JP6578393B2 - Lead-free solder alloy, electronic circuit mounting board, and electronic control device - Google Patents

Description

  The present invention relates to a lead-free solder alloy, an electronic circuit mounting substrate, and an electronic control device.

  As a method for joining an electronic component to a conductor pattern formed on an electronic circuit board such as a printed wiring board or a module board, there is a solder joining method using a solder alloy. Previously, this solder alloy used lead. However, since the use of lead is restricted by the RoHS directive or the like from the viewpoint of environmental load, a solder joining method using a so-called lead-free solder alloy which does not contain lead is becoming common in recent years.

As this lead-free solder alloy, for example, Sn—Cu, Sn—Ag—Cu, Sn—Bi, and Sn—Zn solder alloys are well known. 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.
A lead-free solder alloy is somewhat inferior in solderability compared to a lead-containing solder alloy. However, this solderability problem is covered by improvements in flux and soldering equipment. Therefore, Sn-3Ag-0.5Cu solder alloy is used, for example, in an in-vehicle electronic circuit mounting board that is placed in a relatively mild environment with a difference in temperature as in the interior of an automobile. Even the solder joints formed in this way do not cause any major problems.

However, in recent years, for example, electronic circuit mounting boards used in electronic control devices have been studied and put into practical use, such as placement in an engine compartment, direct mounting on an engine, and placement in an electromechanical integration with a motor. ing. These have a severe temperature difference (for example, a temperature difference of −30 ° C. to 115 ° C., −40 ° C. to 125 ° C.) and a severe environment in which they are subjected to a vibration load. In such an environment where the temperature difference is extremely severe, in the electronic circuit mounting substrate, the mounted electronic component and the substrate (in the case of simply referred to as “substrate” in this specification, the board before the conductor pattern formation, the conductor pattern is formed. Is a board that can be electrically connected to the electronic component, and a board portion that does not include the electronic component among the electronic circuit mounting board on which the electronic component is mounted. Indicates a plate portion not including an electronic component among electronic circuit mounting boards on which electronic components are mounted.) Thermal displacement of the solder joint due to a difference in linear expansion coefficient and stress associated therewith are likely to occur. And the repetition of the plastic deformation due to the temperature difference tends to cause a crack in the solder joint.
Furthermore, since the stress repeatedly applied to the solder joint as time passes concentrates near the tip of the crack that has occurred, the crack easily propagates transversely to the deep part of the solder joint. The crack that has progressed remarkably in this way causes a break in the electrical connection between the electronic component and the conductor pattern formed on the substrate. In particular, in an environment in which vibration is loaded on the electronic circuit mounting board in addition to a severe temperature difference, the cracks and their progress are more likely to occur.

  Therefore, in such a harsh environment, there is a problem in that cracks and cracks are likely to occur at a solder joint formed of Sn-3Ag-0.5Cu solder alloy, and sufficient reliability cannot be ensured.

In addition, the lead parts of electronic parts such as QFP (Quad Flat Package) and SOP (Small Outline Package) mounted on a vehicle-mounted electronic circuit board are conventionally Ni / Pd / Au plated parts or Ni / Au plated parts. Was heavily used. However, with the recent cost reduction of electronic components and downsizing of substrates, electronic components having lead portions replaced with Sn plating and electronic components having Sn-plated bottom electrodes have been studied and put into practical use.
At the time of soldering, the Sn-plated electronic component is likely to cause mutual diffusion between Sn contained in the Sn plating and solder joint and the Cu contained in the lead portion and the lower electrode. Due to this interdiffusion, the Cu ₃ Sn layer, which is an intermetallic compound, is formed in a region near the interface between the solder joint and the lead portion or the lower electrode (hereinafter referred to as “near the interface” in this specification). Grows greatly in an uneven shape. The Cu ₃ Sn layer is originally hard and brittle, and the Cu ₃ Sn layer that has grown greatly in an uneven shape is further brittle. Therefore, particularly in the above-mentioned severe environment, cracks are likely to occur near the interface as compared with the solder joints, and the cracks that have occurred start at once, so that an electrical short circuit is likely to occur.
Therefore, in the future, even when using electronic parts that are not subjected to Ni / Pd / Au plating or Ni / Au plating under the harsh environment described above, lead-free that can exhibit the effect of suppressing crack growth near the interface The demand for solder alloys is also expected to increase.

In recent years, downsizing of on-vehicle electronic circuit mounting boards has begun to be widely studied for the purpose of cost reduction, weight reduction, and layout freedom. As one method for realizing this, a method for reducing the volume of the so-called fillet portion of the solder joint portion is used.
That is, the conventional solder joint portion is generally formed so that the fillet portion covers the side surface of the electronic component and has a gently flared shape as shown in FIG. On the other hand, in recent years, as shown in FIG. 2, a method of reducing the volume of the fillet part by reducing the part of the fillet part that covers the side surface of the electronic component to have a large inclination, or covering the side surface of the electronic component. A method of adopting a structure having no part (no fillet part) is being adopted.
If the volume of the fillet portion of the solder joint portion is reduced, high-density mounting and downsizing of electronic components can be realized. However, the smaller the volume of the fillet part, the lower the strength of the solder joint part. Therefore, especially in a severe environment where the temperature difference is very large and vibration is applied, the solder with a large fillet part volume is required. There is a problem in that the solder joint portion having a smaller volume of the fillet portion is more inferior in crack growth suppressing effect than the joint portion.

  So far, by adding elements such as Ag, Bi, In, and Sb to Sn-Ag-Cu solder alloys, the strength of the solder joints and the thermal fatigue characteristics associated therewith are improved, thereby cracking the solder joints. Several methods for suppressing progress have been disclosed (see Patent Document 1 to Patent Document 3).

Japanese Patent No. 5024380 Japanese Patent No. 5349703 JP 2016-26879 A

  Here, the use of microelectronic components and high-density mounting are being studied and adopted for recent on-vehicle electronic circuit mounting boards. In such electronic circuit mounting boards, there are cases in which electronic components with low heat resistance and large electronic components with large heat capacity are mixedly mounted. In this case, when soldering is performed using a solder alloy having a high melting point, electronic components Therefore, there is a great demand for the heating temperature during soldering to be equal to or lower than that of Sn-3Ag-0.5Cu solder alloy.

The lead-free solder alloy to which Bi is added can make the heating temperature at the time of soldering equal to or lower than that of the Sn-3Ag-0.5Cu solder alloy, and can improve the strength.
However, such a lead-free solder alloy has a possibility that the stretchability is deteriorated, and there is a demerit that the brittleness is increased depending on the content.

In addition, lead-free solder alloys containing Sb or In are distorted in the lattice of the atomic arrangement because Sb or In enters into the lattice of the atomic arrangement of the lead-free solder alloy and replaces Sn, thereby strengthening the Sn matrix. As a result, the strength of the lead-free solder alloy can be improved. Therefore, a solder joint formed using such a lead-free solder alloy can have a high crack growth suppressing effect.
However, in particular, Sb increases the solidus temperature and liquidus temperature of the lead-free solder alloy due to its addition. Therefore, if the heating conditions are the same as those of the Sn-3Ag-0.5Cu solder alloy at the time of soldering, lead-free There is a risk of unmelting of the solder alloy. In addition, since In is an alloy element having a high unit price, the price of the lead-free solder alloy can be increased.
In addition, as described above, depending on the heat resistance condition and heat capacity of the electronic component to be used, the heating condition at the time of soldering may have to be set equal to or lower than that of the Sn-3Ag-0.5Cu solder alloy.

In addition, when solder bonding is performed using an electronic component that is not plated with Ni / Pd / Au or Ni / Au, the Cu ₃ Sn layer, which is an intermetallic compound, grows greatly in an uneven shape near the interface. It is difficult to suppress crack growth near this interface.

  However, there is no mention or suggestion in the above-mentioned patent literature about the crack growth suppression effect of the solder joint and the suppression of the heating temperature at the time of soldering and the crack progress suppression of the solder joint with a small fillet volume. .

  SUMMARY OF THE INVENTION The present invention solves the above-mentioned problem, and the heating conditions during soldering can be made equal to or lower than those of Sn-3Ag-0.5Cu solder alloy, and the environment is severe such that the difference in temperature is intense and vibration is loaded. The crack growth of the solder joint can be suppressed even under the condition, and the crack growth near the interface is suppressed even when soldering is performed using an electronic component that is not subjected to Ni / Pd / Au plating or Ni / Au plating. It is an object of the present invention to provide a lead-free solder alloy, an electronic circuit mounting board, and an electronic control device.

(1) The lead-free solder alloy of the present invention comprises 2.5% by mass to 4% by mass of Ag, 0.6% by mass to 0.75% by mass Cu, and 2% by mass to 6% by mass. Bi, 0.01 mass% or more and 0.04 mass% or less of Ni, and 0.01 mass% or more and 0.04 mass% or less of Co, with the balance being Sn, the total content of Ni and Co The amount is 0.05% by mass or less.

(2) In the configuration described in (1) above, the Ag content is 2.5% by mass or more and 3.8% by mass or less.

(3) In the configuration described in (1) or (2) above, the Bi content is 3% by mass or more and 4.5% by mass or less.

(4) In the configuration described in any one of (1) to (3) above, the Ni content is 0.02 mass% or more and 0.04 mass% or less, and the Co content is 0.00. It is 01 mass% or more and 0.03 mass% or less.

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

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

(7) The solder paste of the present invention is a powdered lead-free solder alloy, the lead-free solder alloy according to any one of (1) to (6), a base resin, a thixotropic agent, and an activity It is characterized by having a flux containing an agent and a solvent.

(8) The electronic circuit mounting board of the present invention is characterized by having a solder joint formed by using the lead-free solder alloy described in any one of (1) to (6) above.

(9) The electronic control device of the present invention is characterized by having the electronic circuit mounting board described in (8) above.

  The lead-free solder alloy of the present invention can reduce the heating condition during soldering to the same level or lower as that of the Sn-3Ag-0.5Cu solder alloy, and can be used in a severe environment where the difference in temperature is intense and vibration is loaded. Propagation of cracks near the interface can be suppressed even when soldering is performed using electronic parts that are not plated with Ni / Pd / Au plating or Ni / Au plating. The electronic circuit mounting board and the electronic control device having such a solder joint portion can exhibit high reliability even in a harsh environment.

The cross-sectional schematic diagram of the electronic circuit mounting board | substrate which has a solder joint part formed so that a fillet part may cover the side surface of an electronic component, and it may become a shape of a gentle hem-expansion. The cross-sectional schematic diagram of the electronic circuit mounting board | substrate which has a solder joint part formed so that the part which covers the side surface of the electronic component of a fillet part may be decreased, and the inclination may become large.

  Hereinafter, an embodiment of a lead-free solder alloy, an electronic circuit mounting 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) Lead-free solder alloy The lead-free solder alloy of this embodiment can contain Ag. By adding Ag to the lead-free solder alloy, an Ag ₃ Sn compound can be precipitated in the Sn grain boundary, and mechanical strength can be imparted.

  In the lead-free solder alloy of the present embodiment, the mechanical strength, thermal shock resistance, and resistance of the lead-free solder alloy are improved while the Ag content is 2.5 mass% or more and 4 mass% or less. It is possible to improve the heat fatigue resistance of the solder joint formed using this.

  Further, when the Ag content is 2.5 mass% or more and 3.8 mass% or less, the balance between the strength and ductility of the lead-free solder alloy can be improved. The particularly preferable Ag content is 3% by mass or more and 3.8% by mass or less.

The lead-free solder alloy of this embodiment can contain Cu. By adding Cu to the lead-free solder alloy, a Cu ₆ Sn ₅ compound can be precipitated in the Sn grain boundary, and the thermal shock resistance of the lead-free solder alloy can be improved.

The lead-free solder alloy of this embodiment can improve the thermal shock resistance without inhibiting the stretchability by setting the Cu content to 0.6 mass% or more and 0.75 mass% or less. Furthermore, it is possible to suppress the concentrated precipitation of the Cu ₆ Sn ₅ compound in the vicinity of the interface of the solder joint portion formed using this, and the joint reliability can be further improved.
When the Cu content is 0.7 mass% or more and 0.75 mass% or less, the thermal shock resistance of the lead-free solder alloy and the bonding reliability of the solder joint can be further improved.

  Bi can be contained in the lead-free solder alloy of this embodiment. By adding Bi to the lead-free solder alloy, a part of the Sn crystal lattice is replaced with Bi, and distortion occurs in the crystal lattice. And by increasing the energy required for dislocations in this crystal, the metal structure of the solder joint formed using such a lead-free solder alloy can be strengthened by solid solution and the Young's modulus can be increased. Therefore, even when the solder joint is subjected to external stress due to temperature difference over a long period of time, deformation of the solder joint can be suppressed and good external stress resistance can be exhibited.

  The lead-free solder alloy of this embodiment improves the effect of the solid solution strengthening by making the Bi content 2% by mass or more and 6% by mass or less, and mechanical strength without hindering the stretchability. Further, the thermal shock resistance and the bonding reliability can be improved. Bi is an alloy element that can hinder the extensibility of a lead-free solder alloy. However, the lead-free solder alloy of the present embodiment inhibits the extensibility by improving the balance of Bi and other alloy elements. The crack growth inhibiting effect can be improved without doing so.

  When the Bi content is 2 mass% or more and 4.5 mass% or less, the effect of solid solution strengthening, mechanical strength, thermal shock resistance and bonding reliability of the lead-free solder alloy can be further improved. The Bi content is more preferably 3% by mass or more and 4.5% by mass or less.

The lead-free solder alloy of this embodiment can contain Ni and Co. By adding Ni and Co to the lead-free solder alloy, fine (Cu, Ni, Co) ₆ Sn ₅ is formed in the molten lead-free solder alloy during soldering and dispersed in the molten solder. . Due to the presence of (Cu, Ni, Co) ₆ Sn ₅ , the solder joint formed using such a lead-free solder alloy exhibits a good crack growth suppressing effect and further improves its thermal fatigue resistance. Can be made. Further, even when an electronic component that has not been subjected to Ni / Pd / Au plating or Ni / Au plating is joined by solder, Ni and Co move to the vicinity of the interface during soldering and become fine (Cu, Ni, Since Co) ₆ Sn ₅ is formed, the growth of the Cu ₃ Sn layer in the vicinity of the interface can be suppressed, and the crack growth suppressing effect in the vicinity of the interface can be improved.

The lead-free solder alloy of the present embodiment has a Ni content of 0.01% by mass or more and 0.04% by mass or less, and a Co content of 0.01% by mass or more and 0.04% by mass or less. By setting the content to 0.05% by mass or less, (Cu, Ni, Co) ₆ Sn ₅ is sufficiently formed at the time of soldering and can be dispersed in a balanced manner in the molten solder and in the vicinity of the interface. Therefore, it is possible to further improve the crack growth suppressing effect and the thermal fatigue resistance in the solder joint to be formed, and the crack growth suppressing effect near the interface.
Moreover, the lead-free solder alloy of this embodiment can suppress the production | generation of the acicular material comprised from Sn, Cu, Ni, and Co in a solder ingot by making content of Ni and Co into the said range. Therefore, precipitation of the needle-like substance at the time of pulverizing this can be suppressed, and a decrease in the printing omission of the solder paste containing the pulverized lead-free solder alloy can be suppressed.

A solder formed using a lead-free solder alloy when the Ni content is 0.02 mass% or more and 0.04 mass% or less, and the Co content is 0.01 mass% or more and 0.03 mass% or less. It is possible to further improve the crack growth suppressing effect and the heat fatigue property at the joint, and the crack growth suppressing effect near the interface.
That is, the lead-free solder alloy of the present embodiment can realize the improvement of the above effect by using Ni and Co together and adjusting the Ni content, the Co content, and the total amount thereof. is there.
In addition, the lead-free solder alloy of this embodiment can further improve the above effect when the content of Ni and Co is 0.04% by mass or less by balancing the content of other alloy elements. .

Further, the lead-free solder alloy of this embodiment can contain at least one of P, Ga, and Ge. By adding at least one of P, Ga and Ge, oxidation of the lead-free solder alloy can be prevented.
The lead-free solder alloy of the present embodiment has a total content of at least one of P, Ga, and Ge of 0.001 mass% or more and 0.05 mass% or less, so that the melting temperature of the lead-free solder alloy (liquid It is possible to improve the oxidation-preventing effect of the lead-free solder alloy while suppressing the rise of the phase wire temperature) and the generation of voids in the solder joints formed using the same.

Furthermore, the lead-free solder alloy of this embodiment can contain at least one of Fe, Mn, Cr, and Mo. By adding at least one of Fe, Mn, Cr, and Mo, it is possible to improve the effect of suppressing crack propagation formed using a lead-free solder alloy.
The lead-free solder alloy of the present embodiment is such that the total content of at least one of Fe, Mn, Cr and Mo is 0.001% by mass or more and 0.05% by mass or less, so that the melting temperature of the lead-free solder alloy While suppressing the rise of (liquidus temperature) and the generation of voids in the solder joint formed using the same, the crack progress suppressing effect of the solder joint can be further improved.

  In addition, the lead-free solder alloy of this embodiment contains other components (elements) such as Cd, Tl, Se, Au, Ti, Si, Al, Mg, Zn, and the like as long as the effect is not hindered. be able to. In addition, the lead-free solder alloy of this embodiment naturally includes unavoidable impurities.

  Moreover, it is preferable that the remainder of the lead-free solder alloy of this embodiment is made of Sn.

  The lead-free solder alloy of this embodiment preferably has a liquidus temperature of 220 ° C. or lower. By setting the liquidus temperature to 220 ° C. or lower, the heating temperature at the time of soldering can be made equal to or lower than that of the Sn-3Ag-0.5Cu solder alloy.

  For the formation of the solder joint portion of the present embodiment, any method may be used as long as the solder joint portion can be formed, such as a flow method, mounting with a solder ball, and a reflow method using a solder paste. Of these, a reflow method using solder paste is particularly preferred.

(2) Solder paste Such a solder paste is produced by, for example, kneading the powdered lead-free solder alloy and flux into a paste.

  As such a flux, for example, a flux containing a base resin, a thixotropic agent, an activator, and a solvent is used.

Examples of the base resin include rosin resins including rosin derivatives such as rosin such as tall oil rosin, gum rosin and wood rosin, hydrogenated rosin, polymerized rosin, heterogeneous rosin, acrylic acid modified rosin and maleic acid modified rosin; Acid, methacrylic acid, various esters of acrylic acid, various esters of methacrylic acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, maleic acid ester, maleic anhydride ester, acrylonitrile, methacrylonitrile, acrylamide, methacrylic acid Examples include acrylic resins obtained by polymerizing at least one monomer such as amide, vinyl chloride, vinyl acetate; epoxy resins; phenol resins. These can be used alone or in combination.
Of these, rosin resins, particularly hydrogenated acid-modified rosin obtained by hydrogenating acid-modified rosin, are preferably used. A combined use of a hydrogenated acid-modified rosin and an acrylic resin is also preferred.

  The acid value of the base resin is preferably 10 mgKOH / g or more and 250 mgKOH / g or less. Moreover, it is preferable that the compounding quantity of the said base resin is 10 to 90 mass% with respect to the flux whole quantity.

  Examples of the thixotropic agent include hydrogenated castor oil, fatty acid amides, and oxy fatty acids. These can be used alone or in combination. The blending amount of the thixotropic agent is preferably 3% by mass or more and 15% by mass or less with respect to the total amount of the flux.

  Examples of the activator include amine salts (inorganic acid salts and organic acid salts) such as organic amine hydrogen halide salts, organic acids, organic acid salts, organic amine salts, and the like. More specifically, diphenylguanidine hydrobromide, cyclohexylamine hydrobromide, diethylamine salt, acid salt, succinic acid, adipic acid, sebacic acid, malonic acid, dodecanedioic acid and the like can be mentioned. These can be used alone or in combination. The blending amount of the activator is preferably 5% by mass or more and 15% by mass or less with respect to the total amount of the flux.

  As said solvent, isopropyl alcohol, ethanol, acetone, toluene, xylene, ethyl acetate, ethyl cellosolve, butyl cellosolve, glycol ether etc. can be used, for example. These can be used alone or in combination. The amount of the solvent is preferably 20% by mass or more and 40% by mass or less based on the total amount of the flux.

  In the flux, an antioxidant can be blended for the purpose of suppressing oxidation of the lead-free solder alloy. Examples of the antioxidant include hindered phenolic antioxidants, phenolic antioxidants, bisphenolic antioxidants, and polymer-type antioxidants. Among these, a hindered phenol antioxidant is particularly preferably used. These can be used alone or in combination. 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 with respect to the total amount of the flux.

Additives such as halogens, matting agents, antifoaming agents and inorganic fillers may be added to the flux.
The blending amount of the additive is preferably 10% by mass or less with respect to the total amount of the flux. Moreover, these more preferable compounding quantities are 5 mass% or less with respect to the flux whole quantity.

  The blending ratio of the alloy powder of the lead-free solder alloy and the flux is preferably 65:35 to 95: 5 in the ratio of solder alloy: flux. A more preferred blending ratio is 85:15 to 93: 7, and a 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 part The solder joint part of this embodiment is formed by, for example, printing the solder paste at a predetermined position on the substrate and performing reflowing at a temperature of, for example, 220 ° C to 250 ° C. This reflow forms a solder residue and a flux residue derived from the flux on the substrate.

  In addition, the electronic circuit mounting substrate having such a solder joint part is formed with electrodes and insulating layers at predetermined positions on the substrate, for example, and the solder paste of this embodiment is printed 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 mounting 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 addition, the solder joint part of this embodiment may be formed so that the solder joint part 30 as shown in FIG. 1, that is, the fillet part covers the side surface of the electronic component and has a gently flared shape. Also, the solder joint portion 40 as shown in FIG. 2, that is, a portion that covers the side surface of the electronic component of the fillet portion (volume of the fillet portion) is small and has a shape with a large inclination. Furthermore, the solder joint part of the present embodiment may have a structure without a fillet part, that is, a structure with no solder joint part covering the side surface of the electronic component.

  In other words, the solder joint formed using the lead-free solder alloy of the present embodiment is a solder joint formed so that the fillet portion has a gently flared shape while covering the side surface of the electronic component. Even if it is a solder joint part formed so that the part (volume of the fillet part) covering the side surface of the electronic component of the fillet part is small and the inclination is large, the crack growth suppressing effect is good in any of them. Can be demonstrated. Therefore, the solder joint part of the present embodiment is used in a severe environment in which the volume of the fillet part is small or the shape has no fillet part and the temperature difference is very large and vibration is applied. be able to.

As described above, the solder joint portion according to the present embodiment can exhibit a good solder crack progress suppressing effect regardless of its shape (volume of the fillet portion), and is an electron in an electronic circuit mounting board used in an environment with a severe temperature difference. High-density mounting of components and downsizing can be realized.
In addition, as described above, the lead-free solder alloy of the present embodiment can set the heating condition during soldering to be equal to or lower than that of the Sn-3Ag-0.5Cu solder alloy. The options for electronic components can be expanded.

  And the electronic circuit mounting board | substrate which has such a solder joint part can be used suitably also for the electronic circuit mounting board | substrate with which high reliability is calculated | required, such as a vehicle-mounted electronic circuit mounting board | substrate.

  Further, by incorporating such an electronic circuit mounting substrate, 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.

Production of Flux The following components were kneaded to obtain fluxes according to Examples and Comparative Examples.
Hydrogenated acid-modified rosin (product name: KE-604, manufactured by Arakawa Chemical Industries, Ltd.) 51% by mass
Hardened castor oil 6% by mass
10% by weight of dodecanedioic acid
Malonic acid 1% by mass
Diphenylguanidine hydrobromide 2% by mass
Hindered phenol antioxidant (Product name: Irganox 245, manufactured by BASF Japan Ltd.) 1% by mass
Diethylene glycol monohexyl ether 29% by mass

Preparation of Solder Paste 11% by mass of the flux and 89% by mass of each lead-free solder alloy powder (powder particle size 20 μm to 38 μm) listed in Tables 1 to 2 were mixed, and Example 1 to Example 21 and Each solder paste according to Comparative Examples 1 to 18 was prepared.
In addition, the liquidus temperature of each lead-free solder alloy of an Example and a comparative example is written together in Table 1 and Table 2.

(1) Shear strength test The following tools were prepared.
・ Chip components with a size of 3.2 mm × 1.6 mm (Ni / Sn plating)
A printed wiring board (two for each solder paste) including a solder resist having a pattern capable of mounting the chip component of the size and an electrode (1.6 mm × 0.55 mm) for connecting the chip component.
-Metal mask with a thickness of 150 μm having the above pattern Each solder paste was printed on two printed wiring boards per one kind of solder paste using the metal mask, and the chip component was mounted on each of the printed wiring boards.
Thereafter, each printed wiring board is heated using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Seisakusho Co., Ltd.) so that the volume of the fillet portion is small as shown in FIG. Each electronic circuit mounting board having a solder joint was produced. The reflow conditions at this time are: preheating from 170 ° C. to 190 ° C. for 110 seconds, peak temperature of 240 ° 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, a thermal shock test apparatus (product name: ES-76LMS, in which each of the electronic circuit mounting substrates is set to -40 ° C. (30 minutes) to 125 ° C. (30 minutes) for each solder paste used. Hitachi Appliances Co., Ltd.) was used and exposed to an environment where the thermal shock cycle was repeated 1,000 cycles, and then this was taken out.
Then, for each solder paste used, the autograph (product name: EZ-L) shows the strength of each chip component of each electronic circuit mounting board to which the above-described thermal shock cycle is applied and each electronic circuit mounting board to which this is not applied. -500N, manufactured by Shimadzu Corporation).
In addition, the measurement conditions of the said shear strength were based on JIS specification C60068-2-21. When measuring the shear strength, the jig is a shear jig with a flat end face and a width equal to or greater than the component dimensions. The jig is abutted against the side of the chip part and parallel to each electronic circuit mounting board at a predetermined shear rate. The maximum test force was calculated and this value was taken as the shear strength. In addition, the shear height in the measurement was ¼ or less of the component height, and the shear rate was 5 mm / min. The number of evaluation chips on each electronic circuit mounting board was five.
The average value of the shear strength of each electronic circuit mounting board that was not given the thermal shock cycle was S0, the average value of the shear strength of the electronic circuit mounting board that was given the thermal shock cycle was S1, and the shear strength based on the following formula The reduction rate was calculated. Tables 3 and 4 show the values of S0, S1 and the shear strength reduction rate in each example and each comparative example.
Share strength reduction rate = (S0−S1) / S0 × 100

(2) Shear strength test A solder resist having a pattern capable of mounting a chip component (Ni / Sn plating) having a size of 3.2 mm × 1.6 mm and an electrode (1.6 mm × 1.2 mm) connecting the chip component Each printed circuit board is mounted under the same conditions as in the above (1) shear strength test, except that a printed wiring board (two per solder paste) is used and a solder joint having a shape as shown in FIG. 1 is formed. A substrate was prepared, the shear strength was measured, and the shear strength reduction rate was calculated. Tables 3 and 4 show the values of S0, S1 and the shear strength reduction rate in each example and each comparative example.

(3) Sn plating SON solder crack test The following tools were prepared.
・ 1.3mm pitch SON (Small Outline Non-leaded package) parts of 6mm x 5mm x 0.8tmm (8 pins, product name: STL60N3LLH5, manufactured by STMicroelectronics)
A printed wiring board having a solder resist having a pattern capable of mounting the SON component and an electrode (compliant with the manufacturer's recommended design) for connecting the SON component. A metal mask having the pattern having a thickness of 150 μm on the printed wiring board. Each solder paste was printed using the metal mask, and the SON component was mounted on each solder paste.
Thereafter, each printed wiring board is heated using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation) to produce each electronic circuit mounting board having a solder joint as shown in FIG. did. The reflow conditions at this time are: preheating from 170 ° C. to 190 ° C. for 110 seconds, peak temperature of 240 ° 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 1,500 ± 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 2, Each of the electronic circuit mounting substrates was exposed to an environment where 000 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). Furthermore, using a wet polishing machine (product name: TegraPol-25, manufactured by Marumoto Struers Co., Ltd.), the cracks occurred in the solder joints so that the center cross section of the SON component mounted on each test board can be seen. Was observed using a scanning electron microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation) as to whether or not the solder joint part completely crossed and reached the fracture.
Based on this observation, evaluation was performed as follows by dividing into cracks that occurred at the interface (intermetallic compound) between the solder joint and the electrode of the SON component. The results are shown in Tables 3 and 4. Note that the number of SONs evaluated in each thermal shock cycle was 20, and one terminal of the gate electrode was observed per SON, and the cross section of a total of 20 terminals was confirmed.

A: Cracks completely crossing the interface are not generated until 3,000 cycles. O: Cracks completely crossing the interface are generated between 2,001 to 3,000 cycles. Cracks completely crossing the interface

(3) Confirmation of needle-like substance 50 g of solder alloy ingot and 890 g of hardened castor oil and hydrogenated acid-modified rosin (product name: KE-604, manufactured by Arakawa Chemical Industries, Ltd.) in a 2 L stainless beaker ) 10 g was added and heated using a mantle heater.
The temperature of the contents of the stainless beaker was measured, and when the temperature reached 160 ° C., stirring of the contents was started using a homogenizer (manufactured by SMT Co., Ltd.) at a rotational speed of 2000 rpm. Then, when the temperature of the contents reached 270 ° C., the heating was stopped, the rotation condition of the homogenizer was changed to 10,000 rpm, and the contents were stirred for 5 minutes.
Thereafter, the rotation of the homogenizer was stopped, and the contents were cooled to room temperature. Next, the solder alloy powder settled in the hardened castor oil was taken out of the contents and washed with ethyl acetate until the hardened castor oil was removed. And the powder of each solder alloy after washing | cleaning was observed with the stereomicroscope, and the state was evaluated on the following references | standards.
○: No acicular substance is generated ×: Acicular substance is generated

As described above, the solder joint portion formed using the lead-free solder alloy according to the embodiment is formed so that the fillet portion has a gently flared shape while covering the side surface of the electronic component. However, even in cases where the fillet part has a small part covering the side surface of the electronic component and has a large inclination, it is a harsh environment where the difference between cold and warm is intense and vibration is applied in any case. It can be seen that a good crack suppressing effect and a shear strength lowering suppressing effect can be exhibited. In addition, the solder joint formed by using the lead-free solder alloy according to the example suppresses crack propagation near the interface even when the printed wiring board electrode is not subjected to Ni / Pd / Au plating or Ni / Au plating. It can be effective.
Furthermore, it can be seen that the lead-free solder alloy according to the example can suppress the generation of acicular substances while exhibiting a good crack suppressing effect and suppressing a decrease in shear strength.

  Therefore, an electronic circuit mounting board having such a solder joint has a severe temperature difference and requires high reliability, such as an in-vehicle electronic circuit mounting board, regardless of the shape of the solder joint, that is, the volume of the fillet. It can also be suitably used for an electronic circuit mounting substrate. Furthermore, such an electronic circuit mounting board can be suitably used for an electronic control device that is required to have higher reliability.

100, 200 Electronic circuit mounting substrate 10 Substrate 11 Electrode 12 Insulating layer 20 Electronic component 30, 40 Solder joint 31, 41 Flux residue

Claims (9)

2.5 mass% or more and 4 mass% or less of Ag, 0.6 mass% or more and 0.75 mass% or less of Cu, 2 mass% or more and 6 mass% or less of Bi, and 0.01 mass% or more and 0.0. Containing 04% by mass or less of Ni and 0.01% by mass or more and 0.04% by mass or less of Co, with the balance being Sn,
A lead-free solder alloy characterized in that the total content of Ni and Co is 0.05% by mass or less.   The lead-free solder alloy according to claim 1, wherein the Ag content is 2.5 mass% or more and 3.8 mass% or less.   The lead-free solder alloy according to claim 1 or 2, wherein the Bi content is 3% by mass or more and 4.5% by mass or less.   The Ni content is 0.02% by mass or more and 0.04% by mass or less, and the Co content is 0.01% by mass or more and 0.03% by mass or less. 4. The lead-free solder alloy according to any one of 3 above.   5. The lead-free solder alloy according to claim 1, further comprising at least one of P, Ga, and Ge in a total amount of 0.001% by mass to 0.05% by mass. .   The lead-free product according to any one of claims 1 to 5, further comprising 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. Solder alloy. A lead-free solder alloy according to any one of claims 1 to 6, which is a powdered lead-free solder alloy,
A solder paste comprising a flux containing a base resin, a thixotropic agent, an activator, and a solvent.   An electronic circuit mounting board having a solder joint formed by using the lead-free solder alloy according to any one of claims 1 to 6. An electronic control device comprising the electronic circuit mounting board according to claim 8.

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