see, for example, Nature 435, 7195 (May 2008), Song et al.). When compressible member 340 is used for the electric insulating base material in this way, even if the packing density (furthermore, the volume fraction) of copper particles 290 and solder particles 300 contained in via paste 260 is to be enhanced, it is difficult to enhance the volume fraction from the viewpoint of the random close packing. Therefore, even when protruding portion 270 is pressurized and compressed to an extent that copper particles 290 and solder particles 300 are deformed and brought into surface contact with each other, it is difficult to exclude thermosetting resin components 310 remaining in the gap among a plurality of copper particles 290 and a plurality of solder particles 300 to the outside of via paste 260.
As a result, a state shown in
As mentioned above, in compressible member 340, the diameter of through-hole 250 is widened or deformed by pressure from via paste 260. Therefore, even when high pressure is applied, via paste 260 may not be sufficiently compressed.
On the other hand, when an incompressible member (for example, a film base material) is used, even when a through-hole acting as a via is provided in a thermosetting adhesive layer an incompressible member, the through holes are filled with via paste to provide a protruding portion, and then pressure is applied thereto, a diameter (or a sectional area) of the through hole after pressure is applied is hardly changed as compared with that before pressure is applied, or the changed amount is suppressed to less than 3%. Then, since the diameter or the sectional area of the through-hole is not changed before and after the through-hole is filled with via paste, the via paste can be sufficiently pressurized and compressed without using specific equipment. This is because when the incompressible member is used, even when a part of the incompressible member is cut by the through-hole, the incompressible member is hardly melted or widened.
However, even when a heat-resistant film like a polyimide film is used, but when the thickness thereof is large as 70 μm, via paste 260 may not be compressed sufficiently even with high pressure applied by using protruding portion 270.
When incompressible member 220 such as a heat-resistant film is used for uncured base material 230, a fluid component (for example, an insulating component such as an organic component) of thermosetting resin component 310 in via paste 260 can be excluded to the outside of via-hole conductor 140. As a result, the volume fraction of thermosetting resin component 310 in via paste 260 can be reduced.
As shown in
Note here that in order to deform and bring copper particles 290 and solder particles 300 into surface contact with each other in a wide area, it is preferable that rigidity of copper particle 290 and rigidity of solder particle 300 are made to be different from each other. For example, by making the rigidity of solder particle 300 be lower than that of copper particle 290, it is possible to reduce powders which slide (or slip) each other. As a result, when pressurizing and compressing shown in
As shown in
That is to say, when the incompressible base material is used for uncured base material 230, the diameter of through-hole 250 is hardly changed between before and after compression. Therefore, according to protruding of via paste 260, via paste 260 can be highly compressed.
Note here that a difference between the diameter (or the sectional area) of the through-hole before pressure is applied and that after pressure is applied is preferably less than 3% and further preferably less than 2%.
Thus, in the present exemplary embodiment, the volume fraction of metal portion 190 after copper particles 290 and solder particles 300 are alloyed can be made to be 74.0 vol % or more and 99.5 vol % or less. Furthermore, in via-hole conductor 140 for electrically connecting a plurality of wirings to each other, the volume fraction of resin portion 200 that is a part excluding metal portion 190 can be reduced to 0.5 vol % or more and 26.0 vol % or less. Herein, resin portion 200 only needs to be a resin portion included in via-hole conductor 140 and may not be thermosetting resin component 310 contained in via paste 260. Furthermore, thermosetting resin component 310 in via paste 260 and thermosetting adhesive layer 210 may be compatible with each other or may be dissolved into each other.
When via paste 260 is filled into through-hole 250 formed in incompressible member 220 and thermosetting adhesive layer 210, and pressed, the content (or volume fraction) of thermosetting resin component 310 in the via paste can be further reduced. Therefore, it is possible to increase the filling rate (or volume fraction) of copper particles 290, solder particles 300, or the like, in via paste 260. As a result, the contact area between copper particles 290 and solder particles 300 is increased, and an alloying reaction is promoted. Thus, the metal portion in via-hole conductor 140 can be increased.
Next, a state in which the alloying reaction between copper particles and solder particles is promoted by reducing the volume fraction of thermosetting resin component 310 is described.
In
Note here that the volume fraction of thermosetting resin component 310 included in via paste 260 is 0.5 vol % or more and 26 vol % or less (furthermore, 20 vol % or less and yet furthermore, 10 vol % or less).
As shown in
Protruding portions 270 are formed on the upper and lower surfaces of via paste 260 in
Note here that the particle diameter of copper particle 290 and the particle diameter of solder particles 300 may be made to be different from each other, and copper particles 290 having different particle diameters may be mixed with each other. However, in such cases, a specific surface area of powder is increased, resulting in increasing the viscosity of via paste 260. As a result, although the volume fraction of the total of copper particles 290 and solder particles 300 in via paste 260 can be increased, the viscosity of via paste 260 is increased, and thus filling property of through-hole 250 may be affected. Therefore, it is preferable that the diameter of copper particle 290 and the diameter of solder particle 300 are the same level as each other.
In order to deform and bring copper particles 290 and solder particles 300 into surface contact with each other, it is desirable that copper particles 290 or solder particles 300 and copper particles 290 are pressurized and compressed such that they are plastically deformed to each other.
It is preferable that heating is carried out at a predetermined temperature in a state in which a compression bonding state is maintained as shown by arrows 280 in
Thus, via-hole conductor 140 is formed as shown in
Metal foils 150 forming a plurality of wirings 120 are electrically coupled to each other by way of second metal region 170. When first metal region 160 and third metal region 180 are scattered in a state in which they are not brought into contact with each other in second metal region 170, the reliability of via-hole conductor 140 is enhanced. In addition, when second metal region 170 includes intermetallic compounds Cu6Sn5 and Cu3Sn and the ratio of Cu6Sn5/Cu3Sn is made to be 0.001 or more and 0.100 or less, the reliability of via-hole conductor 140 is enhanced.
Pressurization and compression shown by arrows 280 are continued also during the alloying reaction, and thereby the height of protruding portion 270 in metal foil 150 after the alloying can be lowered. The height of protruding portion 270 before the alloying reaction is lowered after the alloying reaction, and thereby the volume fraction of resin portion 200 in via-hole conductor 140 can be reduced, and variation in the thickness of multilayer wiring board 110 can be reduced. Furthermore, since flatness or smoothness of multilayer wiring board 110 can be improved, a mounting property of a bare chip such as a semiconductor chip can be enhanced.
In via-hole conductor 140 formed through a reaction between copper particles 290 and solder particles 300, second metal region 170 includes intermetallic compounds Cu6Sn5 and Cu3Sn. Herein, when the ratio of Cu6Sn5/Cu3Sn is reduced to 0.001 or more and 0.100 or less, for example, generation of voids 5a such as Kirkendall voids (see
In order to make the ratio of Cu6Sn5/Cu3Sn be 0.001 or more and 0.100 or less, it is desirable that the contact area between copper particle 290 and solder particle 300 is large. At the time when the alloying reaction (or a formation reaction of an intermetallic compound) is carried out, the volume fraction of thermosetting resin component 310 in via paste 260 is desirably 26 vol % or less (further desirably, 20 vol % or less, and yet further desirably, 10 vol % or less). The smaller the volume fraction of thermosetting resin component 310 is, the larger the contact area between copper particles 290 and solder particles 300 becomes. Thus, the alloying reaction becomes uniform. As a result, in the second metal region including the intermetallic compounds Cu6Sn5 and Cu3Sn, the ratio of Cu6Sn5/Cu3Sn can be suppressed to 0.100 or less. As mentioned above, when a member having incompressibility is used as uncured base material 230, the density of copper particles 290 and Sn—Bi solder particles 300 filled into through hole 250 is increased.
Furthermore, it is useful that compressed via paste 260 is heated in a state in which compression is maintained so as to melt a part of Sn—Bi solder particles 300 at a temperature range of not lower than the eutectic temperature of Sn—Bi solder particle 300 to not higher than a temperature that is higher by 10° C. than the eutectic temperature, and subsequently heated to a temperature range of not lower than a temperature that is higher by 20° C. than the eutectic temperature to not higher than 300° C. Such pressurization and heating can promote growth of second metal region 170. In addition, it is preferable that these are carried out in one process including successive compression bonding and heating. When these are carried out in one continuous process, a formation reaction of each metal region can be stabilized, and the structure of the via itself can be stabilized.
For example, in
Note here that in
Then, when heating is carried out in this state, and a temperature reaches not lower than the eutectic temperature of Sn—Bi solder particle 300, Sn—Bi solder particles 300 start to be partially melted. The composition of the melting solder is determined by a temperature, Sn that is not easily melted at a temperature at the time of heating remains as a Sn solid phase product. Furthermore, copper particles 290 are brought into contact with the melted solder and the surface thereof is wet with the melted Sn—Bi solder, counter diffusion of Cu and Sn proceeds on the interface of the wet portion, and a compound layer of Sn—Cu or the like is formed. Thus, the ratio of second metal region 170 in via-hole conductor 140 can be made to be larger than first metal region 160, and larger than third metal region 180.
On the other hand, when formation of a Sn—Cu compound layer or the like or counter diffusion further proceeds, Sn in the melted solder is reduced. Since Sn that is reduced in the melted solder is supplement from a Sn solid layer, the melting state can be continued to be maintained. When Sn is further reduced and Bi in the ratio of Sn and Bi becomes larger than in Sn-58Bi, segregation of Bi is started, and third metal region 180 as a solid phase product mainly composed of bismuth is deposited and formed.
Note here that well-known solder materials melted at relatively low temperatures include Sn—Pb solder, Sn—In solder, Sn—Bi solder, and the like. Among these materials, In is expensive and Pb has high environmental load. On the other hand, the Sn—Bi solder has a melting point of 140° C. or lower, which is lower than a general solder reflow temperature when an electronic component is surface-mounted. Therefore, when only the Sn—Bi solder as a simple substance is used for a via-hole conductor of a circuit board, solder of the via-hole conductor is melted again at the time of solder reflow, so that the via resistance may be changed.
Further desirably, the ratio is in a region surrounded by a quadrangle having apexes of C (0.79:0.09:0.12), D (0.89:0.10:0.01), E (0.733:0.240:0.027), and F (0.564:0.183:0.253). When the ratio is in the region surrounded by the quadrangle having apexes of C (0.79:0.09:0.12), D (0.89:0.10:0.01), E (0.733:0.240:0.027), and F (0.564:0.183:0.253), the via resistance can be reduced. Furthermore, it becomes easy to include intermetallic compounds Cu6Sn5 and Cu3Sn in the second metal region, and to make the ratio of Cu6Sn5/Cu3Sn be 0.100 or less.
Note here that when the via paste having such a metal composition is used, the Sn composition is larger in the composition of Sn—Bi solder particle 300 than in the eutectic Sn—Bi solder composition (Bi: 58% or less and Sn: 42% or more). When such a via paste is used, a part of the solder composition is melted in a temperature range of not higher than a temperature that is higher by 10° C. than the eutectic temperature of the Sn—Bi solder particle, while Sn that is not melted remains. However, remaining Sn is diffused into copper particle surfaces and reacts therewith. As a result, since Sn concentration is reduced from Sn—Bi solder particle 300, remaining Sn is melted. On the other hand, Sn is melted also because heating is continued and a temperature rises, so that Sn that cannot be melted in the solder composition disappears. When heating is further continued, the reaction with the surface of the copper particle proceeds, and thereby third metal region 180 as a solid phase product mainly composed of bismuth is deposited and formed. When third metal region 180 is deposited in this way, the solder in the via-hole conductor is not easily melted again at the time of solder reflow. Furthermore, when solder particle 300 including a Sn—Bi composition having a higher rate of Sn composition is used, a Bi phase remaining in the via can be reduced. Therefore, a resistance value can be stabilized, and the change of the resistance value does not easily occur even after the solder reflow.
A temperature for heating via paste 260 after compression is not particularly limited as long as it is not lower than the eutectic temperature of Sn—Bi solder particle 300 and is in a temperature range at which components constituting uncured base material 230 are not decomposed. Specifically, when Sn-58Bi solder particle whose eutectic temperature is 139° C. is used as the Sn—Bi solder particle, it is preferable that firstly, a part of Sn-58Bi solder particles 300 is melted by heating it at 139° C. or higher and 149° C. or lower, and then gradually heated to a temperature range of 159° C. or higher and 230° C. or lower. Note here that by appropriately selecting a temperature, the thermosetting resin component included in via paste 260 is cured.
Next, the present exemplary embodiment is specifically described with reference to Examples. Note here that the contents of the Examples are not to be in any way construed as limiting the scope of the present exemplary embodiment. Firstly, raw materials used in Examples are described below.
A metal component including copper particles and Sn—Bi solder particles at a blending ratio as in Table 1, and a resin component including epoxy resin and a curing agent are blended, and then mixed by using a planetary mixer. Thereby, via paste is produced. The blending ratio of the resin components includes 10 parts by weight of the epoxy resin and 2 parts by weight of the curing agent, both relative to 100 parts by weight of a total of the copper particles and the Sn—Bi solder particles.
Protective films are attached to both surfaces of a resin sheet. Then, 100 holes each having a diameter of 150 μm are perforated by using laser from the outer side of the resin sheet to which protective films are attached.
Next, through-holes are filled with the prepared via paste. Then, the protective films on the both surfaces are peeled off, thereby forming protruding portions each formed of the via paste partially protruding from each of the through-holes.
Next, copper foil is disposed on the both surfaces of the resin sheet so as to cover the protruding portions. Then, release paper is disposed on a die below a hot press machine to form a laminated body of the copper foil and the resin sheet, and pressure of 3 MPa is applied to the laminated body. Then, a temperature of the laminated body is increased from an ordinary temperature of 25° C. to a maximum temperature of 220° C. in 60 minutes, kept at 220° C. for 60 minutes, and then cooled to the ordinary temperature over 60 minutes. In this way, a wiring board is obtained.
Resistance values of 100 via-hole conductors formed in the obtained wiring board are measured by a four-terminal method. Then, the initial resistance value and the maximum resistance value are obtained for each of the 100 via-hole conductors. In the initial resistance values, values of 2 mΩ or less are evaluated as “A” and values exceeding 2 mΩ are evaluated as “B.” Also, in the maximum resistance values, values of less than 3 mΩ are evaluated as “A”, and values of more than 3 mΩ are evaluated as “B.”
Herein, the initial resistance value (initial average resistance value) is calculated by forming a daisy chain including 100 vias, measuring the total resistance values of the 100 vias, and dividing the measured values by 100. Furthermore, the maximum resistance value is a maximum value among the average resistance values of 100 daisy chains each including 100 vias. Note here that Table 1 shows resistance values (mΩ) and specific resistance values (m·Ω).
The wiring board whose initial resistance value has been measured is subjected to 500 cycles of heat cycle tests. The via-hole conductors having 10% or less of change rate with respect to the initial resistance value are evaluated as “A,” and those having more than 10% of change rate are evaluated as “B”.
The results are shown in Table 1. Furthermore,
From
Furthermore, a quadrangle having apexes at points C (0.79:0.09:0.12), D (0.89:0.10:0.01), E (0.733:0.240:0.027), and F (0.564:0.183:0.253) is evaluated as “A” in evaluation of all of the initial resistance value, the maximum resistance value, and the connection reliability. Herein, the point E shows Example 14 (E14), and the point F shows Example 17 (E17). In this way, the weight ratio (Cu:Sn:Bi) in the ternary diagram is made to be in the range surrounded by the quadrangle having apexes at points C (0.79:0.09:0.12), D (0.89:0.10:0.01), E (0.733:0.240:0.027), and F (0.564:0.183:0.253), and thereby the weight ratio of Cu having a lower resistance value is increased, so that low resistance of the via hole is achieved. Furthermore, all of Cu and Sn are subjected to an alloying reaction, and thereby Sn—Bi is not melted again. Thus, a printed wiring board having high reliability is achieved.
Furthermore, in a composition region of Comparative Example 7 (C7) which is plotted with the “white triangle” in
Furthermore, in regions of Comparative Examples 2, 4, 6, and 9 (C2, C4, C6, and C9) each of which is plotted with the “white square” in
Furthermore, in the composition in a composition region of Comparative Example 1 (C1) which is plotted with the “black circle” in
Furthermore, in a composition region of Comparative Examples 3, 5, and 8 (C3, C5, and C8) each of which is plotted with the “black triangle” in
Note here that samples used for measurement are pellets made of via paste and having different treatment temperatures. For the X-ray diffraction, “RINT-2000” manufactured by Rigaku Corporation is used.
From the graph of the X-ray diffraction shown in
When the temperature is 150° C., peak V of Cu6Sn5 appears, although it is only slight, in addition to peak I of Cu, peak II of Bi, and peak III of Sn.
When the temperature is 175° C., peak IV of Cu3Sn appears in addition to peak I of Cu, peak II of Bi, and peak V of Cu6Sn5. Peak III of Sn almost disappears. From the above mention, it is shown that an alloying reaction between Cu particles and Sn—Bi solder particles, and furthermore, a formation reaction of an intermetallic compound uniformly proceed.
In the graph of
As mentioned above, in the present exemplary embodiment, the intermetallic compound is not Cu6Sn5 but Cu3Sn that is more stable, and thereby the reliability of the via-hole conductor is enhanced. In other words, in the present exemplary embodiment, it is possible to carry out an alloying reaction (or a formation reaction of an intermetallic compound) in which an intermetallic compound is Cu3Sn that is more stable than Cu6Sn5.
Note here that the thickness of the heat-resistant film that is incompressible member 220 is desirably 3 μm or more and 55 μm or less, further desirably, 50 μm or less and yet further desirably 35 μm or less. When the thickness of the heat-resistant film is less than 3 μm, the film strength is deteriorated, and a compression effect of via paste 260 may not be obtained. Since a heat-resistant film having a thickness of more than 55 μm is specific and expensive, a heat-resistant film having a thickness of 55 μm or less is preferably used.
Furthermore, the thickness per one side of thermosetting adhesive layer 210 provided on the surface of incompressible member 220 is desirably 1 μm or more and 15 μm or less. When the thickness is less than 1 μm, predetermined adhesion strength may not be obtained. Furthermore, when the thickness is more than 15 μm, a compression effect of via paste 260 may not be obtained. Note here that it is useful that the thickness of incompressible member 220 is larger than the thickness of one side of thermosetting adhesive layer 210.
When the thickness of incompressible member 220 is 75 μm, the volume fraction of metal portion 190 in via-hole conductor 140 may be able to be increased only to about 60 vol % or more and 70 vol % or less.
For example, when the thickness of incompressible member 220 is 50 μm (when 10 μm-thick thermosetting adhesive layer 210 is formed on each of both surfaces thereof, the thickness of electric insulating base material 130 is 70 μm), the volume fraction of metal portion 190 in via-hole conductor 140 is 80 vol % or more and 82 vol % or less.
When the thickness of incompressible member 220 is 40 μm (when 10 μm-thick thermosetting adhesive layer 210 is formed on each of both surfaces thereof, the thickness of electric insulating base material 130 is 60 μm), the volume fraction of metal portion 190 in via-hole conductor 140 becomes 83 vol % or more and 85 vol % or less.
When the thickness of incompressible member 220 is 30 μm (when 10 μm-thick thermosetting adhesive layer 210 is formed on each of both surfaces thereof, the thickness of electric insulating base material 130 is 50 μm), the volume fraction of metal portion 190 in via-hole conductor 140 becomes 89 vol % or more and 91 vol % or less.
When the thickness of incompressible member 220 is 20 μm (when 10 μm-thick thermosetting adhesive layer 210 is formed on each of both surfaces thereof, the thickness of electric insulating base material 130 is 40 μm), the volume fraction of metal portion 190 in via-hole conductor 140 becomes 87 vol % or more and 95 vol % or less.
When the thickness of incompressible member 220 is 10 μm (when 10 μm-thick thermosetting adhesive layer 210 is formed on each of both surfaces thereof, the thickness of electric insulating base material 130 is 30 μm), the volume fraction of metal portion 190 in via-hole conductor 140 becomes 98 vol % or more and 99.5 vol % or less.
As mentioned above, the smaller the thickness of incompressible member 220 is, the higher the effect is. However, the thickness is appropriately selected according to the diameter, density, and application of use, or the like, of via-hole conductor 140.
From the above mention, it is shown that when incompressible member 220 is used, the volume fraction of metal portion 190 in via-hole conductor 140 is increased.
A wiring board in accordance with the present exemplary embodiment has effects in reducing a cost, reducing a size, improving performance, and enhancing reliability, and therefore it is used for portable telephones or the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-283189 | Dec 2011 | JP | national |
| 2012-128574 | Jun 2012 | JP | national |
| 2012-210809 | Sep 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/008234 | 12/25/2012 | WO | 00 | 5/20/2013 |
