JP2016159325A - Flux for solder, and solder paste - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flux for a solder, which has little aging change such as a viscosity or reactivity while being stored, and which is excellent in wet expandability at a solder joint time, thereby to suppress the joint failure of the solder.SOLUTION: A solder flux contains: a flux main component of rosin of less than 35 to 70 mass% having a softening point of 70°C to 170°C and an acid value of 150 to 240 mgKOH/g; and a carboxylate ester of less than 0.2 to 5 mass%; and has an acid value of 70 to 110 mgKOH/g.SELECTED DRAWING: None

Description

  The present invention relates to a soldering flux used for soldering and a solder paste using the soldering flux.

  Circuit elements can be mounted on the printed circuit board by using a flow method in which the lower surface of the printed circuit board is immersed in a solder bath, or by adjusting the solder paste (solder flux to the solder alloy powder to an appropriate viscosity) on the printed circuit board. And a reflow method in which solder is heated and melted after the circuit element is placed on the printed circuit element. In either method, it is essential to use solder flux in order to remove the oxide film on the base material surface, prevent oxidation of the base material and solder, and ensure the wettability of the solder surface. Yes.

  Resin flux mainly using rosin, modified rosin, synthetic resin, etc. as the main component is used as the soldering flux used for board mounting. However, since the active force is weak only with the main component, resin flux is usually used. An activator such as an organic acid or an organic halogen compound is added. The carboxyl group in the rosin compound contained in these resin fluxes and the carboxyl group in the organic acid used as the activator remove the metal oxide on the metal surface and contribute to the improvement of solderability. The solder flux may be added with a thixotropy imparting agent, an activator solvent, a dispersion stabilizer, and the like depending on the application.

  In a solder paste using such a solder flux, the solder alloy powder and the solder flux are in a mixed state, so that the carboxyl group in the solder alloy powder and the solder flux may react during storage. . As a result, metal salts are generated in the solder paste, and the viscosity of the solder paste and the reactivity at the time of bonding change over time, so problems such as poor solder paste printing, poor solder wetting and poor bonding Occurs. In particular, the flux for lead-free solder uses a large amount of relatively strong activator to improve the wettability of lead-free solder, which impairs the storage stability of solder paste and the reliability of solder joints. It is easy. Thus, wetting spreadability required for solder flux and solder paste and its reliability and storage stability are in a conflicting relationship, and currently, solder flux that can satisfy these characteristics at the same time. Development is ongoing, but its realization is extremely difficult.

  On the other hand, in Japanese Patent Nos. 3945408 and 4192784, a compound (X) obtained by reacting a carboxylic acid compound with a vinyl ether compound, obtained by reacting a carboxylic anhydride compound with a hydroxy vinyl ether compound. At least one compound having a blocked carboxyl group selected from the group consisting of compound (Y) and a compound (Z) obtained by addition polymerization of a reaction product of an acid anhydride and a polyhydric alcohol with a divinyl ether compound A non-hardening solder flux containing (A) has been proposed.

  In these solder fluxes, normally, the carboxyl group of the carboxylic acid compound constituting the main component of the flux is blocked with a protective group such as vinyl ether, and the blocked carboxylic acid from which the protective group is released from the carboxyl group only during heating is Used as the main component of flux. Therefore, in the solder paste using this soldering flux, the reaction between the carboxyl group and the solder powder alloy is suppressed during storage, and the carboxyl group exhibits its function only at the time of soldering. Stability has been improved. However, the solder paste using these solder fluxes does not have sufficient wettability.

  On the other hand, in Japanese Patent No. 5387844, a flux for lead-free solder containing a specific polyamide resin and rosin esters and / or polymerized rosin and having an acid value of 40 mgKOH / g to 100 mgKOH / g in the flux composition Has been proposed. Since the solder paste using this soldering flux is excellent in wetting and spreading properties, it is possible to suppress heating and solder balls. However, in this solder paste, since a polyamide resin is used as an activator, a residue of the polyamide resin is likely to occur at the time of solder joining, and there is a high possibility that joint failure will occur in the solder.

Japanese Patent No. 3945408 Japanese Patent No. 4192784 Japanese Patent No. 5387844

  An object of the present invention is to provide a soldering flux and a soldering paste that can improve wet spreading and storage stability at the same time.

  The solder flux of the present invention comprises a flux main component having a softening point of 70 ° C. to 170 ° C. and a rosin having an acid value of 150 mgKOH / g to 240 mgKOH / g and a flux main component of 35% by mass or more and less than 70% by mass. 2% by mass or more and less than 5% by mass of carboxylic acid ester, and has an acid value of 70 mg KOH / g to 110 mg KOH / g.

  The rosins are preferably at least one selected from the group of rosin, modified rosin, abietic acid and modified abietic acid.

  The carboxylic acid ester is preferably a polycarboxylic acid ester that generates two or more carboxyl groups per molecule by thermal decomposition. In particular, the carboxylic acid ester is preferably a carboxylic acid ester that generates a vinyl ether containing an alkyl group having 2 to 4 carbon atoms by thermal decomposition, and the alkyl group is a primary or secondary alkyl group. It is more preferable that it is a carboxylic acid ester that produces a vinyl ether.

  The carboxylic acid ester preferably has a thermal decomposition starting temperature of 130 ° C. or higher.

  The soldering flux of the present invention may further include a solvent for dissolving 0.2% by mass or more and 10% by mass or less of the carboxylic acid ester.

  The solder flux of the present invention may further contain an organic solvent having a boiling point of 50% by mass or less in the range of 200 ° C. to 300 ° C. and a thixotropic agent of 10% by mass or less.

  The solder paste of the present invention is characterized by containing 5% by mass or more and 20% by mass or less of the solder flux and 80% by mass or more and 95% by mass or less of the solder alloy powder.

  The solder alloy powder is preferably at least two selected from the group consisting of bismuth, zinc, silver, aluminum, lead and tin.

The solder paste preferably has a change rate α of less than 10% when the viscosity immediately after the solder paste is produced is η ₁ and the viscosity after two weeks is η ₂ .

  The solder paste preferably has a spreading rate of 70% or more determined by a soldering flux test method based on JIS Z 3197.

  By using the solder flux of the present invention, the reaction between the solder flux and the solder alloy powder is suppressed, there is little change over time in viscosity, reactivity, etc., and excellent wetting and spreading properties during solder joining. It becomes possible to provide a solder paste. For this reason, the industrial significance of the present invention is extremely large.

  The present inventors have disclosed a solder obtained by mixing a solder flux using a block carboxylic acid as a flux main component and various solder powder alloys described in Japanese Patent No. 3945408 and Japanese Patent No. 4192784. The paste contains a specific polyamide resin and rosin esters and / or polymerized rosin described in Japanese Patent No. 5387844, and the acid value of the flux composition is 40 mgKOH / g or more and 100 mgKOH / g or less Test research on the wetting and spreading of solder paste, joint reliability and storage stability was repeated, focusing on the solder paste obtained by mixing solder flux and various solder powder alloys. As a result, it was found that the acid value of the soldering flux is greatly related to the change in the viscosity of the solder paste with time.

  As a result of further research on this point, the present inventors have used rosins having specific characteristics as a flux main component, used carboxylic acid ester as an activator, and the acid value of the entire solder flux. The knowledge that the above-mentioned problems can be solved at the same time was obtained by controlling to a specific range. The present invention has been made based on such knowledge.

  Hereinafter, the present invention will be described by dividing it into “1. solder flux” and “2. solder paste”. The present invention can be applied to any of organic solvent cleaning fluxes, water-washing fluxes, and non-cleaning fluxes by appropriately selecting components such as carboxylic acid esters. However, the present invention can remarkably exhibit the effect particularly when applied to a non-cleaning type flux. For this reason, below, it demonstrates on the assumption that the flux for solder of the present invention is applied to a non-cleaning type flux.

1. Soldering flux The soldering flux of the present invention contains 35 mass% or more and less than 70 mass% of a flux main component and 0.2 mass% or more and less than 5 mass% of a carboxylic acid ester. As the flux main component, rosins having a softening point of 70 ° C. to 170 ° C. and an acid value of 150 mgKOH / g to 240 mgKOH / g are used. With such a configuration, the acid value of the solder flux of the present invention is restricted to a range of 70 mgKOH / g to 110 mgKOH / g.

  In the solder paste using the solder flux of the present invention, the carboxyl group in the carboxylic acid ester as the activator is blocked during storage, so that the function is not exhibited. On the other hand, during heating, protective groups such as vinyl ether groups are eliminated from the carboxyl groups constituting the carboxylic acid ester as vinyl ethers, etc., and the carboxyl groups activate the action of the main flux component. Can be removed. Moreover, this solder paste can suppress the generation of flux residue during solder joining, and even when a flux residue is generated, it can be used as an insulating coating.

(1) Acid Value of Soldering Flux In the present invention, the acid value of the entire soldering flux is controlled in the range of 70 mgKOH / g or more and less than 110 mgKOH / g, preferably 70 mgKOH / g or more and 100 mgKOH / g or less. By controlling the acid value of the entire solder flux in such a range, it is possible to improve wettability at the time of soldering while suppressing changes in the viscosity and reactivity of the solder paste during storage. It becomes possible.

  On the other hand, when the acid value as the solder flux is less than 70 mgKOH / g, the solder cannot be sufficiently activated, so that the wettability of the solder with respect to the base material (bonding agent) is insufficient, The conductivity of the wiring board is impaired. On the other hand, when the acid value as the solder flux is 110 mgKOH / g or more, the reaction between the solder alloy powder and the carboxyl group constituting the solder flux cannot be suppressed, and the viscosity and reactivity of the solder paste change over time. End up. Moreover, when the reaction between the solder alloy powder and the solder flux preferentially progresses, the alloying of the base material and the solder alloy powder may not sufficiently proceed.

  The acid value of the soldering flux of the present invention means the mass (mg) of potassium hydroxide (KOH) required to neutralize free fatty acid contained in 1 g of the soldering flux, and is a soldering flux test. It is determined based on a method (conforming to JIS Z 3197).

Specifically, first, 1.875 g of solder flux and 12.5 mL of ethanol are put into a 100 mL beaker, and the flux / ethanol solution is adjusted by stirring at room temperature. Next, 3 drops of phenolphthalein indicator are added to this flux / ethanol solution and further stirred to make a uniform solution. Subsequently, 0.1 N potassium hydroxide / ethanol solution (titer: 1.007) was dropped into the flux / ethanol solution with a burette and stirred for 30 seconds repeatedly, and red coloring was observed after 30 seconds. When remaining, the dropping of the potassium hydroxide / ethanol solution is terminated. During this time, assuming that the total amount of the potassium hydroxide / ethanol solution dropped is V _k (mL), the acid value A _V1 of the solder flux is calculated from the following mathematical formula (a).

(2) Constituent Components of Solder Flux a) Flux Main Component The solder flux of the present invention has a specific rosin, that is, a softening point of 70 ° C. to 170 ° C. and an acid value of 150 mgKOH / g to 240 mgKOH / g. Rosin can be widely used. Rosin contains carboxylic acid in its skeleton, and this carboxyl group reacts with the oxide formed on the surface of the solder alloy powder to remove the oxide film and realize a high spreading rate. is there.

  When the softening point and acid value of the flux main component are in the above range, it is possible to suppress the amount of flux residue generated without sacrificing the solderability such as the wet spreadability of the solder paste. Become. Further, the generated minute amount of flux can be effectively used as an insulating film. On the other hand, when the softening point is less than 70 ° C. or the acid value is less than 150 mgKOH / g, the oxide film formed on the surface of the solder alloy powder at the time of joining may not be completely removed. On the other hand, when the softening point exceeds 170 ° C. or the acid value exceeds 240 mgKOH / g, characteristics including the viscosity of the solder paste deteriorate over time due to the reaction between the solder alloy powder and the main flux component. There is.

  As rosins, publicly known ones can be widely applied. However, rosin, modified rosin, abietic acid, which is the main component of rosin, and modified abietic acid obtained by modifying rosin are used because of heat resistance and oxide film removal. It is preferable to use at least one selected from the group of

  As the flux main component, rosins having a softening point in the range of 70 ° C. to 170 ° C. and an acid value in the range of 150 mgKOH / g to 240 mgKOH / g are used for the solder joint temperature and the acid value of the entire flux. It is for adjusting. The softening point of the main flux component is preferably 70 ° C to 170 ° C, more preferably 80 ° C to 170 ° C. Moreover, it is preferable that the acid value of a flux main component is 150 mgKOH / g-240 mgKOH / g. As for the acid value of the flux main component, the mass (mg) of potassium hydroxide (KOH) required to neutralize the free fatty acid contained in 1 g of the flux main component is the same as the acid value of the solder flux as a whole. It is calculated | required based on the flux test method for soldering (JIS Z 3197 conformity).

  Specific examples of such a flux main component include natural rosin (softening point: 76 ° C. to 85 ° C., acid value: 167 mg KOH / g to 175 mg KOH / g), polymerized rosin (softening point: 96 ° C. to 99 ° C., acid Value: 159 mg KOH / g to 163 mg KOH / g), hydrogenated rosin (softening point: 74 ° C. to 75 ° C., acid value: 160 mg KOH / g to 170 mg KOH / g), abietic acid (softening point: 129 ° C. to 137 ° C., acid value 179 mg KOH / g to 182 mg KOH / g), dihydroabietic acid (softening point: 150 ° C. to 155 ° C., acid value: 154 mg KOH / g to 158 mg KOH / g), dehydroabietic acid (softening point: 160 ° C. to 168 ° C., acid value) : 174 mg KOH / g to 178 mg KOH / g) can be used.

  In addition, content of the flux main component is 35 mass% or more and less than 70 mass%, Preferably it is the range of 45 mass% or more and 60 mass% or less. If the content of the flux main component is less than 35% by mass, the activity of the obtained solder paste is insufficient, and the conductivity between the electronic component and the wiring board may not be sufficiently ensured. On the other hand, when the content of the flux main component is 70% by mass or more, the flux residue after joining increases and the base material and the conductor may be corroded.

b) Carboxylic acid ester [Carboxylic acid ester structure]
The solder flux of the present invention is characterized in that a carboxylic acid ester is mixed as an activator. The carboxylic acid ester is a compound obtained by a condensation reaction between a carboxylic acid and a compound containing a hydroxyl group. For example, as represented by the following chemical formula (1), the carboxylic acid is blocked by a protecting group such as a vinyl ether group. To form a blocked carboxylic acid.

  Thus, in the solder paste using the solder flux containing the carboxylic acid ester (block carboxylic acid) in which the carboxylic acid is blocked, during storage (before heating), since the carboxyl group is blocked by the protective group, The action of the flux main component is not activated. On the other hand, as shown in the chemical reaction formula (A) below, this protecting group can be easily detached from a carboxyl group as vinyl ether or the like by heating, so when heated at the time of soldering, The action of the main component of the flux is activated by the carboxyl group. As a result, changes in the viscosity and reactivity of the solder paste over time are suppressed under normal conditions, and at the time of soldering, the oxide film formed on the surface of the solder alloy powder is effectively removed, resulting in high wetting and spreading. Sex is realized.

  In order to impart a high oxide film removal function to the soldering flux, it is preferable that the carboxylic acid ester generates a carboxylic acid containing two or more carboxyl groups per molecule by thermal decomposition. For example, what produces | generates the carboxylic acid represented by following Chemical formula (2) is preferable, and in the following (Chemical formula 2), succinic acid (n = 2), glutaric acid (n = 3) or those that produce adipic acid (n = 4) are more preferred.

  When a carboxylic acid ester in which a carboxylic acid is blocked with a vinyl ether group is used, the vinyl ether produced during thermal decomposition is preferably represented by the following chemical formula (3), and the thermal decomposition start temperature of the blocked carboxylic acid is described below. From the viewpoint of controlling within a range (130 ° C. or higher), it is more preferable that R is an alkyl group having 2 to 4 carbon atoms. In particular, considering that an alkyl group having a lower electron donating property exhibits higher temperature dissociation properties, among these, R represented by the following chemical formulas (4) to (8) may be primary or secondary. Preferred are those of a secondary alkyl group.

[Pyrolysis start temperature]
The thermal decomposition starting temperature of the carboxylic acid ester described above (the temperature at which the function as a carboxyl group starts to develop by thermal decomposition) is preferably 130 ° C. or higher, more preferably 140 ° C. or higher, more preferably 150 ° C. or higher, most preferably It is necessary to be 180 ° C. or higher. When the thermal decomposition starting temperature is less than 130 ° C., the function as a carboxyl group is exhibited even in a normal state, and the viscosity and reactivity of the solder flux may change over time when stored for a long period of time. . The thermal decomposition starting temperature is preferably a high temperature, and the upper limit thereof is not limited, but is usually 200 ° C. or lower.

[Content]
The content of the carboxylic acid ester with respect to the entire soldering flux is 0.2% by mass or more and less than 5% by mass, preferably 0.2% by mass or more and 3% by mass or less. If the carboxylic acid ester content is less than 0.2% by mass, the amount of carboxyl groups generated by thermal decomposition is insufficient, and the oxide film cannot be sufficiently removed during solder joining. Will fall. On the other hand, when the content of the carboxylic acid ester is 5% by mass or more, the amount of the solvent for dissolving the carboxylic acid ester increases, so that the viscosity and reactivity of the solder flux may change with time. is there.

c) Solvent for dissolving carboxylic acid ester The solder flux of the present invention can contain a solvent for carboxylic acid ester for the purpose of dissolving the carboxylic acid ester. Such a solvent is not particularly limited and may be either inorganic or organic, but is preferably an organic solvent from the viewpoint of solubility. In particular, methyl ethyl ketone, propylene glycol It is preferable to use monomethyl ether acetate, tetraethylene glycol dimethyl ether, or the like.

  In addition, it is preferable that the addition amount of the solvent for melt | dissolving carboxylic acid ester is 1-2 by mass ratio with respect to carboxylic acid ester. That is, the content of the carboxylic acid ester solvent with respect to the entire solder flux is preferably 0.2% by mass to 10% by mass, and more preferably 0.2% by mass to 6% by mass. When the content of the solvent for the carboxylic acid ester is less than 0.2% by mass, the carboxylic acid ester cannot be sufficiently dissolved. On the other hand, when the content of the solvent exceeds 10% by mass, it is impossible to suppress the change with time of the viscosity of the soldering flux.

d) Organic Solvent The solder flux of the present invention can contain an organic solvent in order to facilitate mixing of the solder alloy powder and the carboxylic acid ester and making them into a paste. Such an organic solvent is not particularly limited, and a known one can be used. However, it is preferable to use one having a boiling point of 200 ° C. to 300 ° C. so that the viscosity of the solder paste obtained by mixing the solder flux and the solder alloy powder is in a suitable range at the time of soldering. It is more preferable to use what is 220 degreeC-300 degreeC, and it is still more preferable to use what is 230 degreeC-300 degreeC.

  Specifically, as such an organic solvent, 2-phenoxyethanol (boiling point: 244 to 246 ° C.), 2-ethyl-1,3-hexanediol (boiling point: 243 to 244 ° C.), diethylene glycol monohexyl ether (boiling point: 261 ° C. to 265 ° C.) and diethylene glycol monobutyl ether (boiling point: 230.4 ° C.).

  The content of the organic solvent is preferably less than 50% by mass, more preferably 20% by mass or more and less than 50% by mass, and further preferably 25% by mass or more and 45% by mass or less. When the content of the organic solvent is 50% by mass or more, sagging or the like is likely to occur when the flux is applied.

e) Thixo Agent In the solder flux of the present invention, a thixo agent can be added from the viewpoint of improving the thixotropy. Such a thixotropic agent is not particularly limited, and common ones such as hardened castor oil, beeswax, carnauba wax, stearamide, hydroxystearic acid ethylenebisamide and the like can be used.

  The thixotropic agent content is preferably 10% by mass or less, more preferably 1% by mass or more and 7% by mass or less. If the content of the thixotropic agent is within such a range, the thixotropy can be improved while ensuring the properties of the solder flux of the present invention.

f) Other Additives In addition to the components described above, the solder flux of the present invention may contain a surface protective agent or a component that improves the temporary fixing adhesiveness, if necessary. Specifically, silane coupling agents and surfactants can be added as surface protecting agents, and phosphoric acid-based (meth) acrylic monomers and triazine dithiol-based monomers can be added as components for improving temporary adhesion. it can.

  In the solder flux of the present invention, a thermoplastic resin can be used as a material to be added to or partially substituted for the rosin component from the viewpoint of improving crack resistance in heat cycle. Specifically, a polymer such as polyamide, butadiene, isoprene, styrene, acrylic acid or an ester thereof, methacrylic acid or an ester thereof, ethylene, or a copolymer composed of two or more of these may be used.

  The total content of these additives is preferably 10% by mass or less, more preferably 1% by mass or more and 7% by mass or less. When the content of these additives is within the above range, characteristics such as temporary fixing adhesiveness and crack resistance can be improved while securing the characteristics of the solder flux.

(3) Method for Producing Soldering Flux The soldering flux of the present invention can be obtained by weighing each constituent component so as to have the above-described content and mixing them uniformly. The mixing means is not particularly limited, but is preferably mixed using a self-revolving mixer, a homogenizer, a Hayshell mixer or the like. In addition, from the viewpoint of uniformly mixing the constituent components, it is preferable to first mix only the solid components and then add an organic solvent and further mix them.

2. Solder paste (1) Solder alloy powder The solder alloy powder to be mixed with the solder flux of the present invention is not particularly limited, but bismuth (Bi), zinc (Zn), silver (Ag), aluminum (Al) An alloy made of an alloy containing at least two selected from the group consisting of lead (Pb) and tin (Sn) can be suitably used. For example, Pb-free solder such as Pb—Sn alloy solder, Bi—Zn alloy solder, Sn—Ag alloy solder, or a solder alloy obtained by adding Cu, Bi, In, Al, or the like is used. be able to.

  The average particle size of the solder alloy powder is preferably in the range of 1 μm to 100 μm, more preferably less than 50 μm. When the average particle size of the solder alloy powder is less than 1 μm, there is a possibility that the solder alloy powder aggregates in the solder paste. On the other hand, when the thickness exceeds 100 μm, a new conductive layer containing a resin is formed between the materials to be bonded at the time of bonding, so that the conductivity may be lowered. In the present invention, the average particle size means a particle size having an integrated value of 50%, and can be measured by a particle size distribution measuring device such as SALD-7000 (Shimadzu Corporation).

(2) Mixing ratio The solder paste of the present invention has a solder flux of 5% by mass to 20% by mass, preferably 10% by mass to 15% by mass, and 80% by mass to 95% by mass, preferably 85% by mass. % Or more and 90% by mass or less of the solder alloy powder, and the solder flux of the present invention is used as the solder flux. By controlling the mixing ratio of the soldering flux and the solder alloy powder in such a range, the wet spreadability at the time of joining of the obtained solder paste can be made excellent. Moreover, since a flux residue can be utilized as an insulating film, corrosion of the base material can be effectively prevented.

  In addition, the mixing method of the soldering flux and the solder alloy powder is not particularly limited, and a known method can be adopted. For example, the mixing can be performed by a self-revolving mixer, a homogenizer, a Hayshell mixer, or the like.

(3) Characteristics Since the solder paste of the present invention uses the solder flux of the present invention, there is little change over time in viscosity, reactivity, etc., and excellent long-term storage stability. Moreover, since the oxide film on the surface of the solder alloy powder can be effectively removed at the time of soldering, it has excellent wet spreading properties. Furthermore, the generation of flux residue during solder bonding can be suppressed, and even when a flux residue is generated, it can be used as an insulating coating.

a) Change with time When the viscosity of the solder paste of the present invention is η ₁ and the viscosity after storage at room temperature for 2 weeks is η ₂ , the rate of change α (= (η ₂ −η ₁ ) / Η ₁ × 100) can be less than 10%, preferably less than 5%. Note that the viscosity of the solder paste can be measured by, for example, a spiral viscometer.

b) wettability solder paste of the present invention is excellent in wettability, spreading rate S _R is 70% or more, preferably characterized in that 90% or more. Here, the spreading ratio S _R can be obtained by a soldering test method (based on JIS Z 3197).

Specifically, first, a solder paste is applied to the center of a steel sheet using a metal mask having a thickness of 4 mm and a hole having a diameter of 4 mm. Next, this steel plate is floated on a solder layer set at 320 ° C. and pulled up 30 seconds after the solder melts. After cooling to room temperature, and measuring the height of the solder spread on the steel sheet H (mm), by substituting the obtained value into Equation (b) below, can be determined spreading rate S _R .

  Hereinafter, the present invention will be described in more detail with reference to examples.

(Examples 1-18 and Comparative Examples 1-7)
As a flux for solder, an organic solvent, a thixotropic agent, and an activator were prepared as a flux main component, a carboxylic acid ester, and other constituents described in Table 1. The composition ratio of each component was adjusted as described in Table 2 to produce a solder flux.

  87.5% by mass of a solder alloy powder (Mitsui Metal Industries, Ltd.) made of a Bi—Zn—Al alloy having an average particle diameter of 19 μm to 27 μm is added to 12.5% by mass of the solder flux. Using a revolving mixer (AR-250, manufactured by Shinky Corporation), mixing was performed for 10 minutes to prepare a solder paste.

  The acid value of the solder flux thus prepared, and the change and spread rate of the solder paste over time were evaluated as follows. The results are shown in Table 3.

a) Acid Value The acid value of the solder flux was measured based on a soldering flux test method (based on JIS Z 3197).

First, 1.875 g of solder flux and 12.5 mL of ethanol were put into a 100 mL beaker, and the flux / ethanol solution was adjusted by stirring at room temperature using stirring. Next, 3 drops of phenolphthalein indicator were added to this flux / ethanol solution and further stirred to obtain a uniform solution. Subsequently, a 0.1 N potassium hydroxide / ethanol solution (titer: 1.007) was dropped into the flux / ethanol solution with a burette and the operation of stirring for 30 seconds was repeated. When red coloring remains even after 30 seconds, the dropping of the potassium hydroxide / ethanol solution is terminated, and during this time, the total amount of the dropped potassium hydroxide / ethanol solution is _defined as V _k (mL), and the above formula ( From a), the acid value A _V1 of the solder flux was calculated.

b) Time-dependent change The time-dependent change of the solder paste was determined by measuring the viscosity η ₁ immediately after the solder paste was produced and the viscosity η ₂ after being stored for 2 weeks by using a spiral viscometer (Malcom Co., Ltd., PCU-205). Was evaluated by determining the rate of change α (= (η ₂ −η ₁ ) / η ₁ × 100). Specifically, the rate of change α is less than 5% is “excellent (◎)”, 5% or more and less than 10% is “good (◯)”, and 10% or more. Evaluated as “bad (×)”.

c) Spreading rate The spreading rate of the solder paste was measured based on a soldering flux test method (based on JIS Z 3197).

First, a solder paste was applied to the center of the copper plate using a metal mask having a thickness of 0.5 mm having a hole having a diameter of 4 mm. Next, a copper plate was floated on the solder layer set at 320 ° C., and was pulled up 30 seconds after the solder melted. After cooling to room temperature, the spread rate S _R of the solder paste was calculated from the above mathematical formula (b), and the wet spread property was evaluated. Specifically, “excellent (◎)” means that the spreading rate was 90% or more, “good” that means 70% or more and less than 90%, It was evaluated as “bad (×)”.

(Comprehensive evaluation)
From Tables 1 to 3, it was confirmed that all of the solder fluxes of Examples 1 to 18 had a total acid value in the range of 70 mgKOH / g to 110 mgKOH / g. Further, in the solder paste using these solder flux, it was confirmed that achieved simultaneously spreading rate S _R of viscosity change of less than 10% alpha and 70%. That is, it can be said that the solder pastes of Examples 1 to 18 have little change over time and are excellent in wet spread.

  In addition, in the solder pastes of Examples 1 to 18, the generation of the flux residue was extremely small, and the flux residue generated in a small amount could be used as an insulating coating. For this reason, it is considered that the bonding failure can be greatly reduced in the solder bonding using this solder paste.

In contrast, the solder fluxes of Comparative Examples 1 to 4 had an overall acid value exceeding 110 mgKOH / g due to the addition of adipic acid as another additive. Therefore, in the solder paste of Comparative Examples 1 to 4 were able to spread ratio S _R and 70%, the viscosity change rate α is greater than 10% inhibition of temporal change in the viscosity of the solder paste I couldn't.

In addition, the solder pastes of Comparative Examples 5 to 7 did not contain either the block carboxylic acid or adipic acid as another additive, so the acid value at the time of soldering was insufficient or the flux was sufficient. The spreading rate S _R was less than 70%.

Claims (13)

A flux main component having a softening point of 70 ° C. to 170 ° C. and a rosin having an acid value of 150 mgKOH / g to 240 mgKOH / g, a flux main component of 35% to less than 70% by mass, and 0.2% to less than 5% by mass And having an acid value of 70 mg KOH / g to 110 mg KOH / g,
Solder flux.   2. The solder flux according to claim 1, wherein the rosins are at least one selected from the group consisting of rosin, modified rosin, abietic acid, and modified abietic acid.   The solder flux according to claim 1, wherein the carboxylic acid ester is a polycarboxylic acid ester that generates carboxylic acid containing two or more carboxyl groups per molecule by thermal decomposition.   3. The solder flux according to claim 1, wherein the carboxylic acid ester is a carboxylic acid ester that generates vinyl ether containing an alkyl group having 2 to 4 carbon atoms by thermal decomposition.   The solder flux according to claim 4, wherein the alkyl group is a primary or secondary alkyl group.   The solder flux according to claim 1, wherein the carboxylic acid ester has a thermal decomposition start temperature of 130 ° C. or higher.   The solder flux according to claim 1, further comprising a solvent for dissolving 0.2% by mass or more and 10% by mass or less of the carboxylic acid ester.   The solder flux according to any one of claims 1 to 7, further comprising an organic solvent having a boiling point of 50% by mass or less in a range of 200 ° C to 300 ° C.   The solder flux according to claim 1, further comprising 10% by mass or less of a thixotropic agent.   A solder paste comprising 5% by mass or more and 20% by mass or less of the solder flux according to claim 1 and 80% by mass or more and 95% by mass or less of the solder alloy powder.   The solder paste according to claim 10, wherein the solder alloy powder is at least two selected from the group of bismuth, zinc, silver, aluminum, lead and tin. 12. The solder paste according to claim 10, wherein the rate of change α is less than 10% when the viscosity immediately after the solder paste is produced is η ₁ and the viscosity after two weeks is η ₂ .   The solder paste according to any one of claims 10 to 12, wherein a spreading ratio obtained by a soldering flux test method based on JIS Z 3197 is 70% or more.