Flux, solder paste, and method for manufacturing joint structure

Abstract

To provide a flux which can reduce voids at a junction and can suppress occurrence of large voids, a solder paste, and a joint structure manufacturing method.SOLUTION: A flux, that is used for soldering, contains: a polyvalent solvent which has three or more hydroxy groups, and has a molecular weight of 150 or less; a liquid solvent which has two hydroxy groups, in which the number of carbon atoms linked in a straight chain between two hydroxy groups is 4 or more, and which is a liquid at a normal temperature; a solid solvent which has two hydroxy groups, and is a solid at a normal temperature; and a thixotropic agent.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a flux, a solder paste containing the flux, and a method for manufacturing a joint structure using the solder paste. [Background technology]

[0002] A solder paste, which is a mixture of a solder alloy and flux, is used to join an electronic circuit board such as a printed wiring board to a joining component. The solder paste is applied to the electrode portion on the surface of the board, and heated (reflowed) while the electrode portion of the joining component is in contact with the electrode portion. This melts the solder alloy to form a solder joint, resulting in a joining structure in which the board and joining component are joined via the solder joint.

[0003] In the past, many rosin-free fluxes have been developed with the aim of reducing flux residue after reflow. For example, Patent Document 1 discloses a flux that uses a liquid solvent and a solid solvent in combination instead of rosin. Such rosin-free fluxes aim to reduce voids by incorporating a decompression step during reflow to promote void removal, or by performing reflow in a reducing atmosphere to improve solder wettability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-38026 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, power devices have been widely used as semiconductors capable of handling high voltages and large currents. Power devices can become very hot during operation, which can lead to breakdowns. For this reason, efforts are being made to efficiently dissipate heat by using metal-based high-heat dissipation substrates or heat sinks that have heat dissipation measures in place.

[0006] In the case of metal-based high-heat-dissipation substrates and heat sinks with large joint areas, further reduction of voids at the joints is required to improve cooling efficiency. Furthermore, because power devices generate a large amount of heat, the presence of large voids in the joints can hinder heat conduction.

[0007] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a flux, a solder paste, and a method for manufacturing a joint structure that can reduce voids in a joint and suppress the occurrence of large voids. [Means for solving the problem]

[0008] The flux according to the present invention is a flux used for soldering, and contains: a polyhydric solvent having three or more hydroxy groups and a molecular weight of 150 or less; a liquid solvent having two hydroxy groups, with four or more carbon atoms bonded in a linear chain between the two hydroxy groups, and which is liquid at room temperature; a solid solvent having two hydroxy groups and which is solid at room temperature; and a thixotropic agent.

[0009] With this configuration, the flux can reduce voids in the joint and suppress the occurrence of large voids.

[0010] In the flux according to the present invention, the content of the polyhydric solvent may be 0.5% by mass or more and 60.0% by mass or less.

[0011] With this configuration, the flux according to the present invention can further reduce voids in the joint and further suppress the occurrence of large voids.

[0012] In the flux according to the present invention, the polyhydric solvent may be at least one selected from glycerin, 1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, and erythritol.

[0013] The flux according to the present invention, having such a configuration, improves the wettability of the solder alloy, thereby further reducing voids in the joint and further suppressing the occurrence of large voids.

[0014] In the flux according to the present invention, the content of the liquid solvent may be 5.0 mass % or more and 78.0 mass % or less with respect to the entire flux.

[0015] With this configuration, the flux according to the present invention can further reduce voids in the joint and further suppress the occurrence of large voids.

[0016] In the flux according to the present invention, the liquid solvent may be at least one selected from 3-methyl-1,5-pentanediol, 1,4-butanediol, 1,5-pentanediol, 2,5-hexanediol, and 2,4-diethyl-1,5-pentanediol.

[0017] The flux according to the present invention has such a constitution that the viscosity of the flux is increased, thereby preventing components from falling off when mounted. Also, the flux improves the printability of the solder paste.

[0018] The flux according to the present invention may further contain a monovalent solvent having one hydroxy group.

[0019] With this configuration, the flux according to the present invention can reduce the load during continuous printing of solder paste and suppress a decrease in the viscosity of the solder paste.

[0020] The solder paste according to the present invention contains the flux and a solder alloy.

[0021] With this configuration, the solder paste according to the present invention can reduce voids in the joint and prevent large voids from occurring.

[0022] The method for manufacturing a joint structure according to the present invention uses solder paste to join a substrate and a joining component.

[0023] The method for manufacturing a bonded structure according to the present invention can reduce voids in the bonded portion and obtain a bonded structure in which the occurrence of large voids is suppressed. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a flux, a solder paste, and a method for manufacturing a joint structure that can reduce voids in a joint and prevent the occurrence of large voids. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, a flux, a solder paste, and a method for manufacturing a joint structure according to an embodiment of the present invention will be described.

[0026] <Flux> The flux according to this embodiment contains a polyhydric solvent having three or more hydroxy groups and a molecular weight of 150 or less, a liquid solvent having two hydroxy groups, with four or more carbon atoms bonded in a linear chain between the two hydroxy groups, and which is liquid at room temperature, a solid solvent having two hydroxy groups and which is solid at room temperature, and a thixotropic agent.

[0027] (polyhydric solvent) The flux according to this embodiment contains a polyhydric solvent having three or more hydroxy groups and a molecular weight of 150 or less. Examples of such polyhydric solvents include glycerin, 1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, erythritol, pentaerythritol, and threitol. Among these, the polyhydric solvent is preferably at least one selected from glycerin, 1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, and erythritol. Furthermore, the flux preferably contains at least two polyhydric solvents. From the viewpoints of suppressing an increase in the viscosity of the solder paste and improving printability, the polyhydric solvent preferably has four or less hydroxy groups. By including a polyhydric solvent, the flux according to this embodiment can improve the wettability of the solder. Furthermore, since the molecular weight of the polyhydric solvent is 150 or less, the boiling point is lower and the polyhydric solvent volatilizes more easily than polyhydric solvents with a molecular weight exceeding 150. This reduces voids in the joint and prevents the occurrence of large voids.

[0028] The polyhydric solvent may be liquid or solid at room temperature. In this specification, room temperature means 25°C to 30°C. That is, the polyhydric solvent is liquid or solid over the entire range of 25°C to 30°C.

[0029] The content of the polyhydric solvent is preferably 0.5% by mass or more, and more preferably 1.5% by mass or more, based on the total amount of the flux. Furthermore, the content of the polyhydric solvent is preferably 85.0% by mass or less, more preferably 60.0% by mass or less, and particularly preferably 10.0% by mass or less, based on the total amount of the flux. By ensuring that the content of the polyhydric solvent is 60.0% by mass or less, printability can be improved. When two or more polyhydric solvents are contained, the content is the total content of the polyhydric solvents.

[0030] (liquid solvent) The flux according to this embodiment contains a liquid solvent that has two hydroxy groups, four or more carbon atoms bonded in a linear chain between the two hydroxy groups, and is liquid at room temperature. Such a liquid solvent is preferably at least one selected from the group consisting of 3-methyl-1,5-pentanediol, 1,4-butanediol, 1,5-pentanediol, 2,5-hexanediol, and 2,4-diethyl-1,5-pentanediol. In this specification, room temperature refers to 25°C to 30°C. This means that the liquid solvent is liquid over the entire temperature range from 25°C to 30°C.

[0031] The content of the liquid solvent is preferably 2.4% by mass or more, more preferably 5.0% by mass or more, and particularly preferably 30.0% by mass or more, based on the total mass of the flux. The content of the liquid solvent is preferably 78.0% by mass or less, more preferably 50.0% by mass or less, based on the total mass of the flux. When two or more liquid solvents are contained, the above content is the total content of the liquid solvents.

[0032] (solid solvent) The flux according to this embodiment contains a solid solvent that has two hydroxy groups and is solid at room temperature, such as 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, 1,2-octanediol, 1,8-octanediol, 1,10-decanediol, 1,2-dodecanediol, 1,6-hexanediol, 1,9-nonanediol, 1,2-nonanediol, threo-5,6-dodecanediol, erythro-5,6-dodecanediol, and 1,16-hexadecanediol. Among these, the solid solvent is preferably at least one selected from 2,2-dimethyl-1,3-propanediol and 2,5-dimethyl-2,5-hexanediol. The flux according to this embodiment contains a solid solvent, which can prevent the flux from separating from the solder alloy. Note that, in this specification, room temperature means 25°C to 30°C. This means that the solid solvent is solid over the entire temperature range from 25°C to 30°C.

[0033] From the viewpoint of reducing voids at the joint and suppressing the occurrence of large voids, it is preferable to use two or more solid solvents having a quaternary carbon or two or more solid solvents not having a quaternary carbon. Examples of solid solvents having a quaternary carbon include 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,2,4-trimethyl-1,3-pentanediol, and 2-butyl-2-ethyl-1,3-propanediol. Examples of solid solvents not having a quaternary carbon include 1,2-octanediol, 1,8-octanediol, 1,10-decanediol, 1,2-dodecanediol, 1,6-hexanediol, and 1,9-nonanediol. Among these, the solid solvent is preferably 2,2-dimethyl-1,3-propanediol and 2,5-dimethyl-2,5-hexanediol, 1,6-hexanediol and 1,9-nonanediol, or 1,2-octanediol and 1,8-octanediol, and more preferably 2,2-dimethyl-1,3-propanediol and 2,5-dimethyl-2,5-hexanediol.

[0034] The content of the solid solvent is preferably 2.0 mass% or more, more preferably 30.0 mass% or more, based on the total mass of the flux. The content of the solid solvent is preferably 75.0 mass% or less, more preferably 50.0 mass% or less, based on the total mass of the flux. When two or more solid solvents are contained, the above content is the total content of the solid solvents.

[0035] (thixotropic agent) The flux according to this embodiment includes a thixotropic agent. Examples of thixotropic agents include castor oil, fatty acid amides, fatty acid bisamides, polyamide compounds, kaolin, colloidal silica, organic bentonite, and glass frit. Among these, the thixotropic agent preferably includes a fatty acid amide, and more preferably consists of a fatty acid amide, from the viewpoint of reducing voids and the maximum void size. Examples of fatty acid amides include stearic acid amide, lauric acid amide, palmitic acid amide, oleic acid amide, erucic acid amide, behenic acid amide, hydroxystearic acid amide, and myristic acid amide. Among these, the fatty acid amides are preferably stearic acid amide, lauric acid amide, palmitic acid amide, oleic acid amide, erucic acid amide, and behenic acid amide. The thixotropic agents may be used alone or in combination.

[0036] The content of the thixotropic agent is preferably 2.0 mass% or more, more preferably 10.0 mass% or more, based on the total mass of the flux. Furthermore, the content of the thixotropic agent is preferably 40.0 mass% or less, more preferably 20.0 mass% or less, based on the total mass of the flux. When two or more thixotropic agents are contained, the above content is the total content of the thixotropic agents.

[0037] (monovalent solvent) The flux according to this embodiment may contain a monohydric solvent having one hydroxy group. Examples of the monohydric solvent include α-terpineol, hexyldecanol, isostearyl alcohol, 2-octyldodecanol, 2-decyltetradodecanol, and oleyl alcohol. Among these, the monohydric solvent is preferably hexyldecanol or 2-decyltetradodecanol. The monohydric solvent may be used alone or in combination of two or more.

[0038] The content of the monovalent solvent is preferably 1.0 mass% or more, more preferably 2.0 mass% or more, based on the total mass of the flux. The content of the monovalent solvent is preferably 10.0 mass% or less, more preferably 5.0 mass% or less, based on the total mass of the flux. When two or more monovalent solvents are contained, the content is the total content of the monovalent solvents.

[0039] (resin) The flux according to this embodiment may contain a resin to the extent that it does not affect the effects of the present invention. Examples of resins include rosin-based resins and synthetic resins. Rosin-based resins are not particularly limited, and examples include rosin and rosin derivatives (e.g., hydrogenated rosin, polymerized rosin, disproportionated rosin, acrylic acid-modified rosin, etc.). Examples of synthetic resins include known synthetic resins such as terpene phenol resins. When a resin is contained in the flux, the content of the resin is preferably 5.0 mass% or less, more preferably 3.0 mass% or less, based on the total weight of the flux. The content of the resin is preferably 0.5 mass% or more, more preferably 1.0 mass% or more, based on the total weight of the flux. Note that one type of resin may be used alone, or two or more types may be used in combination. When two or more types of resins are contained, the above content refers to the total content of the resins.

[0040] (activator) The flux according to this embodiment may contain an activator. The activator is not particularly limited, and examples thereof include organic acid activators, amine compounds, amino acids, and halogen-based activators such as amine halogen salts and halogen compounds. The activators may be used alone or in combination of two or more.

[0041] The organic acid surfactant is not particularly limited, and examples thereof include monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, capric acid, lauric acid, myristic acid, pentadecylic acid, palmitic acid, margaric acid, stearic acid, tuberculostearic acid, arachidic acid, behenic acid, lignoceric acid, and glycolic acid; dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, methylsuccinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, tartaric acid, and diglycolic acid; and other organic acids such as dimer acid, levulinic acid, lactic acid, acrylic acid, benzoic acid, salicylic acid, anisic acid, citric acid, and picolinic acid.

[0042] The amine-based compound is not particularly limited, and examples thereof include imidazole-based compounds and triazole-based compounds. Examples of the imidazole-based compounds include benzimidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole (2E4MZ), 2-heptadecylimidazole, 2-undecylimidazole, 1-(4,6-diamino-s-triazin-2-yl)ethyl-2-undecylimidazole, 1-butylimidazole, 2-phenylimidazole, and 4-methyl-2-phenylimidazole. Examples of the triazole-based compounds include benzotriazole, 1H-benzotriazole-1-methanol, and 1-methyl-1H-benzotriazole. Other amine compounds include, for example, cetylamine, erucic acid amide, 3-(dimethylamino)-1,2-propanediol, 3,5-dimethylpyrazole, dimethylurea, hexahydro-1,3,5-triphenyl-1,3,5-triazine, pyrazinamide, N-phenylglycine, 3-methyl-5-pyrazolone, N-lauroylsarcosine, and 1,3-diphenylguanidine.

[0043] The amino acid is not particularly limited, and examples thereof include N-acetylphenylalanine (N-acetyl-L-phenylalanine, N-acetyl-DL-phenylalanine, N-acetyl-D-phenylalanine), N-acetylglutamic acid (N-acetyl-L-glutamic acid), N-acetylglycine, N-acetylleucine (N-acetyl-L-leucine, N-acetyl-DL-leucine, N-acetyl-D-leucine), and N-acetylphenylglycine (N-acetyl-N-phenylglycine, N-acetyl-L-phenylglycine, N-acetyl-DL-phenylglycine).

[0044] The halogen-based activator is not particularly limited, and examples thereof include amine halogen salts and halogen compounds. Examples of amines in amine halogen salts include diethylamine, dibutylamine, tributylamine, diphenylguanidine, and cyclohexylamine. Examples of halogens in amine halogen salts include fluorine, chlorine, bromine, and iodine. Examples of halogen compounds include tris(2,3-dibromopropyl)isocyanurate, 2,3-dibromo-2-butene-1,4-diol, 2-bromo-3-iodo-2-butene-1,4-diol, TBA-bis(2,3-dibromopropyl ether), and 4,4'-diiodobiphenyl.

[0045] The content of the activator is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, based on the total mass of the flux. The content of the activator is preferably 5.0% by mass or less, more preferably 3.0% by mass or less, based on the total mass of the flux. When two or more activators are contained, the above content is the total content of the activators.

[0046] The flux according to the present embodiment may contain at least one other additive selected from the group consisting of a stabilizer, a surfactant, an antifoaming agent, and a corrosion inhibitor. The total content of the other additives is not particularly limited and may be, for example, 5.0 mass % or less with respect to the total amount of the flux.

[0047] The flux according to this embodiment is a flux used for soldering, and contains a polyhydric solvent having three or more hydroxy groups and a molecular weight of 150 or less, a liquid solvent having two hydroxy groups, with four or more carbon atoms bonded in a linear chain between the two hydroxy groups, and which is liquid at room temperature, a solid solvent having two hydroxy groups and which is solid at room temperature, and a thixotropic agent, thereby reducing voids in the joint and suppressing the occurrence of large voids.

[0048] The flux according to this embodiment has a polyhydric solvent content of 0.5 mass % or more and 60.0 mass % or less, thereby further reducing voids in the joint and further suppressing the occurrence of large voids.

[0049] In the flux according to the present embodiment, the polyhydric solvent is at least one selected from glycerin, 1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, and erythritol, which improves the wettability of the solder alloy, thereby further reducing voids in the joint and further suppressing the occurrence of large voids.

[0050] The flux according to this embodiment has a liquid solvent content of 5.0 mass % or more and 78.0 mass % or less relative to the total flux, thereby further reducing voids in the joint and further suppressing the occurrence of large voids.

[0051] The flux according to this embodiment uses at least one liquid solvent selected from the group consisting of 3-methyl-1,5-pentanediol, 1,4-butanediol, 1,5-pentanediol, 2,5-hexanediol, and 2,4-diethyl-1,5-pentanediol, which increases the viscosity of the flux, preventing components from falling off during mounting. The flux also improves the printability of the solder paste.

[0052] The flux according to this embodiment further contains a monovalent solvent having one hydroxy group, which reduces the load during continuous printing of the solder paste and also prevents a decrease in the viscosity of the solder paste.

[0053] <Solder paste> The solder paste according to this embodiment contains the flux and a solder alloy. Examples of the solder alloy include lead-free solder alloys and eutectic solder alloys containing lead. However, from the viewpoint of reducing environmental impact, lead-free solder alloys are preferred. Examples of lead-free solder alloys include alloys containing tin, silver, copper, indium, zinc, bismuth, antimony, etc. More specifically, examples of the lead-free solder alloy include alloys containing Sn / Ag, Sn / Ag / Cu, Sn / Cu, Sn / Ag / Bi, Sn / Bi, Sn / Ag / Cu / Bi, Sn / Sb, Sn / Zn / Bi, Sn / Zn, Sn / Zn / Al, Sn / Ag / Bi / In, Sn / Ag / Cu / Sb, Sn / Ag / Sb, Sn / Ag / Cu / Bi, Sn / Ag / Cu / Bi / In / Sb, In / Ag, and Sn / In. The alloys also contain unavoidable impurities. The unavoidable impurities refer to components that are inevitably mixed in during the manufacturing process and are acceptable within a range that does not affect the effects of the present invention.

[0054] The content of the flux is preferably 5% by mass or more and 20% by mass or less of the total solder paste, and the content of the solder alloy is preferably 80% by mass or more and 95% by mass or less of the total solder paste.

[0055] The solder paste according to this embodiment contains the flux and the solder alloy, and thus can reduce voids in the joint and suppress the occurrence of large voids.

[0056] <Method of manufacturing a bonded structure> In the method for manufacturing a bonded structure according to this embodiment, a substrate (first member) and a bonding component (second member) are bonded using the solder paste. Specifically, the substrate (first member) and the bonding component (second member) are heated in a reflow furnace while in contact with each other via the solder paste, melting the solder alloy that constitutes the solder paste. The substrate (first member) and the bonding component (second member) are then bonded by cooling.

[0057] First, the solder paste is placed on the surface of the substrate (first member). The solder paste can be placed using known methods such as printing with a solder printing device, transfer printing, applying with a dispenser, or mounting with a mounter. When printing the solder paste using a solder printing device, the thickness of the applied solder paste can be 100 μm or more and 600 μm or less. The substrate (first member) is not particularly limited, and known substrates such as a printed circuit board, DBC board, base plate, lead frame, and silicon wafer can be used.

[0058] Next, the joining component (second component) is placed so as to be in contact with the substrate (first component) via the solder paste. The joining component (second component) is not particularly limited, and may be, for example, a known joining component such as a chip component (e.g., IC chip), resistor, diode, capacitor, transistor, semiconductor chip (e.g., Si chip), or heat sink.

[0059] Next, the substrate (first member) and the joining component (second member) are heated while being in contact with each other via the solder paste. The heating temperature is not particularly limited and can be selected appropriately depending on the solder paste, and is in a range equal to or higher than the melting point of the solder paste. The heating step may be performed in an inert gas atmosphere using nitrogen gas or the like in order to suppress oxidation of the metal. Furthermore, the atmosphere may be reduced to a vacuum using a vacuum pump in order to promote gas release. Furthermore, a preheating step may be performed before the heating step. The preheating temperature is not particularly limited and can be selected appropriately depending on the solder paste, and is in a range below the melting point of the solder paste. The preheating step may be performed in an inert gas atmosphere in a reduced pressure or vacuum, or may be performed in a reducing atmosphere using hydrogen gas or formic acid gas.

[0060] Finally, the substrate (first member) and the joining component (second member) that are in contact with each other via the solder paste are cooled, for example, to 20°C or higher and 200°C or lower at a temperature drop rate of 50°C / min or higher and 5°C / min or lower.

[0061] In the manufacturing method of the bonded structure according to this embodiment, the first member is not limited to being a substrate. Examples of first members other than a substrate include chip components (such as IC chips), resistors, diodes, capacitors, transistors, semiconductor chips (such as Si chips), and heat sinks. Furthermore, in the manufacturing method of the bonded structure according to this embodiment, the second member is not limited to being a bonding component. Examples of second members other than a bonding component include a printed circuit board, a DBC substrate, a base plate, a lead frame, and a heat sink.

[0062] The manufacturing method of the joined structure according to this embodiment uses the solder paste to join a substrate and a joining component, thereby reducing voids in the joint and obtaining a joined structure in which the occurrence of large voids is further suppressed. [Example]

[0063] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0064] <Flux materials> (polyhydric solvent) Glycerin: Kao Corporation 1,2,4-Butanetriol: Tokyo Chemical Industry Co., Ltd. Trimethylolethane: Mitsubishi Gas Chemical Company, Inc. Trimethylolpropane: Mitsubishi Gas Chemical Company, Inc. 1,2,6-Hexanetriol: Tokyo Chemical Industry Co., Ltd. Erythritol: Mitsubishi Chemical Foods Corporation (liquid solvent) 3-Methyl-1,5-pentanediol: Kuraray Co., Ltd. 1,4-Butanediol: Tokyo Chemical Industry Co., Ltd. 2,5-Hexanediol: Tokyo Chemical Industry Co., Ltd. 1,5-Pentanediol: Tokyo Chemical Industry Co., Ltd. 2,4-Diethyl-1,5-pentanediol: Tokyo Chemical Industry Co., Ltd. (solid solvent) 2,2-Dimethyl-1,3-propanediol: Mitsubishi Gas Chemical Company, Inc. 2,5-Dimethyl-2,5-hexanediol: Tokyo Chemical Industry Co., Ltd. 2,2,4-trimethyl-1,3-pentanediol: Tokyo Chemical Industry Co., Ltd. 2-Butyl-2-ethyl-1,3-propanediol: Tokyo Chemical Industry Co., Ltd. 1,2-Octanediol: Tokyo Chemical Industry Co., Ltd. 1,8-Octanediol: Tokyo Chemical Industry Co., Ltd. 1,10-Decanediol: Tokyo Chemical Industry Co., Ltd. 1,2-Dodecanediol: Tokyo Chemical Industry Co., Ltd. 1,6-Hexanediol: Tokyo Chemical Industry Co., Ltd. 1,9-Nonanediol: Tokyo Chemical Industry Co., Ltd. (thixotropic agent) Stearic acid amide: manufactured by Kao Corporation Lauric acid amide: Mitsubishi Chemical Corporation Palmitic acid amide: Mitsubishi Chemical Corporation Oleic acid amide: Mitsubishi Chemical Corporation Erucic acid amide: Nippon Fine Chemical Co., Ltd. Behenamide: Nippon Fine Chemical Co., Ltd. (monovalent solvent) Hexyldecanol: manufactured by Kokyu Alcohol Kogyo Co., Ltd. Isostearyl alcohol: manufactured by Kokyu Alcohol Kogyo Co., Ltd. 2-Octyldodecanol: manufactured by Kokyu Alcohol Kogyo Co., Ltd. 2-Decyltetradodecanol: manufactured by Kokyu Alcohol Kogyo Co., Ltd. Oleyl alcohol: manufactured by Kokyu Alcohol Kogyo Co., Ltd. (resin) Hydrogenated rosin: Arakawa Chemical Industries, Ltd. (activator) Phenylsuccinic acid: Tokyo Chemical Industry Co., Ltd.

[0065] <Flux Preparation> The flux materials were placed in a heating vessel in the formulations shown in Tables 1 to 11 and heated to 120°C until all materials were dissolved. The materials were then cooled to room temperature to obtain uniformly dispersed fluxes for each Example and Comparative Example. The formulations shown in Tables 1 to 11 are equal to the content of each component in the flux.

[0066] <Preparing solder paste> The flux of each example and comparative example was mixed at 9 mass % with solder alloy powder (95Sn / 5Sb, Type 3, manufactured by Koki Co., Ltd.) at 91 mass % to obtain a solder paste of each example and comparative example.

[0067] [Table 1]

[0068]

Table 2

[0069]

Table 3

[0070]

Table 4

[0071]

Table 5

[0072]

Table 6

[0073]

Table 7

[0074]

Table 8

[0075]

Table 9

[0076]

Table 10

[0077]

Table 11

[0078] <Production of trial substrate> The solder paste of each example and comparative example was applied to an electrolytic Ni-plated oxygen-free Cu plate (size: 60 × 55 mm) substrate using a 300 μm thick metal mask so as to achieve a 100% aperture ratio. The thickness of the applied solder paste was 300 μm. An oxygen-free Cu plate (size: 44 × 35 mm) was then mounted, and the electrolytic Ni-plated oxygen-free Cu plate (size: 60 × 55 mm) substrate and the oxygen-free Cu plate (size: 44 × 35 mm) were placed in a reflow furnace of a reflow device (product name: VS1, manufactured by Origin Co., Ltd.) with the solder paste in contact with each other. Test substrates were then fabricated by performing the preheating and reflow processes described below.

[0079] In the preheating step, first, the product was heated in a nitrogen atmosphere at a temperature increase rate of 2°C / s up to a preheat temperature of 205°C. Then, at the preheat temperature, the gas in the reflow furnace was evacuated using a vacuum pump and the temperature was maintained for 60 seconds, after which formic acid (3%) was introduced to pressurize the atmosphere in the reflow furnace to atmospheric pressure and the temperature was maintained for 60 seconds.

[0080] In the reflow process, the atmosphere in the reflow furnace was first reduced to 1000 Pa, then heated to a peak temperature of 270°C at a heating rate of 1°C / s and held for 30 seconds. Next, the atmosphere in the reflow furnace was reduced to 200 Pa and held for 30 seconds. Finally, at a peak temperature of 270°C, nitrogen was introduced into the reflow furnace to pressurize it to atmospheric pressure.

[0081] <Void evaluation> X-ray photographs were taken of the area of ​​each test substrate where the oxygen-free Cu plate was mounted. The photographs were then binarized to calculate the void ratio of the joint. The ratio of the largest void area among the detected voids to the soldered area (maximum void ratio) was also calculated. The results are shown in Tables 1 to 11.

[0082] The voids were evaluated based on the following criteria. The results are shown in Tables 1 to 11. Evaluations 1 to 4 were judged as "pass", and evaluation 5 was judged as "fail". Grade 1: Void rate is 3.0% or less and maximum void rate is 0.6% or less Grade 2: Void ratio is 4.0% or less and maximum void ratio is 0.9% or less Grade 3: Void rate is 4.5% or less and maximum void rate is 1.2% or less Grade 4: Void ratio is 5.0% or less and maximum void ratio is 1.5% or less Grade 5: Void ratio exceeds 5.0% or maximum void ratio exceeds 1.5%

[0083] As can be seen from the results in Tables 1 to 11, the joints made using the solder paste of each example that meets all the requirements of the present invention have a void rate of 5.0% or less and a maximum void rate of 1.5% or less, thereby reducing voids in the joints and suppressing the occurrence of large voids.

[0084] On the other hand, the joints using the solder pastes of each comparative example that do not satisfy the constituent requirements of the present invention have a void ratio exceeding 5.0% or a maximum void ratio exceeding 1.5%, making it difficult to reduce voids in the joints and to suppress the occurrence of large voids.

Claims

1. A flux used in soldering, a polyhydric solvent having three or more hydroxy groups and a molecular weight of 150 or less; a liquid solvent that has two hydroxy groups, with four or more carbon atoms bonded in a linear chain between the two hydroxy groups, and is liquid at room temperature; a solid solvent having two hydroxy groups and being solid at room temperature; A thixotropic agent is contained, The content of the polyhydric solvent is 0.5 mass % or more and 85.0 mass % or less with respect to the entire flux, The content of the liquid solvent is 2.4 mass% or more and 78.0 mass% or less with respect to the entire flux, The content of the solid solvent is 2.0 mass% or more and 75.0 mass% or less with respect to the entire flux, A flux in which the content of the thixotropic agent is 2.0 mass % or more and 40.0 mass % or less with respect to the entire flux.

2. 2. The flux according to claim 1, wherein the content of the polyhydric solvent is 0.5 mass % or more and 60.0 mass % or less with respect to the entire flux.

3. The flux according to claim 1 or 2, wherein the polyhydric solvent is at least one selected from the group consisting of glycerin, 1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,2,6-hexanetriol, and erythritol.

4. The flux according to any one of claims 1 to 3, wherein the content of the liquid solvent is 5.0 mass% or more and 78.0 mass% or less with respect to the entire flux.

5. The flux according to any one of claims 1 to 4, wherein the liquid solvent is at least one selected from 3-methyl-1,5-pentanediol, 1,4-butanediol, 1,5-pentanediol, 2,5-hexanediol, and 2,4-diethyl-1,5-pentanediol.

6. Further, it contains a monohydric solvent having one hydroxy group, The flux according to any one of claims 1 to 5, wherein the content of the monovalent solvent is 1.0 mass% or more and 10.0 mass% or less with respect to the entire flux.

7. A solder paste comprising the flux according to any one of claims 1 to 6 and a solder alloy.

8. A method for manufacturing a bonded structure, comprising bonding a substrate and a bonding component using the solder paste according to claim 7.

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