TWI686259B - Flux for solder paste, solder paste, method of forming solder bump using solder paste, and method of producing bonded body - Google Patents

Abstract

The present invention is directed to a flux for a solder paste. The flux includes a viscous agent, a solvent and a thixotropic agent. The acid value of the flux is 100mgKOH/g or less. In the thermogravimetric measurement, the reduction rate at 300ºC is 80 mass% or more. The flux for solder paste has a viscosity of 0.5 Pa·s or more and a tacking force of 1.0 N or more.

Description

Method for forming soldering flux for solder paste, solder paste, solder bump using solder paste, and method for manufacturing joint body

The present invention relates to a method of forming solder paste flux, solder paste, and solder bumps using solder paste that can be used when joining electronic components to a substrate, etc., and a method of manufacturing a bonded body. This case is based on the Japanese Patent Application No. 2017-147084 filed in Japan on July 28, 2017 and the Japanese Patent Application No. 2018-133872 filed on July 17, 2018 in Japan. This invokes the content.

Previously, solder pastes composed of flux and solder powder containing rosin, solvents, thixotropic agents, and active agents were used for soldering. In the case of using such solder paste to form solder bumps, if the solder paste is applied on the substrate and reflowed, residues based on rosin etc. will remain on the solder, so this residue may be caused by The necessity of washing medicines and so on is troublesome. In order to save the workload of cleaning such residues, fluxes that do not generate residues after soldering and solder pastes containing such fluxes are known (refer to Patent Document 1). The flux described in this Patent Document 1 contains ammonium formate and an aliphatic polyhydric alcohol which is a liquid at normal temperature and has a boiling point of 150° C. or higher at atmospheric pressure, so it has reducibility and efficiently reduces the oxide film to remove the generated It is used for the oxide coating on the substrate. The structure of Patent Document 1 is to suppress the residue of flux after soldering electronic parts and the like to the substrate.

However, the flux described in Patent Document 1 and the solder paste containing the flux are decomposed at a low temperature, so the solder surface is exposed without being covered by the flux. Therefore, there is a concern that the surface of the solder is re-oxidized.　　 For this, a method of forming solder bumps that can suppress the reoxidation of the surface of the solder is proposed (see Patent Document 2).

The method of forming a solder bump described in this Patent Document 2 is to arrange a mask having an opening on a substrate, and print solder paste in the opening by filling a solder paste containing flux and solder powder, in After peeling off the mask, the bump precursor on the substrate is reflowed to form a solder bump. The flux contains rosin, solvent and thixotropic agent. The acid value of the flux is 100mg KOH/g or less. The halogen content of the flux is 0.03% by mass or less, and it is configured to perform the reflow treatment in a formic acid gas environment and/or in an environment in which formic acid is decomposed by heat. Here, the method of forming solder bumps is to melt and smooth the solder by performing the reflow process under the formic acid gas environment and/or the environment where the formic acid gas is decomposed by heat, reducing the oxide film of the solder powder, etc. . [Prior Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. 2011-83809 (A) 　　 [Patent Document 2] Japanese Patent Laid-Open No. 2016-78095 (A)

[Problems to be solved by the invention]

However, the method of forming solder bumps described in Patent Document 2 uses a flux containing rosin as the main component, so if a solder paste containing solder flux used in this forming method is reflowed, a rosin-based Residue, so it is necessary to clean the solder. In addition, even if the solder is cleaned, the residue may not be completely removed. In this case, there is a concern that the adhesion between the electronic component and the substrate is low.

The present invention is made in view of such circumstances, and an object of the present invention is to provide a method for forming a solder paste flux, a solder paste, a solder bump using solder paste, and a method for manufacturing a bonded body that can obtain solder that can suppress generation of residues . [Means to solve the problem]

The flux for solder paste of one aspect of the present invention (hereinafter, referred to as "flux for solder paste of the present invention") is a flux for solder paste containing a thickener, a solvent, and a thixotropic agent, and the acid value is 100 mg KOH /g or less, the reduction rate at 300° C. in thermogravimetric measurement is 80% by mass or more, the viscosity is 0.5 Pa･s or more, and the tacking force is 1.0 N or more.　　Still, the adhesive force system means that the adhesive strength to the substrate when the solder flux is applied to the substrate, etc., is measured in accordance with JISZ 3284.

In the present invention, the flux for solder paste is measured by thermogravimetry, and the reduction rate at 300° C. is 80% by mass or more. Therefore, when the solder paste containing the solder paste flux is reflowed, it is because of the volatile content of the flux Many, therefore, in the case where the solder paste containing the solder paste flux is reflowed to form solder bumps, the generation of residues can also be suppressed. If the viscosity of the flux for solder paste is less than 0.5 Pa･s, the viscosity of this flux is too small to constitute a solder paste, and there is a concern that it cannot be applied to a substrate. Furthermore, if the adhesive strength of the solder paste flux is less than 1.0N, the adhesive strength is low, so when the solder paste containing this flux is applied to a substrate, etc., the applied solder paste may come off from the substrate, etc. doubt. In this regard, the viscosity of the flux of the present invention is 0.5 Pa･s or more, and because the adhesive force is set to 1.0 N or more, it is possible to ensure that the shape of the solder paste composed of the solder paste flux and the solder powder is maintained Sex.

In addition, by setting the acid value of the flux for solder paste to 100 mg KOH/g or less, the reaction of the slightly remaining flux residue with the surrounding metal parts, such as copper of wiring, etc. can be suppressed, corrosion can be suppressed, and bonding can be ensured The long-term reliability of the body. In addition, it suppresses the reaction between the solder powder and the flux when it is set as a solder paste. Since the viscosity of the solder paste changes less, it is possible to store the solder paste that usually needs to be stored in the refrigerator at room temperature for a long period of time (for example, 6 (More than months) storage, can improve long-term storage. In addition, if the acid value exceeds 100 mg KOH/g, the generation of reduced water due to the reduction reaction of the solder paste flux and the oxide of the solder powder will increase, because the pores in the solder bumps will increase, so the above The acid value is set to 100 mg KOH/g or less. It is preferred that the flux for solder paste of the present invention does not contain the rosin or active agent contained in the ordinary flux, but it may also be slightly contained if it meets the range of the requirements of the above thermogravimetric measurement and acid value .

As a preferred aspect of the solder paste flux of the present invention, the content of rosin is preferably 10% by mass or less.　　 Even if the amount of rosin is contained in the flux for solder paste, if the amount is less than 10% by mass, the acid value of the flux for solder paste can be 100 mg KOH/g or less. That is, it is okay if the flux for solder paste contains rosin in the range of 10% by mass or less.

The solder paste of another aspect of the present invention (hereinafter, referred to as "solder paste of the present invention") is formed by mixing the above-mentioned solder paste flux and solder powder. Since the solder paste of the present invention contains the above-mentioned flux for solder paste, even when the solder paste is reflowed, the generation of residues can be suppressed, and the deterioration of the adhesion caused by the residues can be suppressed.

As a preferable aspect of the solder paste of the present invention, the content of the flux for the solder paste is preferably 30% by volume or more and 90% by volume or less.　　When the content of the flux for the solder paste is less than 30% by volume, the paste cannot be formed or becomes a dry paste, and there is a concern that the solder paste cannot be applied to the substrate. On the other hand, if the content of the solder paste flux exceeds 90% by volume, the viscosity of the solder paste becomes too low, the coating performance deteriorates, and the flux and the solder powder may be easily separated. On the other hand, in the above-mentioned aspect, because the content of the flux for the solder paste is 30% by volume or more and 90% by volume or less, the solder paste with appropriate viscosity can be formed, and the deterioration of the coating performance can be suppressed, or the flux and Separation of solder powder.

A preferred aspect of the solder paste of the present invention is that the aforementioned solder powder is Sn-Ag-Cu solder powder, Sn-Cu solder powder, Sn-Ag solder powder, Pb-Sn solder powder, Au-Sn solder powder, Au -Either Ge solder powder or Au-Si solder powder is preferred.　　 If it is the above-mentioned aspect, any of the above-mentioned various powders can be used as the solder powder. In particular, in the case where the solder powder is Au-Sn solder powder, the solder paste containing Au-Sn solder powder is a high-melting-point solder paste, so it is easy to generate flying residues during reflow, but because of the use of the above The flux for solder paste suppresses the generation of residues, so it can prevent the residues from flying away during reflow.

In another aspect of the present invention, a method of forming solder bumps using solder paste (hereinafter, referred to as "a method of forming solder bumps using the solder paste of the present invention") is to arrange a mask having an opening on a substrate Printing the solder paste in the opening so as to fill the solder paste, and after peeling off the mask, the bump precursor on the substrate is subjected to a reflow process in a formic acid gas environment to form solder bumps. With such a configuration, the generation of the above-mentioned residues can be suppressed, and the solder bump cleaning step can be omitted. In addition, because the reflow process is performed in a formic acid gas environment, even if the rosin that performs reduction to remove oxides is not included Also, the oxide film on the substrate and the oxide film on the surface of the solder are reduced to melt and smooth the solder.

The manufacturing method of a bonded body using solder paste in another aspect of the present invention (hereinafter, referred to as "the manufacturing method of a bonded body using the solder paste of the present invention") is a manufacturing method of a bonded body using solder paste, and The solder paste is arranged between the object and the object to be joined, and the object to be joined and the object to be joined are welded by heating in a formic acid gas environment. With such a configuration, in order to suppress the generation of the above-mentioned residue, the step of cleaning the joint can be omitted, and because the reflow process is performed in a formic acid gas environment, even if rosin that does not reduce oxides is included, It also reduces the oxide film on the bonding object and the bonded object and the oxide film on the surface of the solder to melt and smooth the solder, so that the bonding object and the bonded object can be bonded more strongly. There is no particular limitation on the method of disposing the solder paste. For example, it can be arranged by a printing method, coating by a dispenser, or needle transfer. In addition, the bonded object system is, for example, a substrate, and the bonded object system is, for example, a semiconductor element such as an LED element. [Effect of invention]

In the solder paste flux, solder paste, solder bump using solder paste forming method and joint manufacturing method of the present invention, generation of residue and reoxidation of the solder surface can be suppressed.

Hereinafter, a method of forming solder bumps, solder paste, and solder bumps using solder paste according to the present invention will be described. The flux for solder paste of the present embodiment (hereinafter, sometimes referred to simply as flux) is a flux containing a thickener, a solvent, and a thixotropic agent, and has an acid value of 100 mg KOH/g or less, in thermogravimetric measurement The reduction rate at 300°C is 80% by mass or more, the viscosity is 0.5 Pa･s or more, and the adhesive force is 1.0 N or more.　　The composition of the flux, the acid value, the reduction rate at 300°C measured by thermogravimetry, the viscosity, and the adhesive force will be explained for the reasons specified above.

[Structure of Flux] 　　 Flux consists of a thickener, a solvent and a thixotropic agent. In addition, in this embodiment, the flux system is composed only of a thickener, a solvent, and a thixotropic agent. The acid value of the flux is set to 100 mg KOH/g or less. By setting the acid value to 100 mg KOH/g or less, the reaction between the solder powder and the flux when the solder paste is set is suppressed, and the viscosity change as a solder paste becomes less. In addition, if the acid value exceeds the above value, the generation of reduced water due to the reduction reaction between the flux and the oxide of the solder powder increases, and the voids in the formed solder bumps increase. Still, the acid value of the flux is preferably 50 mg KOH/g or less, and more preferably 10 mg KOH/g or less. There is no particular limit to the lower limit of the acid value, and it may also be below the detected lower limit of detection, for example, 0.01 mg KOH/g.　　 Furthermore, in the thermogravimetric measurement, the reduction rate of the flux system at 300°C is 80% by mass or more. This is because if the reduction rate at 300°C by thermogravimetric measurement is less than 80% by mass, the amount of residue increases when the solder paste containing flux is reflowed. In addition, the reduction rate of the flux system at 300°C in thermogravimetric measurement is preferably 85% by mass or more, and more preferably 90% by mass or more. The upper limit of the amount of reduction is not particularly limited, but as a value when a generally available thickener is used, for example, it is 99.5% by mass.

As the solvent system included in the flux, solvents such as alcohols, ketones, esters, ethers, aromatic systems, hydrocarbons, terpene systems, terpene systems, and the like can be used. Specifically, benzyl alcohol, ethanol, ethyl alcohol, isopropyl alcohol, butanol, diethylene glycol, ethylene glycol, ethyl cellosolve, butyl cellosolve, butyl carbitol, isopropanol , Ethyl acetate, butyl 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, diisopropyl adipate Butyl ester, hexanediol, cyclohexanedimethanol, 2-terpineoloxyethanol, 2-dihydroterpineoloxyethanol, citral, linalool, limonene, carvacrol, pinene, farnes Alkenes are used alone or in combination.

In addition, as the thixotropic agent system included in the flux, hardened castor oil, hydrogenated castor oil, carnauba wax, amide, hydroxy fatty acids, dibenzylidene sorbitol, bis(p-methylbenzylidene ) Sorbitol, beeswax, stearic acid amide, hydroxystearic acid ethyl bis bisamide, used alone or in combination. Furthermore, fatty acids such as caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, hydroxy fatty acids such as 1,2-hydroxystearic acid, antioxidants, and interfaces are added as necessary. Used as active agents, amines, etc.

The viscosity of the flux is 0.5Pa･s or more. If the viscosity of the flux is less than 0.5 Pa･s, the viscosity of the flux is too small to constitute a solder paste, and there is a concern that it cannot be applied to the substrate. Still, the viscosity of the flux is preferably 1.0 Pa･s or more. Still, the viscosity is the viscosity at room temperature (25°C). The upper limit of the viscosity is not particularly limited, but from the viewpoint of coatability, etc., for example, it is 100 Pa･s.　　Furthermore, the adhesion of flux is 1.0N or more. If the adhesive strength of the flux is less than 1.0N, the adhesive strength is low, so that when the solder paste containing this flux is applied to a substrate or the like, the applied solder paste may fall off from the substrate or the like. Still, the adhesion of the flux is preferably 1.2 N or more. Still, the adhesive force is the value at room temperature (25 ℃). The upper limit of the adhesive force is not particularly limited, but it is 100N, for example, from the standpoint of the releasability of the mask.

[Structure of Viscosity Agent] 　　 Viscosity agent is a solid at room temperature (25℃) or a liquid with a viscosity of 1Pa･s or more. This is because if the viscosity is a liquid with a viscosity of less than 1 Pa･s at room temperature, the viscosity of the flux is too small to constitute a solder paste, and there is a concern that it cannot be applied to a substrate. Such a viscosity agent has a viscosity of 1 Pa･s or more at room temperature, or is solid at room temperature, so a solder paste composed of flux and solder powder containing the viscosity agent has shape retention. The upper limit of the viscosity is not particularly limited, but from the viewpoint of coatability, etc., for example, it is 400 Pa･s.　　Furthermore, the reduction of the viscosity agent at 300°C in thermogravimetric measurement is 90% by mass or more. This is because most of the flux components are viscous, so if the reduction rate at 300°C in thermogravimetric measurement does not reach 90% by mass, the reduction rate at 300°C in thermogravimetric measurement of flux cannot be As more than 80%. There is no particular limitation on the upper limit of the reduction rate, and a thickener with a reduction of 100% by mass can also be used.　　 The viscosity of this thickener is set to 1.1 N or more. As described above, since most of the flux components are thickeners, if the adhesive strength of the thickener is less than 1.1 N, the adhesive strength of the flux cannot be made 1.0 N or more. The upper limit of the adhesive force is not particularly limited, but it is 200N, for example, from the standpoint of the release of the mask.

The viscous system contained in the flux has a low decomposition temperature and is preferably viscous. For example, as the viscous agent, in addition to isobornylcyclohexanol, isobornylphenol, and derivatives thereof, polybutene having a number average molecular weight of 700 or more and 1500 or less can be used. Still, the number average molecular weight of polybutene is set to 700 or more and 1500 or less because if the number average molecular weight is less than 700, the viscosity becomes less than 1 Pa･s and the viscosity effect is low, and the printability of the solder paste is low, because if the quantity If the average molecular weight exceeds 1500, the heat resistance becomes high and it tends to remain as a residue. In this way, in this embodiment, since the viscosity agent is not composed of rosin, dimer acid, and polybutene having a number average molecular weight of more than 1500, the reduction rate at 300°C by thermogravimetric measurement becomes 90% by mass or more , While maintaining the residual residue on the solder bumps, while ensuring the viscosity.　　Further, the flux of the present embodiment does not contain rosin or the like as the main component as described above, so it reduces and removes the oxide to infiltrate the underlying layer, and lacks the effect of covering the solder bumps to prevent reoxidation. Such rosin-like functions are filled by formic acid gas described later.

The composition of such flux is, for example, a solvent of 19% by mass to 60% by mass, a viscosity of 30% by mass to 80% by mass, and a thixotropic agent of 1.0% by mass to 10% by mass. When the solvent is less than 19% by mass, it is difficult for the solder paste to become a paste. If the solvent exceeds 60% by mass, the solder paste is printed on the substrate (hereinafter referred to as a bump precursor) The shape retention property becomes poor. When the thixotropic agent is less than 1.0% by mass, the shape retention of the solder paste becomes poor, and if it exceeds 10% by mass, the solder paste becomes too hard. In addition, when the viscosity agent is less than 30% by mass, the paste cannot be formed, or it becomes a dry paste, and there is a concern that the solder paste cannot be applied to the substrate. On the other hand, if the viscosity exceeds 80% by mass, the viscosity of the solder paste becomes too high, or the adhesion becomes too high, the scraping property during printing deteriorates, or the coating is caused by the dispenser Do not worry about the shape deterioration at the time or needle transfer. The preferred composition of the flux composition is that the viscosity is 35% by mass or more and 80% by mass or less, the thixotropic agent is 2% by mass or more and 6% by mass or less, and the remainder is the solvent. Furthermore, the composition of a further preferable flux is a thickener of 35% by mass or more and 70% by mass or less, a thixotropic agent of 2.5% by mass or more and 5.5% by mass or less, and the remainder being a solvent.　　Still, if a large amount of active agent is contained in the solder paste flux, the solder powder when used as a solder paste reacts with the flux, and the viscosity change becomes large, so it can only be stored for a few months even if it is refrigerated. Therefore, in this embodiment, the flux system for solder paste does not contain an active agent.

[Structure of Solder Paste] 　　 Solder paste is a mixture of the above flux and solder powder, and the flux content is set to 30% by volume or more and 90% by volume or less. When the content of the flux for the solder paste is less than 30% by volume, the paste cannot be formed, or it becomes a dry paste, and there is a concern that the solder paste cannot be printed and applied to the substrate. On the other hand, if the content of the flux for solder paste exceeds 90% by volume, the viscosity of the solder paste becomes too high, or the adhesive force becomes too high, and the scraping property during printing deteriorates, or There is a concern that the shape of the dispenser will deteriorate when it is applied or when the needle is transferred. Therefore, in this embodiment, the flux content is set to 30% by volume or more and 90% by volume or less. Still, the content of the flux is preferably 40% by volume or more and 90% by volume or less.　　 As a result, the viscosity of the solder paste becomes 0.4 Pa･s or more, and the adhesive force of the solder paste becomes 0.8 N or more.

In addition, examples of the solder powder system include Sn-Ag-Cu solder powder, Sn-Cu solder powder, Sn-Ag solder powder, Pb-Sn solder powder, Au-Sn solder powder, and Au-Ge solder powder. In addition, the average particle diameter of the solder powder is, for example, in the range of 0.1 to 30.0 μm, thereby improving the paste filling property to the opening of the mask and the shape retention property of the bump precursor. In order to form the bumps as a narrow pitch, the average particle diameter of the solder powder is preferably in the range of 0.1 to 10.0 μm.

[Method of forming solder bumps] Next, a method of forming solder bumps using solder paste will be described.　　 This forming method includes a printing step of printing solder paste, and a reflow step of heating the solder paste in a formic acid gas environment. Hereinafter, the printing step and the reflow step will be described in detail in order.

(Printing step) 　　In the printing step, a mask having an opening is arranged on a substrate such as a silicon wafer or a glass epoxy substrate, and the opening is filled with solder paste to print and apply the solder paste. After this printing and coating, the mask is peeled off from the substrate to form a bump precursor on the substrate. Although the solder paste is applied for printing, it can also be supplied by discharge by a dispenser or the like, and also by needle transfer by a needle transfer device or the like.　　In this case, because the thickener is a liquid that is solid at room temperature or has a viscosity of 1 Pa･s or more, the shape retention of the solder paste composed of the solder paste flux containing the thickener and the solder powder can be ensured. In addition, since the content rate of the solder paste flux is 30% by volume or more and 90% by volume or less, it is possible to construct a solder paste with appropriate viscosity and adhesion, and it is possible to suppress the deterioration of scraping properties during printing.

(Reflow step) The reflow step is first used as preliminary heating. In the formic acid gas environment, the bump precursor formed on the substrate is heated at a temperature lower than the melting point of the solder powder for 30 seconds to 2 minutes (pre- Thermal step) to evaporate the solvent of the pore source in the flux. Each heating step in this formic acid gas environment generates a gas in which formic acid is dissolved in N ₂ gas by ventilating N ₂ to 99% purity formic acid at normal temperature, and this N ₂ gas dissolved in formic acid is supplied to Implemented in the furnace. The formic acid concentration of the N ₂ gas (formic acid gas) in which formic acid has been dissolved is set to, for example, about 3% by volume. Still, the formic acid gas environment can also be generated by formic acid being built into the furnace. After that, the solder powder is melted by heating at a temperature higher than the melting point of the solder powder, for example, at a temperature of the melting point of the solder powder + 30°C for 10 seconds to 1 minute (formal heating step). At this time, the formic acid system reacts with metal oxides such as Sn contained in the solder powder to generate formate, and then the temperature is increased to formate is reduced by formic acid. In such a manner, under the formic acid gas environment, if each heating step is performed, the oxide film of the solder powder or the like is reduced by the reducing power of formic acid. Furthermore, if the molten solder is cooled, a slightly hemispherical solder bump is formed by surface tension.

In this case, in this embodiment, the flux for solder paste is measured by thermogravimetry, and the reduction rate at 300°C is 80% by mass or more. Among them, the reduction amount at 300°C is especially when the viscosity is measured by thermogravimetry. 90% by mass or more, so when the solder paste containing the flux for solder paste is reflowed, the volatile content of the flux is large, and the bump precursor is reflowed to form a solder bump, which can also suppress the generation of residue , The solder bump cleaning step can be omitted. In addition, because the reflow process is performed in a formic acid gas environment, even if the rosin that reduces and removes oxides is not included in the flux constituting the solder paste, the solder powder or the substrate can be reduced by the reducing power of the formic acid gas The coating is oxidized to melt and smooth the solder.

In addition, since the generation of residues is suppressed by using the flux for solder paste of the present embodiment, even if the solder powder is a high-melting point Au-Sn solder powder, it is possible to prevent the scattering of the residues during reflow. Furthermore, because the acid value of the solder paste flux is set to 100 mg KOH/g or less, in addition to suppressing the generation of voids in the solder bumps, the solder paste can be stored for a long period of time (for example, more than 6 months ), can improve long-term storage.

However, the present invention is not limited to the above-mentioned embodiment, and various changes can be made within the scope that does not deviate from the gist of the present invention. In the above-mentioned embodiment, the flux is composed only of a thickener, a solvent, and a thixotropic agent, but it is not limited to this. If the acid value is 100 mg KOH/g or less, the reduction rate at 300° C. by thermogravimetric measurement is 80 Within the range of mass% or more, the viscosity is 0.5 Pa･s or more, and the adhesive force is 1.0 N or more, the flux may also contain a small amount of rosin and active agent. For example, if it is 10% by mass or less in the case of rosin and 0.1% by mass or less in the case of the active agent, then this flux may be included. In the above embodiment, the preheating step and the main heating step are performed in the reflow step to increase the heating temperature in two stages, but it is not limited to this, and only the main heating step may be performed. In addition, the heating temperature may be increased stepwise in three or more stages. Furthermore, the above embodiment describes the method of manufacturing solder bumps using solder paste, but it is not limited to this. The solder paste of the present invention can also be used in a method of manufacturing a bonded body, which is arranged in a bonded object Between the object and the object to be joined, the object to be joined is welded by heating in a formic acid gas environment. [Example]

Various conditions were changed, and solder paste was produced at the same time. Experiments were conducted on the amount of residue after solder reflow obtained from the solder paste, the long-term storage property and shape retention of the solder paste. The obtained samples of Examples 1 to 9 and Comparative Examples 1 to 6 will be described while referring to Tables 1 and 2.

In addition, in Table 1, the reduction rate at 300°C (hereinafter referred to as 300°C TG reduction in Table 1) measured by the thermogravimetric measurement of individual fluxes and thickeners is measured using a general thermogravimetric measuring device . For example, in the thermogravimetric measurement of flux, the weight change of 10 mg of flux in a N ₂ environment is measured by raising the temperature by 10° C./min and increasing the temperature from room temperature (25° C.) to 300° C. The above-mentioned reduction rate. The thickener is the same. In addition, the viscosity of the flux and the thickener is measured in accordance with JIS Z 8803. In addition, Table 1 shows the viscosity measured according to the above-mentioned JIS Z 8803 for the liquid at normal temperature (25°C), and Table 1 shows the solid for the solid at normal temperature. Furthermore, the acid value of the flux is measured in accordance with JIS Z 3197. Still, those whose acid value is too small and exceeds the detection limit are indicated below the detection limit.

By mixing the solder powder and the flux shown in Table 1 in the proportions shown in Table 1, solder paste was produced. In addition, the flux is prepared by mixing a thickener, a thixotropic agent, a solvent, and an active agent. The thickener, thixotropic agent, solvent, and active agent are as described in Table 1, and the thixotropic agent is 5 mass. %, set the remaining part as the solvent.　　 The evaluation of the presence or absence of residue after reflow, the long-term storage of solder paste and shape retention is carried out by the following method.

(The presence or absence of residue after reflow) Using a 200 μm thick stencil mask for printing on the surface of the copper plate, the solder pastes of each of Examples 1 to 9 and Comparative Examples 1 to 6 were printed and applied to a circle with a diameter of 6.5 mm. The template mask for printing is removed from the copper plate. In this way, for each sample, a circular bump precursor was formed on the copper plate and used as the evaluation substrate. Next, the substrate for evaluation is heated in a formic acid gas environment, and the bump precursor is subjected to a reflow process. At this time, the peak temperature is set to the melting point of each solder powder + 30°C, and the heating time is set to 1 minute. In this acid-based heating atmosphere of N ₂ by ventilation at room temperature to produce the carboxylic acid dissolved gas in N ₂ gas, this acid was dissolved in N ₂ gas supplied to the furnace and implementation. The concentration of formic acid in the N ₂ gas (formic acid gas) into which formic acid has been dissolved is set to about 3% by volume. Then, using an electron microscope (SEM), observe the bump precursors (solder bumps) that have been reflowed, and determine that the area of the residue is less than 30% of the coating area of the solder paste, and the residue covers the solder A case where 30% or more of the application area of the paste is judged as defective.

(Long-term storage properties of solder paste) After the paste was stored at room temperature for 6 months, the paste was used to prepare a substrate for evaluation in the same manner as described above, and then the reflow process was performed to form solder bumps. After that, observation with an optical microscope determined that the amount of unmelted powder of the solder powder was the same as that immediately after the solder paste was produced, and the amount of unmelted powder of the solder powder was increased more than that after the solder paste was produced. The situation is judged to be bad.

(Shape retention) 　　The solder paste was applied on the evaluation substrate with a mask having a pitch of 200 μm, an opening diameter of 120 μm, and a thickness of 25 μm to form 100 bump precursors. Evaluate the substrate with 100 bump precursors visually or observe with an optical microscope. Of the 100 solder bump precursors, the omission is less than 5 places, and the bridge caused by printing collapse is less than 5 places. The case where the defect of the block precursor is 5 places or less is judged to be good, and the case where any one of the omissions, bridges, and defects is generated in more than 5 places is judged to be bad.

As can be understood from Tables 1 and 2, Comparative Examples 1, 3, and 6 are based on the thermogravimetric determination of flux at 300°C and the amount of reduction is as low as 60% by mass or less, so the area generated by reflow residues exceeds solder paste. 30% of the coating area, there are many residues after reflow. On the other hand, in Examples 1 to 9, the reduction rate of the flux at 300°C in thermogravimetric measurement was 80% by mass or more, so there was little residue after reflow. Also, in Comparative Examples 2, 4 and 5, the reduction rate at 300°C was also 80% by mass or more in the thermogravimetric measurement of the flux, so there was little residue after reflow. Therefore, it can be understood that in the thermogravimetric measurement of the flux, the reduction at 300° C. is 80% by mass or more, which is an effective range.

In addition, in Comparative Examples 1, 2 and 5, the acid value of the flux is as high as 200 to 1500 mg KOH/g, so the long-term storage property is poor. On the other hand, in Examples 1-9, since the acid value of the flux is 100 mg KOH/g or less, the solder paste has excellent long-term storage properties. In addition, in Comparative Examples 3, 4 and 6, since the acid value of the flux is below the detection limit, the solder paste has excellent long-term storability. Therefore, it can be understood that the acid value of the flux is 100 mg KOH/g or less as an effective range.　　Also, in Example 2, the flux contains 0.1% by mass of the active agent, but since the acid value of the flux is 100 mg KOH/g or less, the long-term storage property is excellent. On the other hand, in Comparative Example 2, the flux contains 0.2% by mass of the active agent, so the acid value of the flux is as high as 200 mg KOH/g, and the long-term storage property is poor. Therefore, it can be understood that if the acid value of the flux is within the above range, an active agent of 0.1% by mass or less may be included.

In Comparative Example 5, the flux contains 40% by mass of polymerized rosin, so the acid value of the flux is as high as 800 mg KOH/g, and the long-term storage property is poor. On the other hand, in Example 8, although the flux system contained 5% by mass of polymerized rosin, it was excellent in long-term storage.　　 Furthermore, in Comparative Example 6, since the flux contains 40% by mass of rosin ester, the reduction at 300°C in thermogravimetric measurement is as low as 60% by mass or less, and there are many residues after reflow. On the other hand, in Example 9, although the flux system contains 10% by mass of rosin ester, the residual after reflow is small.　　 From these facts, it can be understood that, for example, the acid value of the flux and the reduction at 300°C in the thermogravimetric measurement are within the above range, and rosin of 10% by mass or less can also be included.

Furthermore, in Comparative Example 4, the flux has a viscosity as low as 0.3 Pa･s and an adhesive force as small as 0.8 N. Therefore, printing sags and the like are generated, and shape retention is poor. On the other hand, the viscosity of the flux of Examples 1 to 9 is 1 Pa·s or more, and the adhesive force is also 1.0 N or more, so the shape retention property is excellent. On the other hand, Comparative Examples 1 to 3 and 5 series fluxes have a viscosity of 0.5 Pa･s or more and an adhesive force of 1.0 N or more, so they have excellent shape retention. Therefore, it can be understood that the viscosity of the flux is 0.5 Pa･s or more, and that the adhesive force is 1.0 N or more is an effective range. [Industry availability]

It can effectively improve the reliability of electronic components to be bonded to the substrate.

Claims (7)

A soldering flux for solder paste, which contains a thickener, a solvent, and a thixotropic flux, and is characterized in that the acid value is 100 mg KOH/g or less, and 10 mg of the soldering flux is used under an N ₂ environment. When the temperature is increased by 10℃/min, the weight reduction rate when the temperature is increased from 25℃ to 300℃ is 80% by mass or more, and the viscosity is 0.5Pa. s or more, and the tacking force is 1.0 N or more. The flux for solder paste according to claim 1, wherein the content of rosin is 10% by mass or less. A solder paste characterized by mixing flux for solder paste according to claim 1 or 2 with solder powder. The solder paste according to claim 3, wherein the content rate of the solder paste flux is 30% by volume or more and 90% by volume or less. The solder paste according to claim 3 or 4, wherein the aforementioned solder powder is Sn-Ag-Cu solder powder, Sn-Cu solder powder, Sn-Ag solder powder, Pb-Sn solder powder, Au-Sn solder powder, Au- Either Ge solder powder or Au-Si solder powder. A method of forming solder bumps using solder paste is characterized by: A mask having an opening is arranged on the substrate, the solder paste is printed in the opening so as to be filled with the solder paste according to any one of claims 3 to 5, and after the mask is peeled off, the protrusions on the substrate The block precursor is reflowed in a formic acid gas environment to form solder bumps. A method of manufacturing a bonded body using solder paste, characterized in that the solder paste as described in any one of claims 3 to 5 is arranged between the bonded object and the bonded object, and the foregoing is soldered by heating in a formic acid gas environment The bonded object and the aforementioned bonded object.