WO2019235001A1 - Lead-free solder alloy, solder paste, electronic circuit-mounted substrate, and electronic control device - Google Patents

Abstract

Provided are: a lead-free solder alloy, which is a means that can achieve both of a cracking propagation prevention effect and resistance to impact associated with high-speed deformation at a solder joint part under an environment where the temperature difference is extreme, and which can be used suitably particularly in an in-vehicle electronic mounted substrate and an in-vehicle electronic control device, the lead-free solder alloy being characterized by containing 2 to 4% by mass inclusive of Ag, 0.3 to 1% by mass inclusive of Cu, 1.5% by mass or more and less than 3% by mass of Bi, and 1% by mass or more and less than 3% by mass of In, with the remainder made up by Sn; a solder paste; an electronic circuit-mounted substrate; and an electronic control device.

Description

Lead-free solder alloy, solder paste, electronic circuit mounting board and electronic control device

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

There is a solder joining method using a solder alloy 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. 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 that 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, consumer electronic devices such as televisions and mobile phones are soldered using Sn-3Ag-0.5Cu solder alloy (a solder joint is formed using Sn-3Ag-0.5Cu solder alloy). In many cases, an electronic circuit mounting board is used.
Here, the lead-free solder alloy is somewhat inferior in solderability as compared with the lead-containing solder alloy. However, since the solderability problem has been overcome by improving the flux and soldering apparatus, the Sn-3Ag-0.5Cu solder is used in a device that is placed in a relatively mild environment such as a consumer electronic device. Even with solder bonding using an alloy, a certain degree of reliability of the electronic circuit mounting board can be maintained.

However, for example, an electronic circuit mounting board used for an electronic control device mounted in an engine room or the like, an electronic control device mounted on a motor or the like (mechanical integration) (so-called onboard electronic control device), and an engine directly. The mounted electronic circuit board can be exposed to extremely harsh environments that are subject to severe temperature differences (eg, −30 ° C. to 115 ° C., −40 ° C. to 125 ° C.) and vibration loads.
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 A board that is formed and can be electrically connected to an electronic component, and a board part that does not include an electronic component among electronic circuit mounting boards on which the electronic component is mounted. In this case, the strain due to the difference in linear expansion coefficient from "the board portion of the electronic circuit mounting board on which the electronic component is mounted" that does not include the electronic component) and the stress due to the thermal displacement of the solder joint are generated. These are easy to apply and cause a large load on the solder joint.
And the load repeatedly given to a solder joint part in the use process of a car causes the plastic deformation of a solder joint part many times, and can become a cause of crack generation of a solder joint part.

In addition, since the load, particularly stress, repeatedly applied to the solder joint described above is concentrated near the tip of the crack generated in the solder joint, the crack is easily propagated 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 where vibration is loaded on the electronic circuit mounting substrate in addition to a severe temperature difference, there is a problem that the crack and its progress are more likely to occur.

Until now, several methods for adding elements such as Bi and In to Sn—Ag—Cu based solder alloys for the purpose of improving thermal fatigue characteristics and strength in order to suppress such crack growth in solder joints have been disclosed. (See Patent Document 1 to Patent Document 5).

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

When Bi is added to the solder alloy, Bi enters the lattice of the atomic arrangement of the solder alloy and substitutes Sn to distort the atomic arrangement of the lattice. As a result, the Sn matrix is strengthened and the alloy strength is improved, so that a certain improvement in the crack growth suppressing effect in the bulk portion of the solder joint portion is expected.
However, a lead-free solder alloy that has been strengthened by the addition of Bi has the demerit that extensibility deteriorates and brittleness increases. This is presumably because the addition of Bi makes it difficult for slip deformation in a specific crystal orientation to occur. Accordingly, in a solder joint formed by such a solder alloy, when an impact accompanied by high-speed deformation such as a drop impact is applied, plastic deformation due to slip deformation is compared with a case where a solder alloy not containing Bi is used. Therefore, there is a problem in that brittle fracture without plastic deformation is likely to occur.

On the other hand, as described above, the suppression of crack propagation at the solder joint in an environment with a severe temperature difference is still one of the important issues particularly in the on-vehicle electronic control device and the on-vehicle electronic circuit mounting substrate used therefor. Yes.
However, if the Bi content is suppressed in order to suppress the brittle fracture, the crack growth suppressing effect is lowered, and if the content is increased, the brittle fracture is likely to occur. In addition, when In is added to a solder alloy together with Bi to increase the strength of the solder alloy, since In is an easily oxidizable alloy element, depending on its content or combination with other alloy elements, etc. There is a problem that voids are easily generated in the solder joints, and that cracks caused by the voids and their progress are likely to occur.

Thus, it is desired to realize a lead-free solder alloy that can achieve both the effect of suppressing the crack growth of a solder joint in an environment with a severe temperature difference and the resistance to impact accompanied by high-speed deformation.
However, there is no mention or suggestion in the above-mentioned patent document about such a balance of effects.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems, and provides a lead-free solder alloy, a solder paste, and an electronic circuit that can achieve both crack growth suppression effect of solder joints and resistance to impact accompanied by high-speed deformation in an environment with a severe temperature difference It is an object of the present invention to provide a mounting board and an electronic control device.

(1) The lead-free solder alloy according to the present invention comprises 2% by mass to 4% by mass of Ag, 0.3% by mass to 1% by mass Cu, and 1.5% by mass to less than 3% by mass. It contains Bi and 1% by mass or more and less than 3% by mass of In, with the remainder being made of Sn.

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

(3) In the configuration described in (1) above, the Bi content is 2% by mass or more and 2.8% by mass or less, and the In content is 2% by mass or more and 2.8% by mass or less. Its features.

(4) In the configuration described in (1) above, the Bi content is 2.3 mass% or more and 2.8 mass% or less, and the In content is 2.3 mass% or more and 2.8 mass% or less. It is the feature.

(5) In the configuration described in any one of (1) to (4) above, the Ag content is 2.5% by mass or more and 3.5% by mass or less.

(6) In the configuration according to any one of (1) to (5), the content of Cu is 0.4% by mass or more and 0.8% by mass or less.

(7) The lead-free solder alloy according to the present invention, in the configuration described in any one of (1) to (6) above, further includes at least one of Ni and Co in a total of 0.001% by mass or more. It is characterized by containing 10% by mass or less.

(8) The lead-free solder alloy according to the present invention, in the configuration described in any one of (1) to (7) above, further includes at least one of Fe, Mn, Cr, and Mo in total 0.001 mass. % To 0.05% by mass or less.

(9) The lead-free solder alloy according to the present invention is the structure described in any one of (1) to (8) above, and at least one of P, Ga, and Ge is further 0.001% by mass or more in total. It is characterized by containing 0.05 mass% or less.

(10) The solder paste according to the present invention is a powdered lead-free solder alloy, the lead-free solder alloy according to any one of (1) to (9), a base resin, a thixotropic agent, It is characterized by having a flux containing an activator and a solvent.

(11) An electronic circuit mounting board according to the present invention has a solder joint formed by using the lead-free solder alloy described in any one of (1) to (9).

(12) An electronic control device according to the present invention includes the electronic circuit mounting board described in (11) above.

The lead-free solder alloy and solder paste of the present invention can achieve both the effect of suppressing the crack growth of the solder joint and the resistance to impact accompanied by high-speed deformation under an environment where the temperature difference is severe. The electronic circuit mounting board and the electronic control device having such a solder joint portion can exhibit high reliability even in a severe environment with a severe temperature difference, and particularly the in-vehicle electronic circuit mounting board and the in-vehicle electronic It can be suitably used for a control device.

In order to show the “under-electrode region” and “fillet region” in which the presence or absence of voids is observed in the examples and comparative examples of the present invention, a general chip component mounting board is used from the chip component side using an X-ray transmission device. Photo taken.

Hereinafter, an embodiment of the lead-free solder alloy, solder paste, electronic circuit mounting board, and 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 2 mass% or more and 4 mass% or less of Ag. By adding Ag to the lead-free solder alloy within this range, it is possible to precipitate the Ag ₃ Sn compound in its Sn grain boundary and impart mechanical strength, as well as thermal shock resistance, thermal fatigue characteristics and high speed. It is possible to exhibit resistance to impact accompanied by deformation.

Moreover, when the Ag content is 2% by mass or more and 3.8% by mass or less, the mechanical strength and the strength, ductility, and cost balance of the lead-free solder alloy are improved while improving the stretchability of the lead-free solder alloy. It is possible to improve the resistance to impact accompanied by high-speed deformation. The particularly preferable Ag content is 2.5% by mass or more and 3.5% by mass or less.

The lead-free solder alloy of this embodiment can contain 0.3 mass% or more and 1 mass% or less of Cu. By adding Cu to the lead-free solder alloy within this range, an effect of preventing copper erosion to the Cu land of the conductor pattern (electronic circuit) on the substrate is exhibited and Cu ₆ Sn ₅ compound is added to the Sn grain boundary. Precipitation can improve the thermal shock resistance of the lead-free solder alloy. Moreover, the heat-resistant fatigue characteristics of the solder joint part formed using this can be improved without inhibiting the stretchability of the lead-free solder alloy.

More preferable Cu content is 0.4 mass% or more and 0.8 mass% or less, and particularly preferable content is 0.5 mass% or more and 0.8 mass% or less. By making the content of Cu within this range, it is possible to further balance the effect of suppressing the progress of cracks in the solder joint, the resistance to impact accompanied by high-speed deformation, the suppression of voids, and the effect of suppressing copper erosion.

The lead-free solder alloy of this embodiment can contain Bi. When Bi is added to a 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.

In the lead-free solder alloy of this embodiment, the mechanical strength and thermal shock resistance of the lead-free solder alloy are improved by setting the Bi content to 1.5 mass% or more and less than 3 mass%, and solder joining It is possible to obtain solid solution strengthening of the metal composition of the part, and to exhibit good ductility and resistance to impact accompanied by high-speed deformation.
On the other hand, when the Bi content is 3% by mass or more, a solder joint formed using the Bi is likely to break due to an impact accompanied by high-speed deformation.

Moreover, when the Bi content is 2% by mass or more and less than 3% by mass, the lead-free solder alloy can be strengthened (crack progress suppressing effect) and the resistance to impact with high-speed deformation can be improved, and the balance between the two can be further exhibited. Can do.
The more preferable content of Bi is 2% by mass or more and 2.8% by mass or less, and the particularly preferable content thereof is 2.3% by mass or more and 2.8% by mass or less.

The lead-free solder alloy of this embodiment can contain 1% by mass or more and less than 3% by mass of In. By adding In within this range, the solder structure of the lead-free solder alloy can be polycrystallized to cancel anisotropy. Therefore, the embrittlement phenomenon in a specific crystal orientation in a lead-free solder alloy can be suppressed, and even when an impact accompanied by high-speed deformation is applied to a solder joint formed using this, the occurrence of breakage is prevented. Can be suppressed.
On the other hand, when the In content is 3% by mass or more, voids are likely to be generated in a solder joint portion formed using the solder paste, and the crack growth suppressing effect may be hindered.

Moreover, the preferable content of In is 2% by mass or more and less than 3% by mass, the more preferable content is 2% by mass or more and 2.8% by mass or less, and the particularly preferable content is 2.3% by mass or more and 2. It is 8 mass% or less.

In the lead-free solder alloy according to the present embodiment, the crack of the solder joint portion in an environment with a severe temperature difference is achieved by balancing the contents of Bi and In, and other alloy elements and these contents. It is possible to achieve both the effect of suppressing the progress and the resistance to impact accompanied by high-speed deformation, or the effect of suppressing the void at the soldered joint.

The lead-free solder alloy of this embodiment preferably contains 2% by mass or more and less than 3% by mass of Bi and 2% by mass or more and less than 3% by mass of In, and preferably 2% by mass or more and less than 3% by mass of Bi. It is more preferable to contain 2% by mass to 2.8% by mass of In, and 2.3% by mass to 2.8% by mass of Bi and 2.3% by mass to 2.8% by mass of In. It is particularly preferable to contain.
By limiting the content of Bi and In to this range and balancing the content of these with other alloy elements, the effect of suppressing crack growth in solder joints in environments with severe temperature differences and impact with high-speed deformation These effects can be improved while at the same time satisfying the above-mentioned resistance.

The lead-free solder alloy of this embodiment can contain 0.001% by mass or more and 0.10% by mass or less of at least one of Ni and Co. By adding at least one of Ni and Co to the lead-free solder alloy within this range, it is possible to improve the crack growth suppressing effect of the solder joint while suppressing the generation of voids.

Furthermore, the lead-free solder alloy of this embodiment can contain 0.001% by mass or more and 0.05% by mass or less of at least one of Fe, Mn, Cr, and Mo in total. By adding at least one of Fe, Mn, Cr, and Mo within this range, it is possible to improve the crack growth suppressing effect of the solder joint portion while suppressing the generation of voids.

Moreover, the lead-free solder alloy of this embodiment can contain 0.001% by mass or more and 0.05% by mass or less of at least one of P, Ga, and Ge in total. By adding at least one of P, Ga and Ge within this range, it is possible to prevent oxidation of the lead-free solder alloy while suppressing the generation of voids.

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.

For the formation of the solder joint portion of this 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.

As the activator, for example, an amine salt (inorganic acid salt or organic acid salt) such as an organic amine hydrogen halide salt, an organic acid, an organic acid salt, an organic amine salt, or the like can be blended. 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 the solvent, for example, isopropyl alcohol, ethanol, acetone, toluene, xylene, ethyl acetate, ethyl cellosolve, butyl cellosolve, glycol ether, diethylene glycol monohexyl ether, or the like can be used. 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.

¡An antioxidant may be added to the flux 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 based on 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 compounding ratio of the lead-free solder alloy powder 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.

(4) Electronic circuit mounting board An electronic circuit mounting board having such a solder joint is formed by using, for example, a mask having a predetermined pattern by forming electrodes and insulating layers at predetermined positions on the board. This solder paste is printed, an electronic component conforming to the pattern is mounted at a predetermined position, and this 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.

And the solder joint part concerning this embodiment can exhibit the favorable crack progress inhibitory effect, and also has the tolerance to the impact accompanied by high-speed deformation. Therefore, an electronic circuit mounting board having such a solder joint can be suitably used for an electronic circuit mounting board that is required to have high reliability, such as an in-vehicle electronic circuit mounting board.

(5) Electronic control device A highly reliable electronic control device is manufactured by incorporating such an electronic circuit mounting board. And such an electronic control apparatus can be used suitably for the vehicle-mounted electronic control apparatus by which especially high reliability is calculated | required.

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) shown in Tables 1 and 2 were mixed, and Example 1 to Example 30 and Each solder paste according to Comparative Examples 1 to 18 was prepared.

(1) Solder crack test The following tools were prepared.
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 connecting the chip component (1.6 mm × 1.0 mm, gap between Cu electrodes) : 150 mm thick metal mask having the above pattern, each solder paste is printed on the printed wiring board using the metal mask, and each of the 10 chip components is mounted. did.
Thereafter, each of the printed wiring boards is heated using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation), and each of them has a solder joint for electrically joining the electrode and each chip component. An electronic circuit mounting board was produced. 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 1,500 ± 500 ppm.
Next, a liquid tank type thermal shock test apparatus (product name: ETAC WINTECH LT80, manufactured by Enomoto Kasei Co., Ltd.) in which each electronic circuit mounting substrate is set to a condition of −40 ° C. (5 minutes) to 125 ° C. (5 minutes). ), And was exposed to an environment where the thermal shock cycle was repeated 3,000 cycles.
And the target part of each said test board | substrate was cut out, and this was sealed using the epoxy resin (Product name: Epomount (main agent and hardening | curing agent), refined tech Co., Ltd. product). Further, a scanning electron set to 200 times with a wet polishing machine (product name: TegraPol-25, manufactured by Marumoto Struers Co., Ltd.) in a state in which the center cross section of the chip component mounted on each test substrate can be seen. These were observed using a microscope (product name: TM-1000, manufactured by Hitachi High-Technologies Corporation), and the crack rates generated at the solder joints formed under the electrodes of the chip components of the test substrates were as follows. Calculated based on the formula. The “crack rate” is an index for determining how many cracks actually occur with respect to the assumed crack length.
Crack rate (%) = (total length of cracks / total length of assumed line cracks) x 100
Of the solder joints formed under the two electrodes of each chip component, the one with the longer crack length was used as the target for calculating the crack rate of the chip component.
The “total length of cracks” is the sum of the lengths of a plurality of cracks generated at each solder joint to be evaluated (calculated) on each test substrate.
The “assumed total crack length” is the total length of the expected crack propagation paths (cracks that have reached complete fracture) in each solder joint to be evaluated (calculated) in each test substrate.
And the crack rate of ten chip components mounted in each test board | substrate was calculated, respectively, and the average value was evaluated based on the following references | standards. The results are shown in Tables 3 and 4.
A: Average value of crack rate is 50% or less B: Average value of crack rate is more than 50%, 70% or less Δ: Average value of crack rate is more than 70%, 90% or less ×: Average value of crack rate is 90 Over 100% and below 100%

(2) Shear strength test for impact with high-speed deformation (high-speed shear test)
Except that the printed wiring board electrode is 1.6 mm x 0.5 mm (gap between Cu electrodes: 2.2 mm) and that the five chip components are mounted, the same conditions as in the above (1) solder crack test Thus, each electronic circuit mounting board having a solder joint was produced.
Then, the shear strength of the chip parts of each electronic circuit mounting substrate was measured using an autograph (product name: EZ-L-500N, manufactured by Shimadzu Corporation).
The measurement conditions for the shear strength conformed to JIS standard 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 100 mm / min.
Then, an average value (total of calculated share strengths ÷ 5 (number of chip components)) was calculated and evaluated based on the following criteria for the measured share strength of each electronic circuit mounting board. The results are shown in Tables 3 and 4.
○: Average value of share strength is 7.0N or more △: Average value of share strength is 6.5N or more and less than 7.0N ×: Average value of share strength is less than 6.5N

(3) Void test The following tools were prepared.
A chip component having a size of 2.0 mm × 1.2 mm. A print including 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. Wiring board A 150 μm thick metal mask having the above pattern Each solder paste was printed on the printed wiring board using the metal mask, and 10 chip components were mounted on each of the solder pastes.
Thereafter, each of the printed wiring boards is heated using a reflow furnace (product name: TNP-538EM, manufactured by Tamura Corporation), and each of them has a solder joint for electrically joining the electrode and each chip component. An electronic circuit mounting board (test board) was prepared. 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 1,500 ± 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 each chip component electrode on each test substrate (enclosed by the broken line in FIG. 1). The ratio of the total area of voids generated in the area (a), hereinafter referred to as “under-electrode area”) to the land area (void area ratio of under-electrode area: total void area of under-electrode area / land area × 100) ) And the ratio of the total area of voids generated in the area where the fillet is formed (area (b) surrounded by the broken line in FIG. 1, hereinafter referred to as “fillet area”) to the land area (void area of the fillet area) Rate: total void area of fillet region / land area × 100) was measured and calculated.
And the average value of the void area ratio (the total of the calculated void area ratios ÷ 20 (number of lands)) was calculated and evaluated based on the following criteria. The results are shown in Tables 3 and 4.
○: Average value of void area ratio is more than 3% and not more than 5% △: Average value of void area ratio is more than 5% and not more than 8% ×: Average value of void area ratio is more than 8%

(4) Copper erosion test A test substrate provided with a 35 μm thick copper wiring on the FR-4 substrate was cut into an appropriate size.
After applying a preflux on the copper wiring surface of each test substrate, it was preheated for 60 seconds to bring the temperature of each test substrate to about 120 ° C.
Next, each pre-heated test substrate is placed 2 mm above the jet port of the jet solder bath in which the lead-free solder alloys according to the examples and comparative examples are melted, and immersed in each jet of molten solder for 3 seconds. I let you. And this immersion process was repeated, the frequency | count of immersion until the size (area) of the copper wiring of each test board | substrate was halved was measured, and it evaluated based on the following references | standards. The results are shown in Tables 3 and 4.
○: The size of the copper wiring is not halved even when the number of immersions is 4 times or more. Δ: The size of the copper wiring is halved after 3 times of immersion.

As shown above, the solder joint formed using the lead-free solder alloy according to the example exhibits an effect of suppressing crack growth even in an environment where the temperature difference is severe, and at the same time has resistance to impact accompanied by high-speed deformation. I can see that

In the above (1) solder crack test, a “liquid tank type” thermal shock test apparatus is used as described above. This liquid tank-type thermal shock test apparatus alternates test substrates in two liquid tanks (set at -40 ° C and 125 ° C in the above (1) Solder crack test), which can quickly transfer the test substrate. It is immersed and gives a rapid temperature change to the test substrate. This is a very severe test content compared to other test conditions (for example, “air tank type”).
And the lead-free solder alloy which concerns on an Example can exhibit the crack growth inhibitory effect also under such very severe conditions.

As for the shear rate of the above (2) high-speed shear test, JIS standard C60068-2-21 does not specify the shear rate, but for example, the conditions of the shear test of other metals (the method of tensile shear test and bending of thin clad steel) (Test method: JIS standard Z2288, etc.) specifies a shear (tensile) speed of 1 mm to 5 mm / min. Usually, a shear test is performed within this range. On the other hand, in the above (2) high-speed shear test, the test is performed under the condition of “100 mm / min”, which is at least 20 times the normal shear rate.
And the lead-free solder alloy which concerns on an Example can exhibit the tolerance with respect to the impact accompanying a high-speed deformation also under such severe conditions.

Furthermore, the lead-free solder alloy according to this example can exhibit a void suppressing effect in both the “under-electrode region” and the “fillet region” in the solder joint formed by using this, and the copper corrosion It turns out that a crack suppression effect can be demonstrated.

And the content of Bi is 2.3% by mass or more and 2.8% by mass or less, and the content of In is 2.3% by mass or more and 2.8% by mass or less. In Examples 9, 10, 11, 14, and 18 to 20 in which the balance between the content of other alloy elements and the content thereof was achieved, good crack growth suppressing effect, good resistance to impact with high-speed deformation, good voids It turns out that suppression and the favorable copper erosion suppression effect can be exhibited.

As described above, the electronic circuit mounting board and the electronic control device having the solder joint portion formed by using the lead-free solder alloy according to the present invention can exhibit high reliability even in a severe environment with a severe temperature difference. In particular, it can be suitably used for a vehicle-mounted electronic mounting substrate and a vehicle-mounted electronic control device.

Claims (12)

1. 2% by mass or more and 4% by mass or less of Ag, 0.3% by mass or more and 1% by mass or less of Cu, 1.5% by mass or more and less than 3% by mass of Bi, and 1% by mass or more and less than 3% by mass of In A lead-free solder alloy characterized in that the balance is made of Sn.
2. The lead-free solder alloy according to claim 1, wherein the Bi content is 2% by mass or more and less than 3% by mass, and the In content is 2% by mass or more and less than 3% by mass.
3. The lead-free solder alloy according to claim 1, wherein the Bi content is 2% by mass or more and 2.8% by mass or less, and the In content is 2% by mass or more and 2.8% by mass or less. .
4. The Bi content is 2.3% by mass or more and 2.8% by mass or less, and the In content is 2.3% by mass or more and 2.8% by mass or less. Lead-free solder alloy.
5. The lead-free solder alloy according to any one of claims 1 to 4, wherein an Ag content is 2.5 mass% or more and 3.5 mass% or less.
6. The lead-free solder alloy according to any one of claims 1 to 5, wherein a Cu content is 0.4 mass% or more and 0.8 mass% or less.
7. The lead-free solder alloy according to any one of claims 1 to 6, further comprising 0.001% by mass or more and 0.10% by mass or less of at least one of Ni and Co.
8. The lead-free material according to any one of claims 1 to 7, further comprising at least one of Fe, Mn, Cr, and Mo in a total amount of 0.001% by mass to 0.05% by mass. Solder alloy.
9. The lead-free solder alloy according to any one of claims 1 to 8, further comprising at least one of P, Ga, and Ge in a total amount of 0.001 mass% to 0.05 mass%. .
10. A lead-free solder alloy according to any one of claims 1 to 9, which is a powdered lead-free solder alloy,
    A solder paste comprising a flux including a base resin, a thixotropic agent, an activator, and a solvent.
11. An electronic circuit mounting board comprising a solder joint formed using the lead-free solder alloy according to any one of claims 1 to 9.
12. An electronic control device comprising the electronic circuit mounting board according to claim 11.
    
    

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