Solder paste, solder joined body, and method for producing solder joined body

Abstract

This solder paste contains solder powder and a spacer. The spacer content is 1 mass% or less. The spacer does not dissolve in the solder even if the melting temperature is higher than the melting temperature of the solder powder and the spacer is heated for 5 minutes at a temperature that is 20°C higher than the melting point of the solder powder. The spacer has a columnar shape or a tube shape, and the diameter variation σ / Da of the spacer calculated from the average diameter Da of the spacer and the standard deviation σ is 6.0% or less.

Description

Solder paste, solder joint, and method for manufacturing solder joint

[0001] The present invention relates to a solder paste containing solder powder, a soldered joint in which a first member and a second member are joined via a solder layer, and a method for manufacturing the soldered joint. This application claims priority to Japanese Patent Application No. 2024-055332, filed on March 29, 2024, the contents of which are incorporated herein by reference.

[0002] For example, various devices such as LEDs and power modules have a structure in which electronic components such as semiconductor elements are bonded onto a circuit board. When bonding electronic components such as semiconductor elements onto a circuit board, a method using a solder material is widely used, as shown in Patent Documents 1 and 2, for example.

[0003] When the electronic component is a semiconductor chip with a large area, a power semiconductor that generates high heat, or a flip-chip-mounted chip with multiple electrical connections, the thickness of the solder layer between the electronic component and the substrate varies, making it difficult to maintain a constant distance between the electronic component and the substrate. When the solder layer thickness varies and the electronic component is bonded to the substrate at an angle, thermal resistance changes within the chip, which is the electronic component, resulting in the problem of not achieving the expected heat dissipation. Furthermore, when the electronic component is a high-brightness LED, the light emission angle from the LED does not meet the expected angle.

[0004] Therefore, for example, Patent Document 3 discloses a solder paste that uses Ni balls with a diameter of 1 to 300 μm and a sphericity of 0.90 or more that do not melt at soldering temperatures. Patent Document 1 describes that self-alignment properties can be ensured when the Ni balls are mounted on electrodes of semiconductor elements, and that because the Ni balls do not melt at soldering temperatures, variations in height in solder joints can be suppressed.

[0005] Japanese Patent Application Publication No. 2000-271782 Japanese Patent Application Publication No. 2021-010152 Japanese Patent Application Publication No. 5585752

[0006] However, Patent Document 3 describes a solder paste using Ni balls with a diameter of 1 to 300 μm and a sphericity of 0.90 or more that do not melt at soldering temperatures, but does not disclose details such as the percentage of Ni balls contained in the solder paste. In particular, in the case of Ni powder, which is an aggregate of Ni balls, the percentage of Ni powder contained in the solder paste was not disclosed. As a result, there was a risk that the effect of suppressing tilt of electronic components due to Ni balls in the solder layer would not be fully achieved. Furthermore, if the content of Ni balls was increased in order to achieve the effect of suppressing tilt of electronic components, voids would be more likely to form in the solder layer, and there was a risk that heat generated from the electronic components would not be efficiently transferred to the board through the solder layer.

[0007] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a solder paste that can make the thickness of the solder layer formed when joining components uniform and can suppress the occurrence of voids in the solder layer, a solder joint in which a first component and a second component are joined via a solder layer of uniform thickness and with few voids, and a method for manufacturing a solder joint.

[0008] In order to solve the above problems, the solder paste of aspect 1 of the present invention contains solder powder and spacers, the content of the spacers is 1 mass % or less, the melting temperature of the spacers is higher than the melting temperature of the solder powder, and the spacers do not dissolve in the solder even when heated for 5 minutes at a temperature of the melting point of the solder powder + 20°C, the spacers are cylindrical or tubular, and the diameter variation σ / Da of the spacers calculated from the average diameter Da and standard deviation σ of the spacers is 6.0% or less.

[0009] According to a first aspect of the present invention, the solder paste contains a spacer whose melting temperature is higher than that of the solder powder and which does not dissolve in solder even when heated for 5 minutes at a temperature 20°C above the melting point of the solder powder. The spacer diameter variation σ / Da, calculated from the average diameter Da and standard deviation σ of the spacers, is 6.0% or less. This enables the thickness of the solder layer formed by applying and heating the solder paste to be uniform. Furthermore, the spacer content is 1% by mass or less, thereby preventing voids from forming in the solder layer. Furthermore, since the spacer is cylindrical or tubular, cutting a cylindrical or tubular material with minimal diameter variation in the longitudinal direction can minimize the spacer diameter variation σ / Da, calculated from the average diameter Da and standard deviation σ of the spacers. Furthermore, the spacer content can be kept low, thereby preventing voids from forming in the solder layer.

[0010] The solder paste of Aspect 2 of the present invention is characterized in that, in the solder paste of Aspect 1 of the present invention, the average diameter Da of the spacers is in the range of 1 μm or more and 300 μm or less. According to the solder paste of Aspect 2 of the present invention, since the average diameter Da of the spacers is in the range of 1 μm or more and 300 μm or less, it is possible to form a solder layer with a uniform thickness in the range of 1 μm or more and 300 μm or less.

[0011] A soldered joint according to a third aspect of the present invention comprises a first member, a second member, and a solder layer, the first member and the second member being joined via the solder layer, spacers having a melting temperature higher than that of the solder layer and not melting even when heated for 5 minutes at a temperature 20°C above the melting point of the solder layer dispersed in the solder layer, the spacers having a cylindrical or tubular shape, a diameter variation σ / Da of the spacers calculated from an average diameter Da and a standard deviation σ of the spacers being 6.0% or less, a content of the spacers in the solder layer being 1% by mass or less, an inclination angle of the joining surface of the second member relative to the joining surface of the first member being less than 1.0°, and a void fraction of the solder layer being 20% ​​or less.

[0012] According to the soldered joint of the third aspect of the present invention, the solder layer contains dispersed spacers whose melting temperature is higher than that of the solder layer and that do not melt even when heated for 5 minutes at a temperature 20°C above the melting point of the solder layer. This allows the solder layer to be formed with a uniform thickness, and the inclination angle of the joining surface of the second member relative to the joining surface of the first member can be less than 1.0°. Furthermore, the void fraction of the solder layer is 20% or less, allowing stable heat transfer between the first member and the second member via the solder layer.

[0013] A soldered joint according to a fourth aspect of the present invention is characterized in that, in the soldered joint according to the third aspect of the present invention, the average height of the spacers is within the range of 1 μm or more and 300 μm or less. According to the soldered joint according to the fourth aspect of the present invention, since the average height of the spacers is within the range of 1 μm or more and 300 μm or less, the thickness of the solder layer is within the range of 1 μm or more and 300 μm or less, and the distance between the first member and the second member can be kept constant.

[0014] A soldered joint of Aspect 5 of the present invention is characterized in that, in the soldered joint of Aspect 3 or Aspect 4 of the present invention, the first member is a circuit board and the second member is a semiconductor element. According to the soldered joint of Aspect 5 of the present invention, the first member is a circuit board and the second member is a semiconductor element, so that the semiconductor element and the circuit board are prevented from being joined at an angle, and heat generated in the semiconductor element can be efficiently transferred to the circuit board via the solder layer, making it possible to configure a semiconductor device with stable performance.

[0015] A method for manufacturing a soldered joint of aspect 6 of the present invention is a method for manufacturing a soldered joint in which a first member and a second member are joined via a solder layer, and includes a paste application process of applying the solder paste of aspect 1 or aspect 2 to one or both of the joining surfaces of the first member and the second member, a lamination process of stacking the first member and the second member via the applied solder paste, and a solder joining process of heat-treating the first member and the second member stacked via the solder paste to form a solder layer and join the first member and the second member via the solder layer, characterized in that the heating temperature in the solder joining process is equal to or higher than the melting temperature of the solder powder and lower than the melting temperature of the spacer.

[0016] According to the method for manufacturing a soldered joint of the sixth aspect of the present invention, the solder paste of the first or second aspect of the present invention is used, so that in the soldering step, spacers are dispersed in the molten solder powder, and the thickness of the solder layer can be determined by these spacers. Furthermore, since a solder paste with a low spacer content is used, the occurrence of voids in the solder layer can be suppressed.

[0017] According to the aspects of the present invention, it is possible to provide a solder paste that can make the thickness of the solder layer formed when joining components uniform and can suppress the occurrence of voids in the solder layer, a solder joint in which a first component and a second component are joined via a solder layer of uniform thickness and with few voids, and a method for manufacturing a solder joint.

[0018] 1 is a schematic explanatory diagram of a soldered joint according to one embodiment of the present invention; 2 is an enlarged explanatory diagram of a solder layer according to one embodiment of the present invention; 3 is a schematic explanatory diagram of a solder paste according to one embodiment of the present invention and a spacer included in the solder layer of the soldered joint; 4 is a flow chart showing a method for manufacturing a soldered joint according to one embodiment of the present invention;

[0019] Hereinafter, a solder paste, a soldered joint, and a method for manufacturing a soldered joint according to embodiments of the present invention will be described with reference to the drawings.

[0020] 1 , the soldered joint 10 according to this embodiment has a first member 11, a second member 12, and a solder layer 20, and the first member 11 and the second member 12 are joined via the solder layer 20. In this embodiment, the soldered joint 10 is a semiconductor device in which a circuit board (first member 11) and a semiconductor element (second member 12) are joined via the solder layer 20.

[0021] Here, the circuit board (first member 11) is made of a material with excellent thermal conductivity, such as copper or a copper alloy, aluminum or an aluminum alloy, iron or an iron alloy, or aluminum nitride, and in this embodiment, it is made of an iron alloy (Kovar). It is preferable that a film of Au or the like is formed on the joining surface of the circuit board (first member 11) to ensure wettability with solder. It is also preferable that a film of Au or the like is formed on the joining surface of the semiconductor element (second member 12) to ensure wettability with solder.

[0022] The solder layer 20 is formed from the solder paste of this embodiment, and as shown in Fig. 2, spacers 21 are dispersed inside the solder layer 20. Here, in this embodiment, the spacers 21 have a cylindrical or tubular shape, as shown in Fig. 3.

[0023] The solder paste of this embodiment will be described below. The solder paste of this embodiment contains solder powder and spacers 21. In this embodiment, the content of the spacers 21 in the solder paste is 1% by mass or less. The upper limit of the content of the spacers 21 is preferably 0.5% by mass or less. On the other hand, the lower limit of the content of the spacers 21 is preferably 0.01% by mass or more, and more preferably 0.1% by mass or more.

[0024] The solder powder is appropriately selected depending on the material of the joining surfaces of the first member 11 and the second member 12 to be joined. In this embodiment, when the joining surfaces of the circuit board (first member 11) and the semiconductor element (second member 12) are made of a precious metal (Au), for example, Sn—Ag—Cu solder, Au—Sn solder, Sn—Cu solder, etc. can be used as the solder powder. In this embodiment, the solder powder is Au—Sn solder, which contains 10 mass% to 80 mass% Sn, with the remainder being Au and unavoidable impurities. The average particle size of the solder powder is preferably in the range of 1 μm to 300 μm.

[0025] The spacer 21 is made of a material whose melting temperature is higher than that of the solder powder and that does not melt even when heated for 5 minutes at a temperature 20°C above the melting point of the solder powder. Examples of materials that make up the spacer 21 include Cu, Ag, Au, Ni, Pt, Pd, W, and composites in which these metals are coated on the surfaces of different materials. In this embodiment, the solder powder is the Au-Sn solder described above, and therefore the spacer 21 has a melting temperature higher than the melting temperature (280°C) of the solder powder (Au-Sn solder).

[0026] As shown in FIG. 3 , in this embodiment, the spacers 21 are cylindrical or tubular, and the diameter variation σ / Da of the spacers 21, calculated from the average diameter Da (average value of diameters D) of the spacers 21 and the standard deviation σ of the diameters D, is 6.0% or less. It is more preferable that the diameter variation σ / Da is 2.0% or less. Furthermore, the diameter variation σ / Da may be 0% or more, 1.0% or more, or 1.5% or more. Here, the diameter D of the cylindrical or tubular spacer 21 is the distance between the two most distant points within one spacer on a cross section perpendicular to the longitudinal direction of the cylinder or tube.

[0027] In this embodiment, the average diameter Da of the spacers 21 is preferably in the range of 1 μm to 300 μm, more preferably in the range of 1 μm to 100 μm. Furthermore, in this embodiment, the aspect ratio L / Da of the spacers 21 (the ratio of the average diameter Da to the average length L) is preferably 1 to 50.

[0028] Furthermore, it is preferable that the surface of the spacer 21 is made of a material that has good wettability with the molten solder powder. In this embodiment, since the solder powder is the Au—Sn solder described above, it is preferable that at least the surface of the spacer 21 is made of a metal material consisting of one or two types selected from Ni and Cu.

[0029] Next, a method for manufacturing a semiconductor device (soldered joint 10) according to this embodiment will be described with reference to the flow chart of FIG.

[0030] (Paste application step S01) First, the solder paste of the present embodiment is applied to one or both of the joining surface of the circuit board (first member 11) and the joining surface of the semiconductor element (second member 12). The method for applying the solder paste is not particularly limited, but for example, a metal mask method, a screen printing method, a dispensing method, etc. can be applied.

[0031] (Laminating Step S02) The circuit board (first member 11) and the semiconductor element (second member 12) are laminated together via the solder paste applied as described above.

[0032] (Soldering step S03) The circuit board (first member 11) and the semiconductor element (second member 12) laminated with the solder paste interposed therebetween are subjected to a heat treatment. The heating temperature during this heat treatment is set to be equal to or higher than the melting temperature of the solder powder and lower than the melting temperature of the spacer 21.

[0033] The molten solder paste is solidified to form a solder layer 20, and the circuit board (first member 11) and the semiconductor element (second member 12) are joined via the solder layer 20. Here, since spacers 21 are dispersed in the molten solder powder, the thickness of the solder layer 20 (the distance between the circuit board (first member 11) and the semiconductor element (second member 12)) is constant.

[0034] The solder paste of this embodiment configured as described above contains spacers 21 whose melting temperature is higher than that of the solder powder and which do not dissolve in solder even when heated for 5 minutes at a temperature 20°C above the melting point of the solder powder, and the diameter variation σ / Da of the spacers 21, calculated from the average diameter Da and the standard deviation σ of the diameter D, is 6.0% or less, making it possible to uniformize the thickness of the solder layer 20 formed by applying and heating the solder paste. Furthermore, the content of spacers 21 in the solder paste is 1% by mass or less, making it possible to suppress the formation of voids in the formed solder layer 20.

[0035] Furthermore, in this embodiment, when the average diameter Da of the spacers 21 is in the range of 1 μm or more and 300 μm or less, the thickness of the formed solder layer 20 can be formed uniformly within the range of 1 μm or more and 300 μm or less.

[0036] Furthermore, in this embodiment, when the spacers 21 are cylindrical or tubular, cutting a cylindrical or tubular material with little variation in diameter in the longitudinal direction can reduce the diameter variation σ / Da of the spacers 21, which is calculated from the average diameter Da of the spacers 21 and the standard deviation σ of the diameter D. In addition, the content of the spacers 21 can be reduced, making it possible to further reduce the occurrence of voids in the formed solder layer 20.

[0037] In the soldered joint 10 of this embodiment, spacers 21 are dispersed in the solder layer 20. The spacers 21 have a melting temperature higher than the melting temperature of the solder layer 20 and do not melt even when heated for 5 minutes at a temperature 20°C above the melting point of the solder layer 20. The spacer diameter variation σ / Da, calculated from the average diameter Da and standard deviation σ of the spacers 21, is 6.0% or less. This allows the solder layer 20 to be formed with a uniform thickness, prevents the circuit board (first member 11) and the semiconductor element (second member 12) from being joined at an angle, and ensures that the angle of inclination of the joining surface of the semiconductor element (second member 12) relative to the joining surface of the circuit board (first member 11) is less than 1.0°. The angle of inclination of the joining surface of the semiconductor element (second member 12) relative to the joining surface of the circuit board (first member 11) is preferably 0.5° or less, and more preferably 0.3° or less. Furthermore, the inclination angle of the joining surface of the semiconductor element (second member 12) relative to the joining surface of the circuit board (first member 11) may be 0° or more, 0.1° or more, or 0.2° or more. Furthermore, since the content of the spacer 21 in the solder paste is 1 mass% or less, the void fraction of the solder layer 20 is 20% or less. The void fraction of the solder layer 20 is preferably 15% or less. Furthermore, the void fraction of the solder layer 20 may be 0% or more, 5% or more, or 10% or more. Therefore, it is possible to form a semiconductor device (soldered joint 10) with stable performance.

[0038] Furthermore, in this embodiment, when the average diameter Da of the spacers 21 is within the range of 1 μm or more and 300 μm or less, the solder layer 20 can be formed to have a uniform thickness within the range of 1 μm or more and 300 μm or less, the distance between the circuit board (first member 11) and the semiconductor element (second member 12) becomes constant, and a semiconductor device (soldered joint 10) with stable performance can be manufactured. The average diameter Da of the spacers 21 can also be said to be the average height of the spacers 21 along the thickness direction of the solder layer 20.

[0039] Although the embodiments of the present invention have been described above, the present invention is not limited thereto and can be modified as appropriate within the scope of the technical requirements of the invention. In this embodiment, the soldered joint 10 has been described as a semiconductor device in which a circuit board (first member 11) and a semiconductor element (second member 12) are joined via a solder layer 20, but this is not limited thereto and other soldered joints may also be used. Furthermore, while the solder powder has been described as being made of Au-Sn solder, this is not limited thereto and other solder materials such as Sn-based solder and Pb-based solder may also be used. It is preferable to select the material of the spacer 21 appropriately depending on the material of the solder powder.

[0040] The results of confirmation experiments conducted to confirm the effectiveness of the present invention will be described below.

[0041] A solder paste containing the solder powder and spacers shown in Table 1 was prepared. The average particle size of the solder powder listed in Table 1 is the particle size (d50) at which the cumulative volume frequency of the particles is 50% in the volume-based particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer. The average diameter Da of the cylindrical or tubular spacers was measured as follows: The spacers were placed on the surface of a carbon tape, and SEM images of the spacers were taken at 500x magnification using a Hitachi tabletop microscope TM303. Five spacers were randomly selected from the obtained SEM images so that the overall shape of the surface perpendicular to the length of each spacer could be recognized. The distance between the two furthest points within each spacer in the direction perpendicular to the length of the spacer was measured, and this was defined as the diameter D of the spacer. The average of the diameters D measured for the five spacers was defined as the average diameter Da. The standard deviation σ was calculated from the diameters D measured for the five spacers. The lengths of the five spacers were also measured, and the average of these lengths was defined as the average length L.

[0042] Al with Au metallization on the surface ２ O ３ The above solder paste was applied to a substrate consisting of the above, so that the thickness after reflow heating would be the diameter D of the spacer. The formula for calculating the appropriate paste application thickness: ((solder volume excluding spacers under the chip) / (volume ratio of solder in the solder paste excluding spacers) + (spacer volume under the chip)) / (chip area). Here, the solder volume was calculated assuming that the thickness of the solder layer was the thickness of one spacer, and the spacer volume was calculated from the spacer concentration in the solder paste. A chip (1 mm x 1 mm) was stacked on top of the applied solder paste.

[0043] Then, a reflow oven (SRS-1C manufactured by Malcom) was used to solder-bond the circuit board and chip in a nitrogen atmosphere using the following heating pattern: heating from room temperature to 200°C at a heating rate of 1.5°C / sec, holding at 200°C for 2 minutes, heating from 200°C to 300°C at a heating rate of 1.5°C / sec, and holding at 300°C for 5 seconds.

[0044] The soldered joints obtained as described above were evaluated for voids in the solder layer and tilt of the chip as follows: Furthermore, a comprehensive evaluation of the soldered joints was made from these evaluations.

[0045] (Voids in the solder layer) The chip was observed from directly above using a transmission X-ray device, and the ratio of the total area of ​​the voids observed by projecting them to the area of ​​the chip was taken as the void ratio. The measurement was performed three times, and the average value of the void ratio was calculated.

[0046] (Chip tilt) The soldered joint was cut along a plane perpendicular to the center line of the chip surface, and the angle between the substrate surface and the chip surface (the tilt angle of the chip bonding surface relative to the substrate bonding surface) was measured and used as the chip tilt. This measurement was performed three times to measure the chip tilt.

[0047]

[0048] In Comparative Example 1, the solder paste did not contain spacers, and the chip tilt was large at 1.6°. In Comparative Example 2, the spacer content was high at 30 mass%, and the void rate in the solder layer was high at 35%. In Comparative Example 3, the spacer diameter variation σ / Da was large at 10.0%, the void rate in the solder layer was high at 25%, and the chip tilt was large at 1.0°. In Comparative Example 4, the spacer diameter variation σ / Da was large at 10.0%, and the chip tilt was large at 1.4°.

[0049] In contrast to this, in Examples 1 to 9 of the present invention, it was possible to suppress the occurrence of voids in the solder layer and also to suppress the tilt of the chip.

[0050] From the results of the above confirmation experiments, it was confirmed that the present invention can provide a solder paste that can make the thickness of the solder layer formed when joining components uniform and can suppress the occurrence of voids in the solder layer, a solder joint in which a first component and a second component are joined via a solder layer of uniform thickness and with few voids, and a method for manufacturing a solder joint.

[0051] The solder paste of this embodiment is suitably applied to various devices, such as LEDs and power modules, in which electronic components such as semiconductor elements are bonded onto a circuit board.

[0052] REFERENCE SIGNS LIST 10 Soldered joint 11 First member 12 Second member 20 Solder layer 21 Spacer S01 Paste application step S02 Lamination step S03 Solder joining step

Claims

1. A solder paste comprising solder powder and spacers, wherein the spacer content is 1% by mass or less, the spacers have a melting temperature higher than that of the solder powder and do not dissolve in the solder even when heated for 5 minutes at a temperature 20°C above the melting point of the solder powder, the spacers are cylindrical or tubular, and the diameter variation σ / Da of the spacers calculated from the average diameter Da and standard deviation σ of the spacers is 6.0% or less.

2. The solder paste according to claim 1, wherein the average diameter Da of the spacers is in the range of 1 μm to 300 μm.

3. A soldered joint comprising a first member, a second member, and a solder layer, the first member and the second member being joined via the solder layer, spacers having a melting temperature higher than that of the solder layer and not melting even when heated for 5 minutes at a temperature of the melting point of the solder layer + 20°C dispersed in the solder layer, the spacers being cylindrical or tubular, the spacers having a diameter variation σ / Da calculated from the average diameter Da and standard deviation σ of the spacers being 6.0% or less, the content of the spacers in the solder layer being 1% by mass or less, the inclination angle of the joining surface of the second member relative to the joining surface of the first member being less than 1.0° and the void ratio of the solder layer being 20% ​​or less.

4. A solder joint according to claim 3, wherein the average height of the spacers is in the range of 1 μm to 300 μm.

5. A solder joint according to claim 3 or 4, wherein the first member is a circuit board and the second member is a semiconductor element.

6. A method for manufacturing a soldered joint in which a first member and a second member are joined via a solder layer, comprising: a paste application step of applying the solder paste according to claim 1 or claim 2 to one or both of the joining surfaces of the first member and the second member; a lamination step of laminating the first member and the second member via the applied solder paste; and a solder joining step of heat-treating the first member and the second member laminated via the solder paste to form a solder layer and joining the first member and the second member via the solder layer, wherein the heating temperature in the solder joining step is equal to or higher than the melting temperature of the solder powder and lower than the melting temperature of the spacer.

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