Lead-free solder alloy

Abstract

The object of the present invention is to provide a lead-free solder alloy that has good ductility even in extremely low temperature environments, such as below -20°C. [Solution] The above problem can be solved by a lead-free solder alloy having an alloy composition consisting of Sn: 17.0-18.0 mass%, Bi: 17.0-19.0 mass%, and the remainder being In. The alloy composition of this lead-free solder alloy may further contain at least one selected from Ni, Co, Au, Zn, Sb, and Ga in amounts of 0.01-0.1 mass% each.

Description

Technical Field

[0001] The present invention relates to a lead-free solder alloy, and more particularly to an In-Sn-Bi-based lead-free solder alloy applicable to extremely low temperature environments such as -20°C or lower.

Background Art

[0002] Conventionally, a solder material mainly composed of lead (Pb) has been used as a bonding material for joining electronic components to a substrate. However, as the toxicity of lead has become widely known, in recent years, lead-free solder materials have been widely popularized from the perspective of environmental protection and the like. Examples of such lead-free solder materials include Sn-Ag-Cu-based solder alloys and Sn-Cu-Ni-based solder alloys. However, the melting point of this type of lead-free solder alloy is as high as 200°C or more, and during mounting, the electronic components and the substrate are exposed to high temperature conditions. As a result, it may affect the functions of the electronic components and the substrate. In particular, due to the increasing functionality of portable terminals such as smartphones recently, the use of wiring boards and electronic elements with low heat resistance has increased, and in some cases, it is necessary to set the solder bonding temperature during mounting to a low temperature condition of, for example, 140°C or lower.

[0003] As a lead-free solder alloy applicable to such low temperature bonding, a lead-free solder alloy containing Sn, Bi, and In (Non-Patent Document 1, Patent Document 1) has been studied.

[0004] In addition, an electronic device including a substrate on which electronic components are mounted may constitute, for example; an engine control device of a transportation machine such as an automobile that is used in a severe temperature environment. Therefore, electronic devices applied to this type of use are required to have reliability in such a severe temperature environment. Thus, as a solder alloy applied to such electronic devices, a solder alloy that exhibits stable mechanical properties particularly in an extremely low temperature environment and can obtain sufficient reliability has been proposed (Patent Document 2).

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-43721 [Patent Document 2] Japanese Patent Publication No. 2020-49543 [Non-patent literature]

[0006] [Non-Patent Document 1] Y. Liu, KN Tu, Low melting point solder based on Sn, Bi, and In elements, Materials Today Advances, 8, (2020) 100115 [Overview of the project] [Problems that the invention aims to solve]

[0007] Patent Document 1 and Non-Patent Document 1 describe various low-melting-point lead-free solder alloys containing Sn, Bi, and In, but their properties in cryogenic environments have not been investigated. Furthermore, Patent Document 2 shows that the variation in tensile strength is small in environments such as -40°C, but ductility has not been considered. In cryogenic environments, durability against external loads such as vibration is considered to be particularly important, and the ductility of the solder alloy is thought to improve the toughness of the solder joint, thereby improving durability against external loads and contributing to improved reliability of the solder joint.

[0008] Therefore, the object of the present invention is to provide a lead-free solder alloy that has good ductility even in extremely low temperature environments such as -20°C or below. [Means for solving the problem]

[0009] The inventors of the present invention conducted diligent research to solve the aforementioned problems. As a result, they found that the aforementioned problems can be solved by using a lead-free solder alloy in which In is the main component and the content of Sn and Bi is within a predetermined range. The gist of the present invention is as follows.

[0010] (1) A lead-free solder alloy having an alloy composition consisting of Sn: 17.0-18.0 mass%, Bi: 17.0-19.0 mass%, with the remainder being In. (2) The lead-free solder alloy according to item (1) above, wherein the alloy composition further contains 0.01 to 0.1 mass% each of at least one selected from Ni, Co, Au, Zn, Sb, and Ga. (3) A solder joint formed using the lead-free solder alloy described in item (1) or (2) above. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a lead-free solder alloy that has good ductility even in low-temperature environments such as -20°C or below. [Brief explanation of the drawing]

[0012] [Figure 1] This figure shows the elongation results under each test condition for Example 1 and Comparative Examples 1-3. [Figure 2] This figure shows the maximum tensile strength (UTS) results under each test condition in Example 1 and Comparative Examples 1-3. [Modes for carrying out the invention]

[0013] The lead-free solder alloy according to an embodiment of the present invention (hereinafter sometimes simply referred to as "solder alloy") has an alloy composition consisting of Sn: 17.0-18.0 mass%, Bi: 17.0-19.0 mass%, and the remainder being In.

[0014] By using In as the main component and maintaining the content of Sn and Bi within a predetermined range, it becomes possible to improve ductility under low-temperature conditions. The reason for this is not entirely clear, but it is presumed to be due to the microstructure of the alloy under low-temperature conditions. In solder alloys with a predetermined alloy composition, at room temperature, the initial phase is β-In3Sn(Bi), and a lamellar eutectic structure is formed in which BiIn2 exists in this β phase. Above 0°C, γ-InSn4(Bi) is either absent or present in negligible trace amounts. Furthermore, in solder alloys with a predetermined alloy composition, in temperature environments below -20°C, trace amounts of BiIn2 and γ-InSn4(Bi) precipitate in the β phase due to phase transformation. BiIn2 has an anti-brittlement effect, but γ-InSn4(Bi) has brittle properties, so it is thought that if it is present in the microstructure at a concentration exceeding 20% ​​by mass, it will have a significant impact on ductility. However, if the amount of precipitated γ-InSn4(Bi) is 20% by mass or less, preferably 15% by mass or less, then the coexistence of the γ phase, BiIn2, and the initial β phase in the microstructure, combined with the anti-brittlement effect of BiIn2, is thought to dramatically improve the ductility of the solder alloy in low-temperature environments. Solder alloys with a predetermined alloy composition are thought to be able to have such a microstructure in low-temperature environments and thus possess good ductility. Furthermore, while γ-InSn4(Bi) has low conductivity, BiIn2 has good conductivity and superconductivity. Even if a small amount of γ-InSn4(Bi) precipitates and is included in the low-temperature environment, BiIn2 also precipitates, so good conductivity can be maintained even in low-temperature environments. The content of γ-InSn4(Bi) varies with temperature; for example, at -90°C, 5% by mass or less is preferred, and at -196°C or below (liquid nitrogen temperature), 10% by mass or less is preferred.

[0015] The Sn content can be 17.0 to 18.0% by mass, but 17.5% by mass is particularly preferred from the viewpoint of improving ductility in low-temperature environments.

[0016] The content of Bi may be 17.0 to 19.0% by mass, but from the perspective of improving ductility in a low-temperature environment, 17.0 to 18.0% by mass is preferable, and 17.5% by mass is particularly preferable.

[0017] In an embodiment of the present invention, it is further possible to contain at least one selected from Ni, Co, Au, Zn, Sb, and Ga. The intermetallic compound formed between the solder alloy mainly composed of In and having the content of Sn and Bi within a predetermined range and, for example, a Cu substrate is Cu6(In,Sn)5. This intermetallic compound has a hexagonal crystal structure and is stable. However, by further containing Ni, Co, or Au, it becomes (Cu,Ni)6(Sn,In)5, (Cu,Co)6(Sn,In)5, or (Cu,Au)6(Sn,In)5, and is considered to be more stable. Also, by adding Zn or Sb to Cu6(In,Sn)5, it becomes Cu6(Sn,In,Zn)5 or Cu6(Sn,In,Sb)5, and the crystal structure is considered to be more stable. When the predetermined solder alloy further contains Ga, the intermetallic compounds formed between the Cu substrate are CuGa2 in addition to Cu6(Sn,In)5. Since CuGa2 is formed at a lower temperature than Cu6Sn5, it is considered that the liquid phase is less likely to remain during solidification of the solder alloy and the microstructure becomes smaller.

[0018] From the perspective of enabling each function to be exhibited well, the contents of Ni, Co, Au, Zn, Sb, and Ga are preferably 0.01 to 0.1% by mass, respectively.

[0019] The balance of the alloy composition is In, but it may contain unavoidable impurities. Even when unavoidable impurities are contained, it does not affect the effect of improving ductility in a low-temperature environment.

[0020] The solder alloy as described above can be obtained, for example, by mixing ingots of each metal so as to have a predetermined alloy composition, melting and stirring, and then cooling. The form of the solder alloy is not particularly limited and can be appropriately determined according to the application, etc., and examples include powder form and块状 form.

[0021] The solder joint according to the embodiment of the present invention is formed using the aforementioned solder alloy. Specifically, the solder joint includes a predetermined substrate and a solder joint portion (also referred to as a solder layer) composed of a solder alloy joined to this substrate. Furthermore, when forming a solder joint by soldering using the aforementioned solder alloy, this can be done using known reflow soldering or under a formic acid gas atmosphere. [Examples]

[0022] The embodiments of the present invention will be described in detail below based on the examples provided.

[0023] (Example 1) Using pure ingots of In, Sn, and Bi, a lead-free solder alloy was obtained with an alloy composition of 65% by mass of In and 17.5% by mass of Sn and Bi, respectively. Specifically, each ingot, formulated to achieve the aforementioned alloy composition, was melted in an electric resistance furnace at 450°C for 1 hour with frequent stirring. After that, the dross on the surface of the molten metal was carefully removed, and the molten metal was filled into a preheated mold. The cooled casting was used as an evaluation sample for the lead-free solder alloy.

[0024] (Example 2) Using the lead-free solder alloy evaluation sample obtained in Example 1, copper substrates were joined together according to a standard method to obtain a solder joint. The obtained solder joint was left to stand at -60°C for 5 hours to confirm that there were no problems such as crack formation at the joint interface.

[0025] (Comparative Examples 1-3) In the same manner as in Example 1, evaluation samples of lead-free solder alloys having the alloy compositions shown in Table 1 were obtained.

[0026] (evaluation) <Tensile Test> Using the evaluation samples obtained in Example 1 and Comparative Examples 1-4, Tensile tests were performed in accordance with ASTM E8M, and elongation and maximum tensile strength (UTS) were measured.

[0027] The evaluation results are shown in Table 1, Figures 1 and 2.

[0028] [Table 1]

[0029] As shown in Table 1 and Figures 1 and 2, the lead-free solder alloy of Example 1, which has a predetermined alloy composition, exhibits significantly less elongation at cryogenic conditions of -90°C and -20°C compared to room temperature conditions, compared to each sample in the comparative examples. Furthermore, the maximum tensile strength of Example 1 at these cryogenic conditions was 58 MPa, which is comparable to the room temperature values ​​of other common solder alloys, indicating good strength.

Claims

1. A lead-free solder alloy having an alloy composition consisting of Sn: 17.0–18.0 mass%, Bi: 17.0–19.0 ​​mass%, and the remainder being In.

2. The lead-free solder alloy according to claim 1, wherein the alloy composition further contains 0.01 to 0.1 mass% each of at least one selected from Ni, Co, Au, Zn, Sb, and Ga.

3. A solder joint formed using the lead-free solder alloy described in claim 1 or 2.

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