JP6560272B2 - Solder paste, electronic circuit board and electronic control device - Google Patents

Description

  The present invention relates to a solder paste and an electronic circuit board and an electronic control device having a solder joint formed using the solder paste.

  In order to join an electronic component to an electronic circuit formed on a substrate such as a printed wiring board or a silicon wafer, a solder joining method using a solder paste is employed. 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.

For this lead-free solder alloy powder, for example, Sn—Cu, Sn—Ag—Cu, Sn—Bi, Sn—Zn solder alloys and the like are often used. Among them, Sn-3Ag-0.5Cu solder alloy is often used in consumer electronic devices used for televisions, mobile phones, etc. and in-vehicle electronic devices mounted on automobiles.
Although the lead-free solder alloy is somewhat inferior in solderability as compared with the lead-containing solder alloy, the problem of solderability is covered by the improvement of the flux and the soldering apparatus. For this reason, for example, even in an in-vehicle electronic circuit board, a Sn-3Ag-0.5Cu solder alloy is used in an in-vehicle electronic circuit board that is placed in a relatively mild environment although there is a difference in temperature. There is no major problem with the solder joints formed.

  However, in recent years, for example, an electronic circuit board used in an electronic control device has a particularly severe temperature difference such as engine compartment, engine direct mounting, or mechanical / electrical integration with a motor (for example, −30 ° C. to 110 ° C., −40 ° C. In addition, the arrangement and implementation of electronic circuit boards have been studied in a severe environment in which a vibration load such as a temperature difference of from 125 to 125 ° C. and from −40 ° C. to 150 ° C. is applied.

  Under such an environment where the temperature difference is very severe, thermal displacement of the solder joint and stress accompanying it due to the difference in coefficient of linear expansion between the mounted electronic component and the substrate are likely to occur. Repeated plastic deformation tends to cause cracks in the solder joints, and stresses repeatedly applied are concentrated near the tips of the cracks, so that the cracks easily propagate transversely to the deep part of the solder joints. The crack that has progressed remarkably in this way causes the disconnection of the electrical connection between the electronic component and the electronic circuit formed on the substrate. In particular, in an environment in which vibration is loaded on the electronic circuit board in addition to a severe temperature difference, the crack and its development are more likely to occur.

  Several methods for improving the strength of the solder joint and the accompanying thermal fatigue characteristics by adding an element such as Bi to the Sn—Ag—Cu based solder alloy in order to suppress crack growth in the solder joint are disclosed. (See Patent Document 1 to Patent Document 7).

Japanese Patent Laid-Open No. 5-228685 Japanese Patent Laid-Open No. 9-326554 Japanese Patent Laid-Open No. 2000-190090 JP 2000-349433 A JP 2008-28413 A International Publication Pamphlet WO2009 / 011341 JP 2012-81521 A

  However, the Sn-3Ag-0.5Cu solder alloy has a higher solidus temperature / liquidus temperature of about 40 ° C. or higher than that of conventional Sn—Pb eutectic solder, and also contains highly viscous Cu. ing. Therefore, if the oxide film of the alloy powder using this cannot be removed sufficiently, voids are likely to occur during solder bonding, and voids are likely to remain in the formed solder joint.

For example, if a void occurs near the interface with the electronic component in the solder joint, the electronic component is not symmetrical (the thickness of the solder joint located above the electrode on the substrate and below the bottom electrode of the electronic component is It becomes easy to be bonded to the substrate (in a non-uniform state). For this reason, among the solder joints, the joint life of the thin (thin) part is further shortened. Such a solder joint part is liable to crack from a portion having no thickness, particularly in an environment where the difference between the temperature and the temperature is severe, and the crack progresses easily.
Furthermore, if a void occurs in the fillet portion of the solder joint, the void volume and crack path are shortened compared to a solder joint that does not have a void in the fillet portion, so a crack occurs across the solder joint. There is a risk of becoming easier.

  Especially when adding highly oxidizable elements such as Bi, In and Sb to the Sn-Ag-Cu solder alloy, the surface oxide film of the alloy powder is sufficiently removed than the Sn-3Ag-0.5Cu solder alloy. It tends to be difficult. For this reason, if the active force of the flux composition to be used is insufficient, the viscosity of the molten alloy powder is difficult to decrease, voids are likely to remain in the solder joints, and it is difficult for the alloy powders to agglomerate and fuse together, resulting in solder balls. There is a problem that it is easy to do. Solder balls cause open defects such as an unfused phenomenon between the electrodes of the electronic components mounted on the board and the solder paste, and short-circuits. Therefore, solder balls are particularly useful for automotive electronic circuit boards that require high reliability. Suppression of occurrence is one of the important issues.

  In order to suppress the generation of solder balls, it is conceivable to add an activator having a strong activity, but such an activator is likely to react from the mixing of the alloy powder and the flux composition, so that during preheating or There is a risk that most of it will volatilize during reflow. In order to prevent such a situation, it is conceivable to add a large amount of the active agent to the flux composition. However, such a large amount of the active agent may inhibit the printability of the solder paste.

  In addition, as described above, the solder paste using the highly oxidizable solder alloy powder easily generates voids, which lead to the crack progress of the solder joints in an environment where the temperature varies greatly. There is a risk of lowering the reliability.

  The present invention solves the above-described problem, and even if a solder alloy powder containing a highly oxidizable alloy element is used, it is possible to suppress the generation of voids in the solder joints, so that the difference between cold and warm is greatly affected by vibration. Solder paste capable of further suppressing crack growth of solder joints in a severe environment and exhibiting good printability while suppressing generation of solder balls, and solder joints formed using the same It is an object of the present invention to provide an electronic circuit board and an electronic control device having the above.

(1) The solder paste according to the present invention has Ag of 1% by mass to 3.1% by mass, Cu of more than 0% by mass of 1% by mass and less, Sb of 1% by mass to 5% by mass of Bi, 0.5% by mass or more and 4.5% by mass or less, Ni containing 0.01% by mass or more and 0.25% by mass or less, an alloy powder made of a lead-free solder alloy with the balance made of Sn, and (A) base A flux composition containing a resin, (B) an activator, (C) a thixotropic agent, and (D) a solvent, wherein the blending amount of the activator (B) is 4. 5% by mass or more and 35% by mass or less, and (B-1) a straight-chain saturated dicarboxylic acid having 3 to 4 carbon atoms as the activator (B) is 0.5% by mass with respect to the total amount of the flux composition. 3 mass% or less, (B-2) flux group containing 5 to 13 carbon dicarboxylic acids A flux composition containing 2% by mass to 15% by mass with respect to the total amount of the product, and (B-3) 2 to 15% by mass of the dicarboxylic acid having 20 to 22 carbon atoms with respect to the total amount of the flux composition; It is characterized by including.

(2) In the configuration described in (1) above, the lead-free solder alloy further includes Co in an amount of 0.001% by mass to 0.25% by mass.

(3) In the configuration described in (1) or (2) above, the lead-free solder alloy has a Bi content of 3.1% by mass or more and 4.5% by mass or less. To do.

(4) The solder paste according to the present invention has Ag of 1% by mass to 3.1% by mass, Cu of more than 0% by mass of 1% by mass and less, Sb of 1% by mass to 5% by mass of Bi, 0.5 mass% or more and 4.5 mass% or less, Ni 0.01 mass% or more and 0.25 mass% or less, Co 0.001 mass% or more and 0.25 mass% or less, and remainder is Sn An alloy powder made of a lead-free solder alloy in which each content (mass%) of Ag, Cu, Sb, Bi, Ni, and Co satisfies all of the following formulas A to D, (A) a base resin, B) A flux composition comprising an activator, (C) a thixotropic agent, and (D) a solvent, wherein the blending amount of the activator (B) is 4.5% by mass or more based on the total amount of the flux composition. 30% by mass or less, and (B-1) carbon number is 3 to 4 as the activator (B). The linear saturated dicarboxylic acid is 0.5% by mass or more and 3% by mass or less based on the total amount of the flux composition, and (B-2) the dicarboxylic acid having 5 to 13 carbon atoms is 2% by mass based on the total amount of the flux composition. And 15% by mass or less, and (B-3) a flux composition containing 2 to 15% by mass of a dicarboxylic acid having 20 to 22 carbon atoms based on the total amount of the flux composition. .
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 / 0.25) + (Co content / 0.25) ≦ 1.19... D

(5) In the configuration according to any one of (1) to (4), the lead-free solder alloy further includes more than 0 mass% and 6 mass% or less of In.

(6) In the configuration according to any one of (1) to (5), 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.

(7) In the configuration according to any one of (1) to (6), 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.

(8) In the configuration according to any one of (1) to (7) above, the linear saturated dicarboxylic acid (B-1) having 3 to 4 carbon atoms is at least malonic acid and succinic acid On the other hand, the dicarboxylic acid (B-2) having 5 to 13 carbon atoms is glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, 2-methyl azelaic acid, sebacic acid, undecanedioic acid, 2, It is at least one selected from 4-dimethyl-4-methoxycarbonylundecanedioic acid, dodecanedioic acid, tridecanedioic acid and 2,4,6-trimethyl-4,6-dimethoxycarbonyltridecanedioic acid, the carbon number Dicarboxylic acids (B-3) having a valence of 20 to 22 are eicosadioic acid, 8-ethyloctadecanedioic acid, 8,13-dimethyl-8,12-eicosadienedioic acid and 11-vinyl. And its features is at least one selected from 8-octadecenoic diacid.

(9) An electronic circuit board according to the present invention is characterized by having a solder joint formed by using the solder paste according to any one of (1) to (8) above.

(10) An electronic control device according to the present invention includes the electronic circuit board according to (9).

  The solder paste of the present invention, and the electronic circuit board and the electronic control device having a solder joint formed by using the solder paste can generate voids in the solder joint even when using a solder alloy powder containing a highly oxidizable alloy element. Can suppress the crack progress of the solder joint in a harsh environment where the difference in temperature is intense and the vibration is applied, and also exhibits good printability while suppressing the generation of solder balls. be able to.

The photograph taken from the chip component side using the X-ray transmissive apparatus showing the area | region under the electrode of a chip component, and the area | region in which the fillet is formed in the test board which concerns on the Example and comparative example of this invention.

  Hereinafter, an embodiment of a solder paste, an electronic circuit board, and an electronic control device 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 may contain 1% by mass to 3.1% by mass of Ag. By adding Ag, the Ag ₃ Sn compound can be precipitated in the Sn grain boundary of the lead-free solder alloy, and mechanical strength can be imparted.
However, when the Ag content is less than 1% by mass, precipitation of the Ag ₃ Sn compound is small, and the mechanical strength and thermal shock resistance of the lead-free solder alloy are lowered, which is not preferable. Moreover, even if Ag is added in an amount exceeding 3.1% by mass, the tensile strength is not significantly improved, and it does not lead to a dramatic improvement in thermal fatigue resistance. It is economically undesirable to increase the content of expensive Ag. Furthermore, when the Ag content exceeds 4% by mass, the stretchability of the lead-free solder alloy is hindered, and a solder joint formed using this may cause an electrode peeling phenomenon of an electronic component, which is not preferable.
Further, 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 may contain more than 0 mass% 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.
When the Cu content is 0.5% by mass to 1% by mass, a good Cu erosion preventing effect can be exhibited. 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 reflow 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.
Here, generally, a solder joint formed using a lead-free solder alloy containing Sn, Ag, and Cu has an intermetallic compound (for example, Ag ₃ Sn, Cu ₆ Sn _5, etc.) at the interface between Sn particles. Even when a tensile force is applied to the solder joint, the structure can prevent the phenomenon that the Sn particles slip and deform, and so-called mechanical characteristics can be exhibited. That is, the intermetallic compound plays a role of preventing Sn particles from slipping.
Therefore, in the case of the lead-free solder alloy, the balance of the contents of Ag and Cu is such that Ag is not less than 1% by mass and not more than 3.1% by mass, Cu is more than 0% by mass and not more than 1% by mass, and the Ag content is Cu. By setting it to the same amount or more than the content, Ag ₃ Sn can be easily formed as the intermetallic compound, and the Cu content can exhibit at least relatively good mechanical properties. That is, even if the Cu content is 1% by mass or less, it contributes to the anti-slipping effect of Ag ₃ Sn while part of it becomes an intermetallic compound, so it is good in both Ag ₃ Sn and Cu. It is thought that it can exhibit a good mechanical property.

The lead-free solder alloy can contain 1% by mass to 5% by mass of Sb. By adding Sb within this range, the crack growth suppression effect of the solder joint can be improved without impairing the stretchability of the Sn-Ag-Cu solder alloy, and sufficient toughness against external stress can be secured. Residual stress can also be relieved because it is possible. In particular, when the Sb content is 2% by mass or more and 4% by mass or less, the effect of suppressing crack propagation can be further improved.
However, if the content of Sb exceeds 5% by mass, the melting temperature (solidus temperature / liquidus temperature) of the lead-free solder alloy increases, and Sb does not re-dissolve at high temperatures. Therefore, when exposed to a harsh environment where the difference between the temperature and the temperature is severe, only precipitation strengthening by the SnSb, ε-Ag ₃ (Sn, Sb) compound is performed, so that these intermetallic compounds become coarse over time. And the effect of suppressing slip deformation at the Sn grain boundary is lost. Further, in this case, the heat resistance temperature of the electronic component becomes a problem due to an increase in the melting temperature of the lead-free solder alloy, which is not preferable.

The lead-free solder alloy can contain 0.5 mass% or more and 4.5 mass% or less of Bi. If it is the structure of the lead-free solder alloy used for the solder paste of this embodiment, by adding Bi within this range, its strength will be improved and the Sb addition will increase without affecting its stretchability. The melting temperature can be lowered. 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.
However, if the Bi content exceeds 4.5% by mass, the extensibility of the lead-free solder alloy is lowered and the brittleness is increased. Therefore, when the Bi is exposed to a harsh environment where the temperature difference is severe, It is not preferable because a deep crack is likely to occur in a solder joint formed by a free solder alloy.
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. Moreover, when using together with Ni and / or Co mentioned later, preferable content of Bi is 3.1 mass% or more and 4.5 mass% or less.

The lead-free solder alloy can contain 0.01 mass% or more and 0.25 mass% or less of Ni. If it is the structure of the lead-free solder alloy used for the solder paste of this embodiment, by adding Ni in this range, fine (Cu, Ni) ₆ Sn ₅ is formed in the molten lead-free solder alloy. Since it disperses in the base material, it is possible to suppress the progress of cracks in the solder joint and to further improve the 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. On the other hand, if the Ni content exceeds 0.25% by mass, it is difficult for supercooling to occur as compared with the conventional Sn-3Ag-0.5Cu alloy, and the timing at which the solder alloy solidifies becomes earlier. Therefore, in the fillet of the solder joint portion to be formed, the gas that tried to escape to the outside during melting of the solder alloy is solidified while remaining in it, and a hole (void) due to the gas is generated in the fillet. The case that ends up 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.
Note that, as described above, Ni is likely to generate voids in the fillet, but the solder paste of the present embodiment has a balance between the content of Ni and other elements in the lead-free solder alloy used for this and will be described later. By using the flux composition to be used, the generation of the voids can be suppressed even when Ni is contained in an amount of 0.25% by mass or less.

Further, when the Ni content is 0.01% by mass or more and 0.15% by mass or less, it is possible to improve the suppression of void generation while improving the effect of suppressing crack propagation near the interface and heat fatigue resistance.

The lead-free solder alloy can contain 0.001 mass% or more and 0.25 mass% or less of Co in addition to Ni. If it is the structure of the lead-free solder alloy used for the solder paste of this embodiment, by adding Co in this range, the above-described effect due to the addition of Ni is enhanced and fine (Cu, Co ) Since ₆ Sn ₅ is formed and dispersed in the base metal, it suppresses the creep deformation of the solder joint and suppresses the progress of cracks. Can be improved.
In addition, such a lead-free solder alloy enhances the above-described effect due to the addition of Ni even when soldering an electronic component that is not subjected to Ni / Pd / Au plating or Ni / Au plating. Since it moves to the vicinity of the interface during bonding to form fine (Cu, Co) ₆ Sn ₅ , the growth of the Cu ₃ Sn layer in the vicinity of the interface can be suppressed, and the crack growth suppression effect in the vicinity of the interface can be improved. Can do.

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. On the other hand, if the Co content exceeds 0.25% by mass, it is difficult for supercooling to occur as compared with the conventional Sn-3Ag-0.5Cu alloy, and the timing at which the solder alloy solidifies becomes earlier. Therefore, in the fillet of the solder joint portion to be formed, there is a case in which the gas that tried to escape outside during melting of the solder alloy is solidified while remaining in it, and a void due to the gas is generated in the fillet. It 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.
Note that, as described above, Co is likely to generate voids in the fillet, but the solder paste of the present embodiment has a balance between the content of Co and other elements in the lead-free solder alloy used for this, and will be described later. By using the flux composition, the generation of the voids can be suppressed even when Co is contained in an amount of 0.25% by mass or less.

Further, when the Co content is 0.001% by mass or more and 0.15% 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 Ni and Co are used together in the lead-free solder alloy, it is preferable that the contents (mass%) of Ag, Cu, Sb, Bi, Ni, and Co satisfy all of the following formulas A to D. .
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 / 0.25) + (Co content / 0.25) ≦ 1.19... D

  Thus, by making the contents of Ag, Cu, Sb, Bi, Ni, and Co within the above ranges, the stretchability of the solder joints is inhibited and the brittleness is suppressed, and the strength and thermal fatigue characteristics of the solder joints are improved. , Suppression of voids generated in fillets, Suppression of cracks in solder joints under severe environments with severe temperature differences, and soldering of electronic parts that are not plated with Ni / Pd / Au or Ni / Au All of the effects of suppressing crack growth near the interface can be exhibited in a balanced manner, and the reliability of the solder joint can be further improved.

The lead-free solder alloy may contain more than 0 mass% and 6 mass% or less 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 also SnSnIn and InSb compounds are formed and precipitated at the Sn grain boundaries. It has the effect of suppressing slip deformation at grain boundaries.
However, if the In content exceeds 6% by mass, the extensibility of the lead-free solder alloy is inhibited, and γ-InSn ₄ is formed during a long period of exposure to a harsh environment where the difference in temperature is high. This is not preferable because the lead-free solder alloy is deformed by itself.
A more preferable content of In is more than 0% by mass and 4% by mass or less, and particularly preferably 1% by mass to 2% by mass.

  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.

  Moreover, it is preferable that the remainder of the lead-free solder alloy is made of Sn. In addition, preferable Sn content is 79.8 mass% or more and less than 97.49 mass%.

  The lead-free solder alloy itself can suppress the generation of voids in the formed solder joint due to the balance between the composition and content described above. Further, by using a flux composition described later, although a highly oxidizable element such as Bi, In and Sb is added, the surface oxide film of the alloy powder can be sufficiently removed. Residual voids in the joints can be suppressed, crack growth in the solder joints can be suppressed, solder balls can be prevented from being agglomerated and fused, and good printability can be achieved. can do.

<Flux composition>
The flux composition used for the solder paste of the present embodiment preferably contains (A) a base resin, (B) an activator, (C) a thixotropic agent, and (D) a solvent.

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

  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. And a modified rosin resin obtained by subjecting these rosin or rosin derivative and an unsaturated carboxylic acid (acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, etc.) to a Diels-Alder reaction. 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.

  Examples of the synthetic resin (A-2) include acrylic resin, styrene-maleic acid resin, epoxy resin, urethane resin, polyester resin, phenoxy resin, terpene resin, polyalkylene carbonate, rosin resin having a carboxyl group, and dimer acid. Derivative compounds formed by dehydration condensation with a derivative flexible alcohol compound are exemplified. In addition, you may use these individually by 1 type or in mixture of multiple types. 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.

Regarding a derivative compound obtained by dehydrating and condensing the rosin resin having a carboxyl group and a dimer acid derivative flexible alcohol compound (hereinafter referred to as “rosin derivative compound”), as the rosin resin having a carboxyl group, for example, Rosin such as tall oil rosin, gum rosin, wood rosin; hydrogenated rosin, polymerized rosin, heterogeneous rosin, acrylic acid modified rosin, rosin derivatives such as maleic acid modified rosin, etc. Can be used. These may be used alone or in combination of two or more.
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 is obtained by dehydrating and condensing 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 synthetic resin (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.

  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 achieved.

  Moreover, when using the said synthetic resin (A-2) 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.

  Further, when the rosin resin (A-1) and the synthetic resin (A-2) are used in combination, the blending ratio is preferably 20:80 to 50:50, and 25:75 to 40:60. More preferably.

  The base resin (A) is preferably a rosin resin (A-1) alone or an acrylic resin in combination as the rosin resin (A-1) and the synthetic resin (A-2).

(B) Activator As the activator (B), (B-1) a linear saturated dicarboxylic acid having 3 to 4 carbon atoms is 0.5% by mass or more and 3% by mass or less, based on the total amount of the flux composition. (B-2) The dicarboxylic acid having 5 to 13 carbon atoms is 2% by mass to 15% by mass with respect to the total amount of the flux composition, and (B-3) the dicarboxylic acid having 20 to 22 carbon atoms is the flux composition. It is preferable to contain 2 mass% or more and 15 mass% or less with respect to the whole quantity.
It is preferable that the compounding quantity of the said activator (B) is 4.5 to 35 mass%, and it is more preferable that it is 4.5 to 20 mass%.

The linear saturated dicarboxylic acid (B-1) having 3 to 4 carbon atoms is preferably at least one of malonic acid and succinic acid.
Moreover, the more preferable compounding quantity of the said C3-C4 linear saturated dicarboxylic acid (B-1) is 0.5 mass% to 2 mass% with respect to the flux composition whole quantity.

The carbon chain in the dicarboxylic acid (B-2) having 5 to 13 carbon atoms may be either linear or branched, but glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid 2-methyl azelaic acid, sebacic acid, undecanedioic acid, 2,4-dimethyl-4-methoxycarbonylundecanedioic acid, dodecanedioic acid, tridecanedioic acid, and 2,4,6-trimethyl-4,6-dimethoxy It is preferably at least one selected from carbonyl tridecanedioic acid. Of these, adipic acid, suberic acid, sebacic acid and dodecanedioic acid are particularly preferably used.
Moreover, the more preferable compounding quantity of the said C5-C13 dicarboxylic acid (B-2) is 3 mass% to 12 mass% with respect to the flux composition whole quantity.

  The carbon chain in the dicarboxylic acid (B-3) having 20 to 22 carbon atoms may be either a straight chain or a branched chain, but eicosadioic acid, 8-ethyloctadecanedioic acid, 8,13 It is preferably at least one selected from dimethyl-8,12-eicosadiene diacid and 11-vinyl-8-octadecenedioic acid.

Further, as the dicarboxylic acid (B-3) having 20 to 22 carbon atoms, those that are liquid or semi-solid at room temperature are more preferably used. In this specification, normal temperature refers to a range of 5 ° C to 35 ° C. The semi-solid state refers to a state corresponding to a liquid state and a solid state, a part of which has fluidity and a state of non-fluidity but deformed when external force is applied. As such a dicarboxylic acid (B-3) having 20 to 22 carbon atoms, 8-ethyloctadecanedioic acid is particularly preferably used.
In addition, the more preferable compounding quantity of the said C20-C22 dicarboxylic acid (B-3) is 3 mass% to 12 mass% with respect to the flux composition whole quantity.

  As the activator (B), a dicarboxylic acid corresponding to each carbon number range of the activators (B-1), (B-2) and (B-3) is blended according to the blending amount. The solder paste according to the embodiment can sufficiently remove the oxide film even when an alloy powder made of a lead-free solder alloy to which highly oxidizable elements such as Bi, In, and Sb are added is used. It is possible to improve the cohesive force between the powders and reduce the viscosity at the time of melting the solder, thereby reducing the solder balls generated on the side of the electronic component and the voids generated in the solder joint.

That is, the linear saturated dicarboxylic acid (B-1) having 3 to 4 carbon atoms partially coats the surface of the alloy powder when the flux composition and the alloy powder are kneaded. The surface oxidation can be suppressed. Further, the dicarboxylic acid (B-3) having 20 to 22 carbon atoms is slow in reactivity, so that it is stable in a solder paste printing process on a substrate for a long time and hardly volatilizes even during reflow heating. Therefore, it is possible to cover the surface of the molten alloy powder and suppress oxidation by a reducing action.
However, the dicarboxylic acid (B-3) having 20 to 22 carbon atoms has low activity, and the oxide film on the surface of the alloy powder can be obtained only in combination with the linear saturated dicarboxylic acid (B-1) having 3 to 4 carbon atoms. May not be sufficiently removed. Therefore, when an alloy powder made of the lead-free solder alloy containing a large amount of Bi, In, Sb, etc. is used, the oxidizing action on the alloy powder tends to be insufficient, and the effect of suppressing solder balls and voids is hardly exhibited. However, since the flux composition contains the dicarboxylic acid (B-2) having 5 to 13 carbon atoms exhibiting a strong activity from the preheat within the above range, the reliability of the flux residue is ensured while ensuring the reliability of the flux residue. Even when such an alloy powder is used, the oxide film can be sufficiently removed. Therefore, the solder paste of the present embodiment improves the cohesive force between the alloy powders and reduces the viscosity at the time of melting the solder, thereby causing voids generated in the solder balls and solder joints generated beside the electronic components. Can be reduced.
Moreover, the flux composition combined with such an activator can also exhibit good printability.

  That is, the flux composition is formulated by combining the activators (B-1), (B-2) and (B-3) with dicarboxylic acids corresponding to a specific carbon number range in a predetermined blending amount. By doing so, a good redox effect on the solder alloy can be exhibited. Although the lead-free solder alloy has an effect of suppressing the crack growth of the solder joint formed by the alloy composition, the use of such a flux composition uses a solder alloy powder containing an alloy element having a high oxidizing property as described above. However, the generation of voids in the solder joints can be suppressed, and therefore the crack propagation can be further suppressed.

Other active agents can be blended with the flux composition as long as the above effects are not impaired.
Examples of such other activators include amine salts (inorganic acid salts and organic acid salts) such as organic acid hydrogen halide salts, organic acids, organic acid salts, and organic amine salts. Specifically, diphenylguanidine hydrobromide, cyclohexylamine hydrobromide, diethylamine salt, dimer acid, levulinic acid, lactic acid, acrylic acid, benzoic acid, salicylic acid, anisic acid, citric acid, 1,4- Examples include cyclohexanedicarboxylic acid, anthranilic acid, picolinic acid, and 3-hydroxy-2-naphthoic acid. You may use these individually by 1 type or in mixture of multiple types.
It is preferable that the compounding quantity of the said other active agent is more than 0 mass% and 20 mass% or less with respect to the flux composition whole quantity.

(C) Thixotropic agent 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.

(D) Solvent 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, and butyl carbitol. , Octanediol, α-terpineol, β-terpineol, tetraethylene glycol dimethyl ether, trimellitic acid tris (2-ethylhexyl), bisisopropyl sebacate and the like. You may use these individually by 1 type or in mixture of multiple types.
The blending amount of the solvent (D) is preferably 20% by mass or more and 50% by mass or less, and more preferably 25% by mass or more and 40% by mass or less with respect to the total amount of the flux composition.

In the flux composition, an antioxidant may be blended for the purpose of suppressing oxidation of the 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 average particle size 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) Electronic circuit board It is preferable that the electronic circuit board of this embodiment has the solder joint part and flux residue which are formed using the said solder paste. The electronic circuit board includes, for example, an electrode and a solder resist film formed at a predetermined position on the substrate, the solder paste of this embodiment is printed using a mask having a predetermined pattern, and an electronic component conforming to the pattern is obtained. It is manufactured by mounting it at a predetermined position and reflowing it.
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.

In the electronic circuit board of this embodiment, the solder joint and the flux residue are formed using the solder paste, so that the solder joint can be removed even in a harsh environment where the difference in temperature is intense and vibration is applied. The crack progress can be suppressed, and good printability can be exhibited while suppressing the generation of voids and solder balls in the solder joint.
An electronic circuit board having such a solder joint can be suitably used for an electronic circuit board that requires high reliability, such as an in-vehicle electronic circuit board.

  Further, by incorporating such an electronic circuit board, the electronic control device of this embodiment 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 weight average molecular weight of the synthetic resin was 7,800 Mw, the acid value was 40 mgKOH / g, and the glass transition temperature was −47 ° C.

  Each component described in Table 1 and Table 2 was kneaded to obtain each flux composition according to Examples 1 to 12 and Comparative Examples 1 to 14. In addition, unless otherwise indicated, the unit of the compounding amount in Tables 1 and 2 is mass%.

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

Preparation of Solder Paste 11.2% by mass of the flux composition and 88.8% by mass of the following solder alloy powder were kneaded to prepare each solder paste according to Examples 1 to 12 and Comparative Examples 1 to 14. .
<Example>
Alloy (a): Sn-3Ag-0.7Cu-3.5Bi-3Sb-0.04Ni-0.01Co solder alloy Alloy (b): Sn-3Ag-0.5Cu-4.5Bi-3Sb-0.03Ni Solder Alloy Alloy (c): Sn-3Ag-0.5Cu-3.0Bi-2Sb-0.03Ni Solder Alloy Alloy (d): Sn-3Ag-0.7Cu-3.2Bi-3Sb-0.03Ni-0 .01Co-0.05Fe Solder Alloy <Comparative Example>
Alloy (a): Sn-3Ag-0.7Cu-3.5Bi-3Sb-0.04Ni-0.01Co solder alloy Alloy (e): Sn-0.5Ag-0.5Cu-3.0Bi-2Sb-0 .04Ni solder alloy * The particle size of the solder alloy powder is 20 μm to 36 μm.

(1) Void test A chip component having a size of 2.0 mm × 1.2 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) for connecting the chip component 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 chip component was mounted on each glass paste substrate.
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, the surface state of each test substrate 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 ratio of voids (ratio of the total area of voids; the same applies hereinafter) and the area where fillets are formed (area (b) surrounded by broken lines in FIG. 1) The average value of the area ratio of voids in the area was determined and evaluated as follows. The results are shown in Table 3 and Table 4, respectively.
A: The average value of the void area ratio is 3% or less and the effect of suppressing the generation of voids is very good. ○: The average value of the void area ratio is more than 3% and 5% or less, and the effect of suppressing the generation of voids. △: The average value of the void area ratio is more than 5% and 8% or less, and the effect of suppressing the generation of voids is sufficient. ×: The average value of the void area ratio exceeds 8%, and the effect of suppressing the generation of voids insufficient

(2) Solder ball test Each test substrate under the same conditions as in the above (1) void test except that the peak temperature of the reflow condition is 260 ° C., the time of 200 ° C. or higher is 70 seconds, and the time of 220 ° C. or higher is 60 seconds. The surface state of each test substrate was observed with an X-ray transmission device (product name: SMX-160E, manufactured by Shimadzu Corporation), and the number of solder balls generated on the periphery and lower surface of the chip component was counted. And evaluated as follows. The results are shown in Table 3 and Table 4, respectively.
A: No. of balls generated around 10 chip resistors of 2.0 mm × 1.2 mm. O: No. of balls generated around 10 chip resistors of 2.0 mm × 1.2 mm exceeds 0 and less than 5. : The number of balls generated around 10 chip resistors of 2.0 mm × 1.2 mm exceeds 5 and less than 10 ×: The number of balls generated around 10 chip resistors of 2.0 mm × 1.2 mm exceeds 10

(3) Copper plate corrosion test A test was conducted in accordance with the conditions specified in JIS standard Z 3284 (1994) and evaluated as follows. The results are shown in Table 3 and Table 4, respectively.
○: No discoloration of Cu plate ×: Discoloration of Cu plate (4) Printability test Glass epoxy substrate having solder resist and electrode (diameter 0.25 mm) having a pattern capable of mounting 100-pin 0.5 mm pitch BGA A 120 μm thick metal mask having the same pattern was prepared.
6 pieces of each solder paste are printed continuously on the glass epoxy substrate using the metal mask, and the transfer volume ratio at a diameter of 0.25 mm is measured by an image inspection machine (product name: aspire2, manufactured by Koyon Technology Co., Ltd.) Was evaluated according to the following criteria. The results are shown in Table 3 and Table 4, respectively.
A: Number of transfer volume ratio of 35% or less is 0; O: Transfer volume ratio of 35% or less is more than 0 and 10 or less Δ: Number of transfer volume ratio of 35% or less is more than 10 and 50 or less × : Number of transfer volume ratio 35% or less exceeds 50

(4) Solder crack test A chip component 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 (1.6 mm × 1.2 mm) 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, the chip components were mounted on each glass paste substrate, and solder joints were formed. 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.
Liquid bath type thermal shock test apparatus (product name: ETAC WINTECH LT80, Enomoto Co., Ltd.) in which each glass epoxy board after the formation of the solder joint is set to a condition of -40 ° C. (5 minutes) to 150 ° C. (5 minutes) Each test substrate was manufactured by exposing the thermal shock cycle to an environment where 1,000, 2,000, and 3,000 cycles were repeated.
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, using a wet polishing machine (product name: TegraPol-25, manufactured by Marumoto Struers Co., Ltd.), the chip parts mounted on each test board are in a state where the central cross section of the chip parts can be seen, and the formed solder joints Whether or not the generated crack completely crosses the solder joint and reaches the fracture is observed using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation), and is based on the following criteria. evaluated. The results are shown in Tables 3 and 4. The number of evaluation chips in each thermal shock cycle was 10.
A: No crack that completely crosses the solder joint until 3,000 cycles ○: A crack that completely crosses the solder joint occurs between 2,001 to 3,000 cycles Δ: 1,001 to 2 A crack that completely traverses the solder joint occurs during 1,000 cycles. ×: A crack that completely traverses the solder joint occurs in 1,000 cycles or less.

  As described above, the solder joint portion formed using the solder paste according to the example includes (B-1) 3 carbon atoms as the alloy powder made of the lead-free solder alloy and the activator (B). A flux composition containing a predetermined amount of 4 straight-chain saturated dicarboxylic acid, (B-2) a dicarboxylic acid having 5 to 13 carbon atoms, and (B-3) a dicarboxylic acid having 20 to 22 carbon atoms. Therefore, it is possible to suppress the generation of voids in the solder joints, and thereby, in a severe environment where the difference in temperature is intense and vibration is loaded, particularly in the severe temperature difference of −40 ° C. to 150 ° C. The crack propagation of the solder joint can be further suppressed. Furthermore, it can be seen that these solder pastes can exhibit good printability while suppressing the generation of solder balls.

As mentioned above, the solder paste of this invention 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. Furthermore, such an electronic circuit board can be suitably used for an electronic control device that requires higher reliability.

Claims (10)

Ag is 1% by mass to 3.1% by mass, Cu is more than 0% by mass and 1% by mass or less, Sb is 1% by mass to 5% by mass, and Bi is 0.5% by mass to 4.5% by mass. %, And an alloy powder comprising a lead-free solder alloy containing Ni in an amount of 0.01% by mass to 0.25% by mass, with the balance being Sn.
(A) A flux composition containing a base resin, (B) an activator, (C) a thixotropic agent, and (D) a solvent, wherein the blending amount of the activator (B) is the total amount of the flux composition. 6 % by mass or more and 35% by mass or less, and (B-1) a linear saturated dicarboxylic acid having 3 to 4 carbon atoms as the activator (B) is 1 % by mass with respect to the total amount of the flux composition. 2 % by mass or less, (B-2) 3 to 9 % by mass of a dicarboxylic acid having 5 to 13 carbon atoms based on the total amount of the flux composition, and (B-3) 20 to 22 carbon atoms. A solder paste comprising a flux composition containing 2% by mass or more and 15% by mass or less of dicarboxylic acid with respect to the total amount of the flux composition.   2. The solder paste according to claim 1, wherein the lead-free solder alloy further contains Co in an amount of 0.001 to 0.25 mass%.   The solder paste according to claim 1 or 2, wherein the lead-free solder alloy has a Bi content of 3.1 mass% or more and 4.5 mass% or less. Ag is 1% by mass to 3.1% by mass, Cu is more than 0% by mass and 1% by mass or less, Sb is 1% by mass to 5% by mass, and Bi is 0.5% by mass to 4.5% by mass. %, Ni is 0.01 mass% or more and 0.25 mass% or less, Co is 0.001 mass% or more and 0.25 mass% or less, and the balance is Sn, and Ag, Cu, Sb, Bi, Ni And an alloy powder made of a lead-free solder alloy in which the respective contents (mass%) of and Co satisfy all of the following formulas A to D;
(A) A flux composition containing a base resin, (B) an activator, (C) a thixotropic agent, and (D) a solvent, wherein the blending amount of the activator (B) is the total amount of the flux composition. 6 % by mass or more and 35% by mass or less, and (B-1) a linear saturated dicarboxylic acid having 3 to 4 carbon atoms as the activator (B) is 1 % by mass with respect to the total amount of the flux composition. 2 % by mass or less, (B-2) 3 to 9 % by mass of a dicarboxylic acid having 5 to 13 carbon atoms based on the total amount of the flux composition, and (B-3) 20 to 22 carbon atoms. A solder paste comprising a flux composition containing 2% by mass or more and 15% by mass or less of dicarboxylic acid with respect to the total amount of the flux composition.
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 / 0.25) + (Co content / 0.25) ≦ 1.19... D   5. The solder paste according to claim 1, wherein the lead-free solder alloy further contains more than 0 mass% and 6 mass% or less of In.   The lead-free solder alloy further includes 0.001% by mass or more and 0.05% by mass or less of at least one of P, Ga, and Ge in total. Solder paste as described in 1.   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 or more and 0.05% by mass or less. The solder paste according to item 1. The linear saturated dicarboxylic acid (B-1) having 3 to 4 carbon atoms is at least one of malonic acid and succinic acid,
The carbon atoms from 5 to 13 dicarboxylic acid (B-2) is glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, 2-methyl azelaic acid, sebacic acid, undecane diacid, dodecane diacid and tridecane It is at least one selected or diacid et al,
The dicarboxylic acid (B-3) having 20 to 22 carbon atoms is eicosadioic acid, 8-ethyloctadecanedioic acid, 8,13-dimethyl-8,12-eicosadienedioic acid and 11-vinyl-8-octadecenedioic acid. The solder paste according to claim 1, wherein the solder paste is at least one selected from the group consisting of:   An electronic circuit board having a solder joint formed by using the solder paste according to claim 1.   An electronic control device comprising the electronic circuit board according to claim 9.