Flux, solder paste, and method for manufacturing bonded structures

Abstract

To provide a flux, a solder paste, and a manufacturing method of a joined body which can reduce a reflow process time for achieving a low residue.SOLUTION: A flux is a flux used in soldering, contains at least one selected from aliphatic alcohol which has one hydroxy group and is solid at 20°C and aliphatic ester that is solid at 20°C, a liquid solvent that has 1 to 3 hydroxy groups and is liquid at 20°C, and a solid solvent that has 2 to 4 hydroxy groups and is solid at 20°C, and does not contain a thixotropic agent and contains 20.0 mass% or less of a thixotropic agent with respect to the total flux.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a flux, a solder paste, and a joined structure.

Background Art

[0002] For joining an electronic circuit board such as a printed wiring board and a joining component, a solder paste obtained by mixing solder alloy powder and a flux is used. The solder paste is applied to the electrode portion on the surface of the substrate, and heated (reflowed) in a state where the electrode portion of the joining component is brought into contact with the electrode portion. Thereby, the solder alloy powder melts to form a solder joint portion, and a joined structure in which the substrate and the joining component are joined through the solder joint portion can be obtained.

[0003] As the flux contained in such a solder paste, a flux composed of a resin such as rosin, a solvent, a thixotropic agent, an activator, etc. is generally used. However, when soldering is performed using a solder paste containing such a flux, flux residue may remain around the soldered portion. Therefore, for example, a method of reducing flux residue is attempted by a method of selecting components of the flux from highly volatile materials, vacuum reflow, or vacuum reduction reflow to sufficiently volatilize the flux in the reflow process.

[0004] For example, Patent Document 1 discloses a flux that does not contain a resin such as rosin that is difficult to volatilize. Such a flux attempts to reduce flux residue by using a liquid solvent and a solid solvent in combination instead of a resin such as rosin.

[0005] Traditionally, solder foil has been widely used in soldering for power device manufacturing. However, in recent years, solder paste has been attracting attention as an alternative material to solder foil in the field of power device manufacturing. Solder paste allows for the simultaneous supply of solder to the substrate by printing. Furthermore, soldering using solder paste eliminates the need for component fixing jigs to prevent component shifting and falling during substrate transport. This makes it easier to automate the manufacturing process of joined structures, and as a result, attempts to apply solder paste to soldering in power device manufacturing are being actively pursued. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-38026 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The reflow soldering process suffers from the problem of long reflow times, which are necessary to reduce flux residue after reflow. In particular, in the soldering of power devices, flux residue can cause defects in subsequent processes, making the reduction of flux residue even more critical. Therefore, the reflow process time tends to be longer in the soldering of power devices.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a flux, solder paste, and a method for manufacturing a bonded structure that can shorten the reflow process time for achieving low residue. [Means for solving the problem]

[0009] The flux according to the present invention is a flux used for soldering, and contains at least one selected from an aliphatic alcohol having one hydroxyl group and being solid at 20°C, and a fatty acid ester being solid at 20°C, a liquid solvent having one to three hydroxyl groups and being liquid at 20°C, and a solid solvent having two to four hydroxyl groups and being solid at 20°C, and either does not contain a thixotropic agent or contains a thixotropic agent in an amount of 20.0% by mass or less relative to the entire flux.

[0010] The solder paste according to the present invention comprises the flux and solder alloy powder.

[0011] The method for manufacturing a bonded structure according to the present invention involves joining a substrate and a bonded component using solder paste. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a flux, solder paste, and a method for manufacturing a bonded structure that can shorten the reflow process time for achieving low residue. [Modes for carrying out the invention]

[0013] The following describes a method for manufacturing a flux, solder paste, and a bonded structure according to an embodiment of the present invention.

[0014] <Flux> The flux according to this embodiment contains at least one selected from aliphatic alcohols and fatty acid esters, a liquid solvent, and a solid solvent.

[0015] (Aliphatic alcohols) Aliphatic alcohols have one hydroxyl group and are solid at 20°C. Examples of such aliphatic alcohols include 1-tetradecanol, 1-hexadecanol, 1-octadecanol, 1-eicosanol, 1-docosanol, 1-dodecanol, and 1-tetracosanol. Among these, it is preferable that the aliphatic alcohol be at least one selected from 1-tetradecanol, 1-hexadecanol, 1-octadecanol, 1-eicosanol, and 1-docosanol.

[0016] The aliphatic alcohol content is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and particularly preferably 5.0% by mass or more, relative to the total flux. Furthermore, the aliphatic alcohol content is preferably 45.0% by mass or less, more preferably 30.0% by mass or less, and particularly preferably 20.0% by mass or less, relative to the total flux. If two or more types of aliphatic alcohols are present, the above content refers to the total content.

[0017] (Fatty acid esters) Fatty acid esters are solid at 20°C. Examples of such fatty acid esters include methyl stearate, ethyl stearate, butyl stearate, methyl palmitate, ethyl palmitate, methyl arachidinate, ethyl arachidinate, and methyl behenate. Among these, it is preferable that the fatty acid ester is at least one selected from methyl stearate, ethyl stearate, and butyl stearate.

[0018] The content of the fatty acid ester is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and particularly preferably 3.0% by mass or more, relative to the total flux. Furthermore, the content of the fatty acid ester is preferably 45.0% by mass or less, more preferably 20.0% by mass or less, and particularly preferably 10.0% by mass or less, relative to the total flux. If two or more types of fatty acid esters are present, the above content refers to the total content.

[0019] In this embodiment, the flux preferably contains both the aliphatic alcohol and the fatty acid ester, from the viewpoint of suppressing flux separation. The total content of the aliphatic alcohol and the fatty acid ester is preferably 1.0% by mass or more, more preferably 5.0% by mass or more, and particularly preferably 10.0% by mass or more, relative to the total flux. Furthermore, the total content of the aliphatic alcohol and the fatty acid ester is preferably 45.0% by mass or less, more preferably 40.0% by mass or less, and particularly preferably 30.0% by mass or less, relative to the total flux.

[0020] (Liquid solvent) The flux according to this embodiment contains a liquid solvent that has one to three hydroxyl groups and is liquid at 20°C. Examples of such liquid solvents include 3-methyl-1,5-pentanediol, 1,4-butanediol, 2,5-hexanediol, 1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 1,2,6-hexanetriol, 1,2,4-butanetriol, hexyldecanol, isostearyl alcohol, 2-octyldodecanol, and 2-decylteto Rade Examples include canol, oleyl alcohol, 2-ethyl-1,3-hexanediol, 3-methyl-1,3-butanediol, and glycerin.

[0021] From the perspective of enhancing the volatility of the flux, it is preferable to use at least one type of the liquid solvent. From the perspective of enhancing the viscosity stability of the solder paste, it is more preferable to use two or more types. When using at least one type of the liquid solvent, it is preferably at least one type selected from 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 3-methyl-1,3-butanediol, 1,2,6-hexanetriol, and 1,2,4-butanetriol.

[0022] The content of the liquid solvent is preferably 5.0% by mass or more, more preferably 20.0% by mass or more, with respect to the entire flux. Also, the content of the liquid solvent is preferably 80.0% by mass or less, more preferably 60.0% by mass or less, with respect to the entire flux. When two or more types of liquid solvents are included, the content is the total content.

[0023] (Solid solvent) The flux according to this embodiment has two or more and four or less hydroxy groups and contains a solid solvent that is solid at 20°C. Examples of such solid solvents include 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, trimethylolethane, trimethylolpropane, erythritol, and the like. Among these, from the perspective of improving the printability of the solder paste, the solid solvent is preferably at least three selected from 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, trimethylolethane, trimethylolpropane, and erythritol.

[0024] The content of the solid solvent is preferably 5.0% by mass or more, more preferably 25.0% by mass or more, with respect to the entire flux. Also, the content of the solid solvent is preferably 92.0% by mass or less, more preferably 65.0% by mass or less, with respect to the entire flux. When two or more solid solvents are included, the content is the total content.

[0025] (Thixotropic agent) The flux according to this embodiment either does not contain a thixotropic agent or contains a thixotropic agent in an amount of 20.0% by mass or less relative to the total flux. Examples of thixotropic agents include hydrogenated castor oil, fatty acid amides, fatty acid bisamides, polyamide compounds, kaolin, colloidal silica, organic bentonite, and glass frit. Among these, the thixotropic agent preferably contains a fatty acid amide, and more preferably consists of a fatty acid amide, from the viewpoint of reducing flux residue. 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 amide is preferably stearic acid amide, lauric acid amide, palmitic acid amide, oleic acid amide, erucic acid amide, or behenic acid amide. Note that one type of thixotropic agent may be used alone, or two or more types may be used in combination.

[0026] From the viewpoint of increasing the volatility of the flux, the content of the thixotropic agent is preferably 20.0% by mass or less, more preferably 15.0% by mass or less, and more preferably 10.0% by mass or less, relative to the total flux. Furthermore, from the viewpoint of increasing the volatility of the flux, it is preferable that the flux according to this embodiment does not contain a thixotropic agent. Note that if two or more thixotropic agents are included, the above content is the total content.

[0027] (Activating agent) The flux according to this embodiment may contain an activator. The activator is not particularly limited and examples include organic acid-based activators, amine compounds, amino acids, halogen-based activators such as amine halogen salts and halogen compounds. The activator may be used alone or in combination of two or more.

[0028] Organic acid-based surfactants are not particularly limited and include, for example, monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, capric acid, lauric acid, myristic acid, pentadecyl acid, palmitic acid, margaric acid, stearic acid, tubercurostearic 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.

[0029] The amine compounds are not particularly limited and include imidazole compounds, triazole compounds, etc. Examples of the imidazole compounds include benzimidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole (2E4MZ), 2-heptadecylimidazole, 2-undecylimidazole, 1-(4,6-diamino-s-triazine-2-yl)ethyl-2-undecylimidazole, 1-butylimidazole, 2-phenylimidazole, and 4-methyl-2-phenylimidazole. Examples of the triazole compounds include benzotriazole, 1H-benzotriazole-1-methanol, and 1-methyl-1H-benzotriazole. Other amine compounds include, for example, cetylamine, 3-(dimethylamino)-1,2-propanediol, 3,5-dimethylpyrazole, dimethylurea, hexahydro-1,3,5-triphenyl-1,3,5-triazine, pyrazineamide, N-phenylglycine, 3-methyl-5-pyrazolone, N-lauroyl sarcosine, and 1,3-diphenylguanidine.

[0030] The amino acids are not particularly limited, but examples 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), or N-acetylphenylglycine (N-acetyl-N-phenylglycine, N-acetyl-L-phenylglycine, N-acetyl-DL-phenylglycine), etc.

[0031] The halogenated activators are not particularly limited and 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.

[0032] The content of the activator is preferably 0.5% by mass or more, and more preferably 1.0% by mass or more, relative to the total flux. Furthermore, the content of the activator is preferably 5.0% by mass or less, and more preferably 3.0% by mass or less, relative to the total flux. If two or more types of activators are included, the above content refers to the total content.

[0033] The flux according to this embodiment may also contain, as other additives, at least one selected from, for example, stabilizers, surfactants, defoamers, and corrosion inhibitors. The total content of the other additives is not particularly limited and can be, for example, 5.0% by mass or less of the total flux.

[0034] The flux according to this embodiment contains at least one selected from an aliphatic alcohol having one hydroxyl group and being solid at 20°C, and a fatty acid ester that is solid at 20°C, and either does not contain a thixotropic agent or contains a thixotropic agent in an amount of 20.0% by mass or less relative to the entire flux, thereby shortening the reflow process time to achieve low residue.

[0035] In this embodiment, the flux contains the aliphatic alcohol, and the aliphatic alcohol content is 0.5% by mass or more and 45.0% by mass or less of the total flux, thereby further shortening the reflow process time required to achieve low residue. In addition, the viscosity of the flux is increased, ensuring the viscosity of the solder paste.

[0036] In this embodiment, the flux contains the fatty acid ester, and the fatty acid ester content is 0.5% by mass or more and 45.0% by mass or less of the total flux, thereby further shortening the reflow process time required to achieve low residue. In addition, the viscosity of the flux is increased, ensuring the viscosity of the solder paste.

[0037] In this embodiment, the flux can further shorten the reflow process time to achieve low residue because the aliphatic alcohol is at least one selected from 1-dodecanol, 1-tetradecanol, 1-hexadecanol, 1-octadecanol, 1-eicosanol, and 1-docosanol.

[0038] In this embodiment, the flux, by having the fatty acid ester selected from methyl stearate, ethyl stearate, and butyl stearate, can further shorten the reflow process time required to achieve low residue.

[0039] <Solda paste> The solder paste according to this embodiment contains the flux described above and solder alloy powder. Examples of solder alloys in the solder alloy powder include lead-free solder alloys and lead-containing eutectic solder alloys, but 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 alloys include 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, Sn / In, etc. Furthermore, the alloy contains unavoidable impurities. Unavoidable impurities refer to components that are inevitably mixed in during the manufacturing process and are acceptable to the extent that they do not affect the effects of the present invention.

[0040] The flux content is preferably 5% by mass or more and 20% by mass or less relative to the total solder paste. Furthermore, the solder alloy powder content is preferably 80% by mass or more and 95% by mass or less relative to the total solder paste.

[0041] The solder paste according to this embodiment, by including the flux and solder alloy powder, can shorten the reflow process time to achieve low residue.

[0042] <Method for manufacturing a jointed structure> In the manufacturing method of the bonded structure according to this embodiment, the substrate and the bonded component are joined using the solder paste. Specifically, the substrate and the bonded component are heated in a reflow oven while in contact with each other via the solder paste, and the solder alloy constituting the solder paste is melted. The substrate and the bonded component are then joined by cooling.

[0043] First, the solder paste is placed on the surface of the substrate. The solder paste can be placed using known methods such as printing with a solder printing device, transfer printing, application 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 30 μm or more and 600 μm or less. Furthermore, the substrate is not particularly limited, and known substrates such as printed circuit boards, DBC substrates, base plates, lead frames, and silicon wafers can be used.

[0044] Next, the bonding component is positioned so as to contact the substrate via the solder paste. The bonding component is not particularly limited, and known bonding components such as chip components (IC chips, etc.), resistors, diodes, capacitors, transistors, semiconductor chips (Si chips, etc.), and heat sinks can be used.

[0045] Next, the substrate and the bonding component are heated while in contact via the solder paste. The heating temperature is not particularly limited and can be appropriately selected depending on the solder paste, and should be above the melting point of the solder paste. The heating process may be carried out under an inert gas atmosphere using nitrogen gas or the like to suppress metal oxidation. Alternatively, the atmosphere may be reduced in pressure or evacuated using a vacuum pump to promote gas release. Furthermore, a preheating process may be performed before the heating process. The preheating temperature is not particularly limited and can be appropriately selected depending on the solder paste, and should be below the melting point of the solder paste. The preheating process may be carried out under reduced pressure or evacuated in an inert gas atmosphere, or under a reducing atmosphere using hydrogen gas or formic acid gas.

[0046] Finally, the substrate and the bonded components that are in contact via the solder paste are cooled. In the cooling process, for example, the temperature is cooled to between 20°C and 200°C at a cooling rate of 120°C / min or more and 5°C / min or less.

[0047] The manufacturing method of the bonded structure according to this embodiment can shorten the reflow process time, which is necessary to achieve low residue, by using the solder paste to bond the substrate and the bonded component.

[0048] The present invention includes the following embodiments. [1] A flux used for soldering, At least one selected from aliphatic alcohols having one hydroxyl group and being solid at 20°C, and fatty acid esters that are solid at 20°C, It has one to three hydroxyl groups and is a liquid solvent that is liquid at 20°C. It contains a solid solvent having two to four hydroxyl groups and being solid at 20°C, A flux that does not contain a thixotropic agent, or contains a thixotropic agent at a concentration of 20.0% by mass or less relative to the total flux. [2] The flux comprises the aliphatic alcohol, The flux according to [1], wherein the content of the aliphatic alcohol is 0.5% by mass or more and 45.0% by mass or less of the total flux. [3] The flux comprises the fatty acid ester, The flux according to [1] or [2], wherein the content of the fatty acid ester is 0.5% by mass or more and 45.0% by mass or less of the total flux. [4] The flux according to any one of [1] to [3], wherein the aliphatic alcohol is at least one selected from 1-dodecanol, 1-tetradecanol, 1-hexadecanol, 1-octadecanol, 1-eicosanol, and 1-docosanol. [5] The flux according to any one of [1] to [4], wherein the fatty acid ester is at least one selected from methyl stearate, ethyl stearate, and butyl stearate. A solder paste comprising the flux described in any one of [6][1] to [5] and solder alloy powder. A method for manufacturing a bonded structure, comprising joining a substrate and a bonding component using a solder paste described in any one of [7][1] to [6]. [Examples]

[0049] The following describes embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0050] <Flux materials> (Aliphatic alcohols) 1-Tetradecanol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1-Hexadecanol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1-Octadecanol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1-Eicosanol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1-Docosanol: Manufactured by Tokyo Chemical Industry Co., Ltd. (Fatty acid esters) Methyl stearate: Manufactured by Tokyo Chemical Industry Co., Ltd. Ethyl stearate: Manufactured by Tokyo Chemical Industry Co., Ltd. Butyl stearate: Manufactured by Tokyo Chemical Industry Co., Ltd. (Liquid solvent) 3-Methyl-1,5-pentanediol: Manufactured by Kuraray Co., Ltd. 1,4-Butanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 2,5-Hexanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,5-Pentanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 2,4-Diethyl-1,5-pentanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,2,6-Hexanetriol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,2,4-Butanetriol: Manufactured by Tokyo Chemical Industry Co., Ltd. Hexyldecanol: Manufactured by Higher Alcohol Industry Co., Ltd. Isostearyl alcohol: Manufactured by Higher Alcohol Industry Co., Ltd. 2-Octyldodecanol: Manufactured by Higher Alcohol Industry Co., Ltd. 2-Deciltet RadeCanol: Manufactured by Higher Alcohol Industry Co., Ltd. Oleyl alcohol: Manufactured by Higher Alcohol Industry Co., Ltd. (Solid solvent) 2,2-Dimethyl-1,3-propanediol: Manufactured by Mitsubishi Gas Chemical Company, Inc. 2,5-Dimethyl-2,5-Hexanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 2,2,4-trimethyl-1,3-pentanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 2-Butyl-2-ethyl-1,3-propanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,2-Octanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,8-Octanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,10-Decanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,2-Dodecanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,6-Hexanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. 1,9-nonanediol: Manufactured by Tokyo Chemical Industry Co., Ltd. Trimethylolethane: Manufactured by Mitsubishi Gas Chemical Company, Inc. Trimethylolpropane: Manufactured by Mitsubishi Gas Chemical Company, Inc. Erythritol: Manufactured by Tokyo Chemical Industry Co., Ltd. (Tixotropic agent) Stearic acid amide: Manufactured by Kao Corporation Lauric acid amide: Manufactured by Mitsubishi Chemical Corporation Palmitic acid amide: Manufactured by Mitsubishi Chemical Corporation Oleamide: Manufactured by Kao Corporation Erucic acid amide: Manufactured by Nippon Seika Co., Ltd. Behenamide: Manufactured by Nippon Seika Co., Ltd.

[0051] <Preparation of flux> The flux materials described above were placed in a heating container according to the formulations shown in Tables 1 to 10, and heated to 120°C to dissolve all the materials. Afterward, the mixture was cooled to room temperature to obtain uniformly dispersed fluxes for each example and comparative example. Note that the formulation amounts shown in Tables 1 to 10 are equivalent to the content of each component in the flux.

[0052] <Preparation of Solder Paste> The fluxes for each example and comparative example were mixed at a ratio of 9% by mass, and solder alloy powder (96.5Sn / 3Ag / 0.5Cu, Type 3, manufactured by Koki Co., Ltd.) at a ratio of 91% by mass, to obtain the solder pastes for each example and comparative example.

[0053] [Table 1]

[0054] [Table 2]

[0055] [Table 3]

[0056] [Table 4]

[0057] [Table 5]

[0058] [Table 6]

[0059] [Table 7]

[0060] [Table 8]

[0061] [Table 9]

[0062] [Table 10]

[0063] <Evaluation of flux volatility> First, the weight of the evaporation dish was measured and designated as W1. Next, 0.3 to 0.303 g of flux from each example and comparative example was weighed into the evaporation dish, and the measured value was designated as W2. Subsequently, the evaporation dish containing the measured flux was placed on a hot plate heated to 350°C, and the time until all the flux evaporated was measured visually. After that, the evaporation dish was removed from the hot plate and cooled to room temperature. The maximum heating time was 240 seconds, and even if all the flux had not evaporated, the evaporation dish was removed from the hot plate and cooled to room temperature. The weight of the evaporation dish after cooling to room temperature was measured and designated as W3. The flux evaporation amount [%] for each example and comparative example was calculated from the following formula (1). The results are shown in Tables 1 to 10.

[0064] Flux volatilization amount [%] = (W2 - (W3 - W1)) / W2 × 100 (1) In the formula, W1 is the weight of the evaporating dish, W2 is the weight of the evaporating dish after the flux has been measured out, and W3 is the weight of the evaporating dish after the flux has been measured out, heated, and then cooled to room temperature.

[0065] Flux volatility was evaluated based on the following criteria. The results are shown in Tables 1 to 10. ◎: Flux volatilization rate [%] is 97% or higher, and the time until the flux volatilizes is within 180 seconds. ○: Flux volatilization rate [%] is 97% or higher, and the time until the flux volatilizes is within 240 seconds. ×: Flux volatilization [%] is 97% or less

[0066] <Evaluation of Solder Paste> The solder pastes of each example and comparative example were left for 1 hour in an environment of 20-30°C, and then the state of the solder paste was checked. The evaluation of the solder paste was performed based on the following criteria. The results are shown in Tables 1 to 10. Note that flux separation refers to the state in which the solder alloy powder settles in the solder paste due to the difference in specific gravity between the solder alloy powder and the flux, and the flux accumulates on top. A: No change (flux and solder alloy powder remain dispersed) B: Flux separation occurs, and stirring with a spatula returns it to state A. C: The solder paste is hard, but stirring it with a spatula will return it to state A.

[0067] As can be seen from the results in Tables 1 to 10, the fluxes of each embodiment that satisfy all the requirements of the present invention have a flux volatilization rate of 97% or more and a time until the flux volatilizes of 240 seconds or less, thus enabling a reduction in the reflow process time required to achieve low residue.

Claims

1. A flux used for soldering, At least one selected from aliphatic alcohols having one hydroxyl group and being solid at 20°C, and fatty acid esters that are solid at 20°C, It has one to three hydroxyl groups and is a liquid solvent that is liquid at 20°C. It contains a solid solvent having two to four hydroxyl groups and being solid at 20°C, Either does not contain a thixotropic agent, or contains a thixotropic agent at a concentration of 20.0% by mass or less relative to the total flux. The flux comprises the aliphatic alcohol, The aliphatic alcohol is at least one selected from 1-tetradecanol, 1-hexadecanol, 1-octadecanol, 1-eicosanol, and 1-docosanol. The aliphatic alcohol content is 0.5% by mass or more and 45.0% by mass or less relative to the total flux. A flux in which the total content of the aliphatic alcohol and the fatty acid ester is 1.0% by mass or more and 45.0% by mass or less of the total flux.

2. The flux according to claim 1, wherein the fatty acid ester is at least one selected from methyl stearate, ethyl stearate, and butyl stearate.

3. A solder paste comprising the flux according to claim 1 or 2 and solder alloy powder.

4. A method for manufacturing a bonded structure, comprising joining a substrate and a bonding component using the solder paste described in claim 3.

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