HK40090364A - Flux and solder paste - Google Patents

Description

Flux and solder paste

Technical Field

This invention relates to fluxes for welding and solder pastes using the fluxes.

Background Technology

Generally, flux is used in soldering. Flux chemically removes metal oxides present on the surface of the solder and on the surface of the metal being soldered. This allows for the movement of metal elements at the boundary between the solder and the workpiece, resulting in a strong bond between them.

Fluxes are classified into resin-based fluxes, water-soluble fluxes, and inorganic fluxes. Resin-based fluxes are made by adding activators to resins such as rosin or synthetic resins. Water-soluble fluxes are made by dissolving organic acid-based activators in solvents such as water or organic solvents. In addition to organic acid-based activators, polyethylene glycol, water-soluble base agents, etc., are sometimes added to water-soluble fluxes. Inorganic fluxes use inorganic materials such as hydrochloric acid and zinc chloride.

Solder paste is a composite material obtained by mixing solder alloy powder with flux. Soldering using solder paste is performed as follows: First, solder paste is printed onto the soldering portions of the substrate, such as electrodes. Next, components are mounted on the soldering portions. Then, the substrate is heated in a reflow oven. Thus, the components are joined at the soldering portions.

As prior art related to this application, the techniques of Patent Documents 1 to 3 can be cited as examples. Patent Document 1 discloses a water-soluble flux containing alkane sulfonic acid as an activator. Example 6 of Patent Document 1 shows that excellent results were obtained in the wettability test of molten solder using a water-soluble flux composed of 5 wt% methanesulfonic acid and 95 wt% water.

Patent Document 2 discloses a solder paste comprising a solder alloy, a non-halogenated amine, and an organic component. Paste No. 27 of Patent Document 2 shows a sample containing 0.956 parts by mass of triethanolamine as a non-halogenated amine, 1.00 parts by mass of methanesulfonic acid as an organic component, and solder powder.

Example 4 of Patent Document 3 discloses a flux for rosin-core solder, which includes activators such as methanesulfonic acid, surfactants such as hexadecyl sulfobetaine, octyl benzoate as a gloss agent, and antioxidants such as diethanolamine.

Existing technical documents

Patent documents

Patent Document 1: Japanese Patent Application Publication No. 7-136794

Patent Document 2: US Patent No. 5011546

Patent Document 3: Chinese Patent Application Publication No. 104070308

Summary of the Invention

The problem that the invention aims to solve

The methanesulfonic acid commonly used in Patent Documents 1-3 belongs to the organic sulfonic acid surfactant category. The activity (i.e., oxide film removal) of organic sulfonic acid surfactants is generally higher than that of organic carboxylic acid surfactants. Therefore, if a flux containing the former is used, the wettability of the molten solder can be expected to be improved compared to a flux containing the latter. Therefore, it is believed that using a flux with a high content of organic sulfonic acid surfactants will improve the wettability of the molten solder.

However, organic sulfonic acid surfactants sometimes form salts with Sn, a major 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 their solubility in water. Therefore, cleanability decreases when the organic sulfonic acid surfactant content is high. Thus, there is room for improvement in terms of minimizing the decrease in cleanability without compromising the wetting properties gained from using organic sulfonic acid surfactants.

Furthermore, fluxes composed of organic sulfonic acid activators and solvents such as water have low thixotropy, thus limiting the amount of flux transferred onto the circuit board. Therefore, there is room for improvement in ensuring transferability.

One object of the present invention is to preserve the wettability advantages of organic sulfonic acid surfactants in fluxes containing such surfactants, and to suppress any decrease in cleanability. Another object of the present invention is to ensure the transferability of fluxes containing organic sulfonic acid surfactants to circuit boards. A further object of the present invention is to provide solder paste using such fluxes.

Methods for solving problems

The inventors focused on the ability to ensure flux transferability without compromising wettability by adding nonionic surfactants to organic sulfonic acid-based surfactants. However, no improvement in cleanability was observed depending on the type of nonionic surfactant. Therefore, focusing on the mass-average molecular weight of the nonionic surfactants, it was found that using two nonionic surfactants with different mass-average molecular weights simultaneously ensured transferability and improved cleanability. Therefore, the inventors conducted further research on two nonionic surfactants, resulting in this invention.

The first invention is a flux having the following characteristics.

The above flux includes:

Organic sulfonic acid surfactants: 1–10 wt%;

10–40 wt% of high molecular weight nonionic surfactants with a mass average molecular weight exceeding 1200; and

Low molecular weight nonionic surfactants with a mass average molecular weight of less than 1200, comprising 5-75 wt%, are used as nonionic surfactants.

The content of the aforementioned low-molecular-weight nonionic surfactant is greater than or equal to the content of the aforementioned organic sulfonic acid surfactant.

The flux does not contain cationic surfactants, or contains more than 0 wt% but less than 5 wt% of the aforementioned cationic surfactants.

The second invention also has the following features as described in the first invention.

The above flux also contains more than 0 wt% and less than 10 wt% of active additives.

The aforementioned active additives include at least one of organic acids, amines, organophosphorus compounds, organohalides, and amine hydrohalides.

The third invention also has the following features as described in the second invention.

The above-mentioned flux does not contain the above-mentioned amines, which are used as the above-mentioned active additives.

The fourth invention also has the following features in any one of the first to third inventions.

The above flux also contains more than 0 wt% and less than 60 wt% of solvent.

The fifth invention is a solder paste having the following characteristics.

The above solder paste contains

The flux in any one of the inventions 1 to 4; and

Sn-based solder metals.

The sixth invention also has the following features as described in the fifth invention.

The Sn-based solder metals mentioned above have melting points below 210°C.

Detailed Implementation

The embodiments of the present invention will be described in detail below. It should be noted that in this application, "wt%" refers to "mass %". Furthermore, the wt% of the components constituting the flux is based on the total mass of the flux. Additionally, when using "~" to indicate a numerical range, the range includes the values at both ends.

1. Flux

The flux of this embodiment contains an organic sulfonic acid surfactant, a high molecular weight nonionic surfactant, and a low molecular weight nonionic surfactant as essential components. "High molecular weight nonionic surfactant" is defined as a nonionic surfactant with a mass-average molecular weight (Mw) exceeding 1200. "Low molecular weight nonionic surfactant" is defined as a nonionic surfactant with a mass-average molecular weight (Mw) of less than 1200. It should be noted that the mass-average molecular weight (Mw) is a standard polystyrene conversion value based on gel permeation chromatography (GPC) measurements using tetrahydrofuran (THF) as a solvent. The following provides a detailed description of these components and their content (proportion).

1-1. Organic sulfonic acid surfactants

Examples of organic sulfonic acid surfactants include alkane sulfonic acids, alkane alcohol 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, pentasulfonic acid, hexanesulfonic acid, decanesulfonic acid, and dodecanesulfonic acid. Examples of alkane alcohol 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.

The content of the organic sulfonic acid surfactant (or its total content when using two or more organic sulfonic acid surfactants) is 1 to 10 wt%. A higher content results in better wettability of the molten solder. Therefore, the lower limit of the content is preferably 2.5 wt%. That is, when 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 easy to form salts with Sn contained in the molten solder, leading to reduced cleanability. Therefore, the upper limit of the content is preferably 5 wt%. That is, when ensuring cleanability is important, the content is preferably 1 to 5 wt%.

1-2. Polymer nonionic surfactants

As a high molecular weight nonionic surfactant, examples include polyalkylene glycols, alcohol polyalkylene glycol adducts, and carboxylic acid polyalkylene glycol adducts with a mass-average molecular weight (Mw) exceeding 1200.

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

Examples of polyalkylene glycol adducts include EO adducts of polyalkylene glycol obtained by the addition polymerization of ethylene oxide onto polyalkylene glycol, and EO/PO adducts of polyalkylene glycol obtained by the addition polymerization of ethylene oxide and propylene oxide onto polyalkylene glycol. Examples of such polyalkylene glycol adducts include EO adducts and EO/PO adducts of cetyl alcohol (16 carbon atoms), EO adducts and EO/PO adducts of stearyl alcohol (18 carbon atoms), and EO adducts and EO/PO adducts of behenol (22 carbon atoms). Additionally, examples of EO adducts and EO/PO adducts of resorcinol (6 carbon atoms) are also provided.

Carboxylic acid polyalkylene glycol adducts have a structure in which a polyalkylene glycol is added to an aliphatic or aromatic carboxylic acid. Examples of carboxylic acid polyalkylene glycol adducts include EO adducts and EO/PO adducts. Examples of such carboxylic acid polyalkylene glycol adducts include EO adducts and EO/PO adducts of palmitic acid with 16 carbon atoms, EO adducts and EO/PO adducts of stearic acid with 18 carbon atoms, and EO adducts and EO/PO adducts of behenic acid with 22 carbon atoms.

The content of polymeric nonionic surfactants (when using two or more polymeric nonionic surfactants, their total content) is 10–40 wt%. The lower limit of the content can be 20 wt%.

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 with a mass-average molecular weight (Mw) of 200–1200. Examples of these compounds also include those used as high-molecular-weight nonionic surfactants. Therefore, compounds used as low-molecular-weight nonionic surfactants sometimes share repeating structures with compounds used as high-molecular-weight nonionic surfactants. The lower limit of the mass-average molecular weight (Mw) can be 300 or 400.

The content of the low-molecular-weight nonionic surfactant (the total content of two or more low-molecular-weight nonionic surfactants) is 5 to 75 wt%. The upper limit of the content can be 55 wt%. As shown in Comparative Example 1 described later, when the content of the low-molecular-weight nonionic surfactant is less than the content of the organic sulfonic acid surfactant, it is difficult to realize the advantages of adding the surfactant. Therefore, the content of the low-molecular-weight nonionic surfactant is preferably higher than the content of the organic sulfonic acid surfactant.

1-4. Cationic surfactants

The flux of the embodiments may contain cationic surfactants. That is, the flux of the embodiments contains cationic surfactants as an optional component. Examples of cationic surfactants include organic amine epoxide (AO) type cationic surfactants and polyoxyethylene amine type cationic surfactants.

Organic amine AO type surfactants have 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)).

Polyoxyalkylene amine surfactants have repeating units with olefinic blocks such as ethylene oxide and propylene oxide within the molecule, and an amino group is bonded to the terminal carbon atom. Based on the total number of terminal amino groups, they are classified into monoamine, diamine, and triamine types.

The content of cationic surfactants (or their total content when using two or more cationic surfactants) is greater than 0 wt% and less than 5 wt%. As shown in Comparative Example 1 described later, if the content of cationic surfactants exceeds 5 wt%, the advantages of adding low-molecular-weight nonionic surfactants may be negated. Furthermore, if the content of cationic surfactants exceeds 5 wt%, the advantages of adding organic sulfonic acid surfactants may also be negated. For this reason, an upper limit of 5 wt% for the content of cationic surfactants was set.

1-5. Active additives

The flux of the embodiments may contain active additives. That is, the flux of the embodiments contains active additives as an optional component. Active additives are additives that assist in the reduction of oxides using organic sulfonic acid-based active agents. Examples of active additives include organic acids, amines, organophosphorus compounds, organohalides, and amine hydrohalides, in addition to organic sulfonic acid-based active agents. Two or more of these active additives may be used simultaneously.

The active ingredient (in the case of using two or more active ingredients, their total content) is more than 0 wt% and less than 10 wt%. The upper limit of the content can be 6 wt% or 5 wt%. The content of the active ingredient can be more than or less than the content of the organic sulfonic acid surfactant.

1-5-1. Other organic acids

Other examples of organic acids include glutaric acid, adipic acid, azelaic acid, eicosanoic acid, citric acid, glycolic acid, lactic acid, succinic acid, salicylic acid, diethylene glycol, pyridinedicarboxylic acid, dibutylaniline diethylene glycol, octanoic acid, sebacic acid, mercaptoacetic acid, phthalic acid, isophthalic acid, terephthalic acid, dodecanoic acid, p-hydroxyphenylacetic acid, pyridinecarboxylic acid, phenylsuccinic acid, fumaric acid, maleic acid, malonic acid, lauric acid, benzoic acid, and alcohol. The organic acids include tartaric acid, tris(2-carboxyethyl) isocyanurate, glycine, 1,3-cyclohexanedicarboxylic acid, 2,2-bis(hydroxymethyl)propionic acid, 2,2-bis(hydroxymethyl)butyric acid, 4-tert-butylbenzoic acid, 2,3-dihydroxybenzoic acid, 2,4-diethylglutaric acid, 2-quinolinecarboxylic acid, 3-hydroxybenzoic acid, malic acid, p-methoxybenzoic acid, palmitic acid, stearic acid, 12-hydroxystearic acid, oleic acid, linoleic acid, and linolenic acid. Other examples of organic acids include dimer acids as reactants of oleic acid and linoleic acid, hydrogenated dimer acids obtained by hydrogenating these dimer acids, trimer acids as reactants of oleic acid and linoleic acid, and hydrogenated trimer acids obtained by hydrogenating these trimer acids. Other examples of organic acids include dimer acids other than the reactants of oleic acid and linoleic acid, hydrogenated dimer acids obtained by hydrogenating the dimer acids, trimer acids other than the reactants of oleic acid and linoleic acid, and hydrogenated trimer acids obtained by hydrogenating the trimer acids. Two or more of these other organic acids may be used simultaneously.

1-5-2. Organophosphorus compounds

Examples of organophosphorus compounds include methyl phosphate, ethyl phosphate, isopropyl phosphate, monobutyl phosphate, butyl phosphate, dibutyl phosphate, butoxyethyl phosphate, 2-ethylhexyl phosphate, bis(2-ethylhexyl) phosphate, monoisodecyl phosphate, isodecanyl phosphate, lauryl phosphate, isotracene phosphate, stearyl phosphate, oleyl phosphate, tallow phosphate, coconut oil phosphate, isostearyl phosphate, alkyl phosphate, tetracosyl phosphate, ethylene glycol phosphate, 2-hydroxyethyl methacrylate phosphate, dibutyl pyrophosphate phosphate, 2-ethylhexylphosphonate mono-2-ethylhexyl ester, and alkyl (alkyl)phosphonates. Two or more of these organophosphorus compounds can be used simultaneously.

1-5-3. Organohalides

Examples of organohalides 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-1,4-propanol, 2,3-dibromo-1,4-butanediol, 2,3-dibromo-2-butenediol, trans-2,3-dibromo-2-butenediol, cis-2,3-dibromo-2-butenediol, tetrabromophthalic acid, bromosuccinic acid, and 2,2,2-tribromoethanol. Other examples of organohalides include chlorinated alkanes, chlorinated fatty acid esters, chloramphenicol, and chloramphenicol anhydride. Examples of organohalides also include fluorinated surfactants, surfactants with perfluoroalkyl groups, and organofluorine compounds such as polytetrafluoroethylene. Two or more of these organohalides can be used simultaneously.

1-5-4. Aminohydride halides

Aminohydrohalides are compounds obtained by reacting amines with hydrogen halides. Examples of aminohydrohalides include stearylamine hydrochloride, diethylaniline hydrochloride, diethanolamine hydrochloride, 2-ethylhexylamine hydrobromide, pyridine hydrobromide, isopropylamine hydrobromide, cyclohexylamine hydrobromide, diethylamine hydrobromide, monoethylamine hydrobromide, 1,3-diphenylguanidine hydrobromide, dimethylamine hydrobromide, dimethylamine hydrobromide, rosinamine hydrobromide, 2-ethylhexylamine hydrochloride, isopropylamine hydrochloride, cyclohexylamine hydrobromide, 2-methylpiperidine hydrobromide, 1,3-diphenylguanidine hydrobromide, dimethylbenzylamine hydrobromide, hydrazine hydrobromide hydrate, dimethylcyclohexylamine hydrobromide, trinonylamine hydrobromide, diethylaniline hydrobromide, 2-diethylaminoethanol hydrobromide, 2-diethylaminoethanol hydrobromide, ammonium chloride, diallylamine hydrochloride, diallylamine hydrobromide, monoethylamine hydrobromide, etc. Amino acid hydrochloride, diethylamine hydrochloride, triethylamine hydrobromide, triethylamine hydrochloride, hydrazine monohydrochloride, hydrazine dihydrobromide, hydrazine monohydrobromide, hydrazine dihydrobromide, pyridine hydrochloride, aniline hydrobromide, butylamine hydrochloride, hexylamine hydrochloride, n-octylamine hydrochloride, dodecylamine hydrochloride, dimethylcyclohexylamine hydrobromide, ethylenediamine dihydrobromide, rosin amine hydrobromide, 2-phenylimidazolium hydrobromide, 4-benzylpyridine hydrobromide, L-glutamic acid hydrochloride, N-methylmorpholine hydrochloride, betaine hydrochloride, 2-methylpiperidine hydroiodide, cyclohexylamine hydroiodide, 1,3-diphenylguanidine hydrofluoric acid, diethylamine hydrofluoric acid, 2-ethylhexylamine hydrofluoric acid, cyclohexylamine hydrofluoric acid, ethylamine hydrofluoric acid, rosin amine hydrofluoric acid, cyclohexylamine tetrafluoroborate, dicyclohexylamine tetrafluoroborate.

1,5-5.amine

Examples of amines include monoethanolamine, diphenylguanidine, xylylguanidine, ethylamine, triethylamine, cyclohexylamine, ethylenediamine, triethylenetetramine, imidazole, 2-methylimidazolium, 2-ethylimidazolium, 1,2-dimethylimidazolium, 2-ethyl-4-methylimidazolium, 2-phenylimidazolium, 2-phenyl-4-methylimidazolium, 1-benzyl-2-methylimidazolium, 1-benzyl-2-phenylimidazolium, 1-cyanoethyl-2-methylimidazolium, 1-cyanoethyl-2-undecylimidazolium, 1-cyanoethyl-2-ethyl-4-methylimidazolium, 1-cyanoethyl-2-phenylimidazolium, 1-cyanoethyl-2-undecylimidazolium trimellitate, 1-cyanoethyl-2-phenylimidazolium trimellitate, and 2,4-diamino-6-[2’-methylimidazolium-(1’)] 2,4-Diamino-6-[2’-undecylimidazolyl-(1’)]-ethyl-triazine, 2,4-diamino-6-[2’-ethyl-4’-methylimidazolyl-(1’)]-ethyl-triazine, 2,4-diamino-6-[2’-methylimidazolyl-(1’)]-ethyl-triazine isocyanuric acid adduct, 2-phenylimidazolyl isocyanuric acid adduct, 2-phenyl-4,5-dihydroxymethylimidazolium, 2-phenyl-4-methyl-5-hydroxymethylimidazolium, 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-triazine, 2,4-di- Amino-6-vinyl-triazine isocyanuric acid adduct, 2,4-diamino-6-methacryloyloxyethyl-triazine, epoxy-imidazolium adduct, 2-methylbenzimidazole, 2-octylbenzimidazole, 2-pentylbenzimidazole, 2-(1-ethylpentyl)benzimidazole, 2-nonylbenzimidazole, 2-(4-thiazolyl)benzimidazole, benzimidazole, 2-(2’-hydroxy-5’-methylphenyl)benzotriazole, 2-(2’-hydroxy-3’-tert-butyl-5’-methylphenyl)-5-chlorobenzotriazole, 2-(2’-hydroxy-3’,5’-di-tert-pentylphenyl)benzotriazole, 2-(2’-hydroxy-5’-tert-octylphenyl)benzotriazole, 2,2’-methylenebis[6-(2H-benzotriazol-2-yl)-4-tert- Octylphenol, 6-(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]diethanol, 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 can be used simultaneously.

Amines can reduce the activity of organic sulfonic acid activators. Therefore, when amines, as activators, are used in flux, the wettability of the molten solder provided by the organic sulfonic acid activators may be impaired. Therefore, the amine content is preferably 0 wt%. That is, the flux of the embodiments preferably does not contain amines.

1-6. Solvents

The flux of the embodiments may contain a solvent. That is, the flux of the embodiments contains a solvent as an optional component. In order to effectively exert the reducing effect of the organic sulfonic acid activator and the active agent, the solvent is preferably non-volatile at temperatures below 70°C. If the solvent evaporates, the flux dries and it is difficult for the flux to wet and diffuse to the solder joint. Therefore, the boiling point of the solvent is preferably 200°C or higher. However, it is required that the solvent evaporates upon heating. Therefore, the boiling point of the solvent is preferably 270°C or lower.

Examples of solvents include water, alcohols, glycol ethers, and terpenoids. Examples of alcohols include isopropanol, 1,2-butanediol, isoborneol, 2,4-diethyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2,5-dimethyl-3-hexyn-2,5-diol, 2,3-dimethyl-2,3-butanediol, 1,1,1-tris(hydroxymethyl)ethane, 2-ethyl-2-hydroxymethyl-1,3-propanediol, and 2,2′-oxobis(methylene)ethane. The solvents include bis(2-ethyl-1,3-propanediol), 2,2-bis(hydroxymethyl)-1,3-propanediol, 1,2,6-trihydroxyhexane, bis[2,2,2-tri(hydroxymethyl)ethyl] ether, 1-ethynyl-1-cyclohexanol, 1,4-cyclohexanediol, 1,4-cyclohexanediol, erythritol, threitol, guaiacol glycerol ether, 3,6-dimethyl-4-octyne-3,6-diol, and 2,4,7,9-tetramethyl-5-decyn-4,7-diol. Examples of glycol ether solvents include hexyl diethylene glycol, 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. One or more of the above solvents may be used simultaneously.

The solvent content (or their combined content when using two or more solvents) is greater than 0 wt% and less than 60 wt%. This content can be capped at 30 wt% or 20 wt%.

1-7. Other additives

The flux of the embodiment may include antioxidants, defoamers, and colorants as other additives. Examples of antioxidants include hindered phenolic antioxidants. Examples of defoamers include acrylic polymers, vinyl ether polymers, butadiene polymers, and silicones. One or more of the above additives may be used as other additives. The content of the other additives (the total content when using two or more) is greater than 0 wt% and less than 5 wt%.

2. Solder paste

The solder paste of the embodiment comprises the above-mentioned flux and Sn-based solder metal.

Examples of Sn-based solder metals include elemental Sn and Sn-based alloys. Examples of Sn-based alloys include binary alloys and multi-component alloys (ternary or higher). 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 multi-component alloys include alloys in 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 are added to the aforementioned binary alloys.

Sn-based solder metals are classified into low-temperature solder metals and high-temperature solder metals. The former is defined as Sn-based solder metals that have a melting point (referring to the solidus or liquidus temperature, the same applies below) in a low-temperature region (specifically, a temperature range below 210°C). The latter is defined as Sn-based solder metals that have a melting point in a high-temperature region (specifically, a temperature range above 210°C). Sn-Bi alloys or multi-component alloys containing Cu, Ag, Sb, or Ni are examples of low-temperature solder metals. Sn-Sb alloys and Sn-Ag-Cu alloys are examples of high-temperature solder metals.

Here, regarding Ni electrodes formed on typical circuit boards, the following problem exists: the oxide film on the surface of the Ni electrode (i.e., the Ni oxide film) is difficult to remove during soldering using solder metals with melting points in the low-temperature region. This is because the heating temperature during soldering is low, resulting in insufficient flux activity.

In this respect, the aforementioned flux contains an organic sulfonic acid activator as an essential component. Therefore, as will be seen from the examples described later, even in soldering using low-temperature solder metals, the Ni oxide film can be removed. Thus, the aforementioned flux, especially when combined with low-temperature solder metals, enjoys the advantages of the organic sulfonic acid activator. Therefore, the solder paste of the embodiment is preferably a combination of low-temperature solder metal and the aforementioned flux.

There is no limit to the content of Sn-based solder metal and flux relative to the total mass of the solder paste. For example, the content of Sn-based solder metal is 5–95% by mass, and the content of flux is 5–95% by mass.

There are no limitations on the manufacturing method of solder paste; it can be manufactured by mixing the raw materials simultaneously or sequentially using any method. In manufacturing solder paste, all components of the flux and solder powder are ultimately mixed together. That is, solder powder can be mixed into all components of the prepared flux beforehand, or the remaining components of the flux can be further mixed after a portion of the flux components have been mixed with the solder powder. Alternatively, all components of the solder paste can be mixed simultaneously.

3. Example

The flux and solder paste of the embodiments will be described in detail below based on examples.

The flux compositions of Examples 1 to 29 were prepared according to the mixing ratios shown in Tables 1 to 4 below. Additionally, flux compositions of Comparative Examples 1 to 5 were prepared. Using these flux compositions, the following items (i) to (iv) were evaluated. The evaluation results are also shown in Tables 1 to 4 below.

(i) Utilizing the removeability of Ni oxide film in flux compositions

(ii) Utilizing the wettability of the flux composition in the molten solder

(iii) Cleanability of flux residue

(iv) Transferability of flux compositions

3-1. Evaluation of the removability of Ni oxide film

(1) Verification method

The removal capability was verified using the following method. First, the Ni-plated copper plate was ultrasonically cleaned using a hydrocarbon solvent. Next, the Ni-plated copper plate was heated to 400°C for 1 minute on a hot plate. The surface of the heated copper plate turned light yellow due to Ni oxidation. Then, various flux compositions were applied to the copper plate, and it was heated to 200°C for 30 seconds. Afterward, the copper plate was rinsed with water, and the surface condition was observed.

(2) Judgment Criteria

〇: The metallic luster of Ni can be confirmed.

×: The metallic luster of Ni cannot be confirmed. Alternatively, the surface of the copper plate remains pale yellow.

3-2. Evaluation of wettability

(1) Verification method

Wetting performance was verified using the wetting balance method with a Rhesca SAT-5200 solderability tester. First, a test piece (a Ni-plated copper plate, 5 mm wide × 30 mm long × 0.3 mm thick) was heated for 1 hour in a constant temperature bath set to 300°C. Next, various flux compositions were applied from the tip of the test piece to a depth of approximately 1 mm. Then, the temperature of the constant temperature bath was varied, and the test piece was immersed in the solder bath. The temperature of the constant temperature bath was adjusted appropriately according to the melting point of the solder alloy in the solder bath. For example, it was 190°C in Examples 1-25 and Comparative Examples 1-5, and 250°C in Example 29. The immersion conditions were an immersion depth of 2 mm and an immersion time of 10 seconds.

It should be noted that the composition of the solder alloy is as follows.

Examples 1-25, Comparative Examples 1-5: 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

(2) Judgment Criteria

〇: Zero crossover was observed.

×: No zero crossover was observed.

3-3. Evaluation of Cleanability

(1) Verification method

Cleanability was verified using the following method. First, each flux composition was printed onto the pads of the test substrate. Then, solder balls were mounted on the printed areas and reflow soldering was performed. Immediately after reflow, the test substrate was immersed in deionized water for cleaning. Flux residue was then identified using SEM. The composition of the solder balls was the same as that of the solder alloy used in the wettability evaluation.

The reflow conditions were adjusted appropriately based on the composition of the solder balls. For example, in Examples 1-25 and Comparative Examples 1-5, the temperature was increased from 30°C to a peak temperature (190°C) at a rate of 1°C per second, and heating was continued for 30 seconds after reaching the peak temperature. In Example 29, the peak temperature of the above 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 heated stirrer. The temperature of the ion-exchanged water was set to 50°C ± 10°C, and the rotation speed of the stirrer was set to 300 rpm. Afterward, the printed area was observed.

(2) Judgment Criteria

〇: No flux residue remains after cleaning.

×: There is flux residue after cleaning.

3-4. Evaluation of transferability

(1) Verification method

The transferability of each flux composition was verified by transferring it onto a test substrate using a needle transfer device. The difference between the substrate weight before and after flux composition transfer was measured. The test substrate had 1200 pads, and the needle diameter of the transfer device was 0.1 mm. During flux composition transfer, a kneading process was performed for 5–10 minutes to adjust the film thickness to 0.1 ± 0.01 mm. The weight difference (transfer weight) for each flux composition was measured 9 times, and the average value was calculated.

(2) Judgment Criteria

〇: The average transfer weight is 1 mg or more.

×: The average transfer weight is 1 mg. Or bridging occurred.

3-5. Overall Evaluation

A comprehensive evaluation will be conducted based on the evaluation results of items (i) to (iv) above. The judgment criteria are as follows.

〇: The results for items (i) to (iv) are all 〇.

×: Any one or all of the results of items (i) to (iv) are ×.

As shown in Tables 1-4, the flux compositions of Examples 1-29 exhibited excellent results in all items (i)-(iv). Although the details of the reason are unclear, the inventors speculate as follows: That is, due to the appropriate content of the organic sulfonic acid surfactant, (i) the removal of the Ni oxide film is ensured, thereby also ensuring (ii) wettability. Furthermore, due to the appropriate content of the organic sulfonic acid surfactant and the low-molecular-weight nonionic surfactant, the formation of Sn salt is suppressed, thereby ensuring (iii) cleaning properties. Additionally, due to the appropriate content of the low-molecular-weight nonionic surfactant and the high-molecular-weight nonionic surfactant, the flux composition is endowed with moderate thixotropy, thereby ensuring (iv) transferability.

On the other hand, the flux compositions of Comparative Examples 2 and 3 showed poor results in terms of (i) Ni oxide film removal and (iv) wettability. The inventors speculate that this is because these comparative examples do not contain organic sulfonic acid-based surfactants.

Furthermore, the flux compositions of Comparative Examples 1 and 4 showed poor results in terms of (iii) cleanability and (iv) transferability. The inventors speculate the reasons for this as follows: In Comparative Example 1, the content of the low-molecular-weight nonionic surfactant was less than the content of the organic sulfonic acid surfactant. Therefore, the advantages of adding the low-molecular-weight nonionic surfactant could not be realized. In Comparative Example 4, no low-molecular-weight nonionic surfactant was added. Therefore, the inhibition of Sn salt formation and the imparting of thixotropy were insufficient. Additionally, in Comparative Example 1, the excessive content of the cationic surfactant had many negative effects.

Furthermore, the flux composition of Comparative Example 5 showed poor results in terms of (iv) transferability. The inventors speculate that this is because a high molecular weight nonionic surfactant was not added in Comparative Example 5, thus the imparting of thixotropy was insufficient.

Claims (6)

1. A flux, characterized in that it comprises: Organic sulfonic acid surfactants, 1wt% to 10wt%; High molecular weight nonionic surfactants with a mass-average molecular weight (Mw) exceeding 1200, ranging from 10 wt% to 40 wt%; and Low molecular weight nonionic surfactants with a mass-average molecular weight (Mw) of less than 1200, ranging from 5 wt% to 75 wt%, are used as nonionic surfactants. The content of the low-molecular-weight nonionic surfactant is greater than or equal to the content of the organic sulfonic acid surfactant. The flux does not contain cationic surfactants, or contains more than 0 wt% but less than 5 wt% of such cationic surfactants. 2. The flux as claimed in claim 1, characterized in that it further comprises more than 0 wt% and less than 10 wt% of an active additive. The active ingredient comprises at least one of the following: organic acids, amines, organophosphorus compounds, organohalides, and amine hydrohalates, other than the organic sulfonic acid-based active ingredient. 3. The flux according to claim 2, characterized in that it does not contain the amine as the active additive. 4. The flux according to any one of claims 1 to 3, characterized in that it further comprises more than 0 wt% and less than 60 wt% of solvent. 5. A solder paste, characterized in that it comprises the flux according to any one of claims 1 to 4 and Sn-based solder metal. 6. The solder paste according to claim 5, wherein the Sn-based solder metal has a melting point below 210°C.