JP6275311B1 - Solder paste and solder joint - Google Patents

Abstract

Even when solder bonding is performed using solder paste using Sn-Sb solder alloy powder containing a certain amount of Sb, formation of a sulfide phase at the interface between the solder joint and the flux residue Providing a solder paste that can prevent the deterioration of the yield in the manufacturing process due to this. SOLUTION A powder comprising a lead-free solder alloy containing Sn and Sb and having a Sb content of 1% by mass to 10% by mass, a base resin (A), an activator (B), a thixotropic agent ( A solder paste comprising C), a solvent (D), and a flux composition containing an acidic phosphate ester (E). [Selection] Figure 1

Description

  The present invention relates to a solder paste and a solder joint.

  In order to join an electronic component to an electronic circuit formed on a substrate such as a printed wiring board, a solder joining method using a solder paste is employed. This solder paste is produced by kneading an alloy powder made of a solder alloy and a flux composition containing a base resin such as rosin resin or acrylic resin, an activator, and the like.

When solder bonding is performed using the solder paste, a solder joint portion formed of solder alloy powder and a residue of the flux composition (flux residue) remain on the substrate. Since the flux residue is mainly composed of a resin component, it has good insulating properties at room temperature, and has a certain hardness, so it plays a role of protecting the solder joint.
However, depending on the state of use of the electronic device with a built-in board, the flux residue absorbs moisture present in the atmosphere, which causes a decrease in the insulation resistance of the flux residue and migration due to moisture adhering to the electrode. It was the cause.

Conventionally, it has been common to use lead for the solder alloy powder used in the solder paste. However, since the use of lead is restricted by the RoHS directive or the like from the viewpoint of environmental load, solder paste using so-called lead-free solder alloy powder that does not contain lead is becoming common in recent years.
This lead-free solder alloy generally contains Sn as a main material, and for example, Sn—Cu, Sn—Ag—Cu, Sn—Bi, Sn—Zn solder alloys, etc. are often used.
The solder joint formed by such a lead-free solder alloy powder containing a large amount of Sn is changed to SnO ₂ by the oxidation of Sn contained in the humidity or the flux residue over time. To do. Since this SnO ₂ has a lower density than Sn, the volume easily expands, and this has been a cause of whisker generation and electrical circuit short-circuiting.

In addition, lead-free solder alloys containing a large amount of Sn are inferior in wettability as compared with conventional lead-containing solder alloys, and therefore organic halides are often used as an activator in the flux composition.
This organic halide is decomposed by heat at the time of solder joining, and the released halogen reacts with the metal oxide to activate the surface of the solder alloy. However, unfreed halogen may remain in the flux residue depending on the heating conditions during soldering. Such organic halide residue reacts with water molecules attached to the surface of the flux residue under high temperature and humidity, and gradually changes to an ionic substance. In particular, since the oxidation of Sn proceeds to the inside, the solder joint portion is discolored black.

  In order to solve such a problem, for non-cleaning type solder paste containing 4% by mass to less than 30% by mass of an acidic phosphate ester that forms a hydrophobic coating for suppressing migration on the metal surface of the soldered part during soldering For lead-free solder containing 0.2 to 4% by mass of at least one compound selected from acid phosphates and derivatives thereof in addition to flux (see Patent Document 1), main resin and activator No solder type resin flux (refer to Patent Document 2) and a solder paste composed of a mixture of a lead-free solder powder and a flux containing an organic halide. By adding a phosphorus compound to the solder paste, the soldering part Has been proposed (see Patent Document 3).

Japanese Patent No. 5445716 Japanese Patent No. 4325746 JP 2006-181635 A

  The flux and solder paste disclosed in the above patent documents are all Sn—Ag—Cu, Sn—Cu, Sn—Ag, Sn—Bi, Sn—Zn, or P or Ge. The addition of acid phosphate ester or phosphorus compound is only for the purpose of eliminating defects caused by Sn contained in lead-free solder alloys and organic halides contained in flux compositions. It is said.

  Here, solder joints formed using solder paste are subject to stresses due to thermal expansion that occur on the substrate due to turning on / off of the power supply of electronic devices, or in environments where there is a large temperature difference such as in-vehicle electronic devices. Cracks due to thermal displacement of the solder joint due to the difference in linear expansion coefficient that occurs when it is placed and stress associated therewith may occur. Therefore, in order to suppress such a phenomenon, a so-called Sn—Sb based solder alloy to which Sb is added for the purpose of improving the crack resistance of the solder joint portion (in this specification, an alloy element other than Sn and Sb is contained). Sn-Sb solder alloys including those to be used) are being used.

Sb distributed in the market can be obtained by refining a sulfide mineral (Sb ₂ S ₃ ) called bright an ore. However, it is difficult to completely remove S even by refining, and S remains in the Sb after refining although it is in a very small amount (about 60 ppm to 100 ppm). For this reason, the Sn—Sb-based solder alloy containing Sb also contains a small amount of S.

When solder joining is performed using a solder paste using Sn—Sb solder alloy powder containing a certain amount of Sb, an activator and a metal sulfide contained in the Sn—Sb solder alloy are present during solder joining. The sulfide phase is formed by the S that is liberated by this reaction or the sulfide generated by the reaction, at the interface between the solder joint and the flux residue.
Here, an appearance inspection is performed on the substrate on which the electronic component or the like is mounted in order to confirm the positional deviation or fillet shape of the electronic component. Many of the visual inspection apparatuses used for this inspection irradiate light onto a substrate using illumination and perform visual inspection based on the reflected light information. In the case of a substrate having a sulfide phase of a certain area or more at the interface between the solder joint and the flux residue, an abnormality occurs in the amount of reflected light due to this sulfide phase, resulting in an error in the appearance inspection device. Therefore, there is a problem that the yield is deteriorated in the manufacturing process.
As described above, none of the above patent documents discloses or suggests the use of a lead-free solder alloy containing a certain amount of Sb for the solder paste and the problems caused thereby.

  The present invention solves the above-described problem, and even when solder bonding is performed using a solder paste using a Sn-Sb solder alloy powder containing a certain amount of Sb, the solder bonding portion and the flux It is an object of the present invention to provide a solder paste capable of suppressing the formation of a sulfide phase at the residual interface and preventing the deterioration of the yield in the manufacturing process due to this.

(1) The solder paste according to the present invention comprises Sn and Sb, a powder comprising a lead-free solder alloy having a Sb content of 1% by mass to 10% by mass, a base resin (A), an activator ( B), a thixotropic agent (C), a solvent (D), and a flux composition containing an acidic phosphate (E) are included.

(2) In the configuration described in (1) above, the lead-free solder alloy further includes Ag 0.1 mass% to 5 mass%, Cu 0.5 mass% to 1 mass%, Bi 0.5 mass% It is characterized by containing at least two alloy elements selected from 5% by mass or less and In 0.1% by mass or more and 6% by mass or less.

(3) In the configuration described in (1) or (2) above, the lead-free solder alloy further includes 0.01 mass% or more and 0.1 mass% or less of Ni.

(4) In the configuration described in any one of (1) to (3) above, the lead-free solder alloy further includes Co in an amount of 0.001% by mass to 0.015% by mass. And

(5) In the configuration according to any one of (1) to (4), the lead-free solder alloy further includes at least one of P, Ga, and Ge in a total of 0.001% by mass or more 0 It is characterized by containing 0.05 mass% or less.

(6) In the configuration according to any one of (1) to (5), the lead-free solder alloy further includes at least one of Fe, Mn, Cr, and Mo in a total amount of 0.001% by mass. It is characterized by containing 0.05% by mass or less.

(7) In the configuration described in any one of (1) to (6) above, the amount of the acidic phosphate ester (E) is 1% by mass or more and 10% by mass with respect to the total amount of the flux composition. % Or less.

(8) In the configuration described in any one of (1) to (7) above, the acidic phosphate ester (E) is represented by the following general formula (1).

(In the formula, R represents a linear or branched alkyl group having 8 to 24 carbon atoms. N is 1 or 2, and when n is 2, R may be the same or different. Good.)

(9) The structure according to any one of (1) to (8), wherein the acidic phosphate ester (E) is 2-ethylhexyl acid phosphate, bis (2-ethylhexyl) phosphate, tetracosyl It is characterized by being at least one selected from acid phosphate and isotridecyl acid phosphate.

(10) In the configuration according to any one of (1) to (9) above, the flux composition has an acid value of 80 mgKOH / g or more, and among the acid values, the activator (B ) Derived acid value is 80 mgKOH / g or more.

(11) A solder joint according to the present invention is characterized in that it is formed using the solder paste according to any one of (1) to (10) above.

  Even if the solder paste of the present invention uses Sn-Sb solder alloy powder containing a certain amount of Sb, the solder phase of the solder phase formed at the interface between the solder joint and the flux residue is used. The formation can be suppressed, and the deterioration of the yield in the manufacturing process due to this can be prevented.

The figure which showed the temperature profile at the time of the reflow in the "sulfide phase generation | occurrence | production test" and the "void test" which concern on an Example and a comparative example. In an example and a comparative example, an X-ray transmission device is used for a general chip component mounting board to indicate “a region under an electrode of a chip component” and “a region where a fillet is formed” for observing whether or not voids are generated. Photo taken from the chip part side. The figure which showed the temperature profile at the time of the reflow in the "connection cracking resistance test" concerning an Example and a comparative example.

  Hereinafter, an embodiment of the solder paste and solder joint of the present invention will be described in detail. Of course, the present invention is not limited to the following embodiments.

(1) Solder paste It is preferable that the solder paste of this embodiment contains the alloy powder which consists of a lead-free solder alloy, and a flux composition.

<Lead-free solder alloy>
The lead-free solder alloy according to the present embodiment preferably includes Sn and Sb, and the Sb content is 1% by mass or more and 10% by mass or less. In addition, the lead-free solder alloy further includes Ag 0.1 mass% to 5 mass%, Cu 0.5 mass% to 1 mass%, Bi 0.5 mass% to 5 mass%, and In 0.1 mass% to 6 mass%. It is more preferable that at least two alloy elements selected from% or less are included.

  In order to improve the toughness of the lead-free solder alloy and the crack resistance of the solder joint formed by using the lead-free solder alloy of the present embodiment, the lead-free solder alloy contains 1% by mass to 10% by mass of Sb. Is preferred.

As described above, although Sb generally remains in a small amount, S remains. Therefore, in the lead-free solder alloy according to the present embodiment containing 1% by mass or more of Sb, the activator and the metal sulfide are liable to react at the time of solder joining, and S released by this reaction or produced by the reaction. The sulfide phase easily forms a sulfide phase at the interface between the solder joint and the flux residue.
However, the solder paste according to the present embodiment uses a flux composition containing an acidic phosphate ester as described in detail below, so that the acidic phosphate ester captures S and sulfide generated during solder joining. Generation of a sulfide phase at the interface between the joint and the flux residue can be suppressed.
In addition, there is a method for completely removing S in the purification of Sb. However, since it takes a lot of work, the cost is high.

  Since Sb increases the melting temperature (solidus temperature / liquidus temperature) of the lead-free solder alloy, if it is contained in a large amount, Sb may not be re-dissolved at a high temperature. Therefore, the Sb content is preferably 1% by mass or more and 5% by mass or less. By adding Sb within this range, it is possible to improve the crack growth suppression effect of the solder joint without impairing the extensibility of the solder alloy, and also to ensure sufficient toughness against external stress, which also reduces residual stress. can do. In addition, in order to improve the said crack progress inhibitory effect, it is preferable that content of Sb shall be 3 mass% or more and 5 mass% or less.

The lead-free solder alloy can contain 0.1 mass% or more and 5 mass% or less of Ag. By adding Ag, it is possible to improve the mechanical strength of the lead-free solder alloy by precipitating an Ag ₃ Sn compound in the Sn grain boundary.

In consideration of the balance between the mechanical strength and cost of the lead-free solder alloy and the suppression of the electrode peeling phenomenon of the electronic component by the formed solder joint, the Ag content is 1% by mass or more and 3.1% by mass or less. It is preferable that
In particular, when the Ag content is 2% by mass or more and 3.1% by mass or less, the balance between strength and stretchability of the lead-free solder alloy can be improved. A more preferable Ag content is 2.5% by mass or more and 3.1% by mass or less.

The lead-free solder alloy can contain 0.5 mass% or more and 1 mass% or less of Cu. By adding Cu in this range, the effect of preventing Cu erosion to Cu lands of electronic circuits is exhibited, and the thermal shock resistance of the lead-free solder alloy is improved by precipitating Cu ₆ Sn ₅ compounds in Sn grain boundaries. Can be improved.

In particular, when the Cu content is 0.7% by mass or less, the Cu erosion preventing effect on the Cu land can be exhibited, and the viscosity of the lead-free solder alloy at the time of melting can be maintained in a good state. Generation | occurrence | production of the void at the time of solder joining can be suppressed more, and the thermal shock resistance of the solder joint part to form can be improved. Furthermore, fine Cu ₆ Sn ₅ is dispersed in the Sn grain boundary of the molten lead-free solder alloy, thereby suppressing changes in Sn crystal orientation and suppressing deformation of the solder joint shape (fillet shape). Can do.
If the Cu content exceeds 1% by mass, the Cu ₆ Sn ₅ compound is likely to be deposited in the vicinity of the interface between the electronic component and the electronic circuit board in the solder joint, thereby improving the joint reliability and the stretchability of the solder joint. Since there exists a possibility of inhibiting, it is not preferable.

  The lead-free solder alloy can contain 0.5 mass% or more and 5 mass% or less of Bi. By containing Bi, the melting temperature increased by the addition of Sb can be lowered and the strength can be improved. That is, since Bi also dissolves into the Sn matrix in the same manner as Sb, the lead-free solder alloy can be further strengthened.

In consideration of the stretchability of the lead-free solder alloy and the improvement of the effect of suppressing the crack growth of the formed solder joint, the Bi content is preferably 0.5% by mass or more and 4.5% by mass or less.
When the Bi content is 2% by mass or more and 4.5% by mass or less, the strength of the solder joint can be further improved. In addition, when adding below-mentioned Ni and / or Co to the said lead-free solder alloy, preferable content of Bi is 3.1 mass% or more and 4.5 mass% or less.

  The lead-free solder alloy can contain 0.1 mass% or more and 6 mass% or less of In. By adding In within this range, it is possible to lower the melting temperature of the lead-free solder alloy that has been raised by the addition of Sb and to improve the crack growth suppressing effect. That is, since In dissolves in the Sn matrix as well as Sb, not only can the lead-free solder alloy be further strengthened, but when Ag is added, AgSnIn and InSb compounds are formed and formed into Sn grain boundaries. By precipitating in this, the effect of suppressing the slip deformation of the Sn grain boundary is exhibited.

The lead-free solder alloy can contain 0.01 mass% or more and 0.1 mass% or less of Ni. In particular, when Cu is added to a lead-free solder alloy, by adding Ni, fine (Cu, Ni) ₆ Sn ₅ is formed in the molten lead-free solder alloy and dispersed in the base material. It is possible to suppress the progress of cracks at the joint and further improve its heat fatigue resistance.
Moreover, even when such a lead-free solder alloy is used when soldering an electronic component that is not subjected to Ni / Pd / Au plating or Ni / Au plating, Ni is not bonded between the solder joint and the electronic component. Growth of the Cu ₃ Sn layer in the vicinity of the interface in order to move to a region near the interface with the lead portion and its lower surface electrode (hereinafter referred to as “near the interface”) to form fine (Cu, Ni) ₆ Sn ₅ Can be suppressed, and the effect of suppressing crack growth near the interface can be improved.

However, if the content of Ni is less than 0.01% by mass, the effect of modifying the intermetallic compound becomes insufficient, so that the effect of suppressing cracks near the interface is not sufficiently obtained. Note that a lead-free solder alloy having a high Ni content is less likely to be overcooled compared to a Sn-3Ag-0.5Cu alloy, and the timing at which the solder alloy solidifies tends to be earlier. For this reason, in a solder joint fillet formed using such a lead-free solder alloy, the gas that tried to escape to the outside during melting of the solder alloy is solidified while remaining in the fillet. A case in which a hole due to gas is generated is confirmed. The voids in the fillet deteriorate the heat-resistant fatigue characteristics of the solder joint particularly in an environment with a severe temperature difference such as -40 ° C to 140 ° C and -40 ° C to 150 ° C.
Therefore, in this embodiment, the Ni content is preferably 0.01% by mass or more and 0.1% by mass or less. Further, when the Ni content is 0.01% by mass or more and 0.05% by mass or less, it is possible to improve the effect of suppressing the generation of voids while improving the effect of suppressing crack growth near the interface and the heat fatigue resistance.

The lead-free solder alloy can contain 0.001 mass% or more and 0.015 mass% or less of Co in addition to Ni. In particular, when Cu is added to a lead-free solder alloy, by adding Co, fine (Cu, Co) ₆ Sn ₅ is formed in the molten lead-free solder alloy and dispersed in the base material. It is possible to improve the thermal fatigue resistance of the solder joint part even in an environment where the temperature difference is particularly severe, while suppressing the creep deformation of the joint part and the progress of cracks.
In addition, when Co is added together with Ni, the above effect due to the addition of Ni can be enhanced, and even when an electronic component that is not subjected to Ni / Pd / Au plating or Ni / Au plating is soldered, Ni addition In addition to enhancing the above-described effects, Co moves to the vicinity of the interface during solder bonding to form fine (Cu, Co) ₆ Sn ₅ , thereby suppressing the growth of the Cu ₃ Sn layer in the vicinity of the interface. The effect of suppressing crack growth in the vicinity can be improved.

However, if the content of Co is less than 0.001% by mass, the effect of modifying the intermetallic compound becomes insufficient, so that the effect of suppressing cracks near the interface due to the addition of Co is hardly obtained.
Further, a lead-free solder alloy having a large Co content is less likely to cause overcooling compared to a Sn-3Ag-0.5Cu alloy, and the timing at which the solder alloy solidifies tends to be accelerated. And in a fillet of a solder joint formed using such a lead-free solder alloy, the gas that tried to escape to the outside during melting of the solder alloy is solidified while remaining in it, and the gas enters the fillet. The case where the void by due to occurs is confirmed. The voids in the fillet deteriorate the thermal fatigue characteristics of the solder joint, particularly in an environment where the temperature difference is severe.
Therefore, in this embodiment, the Co content is preferably 0.001% by mass or more and 0.015% by mass or less. Further, when the Co content is 0.001% by mass or more and 0.01% by mass or less, it is possible to improve the suppression of void generation while improving the good crack growth suppressing effect and heat fatigue resistance.

Here, when Ag, Cu, Sb, Bi, Ni and Co are added to the lead-free solder alloy, the content (mass%) of each alloy element preferably satisfies all of the following formulas A to D. By setting the content of each alloy element within this range, the stretchability of the solder joint and inhibition of brittleness increase, the strength and thermal fatigue characteristics of the solder joint are improved, the void generated in the fillet is suppressed, Balances both crack growth suppression in solder joints in severe environments with severe differences, and crack growth suppression effects near the interface when soldering non-Ni / Pd / Au plated or electronic parts without Ni / Au plating Therefore, the reliability of the solder joint can be further improved.
1.6 ≦ Ag content + (Cu content / 0.5) ≦ 5.9 A
0.85 ≦ (Ag content / 3) + (Bi content / 4.5) ≦ 2.10 B
3.6 ≦ Ag content + Sb content ≦ 8.9… C
0 <(Ni content / 0.1) + (Co content / 0.015) ≦ 1.19... D

  The lead-free solder alloy may contain 0.001% by mass or more and 0.05% by mass or less of at least one of P, Ga, and Ge. By adding them within this range, oxidation of the lead-free solder alloy can be prevented. However, if the content of these exceeds 0.05% by mass, the melting temperature of the lead-free solder alloy is increased, or voids are likely to be generated in the soldered joint, which is not preferable.

  Furthermore, the lead-free solder alloy may contain at least one of Fe, Mn, Cr and Mo in a total amount of 0.001% by mass or more and 0.05% by mass or less. By adding these within this range, the crack progress inhibitory effect of a lead-free solder alloy can be improved. However, if the content of these exceeds 0.05% by mass, the melting temperature of the lead-free solder alloy is increased, or voids are likely to be generated in the soldered joint, which is not preferable.

  The lead-free solder alloy can contain other components (elements) such as Cd, Tl, Se, Au, Ti, Si, Al, Mg, Zn, and the like within a range that does not hinder the effect. . The lead-free solder alloy naturally contains inevitable impurities.

  The main component of the lead-free solder alloy is preferably Sn.

<Flux composition>
The flux composition used for the solder paste of this embodiment contains a base resin (A), an activator (B), a thixotropic agent (C), a solvent (D), and an acidic phosphate ester (E). Is preferred.

Base resin (A)
As the base resin (A), for example, it is preferable to use at least one of rosin resin (A-1), rosin derivative compound (A-2), and synthetic resin (A-3).

  Examples of the rosin resin (A-1) include rosins such as tall oil rosin, gum rosin, and wood rosin; polymerization, hydrogenation, heterogeneity, acrylation, maleation, esterification, or phenol addition reaction of rosin. A modified rosin resin obtained by subjecting these rosin or rosin derivative and unsaturated carboxylic acid (acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, etc.) to a Diels-Alder reaction (however, the following Rosin derivative compound (A-2) is excluded.). Among these, a modified rosin resin is particularly preferably used, and a hydrogenated acrylic acid-modified rosin resin hydrogenated by reacting acrylic acid is particularly preferably used. In addition, you may use these individually by 1 type or in mixture of multiple types.

  The acid value of the rosin resin (A-1) is preferably 140 mgKOH / g to 350 mgKOH / g, and its mass average molecular weight is preferably 200 Mw to 1,000 Mw.

As the rosin derivative compound (A-2), for example, a derivative compound obtained by dehydrating and condensing a rosin resin having a carboxyl group and a dimer acid derivative flexible alcohol compound is preferably used.
Examples of the rosin resin having a carboxyl group include rosins such as tall oil rosin, gum rosin, and wood rosin; hydrogenated rosin, polymerized rosin, heterogeneous rosin, acrylic acid-modified rosin, maleic acid-modified rosin and the like Other than these, any rosin having a carboxyl group can be used. You may use these individually by 1 type or in mixture of multiple types.

  Next, examples of the dimer acid derivative flexible alcohol compound include compounds derived from dimer acid such as dimer diol, polyester polyol, and polyester dimer diol, and those having an alcohol group at the terminal thereof. For example, PRIPOL 2033, PRIPLAST 3197, PRIPLAST 1838 (manufactured by Croda Japan Co., Ltd.) and the like can be used.

  The rosin derivative compound (A-2) is preferably obtained by dehydration condensation between the rosin resin having a carboxyl group and the dimer acid derivative flexible alcohol compound. As the dehydration condensation method, a generally used method can be used. In addition, a preferable mass ratio when dehydrating and condensing the rosin resin having a carboxyl group and the dimer acid derivative flexible alcohol compound is 25:75 to 75:25, respectively.

  The acid value of the rosin derivative compound (A-2) is preferably 0 mgKOH / g to 150 mgKOH / g, and its mass average molecular weight is preferably 1,000 Mw to 30,000 Mw.

  Examples of the synthetic resin (A-3) include acrylic resins, styrene-maleic acid resins, epoxy resins, urethane resins, polyester resins, phenoxy resins, terpene resins, and polyalkylene carbonates. You may mix and use. Among these, an acrylic resin is particularly preferably used.

  The acrylic resin can be obtained by, for example, homopolymerizing (meth) acrylate having an alkyl group having 1 to 20 carbon atoms or copolymerizing a monomer having the acrylate as a main component. Among such acrylic resins, acrylic resins obtained by polymerizing methacrylic acid and monomers containing two saturated alkyl groups having 2 to 20 carbon atoms in which the carbon chain is linear are preferably used. It is done. In addition, you may use the said acrylic resin individually by 1 type or in mixture of multiple types.

  The acid value of the synthetic resin (A-3) is preferably 0 mgKOH / g to 150 mgKOH / g, and its mass average molecular weight is preferably 1,000 Mw to 30,000 Mw.

  Further, the blending amount of the base resin (A) is preferably 10% by mass or more and 60% by mass or less, and more preferably 30% by mass or more and 55% by mass or less with respect to the total amount of the flux composition.

  When the rosin resin (A-1) is used alone, the blending amount is preferably 20% by mass or more and 60% by mass or less, and 30% by mass or more and 55% by mass or less with respect to the total amount of the flux composition. More preferably. By setting the blending amount of the rosin resin (A-1) within this range, good solderability can be obtained.

  When the rosin derivative compound (A-2) is used alone, the amount of the rosin derivative compound (A-2) is preferably 10% by mass or more and 60% by mass or less, and more preferably 15% by mass or more and 50% by mass or less based on the total amount of the flux composition. More preferably. By setting the blending amount of the rosin derivative compound (A-2) within this range, good solderability can be achieved.

  Moreover, when using the said synthetic resin (A-3) independently, it is preferable that the compounding quantity is 10 to 60 mass% with respect to the flux composition whole quantity, and is 15 to 50 mass%. It is more preferable.

  As the base resin (A), for example, the rosin resin (A-1), the rosin derivative compound (A-2) and the synthetic resin (A-3) can be used alone, respectively. It can also be used together.

For example, when the rosin resin (A-1) and the rosin derivative compound (A-2) are used in combination, the blending ratio is preferably 20:80 to 50:50, and 35:65 to 45:55. It is more preferable that
Next, for example, when the rosin derivative compound (A-2) and the synthetic resin (A-3) are used in combination, the blending ratio is preferably 20:80 to 50:50, and 35:65 to 45: More preferably, it is 55.
For example, when the rosin resin (A-1) and the synthetic resin (A-3) are used in combination, the blending ratio is preferably 20:80 to 50:50, and 35:65 to 45:55. It is more preferable that
When the rosin resin (A-1), the rosin derivative compound (A-2) and the synthetic resin (A-3) are used in combination, the mixing ratio is 1: 1: 1 to 1: 1: 8. It is preferable that the ratio is 1: 1: 2 to 1: 1: 4.

As the base resin (A), it is particularly preferable to use an acrylic resin in combination as the rosin derivative compound (A-2) and the synthetic resin (A-3).
The flux composition containing the base resin (A) in which the rosin derivative compound (A-2) and an acrylic resin are used in combination includes a substrate having a flux residue formed using the flux composition, for example, for an in-vehicle device. Even when it is used in equipment that is subject to such a severe temperature difference, it is possible to suppress cracks in the flux residue due to stress caused by the difference in linear expansion coefficient caused by the temperature difference and its progress. And a highly reliable substrate can be provided.

Activator (B)
Examples of the activator (B) include carboxylic acids and halogen-containing compounds. You may use these individually by 1 type or in mixture of multiple types.

Examples of the carboxylic acids include monocarboxylic acids, dicarboxylic acids, and other organic acids.
Examples of the monocarboxylic acid include propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, capric acid, lauric acid, myristic acid, pentadecylic acid, palmitic acid, margaric acid, stearic acid, tuberculostearic acid, arachidic acid , Behenic acid, lignoceric acid, glycolic acid and the like.
Dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, dodecanedioic acid, tartaric acid, diglycolic acid, 1, Examples include 4-cyclohexanedicarboxylic acid.
Examples of the other organic acids include dimer acid, levulinic acid, lactic acid, acrylic acid, benzoic acid, 2-iodobenzoic acid, salicylic acid, anisic acid, citric acid, picolinic acid, and anthranilic acid.
In addition, you may use these individually by 1 type or in mixture of multiple types.

Examples of the halogen-containing compound include a non-dissociative halogen compound (non-dissociative activator) and a dissociative halogen compound (dissociative activator).
Examples of the non-dissociative activator include non-salt organic compounds in which halogen atoms are covalently bonded. For example, chlorine, bromine, iodine, fluorine such as chlorinated compounds, brominated compounds, iodinated compounds, and fluorides. The compound may be a compound by covalent bonding of each single element, or a compound in which two or more different halogen atoms are bonded by a covalent bond. In order to improve the solubility in an aqueous solvent, the compound preferably has a polar group such as a hydroxyl group such as a halogenated alcohol. Examples of the halogenated alcohol include 2,3-dibromopropanol, 2,3-dibromobutanediol, 2,3-dibromo-2-butene-1,4-diol, 1,4-dibromo-2-butanol, Brominated alcohols such as bromoneopentyl alcohol; Brominated organic acids such as 2-bromohexanoic acid; Chlorinated alcohols such as 1,3-dichloro-2-propanol and 1,4-dichloro-2-butanol; 3-fluoro Fluorinated alcohols such as catechol; and other similar compounds. You may use these individually by 1 type or in mixture of multiple types.

  The blending amount of the activator (B) is preferably 10% by mass or more and 30% by mass or less, and more preferably 15% by mass or more and 25% by mass or less with respect to the total amount of the flux.

The above-described metal sulfide derived from Sb easily reacts with the activator (B). And especially when the acid value derived from the activator (B) is 80 mgKOH / g or more in terms of the total amount of the flux composition, the Sb-derived metal sulfide and the activator (B) are more easily reacted. And the formation of sulfides are more likely to occur.
However, for example, when an activator having a low acid value is used or the amount of the activator is reduced, the activation of the surface of the lead-free solder alloy powder becomes insufficient and the wettability decreases.
Therefore, in order to ensure the reliability of the substrate, it is desirable that the acid value (activity) derived from the activator (B) is more than a certain value, for example, the acid value derived from the activator (B) in the flux composition. Is preferably 80 mgKOH / g, more preferably 100 mgKOH / g or more.

  In such a situation, the solder paste according to the present embodiment uses acidic phosphate ester (E) as described in detail below, even when a lead-free solder alloy powder containing a certain amount of Sb is used. By using the flux composition containing the acid phosphate ester (E) captures S and sulfides generated during solder bonding, and thus suppresses the generation of a sulfide phase at the interface between the solder joint and the flux residue. Can do. Therefore, the acid value derived from the activator (B) in the flux composition can be 80 mgKOH / g or more, and good wettability of the lead-free solder alloy can be ensured.

Thixotropic agent (C)
Examples of the thixotropic agent (C) include hydrogenated castor oil, fatty acid amides, saturated fatty acid bisamides, oxy fatty acids, and dibenzylidene sorbitols. You may use these individually by 1 type or in mixture of multiple types.
The blending amount of the thixotropic agent (C) is preferably 2% by mass or more and 15% by mass or less, and more preferably 2% by mass or more and 10% by mass or less with respect to the total amount of the flux composition.

Solvent (D)
Examples of the solvent (D) include isopropyl alcohol, ethanol, acetone, toluene, xylene, ethyl acetate, ethyl cellosolve, butyl cellosolve, hexyl diglycol, (2-ethylhexyl) diglycol, phenyl glycol, butyl carbitol, octanediol, Examples include α-terpineol, β-terpineol, tetraethylene glycol dimethyl ether, trimellitic acid tris (2-ethylhexyl), and bisisopropyl sebacate. You may use these individually by 1 type or in mixture of multiple types.
The blending amount of the solvent (D) is preferably 10% by mass or more and 50% by mass or less, and more preferably 15% by mass or more and 40% by mass or less with respect to the total amount of the flux composition.

Acid phosphate ester (E)
As said acidic phosphate ester (E), what is represented, for example by following General formula (1) is used preferably.

(In the formula, R represents a linear or branched alkyl group having 8 to 24 carbon atoms. N is 1 or 2, and when n is 2, R may be the same or different. Good.)

Examples of such acidic phosphate ester (E) include 2-ethylhexyl acid phosphate, bis (2-ethylhexyl) phosphate, tetracosyl acid phosphate, and isotridecyl acid phosphate. You may use these individually by 1 type or in mixture of multiple types.
Moreover, the compounding amount of the acidic phosphate ester (E) is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 8% by mass or less based on the total amount of the flux composition. Further, the blending amount is more preferably 3% by mass or more and 7% by mass or less.
By blending the acidic phosphoric acid ester (E) within this range, even when a lead-free solder alloy powder containing a certain amount of Sb is used, S and sulfides generated during solder bonding are treated with acidic phosphorus. Since the acid ester (E) is captured, generation of a sulfide phase at the interface between the solder joint and the flux residue can be suppressed.

The flux composition may be blended with an antioxidant for the purpose of suppressing oxidation of the lead-free solder alloy powder. Examples of the antioxidant include hindered phenolic antioxidants, phenolic antioxidants, bisphenolic antioxidants, and polymer-type antioxidants. Of these, hindered phenol-based oxidizing agents are particularly preferably used. You may use these individually by 1 type or in mixture of multiple types.
The blending amount of the antioxidant is not particularly limited, but generally it is preferably about 0.5% by mass or more and about 5% by mass or less based on the total amount of the flux composition.

An additive can be blended with the flux composition as necessary. Examples of the additive include an antifoaming agent, a surfactant, a matting agent, and an inorganic filler. You may use these individually or in mixture of multiple types.
The blending amount of the additive is preferably 0.5% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 15% by mass or less with respect to the total amount of the flux composition.

The solder paste of this embodiment can be obtained, for example, by mixing the alloy powder and the flux composition.
The blending ratio of the alloy powder and the flux composition is preferably 65:35 to 95: 5 in the ratio of alloy powder: flux composition. The more preferred blending ratio is 85:15 to 93: 7, and the particularly preferred blending ratio is 87:13 to 92: 8.

  The particle diameter of the alloy powder is preferably 1 μm or more and 40 μm or less, more preferably 5 μm or more and 35 μm or less, and particularly preferably 10 μm or more and 30 μm or less.

(3) Solder joint The solder joint of the present embodiment is formed by printing the solder paste at a predetermined position on the substrate and performing reflow at a temperature of 230 ° C. to 260 ° C., for example. This reflow forms a flux residue derived from the solder joint and the flux composition on the substrate.

In addition, the electronic circuit board having such a solder joint part, for example, forms an electrode and a solder resist film at a predetermined position on the substrate, and prints the solder paste of the present embodiment using a mask having a predetermined pattern, An electronic component that conforms to the pattern is mounted at a predetermined position and is reflowed.
In the electronic circuit board thus manufactured, a solder joint is formed on the electrode, and the solder joint electrically joins the electrode and the electronic component. And on the said board | substrate, the flux residue has adhered so that it may adhere | attach to a solder joint part at least.

Since the solder joint part and the electronic circuit board having the solder joint part of this embodiment are formed with the solder joint part and the flux residue using the solder paste, a lead-free solder alloy powder containing a certain amount of Sb is used. Even in this case, it is difficult for the sulfide phase to be generated at the interface between the solder joint and the flux residue, so that abnormalities in the appearance inspection can be suppressed and the yield can be prevented from deteriorating.
And the electronic circuit board which has such a solder joint part can be used suitably also for the electronic circuit board by which high reliability is calculated | required, such as a vehicle-mounted electronic circuit board.

  In addition, by incorporating such an electronic circuit board, a highly reliable electronic control device is manufactured.

  Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples. The present invention is not limited to these examples.

<Synthesis of acrylic resin>
A solution in which 10% by mass of methacrylic acid, 51% by mass of 2-ethylhexyl methacrylate, and 39% by mass of lauryl acrylate were mixed was prepared.
Thereafter, 200 g of diethylhexyl glycol was charged into a 500 ml four-necked flask equipped with a stirrer, a reflux tube and a nitrogen introduction tube, and heated to 110 ° C. Next, dimethyl 2,2′-azobis (2-methylpropionate) (product name: V-601, manufactured by Wako Pure Chemical Industries, Ltd.) as an azo radical initiator was added to 300 g of the solution from 0.2% by mass to 5%. Mass% was added to dissolve it.
This solution was added dropwise to the four-necked flask over 1.5 hours, and the components in the four-necked flask were stirred at 110 ° C. for 1 hour, and then the reaction was terminated to obtain a synthetic resin. The synthetic resin had a mass average molecular weight of 7,800 Mw, an acid value of 40 mgKOH / g, and a glass transition temperature of −47 ° C.

<Synthesis of rosin derivative compound>
138.6 g of hydrogenated acid-modified rosin resin (hydrogenated acrylic acid-modified rosin resin hydrogenated by reacting rosin with acrylic acid) in a 500 ml four-necked flask equipped with a stirring blade, Dean-Stark apparatus and nitrogen introduction tube (COOH group: 0.6 mol) and dimer diol 85.5 g (OH group: 0.3 mol) were charged, and these were stirred at 150 ° C. for 1 hour in a nitrogen atmosphere to dissolve the hydrogenated acid-modified rosin resin.
Next, 5.7 g (0.03 mol) of p-toluenesulfonic acid hydrate was added thereto, and this was heated to 180 ° C. to perform a dehydration reaction. This was allowed to react for 3 hours until dehydration ceased, then allowed to cool to room temperature, and further 200 g of ethyl acetate was added to make a homogeneous solution.
Then, the solution was neutralized with a saturated aqueous sodium hydrogen carbonate solution and concentrated after separation to obtain 190.5 g of a rosin derivative compound (acid value: 77 mgKOH / g, mass average molecular weight: 2,400 Mw).

  Each component described in Table 1 was kneaded to obtain each flux composition according to Examples 1 to 10 and Comparative Examples 1 to 5. In addition, unless otherwise indicated, the unit of the blending amount shown in Table 1 is mass%.

* 1 Hydrogenated acid-modified rosin manufactured by Arakawa Chemical Co., Ltd. * 2 Ethylene bishydroxystearic acid amide manufactured by Nippon Kasei Co., Ltd. * 3 Hindered phenolic antioxidant manufactured by BASF Japan

<Acid value derived from activator (B)>
About each Example and each comparative example, the acid value derived from an activator (B) was computed among the acid values of a flux composition. The results are shown in Table 2.

Preparation of Solder Paste 11% by mass of each flux composition and 89% by mass of the following solder alloy powder were kneaded to prepare each solder paste according to Examples 1 to 10 and Comparative Examples 1 to 5.
Examples 1 to 9 and Comparative Examples 1, 3 and 4
Sn-3Ag-0.5Cu-0.5Bi-3Sb-0.03Ni solder alloy / Example 10 and Comparative Example 5
Sn-3Ag-0.5Cu-0.5Bi-5Sb-0.03Ni solder alloy / Comparative Example 2
Sn-3Ag-0.5Cu solder alloy * The particle diameter of the powder of the solder alloy is 19 μm to 41 μm.

(1) Sulfide phase generation test Each solder paste was printed on a 50 mm × 50 mm × 0.3 mm phosphoric acid-deoxidized copper plate using a metal mask having a thickness of 6.5 mm and a thickness of 0.2 mm. Next, each phosphoric acid-deoxidized copper plate after printing was reflowed under the temperature profile conditions shown in FIG. 1 using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation) to prepare each test plate. The oxygen concentration was set to 1500 ± 500 ppm.
The area of the sulfide phase generated on the surface of the solder portion (the solder alloy melted and aggregated) formed on each test plate was measured and evaluated as follows. The results are shown in Table 2. In addition, since the sulfide phase is exhibiting black, the area is measured by setting the blackened portion as the sulfide phase.
○: The area of the sulfide phase is 0%
Δ: The area of the sulfide phase is 10% or less ×: The area of the sulfide phase is more than 10% and 20% or more

(2) Void test A chip component having a size of 3.2 mm × 1.6 mm, a solder resist having a pattern on which the chip component of the size can be mounted, and an electrode (1.25 mm × 1.0 mm) connecting the chip component And a metal mask with a thickness of 150 μm having the same pattern were prepared.
Each solder paste was printed on the glass epoxy substrate using the metal mask, and 20 chip components were mounted. Each glass epoxy substrate is reflowed under a temperature profile condition shown in FIG. 1 using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation), and the glass epoxy substrate and the chip component are electrically connected. Each test substrate having a solder joint part for jointly joining was prepared. The oxygen concentration was set to 1500 ± 500 ppm.
The surface state of each test substrate produced was observed with an X-ray transmission device (product name: SMX-160E, manufactured by Shimadzu Corporation), and the area under the electrode of the chip component (see FIG. The area of the void in the area (a) surrounded by the broken line 2 and the area of the void in the area where the fillet is formed (area (b) surrounded by the broken line in FIG. 2) were measured. Based on the measurement results, a void area ratio (total void area / total land area × 100) was calculated and evaluated as follows. The results are shown in Table 2.
○: Void area ratio is 10% or less △: Void area ratio is more than 10% and 15% or less ×: Void area ratio is more than 15%

(3) Connection crack resistance test A 0.65 mm pitch QFP (Quad Flat Package) component having a size of 16 mm × 22 mm × 1.4 mm, a solder resist having a pattern capable of mounting the QFP component, and an electrode for connecting the QFP component ( A glass epoxy substrate (thickness 1.6 mm) provided with a maker recommended design) and a 150 μm thick metal mask having the same pattern were prepared.
Each solder paste was printed on the glass epoxy substrate using the metal mask, and the QFP component was mounted on each substrate.
Next, each printed substrate is reflowed under a temperature profile condition shown in FIG. 3 using a reflow oven (product name: TNP-538EM, manufactured by Tamura Corporation), and the glass epoxy substrate and the QFP component are combined. Each test substrate having a solder joint part to be electrically joined was produced. The oxygen concentration was set to 1500 ± 500 ppm.
Next, using a thermal shock test apparatus (product name: ES-76LMS, manufactured by Hitachi Appliances Co., Ltd.) set at -40 ° C. (30 minutes) to 125 ° C. (30 minutes), a thermal shock cycle was applied to each test substrate. It was set to give 3,000 cycles repeatedly. Then, each test substrate was taken out of the thermal shock test apparatus every 1,000 thermal shock cycles, and the occurrence of connected cracks in the flux residue was visually confirmed, and evaluated as follows. The results are shown in Table 2. In addition, in this specification, a connection crack refers to the crack of the flux residue which generate | occur | produced so that it might straddle a some solder joint part.
◎: No connection cracking up to 3,000 cycles ○: Connection cracking occurred over 2,000 cycles and 3,000 cycles or less △: Connection cracks generated over 1,000 cycles and 2,000 cycles or less ×: 1,000 cycles or less Connected cracks occur at

(4) Solder cracking test A chip component (Ni / Sn plating) having a size of 3.2 mm × 1.6 mm, a solder resist having a pattern capable of mounting the chip component of the size, and an electrode for connecting the chip component. A glass epoxy substrate provided and a 150 μm thick metal mask having the same pattern were prepared.
Each solder paste composition was printed on the glass epoxy substrate using the metal mask, and the chip component was mounted on each of the solder paste compositions.
Thereafter, using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Seisakusho Co., Ltd.), each of the glass epoxy substrates is heated to electrically bond the glass epoxy substrate and the chip component to each other. A part was formed, and the chip component was mounted. The reflow conditions at this time are: preheating from 170 ° C. to 190 ° C. for 110 seconds, peak temperature of 245 ° C., time of 200 ° C. or higher for 65 seconds, time of 220 ° C. or higher for 45 seconds, peak temperature to 200 ° C. The cooling rate was 3 ° C. to 8 ° C./second, and the oxygen concentration was set to 1500 ± 500 ppm.
Next, using a thermal shock test apparatus (product name: ES-76LMS, manufactured by Hitachi Appliances Co., Ltd.) set to -40 ° C. (30 minutes) to 125 ° C. (30 minutes), the thermal shock cycle is 1, Each glass epoxy substrate was exposed to each other in an environment where 000, 1,500, 2,000, 2,500, and 3,000 cycles were repeated, and then taken out to prepare each test substrate.
Subsequently, the target part of each test board | substrate was cut out, and this was sealed using the epoxy resin (Product name: Epomount (main agent and hardening | curing agent), the refine tech Co., Ltd. product). Further, a solder joint formed by using a wet polishing machine (product name: TegraPol-25, manufactured by Marumoto Struers Co., Ltd.) so that the center cross section of the chip component mounted on each test substrate can be seen. Was observed using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation) to determine whether or not the cracks generated in the sample completely crossed the solder joint and led to the fracture. And evaluated. The results are shown in Table 3. The number of evaluation chips in each thermal shock cycle was 10.
A: No crack that completely traverses the solder joint until 3,000 cycles. O: A crack that completely traverses the solder joint occurs between 2,501 and 3,000 cycles. Δ: 2,001 to 2 , Cracks that completely traverse the solder joints occur during 500 cycles ×: cracks that completely traverse the solder joints occur after 2,000 cycles or less

As described above, in the solder paste according to the example, the acid value derived from the activator (B) is 80 mgKOH / g or more and Sb is constant by adding the acidic phosphate ester (E) to the flux composition. It can be seen that even when the above lead-free solder alloy powder is used, the generation of the sulfide phase can be suppressed to a certain level, and both the suppression of voids and the suppression of cracks in the flux residue can be achieved. Furthermore, since these solder pastes contain Sb in the lead-free solder alloy powder to be used, it can be seen that the solder joint has good crack resistance even in an environment where the temperature difference is high.
That is, the solder paste according to the example exhibits the same or better effect than the solder paste of Comparative Example 2 using the lead-free solder alloy powder containing no Sb in the sulfide phase generation test, void test and joint crack resistance test. In addition, since the lead-free solder alloy powder containing a certain amount or more of Sb is used, the crack resistance of the solder joint can be improved.

On the other hand, when a lead-free solder alloy powder having an acid value derived from the activator (B) in the flux composition of 80 mgKOH / g or more and containing Sb or more as in Comparative Examples 1 and 5 is used, the flux composition It turns out that a sulfide phase will generate | occur | produce on the surface of a solder joint part, when acidic phosphate ester (E) is not mix | blended with.
In addition, although the comparative example 3 mix | blends neutral phosphate ester, it turns out that neutral phosphate ester cannot suppress generation | occurrence | production of a sulfide phase.
Further, as in Comparative Example 4, when the acid value derived from the base resin (A) and the acid value derived from the activator (B) in the flux composition are low, the surface activation of the lead-free solder alloy becomes insufficient. It can be seen that the wettability deteriorates and voids are easily generated in the solder joint.

  As described above, the solder paste of the present invention is economical because it can suppress the deterioration of the yield in the manufacturing process, and can also exhibit the effect of suppressing voids and the effect of suppressing crack propagation of the flux residue, and further includes a lead-free solder alloy containing Sb. Since the powder is used, the crack resistance of the solder joint can be improved, so that it can be suitably used for an electronic circuit board that requires high reliability, such as an in-vehicle electronic circuit board. Furthermore, such an electronic circuit board can be suitably used for an electronic control device that requires higher reliability.

Claims (10)

Ag content is 0.1 mass% or more and 5 mass% or less, Cu content is 0.5 mass% or more and 1 mass% or less, Sb content is 1 mass% or more and 10 mass% or less , and the balance is Sn A powder composed of a lead-free solder alloy,
A base resin (A), the active agent (B), and a thixotropic agent (C), and a solvent (D), acidic phosphoric acid ester (E) A solder paste comprising a including flux composition,
The amount of the acidic phosphate ester (E) is 1% by mass or more and 10% by mass or less based on the total amount of the flux composition,
The solder composition, wherein the flux composition has an acid value of 80 mgKOH / g or more, and the acid value derived from the activator (B) is 80 mgKOH / g or more . Ag content is 0.1 mass% or more and 5 mass% or less, Cu content is 0.5 mass% or more and 1 mass% or less, Sb content is 1 mass% or more and 10 mass% or less, In content Is a powder made of a lead-free solder alloy of 0.1 mass% to 6 mass% with the balance being Sn ,
A solder paste comprising a base resin (A), an activator (B), a thixotropic agent (C), a solvent (D), and a flux composition comprising an acidic phosphate ester (E),
The amount of the acidic phosphate ester (E) is 1% by mass or more and 10% by mass or less based on the total amount of the flux composition,
The solder composition, wherein the flux composition has an acid value of 80 mgKOH / g or more, and the acid value derived from the activator (B) is 80 mgKOH / g or more. 3. The solder paste according to claim 1, wherein the lead-free solder alloy further contains Bi in an amount of 0.5 mass% to 5 mass%. The solder paste according to any one of claims 1 to 3, wherein the lead-free solder alloy further contains 0.01 mass% or more and 0.1 mass% or less of Ni. The solder paste according to claim 4 , wherein the lead-free solder alloy further contains Co in an amount of 0.001% by mass to 0.015% by mass. The lead-free solder alloy may further P, any one of claims 1 to 5, characterized in that it comprises Ga and Ge of at least one of 0.05 wt% 0.001 wt% in total less Solder paste as described in 1. The lead-free solder alloy, any further Fe, Mn, from claim 1, characterized in that it comprises Cr and Mo of at least one of total 0.001 wt% to 0.05 wt% or less of claim 6 The solder paste according to item 1. The said acid phosphate ester (E) is represented by following General formula (1), The solder paste of any one of Claims 1-7 characterized by the above-mentioned.
(In the formula, R represents a linear or branched alkyl group having 8 to 24 carbon atoms. N is 1 or 2, and when n is 2, R may be the same or different. Good.)   The acidic phosphate ester (E) is at least one selected from 2-ethylhexyl acid phosphate, bis (2-ethylhexyl) phosphate, tetracosyl acid phosphate, and isotridecyl acid phosphate. The solder paste according to any one of claims 1 to 8. A solder joint formed by using the solder paste according to any one of claims 1 to 9 .