Flux and solder paste

Abstract

A flux containing 1 to 10 wt% of an organic sulfonic acid activator, 10 to 40 wt% of a high-molecular-weight nonionic surfactant that is a nonionic surfactant having a mass-average molecular weight Mw of more than 1200 and 5 to 75 wt% of a low-molecular-weight nonionic surfactant that is a nonionic surfactant having a mass-average molecular weight Mw of 1200 or less, in which the content of the low-molecular-weight nonionic surfactant is equal to or larger than the content of the organic sulfonic acid activator. This flux contains no cationic surfactant or contains more than 0 wt% and 5 wt% or less of the cationic surfactant. A solder paste containing this flux and a Sn-based solder metal.

Description

Flux and Solder Paste

[0001] The present invention relates to a flux used in soldering and a solder paste using this flux.

[0002] Generally, flux is used in soldering. Flux chemically removes metal oxides present on the surface of the solder and on the metal surface of the object to be soldered. This allows the metal elements to migrate at the interface between the solder and the object, resulting in a strong bond between the two.

[0003] Fluxes are classified into resin-based fluxes, water-soluble fluxes, and inorganic fluxes. Resin-based fluxes are fluxes in which an activator is added to resins such as rosin or synthetic resins. Water-soluble fluxes are fluxes in which an organic acid-based activator is dissolved in a solvent such as water or an organic solvent. In addition to organic acid-based activators, polyethylene glycol, water-soluble base agents, etc. may be added to water-soluble fluxes. Inorganic fluxes are fluxes that use inorganic materials such as hydrochloric acid and zinc chloride.

[0004] Solder paste is a composite material obtained by mixing solder alloy powder and flux. Soldering using solder paste is performed, for example, as follows: First, the solder paste is printed on the soldering areas, such as electrodes on a circuit board. Next, components are mounted on the soldering areas. The circuit board is then heated in a heating furnace called a reflow furnace. This bonds the components to the soldering areas.

[0005] Examples of prior art related to the present application include the technologies of Patent Documents 1 to 3. Patent Document 1 discloses a water-soluble flux containing an alkane sulfonic acid as an activator. Example 6 of Patent Document 1 shows that excellent results were obtained in a wettability test of molten solder using a water-soluble flux consisting of 5 wt % methanesulfonic acid and 95 wt % water.

[0006] Patent Document 2 discloses a solder paste containing a solder alloy, a non-halogenated amine, and an organic portion. Paste No. 27 in Patent Document 2 shows an example sample containing 0.956 parts by mass of triethanolamine as the non-halogenated amine, 1.00 parts by mass of methanesulfonic acid as the organic portion, and solder powder.

[0007] Example 4 of Patent Document 3 discloses a flux for flux-cored solder containing an activator such as methanesulfonic acid, a surfactant such as cetyl sulfobetaine, octyl benzoate as a brightener, and an antioxidant such as diethanolamine.

[0008] Japanese Patent Publication No. 7-136794 U.S. Patent No. 5,011,546 Chinese Patent Application Publication No. 104,070,308

[0009] Methanesulfonic acid, which is common to Patent Documents 1 to 3, is classified as an organic sulfonic acid activator. The activity (i.e., oxide film removal ability) of organic sulfonic acid activators is generally higher than that of organic carboxylic acid activators. Therefore, fluxes containing the former are expected to have higher wettability with molten solder than those containing the latter. Therefore, fluxes with a high content of organic sulfonic acid activators are thought to improve wettability with molten solder.

[0010] However, organic sulfonic acid activators may form salts with Sn, the main component of molten solder, during soldering. Since Sn salts are flux residues, they can be removed by washing with water after soldering. However, the formation of Sn salts reduces the solubility in water. Therefore, when the content of organic sulfonic acid activators is high, the cleaning ability decreases. Therefore, there is room for improvement in terms of suppressing the decrease in cleaning ability without compromising the wettability benefits of using organic sulfonic acid activators.

[0011] Furthermore, fluxes made of organic sulfonic acid activators and solvents such as water have low thixotropy, which means that the amount of flux that can be transferred to a circuit board cannot be increased. Therefore, there is room for improvement in terms of ensuring transferability.

[0012] One object of the present invention is to suppress the deterioration of cleaning ability in a flux containing an organic sulfonic acid activator without impairing the wettability benefits of the organic sulfonic acid activator. Another object of the present invention is to ensure the transferability of a flux containing an organic sulfonic acid activator to a circuit board. Yet another object of the present invention is to provide a solder paste using such a flux.

[0013] The present inventors have noticed that adding a nonionic surfactant to an organic sulfonic acid activator can ensure flux transferability without impairing the benefits of wettability. However, depending on the type of nonionic surfactant, no improvement in cleaning performance was observed. Therefore, by focusing on the mass average molecular weight of the nonionic surfactant, it was discovered that by using two types of nonionic surfactants with different mass average molecular weights in combination, it is possible to ensure transferability and improve cleaning performance at the same time. Therefore, the present inventors have further studied two types of nonionic surfactants and have completed the present invention.

[0014] A first invention is a flux having the following characteristics: The flux contains 1 to 10 wt % of an organic sulfonic acid activator, 10 to 40 wt % of a polymeric nonionic surfactant that is a nonionic surfactant with a mass average molecular weight of more than 1200, and 5 to 75 wt % of a low molecular weight nonionic surfactant that is a nonionic surfactant with a mass average molecular weight of 1200 or less, wherein the content of the low molecular weight nonionic surfactant is equal to or greater than the content of the organic sulfonic acid activator, and the flux does not contain a cationic surfactant, or contains more than 0 wt % but not more than 5 wt % of the cationic surfactant.

[0015] A second aspect of the present invention is the first aspect of the present invention, further characterized in that the flux further contains an activator adjuvant in an amount of more than 0 wt % and not more than 10 wt %. The activator adjuvant includes at least one of an organic acid, an amine, an organic phosphorus compound, an organic halogen compound, and an amine hydrohalide.

[0016] A third aspect of the present invention is the second aspect of the present invention, further characterized in that the flux does not contain the amine as the activation aid.

[0017] A fourth invention is any one of the first to third inventions, further comprising the following feature: the flux further contains more than 0 wt % and not more than 60 wt % of a solvent.

[0018] A fifth aspect of the present invention is a solder paste having the following characteristics: the solder paste comprises the flux according to any one of the first to fourth aspects of the present invention, and an Sn-based solder metal.

[0019] A sixth aspect of the present invention is the fifth aspect of the present invention, further characterized in that the Sn-based solder metal has a melting point of 210° C. or less.

[0020] An embodiment of the present invention will be described in detail below. In this application, "wt %" means "mass %." The wt % of a component constituting a flux is based on the mass of the entire flux. When a numerical range is expressed using "to" (~), the range is inclusive of both ends.

[0021] 1. Flux The flux according to the embodiment contains, as essential components, an organic sulfonic acid activator, a polymeric nonionic surfactant, and a low molecular weight nonionic surfactant. A "polymeric nonionic surfactant" is defined as a nonionic surfactant with a mass average molecular weight Mw of greater than 1200. A "low molecular weight nonionic surfactant" is defined as a nonionic surfactant with a mass average molecular weight Mw of 1200 or less. The mass average molecular weight Mw is a standard polystyrene equivalent value based on measurements obtained by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as a solvent. These components and the content (content ratio) of each component are described in detail below.

[0022] 1-1. Organic Sulfonic Acid Activators Examples of organic sulfonic acid activators include alkane sulfonic acids, alkanol sulfonic acids, and aromatic sulfonic acids. Examples of alkane sulfonic acids include methanesulfonic acid, ethanesulfonic acid, 1-propanesulfonic acid, 2-propanesulfonic acid, 1-butanesulfonic acid, 2-butanesulfonic acid, pentanesulfonic acid, hexanesulfonic acid, decanesulfonic acid, and dodecanesulfonic acid. Examples of alkanol sulfonic acids include 2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid, 2-hydroxybutane-1-sulfonic acid, 2-hydroxypentane-1-sulfonic acid, 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybutane-1-sulfonic acid, 2-hydroxyhexane-1-sulfonic acid, 2-hydroxydecane-1-sulfonic acid, and 2-hydroxydodecane-1-sulfonic acid. Examples of aromatic sulfonic acids include 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, p-toluenesulfonic acid, xylenesulfonic acid, p-phenolsulfonic acid, cresolsulfonic acid, sulfosalicylic acid, nitrobenzenesulfonic acid, sulfobenzoic acid, and diphenylamine-4-sulfonic acid.

[0023] The content of the organic sulfonic acid activator (when two or more types of organic sulfonic acid activators are used, the total content of these activators) is 1 to 10 wt%. The higher the content, the better the wettability of the molten solder. Therefore, the lower limit of the content is preferably 2.5 wt%. In other words, if improving wettability is important, the content is preferably 2.5 to 10 wt%. On the other hand, if the content is too high, it is more likely to form a salt with the Sn contained in the molten solder, which will cause a decrease in cleanability. Therefore, the upper limit of the content is preferably 5 wt%. In other words, if ensuring cleanability is important, the content is preferably 1 to 5 wt%.

[0024] 1-2. Polymeric Nonionic Surfactants Examples of polymeric nonionic surfactants include polyalkylene glycols, alcohol polyalkylene glycol adducts, and carboxylic acid polyalkylene glycol adducts, all of which have a mass average molecular weight Mw of more than 1,200.

[0025] Examples of polyalkylene glycols include polyethylene glycol (PEG), polypropylene glycol (PPG), and polyethylene glycol-polypropylene glycol copolymer (PEG-PPG copolymer).

[0026] Examples of alcohol polyalkylene glycol adducts include alcohol polyalkylene glycol EO adducts obtained by addition polymerization of ethylene oxide to alcohol polyalkylene glycols, and alcohol polyalkylene glycol EO / PO adducts obtained by addition polymerization of ethylene oxide and propylene oxide to alcohol polyalkylene glycols. Examples of such alcohol polyalkylene glycol adducts include cetyl alcohol EO adduct and cetyl alcohol EO / PO adduct, which have 16 carbon atoms, stearyl alcohol EO adduct and stearyl alcohol EO / PO adduct, which have 18 carbon atoms, and behenyl alcohol EO adduct and behenyl alcohol EO / PO adduct, which have 22 carbon atoms. Further examples include resorcinol EO adduct and resorcinol EO / PO adduct, which have 6 carbon atoms.

[0027] A carboxylic acid polyalkylene glycol adduct has a structure in which a polyalkylene glycol is added to an aliphatic carboxylic acid or an aromatic carboxylic acid. Examples of the carboxylic acid polyalkylene glycol adduct include a carboxylic acid polyalkylene glycol EO adduct and a carboxylic acid polyalkylene glycol EO / PO adduct. Examples of such a carboxylic acid polyalkylene glycol adduct include a palmitic acid EO adduct and a palmitic acid EO / PO adduct each having 16 carbon atoms, an EO adduct and a stearic acid EO / PO adduct each having 18 carbon atoms, and an EO adduct and a behenic acid EO / PO adduct each having 22 carbon atoms.

[0028] The content of the polymeric nonionic surfactant (when two or more types of polymeric nonionic surfactants are used, the total content of these surfactants) is 10 to 40 wt %, and the lower limit of the content may be 20 wt %.

[0029] 1-3. Low-Molecular-Weight Nonionic Surfactants Examples of low-molecular-weight nonionic surfactants include polyalkylene glycols, alcohol-polyalkylene glycol adducts, and carboxylic acid-polyalkylene glycol adducts having a mass-average molecular weight Mw of 200 to 1200. Examples of these compounds include the compounds exemplified as high-molecular-weight nonionic surfactants. Therefore, compounds used as low-molecular-weight nonionic surfactants may share a repeating structure with compounds used as high-molecular-weight nonionic surfactants. The lower limit of the mass-average molecular weight Mw may be 300 or 400.

[0030] The content of the low molecular weight nonionic surfactant (when two or more types of low molecular weight nonionic surfactants are used, the total content of these surfactants) is 5 to 75 wt %. The upper limit of the content may be 55 wt %. As can be seen from the results of Comparative Example 1 described below, when the content of the low molecular weight nonionic surfactant is less than that of the organic sulfonic acid surfactant, the benefits of adding the surfactant are less likely to be realized. Therefore, it is preferable that the content of the low molecular weight nonionic surfactant be equal to or greater than that of the organic sulfonic acid surfactant.

[0031] 1-4. Cationic Surfactant The flux according to the embodiment may contain a cationic surfactant. That is, the flux according to the embodiment contains a cationic surfactant as an optional component. Examples of cationic surfactants include organic amine alkylene oxide (AO) type cationic surfactants and polyoxyalkylene amine type cationic surfactants.

[0032] The organic amine AO ​​type surfactant has a structure in which at least one AO ​​selected from ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO) is added to an organic amine such as an aliphatic amine (aliphatic monoamine and polyamine (aliphatic diamine and aliphatic triamine)) or an aromatic amine (aromatic monoamine and polyamine (aromatic diamine and aromatic triamine)).

[0033] Polyoxyalkyleneamine surfactants have repeating units of oxyalkylene blocks such as oxyethylene blocks or oxypropylene blocks in the molecule, and have a structure in which amino groups are bonded to the terminal carbon atoms. They are classified into monoamine, diamine, and triamine types depending on the total number of terminal amino groups.

[0034] The content of the cationic surfactant (when two or more types of cationic surfactants are used, the total content of these) is greater than 0 wt% and not more than 5 wt%. As can be seen from the results of Comparative Example 1 described below, if the content of the cationic surfactant exceeds 5 wt%, the benefits of adding a low-molecular-weight nonionic surfactant may be offset. Furthermore, if the content of the cationic surfactant exceeds 5 wt%, the benefits of adding an organic sulfonic acid surfactant may also be offset. For these reasons, the upper limit of the content of the cationic surfactant (5 wt%) is set.

[0035] 1-5. Activation Adjuvant The flux according to the embodiment may contain an activation adjuvant. That is, the flux according to the embodiment contains an activation adjuvant as an optional component. The activation adjuvant is an additive that assists the reduction of oxides by the organic sulfonic acid activator. Examples of the activation adjuvant include organic acids other than organic sulfonic acid activators, amines, organic phosphorus compounds, organic halogen compounds, and amine hydrohalides. Two or more of these activation adjuvant may be used simultaneously.

[0036] The content of the activator aid (when two or more types of activator aids are used, the total content of all activator aids) is greater than 0 wt % and less than 10 wt %. The upper limit of the content may be 6 wt % or 5 wt %. The content of the activator aid may be greater than or equal to the content of the organic sulfonic acid activator, or may be less than the content of the organic sulfonic acid activator.

[0037] 1-5-1. Other organic acids Other organic acids include glutaric acid, adipic acid, azelaic acid, eicosane diacid, citric acid, glycolic acid, lactic acid, succinic acid, salicylic acid, diglycolic acid, dipicolinic acid, dibutylaniline diglycolic acid, suberic acid, sebacic acid, thioglycolic acid, phthalic acid, isophthalic acid, terephthalic acid, dodecanedioic acid, parahydroxyphenylacetic acid, picolinic acid, phenylsuccinic acid, fumaric acid, maleic acid, malonic acid, lauric acid, benzoic acid, tartaric acid, and tris(2-isocyanuric acid) 12-hydroxystearic acid, oleic acid, linoleic acid, and linolenic acid. Other examples of organic acids include dimer acid, which is a reaction product of oleic acid and linoleic acid, hydrogenated dimer acid obtained by adding hydrogen to this dimer acid, trimer acid, which is a reaction product of oleic acid and linoleic acid, and hydrogenated trimer acid obtained by adding hydrogen to this trimer acid. Further examples of the other organic acid include dimer acids other than the reaction products of oleic acid and linoleic acid, hydrogenated dimer acids obtained by adding hydrogen to these dimer acids, trimer acids other than the reaction products of oleic acid and linoleic acid, and hydrogenated trimer acids obtained by adding hydrogen to these trimer acids. Two or more of these other organic acids may be used simultaneously.

[0038] 1-5-2. Organic phosphorus compounds Examples of organic phosphorus compounds include methyl acid phosphate, ethyl acid phosphate, isopropyl acid phosphate, monobutyl acid phosphate, butyl acid phosphate, dibutyl acid phosphate, butoxyethyl acid phosphate, 2-ethylhexyl acid phosphate, bis(2-ethylhexyl) phosphate, monoisodecyl acid phosphate, isodecyl acid phosphate, lauryl acid phosphate, and isotridecyl acid phosphate. Examples of the organic phosphorus compounds include methyl methacrylate, methyl methacrylate, methyl methacrylate phosphate ...

[0039] 1-5-3. Organic halogen compounds Examples of organic halogen compounds include trans-2,3-dibromo-1,4-butenediol, triallyl isocyanurate hexabromide, 1-bromo-2-butanol, 1-bromo-2-propanol, 3-bromo-1-propanol, 3-bromo-1,2-propanediol, 1,4-dibromo-2-butanol, 1,3-dibromo-2-propanol, 2,3-dibromo-1-propanol, 2,3-dibromo-1,4-butanediol, 2,3-dibromo-2-butene-1,4-diol, trans-2,3-dibromo-2-butene-1,4-diol, cis-2,3-dibromo-2-butene-1,4-diol, tetrabromophthalic acid, bromosuccinic acid, and organic bromo compounds such as 2,2,2-tribromoethanol. Examples of the organic halogen compound include organic chloro compounds such as chloroalkanes, chlorinated fatty acid esters, chlorendic acid, and chlorendic acid anhydride. Examples of the organic halogen compound include organic fluoro compounds such as fluorine-based surfactants, surfactants having perfluoroalkyl groups, and polytetrafluoroethylene. Two or more of these organic halogen compounds may be used simultaneously.

[0040] 1-5-4. Amine Hydrohalides Amine hydrohalides are compounds obtained by reacting amines with hydrogen halides. Examples of amine hydrohalides include stearylamine hydrochloride, diethylaniline hydrochloride, diethanolamine hydrobromide, 2-ethylhexylamine hydrobromide, pyridine hydrobromide, isopropylamine hydrobromide, cyclohexylamine hydrobromide, diethylamine hydrobromide, monoethylamine hydrobromide, 1,3-diphenylguanidine hydrobromide, dimethylamine hydrobromide, dimethylamine hydrochloride, rosinamine hydrobromide, 2-ethylhexylamine hydrochlor ... Pyramine hydrochloride, cyclohexylamine hydrochloride, 2-pipecoline hydrobromide, 1,3-diphenylguanidine hydrochloride, dimethylbenzylamine hydrochloride, hydrazine hydrate hydrobromide, dimethylcyclohexylamine hydrochloride, trinonylamine hydrobromide, diethylaniline hydrobromide, 2-diethylaminoethanol hydrobromide, 2-diethylaminoethanol hydrochloride, ammonium chloride, diallylamine hydrochloride, diallylamine hydrobromide, monoethylamine hydrochloride, diethylamine amine hydrochloride, triethylamine hydrobromide, triethylamine hydrochloride, hydrazine monohydrochloride, hydrazine dihydrochloride, hydrazine monohydrobromide, hydrazine dihydrobromide, pyridine hydrochloride, aniline hydrobromide, butylamine hydrochloride, hexylamine hydrochloride, n-octylamine hydrochloride, dodecylamine hydrochloride, dimethylcyclohexylamine hydrobromide, ethylenediamine dihydrobromide, rosinamine hydrobromide, 2-phenylimidazole hydrobromide, 4-benzylpyridine hydrobromide Examples of the fluoride include benzophenone hydrochloride, L-glutamic acid hydrochloride, N-methylmorpholine hydrochloride, betaine hydrochloride, 2-pipecoline hydroiodide, cyclohexylamine hydroiodide, 1,3-diphenylguanidine hydrofluoride, diethylamine hydrofluoride, 2-ethylhexylamine hydrofluoride, cyclohexylamine hydrofluoride, ethylamine hydrofluoride, rosinamine hydrofluoride, cyclohexylamine tetrafluoroborate, and dicyclohexylamine tetrafluoroborate.

[0041] 1-5-5. Amines Examples of amines include monoethanolamine, diphenylguanidine, ditolylguanidine, ethylamine, triethylamine, cyclohexylamine, ethylenediamine, triethylenetetramine, imidazole, 2-methylimidazole, 2-ethylimidazole, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, and 1-benzyl-2-methylimidazole. imidazole, 1-benzyl-2-phenylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazolium trimellitate, 1-cyanoethyl-2-phenylimidazolium trimellitate, 2,4-diamino-6-[2'-methylimidazole] 2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine isocyanuric acid adduct, 2-phenylimidazole isocyanuric acid adduct, 2-phenyl phenyl-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazole, 1-dodecyl-2-methyl-3-benzylimidazolium chloride, 2-methylimidazoline, 2-phenylimidazoline, 2,4-diamino-6-vinyl-s-triazine, 2,4-diamino-6-vinyl-s-triazine isocyanuric acid adduct, 2,4-Diamino-6-methacryloyloxyethyl-s-triazine, epoxy-imidazole adduct, 2-methylbenzimidazole, 2-octylbenzimidazole, 2-pentylbenzimidazole, 2-(1-ethylpentyl)benzimidazole, 2-nonylbenzimidazole, 2-(4-thiazolyl)benzimidazole, benzimidazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazine azole, 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-3',5'-di-tert-amylphenyl)benzotriazole, 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole, 2,2'-methylenebis[6-(2H-benzotriazol-2-yl)-4-tert-octylphenol], 6- Examples include (2-benzotriazolyl)-4-tert-octyl-6'-tert-butyl-4'-methyl-2,2'-methylenebisphenol, 1,2,3-benzotriazole, 1-[N,N-bis(2-ethylhexyl)aminomethyl]benzotriazole, carboxybenzotriazole, 1-[N,N-bis(2-ethylhexyl)aminomethyl]methylbenzotriazole, 2,2'-[[(methyl-1H-benzotriazol-1-yl)methyl]imino]bisethanol, 1-(1',2'-dicarboxyethyl)benzotriazole, 1-(2,3-dicarboxypropyl)benzotriazole, 1-[(2-ethylhexylamino)methyl]benzotriazole, 2,6-bis[(1H-benzotriazol-1-yl)methyl]-4-methylphenol, 5-methylbenzotriazole, and 5-phenyltetrazole. Two or more of these amines may be used simultaneously.

[0042] Since amines have the effect of reducing the activity of organic sulfonic acid activators, the use of amines as auxiliary activators in flux may impair the wettability of molten solder provided by the organic sulfonic acid activators. Therefore, the amine content is preferably 0 wt %. In other words, the flux according to the embodiment preferably does not contain amines.

[0043] 1-6. Solvent The flux according to the embodiment may contain a solvent. That is, the flux according to the embodiment contains a solvent as an optional component. The solvent preferably does not volatilize below 70°C in order to efficiently bring about the reducing action of the organic sulfonic acid activator and the activator auxiliary. If the solvent volatilizes, the flux will dry out, making it difficult for the flux to wet and spread over the soldered portion. Therefore, the boiling point of the solvent is preferably 120°C or higher. However, the solvent is required to volatilize when heated. Therefore, the boiling point of the solvent is preferably 270°C or lower.

[0044] Examples of the solvent include water, alcohol-based solvents, glycol ether-based solvents, and terpineols. Examples of the alcohol-based solvent include isopropyl alcohol, 1,2-butanediol, isobornylcyclohexanol, 2,4-diethyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,5-dimethyl-3-hexyne-2,5-diol, 2,3-dimethyl-2,3-butanediol, 1,1,1-tris(hydroxymethyl)ethane, 2-ethyl-2-hydroxymethyl-1,3-propanediol, 2,2'-oxybis(methylene)bis(2-ethyl Examples of the glycol ether solvent include hexyl diglycol, diethylene glycol mono-2-ethylhexyl ether, ethylene glycol monophenyl ether, 2-methylpentane-2,4-diol, diethylene glycol monohexyl ether, diethylene glycol dibutyl ether, and triethylene glycol monobutyl ether. As the solvent, one of the above-mentioned solvents may be used alone, or two or more may be used simultaneously.

[0045] The content of the solvent (when two or more types of solvents are used, the total content of the solvents) is more than 0 wt % and not more than 60 wt %. The upper limit of this content may be 30 wt % or 20 wt %.

[0046] 1-7. Other Additives The flux according to the embodiment may contain other additives, such as an antioxidant, an antifoaming agent, and a colorant. Examples of antioxidants include hindered phenol-based antioxidants. Examples of antifoaming agents include acrylic polymers, vinyl ether polymers, butadiene polymers, and silicones. As the other additives, one of the additives described above may be used, or two or more may be used simultaneously. The content of the other additives (when two or more types are used, the total content thereof) is greater than 0 wt % and not more than 5 wt %.

[0047] 2. Solder Paste The solder paste according to the embodiment contains the above-described flux and an Sn-based solder metal.

[0048] Examples of Sn-based solder metals include simple Sn and Sn-based alloys. Examples of Sn-based alloys include binary alloys and ternary or higher-component alloys. Examples of binary alloys include Sn—Sb alloys, Sn—Pb alloys, Sn—Cu alloys, Sn—Ag alloys, Sn—Bi alloys, and Sn—In alloys. Examples of multicomponent alloys include the above-mentioned binary alloys to which one or more metals selected from the group consisting of Sb, Bi, In, Cu, Zn, As, Ag, Cd, Fe, Ni, Co, Au, Ge, and P have been added.

[0049] Sn-based solder metals are classified into low-temperature solder metals and high-temperature solder metals. The former are defined as Sn-based solder metals that have a melting point (referring to the solidus temperature or liquidus temperature; the same applies hereinafter) in the low-temperature range (specifically, a temperature range of 210°C or lower). The latter are defined as Sn-based solder metals that have a melting point in the high-temperature range (specifically, a temperature range higher than 210°C). Sn—Bi-based alloys and multi-component alloys to which Cu, Ag, Sb, or Ni have been added are examples of low-temperature solder metals. Sn—Sb-based alloys and Sn—Ag—Cu-based alloys are examples of high-temperature solder metals.

[0050] Here, Ni electrodes formed on typical circuit boards have the following problem: The oxide film (i.e., Ni oxide film) on the surface of the Ni electrode is difficult to remove during soldering using a solder metal with a melting point in the low temperature range, because the heating temperature during soldering is low and the flux is not sufficiently activated.

[0051] In this regard, the above-mentioned flux contains an organic sulfonic acid-based activator as an essential component. Therefore, as will be apparent from the examples described below, it is possible to remove Ni oxide films even in soldering using low-temperature solder metals. In this way, the above-mentioned flux enjoys the benefits of the organic sulfonic acid-based activator, particularly when combined with low-temperature solder metals. Therefore, the solder paste according to the embodiment is preferably a combination of low-temperature solder metals and the above-mentioned flux.

[0052] There is no limitation on the content of the Sn-based solder metal and flux relative to the total mass of the solder paste, for example, the content of the Sn-based solder metal is 5 to 95 mass % and the content of the flux is 5 to 95 mass %.

[0053] There are no limitations on the manufacturing method of the solder paste, and it can be manufactured by mixing the raw materials simultaneously or sequentially using any method. When manufacturing the solder paste, it is sufficient that all of the components of the flux and the solder powder are finally mixed. That is, the solder powder may be mixed with all of the components of the flux prepared in advance, or some of the components of the flux may be mixed with the solder powder and then the remaining components of the flux may be further mixed. Furthermore, all of the components of the solder paste may be mixed simultaneously.

[0054] 3. Examples The flux and solder paste according to the embodiments will be described in detail below based on examples.

[0055] Flux compositions of Examples 1 to 29 were prepared according to the blending ratios shown in Tables 1 to 4 below. Flux compositions of Comparative Examples 1 to 5 were also prepared. These flux compositions were evaluated for the following items (i) to (iv). The evaluation results are also shown in Tables 1 to 4 below. (i) Removability of Ni oxide film by the flux composition (ii) Wettability of molten solder by the flux composition (iii) Cleanability of flux residue (iv) Transferability of the flux composition

[0056] 3-1. Evaluation of Ni oxide film removability (1) Verification method Verification of removability was performed using the following method. First, a Ni-plated copper plate was subjected to ultrasonic cleaning using a hydrocarbon solvent. Then, the Ni-plated copper plate was heated for 1 minute on a hot plate set at 400°C. After heating, the surface of the copper plate turned pale yellow due to the oxidation of Ni. Next, each flux composition was applied to the copper plate, which was then placed on a hot plate set at 200°C and heated for 30 seconds. After that, the copper plate was washed with water, and the surface condition was observed.

[0057] (2) Evaluation Criteria ◯: The metallic luster of Ni was confirmed. ×: The metallic luster of Ni was not confirmed. Or, the surface of the copper plate remained pale yellow.

[0058] 3-2. Evaluation of Wettability (1) Verification Method Wettability was verified using the wetting balance method using a Rhesca Solder Checker (SAT-5200). First, a test piece (a Ni-plated copper plate measuring 5 mm wide, 30 mm long, and 0.3 mm thick) was heated for 1 hour in a thermostatic chamber set at 300°C. Next, each flux composition was applied to the test piece up to a position approximately 1 mm from the tip. The temperature setting of the thermostatic chamber was then changed, and the test piece was immersed in the solder bath. The temperature setting of the thermostatic chamber was appropriately changed depending on the melting point of the solder alloy in the solder bath. For example, the temperature setting was 190°C for Examples 1 to 25 and Comparative Examples 1 to 5, and 250°C for Example 29. The immersion conditions were an immersion depth of 2 mm and an immersion time of 10 seconds.

[0059] The compositions of the solder alloys are as follows: Examples 1 to 25 and Comparative Examples 1 to 5: Sn-40Bi-Cu-Ni Example 26: Sn-1Ag-57Bi Example 27: Sn-58Bi Example 28: Sn-58Bi-0.5Sb-0.015Ni Example 29: Sn-3.0Ag-0.5Cu

[0060] (2) Evaluation criteria: ◯: Zero crossing was observed. ×: Zero crossing was not observed.

[0061] 3-3. Evaluation of Cleanability (1) Verification Method The verification of cleanability was performed using the following method. First, each flux composition was printed on the pads of the test substrate. Next, a solder ball was mounted on this printed area and reflow was performed. Immediately after reflow, the test substrate was immersed in ion-exchanged water and washed. After that, flux residue was confirmed using an SEM. The composition of the solder ball was the same as the composition of the solder alloy used in the wettability evaluation.

[0062] The reflow conditions were adjusted appropriately depending on the composition of the solder balls. For example, in Examples 1 to 25 and Comparative Examples 1 to 5, the temperature was increased from 30°C by 1°C per second to the peak temperature (190°C), and once this peak temperature was reached, heating was continued for 30 seconds. In Example 29, the peak temperature of the above-mentioned reflow conditions was changed to 250°C. The test substrate was cleaned by immersing it in a beaker containing ion-exchanged water for 3 minutes using a hot stirrer. The temperature of the ion-exchanged water was set to 50°C ± 10°C, and the stirrer rotation speed was set to 300 rpm. The printed area was then observed.

[0063] (2) Evaluation criteria: ◯: No flux was left behind after cleaning. ×: Flux was left behind after cleaning.

[0064] 3-4. Evaluation of Transferability (1) Verification Method The transferability was verified by transferring each flux composition to a test substrate using a pin transfer device. The difference in substrate weight before and after the flux composition was transferred was measured. The test substrate had 1,200 pads, and the diameter of the pins in the transfer device was 0.1 mm. When transferring the flux composition, kneading was performed for 5 to 10 minutes, and the film thickness was adjusted to 0.1 ± 0.01 mm. The weight difference (transferred weight) was measured nine times for each flux composition, and the average value was calculated.

[0065] (2) Evaluation criteria: ◯: The average transferred weight was 1 mg or more. ×: The average transferred weight was 1 mg, or bridging occurred.

[0066] 3-5. Overall evaluation An overall evaluation was made based on the evaluation results of the above items (i) to (iv). The evaluation criteria are as follows: 〇: All results of items (i) to (iv) were 〇. ×: Any one or all of the results of items (i) to (iv) were ×.

[0067]

[0068]

[0069]

[0070]

[0071] As can be seen from Tables 1 to 4, the flux compositions of Examples 1 to 29 showed excellent results in all of the results for items (i) to (iv). While the exact reasons for this are unclear, the inventors speculate as follows: The appropriate content of the organic sulfonic acid activator ensured (i) the removability of Ni oxide films, thereby ensuring (ii) wettability. Furthermore, the appropriate content of the organic sulfonic acid activator and low-molecular-weight nonionic surfactant suppressed the formation of Sn salts, thereby ensuring (iii) cleanability. Furthermore, the appropriate content of the low-molecular-weight nonionic surfactant and high-molecular-weight nonionic surfactant imparted appropriate thixotropy to the flux compositions, thereby ensuring (iv) transferability.

[0072] On the other hand, the flux compositions of Comparative Examples 2 and 3 showed poor results in (i) removability of Ni oxide film and (iv) wettability, which the inventors speculate is due to the absence of an organic sulfonic acid-based activator in these comparative examples.

[0073] Furthermore, the flux compositions of Comparative Examples 1 and 4 exhibited poor results in (iii) cleaning performance and (iv) transferability. The inventors speculate that the reasons for this are as follows: In Comparative Example 1, the content of the low-molecular-weight nonionic surfactant was lower than the content of the organic sulfonic acid activator. Therefore, the benefits of adding the low-molecular-weight nonionic surfactant were not utilized. In Comparative Example 4, no low-molecular-weight nonionic surfactant was added. Therefore, the suppression of Sn salt formation and the impartation of thixotropy were insufficient. Furthermore, in Comparative Example 1, the content of the cationic surfactant was too high, which had a significant impact.

[0074] Furthermore, the flux composition of Comparative Example 5 showed poor results in terms of (iv) transferability. The inventors speculate that the reason for this is that the absence of a polymeric nonionic surfactant in Comparative Example 5 resulted in insufficient thixotropy.

Claims

1. It contains 1 to 10 wt% of an organic sulfonic acid-based surfactant, 10 to 40 wt% of a polymer nonionic surfactant which is a nonionic surfactant with a mass average molecular weight exceeding 1200, and 5 to 75 wt% of a low molecular weight nonionic surfactant which is a nonionic surfactant with a mass average molecular weight of 1200 or less. The content of the low molecular weight nonionic surfactant is not less than the content of the organic sulfonic acid-based surfactant, and it does not contain a cationic surfactant, or contains the cationic surfactant in an amount exceeding 0 wt% and not exceeding 5 wt%. A flux characterized by this.

2. It further contains an active auxiliary agent in an amount exceeding 0 wt% and not exceeding 10 wt%. The active auxiliary agent contains at least one of other organic acids, amines, organic phosphorus compounds, organic halogen compounds, and amine hydrohalide salts excluding the organic sulfonic acid-based surfactant. The flux according to claim 1, characterized by this.

3. The flux according to claim 2, characterized by not containing the amine as the active auxiliary agent.

4. The flux according to any one of claims 1 to 3, further characterized by containing a solvent in an amount exceeding 0 wt% and not exceeding 60 wt%.

5. A solder paste characterized by containing the flux according to any one of claims 1 to 4 and a Sn-based solder metal.

6. The solder paste according to claim 5, characterized in that the Sn-based solder metal has a melting point of 210°C or lower.