JP7488504B1 - Solder alloy, solder paste, solder ball, solder preform, solder joint, on-vehicle electronic circuit, ECU electronic circuit, on-vehicle electronic circuit device, and ECU electronic circuit device - Google Patents

Abstract

[Problem] To provide a solder alloy, solder paste, solder ball, solder preform, solder joint, on-vehicle electronic circuit, ECU electronic circuit, on-vehicle electronic circuit device, and ECU electronic circuit device, which have a low melting point, high shear strength, and an appropriate failure mode. [Solution] The solder alloy has an alloy composition consisting of, by mass%, 2.0 to 3.6% Ag, 1.0 to 5.0% In, 3.0 to 5.0% Sb, 0.001 to 0.030% Fe, 0% to 0.050% Co, and the balance being Sn. Preferably, this alloy composition further contains, by mass%, 0.1% or less of at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si. [Selected Figure] Figure 1

Description

The present invention relates to a solder alloy, a solder paste, a solder ball, a solder preform, a solder joint, an on-board electronic circuit, an ECU electronic circuit, an on-board electronic circuit device, and an ECU electronic circuit device.

Automobiles are equipped with electronic circuits (hereafter referred to as "on-board electronic circuits") that have electronic components soldered to a printed circuit board. On-board electronic circuits are used in devices that electrically control the engine, power steering, brakes, etc., and are extremely important safety components for the operation of an automobile. In particular, on-board electronic circuits called ECUs (Engine Control Units), which are electronic circuits that control the vehicle via computer to improve fuel efficiency, must be able to operate stably without failure for long periods of time. As the mounting area of such on-board electronic circuits has expanded, they are now mounted in places that are subject to various external loads such as shocks and vibrations.

Since the 1980s, Sn-Ag solder alloys, which are an alternative to Sn-Pb solder alloys, have been considered as a solder alloy for use in engine compartments. Sn-Ag solder alloys have long been known as a highly versatile solder alloy, as can be seen from the JIS symbol A35 that has been assigned to them. However, Sn-Ag solder alloys are considered to be a basic lead-free solder alloy to replace Sn-Pb solder alloys, and further consideration has been given to them.

In addition, when a Sn-Ag solder alloy is connected to a Cu electrode, Cu hardly dissolves in Sn, so that a coarse CuSn intermetallic compound layer is formed at the joint interface by Cu diffused from the electrode and Sn in the solder alloy. Furthermore, the large amount of Ag ₃ Sn precipitates, making the solder alloy brittle, and there are various concerns. For this reason, there is a demand for a Sn-Ag solder alloy that does not break the solder joint even in a harsh operating environment, and various studies have been conducted in the past.

In Patent Document 1, in order to improve the tensile strength of the solder alloy, the addition of Sb, Bi, In, Ag, and Ga to a Sn-Co-(Sb, Bi, In, Ag, Ga) solder alloy is investigated. The solder alloy described in the document discloses that by containing Co, fine CoSn and _CoSn2 are dispersed in the Sn matrix, which contributes to improving the tensile strength. In addition, in order to lower the melting point, the addition of further elements such as Sb is investigated.

Patent Document 2 discloses a Sn-Ag-Sb-Ni-Bi-Co solder alloy that suppresses the occurrence of voids and also suppresses the occurrence of cracks even after a heat cycle test. The document states that the absence of Cu reduces the melt viscosity, thereby suppressing the occurrence of voids. Furthermore, the document also discloses that the inclusion of Ni suppresses the excessive diffusion of Cu even without the inclusion of Cu, thereby suppressing the progression of cracks.

Japanese Patent Application Laid-Open No. 6-344180 JP 2018-1179 A

As mentioned above, the inventions described in Patent Documents 1 and 2 aim to improve the tensile strength of the solder alloy, suppress the occurrence of voids, and improve heat cycle resistance. Patent Document 1 also considers lowering the melting point.

However, these evaluations alone are not sufficient for solder alloys to be used in automotive electronic circuits. For example, when a vehicle equipped with electronic circuits runs on rough roads, the electronic circuits are subjected to stress from external shocks and vibrations. For this reason, it is extremely important for solder joints to exhibit high shear strength so that they do not break.

Furthermore, even if a solder joint that is difficult to break is formed, if stress is continuously applied to the solder joint, it will eventually break. It is thought that such continuous stress is continuously applied due to exposure to an environment with extreme temperature differences. This is due to the difference in thermal expansion coefficients of the electrodes, the intermetallic compounds formed at the joint interface, and the bulk. In this case, the fracture mode that indicates the location of the fracture must not be the joint interface. Since the joint interface is joined to the electrodes, it is not easy to relieve stress at the joint interface. However, physical and electrical loads are mainly applied to the joint interface of the solder joint. For this reason, it is thought that fracture can be suppressed by relieving stress in the bulk, which is relatively easy to deform.

However, Patent Documents 1 and 2 do not consider shear strength and fracture modes at all, and it is difficult to say that they reflect the actual situation when using solder joints. Since solder joints electrically connect substrates and electronic components, fracture at the joint interface should be avoided as much as possible.

Thus, Patent Documents 1 and 2 do not consider at all the important properties of solder joints, namely shear strength and fracture mode. Solder alloys that exhibit these properties, even if they do not contain Cu, are desirable. However, as automobiles have become increasingly electrified in recent years and the number of circuit boards mounted on them is expected to continue to increase, there is an urgent need to develop solder alloys that exhibit these properties. Furthermore, in consideration of the heat resistance of electronic components, it is also desirable for them to exhibit a melting point equivalent to that of conventional solder alloys.

The object of the present invention is to provide a solder alloy, solder paste, solder ball, solder preform, solder joint, on-board electronic circuit, ECU electronic circuit, on-board electronic circuit device, and ECU electronic circuit device that have a low melting point, high shear strength, and an appropriate failure mode.

The present inventors have reexamined the solder alloys disclosed in Patent Documents 1 and 2. Among the solder alloys disclosed in both documents, the Sn-Ag-In-Sb-Co-Ga solder alloy in Example 5 of Patent Document 1, the Sn-Ag-In-Sb-Co-Ni-Bi solder alloy in Example 19 of Patent Document 2, and the Sn-Ag-Sb-Co-Fe-Ni-Bi solder alloy in Example 24 of the same document have the same shear strength as conventional solder alloys, and have found that there is room for improvement.

These solder alloys do not have alloy compositions designed for the purpose of improving shear strength. For each solder alloy, if the content of even one of the compositional components differs, the overall characteristics will usually differ, and it should be understood that the entire combination of alloying elements with a specified content is technically evaluated as a single entity.

The inventors therefore conducted detailed research into the improvement of shear strength and the fracture mode while suppressing the rise in melting point. Here, Patent Document 1 explains that the melting point can be lowered by adding Ag, In, Sb, and Ga to a Sn-Co solder alloy. However, since the melting point varies greatly depending on the content of these elements, it is not necessarily the case that the composition described in Example 5 of Patent Document 1 is appropriate.

Patent Document 1 also discloses the addition of Co to improve tensile strength. However, no consideration is given to improving shear strength. Furthermore, if the tensile strength of the solder alloy is increased more than necessary, the fracture mode becomes the joint interface. For this reason, it is presumed that the addition of Co is not desirable in Sn-Ag-In-Sb-Co-Ga solder alloys.

Patent Document 2 describes that the occurrence of voids can be suppressed if Ni and Co are within a specified range. In light of the description in Patent Document 2, Co can be effective when it coexists with Ni. Therefore, based on the knowledge that the alloy composition in Patent Document 1 had low shear strength and an inappropriate fracture mode, it is presumed that the addition of Ni is also undesirable in Patent Document 2.

Furthermore, Patent Document 2 states that thermal shock resistance can be maintained if the Bi content is below a certain amount. However, in Patent Document 2, Bi dissolves in Sn up to about 3%, so the tensile strength of the solder alloy increases due to the solid solution strengthening of Sn, and the fracture mode becomes the joint interface.

In view of the above findings, the inventors focused on the Sn-Ag-In-Sb solder alloy in Patent Documents 1 and 2, in which Ni, Co, and Bi were excluded from the additive elements to Sn. The inventors then conducted detailed studies on the selection of additional additive elements and the contents of Ag, In, and Sb in order to improve the shear strength of this solder alloy and ensure that the fracture mode is bulk.

First, in order to ensure an appropriate fracture mode, it is necessary to suppress the growth of intermetallic compounds at the joint interface in solder alloys that do not contain Cu or Ni. Intermetallic compounds grow as Cu diffuses from the electrode into the solder alloy. Here, intermetallic compounds are mainly composed of compounds of Sn and Cu. If the intermetallic compound layer at the interface between the solder alloy and the electrode is coarse, the fracture mode is likely to be inappropriate.

Therefore, focusing on the fact that the interface is modified by including Fe, the content of the additive elements in the Sn-Ag-In-Sb-Fe solder alloy was investigated in detail. As a result, it was found that the shear strength can be improved and the fracture mode can be optimized only when the content of the additive elements is within a predetermined range.
The present invention, which was made based on these findings, is as follows.

(0) A solder alloy comprising, by mass%, 2.0 to 3.6% Ag, 1.0 to 5.0% In, 3.0 to 5.0% Sb, 0.0010 to 0.0300% Fe, 0% or more and 0.050% or less Co, and the balance being Sn.
(1) A solder alloy having an alloy composition consisting of, in mass%, 2.0 to 3.6% Ag, 1.0 to 5.0% In, 3.0 to 5.0% Sb, 0.0010 to 0.0300% Fe, 0% or more and 0.050% or less Co, and the balance being Sn.

(2) The solder alloy according to (0) or (1) above, further comprising, by mass%, at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si in a total amount of 0.1% or less.

(3) A solder alloy according to any one of (0) to (2) above, wherein the alloy composition (solder alloy) satisfies the following formulas (1) and (2):
0.36≦Ag×In×Sb×Fe≦1.19 (1)
47≦(In×Sb)/(Ag×Fe)≦319 (2)
In the above formulas (1) and (2), Ag, In, Sb, and Fe each represent the content in mass % of the solder alloy.

(4) A solder paste having a solder powder made of the solder alloy described in any one of (0) to (2) above.

(5) A solder ball made of a solder alloy described in any one of (0) to (2) above.

(6) A solder preform made of the solder alloy described in any one of (0) to (2) above.

(7) A solder joint having a solder alloy described in any one of (0) to (2) above.

(8) An in-vehicle electronic circuit comprising the solder alloy described in any one of (0) to (2) above.

(9) An ECU electronic circuit comprising the solder alloy described in any one of (0) to (2) above.

(10) An on-vehicle electronic circuit device comprising the on-vehicle electronic circuit described in (8) above.

(11) An ECU electronic circuit device comprising the ECU electronic circuit described in (9) above.

FIG. 1 shows optical microscope photographs of samples after measuring the shear strength, where FIG. 1(a) is Example 14, FIG. 1(b) is Example 2, and FIG.

The present invention is described in more detail below. In this specification, "%" in relation to the solder alloy composition means "mass %" unless otherwise specified.

1. Solder alloy (1) Ag: 2.0-3.6%
Ag contributes to improving the shear strength, optimizing the fracture mode by precipitation of Ag ₃ Sn, and lowering the melting point. If the Ag content is less than 2.0%, the amount of compound precipitated is small and the shear strength is reduced. The lower limit of the Ag content is 2.0% or more, preferably 2.5% or more, more preferably 2.7% or more, and even more preferably 3.0% or more.

On the other hand, when the Ag content exceeds 3.6%, a large amount of Ag ₃ Sn precipitates due to hypereutectic, and the strength of the bulk increases excessively, so that the fracture mode becomes the bonding interface. Furthermore, the fracture mode becomes the bonding interface, and the shear strength decreases. The upper limit of the Ag content is 3.6% or less, preferably 3.5% or less, and more preferably 3.4% or less.

(2) In: 1.0 to 5.0%
In contributes to improving the shear strength and optimizing the fracture mode. If the In content is less than 1.0%, the wettability is reduced, the wetting spread is insufficient, and the effect of solid solution strengthening is insufficient, so the shear strength is inferior and the fracture mode is inappropriate. The lower limit of the In content is 1.0% or more, preferably 1.5% or more, more preferably 2.0% or more, even more preferably 2.5% or more, and particularly preferably 3.0% or more.

On the other hand, if the In content exceeds 5.0%, a large amount of compounds are precipitated, causing the melting point to rise. Furthermore, the bulk strength increases, raising concerns about fracture at the joint interface or at the component. The upper limit of the In content is 5.0% or less, preferably 4.5% or less, more preferably 4.0% or less, and even more preferably 3.5% or less.

(3) Sb: 3.0 to 5.0%
Sb contributes to improving the shear strength and optimizing the fracture mode. If the Sb content is less than 3.0%, the shear strength is inferior because the solid solution strengthening with respect to Sn and the precipitation strengthening of the Sn-Sb compound are insufficient. The lower limit of the Sb content is 3.0% or more, preferably 3.5% or more, more preferably 3.6% or more, even more preferably 3.8% or more, particularly preferably 3.9% or more, and most preferably 4.0% or more.

On the other hand, if the Sb content exceeds 5.0%, coarse SnSb compounds are formed, resulting in poor shear strength. Furthermore, wettability deteriorates, and the failure mode becomes the joint interface or component failure, which is inappropriate. The upper limit of the Sb content is 5.0% or less, preferably 4.8 or less, more preferably 4.6% or less, even more preferably 4.5% or less, particularly preferably 4.3% or less, and most preferably 4.1% or less.

(4) Fe: 0.0010 to 0.0300%
Fe contributes to improving the shear strength and optimizing the fracture mode. If the Fe content is less than 0.0010%, the effect of strengthening the interface by modifying the intermetallic compound layer formed at the interface is insufficient, so the shear strength is poor and the fracture mode is at the bonded interface, which is not appropriate. The lower limit of the Fe content is 0.0010% or more, preferably 0.0050% or more, more preferably 0.0100% or more, even more preferably 0.0150% or more, and particularly preferably 0.0200% or more.

On the other hand, if the Fe content exceeds 0.0300%, Sn and Fe compounds precipitate, which increases the bulk strength excessively, raising concerns about fracture at the bonding interface. The upper limit of the Fe content is 0.0300% or less, preferably 0.0270% or less, and more preferably 0.0250% or less.

(5) Co: 0% to 0.050% Co is an optional element that contributes to suppressing the rise in melting point, improving the shear strength, and optimizing the fracture mode. In conventional solder alloys, it was considered better not to include Co from the viewpoint of improving the shear strength and optimizing the fracture mode. However, when Co was added, the high shear strength was maintained and the fracture mode remained bulk. In Sn-Ag-In-Sb solder alloys, Co is thought to finely disperse CoSn and the like. However, it does not reach the point of making the alloy structure of Sn fine. In this alloy system, since the alloy structure of Sn is made fine by Fe, it is presumed that when the fine structure of CoSn and the like is dispersed, the entire alloy structure becomes finer. Therefore, in the solder alloy according to the present invention, Co can exert a synergistic effect in the presence of Fe.

In addition, since Co is an optional element in the solder alloy of the present invention, even if Co is not contained, high shear strength and optimization of the fracture mode can be maintained. The lower limit of the Co content is 0% or more, preferably more than 0%, more preferably 0.0010% or more, even more preferably 0.0030% or more, particularly preferably 0.0060% or more, and most preferably 0.0080% or more.

On the other hand, if the Co content exceeds 0.0500%, Sn and Co compounds precipitate, causing the bulk strength to increase excessively, and the fracture mode becomes the bonding interface. In addition, the melting point increases significantly due to the large amount of compounds precipitated, and the wettability deteriorates, resulting in a decrease in shear strength. The upper limit of the Co content is 0.0500% or less, preferably 0.0300% or less, and more preferably 0.0100% or less.

(6) Balance: Sn
The balance of the solder alloy according to the present invention is Sn. In addition to the above elements, unavoidable impurities may be contained. Even if unavoidable impurities are contained, the above-mentioned effects are not affected. In the present invention, when the Sn-Ag-In-Sb-Fe solder alloy contains Ni, SnNi compounds are precipitated. Since the SnNi compounds precipitate using the intermetallic compounds formed at the interface as nuclei, the intermetallic compound layer formed at the interface becomes thick. As a result, the shear strength decreases. Therefore, in the present invention, it is better not to contain Ni. In addition, when Bi coexists with In, it forms a Sn-In-Bi low melting point phase. In the low melting point phase, in view of creep deformation, the room temperature environment is a very high temperature environment relative to the melting point, so it is easy to creep deform and the shear strength decreases. For this reason, in the present invention, it is better not to contain Bi.

(7) At least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si is 0.1% or less in total. The solder alloy according to the present invention can contain at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si in a total amount of 0.1% or less as an optional element, to the extent that the effect of the present invention is not impaired. Preferably, the total amount is 0.08% or less. The lower limit of the content is not particularly limited, but may be 0.0001% or more, and may be 0.001% or more.

(8) Formula (1) and formula (2) 0.36≦Ag×In×Sb×Fe≦1.19 (1)
47≦(In×Sb)/(Ag×Fe)≦319 (2)
In the above formulas (1) and (2), Ag, In, Sb, and Fe each represent a content in terms of mass % of the solder alloy.

Formula (1) is a formula that takes into consideration the balance of the additive elements of the solder alloy of the present invention. The solder alloy of the present invention can exhibit a low melting point, high shear strength, and appropriate fracture mode due to the synergistic effect of each constituent element. Therefore, the balance of all constituent elements except Sn can further improve all the effects of the present invention. In formula (1), the contents of Ag, In, and Sb are about 10 to 100 times the content of Fe. However, it is considered that the contribution of each of these elements to the solder alloy is about the same. Therefore, in order to further improve the low melting point, high shear strength, and appropriate fracture mode of the present invention in one composition at the same time, it is preferable to have a balanced content.

Formula (2) takes into consideration the balance within the group of In and Sb, which, when exceeded, increase the shear strength to the point of component failure, and the balance within the group of Ag and Fe, which remain at the interface, among the added elements, and also takes into consideration the balance between the two groups. When formula (2) is satisfied, the failure mode may become even more appropriate depending on the alloy composition.

The lower limit of formula (1) is preferably 0.36 or more, more preferably 0.39 or more, even more preferably 0.42 or more, even more preferably 0.43 or more, particularly preferably 0.44 or more, and most preferably 0.52 or more, 0.53 or more, 0.60 or more, 0.656 or more, 0.66 or more, 0.70 or more, 0.75 or more, 0.78 or more, 0.79 or more, 0.84 or more, 0.87 or more, or 0.88 or more. The upper limit of formula (1) is preferably 1.19 or less, more preferably 1.18 or less, even more preferably 1.09 or less, even more preferably 1.08 or less, particularly preferably 1.05 or less, and most preferably 1.02 or less, and may be 0.91 or less, 0.92 or less, or 0.90 or less.

The lower limit of formula (2) is preferably 47 or more, more preferably 51 or more, even more preferably 57 or more, even more preferably 68 or more, particularly preferably 69 or more, and most preferably 85 or more, 86 or more, 91 or more, 102 or more, 103 or more, 114 or more, 120 or more, 133 or more, 137 or more, 141 or more, 142 or more, 143 or more, 154 or more, or 160 or more. The upper limit of formula (2) is preferably 319 or less, more preferably 286 or less, even more preferably 285 or less, even more preferably 267 or less, particularly preferably 266 or less, and most preferably 257 or less, and may be 240 or less, 229 or less, 228 or less, 213 or less, 206 or less, 205 or less, 200 or less, 192 or less, 183 or less, 182 or less, or 171 or less.

In the calculation of formula (1) and formula (2), the numerical values themselves shown in the measured values of the alloy composition shown in Tables 1 and 2 below are used. That is, in the calculation of formula (1) and formula (2), all digits smaller than the number of significant digits in the measured values shown in Tables 1 to 3 below are treated as 0. For example, if the measured Fe content is "0.0250" mass%, the Fe content used in the calculation of formula (1) and formula (2) does not have a range of 0.02505 to 0.02514%, but is treated as "0.025000...". In formula (1), calculation is performed to the third decimal place, and the third decimal place is rounded off to the second decimal place, and in formula (2), calculation is performed to the first decimal place, and the first decimal place is rounded off to the one place.
The same procedure is applied when calculating formulas (1) and (2) from alloy compositions specifically disclosed in patent documents or other documents mentioned in this specification.

As mentioned above, in an alloy, all the constituent elements do not function individually, but rather all the constituent elements form a single entity as a whole, so it is rare for one element to simultaneously exert all of the excellent effects. For this reason, as mentioned above, in order to achieve even better properties within the optimal content range of each constituent element, it is necessary to consider the constituent elements as a whole. In the solder alloy of the present invention, in order to achieve a high level of low melting point, high shear strength, and appropriate fracture mode all in one composition, it is preferable for formulas (1) and (2) to be satisfied.

In the examples described below, if the evaluation result is "◎", it means that it is particularly preferable in practical use compared to "◯". Since "◯" is a more preferable result than the conventional example, if other evaluation results are also excellent, it is within the scope of the present invention and is treated as an example. Since "×" is an insufficient result in the present invention, it is outside the scope of the present invention and is treated as a comparative example.

2. Solder Paste The solder paste according to the present invention is a mixture of solder powder having the above-mentioned alloy composition and flux. The flux used in the present invention is not particularly limited as long as it allows soldering by a normal method. Therefore, a suitable mixture of commonly used rosin, organic acid, activator, thixotropic agent, and solvent may be used. The blending ratio of the metal powder component and the flux component in the present invention is not particularly limited, but is preferably 70 to 90 mass % of the metal powder component and 10 to 30 mass % of the flux component.

3. Solder Ball The solder alloy according to the present invention can be used as a solder ball. When used as a solder ball, the solder alloy according to the present invention can be manufactured by using a dropping method, which is a common method in the industry. Also, a solder joint can be manufactured by processing the solder ball by a common method in the industry, such as mounting one solder ball on one electrode coated with flux and joining it. The particle size of the solder ball is preferably 1 μm or more, more preferably 10 μm or more, even more preferably 20 μm or more, and particularly preferably 30 μm or more. The upper limit of the particle size of the solder ball is preferably 3000 μm or less, more preferably 1000 μm or less, even more preferably 800 μm or less, and particularly preferably 600 μm or less.

4. Solder Preform The solder alloy according to the present invention can be used as a preform. Preform shapes include washers, rings, pellets, disks, ribbons, wires, and the like.

5. Solder joint The solder joint according to the present invention is suitable for use in joining at least two or more members to be joined. The members to be joined are not particularly limited as long as they are electrically connected using the solder alloy according to the present invention, and include, for example, elements, substrates, electronic components, printed circuit boards, insulating substrates, heat sinks, lead frames, semiconductors using electrode terminals, power modules, inverter products, and the like.

The joining method using the solder alloy of the present invention may be performed in the usual manner, for example using the reflow method. The melting temperature of the solder alloy when performing reflow soldering may be a temperature approximately 20°C higher than the liquidus temperature. Furthermore, when joining using the solder alloy of the present invention, the alloy structure can be made finer by taking into consideration the cooling rate during solidification. For example, the solder joint is cooled at a cooling rate of 2 to 3°C/s or more. Other joining conditions can be adjusted as appropriate depending on the alloy composition of the solder alloy.

6. On-vehicle electronic circuit, ECU electronic circuit, on-vehicle electronic circuit device, ECU electronic circuit device As is clear from the above explanation, the solder alloy according to the present invention has a suppressed increase in melting point and an appropriate fracture mode. Therefore, even when used for automobiles exposed to harsh environments, i.e., for on-vehicle use, fracture of the solder joint is suppressed without variation. Therefore, since the solder alloy according to the present invention has such particularly remarkable properties, it is found to be particularly suitable for soldering electronic circuits mounted on automobiles.

Thus, the solder alloy of the present invention is particularly effective when used for soldering in-vehicle electronic circuits or when used for soldering ECU electronic circuits.

An "electronic circuit" is a system that performs a desired function as a whole by combining multiple electronic components, each of which has its own function, in an electronic engineering manner.

Examples of electronic components that make up such electronic circuits include chip resistor components, multi-resistor components, QFP, QFN, power transistors, diodes, capacitors, etc. Electronic circuits incorporating these electronic components are mounted on a substrate to form an electronic circuit device.

In the present invention, the substrate constituting such an electronic circuit device, for example a printed wiring board, is not particularly limited. The material is also not particularly limited, but examples include heat-resistant plastic substrates (e.g. FR-4 with high Tg and low CTE). The printed wiring board is preferably a printed circuit board in which the Cu land surface is treated with an organic substance such as amine or imidazole (OSP: Organic Surface Protection).

7. Others The solder alloy according to the present invention can be manufactured using a low alpha radiation material as its raw material. When such a low alpha radiation alloy is used to form solder bumps around a memory, it is possible to suppress soft errors.

The present invention will be described with reference to the following examples, but the present invention is not limited to the following examples.
In order to verify the effects of the present invention, the solder alloys shown in Tables 1 to 3 were used to evaluate (1) melting point, (2) shear strength, and (3) fracture mode.

(1) Melting point For the solder alloys shown in Tables 1 to 3, the respective temperatures were obtained from the DSC curves. The DSC curves were obtained by raising the temperature at 5°C/min in air using a Seiko Instruments Inc. DSC (model number: 6200). The liquidus temperature was obtained from the obtained DSC curve and was taken as the melting point. When the melting point is 232°C or less, reflow soldering can be performed at a temperature similar to that of the conventional method. When the melting point is more than 232°C, conventional reflow soldering cannot be performed due to the high melting point.

(2) Shear strength (2-1) Sample preparation The solder alloys shown in Tables 1 to 3 were cast to prepare solder sheets (diameter: 1 mm, thickness: 0.15 mm). A reflow furnace (SNR-615: manufactured by Senju Metal Industry Co., Ltd.) was used to solder chip resistors to Cu-OSP electrodes on an FR-4 board. The chip resistors used were 3216CR (CR32-114JV: manufactured by Hokuriku Electric Industry Co., Ltd.). The reflow profile was held at 220°C or higher for 40 seconds in a nitrogen atmosphere, with a peak temperature of 245°C.
(2-2) Evaluation of Shear Strength The shear strength of the samples thus prepared was measured using a shear tester (STR-1000: manufactured by RHESCA) at a shear speed of 6 mm/min. When the shear strength was 84.0 N or more, it was evaluated as "◎". When the shear strength was 70.0 N or more but less than 84.0 N, it was evaluated as "◯". When the shear strength was less than 70.0 N, it was evaluated as "×".

(3) Fracture mode The samples evaluated in "(2) Shear strength" above were observed for fracture mode using an optical microscope (VHX-5000: manufactured by KEYENCE). When the sample fractured in the bulk, it was rated as "◎". When the sample fractured in the bulk and in the intermetallic compound (IMC) at the bonding interface, it was rated as "◯". When the sample fractured in the intermetallic compound, it was rated as "X".
The evaluation results are shown in Tables 1 to 3.

As shown in Tables 1 and 2, Examples 1 to 110 all had appropriate amounts of each constituent element, and all evaluations showed results that were acceptable for practical use. In addition, Examples 3 to 14, 16, 19, 22, 28 to 31, 37 to 48, 50, 54, 57, 66, 68 to 73, and 77 to 110, which satisfy formulas (1) and (2), were found to show extremely excellent results in all evaluations. These were results that were significantly superior among the results that were acceptable for practical use.

On the other hand, as shown in Table 3, Comparative Example 1 did not contain In, Sb, or Fe, so the shear strength was poor and the failure mode was inappropriate. Comparative Example 2 had poor shear strength because the Ag content was low. Comparative Example 3 had poor shear strength and an inappropriate failure mode because the Ag content was high.

Comparative Example 4 did not contain In, and Comparative Example 5 had a low In content, resulting in poor shear strength and an inappropriate fracture mode. Comparative Example 6 had a high In content, resulting in an inappropriate fracture mode.

Comparative Example 7 had poor shear strength and an inappropriate fracture mode because it contained a low Sb content and no Fe. Comparative Example 8 had poor shear strength because it contained a low Sb content. Comparative Example 9 had poor shear strength and an inappropriate fracture mode because it contained a high Sb content.

In Comparative Examples 10 to 12, the Fe content was inappropriate, resulting in poor shear strength and an inappropriate fracture mode. In Comparative Example 13, the Co content was high, resulting in a significant increase in melting point, poor shear strength, and an inappropriate fracture mode. Comparative Examples 14 and 15, which each contained Ni or Bi, resulted in poor shear strength.

Figure 1 shows optical microscope photographs of samples after measuring the shear strength, with Figure 1(a) being Example 14, Figure 1(b) being Example 2, and Figure 1(c) being Comparative Example 3. As is clear from Figure 1, in Example 14, it was found that the solder joint broke due to bulk fracture. Also, in Example 2, it was found that the fracture occurred in the bulk and in the intermetallic compound at the joint interface. On the other hand, in Comparative Example 3, it was found that the solder joint broke due to fracture in the intermetallic compound at the joint interface. Therefore, it was found that the fracture mode was appropriate in this Example 14. This result was similar in the other Examples.

The solder of the present invention can be used in on-board electronic circuits such as ECUs, which are electronic circuits that control automobiles using computers to improve fuel efficiency, but it can also be used in consumer electronic devices such as personal computers with excellent results.

Claims (11)

A solder alloy characterized by an alloy composition consisting of, in mass%, Ag: 2.0-3.6%, In: 1.0-5.0%, Sb: 3.0-5.0%, Fe: 0.0010-0.0300%, Co: 0% or more and 0.050% or less, and the balance being Sn. The solder alloy according to claim 1, further comprising, by mass%, at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si in a total amount of 0.1% or less. The solder alloy according to claim 1 or 2, wherein the alloy composition satisfies the following formulas (1) and (2):
0.36≦Ag×In×Sb×Fe≦1.19 (1)
47≦(In×Sb)/(Ag×Fe)≦319 (2)
In the above formulas (1) and (2), Ag, In, Sb, and Fe each represent the content in mass % of the solder alloy. A solder paste comprising a solder powder made of the solder alloy according to claim 1 or 2. A solder ball made of the solder alloy according to claim 1 or 2. A solder preform made of the solder alloy according to claim 1 or 2. A solder joint having the solder alloy according to claim 1 or 2. An in-vehicle electronic circuit comprising the solder alloy according to claim 1 or 2. An ECU electronic circuit comprising the solder alloy according to claim 1 or 2. An on-vehicle electronic circuit device comprising the on-vehicle electronic circuit according to claim 8. An ECU electronic circuit device comprising the ECU electronic circuit according to claim 9.