阅读说明：本技术 焊膏用助焊剂、焊膏、使用焊膏的焊锡凸块的形成方法及接合体的制造方法 (Flux for solder paste, method for forming solder bump using solder paste, and method for manufacturing bonded body ) 是由 八十嶋司 石川雅之 于 2018-07-26 设计创作，主要内容包括：本发明的焊膏用助焊剂包含粘合剂、溶剂及触变剂,所述焊膏用助焊剂中,酸值为100mgKOH/g以下,热重量测定中在300℃的减少率为80质量％以上,粘度为0.5Pa·s以上,并且粘着力为1.0N以上。(The flux for solder paste of the present invention comprises an adhesive, a solvent, and a thixotropic agent, and has an acid value of 100mgKOH/g or less, a reduction rate of 80 mass% or more at 300 ℃ in thermogravimetric measurement, a viscosity of 0.5 pas or more, and an adhesion of 1.0N or more.)

1. A soldering flux for solder paste, which comprises an adhesive, a solvent and a thixotropic agent,

the acid value of the soldering flux for soldering paste is less than 100mgKOH/g, the reduction rate at 300 ℃ in thermogravimetric measurement is more than 80 mass percent, the viscosity is more than 0.5 Pa.s, and the adhesive force is more than 1.0N.

2. The flux for solder paste according to claim 1,

the content of rosin in the flux for solder paste is 10 mass% or less.

3. A solder paste characterized in that,

the solder paste is obtained by mixing the flux for solder paste according to claim 1 or 2 and solder powder.

4. A solder paste according to claim 3,

the content of the flux for solder paste is 30 vol% or more and 90 vol% or less.

5. A solder paste according to claim 3 or 4,

the solder powder is any one of Sn-Ag-Cu solder powder, Sn-Ag solder powder, Pb-Sn solder powder, Au-Ge solder powder and Au-Si solder powder.

6. A method for forming a solder bump using a solder paste,

disposing a mask having an opening on a substrate, printing the solder paste so that the solder paste of any one of claims 3 to 5 fills the opening, peeling the mask, and then performing reflow processing on the bump precursor on the substrate in a formic acid gas atmosphere to form a solder bump.

7. A method for producing a joined body using a solder paste,

a solder paste according to any one of claims 3 to 5 is placed between a bonding object and an object to be bonded, and the bonding object and the object to be bonded are solder-bonded by heating in a formic acid gas atmosphere.

Technical Field

The present invention relates to a flux for solder paste, which can obtain solder used for bonding an electronic component to a substrate, a solder paste, a method for forming a solder bump using a solder paste, and a method for manufacturing a bonded body.

The present application claims priority based on patent application nos. 2017-147084, 2017-28 and 2018-133872, which were applied in japan at 7-17, 2017, and the contents thereof are incorporated herein by reference.

Background

Conventionally, a solder paste composed of a flux containing rosin, a solvent, a thixotropic agent, and an activator, and solder powder has been used for soldering. When a solder bump is formed using such a solder paste, when the solder paste is applied to a substrate and reflow-processed, a residue based on rosin or the like remains on the upper surface of the solder, and therefore, it is necessary to wash the residue with chemicals or the like, which is troublesome.

In order to save time and effort for cleaning such residue, a flux which does not generate residue after soldering and a solder paste containing the flux are known (see patent document 1).

The flux described in patent document 1 contains ammonium formate and an aliphatic polyol which is liquid at normal temperature and has a boiling point of 150 ℃ or higher under atmospheric pressure, and therefore has a reducing property and reduces an oxide film efficiently, thereby removing the oxide film generated on a substrate or the like. In the structure of patent document 1, the residue of the flux after soldering the electronic components and the like on the substrate can be suppressed.

Since the flux described in patent document 1 and the solder paste containing the flux are decomposed at low temperature, the solder surface is exposed without being coated with the flux. Therefore, the surface of the solder may be re-oxidized.

In contrast, a method of forming a solder bump capable of suppressing reoxidation of the solder surface has been proposed (see patent document 2).

The method for forming a solder bump described in patent document 2 is as follows: a method for forming a solder bump by disposing a mask having an opening on a substrate, printing a solder paste so that the opening is filled with the solder paste containing a flux and a solder powder, peeling off the mask, and then performing reflow processing on a bump precursor on the substrate, the method comprising: the flux contains rosin, a solvent and a thixotropic agent, the acid value of the flux is less than 100mgKOH/g, the halogen content of the flux is less than 0.03 mass%, and reflow treatment is performed in a formic acid gas atmosphere and/or an atmosphere of a gas in which formic acid is decomposed by heat.

In the method for forming a solder bump, reflow treatment is performed in an atmosphere of formic acid gas and/or an atmosphere of a gas in which formic acid is decomposed by heat, thereby reducing an oxide film such as solder powder and smoothly melting the solder.

Patent document 1: japanese patent laid-open publication No. 2011-83809 (A)

Patent document 2: japanese laid-open patent publication No. 2016-78095 (A)

In the method for forming solder bumps described in patent document 2, since a flux containing rosin as a main component is used, when a solder paste containing the flux used in the forming method is reflowed, a residue based on the rosin is generated, and thus the solder needs to be cleaned. Further, even if the solder is cleaned, the residue may not be completely removed, and in this case, the bondability between the electronic component and the substrate may be reduced.

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a flux for solder paste, a method for forming a solder bump using a solder paste, and a method for manufacturing a bonded body, which can obtain a solder capable of suppressing generation of residue.

A flux for solder paste according to an aspect of the present invention (hereinafter referred to as "flux for solder paste of the present invention") includes a binder, a solvent, and a thixotropic agent, and has an acid value of 100mgKOH/g or less, a reduction rate of 80 mass% or more at 300 ℃ in a thermogravimetric measurement, a viscosity of 0.5Pa · s or more, and an adhesion of 1.0N or more.

The adhesive strength is the adhesive strength to a substrate when a solder flux is applied to the substrate or the like, and is measured according to JISZ 3284.

In the present invention, since the reduction rate at 300 ℃ in the thermogravimetric measurement of the flux for solder paste is 80 mass% or more, when reflow soldering is performed on a solder paste containing the flux for solder paste, the volatile component is large as the flux, and therefore, even when the solder paste containing the flux for solder paste is reflow soldered to form a solder bump, generation of residue can be suppressed.

If the viscosity of the flux for solder paste is less than 0.5Pa · s, the viscosity of the flux is too low to constitute a solder paste, and there is a possibility that the flux cannot be applied to a substrate or the like. Further, if the adhesion of the flux for solder paste is less than 1.0N, the adhesive strength is low, and therefore, when the solder paste containing the flux is applied to a substrate or the like, the applied solder paste may fall off the substrate or the like. In contrast, in the present invention, since the viscosity of the flux is 0.5Pa · s or more and the adhesive force is 1.0N or more, the shape retainability of the solder paste composed of the flux for the solder paste and the solder powder can be ensured.

Further, by setting the acid value of the flux for solder paste to 100mgKOH/g or less, it is possible to suppress reaction between flux residue that may remain slightly and surrounding metal parts, for example, copper of wiring, etc., and to suppress corrosion, and to ensure long-term reliability of the joined body. In addition, since the reaction between the solder powder and the flux in the production of the solder paste is suppressed and the viscosity change of the solder paste is small, the solder paste which needs to be stored under refrigeration can be stored at room temperature for a long period of time (for example, 6 months or more), and the long-term storage property can be improved.

When the acid value exceeds 100mgKOH/g, the reduction water generated in the reduction reaction between the solder paste flux and the oxide of the solder powder increases, and the voids in the solder bump increase, so that the acid value is set to 100mgKOH/g or less.

The flux for solder paste of the present invention preferably does not contain a rosin or an activator which is contained in a usual flux, but may contain a trace amount as long as the requirement for the thermogravimetric measurement and the acid value is satisfied.

In a preferred embodiment of the flux for solder paste of the present invention, the content of rosin is preferably 10 mass% or less.

Even if the rosin is contained in the flux for solder paste, the acid value of the flux for solder paste can be set to 100mgKOH/g or less as long as the amount is 10 mass% or less. That is, the flux for solder paste may contain rosin in an amount of 10 mass% or less.

A solder paste according to another aspect of the present invention (hereinafter referred to as "the solder paste of the present invention") is obtained by mixing the flux for the solder paste and the solder powder.

Since the solder paste of the present invention contains the flux for solder paste, generation of residue can be suppressed even when reflow of the solder paste is performed, and a decrease in adhesiveness due to residue remaining can be suppressed.

In a preferred embodiment of the solder paste of the present invention, the content of the flux for solder paste is preferably 30 vol% or more and 90 vol% or less.

When the content of the flux for solder paste is less than 30 vol%, the flux may not form a slurry or may become a dry slurry, and the solder paste may not be applied to a substrate or the like. On the other hand, if the content of the flux for solder paste exceeds 90 vol%, the viscosity of the solder paste becomes too low and the coating performance deteriorates, or the flux and the solder powder are likely to be easily separated.

In contrast, in the above aspect, since the content of the flux for solder paste is 30 vol% or more and 90 vol% or less, a solder paste having an appropriate viscosity can be formed, and deterioration of coating performance and separation of the flux and the solder powder can be suppressed.

In a preferred embodiment of the solder paste of the present invention, the solder powder is one of Sn-Ag-Cu solder powder, Sn-Ag solder powder, Pb-Sn solder powder, Au-Ge solder powder and Au-Si solder powder.

According to the above aspect, any of the various powders described above can be used as the solder powder. In particular, in the case where the solder powder is an Au — Sn solder powder, the solder paste containing the Au — Sn solder powder has a high melting point, and therefore, the residue is likely to splash during reflow.

In a method for forming a solder bump using a solder paste according to another aspect of the present invention (hereinafter referred to as a method for forming a solder bump using a solder paste according to the present invention), a mask having an opening is disposed on a substrate, the solder paste is printed so as to fill the opening with the solder paste, the mask is peeled off, and then a reflow process is performed on a bump precursor on the substrate in a formic acid gas atmosphere to form a solder bump.

According to this configuration, the generation of the residue is suppressed, so that the step of cleaning the solder bump can be omitted, and in addition, the reflow process is performed in a formic acid gas atmosphere, so that the oxide film on the substrate and the oxide film on the surface of the solder can be reduced and the solder can be smoothly melted without including rosin or the like that reduces and removes the oxide.

A method for producing a joined body using a solder paste according to another aspect of the present invention (hereinafter referred to as "a method for producing a joined body using a solder paste according to the present invention") is a method for producing a joined body using a solder paste, in which the solder paste is placed between a joined object and a joined object, and the joined object are solder-joined by heating in a formic acid gas atmosphere.

According to this configuration, the generation of the residue is suppressed, so that the cleaning step of the bonded portion can be omitted, and in addition, the reflow process is performed in the formic acid gas atmosphere, so that even if the rosin or the like for reducing and removing the oxide is not included, the oxide film on the bonded object and the oxide film on the surface of the solder can be reduced and the solder melting can be performed smoothly, and the bonded object can be bonded more firmly.

The method of disposing the solder paste is not particularly limited, and for example, the paste can be applied by a printing method or a dispenser, or transferred by a needleEtc. are configured. The bonding material is, for example, a substrate, and the bonding material is, for example, a semiconductor element such as an LED element.

The invention provides a flux for solder paste, a method for forming solder bumps using the solder paste, and a method for manufacturing a joined body, wherein generation of residue and reoxidation of the surface of solder can be suppressed.

Detailed Description

The following describes a flux for a solder paste, and a method for forming a solder bump using the solder paste according to the present invention.

The flux for solder paste (hereinafter, may be simply referred to as flux) of the present embodiment is a flux comprising a binder, a solvent and a thixotropic agent, and has an acid value of 100mgKOH/g or less, a reduction rate of 80 mass% or more at 300 ℃ in thermogravimetric measurement, a viscosity of 0.5Pa · s or more, and an adhesion of 1.0N or more.

The reason why the composition, acid value, and reduction rate at 300 ℃ in the thermogravimetric measurement of the flux, viscosity, and adhesive force are defined as described above will be described below.

[ constitution of flux ]

The soldering flux comprises a binder, a solvent and a thixotropic agent. In the present embodiment, the flux is composed of only the binder, the solvent, and the thixotropic agent.

The acid value of the flux is set to be 100mgKOH/g or less. This is due to the following reasons: by setting the acid value to 100mgKOH/g or less, the reaction between the solder powder and the flux when the solder paste is prepared is suppressed, and the viscosity change as the solder paste is reduced; when the acid value exceeds the above value, the generation of reduced water generated in the reduction reaction between the flux and the oxide of the solder powder increases, and the number of voids in the solder bump formed increases. The acid value of the flux is preferably 50mgKOH/g or less, and more preferably 10mgKOH/g or less. The lower limit of the acid value is not particularly limited, and may be not more than the lower limit of the detection in the measurement, for example, 0.01 mgKOH/g.

Further, the reduction rate of the flux at 300 ℃ in thermogravimetric measurement is 80 mass% or more. This is because, if the reduction rate at 300 ℃ in the thermogravimetric measurement is less than 80 mass%, the amount of residue increases when reflow soldering paste containing flux is reflowed. The reduction rate of the flux at 300 ℃ in the thermogravimetric measurement is preferably 85 mass% or more, and more preferably 90 mass% or more. The upper limit of the amount of reduction is not particularly limited, but is, for example, 99.5% by mass as a value in the case of using a binder which can be generally used.

As the solvent contained in the flux, solvents such as alcohols, ketones, esters, ethers, aromatic solvents, hydrocarbons, terpenes, and terpenoids can be used. Specifically, benzyl alcohol, Ethanol (Ethanol), Ethanol (Ethyl alcohol), isopropyl alcohol, butanol, diethylene glycol, ethylene glycol, Ethyl cellosolve, butyl carbitol, isopropyl alcohol, Ethyl acetate, butyl benzoate, diethyl adipate, dodecane, tetradecene, α -terpineol, 2-methyl-2, 4-pentanediol, 2-ethyl-1, 3-hexanediol, toluene, xylene, propylene glycol monophenyl ether, diethylene glycol monohexyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, diisobutyl adipate, hexanediol, cyclohexane dimethanol, 2-terpenyloxyethanol, 2-dihydroterpenyloxyethanol, citral, linalool, limonene, carvacrol, pinene, farnesene, and the like, or a mixture thereof.

As the thixotropic agent contained in the flux, hardened castor oil, hydrogenated castor oil, carnauba wax, amides, hydroxy fatty acids, dibenzylidene sorbitol, bis (p-methylbenzylidene) sorbitol, beeswax, stearic acid amide, hydroxystearic acid ethylene bisamide, and the like can be used alone or in combination. Further, if necessary, fatty acids such as octanoic acid, dodecanoic acid, myristic acid, palmitic acid, stearic acid, behenic acid, etc., hydroxy fatty acids such as 1, 2-hydroxystearic acid, etc., antioxidants, surfactants, amines, etc. may be added to these compounds for use.

The viscosity of the flux is 0.5 pas or more. If the viscosity of the flux is less than 0.5Pa · s, the viscosity of the flux is too low to constitute a solder paste, and there is a possibility that the flux cannot be applied to a substrate or the like. The viscosity of the flux is preferably 1.0Pa · s or more. The viscosity was at room temperature (25 ℃ C.). The upper limit of the viscosity is not particularly limited, but is, for example, 100 pas from the viewpoint of coatability and the like.

And the adhesion of the flux is 1.0N or more. If the adhesion of the flux is less than 1.0N, the adhesive strength is low, and therefore, when the solder paste containing the flux is applied to a substrate or the like, the applied solder paste may fall off the substrate or the like. The adhesion of the flux is preferably 1.2N or more. The adhesion was a value at room temperature (25 ℃ C.). The upper limit of the adhesion is not particularly limited, but is, for example, 100N from the viewpoint of plate releasability of the mask.

[ constitution of adhesive ]

The binder is preferably a solid or a liquid having a viscosity of 1 pas or more at normal temperature (25 ℃). This is because, if the binder is a liquid having a viscosity of less than 1Pa · s at normal temperature, the viscosity of the flux is too low to constitute a solder paste, and there is a possibility that the flux cannot be applied to a substrate or the like. As described above, since the binder has a viscosity of 1Pa · s or more at normal temperature or is solid at normal temperature, a solder paste composed of a flux and solder powder including the binder has shape retentivity. The upper limit of the viscosity is not particularly limited, but is, for example, 400 pas from the viewpoint of coatability and the like.

The amount of decrease in the thermogravimetric amount of the binder at 300 ℃ is 90 mass% or more. This is because most of the flux components are binders, and therefore, if the reduction rate at 300 ℃ in the thermogravimetric measurement is less than 90 mass%, the reduction rate at 300 ℃ in the thermogravimetric measurement of the flux cannot be set to 80% or more. The upper limit of the reduction rate is not particularly limited, and a binder reduced by 100 mass% can be used.

The adhesive force of the adhesive is set to be more than 1.1N. As described above, since most of the flux components are adhesives, if the adhesive force of the adhesives is less than 1.1N, the adhesive force of the flux cannot be set to 1.0N or more. The upper limit of the adhesion is not particularly limited, but is, for example, 200N from the viewpoint of plate releasability of the mask.

As the binder included in the flux, a binder having low decomposition temperature and adhesiveness is preferable. For example, as the binder, in addition to isobornyl cyclohexanol, isobornyl phenol, and derivatives thereof, polybutene or the like having a number average molecular weight of 700 or more and 1500 or less can be used. The reason why the number average molecular weight of polybutene is 700 or more and 1500 or less is as follows: if the number average molecular weight is less than 700, the viscosity becomes less than 1Pa · s and the adhesive effect is low, and the printability of the solder paste is lowered, and if the number average molecular weight exceeds 1500, the heat resistance becomes high and residues are likely to remain.

As described above, in the present embodiment, since the adhesive is not composed of rosin, dimer acid, polybutene having a number average molecular weight of more than 1500, and the like, the reduction rate at 300 ℃ in the thermogravimetric measurement becomes 90 mass% or more, and the residue remaining on the solder bump is suppressed, and the adhesiveness is ensured.

Further, since the flux of the present embodiment does not contain rosin or the like as a main component as described above, the effect of reducing and removing oxides to wet the substrate and coating solder bumps to prevent reoxidation is poor. The function of these rosins is compensated by formic acid gas, which will be described later.

The composition of the flux includes, for example, 19 to 60% by mass of a solvent, 30 to 80% by mass of a binder, and 1.0 to 10% by mass of a thixotropic agent. When the solvent is less than 19 mass%, the solder paste is hard to be in a paste state, and when the solvent exceeds 60 mass%, the shape retention of the solder paste (hereinafter, referred to as a bump precursor) in a state of being printed and coated on a substrate becomes poor. If the thixotropic agent is less than 1.0 mass%, the shape retention of the solder paste is poor, and if it exceeds 10 mass%, the solder paste becomes too hard. If the binder is less than 30 mass%, the slurry may not be formed, or the slurry may become dry and the paste may not be applied to a substrate or the like.

On the other hand, if the binder exceeds 80 mass%, the viscosity of the solder paste becomes too high, the adhesive force becomes too high, the scratch property during printing deteriorates, and the shape deteriorates during application by a dispenser or during pin transfer. In the preferred formulation of the flux, the binder is 35 mass% or more and 80 mass% or less, the thixotropic agent is 2 mass% or more and 6 mass% or less, and the balance is a solvent. Further, in a more preferable formulation of the flux, the binder is 35 mass% or more and 70 mass% or less, the thixotropic agent is 2.5 mass% or more and 5.5 mass% or less, and the balance is a solvent.

In addition, when a large amount of an activator is contained in the flux for solder paste, the solder powder in the preparation of the solder paste reacts with the flux to cause a large viscosity change, and therefore the solder paste can be stored for only several months even in a refrigerated storage. Therefore, in the present embodiment, the flux for solder paste does not contain an active agent.

[ constitution of solder paste ]

The solder paste is a mixture obtained by mixing the above-described flux and solder powder, and the content of the flux is set to 30 vol% or more and 90 vol% or less. When the content of the flux for solder paste is less than 30 vol%, the flux may not form a slurry or may become a dry slurry, and the solder paste may not be applied to a substrate or the like by printing. On the other hand, if the content of the flux for solder paste exceeds 90 vol%, the viscosity of the solder paste becomes too high, the adhesive force becomes too high, the scratch property during printing deteriorates, and the shape deteriorates during application by a dispenser or during pin transfer. Therefore, in the present embodiment, the content of the flux is set to 30% by volume or more and 90% by volume or less. The content of the flux is preferably 40% by volume or more and 90% by volume or less.

Thereby, the viscosity of the solder paste becomes 0.4 pas or more, and the adhesive force of the solder paste becomes 0.8N or more.

Examples of the solder powder include Sn-Ag-Cu solder powder, Sn-Ag solder powder, Pb-Sn solder powder, Au-Sn solder powder, and Au-Ge solder powder. Further, the average particle diameter of the solder powder is, for example, in the range of 0.1 to 30.0 μm, whereby the paste filling property to the mask opening and the shape retaining property of the bump precursor can be improved.

In order to form bumps with a narrow pitch, the average particle size of the solder powder is preferably in the range of 0.1 to 10.0 μm.

[ method for Forming solder bumps ]

Next, a method of forming a solder bump using a solder paste will be described.

The forming method comprises: a printing step of printing a solder paste; and a reflow step of heating the solder paste in a formic acid gas atmosphere. The printing step and the reflow step are explained in detail in this order below.

(printing Process)

In the printing step, a mask having an opening is disposed on a substrate such as a silicon wafer or a glass epoxy substrate, and a solder paste is printed and applied so that the opening is filled with the solder paste. After the print coating, the mask is peeled off from the substrate, and a bump precursor is formed on the substrate. The solder paste is printed and applied, but may be discharged and supplied from a dispenser or the like, or may be transferred by a needle of a needle transfer device or the like.

In this case, since the binder is a solid or a liquid having a viscosity of 1Pa · s or more at normal temperature, the shape retention of the solder paste composed of the flux for solder paste and the solder powder including the binder can be ensured. Further, since the content of the flux for solder paste is 30 vol% or more and 90 vol% or less, a solder paste having appropriate viscosity and adhesion can be formed, and deterioration of scratch resistance during printing and the like can be suppressed.

(reflow Process)

In the reflow step, first, the bump precursor formed on the substrate is heated for 30 seconds to 2 minutes in a formic acid gas atmosphere at a temperature lower than the melting point of the solder powder (preheating step)) The solvent in the flux, which is a void source, is volatilized. Each heating process under the formic acid gas atmosphere is performed as follows: at normal temperature, N is reacted₂Bubbling in 99% pure formic acid to generate a solution of formic acid in N₂The gas in the gas will dissolve the N of the formic acid₂Gas is supplied into the furnace. N in which formic acid is dissolved₂The formic acid concentration of the gas (formic acid gas) is set to, for example, about 3 vol%. Alternatively, formic acid gas atmosphere may be generated by placing formic acid in the furnace.

Thereafter, the solder powder is heated at a temperature higher than the melting point of the solder powder, for example, at a temperature of +30 ℃ for 10 seconds to 1 minute (main heating step), and the solder powder is melted. At this time, formic acid reacts with a metal oxide such as Sn contained in the solder powder to generate a formate, and then the formate is reduced by formic acid by further heating. When each heating step is performed in such a formic acid gas atmosphere, the oxide film such as solder powder is reduced by the reducing power of formic acid. Then, when the molten solder is cooled, a solder bump having a substantially hemispherical shape is formed by surface tension.

In this case, in the present embodiment, since the reduction rate at 300 ℃ in the thermal weight change measurement of the flux for solder paste is 80 mass% or more, and the reduction rate at 300 ℃ in the thermal weight change measurement of the binder is 90 mass% or more, when reflow soldering is performed on a solder paste including the flux for solder paste, the flux contains a large amount of volatile components, and therefore, even when reflow soldering is performed on a bump precursor to form a solder bump, generation of residue can be suppressed, and the step of cleaning the solder bump can be omitted.

Further, since the reflow process is performed in the formic acid gas atmosphere, even if the rosin that reduces and removes the oxide is not contained in the flux constituting the solder paste, the solder powder or the oxide film on the substrate can be reduced by the reducing force of the formic acid gas, and the solder melting can be performed smoothly.

Further, since the generation of the residue is suppressed by using the flux for solder paste of the present embodiment, the residue can be prevented from being scattered at the time of reflow even if the solder powder is an Au — Sn solder powder having a high melting point.

Further, since the acid value of the flux for solder paste is set to 100mgKOH/g or less, the generation of voids in the solder bump can be suppressed, and in addition, the solder paste can be stored for a long period of time (for example, 6 months or more), and the long-term storage property can be improved.

The present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

In the above embodiment, the flux is composed of only the binder, the solvent, and the thixotropic agent, but is not limited thereto, and the flux may contain a trace amount of the rosin and the active agent as long as the acid value is 100mgKOH/g or less, the rate of decrease at 300 ℃ in the thermogravimetric measurement is 80 mass% or more, the viscosity is 0.5Pa · s or more, and the adhesion is 1.0N or more. For example, the flux may contain the rosin in an amount of 10 mass% or less and the active agent in an amount of 0.1 mass% or less.

In the above embodiment, the preheating step and the main heating step are performed in the reflow step, and the heating temperature is raised stepwise in two stages, but the present invention is not limited thereto, and only the main heating step may be performed. Further, the heating temperature may be increased stepwise in three or more stages.

Further, although the above embodiment has described the method for producing a solder bump using the solder paste, the solder paste of the present invention is not limited to this, and the solder paste of the present invention may be used for a method for producing a joined body in which the joined body and the joined body are joined by solder by being placed between the joined body and being heated in a formic acid gas atmosphere.