Solder cream

Abstract

This invention relates to a flux that includes rosin, active ingredient, solvent, and thisotrope. An agent containing polyethylene glycol and solder powder, in which the amount of polyethylene glycol is 10 to... 20% by mass relative to the total mass of flux, and the amount of thixotropic agent excluding poly... Ethylene glycol will not exceed 5% by mass relative to the total mass of flux;

Description

Flux, solder paste

[0001] The present invention relates to a flux and a solder paste that are inhibited from burning after heating and from separating powder and flux.

[0002] Solder paste is composed of flux and solder powder. Electrode pads are provided on the board on which electronic components are mounted. Solder paste is printed on the electrode pads, and solder joints are formed by a reflow process. Solder paste is printed by placing a metal mask with openings on the board, and then moving a squeegee while pressing it against the metal mask, so that the solder paste is applied all at once from the openings in the metal mask to the electrode pads on the board.

[0003] In recent years, as electronic components have become increasingly miniaturized, electrode pads have become smaller, resulting in longer times until purchased solder paste is used up. If stored for a long period of time, depending on the storage conditions, the viscosity of the solder paste may increase, preventing the solder paste from being applied through the openings in the metal mask, resulting in a loss of the printing performance it had when first purchased. Furthermore, as electrode pads become smaller and the printing area becomes narrower, the particle size of the solder powder used in the solder paste must be made smaller. This increases the surface area of ​​the solder powder, which can oxidize and cause the solder paste to deteriorate over time.

[0004] Therefore, for example, Patent Documents 1 to 3 disclose fluxes that contain 7 wt % or less of polyethylene glycol (hereinafter referred to as "PEG" as appropriate) to suppress an increase in the viscosity of the solder paste caused by the formation of a complex between the constituent elements of the solder powder and the rosin in the flux, thereby improving printing performance and storage stability.

[0005] Japanese Unexamined Patent Publication No. 5-228690 Japanese Unexamined Patent Publication No. 10-109188 Japanese Unexamined Patent Application No. 10-128573

[0006] The inventions described in Patent Documents 1 to 3 contain a predetermined amount of PEG to suppress an increase in the viscosity of the solder paste, but Patent Document 1 describes that sagging decreases when the PEG content exceeds 7% by mass. Patent Document 2 describes that when the PEG content exceeds 3% by mass, not only does the hydrophilicity of the flux itself increase the risk of migration, but the excessive protection of the solder metal particles inhibits activation, resulting in increased occurrence of solder balls. Patent Document 3 describes that when PEG is contained in an amount of 2% by mass or more, the occurrence of solder balls increases. Thus, in the inventions described in Patent Documents 1 to 3, the PEG content is determined with an eye on sagging and the occurrence of solder balls, which are adverse effects of suppressing viscosity increase and improving printing performance and storage stability.

[0007] However, as described in Patent Documents 1 to 3, increasing the content of thixotropic agent may be thought to suppress sagging and solder balls, but extending the melting time during reflow to reduce void formation increases the peak temperature. As a result, the thixotropic agent burns, making cleaning difficult. Furthermore, these patent documents do not consider at all how to reduce the rate of void formation, which must be suppressed in solder joints.

[0008] Even if scorching occurs, it can be removed by using a cleaning solution with high cleaning power, such as a fluorocarbon-based cleaning solution, but the use of such cleaning solutions should be avoided because they have a large environmental impact.Furthermore, in the inventions described in Patent Documents 1 to 3, if the amount of thixotropic agent is limited to prevent scorching, there is a concern that the solder powder and flux may separate due to a decrease in thixotropy and viscosity.

[0009] As described above, with conventional solder pastes, it is difficult to simultaneously solve the problems of separation of solder powder and flux, the environmental impact during cleaning, and the occurrence of voids, and therefore further investigation is required.

[0010] An object of the present invention is to provide a flux and solder paste that can be easily cleaned using an environmentally friendly cleaning solution, that does not separate the powder and flux, and that suppresses the generation of voids.

[0011] The inventors focused on the substances that cause cleaning residues, so that they could be easily removed using a cleaning solution with low environmental impact while ensuring sufficient melting time to reduce the occurrence of voids. As mentioned above, the substances that cause cleaning residues are generated by the scorching of thixotropic agents. Therefore, the inventors came up with the idea of ​​intentionally reducing the content of thixotropic agents, unlike conventional fluxes. However, even if the content of thixotropic agents is reduced, the scorching of the thixotropic agents still occurs, and residues will remain when using a cleaning solution with low environmental impact and low cleaning power. Furthermore, separation of the solder powder and flux due to a decrease in the viscosity of the solder paste must be avoided.

[0012] Therefore, we conducted extensive research to simultaneously achieve reduced void generation, improved cleaning performance, and suppressed separation of solder powder and flux. While keeping the thixotropic agent content low, we deliberately increased the PEG content, a content that had previously been avoided due to reduced sagging and the occurrence of solder balls. As a result, PEG remains liquid without decomposing even when the peak temperature increases with longer heating times. This suppresses scorching of the thixotropic agent, making it easy to remove residues with environmentally friendly cleaning solutions, such as aqueous or semi-aqueous cleaning solutions. This also allows for a longer heating time to ensure a reduced void generation rate. Furthermore, we discovered that a higher PEG content than conventional methods also suppresses separation of solder powder and flux, leading to the completion of this invention.

[0013] The present invention, which was made based on these findings, is as follows: (1) A flux containing a rosin, an activator, a solvent, and a thixotropic agent containing polyethylene glycol, wherein the content of polyethylene glycol is 10 to 20% by mass with respect to the total mass of the flux, and the content of the thixotropic agents other than polyethylene glycol is 5% by mass or less with respect to the total mass of the flux.

[0014] (2) The flux according to (1) above, wherein the polyethylene glycol is solid at room temperature. (3) A solder paste containing the flux according to (1) above or (2) above and solder powder.

[0015] The present invention will be described in more detail below. In this specification, "%" means "mass %" unless otherwise specified. Each component constituting the flux according to the present invention will be described in detail. 1. Rosin: The flux according to the present invention contains rosin. The inclusion of rosin enables the removal of metal oxides. Examples of rosins used in the present invention include the following: raw rosins such as gum rosin, wood rosin, and tall oil rosin, as well as derivatives obtained from the raw rosins. Examples of the derivatives include purified rosin, hydrogenated rosin, disproportionated rosin, polymerized rosin, and α,β-unsaturated carboxylic acid-modified products (such as acrylated rosin, maleated rosin, and fumarated rosin), as well as purified, hydrogenated, and disproportionated products of the polymerized rosin, and purified, hydrogenated, and disproportionated products of the α,β-unsaturated carboxylic acid-modified products. Two or more of these may be used simultaneously. Polymerized rosin is preferred as the rosin used in the present invention because of its high heat resistance and the improved cleaning properties of the flux.

[0016] The rosin content is preferably 15 to 50% based on the total mass of the flux. The rosin content is more preferably greater than 15%, even more preferably 20% or more, and particularly preferably 25% or more based on the total mass of the flux. Furthermore, the rosin content is more preferably less than 50%, even more preferably 48% or less, and particularly preferably 46% or less based on the total mass of the flux. Rosin is a highly effective material in preventing powder reoxidation during reflow (heating). It is widely used as a soldering flux material and improves basic mounting characteristics. On the other hand, from the perspective of cleanability, for example, using a cleaning solution that poorly dissolves rosin remaining as flux residue can result in poor cleaning. In the present invention, the decrease in cleanability due to the addition of rosin is eliminated by adding polyethylene glycol, allowing the rosin content to be within the above range.

[0017] 2. Activator The flux according to the present invention contains an activator to remove metal oxides. Examples of the activator used in the present invention include organic acids, organic halogen compounds, and amine hydrohalides, with organic acids being preferred.

[0018] Examples of organic acids include malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, phthalic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dimer acid, propionic acid, 2,2-bishydroxymethylpropionic acid, tartaric acid, malic acid, glycolic acid, diglycolic acid, thioglycolic acid, dithioglycolic acid, stearic acid, 12-hydroxystearic acid, palmitic acid, and oleic acid. Preferred are malonic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, and sebacic acid, and more preferred are adipic acid and 2,2-bishydroxymethylpropionic acid. Two or more of these may be contained simultaneously.

[0019] 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, etc. Amine hydrohalides are compounds obtained by reacting an amine with hydrogen halide. Examples of amines in the amine hydrohalide salt include ethylamine, ethylenediamine, triethylamine, methylimidazole, 2-ethyl-4-methylimidazole, diphenylguanidine, and ditolylguanidine. Examples of hydrogen halides include chlorine, bromine, iodine, and fluorine hydrides (hydrogen chloride, hydrogen bromide, hydrogen iodide, and hydrogen fluoride). The content of the activator is preferably 1 to 10% of the total mass of the flux. The content of the activator is more preferably 1.5% or more, even more preferably 2.0% or more, and particularly preferably 2.5% or more, of the total mass of the flux. The content of the activator is more preferably 8% or less, even more preferably 6% or less, and particularly preferably 4% or less, of the total mass of the flux.

[0020] 3. Solvent The flux according to the present invention contains a solvent to melt and uniformly disperse rosin and other components. Examples of solvents used in the present invention include the following: alcohol-based solvents, ester-based solvents, glycol ether-based solvents, and terpineols, with glycol ether-based solvents being preferred. Examples of ester-based solvents include diisobutyl succinate, dibutyl succinate, dimethyl adipate, diethyl adipate, dibutyl adipate, diisopropyl adipate, diisobutyl adipate, diisodecyl adipate, dibutyl maleate, dimethyl sebacate, diethyl sebacate, dibutyl sebacate, dioctyl sebacate, and diisopropyl decanedioate. Examples of alcohol-based solvents 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 2,2-bis(hydroxymethyl)-1,3-propanediol, 2,2-bis(hydroxymethyl)-1,3-propanediol, 1,2,6-trihydroxyhexane, bis[2,2,2-tris(hydroxymethyl)ethyl]ether, 1-ethynyl-1-cyclohexanol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, erythritol, threitol, guaiacol glycerol ether, 3,6-dimethyl-4-octyne-3,6-diol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, and the like.Examples of glycol ether-based solvents include diethylene glycol mono-2-ethylhexyl ether, ethylene glycol monophenyl ether, 2-methylpentane-2,4-diol, diethylene glycol monohexyl ether, diethylene glycol dibutyl ether, triethylene glycol monobutyl ether, 1,3-butylene glycol, phenyl glycol, and hexylene glycol, with diethylene glycol monohexyl ether or diethylene glycol mono-2-ethylhexyl ether being preferred. Two or more of these may be used simultaneously. The solvent content is preferably 20 to 75% of the total mass of the flux. The solvent content is more preferably 25% or more, even more preferably 30% or more, and particularly preferably 35% or more, of the total mass of the flux. Furthermore, the solvent content is more preferably 72% or less, even more preferably 70% or less, and particularly preferably 50% or less, of the total mass of the flux.

[0021] 4. Thixotropic Agent Containing Polyethylene Glycol The flux of the present invention contains a thixotropic agent containing PEG. Regarding the molecular weight of the PEG contained in the thixotropic agent, a low molecular weight is desirable for aqueous cleaning solutions from the standpoint of cleanability. However, low molecular weights often result in a liquid state, which can lead to concerns about poor viscosity control and heat sagging characteristics as a solder paste. Considering the above, the optimal molecular weight is preferably 2700 to 3400. However, since PEGs with different molecular weights can be used in combination, or some properties can be covered by other flux materials, molecular weights between 250 and 9000 can also be used in appropriate combinations.

[0022] PEG is preferably liquid at temperatures above 240°C. The boiling point of PEG is preferably above 200°C, more preferably above 230°C, and even more preferably above 250°C, from the perspective of suppressing void formation and improving cleanability. Because PEG volatilization during reflow can lead to void formation, and leaving a large amount of PEG as flux residue is advantageous for cleanability, a higher boiling point is preferable. Furthermore, a boiling point of 300°C or higher is preferable. Such PEGs have a high boiling point, allowing them to maintain a liquid state at temperatures at which solder alloys known as high-melting-point solders melt. Maintaining a liquid state during heating reduces the adhesion of scorch caused by thixotropic agents due to the fluidity of the liquid PEG, allowing residues to be easily cleaned even with cleaning solutions that have a low environmental impact and low cleaning power. Furthermore, PEG's thixotropy allows it to serve as a substitute for thixotropic agents, which cause scorching. Reducing the content of thixotropic agents reduces the amount of scorch adhesion, allowing residues to be easily cleaned with the above-mentioned cleaning solutions. Furthermore, even if the peak temperature during heating increases due to a longer heating time, the residue can be easily washed away, which results in a reduced void generation rate.In addition, when PEG, which is solid at room temperature, is used, it can adjust the fluidity of the solder paste at room temperature in the same way as a thixotropic agent, making it possible to suppress separation of the solder powder and flux.

[0023] To achieve these effects, the solder paste of the present invention contains 10 to 20% PEG relative to the mass of the flux. If the PEG content is less than 10%, the above effects are not achieved. On the other hand, if the PEG content exceeds 20%, the amount of rosin that can be contained is limited to achieve a viscosity suitable for use, which reduces reflowability and makes it impossible to suppress the occurrence of voids. Furthermore, because PEG is hydrophilic, using a quasi-aqueous cleaning solution suitable for cleaning rosin-based flux may result in poor cleaning performance. The PEG content is preferably 10 to 16%, more preferably 12 to 16%.

[0024] The PEG used in the present invention is preferably solid at room temperature, in order to impart sufficient thixotropy to the solder paste and increase the viscosity of the solder paste, thereby suppressing separation of the flux and solder powder that may occur during storage.

[0025] The solder paste according to the present invention may contain a thixotropic agent in addition to PEG. Examples include wax-based thixotropic agents and amide-based thixotropic agents, with wax-based thixotropic agents being preferred.

[0026] Examples of wax-based thixotropic agents include hydrogenated castor oil, etc. Examples of amide-based thixotropic agents include lauric amide, palmitic amide, stearic amide, behenic amide, hydroxystearic amide, oleic amide, erucic amide, methylene bisstearic amide, ethylene bislauric amide, ethylene bisstearic amide, ethylene bishydroxystearic amide, methylol stearic amide, etc. Two or more of these may be contained simultaneously.

[0027] The content of the thixotropic agent other than PEG must be 5% or less, preferably 3% or less, based on the total mass of the flux, and may not be contained at all.

[0028] 5. Other Components The flux according to the present invention may contain amines, antioxidants, or antifoaming agents. Examples of amines that can be used include aliphatic amines, amino alcohols, and imidazoles.

[0029] Examples of aliphatic amines include dimethylamine, ethylamine, 1-aminopropane, isopropylamine, trimethylamine, n-butylamine, diethylamine, sec-butylamine, tert-butylamine, N,N-dimethylethylamine, isobutylamine, and cyclohexylamine.

[0030] Examples of amino alcohols include 2-aminoethanol, 2-(ethylamino)ethanol, diisopropanolamine, triethanolamine, N-butyldiethanolamine, triisopropanolamine, N,N-bis(2-hydroxyethyl)-N-cyclohexylamine, N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine, and N,N,N',N'',N''-pentakis(2-hydroxypropyl)diethylenetriamine.

[0031] Examples of imidazoles include 2-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 1-benzyl-2-methylimidazole, 1-benzyl-2-phenylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-undecylimidazole, and 1-cyanoethyl-2-ethylimidazole. 1-cyanoethyl-4-methylimidazole, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-undecylimidazolium trimellitate, 1-cyanoethyl-2-phenylimidazolium trimellitate, 2,4-diamino-6-[2'-methylimidazolyl-(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-4,5-dihydroxymethylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazole, 2-methylimidazoline, 2-phenylimidazoline, 2,4-diamino-6- Examples include 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, and benzimidazole.

[0032] Examples of the antioxidant include hindered phenol-based antioxidants. Examples of the defoaming agent include acrylic polymers, vinyl ether polymers, butadiene polymers, and silicones.

[0033] 6. Solder Paste The solder paste according to the present invention contains the above-described flux and solder powder. The solder powder is not particularly limited, and solder powders having various solder alloy compositions, such as Sn-3Ag-0.5Cu, Pb-10Sn, and Sn—Sb solder alloys, can be used. Because the solder paste according to the present invention uses the above-described flux, it can solve the problems that arise in solder pastes that combine conventional flux and high-melting-point solder powder, even when a high-melting-point solder powder is used. The solder paste according to the present invention contains a predetermined amount of PEG, and therefore can suppress poor cleaning due to scorching, separation of the solder powder and flux, and the occurrence of voids, even when the melting point of the solder powder is 240°C or higher or 280°C or higher.

[0034] The particle size of the solder powder is preferably 10 to 50 μm, and the content of the solder powder is preferably 85 to 92% of the total mass of the solder paste.

[0035] The solder paste according to the present invention is manufactured by a method commonly used in the industry. First, the solder powder can be manufactured by a known method, such as a dripping method in which heated molten solder is dripped to obtain particles, a centrifugal atomization method, or a method in which bulk solder material is crushed. In the dripping method or atomization method, the dripping or atomization is preferably carried out in an inert atmosphere or solvent to obtain particles. Then, the components are heated and mixed to prepare a flux, and the solder powder is introduced into the flux, followed by stirring and mixing.

[0036] The present invention will be described with reference to the following examples, but is not limited to these examples. In the examples, "%" means "mass %" unless otherwise specified. Furthermore, the numbers in the alloy composition of the solder powder refer to the content in mass %.

[0037] Solder powders with particle sizes of 20 to 38 μm were prepared, each having an alloy composition of Sn-3Ag-0.5Cu (Ag: 3% by mass, Cu: 0.5% by mass, balance: Sn and impurities, melting point: 220°C) and Pb-10Sn (Sn: 10% by mass, balance: Pb and impurities, melting point: 302°C). Solder pastes were prepared by mixing the flux and solder powder prepared as shown in Tables 1 and 3. The values ​​shown in Tables 1 and 3 are in "mass %." The alloy composition of the solder powder used in Examples 1 to 7 and Comparative Examples 1 to 4 shown in Table 1 was Sn-3Ag-0.5Cu, and the alloy composition of the solder powder used in Examples 8 to 14 and Comparative Examples 5 to 8 shown in Table 3 was Pb-10Sn. The mass ratio of flux to solder powder was 12:88 when Sn-3Ag-0.5Cu solder powder was used, and 9:91 when Pb-10Sn solder powder was used. Each solder paste was evaluated for 1. cleanability, 2. separation of flux and solder powder, and 3. presence or absence of voids. Details are as follows:

[0038] 1. Cleanability 1) Printing The solder paste prepared as described above was printed on a 50 mm x 50 mm Cu plate using a metal mask with an opening size of 5 mm x 5 mm and a thickness of 0.4 mm.

[0039] 2) Reflow conditions - For Sn-3Ag-0.5Cu The preheat temperature was set to 150 to 180°C and held for 200 seconds, the temperature was increased from the preheat temperature to the peak temperature at a rate of 1.34°C / sec, the peak temperature was set to 250°C, the heating time above 220°C was set to 250 seconds, and the oxygen concentration was set to 2000 ppm, and the material was then cooled to room temperature.

[0040] In the case of Pb-10Sn, the heating rate from room temperature to the peak temperature was 4°C / sec, the peak temperature was 380°C, the heating time above 302°C was 50 seconds, and the oxygen concentration was 100 ppm or less, and then the sample was cooled to room temperature.

[0041] 3) Cleaning conditions - In the case of aqueous cleaning solution A cleaning agent with the trade name "VIGON A200" (manufactured by Zestron) was used, and diluted with ion-exchanged water at a ratio of 30:70 (mass ratio) to obtain a cleaning solution. Then, cleaning was performed according to the following procedure: (1) Ultrasonic cleaning in an ultrasonic bath filled with the cleaning solution (50°C, 10 minutes) (2) Rinse in an ultrasonic bath filled with ion-exchanged water (room temperature, 3 minutes twice) (3) Dry for 10 minutes in a constant temperature bath at 50°C

[0042] - In the case of a semi-aqueous cleaning solution, cleaning was performed using a cleaning solution with the trade name "Clean Thru 750J" (manufactured by Kao Corporation) according to the following procedure: (1) Ultrasonic cleaning in an ultrasonic bath filled with the cleaning solution (40°C, 10 minutes) (2) Rinse in an ultrasonic bath filled with ion-exchanged water (room temperature, 3 minutes) (3) Dry for 10 minutes in a thermostatic bath at 50°C

[0043] 4) Evaluation The dried test piece was observed under a scanning electron microscope (SEM). When no cleaning residue was observed, it was marked as "◯", and when cleaning residue was observed, it was marked as "X".

[0044] 2. Separation After leaving the sample at room temperature for 3 days, the surface condition of the paste was visually confirmed. If no separation was observed, it was marked as "Good", and if separation was observed (flux protrusion due to separation), it was marked as "Poor".

[0045] 3. Voids (presence or absence) 1) Printing The solder paste prepared as described above was printed on a 50 mm x 50 mm Cu plate using a metal mask with an opening size of 5 mm x 5 mm and a thickness of 0.4 mm.

[0046] 2) Reflow conditions - For Sn-3Ag-0.5Cu The preheat temperature was set to 150 to 180°C and held for 200 seconds, the temperature was increased from the preheat temperature to the peak temperature at a rate of 1.34°C / sec, the peak temperature was set to 250°C, the heating time above 220°C was set to 250 seconds, and the oxygen concentration was set to 2000 ppm, and the material was then cooled to room temperature.

[0047] In the case of Pb-10Sn, the heating rate from room temperature to the peak temperature was 4°C / sec, the peak temperature was 380°C, the heating time above 302°C was 50 seconds, and the oxygen concentration was 100 ppm or less, and then the sample was cooled to room temperature.

[0048] 3) Void Observation: A transmission image of the soldered portion after reflow was observed using a Microfocus X-ray System XVR-160 manufactured by UNI-HiTE SYSTEM, and the void occurrence rate was determined. The void occurrence rate was determined by observing the soldered portion 3 mm from the Cu plate surface in a direction perpendicular to the Cu plate surface, thereby obtaining a solder joint transmission image. Then, an automatic analysis was performed to distinguish between metal-filled portions and void portions (metal-unfilled portions) based on the color contrast of the transmission image, and the void area rate was calculated, which was taken as the void occurrence rate.

[0049] 4) Evaluation criteria: If the void occurrence rate was more than 3%, it was marked "x", if the void occurrence rate was more than 1% and 3% or less, it was marked "o", and if the void occurrence rate was 1% or less, it was marked "◎". The evaluation results are shown in Tables 2 and 4.

[0050]

[0051]

[0052]

[0053]

[0054] As shown in Tables 2 and 4, it was found that the solder pastes of the examples all satisfied the requirements of the present invention, and therefore had excellent cleanability and suppressed the occurrence of separation and voids.

[0055] In contrast, the solder pastes of Comparative Examples 1, 2, 5, and 6 contained no PEG or only a small amount of PEG, and therefore residue was observed in aqueous cleaning solutions, which are more hydrophilic than quasi-aqueous cleaning solutions. The solder pastes of Comparative Examples 3 and 7 contained a large amount of PEG, which limited the rosin content, resulting in poor reflowability and a high void rate. Furthermore, because PEG is hydrophilic, the solder pastes of Comparative Examples 3 and 7 were found to have poor cleaning properties with quasi-aqueous cleaning solutions. The solder pastes of Comparative Examples 4 and 8 contained a large amount of thixotropic agents other than PEG, which caused scorching and poor cleaning properties with all cleaning solutions.

[0056] According to the present invention, it is possible to provide a flux and a solder paste that can be easily cleaned using an environmentally friendly cleaning solution, that does not separate from the powder and the flux, and that suppresses the generation of voids.

[0057] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2019-183500) filed on October 4, 2019, the contents of which are incorporated herein by reference.

Claims

Solder cream comprising: solder powder; and flux comprising: rosin; activator; solvent; and thixotropic agent containing polyethylene glycol, where the amount of polyethylene glycol is 10% by mass to 20% by mass relative to the total mass of flux, the amount of thixotropic agent excluding polyethylene glycol is 5% by mass or less relative to the total mass of flux, and the amount of rosin is not more than 15% by mass and 50% by mass or less relative to the total mass of flux.

2. Solder cream according to claim 1 where polyethylene glycol is solid at room temperature;