TWI899001B - Solder alloy, solder paste, solder ball, solder preform, solder joint, automotive electronic circuit, ECU electronic circuit, automotive electronic circuit device, and ECU electronic circuit device - Google Patents

Abstract

The present invention provides a solder alloy, solder paste, solder balls, solder preforms, solder joints, automotive electronic circuits, ECU electronic circuits, automotive electronic circuit devices, and ECU electronic circuit devices, all with a low melting point, high shear strength, and a suitable failure mode. The solder alloy comprises, by mass%, 2.0-3.6% Ag, 1.0-5.0% In, 3.0-5.0% Sb, 0.001-0.030% Fe, and 0.0000% to 0.0500% Co, with the remainder being Sn. Preferably, the alloy further comprises at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si in a total mass of 0.100% or less.

Description

Solder alloy, solder paste, solder ball, solder preform, solder joint, automotive electronic circuit, ECU electronic circuit, automotive electronic circuit device, and ECU electronic circuit device

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

Automobiles are equipped with electronic circuits (hereinafter referred to as "automotive electronic circuits") that consist of electronic components soldered to printed circuit boards. These circuits are used to electrically control components such as the engine, power steering, and brakes, and are crucial safety components during vehicle operation. In particular, the ECU (Engine Control Unit), which controls the vehicle's electronic circuits using a computer to improve fuel efficiency, must operate stably and without failure for extended periods of time. As the number of electronic circuits installed in vehicles increases, they are being installed in locations subject to various external loads, such as shock and vibration.

Since the 1980s, Sn-Ag solder alloys have been considered as alternatives to Sn-Pb solder alloys for use in engine compartments. Sn-Ag solder alloys have long been known as widely available solder alloys, as evidenced by their JIS designation A35. However, Sn-Ag solder alloys are considered to be a fundamental counterpart to Sn-Pb solder alloys as lead-free solder alloys, and further research is underway.

Furthermore, when Sn-Ag solder alloys are connected to Cu electrodes, Cu is almost insoluble in Sn. Therefore, at the joint interface, Cu diffused from the electrode forms a coarse Cu-Sn intermetallic compound layer with Sn in the solder alloy. Furthermore, the large amount of _Ag₃Sn precipitated can cause the solder alloy to become brittle, raising various concerns. Therefore, efforts have been underway to develop Sn-Ag solder alloys that resist cracking even under harsh operating conditions.

Patent Document 1 examines the addition of Sb, Bi, In, Ag, and Ga to a Sn-Co solder alloy to improve its tensile strength. It reveals that the inclusion of Co in the solder alloy described in this document contributes to improved tensile strength by dispersing fine CoSn and _CoSn2 within the Sn matrix. Furthermore, the addition of further elements such as Sb was considered to lower the melting point.

Patent Document 2 discloses a Sn-Ag-Sb-Ni-Bi-Co solder alloy that suppresses porosity and cracking even after thermal cycling testing. The document states that excluding Cu reduces the melt viscosity, thereby suppressing porosity. Furthermore, the document also discloses that the inclusion of Ni suppresses excessive Cu diffusion, even without Cu, thereby suppressing crack growth. [Prior Art Document] [Patent Document]

Patent Document 1: Japanese Patent Application Publication No. 6-344180 Patent Document 2: Japanese Patent Application Publication No. 2018-1179

[Problem to be Solved by the Invention] As previously mentioned, the inventions described in Patent Documents 1 and 2 achieve improved tensile strength of solder alloys, suppress the formation of voids, and improve thermal cycling resistance. Furthermore, Patent Document 1 examines the lowering of the melting point.

However, for solder alloys used in automotive electronic circuits, these evaluations alone are insufficient. For example, when a car carrying the electronic circuits travels on uneven roads, external stresses such as shock and vibration are applied to the electronic circuits. Therefore, exhibiting high shear strength to prevent the solder joints from breaking is crucial for solder joints.

Furthermore, even if a solder joint that is difficult to break is formed, it will eventually break if stress is continuously applied to the solder joint. This continuous stress is believed to be continuously applied due to exposure to an environment with a drastic temperature difference. This is due to the difference in thermal expansion coefficients between the intermetallic compound and the bulk formed at the electrode and the joint interface. In this case, the failure mode representing the fracture site is the joint interface. Since the joint interface is bonded to the electrode, 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. Therefore, it is believed that relieving stress with a bulk that is relatively easy to deform can suppress failure.

However, Patent Documents 1 and 2 do not examine shear strength and failure modes, making it difficult to say whether they reflect the actual use of solder joints. Since solder joints electrically connect substrates and electronic components, fractures at the joint interface should be avoided as much as possible.

Therefore, Patent Documents 1 and 2 fail to examine shear strength and failure modes, which are crucial properties of solder joints. A solder alloy that exhibits these properties, even one that does not contain Cu, is desired. However, with the recent advancement of automotive electrification, the number of boards mounted on these components is expected to continue to increase, making the development of solder alloys that exhibit these properties a top priority. Furthermore, given the heat resistance of electronic components, a similar melting point is also desired.

Therefore, the subject of the present invention is to provide a solder alloy, solder paste, solder ball, solder preform, solder joint, automotive electronic circuit, ECU electronic circuit, automotive electronic circuit device, and ECU electronic circuit device with a low melting point, high shear strength, and an appropriate failure mode. [Means for Solving the Problem]

The inventors of the present invention have re-examined the solder alloys disclosed in Patent Documents 1 and 2. They have found that, among the solder alloys disclosed in these two documents, the shear strength of the Sn-Ag-In-Sb-Co-Ge solder alloy of Example 5 of Patent Document 1, the Sn-Ag-In-Sb-Co-Ni-Bi solder alloy of Example 19 of Patent Document 2, and the Sn-Ag-Sb-Co-Fe-Ni-Bi solder alloy of Example 24 of the same patent are all comparable to conventional solder alloys, and therefore have room for improvement.

These solder alloys do not necessarily have alloy compositions designed for enhanced shear strength. It should be understood that solder alloys generally exhibit different overall properties even if the content of one of their components varies. A combination of alloying elements with a specific content is technically evaluated as a whole.

Therefore, the inventors conducted detailed investigations to suppress the rise in melting point while simultaneously improving shear strength and failure mode. Patent Document 1 states that adding Ag, In, Sb, and Ga to a Sn-Co solder alloy can lower the melting point. However, because the melting point varies significantly depending on the content of these elements, the composition described in Example 5 of Patent Document 1 may not be suitable.

Furthermore, Patent Document 1 discloses the addition of Co for the purpose of increasing tensile strength. However, there is no examination of its ability to improve shear strength. Furthermore, when the tensile strength of the solder alloy is increased beyond necessity, the failure mode becomes the bonding interface. Therefore, it is speculated that the addition of Co to the Sn-Ag-In-Sb-Co-Ga solder alloy is not optimal.

Patent 2 states that as long as Ni and Co are within a specific range, porosity can be suppressed. Therefore, based on the description in Patent 2, the coexistence of Co and Ni can achieve this effect. Therefore, based on the observation that the alloy composition in Patent 1 has low shear strength and an inappropriate failure mode, it can be inferred that the addition of Ni in Patent 2 is also unfavorable.

Patent Document 2 also states that thermal shock resistance can be maintained when the Bi content is below a specific amount. However, in Patent Document 2, since Bi forms a solid solution in Sn at a concentration of approximately 3%, the tensile strength of the solder alloy increases due to solid solution strengthening by Sn, making the failure mode the bonding interface.

In light of the above findings, the inventors focused on the Sn-Ag-In-Sb solder alloys described in Patent Documents 1 and 2, which exclude Ni, Co, and Bi from the elements added to Sn. Furthermore, to enhance the shear strength of the solder alloy and achieve a bulk failure mode, they conducted a detailed examination of the selection of additive elements and the contents of Ag, In, and Sb.

First, to achieve an appropriate failure mode, the growth of intermetallic compounds at the joint interface must be suppressed in solder alloys that do not contain Cu and Ni. Intermetallic compounds grow as Cu diffuses from the electrode into the solder alloy. Here, intermetallic compounds primarily consist of compounds of Sn and Cu. Therefore, a coarse intermetallic layer at the interface between the solder alloy and the electrode can easily lead to an inappropriate failure mode.

Therefore, focusing on improving the interface by incorporating Fe, we conducted a detailed investigation into the content of additive elements in Sn-Ag-In-Sb-Fe solder alloys. This led to the discovery that only when the content of the additive elements is within a specific range can the shear strength be improved and the failure mode be optimized. This discovery led to the present invention as follows.

(0) A solder alloy characterized by being composed, in mass%, of Ag: 2.0-3.6%, In: 1.0-5.0%, Sb: 3.0-5.0%, Fe: 0.0010-0.0300%, Co: 0.0000% or more and 0.0500% or less, with the remainder being Sn. (1) A solder alloy characterized by being composed, in mass%, of 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, with the remainder being Sn.

(2) A solder alloy as described in (0) or (1) above, wherein the alloy composition (solder alloy) further contains at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al and Si in a total mass % not exceeding 0.100%.

(3) The solder alloy according to any one of (0) to (2) above, wherein the alloy composition (solder alloy) satisfies the following formulas (1) and (2), In the above formulas (1) and (2), Ag, In, Sb and Fe are the mass % contents of the above solder alloy.

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

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

(6) A solder preform formed from a solder alloy as described in any one of (0) to (2) above.

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

(8) An automotive electronic circuit characterized by having a solder alloy as described in any one of (0) to (2) above.

(9) An ECU electronic circuit characterized by having a solder alloy as described in any one of (0) to (2) above.

(10) An automotive electronic circuit device characterized by having the automotive electronic circuit as described in (8) above.

(11) An ECU electronic circuit device characterized by having the ECU electronic circuit as described in (9) above.

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

1. Solder Alloy (1) Ag: 2.0~3.6% Ag helps to improve shear strength, optimize the failure mode due to Ag ₃ Sn precipitation, and lower the melting point. When the Ag content is less than 2.0%, the amount of compound precipitation 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 hypereutectic is formed, leading to the precipitation of a large amount of _Ag3Sn , which increases the bulk strength and shifts the failure mode to the bond interface. Furthermore, since the failure mode is at the bond interface, the shear strength decreases. The upper limit of the Ag content is 3.6% or less, preferably 3.5% or less, and even more preferably 3.4% or less.

(2) In: 1.0~5.0% In helps to improve shear strength and optimize the failure mode. When the In content is less than 1.0%, the wettability is reduced and the wettability expansion is insufficient. At the same time, the solid solution strengthening effect is insufficient, resulting in poor shear strength and inappropriate failure mode. The lower limit of the In content is 1.0% or more, preferably 1.5% or more, more preferably 2.0% or more, more preferably 2.5% or more, and particularly preferably 3.0% or more.

On the other hand, when the In content exceeds 5.0%, the melting point rises due to the precipitation of large amounts of compounds. There is also concern about increased bulk strength, leading to damage at the joint interface or in components. 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: 3.0~5.0% Sb helps to improve shear strength and optimize the failure mode. When the Sb content is less than 3.0%, the shear strength is poor due to insufficient solid solution strengthening of Sn and precipitation strengthening of Sn-Sb compounds. 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 the best is 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 unsuitable, either at the bond interface or at the component. 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: 0.0010~0.0300% Fe helps to improve shear strength and optimize the failure mode. When the Fe content is less than 0.0010%, the interface strengthening effect caused by the modification of the intermetallic compound layer formed at the interface is insufficient, resulting in poor shear strength and an inappropriate failure mode for the bonding interface. 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-Fe compounds precipitate, excessively increasing the bulk strength and causing concern about damage to the bonding interface. The upper limit of the Fe content is 0.0300% or less, preferably 0.0270% or less, and even more preferably 0.0250% or less.

(5) Co: 0.0000% to 0.0500% Co is an arbitrary element that helps to suppress the rise in melting point, improve shear strength and optimize the failure mode. In the past, solder alloys were considered to be better without Co from the perspective of improving shear strength and optimizing the failure mode. However, as a result of adding Co, high shear strength can be maintained while the failure mode remains bulk. In the Sn-Ag-In-Sb solder alloy, Co is considered to be finely dispersed in the form of CoSn and the like. However, it has not reached the level of making the alloy structure of Sn fine. Since the alloy system makes the alloy structure of Sn finer by Fe, it is speculated that if the fine structure of CoSn and the like is dispersed, the overall alloy structure will further become finer. Therefore, the solder alloy Co of the present invention can exert a multiplying effect in the presence of Fe.

Furthermore, since Co is an optional element in the solder alloy of the present invention, high shear strength and optimized failure mode can be maintained even when Co is not present. The lower limit of the Co content is 0.0000% or greater, preferably exceeding 0.0000%, more preferably 0.0010% or greater, even more preferably 0.0030% or greater, particularly preferably 0.0060% or greater, and most preferably 0.0080% or greater.

On the other hand, if the Co content exceeds 0.0500%, Sn-Co compounds precipitate, excessively increasing the bulk strength and causing the failure mode to be at the bond interface. Furthermore, the large amount of compound precipitation significantly increases the melting point and deteriorates wettability, 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 even more preferably 0.0100% or less.

(6) The remainder: Sn The remainder of the solder alloy of the present invention is Sn. In addition to the above elements, it may also contain unavoidable impurities. When unavoidable impurities are contained, the above effects are not affected. In the present invention, when the Sn-Ag-In-Sb-Fe solder alloy contains Ni, SnNi will precipitate. Since the SnNi compound precipitates with the intermetallic compound formed at the interface as the nucleus, the intermetallic compound formed at the interface becomes thicker. As a result, the shear strength decreases. Therefore, in the present invention, it is better not to contain Ni. Moreover, if Bi coexists with In, a Sn-In-Bi low-melting-point phase is formed. In view of creep deformation, since the low-melting-point phase has a low melting point that changes the room temperature environment to a high-temperature environment, it is easy to creep deform, which reduces the shear strength. Therefore, in the present invention, it is better not to contain Bi.

(7) The solder alloy of the present invention may 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 of 0.100% or less. The solder alloy of the present invention may 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 of 0.100% or less as an arbitrary element, to the extent that the effects of the present invention are not impaired. The preferred total amount is 0.080% or less. The lower limit of the content is not particularly limited, but it is sufficient to be 0.0001% or more, and may be 0.001% or more.

(8) Formula (1) and Formula (2) In the above formulas (1) and (2), Ag, In, Sb, and Fe are the mass % contents of the solder alloy.

Formula (1) is a formula that takes into account the balance of the added 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 an appropriate fracture mode by the multiplication of the constituent elements. Therefore, if the balance of all constituent elements other than Sn is further optimized, all the effects of the present invention can be further improved. In formula (1), the content of Ag, In and Sb is about 10 to 100 times that of Fe. However, it is believed that the contribution to the solder alloy is at the same level. Therefore, in the present invention, in order to further improve the low melting point, high shear strength and appropriate fracture mode with one composition, it is better to set a balanced content.

Equation (2) takes into account the balance between the In and Sb groups, which will increase the shear strength to the point of component failure if the upper limit is exceeded, and the balance between the Ag and Fe groups, which are limited to interface failure, and the balance between these two groups. When Equation (2) is satisfied, the failure mode can be further optimized according to the alloy composition.

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

(2) The lower limit of the formula is preferably 47 or greater, more preferably 51 or greater, even more preferably 57 or greater, still more preferably 68 or greater, particularly preferably 69 or greater, and most preferably 85 or greater, 86 or greater, 91 or greater, 102 or greater, 103 or greater, 114 or greater, 120 or greater, 133 or greater, 137 or greater, 141 or greater, 142 or greater, 143 or greater, 154 or greater, and 160 or greater. (2) The upper limit of the formula is preferably 319 or less, more preferably 286 or less, still more preferably 285 or less, still more preferably 267 or less, particularly preferably 266 or less, and most preferably 257 or less, 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, and 171 or less.

In the calculations of formulas (1) and (2), the values described in the tables are used for the measured values of the alloy compositions shown in Tables 1 and 2 below. That is, in the calculations of formulas (1) and (2), all digits less than the significant digits in the measured values shown in Tables 1 to 3 below are regarded as 0. For example, when the Fe content is measured as "0.0250" mass%, the Fe content used to calculate formulas (1) and (2) is not in the range of 0.02505~0.02514%, but is regarded as "0.025000...". In formula (1), the third decimal place is calculated and rounded to the second decimal place. In formula (2), the first decimal place is calculated and rounded to the first decimal place. Furthermore, when calculating equations (1) and (2) from the alloy compositions specifically disclosed in the patent documents and other documents described in this specification, the same treatment shall be applied.

As mentioned above, an alloy is not made up of all its constituent elements individually, but rather of all the constituent elements combined to form a single object. Therefore, it is rare for a single element to simultaneously exert all its advantageous effects. Therefore, as mentioned above, in order to exhibit superior properties within the optimal content range of each constituent element, it is necessary to examine all the constituent elements. The solder alloy of the present invention preferably satisfies equations (1) and (2) in order to achieve a low melting point, high shear strength, and an appropriate failure mode all at a high level with a single composition.

In the examples described below, a "◎" rating indicates a significantly better performance than a "0" rating. Since "0" represents a superior result compared to conventional methods, if other evaluation results are also excellent, this falls within the scope of the present invention and is treated as an example. Since "×" represents an insufficient result for the present invention, it falls outside the scope of the present invention and is treated as a comparative example.

2. Solder Paste The solder paste of the present invention is a mixture of solder powder composed of the aforementioned alloy and a flux. The flux used in the present invention is not particularly limited as long as it can be soldered using conventional methods. Therefore, a properly blended mixture of commonly used rosin, organic acid, activator, activating agent, and solvent can be used. The ratio of the metal powder component to the flux component in the present invention is not particularly limited, but is preferably 70-90% by mass of the metal powder component and 10-30% by mass of the flux component.

3. Solder Balls The solder alloy of the present invention can be used as solder balls. When used as solder balls, the solder alloy of the present invention can be used to produce solder balls using the dripping method, which is a general method in this technology. In addition, a solder joint can be produced by processing a solder ball by placing it on an electrode coated with flux and joining them together using the general method in this technology. 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 Preforms The solder alloy of the present invention can be used as a preform. Examples of preform shapes include washers, rings, pellets, discs, strips, wires, etc.

5. Solder Joints The solder joints of the present invention are suitable for connecting at least two components. The components to be joined are not particularly limited, as long as they are semiconductors, power modules, inverter products, and other products such as components, substrates, electronic components, printed circuit boards, insulated substrates, heat sinks, lead frames, and electrode terminals, and are electrically connected using the solder alloy of the present invention.

The solder alloy of the present invention can be joined using conventional methods, such as the reflow method. The melting temperature of the solder alloy during reflow soldering is preferably approximately 20°C above the liquidus temperature. Furthermore, when joining using the solder alloy of the present invention, the alloy structure can be further refined by taking into account the cooling rate during solidification. For example, the solder joint can be cooled at a cooling rate of 2-3°C/s or higher. These other joining conditions can be appropriately adjusted based on the composition of the solder alloy.

6. Automotive Electronic Circuits, ECU Electronic Circuits, Automotive Electronic Circuit Devices, and ECU Electronic Circuit Devices: As can be seen from the description thus far, the solder alloy of the present invention suppresses melting point rise and has a suitable failure mode. Therefore, even in automotive applications exposed to harsh environments, i.e., in-vehicle applications, the solder alloy of the present invention consistently suppresses cracking of solder joints. Due to these exceptional properties, the solder alloy of the present invention is particularly suitable for soldering electronic circuits mounted on automobiles.

Therefore, the solder alloy of the present invention is more specifically used for welding automotive electronic circuits or ECU electronic circuits, and exhibits excellent heat cycle resistance.

An "electronic circuit" is a system that achieves its intended function by combining multiple electronic components, each with its own function, using electronic engineering techniques.

Examples of electronic components constituting these electronic circuits include chip resistor components, multiple resistor components, QFP, QFN, power transistors, diodes, capacitors, etc. An electronic circuit composed of these electronic components is provided on a substrate to form an electronic circuit device.

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

7. Others The solder alloy of the present invention can be made into a low-alpha radiation alloy by using a low-alpha radiation material as its raw material. This low-alpha radiation alloy can suppress soft errors when used in the formation of solder bumps around memory devices. [Example]

Although the present invention is described in the following examples, the present invention is not limited to the following examples. To demonstrate the effects of the present invention, the solder alloys listed in Tables 1 to 3 were used to evaluate (1) melting point, (2) shear strength, and (3) failure mode.

(1) Melting Point For the solder alloys shown in Tables 1 to 3, the respective temperatures were determined from the DSC curves. The DSC curves were obtained using a Seiko Instruments DSC (Model: 6200) with a temperature increase of 5°C/min in atmospheric air. The liquidus temperature was determined from the obtained DSC curves and set as the melting point. When the melting point is below 232°C, reflow soldering can be performed at the same temperature as conventional soldering. When the melting point is above 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 produce solder sheets (diameter: 1 mm/, thickness: 0.15 mm). Chip resistors were soldered to Cu-OSP electrodes on FR-4 substrates using a reflow furnace (SNR-615: manufactured by Senju Metal Industries, Ltd.). Chip resistors used were 3216CR (CR32-114JV: manufactured by Hokuriku Electric Industries, Ltd.). The reflow profile was maintained at 220°C or higher for 40 seconds, and the peak temperature was set to 245°C in a nitrogen atmosphere. (2-2) Shear Strength Evaluation 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. A shear strength of 84.0 N or higher was rated as "◎." When the shear strength is 70.0 N or more and less than 84.0 N, the evaluation is "○". When the shear strength is less than 70.0 N, the evaluation is "×".

(3) Failure Mode The failure mode of the samples evaluated in "(2) Shear Strength" above was observed using an optical microscope (VHX-5000: manufactured by KEYENCE). When the sample was destroyed in the bulk, it was evaluated as "◎". When the sample was destroyed in the bulk and the intermetallic compound (IMC) at the bonding interface, it was evaluated as "0". When the sample was destroyed in the intermetallic compound at the bonding interface, it was evaluated as "×". The evaluation results are shown in Tables 1 to 3.

As shown in Tables 1 and 2, in Examples 1-110, since the contents of each constituent element were all appropriate, all evaluations were practically acceptable. Furthermore, Examples 3-14, 16, 19, 22, 28-31, 37-48, 50, 54, 57, 66, 68-73, and 77-110, which satisfied equations (1) and (2), exhibited extremely excellent results in all evaluations. This is a meaningfully superior result among practically acceptable results.

On the other hand, as shown in Table 3, Comparative Example 1 lacks In, Sb, and Fe, resulting in poor shear strength and an inappropriate failure mode. Comparative Example 2 also has poor shear strength due to its low Ag content. Comparative Example 3 also has poor shear strength and an inappropriate failure mode due to its high Ag content.

Comparative Example 4 contains no In, and Comparative Example 5 has a low In content, resulting in poor shear strength and an inappropriate failure mode. Comparative Example 6 has a high In content, resulting in an inappropriate failure mode.

In Comparative Example 7, due to its low Sb content and lack of Fe, the shear strength is poor and the failure mode is inappropriate. In Comparative Example 8, due to its low Sb content, the shear strength is poor. In Comparative Example 9, due to its high Sb content, the shear strength is poor and the failure mode is inappropriate.

Comparative Examples 10-12 exhibit poor shear strength and an inappropriate failure mode due to inappropriate Fe content. In Comparative Example 13, due to the high Co content, the melting point rises significantly, resulting in poor shear strength and an inappropriate failure mode. Comparative Examples 14 and 15 exhibit poor shear strength due to the inclusion of Ni or Bi.

Figure 1 shows optical microscope photographs of samples after shear strength measurement. Figure 1(a) shows Example 14, Figure 1(b) shows Example 2, and Figure 1(c) shows Comparative Example 3. As can be seen from Figure 1, in Example 14, the solder joint fractured due to bulk failure. On the other hand, in Example 2, the intermetallic compound in the bulk and at the bonding interface fractured. On the other hand, in Comparative Example 3, the solder joint fractured due to intermetallic compound failure at the bonding interface. Therefore, it can be seen that the fracture mode in Example 14 is appropriate. The results are similar in the other examples. [Industrial Applicability]

The solder of the present invention can be used in automotive electronic circuits such as ECUs, which are computer-controlled electronic circuits in automobiles to improve fuel efficiency, but can also be used in consumer electronic devices such as personal computers and achieve excellent results.

[Figure 1] shows optical micrographs of samples after shear strength measurement. Figure 1(a) shows Example 14, Figure 1(b) shows Example 2, and Figure 1(c) shows Comparative Example 3.

Claims (11)

A solder alloy is characterized by being composed of, by mass%, 2.0-3.6% Ag, 1.0-5.0% In, 3.0-5.0% Sb, 0.0010-0.0300% Fe, and 0.0000% to 0.0500% Co, with the remainder being Sn. The solder alloy of claim 1, wherein the alloy composition further contains at least one of Zr, Ge, Ga, P, As, Pb, Zn, Mg, Cr, Ti, Mo, Pt, Pd, Au, Al, and Si in a total mass % not exceeding 0.100%. The solder alloy of claim 1 or 2, wherein the alloy composition satisfies the following formula (1) and (2), In the above formulas (1) and (2), Ag, In, Sb and Fe are the mass % contents of the above solder alloy. A solder paste comprising solder powder formed from the solder alloy of claim 1 or 2. A solder ball is made of the solder alloy according to claim 1 or 2. A solder preform is made of the solder alloy of claim 1 or 2. A solder joint comprising the solder alloy of claim 1 or 2. An automotive electronic circuit is characterized by having a solder alloy as claimed in claim 1 or 2. An ECU electronic circuit is characterized by having a solder alloy as claimed in claim 1 or 2. An automotive electronic circuit device is characterized by comprising the automotive electronic circuit as claimed in claim 8. An ECU electronic circuit device is characterized by having the ECU electronic circuit as claimed in claim 9.