Flux, solder paste, and printed circuit board

Abstract

To provide a flux that enables suppression of unmelted solder powder and formation of solder balls even after being allowed to stand for a certain time following application.SOLUTION: A flux comprising a resin, a solvent, and an activator, wherein the solvent contains, in an amount of 4 mass% or more and 16 mass% or less relative to the total amount of the flux, a compound represented by general formula (1), where R1 represents a linear or branched alkylene group having 3 to 5 carbon atoms, or a combination of a linear or branched alkylene group having 3 to 5 carbon atoms and a linear or branched ethylene oxide group having 3 to 5 carbon atoms.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a flux, a solder paste and a printed circuit board. [Background technology]

[0002] One method of soldering materials to be joined (e.g., printed wiring board and electronic component) is to use a solder paste containing flux and solder powder. The flux is a component that improves the solderability of the solder powder, and the active component (e.g., activator) contained in the flux serves to remove oxides present on the solder powder and the surfaces of the materials to be joined when heated.

[0003] However, solder powders containing alloying elements such as Bi that are easily oxidized are prone to forming oxide films on their surfaces, and therefore, solder pastes containing such solder powders in particular have the problem that the active components are unable to completely remove the oxide films when heated, resulting in the solder powder not melting and the formation of solder balls.

[0004] As a solder paste that solves the above problems, for example, the following solder paste has been proposed.

[0005] A lead-free solder paste (see Patent Document 1) is characterized by comprising a rosin-based resin (A) containing polymerized rosins (a1), a non-halogen-based flux containing, as essential components, a dibasic acid-based activator (B) and an ethylene oxide adduct of cyclohexylamine (C), and an Sn-Bi solder alloy powder.

[0006] A solder paste containing solder powder and flux, wherein the solder powder is a lead-free solder alloy having a solidus temperature of 150°C or less, the flux contains rosin, a solvent, a thixotropic agent, and organic acids and imidazoles as activators, the organic acids including one or more dicarboxylic acids having 4 to 6 carbon atoms, and the imidazoles including one or more imidazole compounds having 3 to 6 carbon atoms or derivatives thereof (see Patent Document 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-002553 [Patent Document 2] Brochure for International Publication No. 2022-234690 Summary of the Invention [Problem to be solved by the invention]

[0008] During soldering, solder paste containing flux may be applied to the surfaces of the workpieces and then left to stand (exposed to the atmosphere) for a certain period of time. During this standing time, the solder powder contained in the solder paste reacts with oxygen and water in the atmosphere. As a result, compared to when the paste is not left to stand, an increased amount of oxide film is formed on the surface of the solder powder, which tends to result in unmelted solder powder and solder balls. In particular, when using solder powder containing alloy elements that are easily oxidized, such as Bi, unmelted solder powder and solder balls are even more likely to occur.

[0009] The primary object of the present invention is to provide a flux that can prevent unmelted solder powder and the occurrence of solder balls even when left for a certain period of time after application. Another primary object of the present invention is to provide a flux that can be suitably used for soldering using solder powder containing easily oxidized alloy elements such as Bi. Another main object of the present invention is to provide a solder paste containing the above flux. Another object of the present invention is to provide a printed circuit board having a joint formed using the above solder paste. [Means for solving the problem]

[0010] (1) A flux according to one embodiment of the present invention includes a resin, a solvent, and an activator, and includes, as the solvent, a compound represented by the following general formula (1) in an amount of 4% by mass or more and 16% by mass or less relative to the total mass of the flux: [ka] (In the formula, R1 represents a linear or branched alkylene group having 3 to 5 carbon atoms, or a linear or branched alkylene group having 3 to 5 carbon atoms bonded to a linear or branched ethylene oxide group having 3 to 5 carbon atoms.)

[0011] (2) The flux described in (1) above may further contain an antioxidant, and the antioxidant may include a compound having an aniline skeleton.

[0012] (3) In the flux described in (2) above, the compound having an aniline skeleton may contain at least one compound represented by the following general formula (2) or (3): [ka] (In the above general formula (2), R2 represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms, R3 represents a phenyl group or a naphthyl group, R4 represents a phenyl group or a naphthyl group, and R5 represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms.) [ka] (In the above general formula (3), R6 and R7 each independently represent a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms which may be substituted with an aromatic ring.)

[0013] (4) In the flux described in (2) or (3) above, the compound having an aniline skeleton may be blended in an amount of 0.5% by mass or more and 5% by mass or less.

[0014] (5) The flux according to any one of (1) to (4) above may further contain a thixotropic agent.

[0015] (6) In the flux according to any one of (1) to (5) above, the solvent may contain other solvents in addition to the compound represented by the general formula (1), and the blending ratio of the compound represented by the general formula (1) to the other solvents may be, in mass ratio, 10:90 to 40:60 (compound represented by the general formula (1):other solvent).

[0016] (7) A solder paste according to one aspect of the present invention includes the flux according to any one of (1) to (6) above and a solder powder.

[0017] (8) In the solder paste described in (7) above, the solder powder may be made of a Sn—Bi based solder alloy.

[0018] (9) In the solder paste described in (8) above, the Sn—Bi solder alloy may contain 50 mass % or more of Bi and may further contain at least one of Cu, In, Sb, Ni, and Co.

[0019] (10) In the solder paste described in (9) above, the Sn—Bi solder alloy may contain 0% by mass or more and 5% by mass or less of Ag, 0% by mass or more and 1% by mass or less of Cu, 0% by mass or more and 5% by mass or less of In, 0% by mass or more and 5% by mass or less of Sb, 0% by mass or more and 0.5% by mass or less of Ni, and 0% by mass or more and 0.5% by mass or less of Co.

[0020] (11) A joint according to one aspect of the present invention is formed using the solder paste according to any one of (7) to (10) above.

[0021] (12) A printed circuit board according to one aspect of the present invention has the joint described in (11) above.

[0022] (13) A bonded structure according to one aspect of the present invention includes the bonding portion according to (11) above, and a first bonded body and a second bonded body bonded via the bonding portion.

[0023] (14) An electronic control device according to one aspect of the present invention is mounted with the printed circuit board described in (12) above. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0025] 1. Flux The flux of this embodiment includes a resin, a solvent, and an activator.

[0026] 1) Resin Examples of the resin include rosin-based resins, acrylic resins, styrene-maleic acid resins, epoxy resins, urethane resins, polyester resins, phenoxy resins, terpene resins, polyalkylene carbonates, and phenoxy resins. Among these, rosin-based resins are preferably used, but are not limited thereto. These resins may be used alone or in combination.

[0027] Examples of the rosin-based resin include rosins such as tall oil rosin, gum rosin, and wood rosin; rosin-based modified resins such as hydrogenated rosin, polymerized rosin, heterogenized rosin, acrylic acid-modified rosin, maleic acid-modified rosin, and formylated rosin; and derivatives thereof. Among these, hydrogenated acid-modified rosin is preferably used, but is not limited thereto. These may be used alone or in combination of two or more.

[0028] Examples of the acrylic resin include acrylic resins obtained by polymerizing at least one monomer selected from acrylic acid, methacrylic acid, various esters of acrylic acid, various esters of methacrylic acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, esters of maleic acid, esters of maleic anhydride, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, vinyl chloride, vinyl acetate, etc. These may be used alone or in combination of two or more.

[0029] The blending amount of the resin may be 10% by mass or more and 90% by mass or less, 30% by mass or more and 70% by mass or less, or 35% by mass or more and 60% by mass or less, based on the total amount of the flux.

[0030] 2) Solvent The flux of this embodiment contains, as the solvent, a compound represented by the following general formula (1) in an amount of 4% by mass or more and 16% by mass or less relative to the total amount of the flux. [ka] (In the formula, R1 represents a linear or branched alkylene group having 3 to 5 carbon atoms, or a linear or branched alkylene group having 3 to 5 carbon atoms bonded to a linear or branched ethylene oxide group having 3 to 5 carbon atoms.)

[0031] Examples of the compound represented by the general formula (1) include propylene glycol monophenyl ether, etc. The compounds represented by the general formula (1) can be used singly or in combination.

[0032] The compound represented by the general formula (1) is preferably blended in an amount of 8% by mass or more and 12% by mass or less based on the total amount of the flux.

[0033] The flux of this embodiment may contain a solvent other than the compound represented by the general formula (1). Examples of the other solvents include alcohol-based, ethanol-based, acetone-based, toluene-based, xylene-based, ethyl acetate-based, ethyl cellosolve-based, butyl cellosolve-based, glycol ether-based, ester-based solvents, etc. These may be used alone or in combination of two or more.

[0034] The glycol ether solvent may include, for example, a compound represented by the following general formula (4). [ka] (wherein R8 represents a linear or branched alkyl group having 3 to 10 carbon atoms).

[0035] Examples of the compound represented by the general formula (4) include isopropyl diglycol, butyl diglycol, isobutyl diglycol, hexyl diglycol, 2-ethylhexyl diglycol, etc. Among these, hexyl diglycol and 2-ethylhexyl diglycol are preferably used, but are not limited thereto. These may be used alone or in combination of two or more.

[0036] The total amount of the solvent may be 20% by mass or more and 70% by mass or less, 25% by mass or more and 65% by mass or less, or 30% by mass or more and 60% by mass or less, based on the total amount of the flux. The compounding ratio of the compound represented by the general formula (1) to the other solvents may be, in mass ratio, compound represented by the general formula (1):other solvents=10:90 to 40:60, or 20:80 to 30:70.

[0037] The compound represented by the general formula (1) has a low moisture absorption property. A flux containing a certain amount of this compound is also low in moisture absorption and can maintain a moderate level of tackiness. Therefore, a coating film formed using a solder paste containing the flux maintains a moderate level of tackiness even after being left for a certain period of time, and oxidation of the solder powder in the coating film can be suppressed.

[0038] 3) Activator Examples of the activator include organic acids, halogen-based activators, amine-based activators, etc. These may be used alone or in combination of two or more.

[0039] Examples of organic acids include carboxylic acids and other organic acids. Examples of the carboxylic acid include monocarboxylic acids and dicarboxylic acids. Examples of monocarboxylic acids include 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. Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedioic acid, eicosanedioic acid, fumaric acid, maleic acid, tartaric acid, diglycolic acid, and 1,4-cyclohexanedicarboxylic acid. Examples of other organic acids include dimer acid, levulinic acid, lactic acid, acrylic acid, benzoic acid, salicylic acid, anisic acid, citric acid, picolinic acid, and anthranilic acid. The above components can be used alone or in combination of two or more.

[0040] Examples of the halogen-based activator include bromine-based activators and iodine-based activators. The halogen-based activator may be a compound formed by covalent bonding of a single element such as chlorine, bromine, or fluorine, such as a chloride, bromide, or fluoride, or a compound formed by covalent bonding of two or more different halogen atoms. These may be used alone or in combination.

[0041] Examples of the amine-based surfactant include organic amines, amine salts (inorganic acid salts or organic acid salts) such as hydrogen halide salts of organic amines, organic acid salts, organic amine salts, etc. These can be used alone or in combination of two or more.

[0042] The amount of the activator may be 1% by mass or more and 20% by mass or less, 1% by mass or more and 15% by mass or less, or 1% by mass or more and 10% by mass or less, based on the total amount of the flux.

[0043] 4) Antioxidants The flux of this embodiment may contain an antioxidant. Furthermore, the flux of this embodiment may contain a compound having an aniline skeleton as the antioxidant.

[0044] As the compound having an aniline skeleton, for example, at least one compound represented by the following general formula (2) or (3) can be blended. [ka] (In the above general formula (2), R2 represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms, R3 represents a phenyl group or a naphthyl group, R4 represents a phenyl group or a naphthyl group, and R5 represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms.)

[0045] [ka] (In the above general formula (3), R6 and R7 each independently represent a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms which may be substituted with an aromatic ring.)

[0046] Examples of compounds represented by the general formula (2) include p,p'-dimethyldiphenylamine, p,p'-diethyldiphenylamine, p,p'-dipropyldiphenylamine, p,p'-dibutyldiphenylamine, p,p'-dipentyldiphenylamine, p,p'-dihexyldiphenylamine, p,p'-diheptyldiphenylamine, p,p'-dioctyldiphenylamine, p,p'-dinonyldiphenylamine, p,p'-didecyldiphenylamine, N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, and N-(p-dodecyl)phenyl-2-naphthylamine. Among these, p,p'-dioctyldiphenylamine and N-phenyl-1-naphthylamine are preferred, but are not limited thereto. These compounds may be used alone or in combination.

[0047] Examples of compounds represented by the general formula (3) include phenothiazine, 2-methoxyphenothiazine, 4-methoxyphenothiazine, monooctylphenothiazine, 3,7-dioctylphenothiazine, and 3,7-di-t-butylphenothiazine. Among these, phenothiazine is preferably used, but is not limited thereto. These compounds may be used alone or in combination.

[0048] The compound having an aniline skeleton may be blended in an amount of 0.5% by mass or more and 5% by mass or less relative to the total amount of the flux, and preferably in an amount of 1% by mass or more and 3% by mass or less.

[0049] The flux of this embodiment may contain an antioxidant other than at least one of the compounds having an aniline skeleton. Examples of the other antioxidants include hindered phenol-based antioxidants, phenol-based antioxidants, bisphenol-based antioxidants, polymer-type antioxidants, etc. These can be used alone or in combination of two or more.

[0050] The total amount of the antioxidants may be 0.5% by mass or more and 6% by mass or less, 1% by mass or more and 4% by mass or less, or 2% by mass or more and 4% by mass or less, based on the total amount of the flux.

[0051] When the flux contains the compound having an aniline skeleton, oxidation of the solder powder in the coating film can be further suppressed.

[0052] 5) Thixotropic agents Examples of the thixotropic agent include hydrogenated castor oil, hydrogenated castor oil, bisamide-based thixotropic agents (saturated fatty acid bisamide, unsaturated fatty acid bisamide, aromatic bisamide, etc.), oxyfatty acids, dimethyldibenzylidene sorbitol, etc. These can be used alone or in combination of two or more.

[0053] The blending amount of the thixotropic agent may be 1% by mass or more and 10% by mass or less, 2% by mass or more and 9% by mass or less, or 3% by mass or more and 8% by mass or less, based on the total amount of the flux.

[0054] The flux of this embodiment may further contain additives (antifoaming agents, matting agents, antifungal agents, etc.) The amount of the additives may be 10% by mass or less, or 5% by mass or less, based on the total amount of the flux.

[0055] When used in a solder paste, the flux of this embodiment having the above configuration can suppress moisture absorption of the solder paste while maintaining solder paste properties such as print stability and can maintain appropriate tackiness. Furthermore, a coating film formed using the solder paste can maintain appropriate tackiness even after being left for a certain period of time, and can also suppress oxidation of the solder powder in the coating film. Therefore, even when the coating film is heated after being left for a certain period of time, unmelted solder powder and the occurrence of solder balls can be suppressed. Therefore, such a flux can be suitably used in solder joints using solder powder containing easily oxidized alloy elements such as Bi.

[0056] Furthermore, as described above, the solder paste containing the flux can suppress oxidation of the solder powder in the coating film, enabling proper solder bonding even in the atmosphere, which can contribute to reducing power consumption and costs during solder bonding.

[0057] 2. Solder paste The solder paste of this embodiment contains the flux and solder powder, and can be produced by mixing the flux and solder powder by a known method.

[0058] The solder powder may be a powder of a Sn-Bi solder alloy, such as one containing 50 mass % or more of Bi and at least one of Cu, In, Sb, Ni, and Co. The Sn-Bi solder alloy may contain 0% to 5% by mass of Ag, 0% to 1% by mass of Cu, 0% to 5% by mass of In, 0% to 5% by mass of Sb, 0% to 0.5% by mass of Ni, and 0% to 0.5% by mass of Co. The Sn-Bi solder alloy may contain alloy elements other than these alloy elements.

[0059] The particle diameter of the solder powder may be 1 μm or more and 40 μm or less, 5 μm or more and 35 μm or less, or 10 μm or more and 30 μm or less, but is not limited to these.

[0060] The solder paste of this embodiment can contain, for example, the solder powder and the flux in a compounding ratio (mass %) of solder powder:flux=65:35 to 95:5, 85:15 to 93:7, or 87:13 to 92:8.

[0061] The solder paste of this embodiment contains the flux. Therefore, even when the solder paste uses a solder powder containing an easily oxidized alloy element such as Bi, the formed coating film maintains a suitable level of tackiness even after being left for a certain period of time, and oxidation of the solder powder in the coating film can be suppressed. Therefore, even when the coating film is heated after being left for a certain period of time, the occurrence of unmelted solder powder and solder balls can be suppressed. Furthermore, the solder paste of this embodiment having the above-described configuration can maintain properties such as printing stability.

[0062] Furthermore, the solder paste can suppress oxidation of the solder powder in the coating film, enabling proper solder bonding even in the atmosphere, which can contribute to reducing power consumption and costs during solder bonding.

[0063] 3.Joint part The joint of this embodiment is formed using the above solder paste and joins the materials to be joined together. As a method for forming the joint of this embodiment, a known method such as a reflow method can be used.

[0064] The materials to be joined can be substrates (materials whose surfaces are made of ceramic, metal, alloy, or resin and on which no electronic circuits are formed), printed wiring boards (substrates on which electronic circuits are formed but on which no electronic components are mounted), printed circuit boards (printed wiring boards on which electronic components are mounted), electronic components, silicon wafers, semiconductor packages, semiconductor chips, etc. Different types of materials to be joined may be used, or the same type of materials to be joined may be used.

[0065] 4.Joint structure The bonded structure of this embodiment includes, for example, a first material to be bonded, the bonding portion, and a second material to be bonded, and the first material to be bonded and the second material to be bonded are bonded via the bonding portion. The first and second bonding materials may be the same as those described above. Specific examples of combinations of the first and second bonding materials include a printed wiring board and an electronic component, a printed wiring board and a semiconductor chip, a semiconductor package and a printed circuit board, and a printed wiring board and a printed wiring board.

[0066] The bonded structure of this embodiment is produced, for example, by the following method. When a printed wiring board is used as the first material to be joined and an electronic component is used as the second material to be joined, the solder paste is first printed on a predetermined position of the first material to be joined, for example, on an electronic circuit, to form a coating film. Next, the second material to be joined is placed on the coating film. These are then reflowed at a predetermined heating temperature, for example, a peak temperature of 190°C, to form a joint joining the first material to be joined and the second material to be joined. This produces the joined structure (printed circuit board) of this embodiment. The joined structure of this embodiment, which has a joint formed using the solder paste, has high reliability.

[0067] 5. Electronic control unit The electronic control device of this embodiment is one in which the above-described joining structure is mounted, for example, the joining structure is disposed in a housing, and controls the operation of components that constitute an electronic device. The electronic control device in which the above-described joining structure is mounted has high reliability. [Example]

[0068] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0069] [Table 1]

[0070] [Table 2] *1 Hydrogenated acid-modified rosin (manufactured by Arakawa Chemical Industries, Ltd.) *2 Hindered phenol antioxidant (manufactured by BASF Japan Ltd.) *3 Hexamethylene bishydroxystearic acid amide (Mitsubishi Chemical Corporation)

[0071] <Flux Preparation> Each flux of the examples and comparative examples was prepared by kneading the components according to the composition and blending ratio shown in Tables 1 and 2. In Tables 1 and 2, the units of the numerical values ​​representing the composition are % by mass unless otherwise specified.

[0072] <Preparing solder paste> Examples 1 to 12 and Comparative Examples 1 to 6: Each solder paste was obtained by mixing 10.4 mass% of each flux listed in Tables 1 and 2 with 89.6 mass% of Sn-50Bi-1Sb-0.5In solder powder (size: Type 4 (IPC standard), particle size 20 to 38 μm). Example 13: 10.4 mass % of the flux shown in Table 1 and 89.6 mass % of Sn-50Bi-1Sb-0.5In-0.05Ni-0.1Co solder powder (size: Type 4 (IPC standard), particle size 20 to 38 μm) were mixed to obtain a solder paste. Example 14: 10.4 mass % of the flux shown in Table 1 and 89.6 mass % of Sn-50Bi-1Sb-2Cu-0.5In-0.05Ni-0.1Co solder powder (size: Type 4 (IPC standard), particle size 20 to 38 μm) were mixed to obtain a solder paste. Example 15: 10.4 mass % of the flux shown in Table 1 and 89.6 mass % of Sn-57Bi-1Ag solder powder (size: Type 4 (IPC standard), particle size 20 to 38 μm) were mixed to obtain a solder paste. Example 16: 10.4 mass % of the flux shown in Table 1 and 89.6 mass % of Sn-58Bi solder powder (size: Type 4 (IPC standard), particle size 20 to 38 μm) were mixed to obtain a solder paste.

[0073] (1-1) Solder ball confirmation test (immediately after printing) The following tools were prepared for each solder paste: Ceramic substrate (size: 25mm x 25mm x 0.65mmt) Metal mask (thickness: 0.1 mm, with a pattern that can form a printing layer with a diameter of 6.5 mm) Then, for each solder paste, the test was carried out according to the following procedure. Using the metal mask, solder paste was printed on the ceramic substrate to form a circular printed layer with a diameter of 6.5 mm. Note that only one printed layer was formed. Next, the ceramic substrate with the printed layer was placed on a hot plate set to 180° C. Five seconds after the melting of the printed layer was visually confirmed, the ceramic substrate was removed from the hot plate and, while held horizontally, cooled on a metal net at room temperature (20° C. to 25° C.) to obtain a test piece. The surfaces of the test pieces were then observed using a microscope to check for the presence or absence of solder balls and their number, and were evaluated according to the following criteria. The results are shown in Table 3. ○: One large solder ball is formed, and only solder balls with a diameter of 75 μm or less are generated around the solder, and there are four or more of them, but the solder balls are not arranged in a semi-continuous ring. △: One large solder ball is formed, and only solder balls with a diameter of 75 μm or less are generated around the solder, and there are four or more of them, but the solder balls are arranged in a semi-continuous ring. ×: Other than the above

[0074] (1-2) Solder ball confirmation test (after 1 hour) After forming the printed layer, the ceramic substrate was left in a 25°C, 50% RH environment for 1 hour, and then heated on the hot plate. Test pieces were obtained under the same conditions as in (1-1) Solder Ball Verification Test (Immediately After Printing). The surfaces of the test pieces were then observed and evaluated under the same conditions and with the same criteria as in (1-1) Solder Ball Verification Test (Immediately After Printing). The results are shown in Table 3.

[0075] (2-1) Pin-to-pin ball confirmation test (immediately after printing) The following tools were prepared for each solder paste: Printed wiring board (size: 105mm x 145mm x 1.2mmt, with 80 pins of 0.4mm x 3.0mm Cu pad area at 0.8mm pitch) A metal mask (thickness: 100 μm) with a pattern corresponding to the Cu pad portion. Then, for each solder paste, the test was carried out according to the following procedure. Using the metal mask, solder paste was printed onto the printed wiring board under the following printing conditions: printing speed 30 mm / sec, printing pressure 30 N / mm. The printed wiring board was then reflowed in air using a reflow furnace (product name: TNV-M6110CR, manufactured by Tamura Corporation) to produce a test board with a solder layer. The reflow was performed with a preheat of 70 to 100°C, a peak temperature of 190°C, a time above 140°C of 80 seconds, and a cooling rate from the peak temperature to 100°C of 1 to 4°C / second. The surface of the test board was then observed using a microscope to check for the presence or absence of solder balls between the pins and the number of solder balls between the pins (number of balls between the pins), and evaluated according to the following criteria. The results are shown in Table 3. ◎: The number of balls between pins of 75 μm or less is less than 10 ○: The number of balls between pins of 75 μm or less is 10 or more but less than 20 △: The number of balls between pins of 75 μm or less is 20 or more but less than 30 ×: The number of inter-pin balls with diameters of 75 μm or less is 30 or more, or there are inter-pin balls with diameters exceeding 75 μm. ××: Poor fusion occurred, making it impossible to count the number of balls between pins.

[0076] (2-2) Pin-to-pin ball confirmation test (after 3 hours) After printing the solder paste, the printed wiring board was left in a 25°C, 50% RH environment for 3 hours before reflow. A test board was prepared under the same conditions as in (2-1) Inter-pin ball-forming test (immediately after printing). The surface of the test board was then observed and evaluated under the same conditions and with the same criteria as in (2-1) Inter-pin ball-forming test (immediately after printing). The results are shown in Table 3.

[0077] (3-1) Tackiness confirmation test (immediately after printing) For each solder paste, the test was conducted according to the following procedure, using a method that conforms to the adhesion test of JIS standard Z3284-3 (2014). First, a metal mask (thickness: 0.2 mm) was used to print the solder paste onto a slide glass (size: 76 mm x 25 mm x 1 mmt) in a circular shape with a diameter of 6.5 mm. The adhesive strength of the solder paste (printed layer) on the glass slide was measured using a tackiness measuring device (product name: Tackiness Tester (TK-1), manufactured by Malcom Co., Ltd.) under the following conditions: lowering speed 2.0 mm / sec, pressure 0.05±0.005 Nm, pressure time 0.2 sec, and rising speed 10 mm / sec. The adhesive strength was evaluated according to the following criteria. The results are shown in Table 3. ○: Adhesive strength is 1N or more △: Adhesive strength is 0.8N or more but less than 1N ×: Adhesive strength is less than 0.8N

[0078] (3-2) Tackiness confirmation test (after 8 hours) After printing the solder paste, the slide glass was left to stand for 8 hours at 25°C and 50% RH, after which the adhesive strength was measured and evaluated under the same conditions and criteria as in the above (3-1) Tackiness Test (immediately after printing). The results are shown in Table 3.

[0079] (4) Dewetting test For each solder paste, the test was conducted according to the following procedure, using a method that conforms to the dewetting test of JIS standard Z3284-4 (2014). First, the surface of the copper plate was washed with 2-propanol. Then, one side of the copper plate was polished with abrasive paper while dropping 2-propanol, and then the surface of the copper plate was washed with 2-propanol to remove any surface dirt. The copper plate was then thoroughly dried at room temperature. Then, a solder paste was applied to the center of the copper plate using a metal mask to obtain a test plate. Next, the solder paste (printed layer) on the test plate was melted using a hot plate at 180° C., and then the test plate was lifted up while being kept horizontal, and cooled to room temperature. Thereafter, the flux residue remaining on the copper plate was removed with 2-propanol, and the degree of spreading of the formed solder layer was observed and evaluated according to the following criteria. ○: More than 80% of the area where the solder paste was applied is wet with solder (a solder layer has been formed) △: Between 50% and 80% of the area where the solder paste was applied was wet with solder (a solder layer was formed). ×: Less than 50% of the area where the solder paste was applied was wet with solder (a solder layer was formed)

[0080] (5) Continuous rolling test For each solder paste, the test was carried out according to the following procedure. First, the viscosity η1 of the solder paste at 10 r / m was measured using a method conforming to the viscosity characteristics test (spiral method) of JIS standard Z3284-3 (2014). Next, using a metal squeegee, the solder paste was continuously squeezed for 12 hours on a sealed metal mask under the following conditions: a squeegee angle of 60°, a printing cycle time of 30 seconds, a stroke of 300 mm, and an ambient temperature of 25°C and 50% RH. The sample amount was 500 g. The viscosity η2 of the solder paste after squeegeeing was then measured under the same conditions as the viscosity η1. The viscosity change rate (%) was calculated based on the following formula (A) and evaluated according to the following criteria. The results are shown in Table 3. Viscosity change rate (%) = (η2 / η1-1) × 100 ... Equation (A) ○: Viscosity change rate is ±20% or less △: Viscosity change rate is over ±20% and is ±25% or less ×: Viscosity change rate exceeds ±25%

[0081] [Table 3]

[0082] As described above, the coating film formed using the solder paste containing the flux of this example can suppress the occurrence of unmelted solder powder and solder balls even when heated after being left for a certain period of time. Furthermore, it can be seen that the solder paste using the flux of this example exhibits the above-mentioned effects even when using solder powder containing alloy elements that are easily oxidized, such as Bi. Furthermore, it can be seen that the solder paste of this example has good printing stability, and that the coated film of the solder paste retains a suitable level of tackiness even after 8 hours.

[0083] Thus, the flux of this embodiment can provide a solder paste that can form highly reliable solder joints.

Claims

1. a resin, a solvent, and an activator; A flux containing a compound represented by the following general formula (1) as the solvent in an amount of 4 mass % or more and 16 mass % or less based on the total amount of the flux: 【Chemical 1】 (In the formula, R １ represents a linear or branched alkylene group having 3 to 5 carbon atoms, or a linear or branched alkylene group having 3 to 5 carbon atoms bonded to a linear or branched ethylene oxide group having 3 to 5 carbon atoms.

2. Further, it contains an antioxidant, The flux according to claim 1 , wherein the antioxidant comprises a compound having an aniline skeleton.

3. The flux according to claim 2, wherein the compound having an aniline skeleton includes at least one compound represented by the following general formula (2) or (3): 【change】 (In the above general formula (2), R ２ represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms; R ３ represents a phenyl group or a naphthyl group, R ４ represents a phenyl group or a naphthyl group, R ５ represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms. 【Chemistry 3】 (In the above general formula (3), R ６ and R ７ each independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms which may be substituted with an aromatic ring.

4. A solder paste comprising the flux according to any one of claims 1 to 3 and solder powder.

5. 5. The solder paste according to claim 4, wherein the solder powder is made of a Sn--Bi based solder alloy.

6. 6. The solder paste according to claim 5, wherein the Sn—Bi based solder alloy contains 50 mass % or more of Bi, and further contains at least one of Ag, Cu, In, Sb, Ni, and Co.

7. A printed circuit board having a joint formed using the solder paste of claim 6.

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