Conductive joint, method for manufacturing a conductive joint, and method for joining a first substrate and a second substrate.

Abstract

The present invention aims to provide a conductive bond that exhibits excellent conductivity both during initial testing and after electromigration (EM) testing. [Solution] A conductive bond comprising a first substrate 10, a second substrate 20, a conductive core material 40 located between the first substrate and the second substrate, and solder 30 located at the joint between the first substrate, the second substrate and the core material, containing 20% ​​by mass to 70% by mass of Bi and the remainder being Sn and unavoidable impurities, wherein the conductive bond satisfies the following formula (1): 65% ≤ D B / D A ×100 ≤ 100% (1) (D A : Distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the aforementioned nucleus material.

Description

Technical Field

[0001] The present invention relates to a conductive joint, a method for manufacturing a conductive joint, and a method for joining a first substrate and a second substrate for forming the conductive joint. In particular, the present invention relates to a conductive joint excellent in conductivity after initial and electromigration (EM) tests, a method for manufacturing the conductive joint, further to a conductive joint excellent in conductivity after initial and EM tests and having a small increase rate of electrical resistance value (resistance increase rate) after the EM test, a method for manufacturing the conductive joint, and a method for joining a first substrate and a second substrate for forming the conductive joint.

Background Art

[0002] In recent years, due to the development of small information devices, the electronic components mounted thereon are rapidly miniaturized. In order to cope with the narrowing of connection terminals and the reduction of mounting area due to the demand for miniaturization, ball grid arrays (hereinafter referred to as "BGA") having electrodes on the back surface are applied.

[0003] Examples of electronic components to which BGA is applied include semiconductor packages. In a semiconductor package, a semiconductor chip having electrodes is sealed with resin. Solder bumps are formed on the electrodes of the semiconductor chip. These solder bumps are formed by joining solder balls to the electrodes of the semiconductor chip. A semiconductor package to which BGA is applied is placed on a printed circuit board so that each solder bump contacts a conductive land of the printed circuit board, and is mounted on the printed circuit board by joining the melted solder bumps and the lands by heating.

[0004] Solder bumps that do not cause defects such as the solder balls being crushed by their own weight or being deformed during solder melting have been proposed. In order to prevent such defects, specifically, it has been proposed to use a ball formed of metal or resin as a core and use a core material coated with solder as the solder bump.

[0005] Patent Document 1 discloses a Cu core ball in which a Sn-Bi solder alloy containing Sn and Bi is plated onto the Cu core ball. In recent years, the practical application of joining using Sn-Bi alloys, such as the plating coating used in Patent Document 1, has been progressing. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6969070 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The metallic structure of Sn-Bi low-temperature solder is mainly composed of Sn and Bi phases. However, because the Bi phase has low electrical and thermal conductivity, the overall conductivity of Sn-Bi solder is inferior to that of Sn-based solder.

[0008] Furthermore, when an electric current flows through a material, a phenomenon called electromigration (EM) occurs, in which atoms receive the kinetic energy of the electron flow and are pushed towards the positive electrode (anode). When EM occurs, the mechanical and electrical reliability of the conductive part decreases due to vacancy formation and component segregation. In particular, joints made with Sn-Bi low-temperature solder, which has a low melting point, low conductivity, and a two-phase structure, are generally prone to structural degradation due to EM, and are considered to be prone to a decrease in reliability and performance as a conductive joint. Specifically, in the EM reaction of Sn-Bi solder, Bi, which readily receives kinetic energy from the current, is pushed towards the positive electrode, and Sn is pushed back to the negative electrode as a reaction, resulting in a two-phase separated structure. In this state, the conductivity and heat transfer performance of the joint decreases due to the low conductivity and low thermal conductivity of Bi, and in addition, the structure becomes mechanically fragile. In parallel, recent semiconductor package structures are becoming miniaturized, and the need to handle larger currents is increasing, leading to a trend of increasing current density at solder joints. Consequently, the risk of EM generation is rising in terms of structure and application. As described above, when a conductive joint is formed using a Sn-Bi alloy, an improvement in conductive performance, including EM resistance, is required. Similarly, when a conductive joint is formed using a solder material (such as a metal core ball) having a core material and a solder layer of a Sn-Bi alloy, an improvement in conductive performance is also required. In view of these circumstances, the present invention aims to provide a conductive bond that exhibits excellent conductivity both initially and after electromigration (EM) testing. Another objective of the present invention is to provide a conductive bond that, in addition to exhibiting excellent conductivity both initially and after electromigration (EM) testing, exhibits a small rate of increase in electrical resistance (resistance increase rate) after EM testing. [Means for solving the problem]

[0009] As a result of diligent research to solve the above problems, the inventors have found that the above problems can be solved by using solder of a specific composition and satisfying the conditions of formula (1) described later, and have completed the present invention. Specific embodiments of the present invention are as follows.

[0010] [1] A conductive joint, First substrate, Second substrate, A conductive core material located between the first substrate and the second substrate, and A solder present at the position where the first substrate, the second substrate, and the core material are joined, containing 20% ​​to 70% by mass of Bi and the remainder being Sn, as well as unavoidable impurities. Includes, The conductive joint that satisfies the following formula (1): 65% ≤ D B / D A ×100 ≤ 100% (1) (D A : Distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the aforementioned nucleus material. [2] The above D AThe conductive joint according to [1], wherein the size is 10 to 4620 μm. [3] The D B The conductive joint according to [1] or [2], wherein the size is 10 to 3000 μm. [4] The solder further contains an element selected from the group consisting of Ag, Sb, Cu, Ni, Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, Pb, and combinations thereof, and the conductive joint according to any one of [1] to [3]. [5] The solder contains 0 mass% or more and 5 mass% or less of Ag, 0 mass% or more and 10 mass% or less of Sb, 0 mass% or more and 1 mass% or less of Cu, or 0 mass% or more and 1 mass% or less of Ni, and the conductive joint according to [4]. [6] The nuclear material contains at least one selected from the group consisting of Ni and Co, and the conductive joint according to any one of [1] to [5] includes a barrier layer. [7] The nuclear material contains Cu, and the conductive joint according to any one of [1] to [6]. [8] A method for manufacturing the conductive joint according to any one of [1] to [7], (1) A step of adjusting the distance between the first substrate and the second substrate, and (2) A step of disposing a solder material having a conductive nuclear material and a solder layer that exists around the nuclear material and contains 20 mass% or more and 70 mass% or less of Bi and the balance of Sn between the first substrate and the second substrate, and (3) A step of joining the first substrate, the second substrate, and the nuclear material with the solder layer by melting at least the surface of the solder layer and then solidifying it, including The conductive joint satisfies the following formula (1) 65%≦D B / D A ×100≦100% (1) (D A : The distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the nuclear material) The method. [9] A method for joining a first substrate and a second substrate, (1') A step of adjusting the distance between the first substrate and the second substrate, (2') A step of placing a solder material between the first substrate and the second substrate, the solder material having a conductive core material and a solder layer surrounding the core material containing 20% ​​by mass or more and 70% by mass or less of Bi and the remainder being Sn and unavoidable impurities, (3') A step of forming a conductive joint that satisfies the following formula (1) by melting at least the surface of the solder layer and then solidifying it, thereby joining the first substrate, the second substrate and the core material with the solder layer. The method, including: 65% ≤ D B / D A ×100 ≤ 100% (1) (D A : Distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the aforementioned nucleus material. [Effects of the Invention]

[0011] The conductive bond of the present invention exhibits excellent conductivity both initially and after electromigration (EM) testing. Furthermore, in some cases, in addition to exhibiting excellent conductivity both initially and after electromigration (EM) testing, the conductive bond also exhibits a small rate of increase in electrical resistance (resistance increase rate) after EM testing. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a cross-sectional view showing one embodiment of the conductive bond of the present invention. [Figure 2] Figure 2 is a graph showing the electrical resistance values ​​of the conductive joints in Examples 1-6 and Comparative Examples 1-3 during EM testing. [Figure 3] Figure 3 is a graph showing the resistance increase rate in EM tests for the conductive joints of Examples 1-6 and Comparative Examples 1-3. [Modes for carrying out the invention]

[0013] The conductive joint of the present invention, a method for manufacturing the conductive joint, and a method for joining a first substrate and a second substrate will be described below. In this specification, when "X~Y" is used to represent a numerical range, that range includes the values ​​at both ends.

[0014] 1. Conductive conjugate The conductive bonding body of the present invention is First substrate, Second substrate, A conductive core material located between the first substrate and the second substrate, and A solder present at the position where the first substrate, the second substrate, and the core material are joined, containing 20% ​​to 70% by mass of Bi and the remainder being Sn, as well as unavoidable impurities. Includes, The following equation (1) is satisfied: 65% ≤ D B / D A ×100 ≤ 100% (1) (D A : Distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the aforementioned nucleus material.

[0015] Figure 1 is a cross-sectional view showing one embodiment of the conductive bond of the present invention. In Figure 1, a semiconductor package is used as the first substrate 10, and an electrode 11 is provided on one side of the first substrate 10. A printed circuit board is used as the second substrate 20, and an electrode 21 is provided on one side of the second substrate 20. In Figure 1, both the first substrate 10 and the second substrate 20 are flat plates, and one side of each is parallel to the other. The distance between the first substrate 10 and the second substrate 20 (length of side AB) is D A It is (μm). In Figure 1, a core ball is used as the conductive core material 40, and this core ball is located between the first base material 10 and the second base material 20. The core ball has a center of gravity G, and the diameter of the core ball (the length of the side A'B' passing through the center of gravity G) is DB (μm). The solder 30 contains 20% to 70% by mass of Bi and the remainder of Sn, and is located at the position where the first substrate 10, the second substrate 20, and the core material 30 are joined. And in the conductive joint shown in Figure 1, D A (μm) and D B (μm) satisfies the relationship in equation (1) above.

[0016] The conductive bond is not particularly limited, but can consist of electronic components, electronic circuits made of substrates, or mounting parts for components and circuits. Specifically, examples include outer bump bonds of BGAs as external electrodes of a package, interposer bonds for vertical bonding between substrates or for substrate bonding, and inner bump bonds inside semiconductors as internal circuit formation and standoff adjustment materials.

[0017] [Formula (1)] The conductive bond of the present invention, by satisfying the above formula (1), exhibits excellent conductivity both initially and after electromigration (EM) testing. Furthermore, in some cases, the conductive bond of this embodiment, by satisfying the above formula (1), exhibits a smaller rate of increase in electrical resistance after EM testing (resistance increase rate). In practical applications, the degree of change from the initial state immediately after implementation is often considered important. Therefore, the rate of resistance increase from the initial state (rate of resistance increase after EM testing) is frequently used to determine the degradation of the conductive performance of the junction.

[0018] Regarding the significance of equation (1) above, while not bound by theory, it can be inferred as follows. As a result of our investigations, we found that the inherent low conductivity and low EM resistance of Sn-Bi solder can be dramatically improved by increasing the ratio of the diameter of the core material to the distance between the two substrates. Generally, current from substrate electrodes passes more selectively through highly conductive materials and structures. In this invention, by providing a conductive core material between the two electrodes, the current path is focused to the core material, and the amount of current passing through the low-conductivity Sn-Bi solder is reduced, thereby improving conductivity. For this to work, the core material must have a higher conductivity than the Sn-Bi solder. Furthermore, in order to extract this improvement, the core material must satisfy a certain distance ratio between the two electrodes through which the current flows. This is because if the size of the core material is too small compared to the distance between the electrodes, the distance that the current from the substrate electrodes has to pass through the Sn-Bi solder before reaching the core material increases, and the conductivity of the entire conductive joint decreases.

[0019] Furthermore, the existence of core materials is extremely important from the perspective of ensuring EM resistance. When considering EM resistance, there are two perspectives: one that evaluates the conductivity of the junction itself during the EM reaction, and another that evaluates the rate of change in conductivity from the initial stage. When aiming to improve EM resistance from the former perspective, the presence of a core material reduces the amount of Bi in the entire junction, thus reducing the amount of Bi deposited on the positive electrode that induces a decrease in conductivity and embrittlement, and making it easier to improve the numerical value indicating conductivity. Furthermore, even if Bi is deposited on the positive electrode due to the progression of EM, as before the EM reaction, most of the current preferentially passes through the highly conductive core material, so the increase in the overall resistance of the junction is kept to a minimum. Furthermore, when aiming to improve EM resistance from the latter perspective, it is necessary to delay or suppress the EM reaction itself. As mentioned earlier, current preferentially flows through the core material with high conductivity, but as the amount of current passing through the core material increases, the amount of current passing through the Sn-Bi solder relatively decreases. In particular, when a large portion of the current passes through the core material, insufficient current flows through the Sn-Bi solder to promote the EM reaction, and the movement of Bi is greatly delayed, significantly slowing down the EM reaction.

[0020] Here, if the geometric balance between the distance between the two substrates and the core material, or the solder composition forming the joint, is not appropriate, it is thought that excellent conductivity and EM resistance will not be exhibited, and in some cases, these properties may even deteriorate. Therefore, it is not reasonable to assume that good properties can be obtained simply by inserting a metal core ball between two substrates without considering the geometric balance and solder composition forming the joint mentioned above. In the case of using a Sn-Bi-based solder of a specific composition, as in the present invention, the geometric balance between the distance between the two substrates and the core material shown in equation (1) above is considered particularly suitable.

[0021] D B / D A The value of ×100 represents the percentage (%) of the distance between the two substrates that is occupied by the diameter of the core material. D B / D A The value of ×100 is 65% or more, preferably 70% or more, more preferably 80% or more, and most preferably 85% or more. B / D A A value of 65% or higher for ×100 can improve conductivity and EM resistance. While not strictly theoretical, D B / D A If the value of ×100 is less than 65%, the proportion of the core material with excellent conductivity in the entire conductive path becomes smaller. In this case, as explained above, the current is forced to pass through the metal structure of the solder, which has higher resistance (relatively to the core material), over a longer distance, and the electrical resistance value in the conductive path increases, so it is thought that the conductivity of the conductive joint decreases. Also, D B / D A The value of ×100 is 100% or less, and can also be 99% or less, 95% or less, or 90% or less. B / D A The value of ×100 can also be 100%. D B / D AA value of 100 for ×100 means that the core material is in contact with the first and second substrates, but this does not necessarily mean that there are no gaps; gaps of about 1 to 10 μm may exist. Intermetallic compounds and the like may be present in these gaps. D B / D A The above numerical range of ×100 can be combined in any way. D B / D A If the value of ×100 falls within the above range, it exhibits excellent conductivity both initially and after electromigration (EM) testing. Furthermore, in some cases, the rate of increase in electrical resistance after EM testing may be smaller. D A and D B The definitions of each will be explained later.

[0022] [First substrate and second substrate] The first substrate is not particularly limited, but may consist of a package (semiconductor package), a printed circuit board, an interposer, or an electronic component including a solder joint. The shape of the first substrate is not particularly limited, but can be flat, cylindrical, or lattice-shaped. The first substrate is not particularly limited, but it may have surface electrodes (electrodes on the surface of the substrate).

[0023] The second substrate is not particularly limited, but may consist of a package (semiconductor package), a printed circuit board, an interposer, or an electronic component including a solder joint. The shape of the second substrate is not particularly limited, but can be flat, cylindrical, or lattice-shaped. The second substrate is not particularly limited, but may have surface electrodes (electrodes on the surface of the substrate).

[0024] The positional relationship between the first and second substrates is not particularly limited, but they can be arranged so that one side is parallel to the other. By arranging them in a parallel state, the mechanical and / or electrical reliability during joining can be improved. As a secondary effect of using a core material, this parallelism becomes easier to achieve. If both the first substrate and the second substrate have electrodes, the electrodes of the first substrate and the electrodes of the second substrate can be joined together with a core material and solder to electrically connect them.

[0025] In this specification, D A The distance between the first substrate and the second substrate (in μm) is the distance of the line segment that shows the smallest value among the line segments formed by connecting a point on the surface of the first substrate (or the electrode if the first substrate has an electrode) and a point on the surface of the second substrate (or the electrode if the second substrate has an electrode), and passing through the centroid of the core material. In Figure 1, the first substrate and the second substrate are flat plates with one side parallel to the other, and in this case, the length of the line segment AB passing through the centroid G of the core material is D A This is the result.

[0026] The above D A The particle size is not particularly limited, but is preferably 10 to 4620 μm, more preferably 50 to 1000 μm, and most preferably 150 to 400 μm. A While not particularly limited, the thickness can be 25 μm, 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 500 μm, 1000 μm, 1500 μm, 2000 μm, 3000 μm, or 4000 μm, and may be within a range between any two of these values.

[0027] (nuclear material) In this specification, D B The diameter of the core material (μm) is the distance of the portion of the line segment formed by connecting a point on the surface of the first substrate and a point on the surface of the second substrate, passing through the centroid of the core material, where the core material exists. B This is the above D AThis can also be described as the distance of the portion of the line segment corresponding to the core material that exists. In Figure 1, the length of the line segment A'B' passing through the centroid G of the core material is D. B This is the result.

[0028] The above D B The particle size is not particularly limited, but is preferably 10 to 3000 μm, more preferably 50 to 1000 μm, and most preferably 100 to 300 μm. B While not particularly limited, the thickness can be 25 μm, 50 μm, 100 μm, 140 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 500 μm, 1000 μm, 1500 μm, 2000 μm, or 3000 μm, and may be within a range between any two of these values.

[0029] The position of the core material between the first substrate and the second substrate is not particularly limited and may be at any position. For example, the core material may be located close to the first or second substrate, or it may be located midway between the first and second substrates.

[0030] The core material is electrically conductive and functions as a conductive path material. In this specification, "having conductivity" means having an electrical conductivity of 1.0 × 10⁻⁶. 5 This means that the value is greater than or equal to S / m. The electrical conductivity is based on the information on page 13 of "Metal Data Book, Revised 4th Edition" edited by the Japan Institute of Metals (Maruzen Co., Ltd., published in 2004).

[0031] The core material has a conductivity of Bi (approximately 0.9 × 10⁻¹⁰) as an electrical conduction pathway. 6The material may contain or consist of a metal having conductivity equivalent to or greater than S / m (a value based on the information on page 13 of "Metal Data Book, Revised 4th Edition," edited by the Japan Institute of Metals, published by Maruzen Co., Ltd. in 2004). Such a metal is not particularly limited, but it may include elemental metals selected from the group consisting of Cu, Ni, Ag, Pb, Al, Sn, Fe, Zn, In, Sb, Co, Au, Pt, Cr, La, Mo, Nb, Pd, Ti, Zr, Mg, Rh, Ir, W, and Ru, alloys of two or more of these, metal oxides of these, or mixed oxides of these metals. In particular, the core material may consist of pure Cu or an alloy composition with Cu as the main component. When the core material is composed of a Cu alloy, the Cu content can be 50% by mass or more. Furthermore, the core material can be a spherical core ball or a columnar core column. From the viewpoint of conductivity, the core material is preferably a spherical Cu core ball. Alternatively, a core ball made of Sn-based solder may be selected as one of the practical core materials other than Cu cores. Specifically, a Pb-free solder composition centered on Sn-3.0Ag-0.5Cu, which has excellent strength and processability, can be used as a core ball composition.

[0032] The core material is not particularly limited, but can be a single element (single composition), a shell structure, a fibrous structure, or a porous structure. The core material only needs to have a structure that contributes to conductivity; the remaining non-conductive parts can be made of a low-conductivity material equivalent to or less than that of Bi. For example, metal-plated plastic is also included as such a core material.

[0033] The core material is not particularly limited, but may include a barrier layer containing at least one selected from the group consisting of Ni and Co. The presence of a barrier layer in the core material allows for control of the reaction amount between the outer solder (solder present on the outer periphery of the core material) and the core material. The barrier layer can be formed on the surface of the core material.

[0034] The thickness of the barrier layer (on one side) is not particularly limited, but is preferably 1 to 20 μm, more preferably 1.5 to 18 μm, and most preferably 2 to 16 μm.

[0035] [solder] Solder is present at the joints between the first substrate, the second substrate, and the core material. At least a portion of the solder may be present between the first substrate and the core material, between the second substrate and the core material, or both. Also, at least a portion of the solder may be present around the contact point between the first substrate and the core material, around the contact point between the second substrate and the core material, or both. For example, D B / D A When the value of ×100 is 100%, there is either no solder present at the contact point between the core material and the two substrates, or only a very small amount. At the above contact point, the core material and the two substrates are in a point contact structure cross-linked by an intermetallic compound consisting of solder or electrode material components. The presence of solder around the contact point allows the core material and the two substrates to be joined with sufficient strength. The solder can join the electrode of the first substrate to the electrode of the second substrate.

[0036] The solder contained in the conductive joint may consist of solder derived from the solder layer in the solder material (having a core material and a solder layer) described later. In addition, solder paste may be applied to the electrodes of either the first substrate or the second substrate, or both. The solder paste may be used to bond a core material to the electrodes to form solder bumps, or it may not be used for the purpose of forming such solder bumps. When solder paste is applied to the electrodes, the solder contained in the conductive joint may consist of solder derived from the solder layer, solder derived from the solder paste on the electrodes of the first substrate, and / or solder derived from the solder paste on the electrodes of the second substrate. The solder derived from the solder layer, the solder derived from the solder paste on the electrodes of the first substrate, and / or solder derived from the solder paste on the electrodes of the second substrate may have the same or different compositions. If the solder derived from the solder layer, the solder derived from the solder paste on the electrodes of the first substrate, and / or solder derived from the solder paste on the electrodes of the second substrate have different compositions, the solder contained in the conductive joint will be formed by mixing the compositions of these solders. The amount of solder derived from the solder layer relative to the solder contained in the conductive joint is not particularly limited, but can be 100% by volume, 1 to 99% by volume, or 30 to 80% by volume.

[0037] The composition of the solder derived from the solder paste applied to the first or second substrate is not particularly limited and includes Sn-58Bi-0.5Sb-0.015Ni, Sn-40Bi-0.5Cu-0.03Ni, Sn-20Bi-0.5Sb-0.015Ni, Sn-20Bi-0.5Cu-0.03Ni, Sn-70Bi-0.5Sb-0.015Ni, Sn-70Bi-0.5Cu-0.03Ni, Sn-40Bi-0.03Ni, Sn-58Bi-1Cu-0.5Sb-0.015Ni, Sn-40Bi-1Cu-0.03Ni, and Sn-58Bi-0.5 Sb, Sn-40Bi-0.5Cu, Sn-58Bi-0.5Sb-1Ni, Sn-40Bi-0.5Cu-1Ni, Sn-58Bi-0.015Ni, Sn-58Bi-10Sb-0.015Ni, Sn-40Bi-0.5Cu-0.03Ni-10Sb, Sn-58Bi -0.5Sb-0.015Ni-5Ag, Sn-40Bi-0.5Cu-0.03Ni-5Ag, Sn-58Bi-0.5Sb-0.01 5Ni-0.03Ge, Sn-58Bi-0.5Sb-0.015Ni-0.03P, Sn-58Bi-0.5Sb-0.015Ni-0 .03Ga, Sn-58Bi-0.5Sb-0.015Ni-0.03As, Sn-58Bi-0.5Sb-0.015Ni-0.03Fe, Sn-58Bi-0.5Sb-0.015Ni-0.03Co, Sn-58Bi-0.5Sb-0.015Ni-0.03Pd, S n-58Bi-0.5Sb-0.015Ni-0.03Zn, Sn-58Bi-0.5Sb-0.015Ni-0.03Zr, Sn-58 Bi-0.5Sb-0.015Ni-0.03Pb, Sn-58Bi-0.5Sb-0.015Ni-0.03Ti, Sn-58Bi-0 .5Sb-0.015Ni-0.03Ce, Sn-58Bi-0.5Sb-0.015Ni-0.03In, Sn-58Bi-0.5S b-0.015Ni-1Ag-0.03Ge-0.03P-0.03Ga-0.03As-0.03Fe-0.03Co-0.03Pd- 0.03Zn-0.03Zr-0.03Pb-0.03Ti-0.03Ce-0.03In, Sn-58Bi, Sn-40Bi, Sn-20Bi, Sn-40Bi-1Cu-1Ni-10Sb, Sn-70Bi-1Cu-1Ni-10Sb, Sn-75Bi-0.5Sb-0.The solder paste may consist of or contain 015Ni, Sn-75Bi-0.5Cu-0.03Ni, Sn-15Bi-0.5Sb-0.015Ni, Sn-15Bi-0.5Cu-0.03Ni, or a combination of two or more of these. The solder derived from the solder paste applied to the first substrate and the solder derived from the solder paste applied to the second substrate may have the same or different compositions.

[0038] The Bi content in the solder is 20% by mass or more, preferably 25% by mass or more, more preferably 30% by mass or more, and even more preferably 35% by mass or more. The Bi content in the solder can also be 38% by mass or more, or 40% by mass or more. If the Bi content is less than 20% by mass, the melting point of the solder will not be lowered sufficiently, and it will not achieve its purpose as a solder for low-temperature mounting. Furthermore, when the Bi content is less than 20% by mass, in the high-temperature range the Bi phase is absorbed by Sn, whose solid solubility limit has increased, and the single Bi phase almost disappears from the structure. In addition, as the ratio of Sn and Bi deviates from the eutectic, the melting point rises and the atomic migration speed decreases, segregation of the single Bi phase by EM becomes almost negligible. Furthermore, the Bi content in the solder is 70% by mass or less, preferably 65% ​​by mass or less, more preferably 60% by mass or less, and most preferably 58% by mass or less. The Bi content in the solder can also be 55% by mass or less or 50% by mass or less. If the Bi content exceeds 70% by mass, the melting point of the solder will rise too high, failing to achieve its purpose as a solder for low-temperature mounting. In addition, sufficient conductivity cannot be obtained, and the homogeneity of the structure is impaired. The above numerical ranges for the Bi content can be combined arbitrarily.

[0039] Solder may further contain Ag, Sb, Cu, Ni, Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, Pb, In, or combinations of two or more of these. By adding Ag and / or Sb, resistance to temperature cycling (TCT), hardness, strength, microstructure, and EM resistance can be improved. By adding Cu, Ni, Zn, Ti, In, and / or Ce, the strength and microstructure can be improved, and the occurrence of electrode erosion and the growth of IMC (intermetallic compounds) at the bonding interface can be suppressed. By adding P, Ge, Ga, and / or As, the surface of the solder can be modified and oxidation can be suppressed. The structure and compound state of solder can be modified by adding Fe and / or Co. Specifically, Fe and / or Co refine the solidification structure of the alloy by forming compounds with Sn in the solder, and also refine the interfacial compounds by solid-solving them into the compound. These refinements are useful in improving reliability and electrical properties. Adding PD stabilizes the relationship between the solder and the plating (the barrier layer of the core material), making it easier to form the solder layer. By adding lead (Pb), the rate of missing solder joints can be reduced, and the wetting behavior can be improved.

[0040] Solder may contain 0% to 5% by mass of Ag, 0% to 10% by mass of Sb, 0% to 1% by mass of Cu, or 0% to 1% by mass of Ni. The content of each of these elements can be applied in combination of two or more types. Solder may contain Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, Pb, or In in amounts of 0% by mass or more and 0.1% by mass or less. The content of each of these elements can be applied in combination of two or more types. By keeping the content of each of these elements (Ag, Sb, Cu, Ni, Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, Pb, In) within the above numerical range, it is possible to achieve miniaturization of IMC, improvement of TCT resistance, reduction of melting point, improvement of ductility, improvement of wettability, improvement of conductivity, etc.

[0041] The Sn content in the solder is not particularly limited, but is 30% by mass or more, preferably 35% by mass or more, more preferably 40% by mass or more, and most preferably 42% by mass or more. Furthermore, the Sn content in the solder is not particularly limited, but is 80% by mass or less, preferably 75% by mass or less, more preferably 70% by mass or less, and even more preferably 65% ​​by mass or less. The above numerical ranges for Sn content can be combined arbitrarily.

[0042] The solder may consist of 20% to 70% by mass of Bi, any element (as mentioned above: Ag, Sb, Cu, Ni, Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, In, and / or Pb), and the remainder being Sn, along with unavoidable impurities.

[0043] The electrical resistance value (initial, room temperature) of the conductive joint of the present invention is not particularly limited, but is preferably 0.03 Ω or less, and more preferably 0.02 Ω or less. The electrical resistance value (initial, room temperature) of the conductive joint is not particularly limited, but can be greater than 0 Ω. The above numerical ranges for the electrical resistance value (initial, room temperature) of the conductive joint can be arbitrarily combined. The electrical resistance value (initial, room temperature) of the conductive joint can be measured as the electrical resistance value (initial, room temperature) of the test circuit in accordance with the procedure and method described in "2. Evaluation" and "(1) Conductivity (initial, room temperature)" of the [Examples] described below.

[0044] Electromigration (EM) test of the conductive joint of the present invention: The electrical resistance value after 2500 hours is preferably 0.115 Ω or less, and more preferably 0.075 Ω or less. EM test of the conductive joint: The electrical resistance value after 2500 hours is not particularly limited, but can be greater than 0 Ω. The above numerical ranges for the electrical resistance value after 2500 hours of EM testing of the conductive joint can be arbitrarily combined. EM test of the conductive joint: The electrical resistance value after 2500 hours can be measured as the EM test value of the test circuit after 2500 hours, in accordance with the procedure and method described in "2. Evaluation" and "(2) Electromigration (EM) test" of the [Examples] described below.

[0045] Electromigration (EM) test of the conductive joint of the present invention: The rate of increase in electrical resistance (resistance increase rate) after 2500 hours is preferably 220% or less, and more preferably 180% or less. EM test of the conductive joint: The rate of increase in resistance after 2500 hours is not particularly limited, but can exceed 100%. The above numerical ranges for the rate of increase in resistance after 2500 hours of EM testing of the conductive joint can be arbitrarily combined. EM test of the conductive joint: The rate of increase in resistance after 2500 hours can be measured as the rate of increase in resistance after 2500 hours of EM testing of the test circuit, in accordance with the procedure and method described in "2. Evaluation" and "(2) Electromigration (EM) test" of the [Examples] described below.

[0046] 2. Method for manufacturing a conductive bond The method for manufacturing a conductive joint according to the present invention is: A method for manufacturing the conductive bond described in 1. above, (1) A step of adjusting the distance between the first substrate and the second substrate, (2) A step of placing a solder material between the first substrate and the second substrate, the solder material having a conductive core material and a solder layer surrounding the core material containing 20% ​​by mass or more and 70% by mass or less of Bi, the remainder being Sn, and unavoidable impurities, (3) A step of joining the first substrate, the second substrate, and the core material by melting at least the surface of the solder layer and then solidifying it, Includes, The conductive joint satisfies the following formula (1). 65% ≤ D B / D A ×100 ≤ 100% (1) (D A : Distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the aforementioned nucleus material. The conductive joint manufactured by the method of this embodiment can be the conductive joint described in "1. Conductive Joint" above. In the method of this embodiment, equation (1), DA , and D B This is the formula (1) and D described in "1. Conductive Joint" above. A , and D B These can be treated similarly.

[0047] [Process (1)] In step (1), the method for adjusting the distance between the first substrate and the second substrate is not particularly limited, but one method is to insert a spacer of a specific thickness between the first substrate and the second substrate.

[0048] [(Step (2)] In step (2), the solder material placed between the first substrate and the second substrate comprises a conductive core material and a solder layer surrounding the core material, which contains 20% to 70% by mass of Bi and the remainder of Sn, as well as unavoidable impurities. The solder material can be manufactured by providing the solder layer on the surface of the core material.

[0049] The thickness of the solder layer (on one side) in the solder material is not particularly limited, but is preferably 5 to 200 μm, more preferably 10 to 100 μm, and most preferably 20 to 50 μm. By having the thickness of the solder layer (on one side) within the above numerical range, a sufficient amount of solder bonding can be ensured. The thickness of the solder layer (on one side) is not particularly limited, but can be 5 μm, 10 μm, 15 μm, 17.5 μm, 20 μm, 25 μm, 30 μm, 50 μm, 100 μm, 150 μm, or 200 μm, and may be within the range of any two of these values. The solder layer may consist of or include solder containing 20% ​​to 70% by mass of Bi and the remainder being Sn, as well as unavoidable impurities. The composition of the solder layer described above is not particularly limited and includes Sn-40Bi-0.5Cu, Sn-20Bi-0.5Cu, Sn-70Bi-0.5Cu, Sn-40Bi, Sn-40Bi-1Cu, Sn-40Bi-0.5Cu-1Ni, Sn-40Bi-0.5Cu-10Sb, Sn-40Bi-0.5Cu-5Ag, Sn-58Bi, Sn-20Bi, Sn-40Bi- The solder can consist of 1Cu-1Ni-10Sb, Sn-70Bi-1Cu-1Ni-10Sb, Sn-40Bi-0.5Cu-1Ag, Sn-40Bi-0.5Cu-1Ag-0.5Sb, Sn-40Bi-0.5Cu-0.03Ni, Sn-75Bi-0.5Cu, Sn-15Bi-0.5Cu, or a combination of two or more of these. Among these, Sn-40Bi-0.5Cu is preferred. By using Sn-40Bi-0.5Cu, a solder layer with good coverage and structural homogeneity can be obtained on the outer periphery of the core.

[0050] The composition of the core material in the solder material (diameter, position, type, structure, barrier layer composition, etc.) and the composition of the solder contained in the solder layer (position, content of each element, etc.) can be the same as the composition of the core material and the composition of the solder described in "1. Conductive Joint" above. In particular, the composition of the solder layer in the solder material can be the same as the composition of the solder described in "1. Conductive Joint" (Solder) above.

[0051] [(Step (3)] In step (3), the temperature at which at least the surface of the solder layer is melted is not particularly limited, but is preferably 140 to 220°C, more preferably 150 to 210°C, and most preferably 160 to 200°C. By keeping the temperature within the above numerical range, the following effects can be obtained: improved mounting yield due to reduced warping of the components, reduced energy consumption, and reduced temperature load. The time required to melt at least the surface of the solder layer is not particularly limited, but is preferably 1 to 300 seconds, more preferably 5 to 180 seconds, and most preferably 10 to 60 seconds. By keeping the time within the above numerical range, the effects of preventing missing solder, suppressing oxidation, and reducing energy consumption can be obtained. In step (3), it is sufficient to melt at least the surface of the solder layer. For example, 50% or more of the entire solder layer may be melted, or the entire solder layer may be melted.

[0052] In step (3), the cooling rate when solidifying the molten solder is not particularly limited, but is preferably 1 to 40°C / second, more preferably 3 to 20°C / second, and most preferably 5 to 10°C / second. By keeping the temperature when solidifying the solder within the above numerical range, re-oxidation of the surface can be prevented, and the effect of fine solidification of the metal structure can be obtained.

[0053] 3. Method for joining the first substrate and the second substrate. The method for joining the first substrate and the second substrate of the present invention is as follows: (1') A step of adjusting the distance between the first substrate and the second substrate, (2') A step of placing a solder material between the first substrate and the second substrate, the solder material having a conductive core material and a solder layer surrounding the core material containing 20% ​​by mass or more and 70% by mass or less of Bi, the remainder being Sn, and unavoidable impurities, (3') A step of forming a conductive joint that satisfies the following formula (1) by melting at least the surface of the solder layer and then solidifying it, thereby joining the first substrate, the second substrate and the core material with the solder layer. Includes: 65% ≤ D B / D A ×100 ≤ 100% (1) (D A : Distance (μm) between the first substrate and the second substrate, D B : The diameter (μm) of the aforementioned nucleus material.

[0054] The components (means, temperature, time, etc.) of steps (1') to (3') in the method of this embodiment can be the same as the components of steps (1) to (3) described in "2. Method for manufacturing a conductive bond" above.

[0055] The solder material used in step (2') may be the same as the solder material described in "Step (2)" of "2. Method for Manufacturing a Conductive Joint" above. Furthermore, the components of the core material in the solder material (diameter, position, type, structure, barrier layer configuration, etc.) and the components of the solder contained in the solder layer (position, content of each element, etc.) may be the same as the components of the core material and the solder described in "1. Conductive Joint" or "2. Method for Manufacturing a Conductive Joint" above.

[0056] The conductive joint formed by step (3') may be the conductive joint described in "1. Conductive Joint" above.

[0057] In the method of this embodiment, equation (1), D A , and D B This is the formula (1) and D described in "1. Conductive Joint" above. A , and D B These can be treated similarly.

[0058] The present invention will be described in detail below with reference to examples, but the present invention is not limited to what is described in the examples. [Examples]

[0059] 1. Preparation of evaluation samples [Example 1] A first substrate (a 12mm x 12mm package with a 0.24mm diameter Cu electrode, 1.1mm thick, and flat in shape) and a second substrate (a 29mm x 19mm glass epoxy substrate (FR-4) with a 0.8mm thickness and a 0.24mm diameter Cu electrode, and flat in shape) were prepared. A low-temperature solder paste with a Sn-40Bi composition (numbers are in mass%, the remainder being Sn) was applied to the Cu electrode of the first substrate (mixing ratio: 60% alloy powder by mass, 40% flux by mass; flux composition: flux containing solvent, thixotropic agent, organic acid, etc. (STHi, manufactured by Senju Metal Industry Co., Ltd.)). Using the low-temperature solder paste applied to the first substrate described above, a core ball coated with solder of Sn-40Bi-0.5Cu composition (numbers are in mass%), the remainder being Sn was reflow soldered to the first substrate under conditions of a maximum temperature of 190°C and a maximum temperature holding time of approximately 20 seconds to prepare a package with solder bumps. Meanwhile, a low-temperature solder paste of Sn-58Bi-0.5Sb-0.015Ni composition (numbers are in mass%), the remainder being Sn (mixing ratio: alloy powder 89.5% by mass, flux 10.5% by mass; flux composition: flux containing rosin, solvent, thixotropic agent, organic acid, etc. ("155HF" manufactured by Senju Metal Industry Co., Ltd.)) was printed on one side of the glass epoxy substrate (second substrate). Furthermore, the solder bump package (first substrate) and the printed glass epoxy substrate (second substrate) are parallel, and the distance (D) between one side of the first substrate and one side of the second substrate is... AA laminate was prepared by positioning the first substrate and the second substrate so that the sieve width was 250 μm. In this laminate, the solder bumps of the first substrate and the printed portion (solder paste portion) of the second substrate are in contact. The obtained laminate was reflow mounted under conditions of a maximum temperature of 190°C and a maximum temperature holding time of approximately 20 seconds to prepare a test circuit (conductive joint) including a daisy chain joined with a solder alloy, which was used as an evaluation sample. The final solder alloy composition of the joint is formed by the uniform mixing of the solder coating on the core ball, the solder in the solder paste on the first substrate, and the solder in the solder paste on the second substrate, as shown in Table 1. The amount of low-temperature solder paste applied to the first substrate is minute, and the alloy powder content is also small, so the influence of the composition of the low-temperature solder paste on the final solder alloy composition of the joint is almost negligible.

[0060] [Examples 2-50 and Comparative Examples 1, 2, and 4-9] The final solder alloy composition of the above joint, the solder composition in the solder paste on the second substrate, and the distance (D) between one side of the first substrate and one side of the second substrate. A Evaluation samples for Examples 2-50 and Comparative Examples 1, 2, and 4-9 were prepared in the same manner as in Example 1, except that the configurations of the core ball having the solder layer were changed as shown in Tables 1-6.

[0061] [Comparative Example 3] The final solder alloy composition of the above joint, the solder composition in the solder paste on the second substrate, and the distance (D) between one side of the first substrate and one side of the second substrate. A A sample of Comparative Example 3 was prepared in the same manner as in Example 1, except that the configurations of the core ball having a solder layer were changed as shown in Tables 5 and 6, and the core ball having a solder layer was replaced with a solder ball (containing no core material and consisting only of solder) as shown in Tables 5 and 6.

[0062] [Table 1]

[0063] [Table 2]

[0064] [Table 3]

[0065] [Table 4]

[0066] [Table 5]

[0067] [Table 6]

[0068] 2. Evaluation The evaluation samples prepared as described above for Examples 1-50 and Comparative Examples 1-9 were subjected to the following evaluations. The results of each evaluation are shown in Tables 10-12.

[0069] (1) Conductivity (initial, room temperature) Each test sample was connected to an ultra-high-precision digital resistance meter (RESISTOMAT2305-V205, manufactured by Soken Electric Co., Ltd.), and its electrical resistance was measured at room temperature (25°C) under atmospheric conditions. Conductivity (initial, room temperature) was then evaluated according to the criteria in Table 7 below.

[0070] [Table 7]

[0071] (2) Electromigration (EM) test Each test sample was connected to a compact multi-range DC power supply (Chiyoda Electronics Co., Ltd.: CM30-36 or CM80-13R5) and the current density flowing through the bump section was measured at 25 A / mm² in a silicon oil bath maintained at 120°C. 2 Current was applied in such a manner. The electrical resistance of the energized sample was continuously measured while the current was applied. For Examples 1 to 6 and Comparative Examples 1 to 3, the measurement results (Tables 10 and 12) of the electrical resistance at each time (0, 1250, or 2500 hours) and the resistance increase rate (electrical resistance at each time / electrical resistance at 0 hours × 100 (%)) at each time (1250 or 2500 hours) were graphed and shown in Figures 2 and 3. Then, as a criterion for judging the EM progression suppression force, the electrical resistance and resistance increase rate after 2500 hours were evaluated according to the criteria in Tables 8 and 9 below.

[0072] [Table 8]

[0073] [Table 9]

[0074] [Table 10]

[0075] [Table 11]

[0076] [Table 12]

[0077] Examples 1 to 50 have a solder containing a core material and 20% to 70% by mass of Bi and the remainder being Sn, as well as unavoidable impurities, and the above formula (1) (65% ≤ D B / D AThis relates to a conductive joint that satisfies the condition ×100 ≤ 100%. Comparative Examples 1, 2, 8, and 9 have a solder containing a nuclear material and 20% to 70% by mass of Bi and the remainder being Sn and unavoidable impurities, but do not satisfy the above formula (1) (D B / D A Regarding the joint (×100 < 65%). Comparative Example 3 relates to a joint that uses solder balls and does not contain a core material. Comparative Examples 4-7 are given by the above formula (1) (65% ≤ D B / D A This relates to a joint that satisfies the condition (×100 ≤ 100%), but in which the Bi content in the solder is less than 20% by mass or greater than 70% by mass.

[0078] From the results in Tables 10-12 and Figures 2 and 3, the conductive junctions of Examples 1-50 had low initial electrical resistance values ​​and excellent or good initial conductivity, and also had low electrical resistance values ​​after the EM test (2500 hours) and excellent or good conductivity after the EM test. Furthermore, the conductive junctions of Examples 1-49 had a low resistance increase rate after the EM test and excellent or good resistance increase rate evaluation.

[0079] In particular, similarly good results were obtained when the type of core material was changed to SAC305 (Examples 7-10), when the Bi content in the solder was reduced to 20% by mass or increased to 70% by mass (Examples 11-14), and when various elements such as Ag, Sb, Cu, Ni, Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, Pb, and In were added (Examples 26-41, etc.).

[0080] On the other hand, as shown in Table 12 and Figures 2 and 3, the joint of Comparative Example 1 had a high initial electrical resistance and poor initial conductivity, and also a high electrical resistance after the EM test and poor conductivity after the EM test. Furthermore, the joint of Comparative Example 1 had a high resistance increase rate after the EM test and poor evaluation of the resistance increase rate. This is because the distance between the two electrodes (D A ) relative to the diameter of the nuclear material (D BThis is because the ratio was small, and the improvement of initial conductivity and EM suppression by the core material were not sufficiently achieved.

[0081] The joint in Comparative Example 2 had a high electrical resistance value and poor conductivity after the EM test. This was due to the distance (D) between the two electrodes. A ) relative to the diameter of the nuclear material (D B This is because the ratio was small, and EM suppression was not sufficiently achieved. The composite material of Comparative Example 3 had a high electrical resistance value after the EM test and poor conductivity after the EM test. Furthermore, the composite material of Comparative Example 3 had a high resistance increase rate after the EM test, and the evaluation of the resistance increase rate was poor. This is because, due to the absence of a core material, improvement of initial conductivity and EM suppression by a core material were not achieved.

[0082] The composites of Comparative Examples 4 and 5 had high initial electrical resistance and poor initial conductivity, and also high electrical resistance and poor conductivity after the EM test. This is because the excessively high Bi concentration resulted in low conductivity of the composite from the beginning, and the desired conductivity could not be achieved even with improvement of initial conductivity by the core material and suppression of EM. The bonded structures in Comparative Examples 6 and 7 had poor low-temperature mounting properties, resulting in no circuit formation and making it impossible to measure electrical resistance values, etc. This was because the Bi concentration was excessively low, and insufficient liquid phase components were generated during low-melting-point mounting.

[0083] The joints of Comparative Examples 8 and 9 had high electrical resistance values ​​and poor conductivity after the EM test. Furthermore, the joints of Comparative Examples 8 and 9 had a high resistance increase rate after the EM test, and the evaluation of the resistance increase rate was poor. This is because the distance between the two electrodes (D A ) relative to the diameter of the nuclear material (D B This is because the ratio of ) was insufficient, and EM suppression was not fully achieved.

[0084] From the above, a first base material, a second base material, a core material located between the first base material and the second base material and having conductivity, and solder present at a position joining the first base material, the second base material, and the core material, the solder containing 20% by mass or more and 70% by mass or less of Bi, the balance of Sn, and inevitable impurities, and satisfying the formula (1) (65% ≦ D B / D A ×100 ≦ 100%) has been found to be excellent in conductivity both initially and after an electromigration (EM) test.

Explanation of Symbols

[0085] 10…First base material 11…Electrode 20…Second base material 21…Electrode 30…Solder 40…Core material G…Center of gravity of the core material

Claims

1. A conductive joint, First substrate, Second substrate, A conductive core material located between the first substrate and the second substrate, and A solder present at the position where the first substrate, the second substrate, and the core material are joined, containing 20% ​​to 70% by mass of Bi and the remainder being Sn, and unavoidable impurities. Includes, The conductive joint that satisfies the following formula (1): 65%≦D Ｂ / D Ａ ×100≦100% (1) (D Ａ : Distance between the first substrate and the second substrate (μm), D Ｂ : The diameter of the aforementioned nuclear material (μm).

2. The aforementioned D Ａ The conductive bond according to claim 1, wherein the thickness is 10 to 4620 μm.

3. The aforementioned D Ｂ The conductive bond according to claim 1 or 2, wherein the thickness is 10 to 3000 μm.

4. The conductive bond according to claim 1 or 2, wherein the solder further comprises an element selected from the group consisting of Ag, Sb, Cu, Ni, Zn, Ti, Ce, P, Ge, Ga, As, Fe, Co, Pd, Pb, In, and combinations thereof.

5. The conductive bond according to claim 4, wherein the solder contains 0% to 5% by mass of Ag, 0% to 10% by mass of Sb, 0% to 1% by mass of Cu, or 0% to 1% by mass of Ni.

6. The conductive bond according to claim 1 or 2, wherein the core material comprises a barrier layer containing at least one selected from the group consisting of Ni and Co.

7. The conductive bond according to claim 1 or 2, wherein the core material includes Cu.

8. A method for manufacturing a conductive bond according to claim 1 or 2, (1) A step of adjusting the distance between the first substrate and the second substrate, (2) A step of placing a solder material between the first substrate and the second substrate, the solder material having a conductive core material and a solder layer surrounding the core material containing 20% ​​by mass to 70% by mass of Bi and the remainder being Sn and unavoidable impurities, (3) A step of joining the first substrate, the second substrate, and the core material by melting and then solidifying at least the surface of the solder layer, Includes, The conductive joint satisfies the following formula (1). 65%≦D Ｂ / D Ａ ×100≦100% (1) (D Ａ : The distance (μm) between the first substrate and the second substrate, D Ｂ : The diameter (μm) of the nuclear material) The aforementioned method.

9. A method for joining a first substrate and a second substrate, (1') A step of adjusting the distance between the first substrate and the second substrate, (2') A step of placing a solder material between the first substrate and the second substrate, the solder material having a conductive core material and a solder layer surrounding the core material containing 20% ​​by mass to 70% by mass of Bi and the remainder being Sn and unavoidable impurities, (3') A step of forming a conductive joint that satisfies the following formula (1) by melting at least the surface of the solder layer and then solidifying it, thereby joining the first substrate, the second substrate and the core material with the solder layer. The method, including: 65%≦D Ｂ / D Ａ ×100≦100% (1) (D Ａ : Distance between the first substrate and the second substrate (μm), D Ｂ : The diameter of the aforementioned nuclear material (μm).

Images