Silver alloy wire, and conductive wire, wire for joining, wire for semiconductor, and wire for structure using same

Abstract

Provided is a silver alloy wire that has high tensile strength and that, due to having high resistance against grain boundary deterioration, is able to maintain high strength as an industrial wire even at high temperatures. The silver alloy wire contains not less than 99.8 mass% silver, wherein, in a cross-section perpendicular to the lengthwise direction of the silver alloy wire, the ratio of a special grain boundary length of crystal grains is not less than 30%, and among lengthwise direction crystal orientations <hkl> measured in the cross-section, the ratio of crystal orientations <111> having an angle difference of not more than 15 degrees with respect to the lengthwise direction of the silver alloy wire is 10-60%.

Description

Silver alloy wire and conductive wire, bonding wire, semiconductor wire, and structural wire made from the same

[0001] The present invention relates to a silver alloy wire, and particularly to a silver alloy wire that is suitable for a wide range of industrial applications (for example, conductive wire, bonding wire, semiconductor wire, structural wire, etc.).

[0002] Silver has a high light reflectivity, a beautiful metallic luster, and is inexpensive among precious metals, making it a popular choice for jewelry. However, pure silver is soft and prone to sulfurization. For example, pure silver discolors due to the formation of silver sulfide on its surface caused by sulfur compounds such as automobile exhaust and hydrogen sulfide in hot springs. Since the higher the purity of silver, the more susceptible it is to tarnishing, so when silver is used in jewelry, it is typically mixed with other metals to lower its purity. Silver with purities of 92.5%, 83.5%, and 80% is used for jewelry. These purities are specified as 925, 835, and 800, respectively, in the purity (quality) of metal alloys for jewelry in ISO 9202 (International Organization for Standardization) and JIS H6309 (Japanese Industrial Standards), which are the standards for certifying the quality of precious metals.

[0003] Such silver alloys for jewelry have high electrical resistance due to their low purity and are difficult to process due to their high hardness, making them unsuitable as industrial materials and not yet widely used in industrial applications.

[0004] Traditionally, copper has been widely used as industrial metal wire. However, with the rapid advancement of cutting-edge industrial technology, materials with unprecedented new properties are being sought. For example, in recent years, technologies have been developed for applying industrial fine metal wires to parts and products by processing them, such as twisting, braiding, weaving, tying, connecting, and splicing. Furthermore, even after application to parts and products, depending on the application, the industrial fine metal wires may be subjected to tensile forces for long periods of time or may be heated by passing electricity through them. For example, in high-performance and multi-functional products, the direction and magnitude of the processing and tensile forces described above tend to become complex, and heating due to large currents also tends to increase. Thus, for metal materials used as industrial wires, there is a demand for materials that are compatible with high-performance and multi-functional products, do not lead to breakdowns, and have excellent strength that can withstand the miniaturization of parts and products and complex secondary processing.

[0005] JP 2013-021280 A JP 2018-530900 A JP 2010-167490 A JP 2014-073529 A

[0006] The present invention has been made to solve the above-mentioned problems, and aims to provide a silver alloy wire that has high tensile strength and high resistance to grain boundary degradation, so that it can maintain its high strength even at high temperatures as an industrial wire. Another aim of the present invention is to provide a conductive wire, a bonding wire, a semiconductor wire, or a structural wire using a silver alloy wire that has high tensile strength and high resistance to grain boundary degradation, so that it can maintain its high strength even at high temperatures as an industrial wire.

[0007] Silver has the highest electrical and thermal conductivity of any metal. The higher the purity of silver, the higher its electrical and thermal conductivity, but it also becomes softer (i.e., less stress) and more susceptible to breaking when pulled. Therefore, in conventional silver alloy wires, strength improvement has mainly been achieved by alloying with other metal elements, or by drawing to refine the crystal grains and introduce strain. For example, in drawing, strong processing (increasing the area reduction rate per die or increasing the processing rate) can introduce large strain into the material and refine the crystal grains, thereby improving strength. In conventional silver alloy wires where strength improvement has been achieved through alloying or grain refinement, the lower the silver purity, the greater the number of crystals, and the smaller the crystal grain size, the higher the hardness and strength.

[0008] Silver alloys are polycrystalline bodies made up of multiple crystal grains. Each crystal grain in the polycrystalline body has a different crystal orientation, and the relationship between the pulling direction of the silver alloy wire and the crystal orientation varies for each crystal grain. In the drawing process, the strength can be improved by aligning multiple crystal grains in a specific crystal orientation.

[0009] However, the inventors have found that conventional silver alloy wires, which have increased strength by reducing the size of the crystal grains, suffer from grain boundary degradation. In particular, the inventors have found that although the strength of the silver alloy wire is improved by aligning the crystal grains in a specific crystal orientation, the grain boundary degradation is more likely to occur. Furthermore, they have found that while there is no significant decrease in strength during the progression of grain boundary degradation, accidental fracture can occur. Such unpredictable accidental fracture can lead to failure of parts and equipment manufactured using advanced industrial technology, and can cause serious accidents.

[0010] Furthermore, the inventors have found through their research that by simultaneously controlling the special grain boundaries and the crystal orientation, it is possible to achieve both grain boundary degradation resistance and high strength in a silver alloy wire by achieving a specific special grain boundary length ratio and a crystal structure that is not concentrated in a specific crystal orientation.

[0011] Generally, metallic materials are polycrystalline, consisting of multiple single crystals. The boundaries between crystal grains with different crystal orientations in polycrystalline materials are called grain boundaries (or simply "grain boundaries"). Atoms are regularly and densely arranged within most crystal grains. At grain boundaries, which are the boundaries between crystal grains, the arrangement of atoms is irregular, resulting in an arrangement with many gaps. Grain boundary degradation is a phenomenon in which the strength and corrosion resistance of grain boundaries are reduced due to the effects of high temperatures and stress. During the grain boundary degradation process, no significant changes are observed in the appearance of the material. However, grain boundaries initiate small cracks, which then lead to larger cracks and fractures. Therefore, grain boundary degradation resistance has a significant impact on the lifespan and reliability of materials.

[0012] Generally, most reported cases of grain boundary degradation in iron-based materials are due to corrosion or precipitation of alloying elements. On the other hand, the high-purity silver used in this embodiment is a stable metal that is resistant to oxidation and has high purity, so there are no reported cases of grain boundary degradation. Therefore, due to the lack of reference cases, it has been very difficult to grasp the phenomenon of grain boundary degradation in high-purity silver alloy wires due to the influence of high-temperature usage environments and stress. Under these circumstances, the present inventors have conducted extensive research and discovered that it is effective to confirm the grain boundary degradation of high-purity silver alloy wires by observing the phenomenon of grain boundary fracture in creep tests.

[0013] Furthermore, as a result of intensive research, the inventors have discovered that grain boundary deterioration (grain boundary fracture) caused by creep testing is related to the crystalline structure (special grain boundary, crystal orientation, and crystal grain size) of high-purity silver alloy wire.

[0014] Specifically, the inventors prepared silver alloy wires of various compositions and conducted tensile tests and creep tests. In the tensile tests, the silver alloy wires were pulled at a constant speed at room temperature and at a high temperature (approximately 200°C, the recrystallization temperature of silver) until they broke. In the high-temperature creep tests, the silver alloy wires were pulled at a constant load at a high temperature (approximately 200°C) until they broke. Then, the crystalline structure of the silver alloy wires was analyzed before and after each test.

[0015] (Experimental Example) An experimental example that is an example of the above test will be described. Using the silver alloy wires produced in the examples and comparative examples described below, the relationship between the special grain boundary length ratio (%) of the cross section of the silver alloy wire, the <111> orientation ratio (%), the state of the fracture surface after a tensile test at room temperature or a high-temperature creep test, the fracture elongation (%), and the fracture time (hrs) (time to fracture) was investigated in advance. The results of the silver alloy wires of Example 5 and Comparative Example 1 are shown in Table 1 and Figures 9 and 10. Note that the wire diameters of Example 5 and Comparative Example 1 are both 400 μm.

[0016]

[0017] Figure 9 is a photograph of the fracture surface of the silver alloy round wire of Example 5 after a tensile test. Figure 10 is a photograph of the fracture surface of the silver alloy round wire of Comparative Example 1 after a high-temperature creep test. All of the photographs in Figures 9 and 10 were taken with a field emission scanning electron microscope (FE-SEM). The high-temperature creep test was carried out at a heating temperature of 200°C and a load of 9.7 N. The load was set lower than the 0.2% proof stress of each silver alloy wire.

[0018] As shown in Fig. 9, the fracture surface of the silver alloy wire of Example 5 in the tensile test was smooth, and no intergranular cracks were observed. This indicates that the fractures of the silver alloy wire in the tensile test were all transgranular fractures (similar to Fig. 9).

[0019] In contrast, as shown in Figure 10, many fine grain boundary cracks (voids) were observed on the fracture surface of the silver alloy wire of Comparative Example 1 after the creep test. From this, it was determined that the fracture of the silver alloy wire in the high-temperature creep test was due to grain boundary fracture (Figure 10 is one example). From these results, the inventors considered that the high-temperature creep test of the silver alloy wire can confirm the phenomenon of grain boundary fracture originating from the grain boundary. Furthermore, from the results of the examples shown in Table 1, it can be seen that the silver alloy wire of Example 5 took a longer time to fracture and had a slower rate of grain boundary deterioration than the silver alloy wire of Comparative Example 1.

[0020] It was also found that the grain boundary degradation and strength of silver alloy wire are strongly dependent on the grain boundary structure of the silver alloy wire's crystal grains and are also affected by the crystal orientation. It was discovered that it is possible to achieve both grain boundary degradation resistance and high strength by controlling the crystalline structure of the silver alloy wire through control of the composition and manufacturing method of the silver alloy wire, that is, by controlling the ratio of the special grain boundary length and the specific crystal orientation ratio of the silver alloy wire, or by adding specific alloy elements and controlling the ratio of the special grain boundary length.

[0021] The mechanism of grain boundary degradation in silver alloy wire is unclear, but it is speculated as follows: The higher the special grain boundary length ratio, the more dominant the grain boundaries with high bond strength. Therefore, the recrystallization of the silver alloy wire's crystal grains at high temperatures is slower, and the strength of the silver alloy wire is less likely to decrease. Furthermore, the higher the special grain boundary length ratio, the stronger the grain boundary bond strength, even after recrystallization, making it less likely that fracture due to grain boundary degradation will occur. On the other hand, the lower the special grain boundary length ratio, the more dominant the grain boundaries with low bond strength, making it more likely that the strength of the silver alloy wire will decrease before recrystallization at high temperatures, making it more likely that fracture due to grain boundary degradation will occur. The lower the special grain boundary length ratio, the weaker the grain boundary bond strength, even after recrystallization, making it more likely that fracture due to grain boundary degradation will occur, resulting in earlier fracture from the grain boundary. In this way, the special grain boundary length ratio of silver alloy wire can be used as an indicator to predict the occurrence of grain boundary degradation early in the production of silver alloy wire, thereby preventing failures and serious accidents in parts using silver alloy wire.

[0022] Furthermore, from the above test results, although there is no specific correlation between the ratio of the special grain boundary length and the fracture elongation of the tensile test, there is a correlation between the ratio of the fracture elongation of the high-temperature creep to the fracture elongation of the room-temperature tensile test (i.e., the degradation rate described below = fracture elongation of the high-temperature creep / fracture elongation of the room-temperature tensile test) and the ratio of the special grain boundary length, and it was found that the larger the ratio of the special grain boundary length, the lower the degradation rate. From these results, in this embodiment, the grain boundary degradation resistance of the silver alloy wire is evaluated by the degradation rate.

[0023] Unless otherwise specified, the ratio of the <111> crystal orientation (<111> orientation ratio) means the ratio of the <hkl> crystal orientations in the longitudinal direction of the wire, including those with an angle difference of up to 15 degrees with respect to the longitudinal direction of the wire.

[0024] That is, the silver alloy wire, conductive wire, bonding wire, semiconductor wire, and structural wire according to the embodiments of the present invention are as follows: [1] A silver alloy wire containing 99.8% by mass or more of silver, characterized in that the ratio of the special grain boundary length defined by the following formula (1) of crystal grains in a cross section perpendicular to the longitudinal direction of the silver alloy wire is 30% or more, and the ratio of the crystal orientation <111> having an angular difference of 15 degrees or less with respect to the longitudinal direction of the silver alloy wire among the crystal orientations <hkl> in the longitudinal direction of the silver alloy wire measured in the cross section is 10% or more and 60% or less. Special grain boundary length ratio = grain boundary length of Σ3 to Σ29 / total grain boundary length (1) [2] The silver alloy wire according to [1], wherein the average crystal grain size in the cross section of the silver alloy wire is 2 μm or more and 15 μm or less. [3] The silver alloy wire according to [1] or [2], containing at least one element selected from the group consisting of Pt, Ti, Zn, Au, Cu, Mg, Ni, Al, Sn, Pd, Cd, In, W, Nd, and Bi, and having a total concentration of the element relative to the total amount of the silver alloy wire of 0.003% by mass or more and 0.1% by mass or less. [4] A silver alloy wire containing 99.8% by mass or more of silver, wherein the ratio of the special grain boundary length defined by the following formula (1) of crystal grains in a cross section perpendicular to the longitudinal direction of the silver alloy wire is 30% or more, and containing at least one element selected from the group consisting of Pt, Ti, Zn, Au, Cu, Mg, Ni, Al, Sn, Pd, Cd, In, W, Nd, and Bi, and having a total concentration of the element relative to the total amount of the silver alloy wire of 0.003% by mass or more and 0.1% by mass or less. Ratio of special grain boundary length = grain boundary length of Σ3 to Σ29 / total grain boundary length (1). [5] The silver alloy wire according to [4], wherein, among the crystal orientations <hkl> in the longitudinal direction of the silver alloy wire measured on the cross section, a ratio of crystal orientations <111> having an angular difference of 15 degrees or less with respect to the longitudinal direction of the silver alloy wire is 10% or more and 60% or less. [6] The silver alloy wire according to [4] or [5], wherein the average crystal grain size in the cross section of the silver alloy wire is 2 μm or more and 15 μm or less. [7] A conductive wire using the silver alloy wire according to any one of [1] to [6]. [8] A bonding wire using the silver alloy wire according to any one of [1] to [6]. [9] A semiconductor wire using the silver alloy wire according to any one of [1] to [6].

[10] A structural wire using the silver alloy wire according to any one of [1] to [6]. Note that the symbol "to" indicates a numerical range including the numerical values ​​before and after it.

[0025] The silver alloy wire of the embodiment has high strength and high resistance to grain boundary degradation, so that it can maintain high strength even at high temperatures as an industrial wire. Furthermore, the conductive wire, bonding wire, semiconductor wire, and structural wire have high strength and high resistance to grain boundary degradation, so that it can maintain high strength even at high temperatures as an industrial wire.

[0026] 1 is a diagram schematically showing a cross section of a silver alloy wire in a round wire material; FIG. 2 is a diagram schematically showing a cross section of a silver alloy wire in a plate material; FIG. 3 is a diagram schematically showing an example of a cross-sectional shape of a silver alloy wire; FIG. 4 is a diagram schematically showing a power semiconductor device of an embodiment; FIG. 5 is a diagram schematically showing a part of a lithium ion battery module of an embodiment; FIG. 6 is a diagram schematically showing a part of a lithium ion battery module of another embodiment; FIG. 7 is a photograph of a joint where curling of the silver alloy wire (area surrounded by dotted lines) is observed around the joint; FIG. 8 is a photograph of a joint where curling of the silver alloy wire is not observed around the joint; FIG. 9 is a field emission scanning electron microscope (FE-SEM) photograph of a fracture surface after a tensile test of the silver alloy round wire of Example 5; and FIG. 10 is an FE-SEM photograph of a fracture surface after a creep test of the silver alloy round wire of Comparative Example 1.

[0027] Hereinafter, a silver alloy wire according to an embodiment of the present invention will be described in detail. In the silver alloy wire according to this embodiment, the "special grain boundary length ratio" of the crystal grains in a cross section perpendicular to the longitudinal direction of the silver alloy wire, as defined by the following formula (1), is 30% or more, and the ratio of the crystal orientation <111>, which has an angle difference of 15 degrees or less with respect to the longitudinal direction of the silver alloy wire, among the crystal orientations <hkl> in the longitudinal direction of the silver alloy wire measured on the cross section of the silver alloy wire, is 10% or more and 60% or less. Hereinafter, the ratio of the crystal orientation <111>, which has an angle difference of 15 degrees or less with respect to the longitudinal direction of the silver alloy wire, among the crystal orientations <hkl> in the longitudinal direction of the silver alloy wire, is referred to as the "<111> orientation ratio", and the same applies to other crystal orientations <hkl>.

[0028] Ratio of special grain boundary length = grain boundary length of Σ3 to Σ29 / total grain boundary length (1)

[0029] In addition, in another embodiment of the silver alloy wire of the present invention, the "special grain boundary length ratio" defined by the above formula (1) of the crystal grains in a cross section perpendicular to the longitudinal direction of the silver alloy wire is 30% or more, and the silver alloy wire contains at least one element selected from the group consisting of Pt, Ti, Zn, Au, Cu, Mg, Ni, Al, Sn, Pd, Cd, In, W, Nd, and Bi, and the concentration of the elements relative to the total amount of the silver alloy wire is 0.003 mass% or more and 0.1 mass% or less.

[0030] In the silver alloy wire of this embodiment, the "longitudinal direction" refers to the direction of elongation (wire drawing, rolling) when the silver alloy wire is obtained by elongation. For example, when the silver alloy wire is a round wire, the drawing direction of the round wire corresponds to the longitudinal direction of the round wire (the direction perpendicular to the wire diameter). When the silver alloy wire is a plate, the rolling direction of the rolling corresponds to the longitudinal direction. In the case of a plate, it may be cut into small pieces to a predetermined size after rolling. In this case, the longer distance in the small pieces may not coincide with the direction of elongation of the rolling, and the shorter distance may coincide with the direction of elongation of the rolling. Since the rolling direction of the plate obtained by such small pieces can be confirmed from the processed state of the plate surface, this rolling direction is taken as the longitudinal direction.

[0031] In the silver alloy wire of this embodiment, the "cross section" refers to a cross section perpendicular to the "longitudinal direction." Examples of the cross section of the silver alloy wire of this embodiment include a circle and an approximately rectangular shape. Here, the word "approximately rectangular" is used because the corners of the cross section of the silver alloy wire (the above-mentioned plate material) obtained by rolling processing do not have 90-degree vertices and have a rounded shape. In addition, in this embodiment, a wire with a circular cross section is also called a round wire.

[0032] FIG. 1 schematically shows a cross section of a round wire material among the silver alloy wires of this embodiment. FIG. 2 schematically shows a cross section of a plate material among the silver alloy wires of this embodiment. In FIGS. 1 and 2, the axis of symmetry L of the cross section is represented by a dotted line, the intersection (center) O of the axes of symmetry of the multiple lines of the cross section is represented by a black circle, and the shortest distance from the outer periphery to the center of the cross section is represented by t. In the silver alloy wire of this embodiment, if 2t is defined as the thickness, the thickness is the shortest length in a direction perpendicular to the processing direction of wiredrawing or in the rolling direction in rolling. In the case of the round wire of FIG. 1, the cross section of the silver alloy wire is circular, and the thickness 2t is equal to the diameter of the circle. In the cross section of the round wire shown in FIG. 1, observation of the crystal structure can be performed in a rectangular, preferably approximately square, range (observation region R) with a height (several tens of μm to 800 μm) x width (several tens of μm to 800 μm) centered at the center O of the cross section. In the cross section of the plate material shown in Figure 2, observation can be performed in a rectangular, preferably approximately square, area (observation region R) with a height (several tens to 800 μm) x width (several tens to 800 μm) centered at the center O of the cross section and with all four sides parallel to the four sides of the cross section.

[0033] As described below, the crystalline structure of the cross section can be observed by electron backscattered diffraction (EBSD) (hereinafter referred to as "EBSD"). Since the resolution of the EBSD method depends on the magnification of observation, a magnification of 100 times or more is preferable to clearly observe the crystalline structure. However, the higher the magnification, the narrower the observation area. For a silver alloy wire with a large cross section, if the magnification is increased to a value sufficient for observing the crystalline structure, the entire cross section may not be observed. In this case, taking symmetry into consideration, a portion of the cross section may be used as the observation region R. When the cross section is large and the entire cross section cannot be observed in one observation, for example, in the case of a circular wire in FIG. 1, a region including a quadrant is used as the observation region (observation region R1 shown by a dashed line). In the case of a cross section of a plate material shown in FIG. 2 , which has only two axes of symmetry, observation is performed at two locations (observation region R1 indicated by the dashed dotted line)—near the center and near the edge of the rectangle, as shown in FIG. 2 . In addition, in the case of a cross section having a shape with multiple axes of symmetry, it is preferable to set two observation regions in consideration of symmetry. Thus, it is desirable to optimize the observation magnification, observation region, and observation location so that the crystalline structure can be clearly observed, taking into consideration the observation time, the cross-sectional shape of the sample (e.g., symmetry), and cross-sectional size. Furthermore, when the shape of the cross section is a shape with one or fewer axes of linear symmetry, or when it is difficult to determine the observation region using the above-described method, it is desirable to optimize the observation region so that the crystalline structure can be clearly observed near the center of gravity and near the periphery of the cross-sectional shape.

[0034] Next, we will explain the ratio of special grain boundary length. Crystal grain boundaries observed in crystalline structures are broadly divided into two types: low-angle grain boundaries and high-angle grain boundaries. High-angle grain boundaries are further divided into two types: random grain boundaries and coincidence grain boundaries. Coincidence grain boundaries are low-energy grain boundaries, and in face-centered cubic crystals, they mainly exist from Σ1 to Σ49. The inventors have found that, in silver alloy wires used as industrial wires, among the coincidence grain boundaries, the grain boundaries Σ3 to Σ29 are highly correlated with grain boundary degradation in silver alloy wires, and further, the ratio of the grain boundary lengths of Σ3 to Σ29 to the total grain boundary length (hereinafter referred to as the "special grain boundary length ratio") affects the grain boundary degradation of silver alloy wires. In other words, it was found that the higher the ratio of the sum of the grain boundary lengths of Σ3 to Σ29 to the total grain boundary length (special grain boundary length ratio), the better the resistance to grain boundary degradation (the less likely grain boundary degradation will occur).

[0035] In the processed texture and recrystallization texture of face-centered cubic metals, the orientation toward the <111> and <100> crystal orientations tends to increase as the processing rate increases and as the recrystallization progresses, and the proportion of the <111> crystal orientation increases to nearly 100% as the processing rate increases. The inventors have discovered that a silver alloy wire having a texture with a <111> orientation proportion close to 100% has high strength, but is prone to grain boundary degradation. In order to achieve both grain boundary degradation resistance and high strength, it is effective to keep the <111> orientation proportion within a predetermined range. In the silver alloy wire of this embodiment, by controlling both the special grain boundary length proportion and the <111> orientation proportion, both grain boundary degradation resistance and high strength can be achieved.

[0036] The ratio of the special grain boundary length of the silver alloy wire in this embodiment is 30% or more. If the ratio of the special grain boundary length of the silver alloy wire is less than 30%, grain boundaries with weak bonding strength become dominant, making grain boundary degradation more likely to occur. Therefore, at high temperatures as an industrial wire, if a certain load (e.g., tensile force, bending force, compressive force, etc.) is continuously applied for a long period of time, there is a risk of fracture. The upper limit of the ratio of the special grain boundary length of the silver alloy wire is not particularly limited, and if special grain boundaries other than Σ3 to Σ29 remain, this value excludes them. Note that the ratio of the special grain boundary length of the silver alloy wire is preferably 85% or less, and more preferably 80% or less. This improves resistance to grain boundary degradation while reducing the decrease in ductility and hardness of the material, making secondary processing such as winding, braiding, twisting, and bending easier.

[0037] In the silver alloy wire of this embodiment, in order to achieve both improved resistance to grain boundary degradation and high strength, the <111> orientation ratio of the cross section of the silver alloy wire is preferably 10% or more and 60% or less. When the <111> orientation ratio of the cross section of the silver alloy wire is 10% or more, the strength is high, making it suitable as an industrial wire rod. When the <111> orientation ratio of the cross section of the silver alloy wire is 60% or less, high strength is achieved without becoming too brittle, making it easy to perform secondary processing such as winding, braiding, twisting, and bending.

[0038] For the above reasons, it is preferable that the silver alloy wire of this embodiment has a special grain boundary length ratio of 30% or more and a <111> orientation ratio of 10% to 60%. By having such a metal structure, the silver alloy wire of this embodiment can achieve both grain boundary degradation resistance and high strength.

[0039] In the silver alloy wire of this embodiment, in order to achieve both improved grain boundary degradation resistance and high strength, the ratio of the special grain boundary length is preferably 30% or more, more preferably 50% or more, and even more preferably 70% or more. The <111> orientation ratio is preferably 10% or more, more preferably 30% or more, and even more preferably 40% or more.

[0040] In the silver alloy wire of this embodiment, the average crystal grain size in a cross section perpendicular to the longitudinal direction is preferably 2 μm or more and 15 μm or less. Having an average crystal grain size of 15 μm or less makes it easier to improve the strength of the silver alloy wire. Furthermore, having an average crystal grain size of 2 μm or more makes it easier to obtain sufficient strength while maintaining resistance to grain boundary degradation. The average crystal grain size, the ratio of the special grain boundary length, and the <111> orientation ratio can be adjusted by the wiredrawing conditions and heat treatment conditions during the manufacturing process of the silver alloy wire.

[0041] (Measurement of Crystal Structure) The crystalline structure of the crystal plane in the cross section of the silver alloy wire of this embodiment can be measured using the electron backscattered diffraction (EBSD) method. By measuring the crystalline structure using the EBSD method, it is possible to measure the special grain boundary length, crystal orientation, average crystal grain size, etc. with high accuracy and sufficient reproducibility. In the present invention, a grain boundary is defined as a grain where the orientation difference between adjacent crystal grains is 15 degrees or more, and a crystal grain with an orientation difference of 15 degrees or less and five or more pixels is recognized as one crystal grain.

[0042] In the EBSD method, it is usually difficult to measure the grain boundaries, crystal orientation, crystal grain size, etc. with high precision when the sample has large unevenness or curved surfaces. Therefore, in order to observe the cross section of the silver alloy wire of this embodiment with EBSD, it is useful to smooth the surface of the cross section after cutting the silver alloy wire. Methods for smoothing the surface of the cross section include mechanical polishing, chemical polishing, FIB processing, etc. These methods can remove residual strain on the surface of the cross section and obtain a smooth surface.

[0043] As described above, by optimizing the pretreatment of the sample, it is possible to measure and analyze the grain boundaries, crystal orientation, grain size, etc. of the cross section of the silver alloy wire with high accuracy by the EBSD method. Furthermore, by measuring at three or more different locations, it is possible to obtain average information that takes into account variations.

[0044] In addition, in the EBSD method, standard data (standard pattern file) of a material with a composition similar to that of the structure to be measured is usually used. However, in the silver alloy wire of this embodiment, the concentration of alloy elements other than silver relative to the entire silver alloy wire is extremely small, at 0.2 mass (wt)% or less, so data of pure silver may be used as standard data when measuring the crystal structure by the EBSD method. Note that when measuring the crystal structure by the EBSD method, measurement data is obtained under conditions of an acceleration voltage of 15 kV and a measurement interval of 0.5 to 1.5 μm.

[0045] (Method of calculating the ratio of special grain boundary length) Using the measurement data obtained by the EBSD method, Σ data for the CSL grain boundary (Coincidence Site Lattice) is obtained, and the sum of the grain boundary lengths of Σ3 to Σ29 is taken as the special grain boundary length. The "Total Length" of the CSL grain boundary data is used as the total grain boundary length, and the ratio of the special grain boundary length is calculated. Similar measurements are performed on a total of three randomly selected cross sections of the same sample, and the average value of the ratio of the special grain boundary length is calculated.

[0046] (Method of Measuring Crystal Grain Size) The average crystal grain size in the cross section of the sample is calculated using the measurement data obtained by the EBSD method, and the crystal grain size can be calculated by the circle approximation method (diameter).

[0047] (Alloying Elements) The silver alloy wire of this embodiment is made of a silver alloy having a silver purity (amount of silver relative to the total amount of silver alloy) of 99.8% by mass or more. That is, the silver alloy wire of this embodiment contains 99.8% by mass or more of silver relative to the total amount of the silver alloy wire. Furthermore, the silver content in the silver alloy wire of this embodiment is preferably 99.99% by mass or less relative to the total amount of the silver alloy wire, so that the strength required for industrial wire can be maintained for a long period of time. The silver alloy wire of this embodiment contains specific elements (alloying elements described below), which make it easier to achieve both grain boundary degradation resistance and high strength. In particular, by containing specific elements, it is easier to achieve both grain boundary degradation resistance and high strength without controlling the <111> orientation ratio.

[0048] Silver can contain a variety of alloying metals. The crystalline structure of a silver alloy varies depending on the amount of alloying metal contained in the silver. The lower the alloying metal content, the more stable the crystalline structure of the silver alloy, maintaining the crystalline structure of pure silver. However, the strength of the silver alloy tends to decrease. In contrast, as the alloying metal content increases, the solubility limit of the silver alloy is exceeded, and precipitates with different crystalline structures form within the crystalline structure of pure silver. While the precipitates strengthen the silver alloy, their presence makes the overall crystalline structure less stable than that of pure silver, making them more susceptible to grain boundary degradation. The inventors have found that a silver alloy with a content of 99.8% by mass or more is less likely to produce precipitates and improves the resistance of silver alloy wire to grain boundary degradation. For this reason, the silver alloy wire of this embodiment contains 99.8% by mass or more of silver and 0.2% by mass or less of elements and inevitable impurities relative to the total amount of the silver alloy wire. Hereinafter, elements other than silver and inevitable impurities contained in the silver alloy wire will also be referred to as "alloying elements."

[0049] The alloying elements other than silver contained in the silver alloy wire of this embodiment are at least one element selected from the group consisting of platinum (Pt), titanium (Ti), zinc (Zn), gold (Au), copper (Cu), magnesium (Mg), nickel (Ni), aluminum (Al), tin (Sn), palladium (Pd), cadmium (Cd), indium (In), tungsten (W), neodymium (Nd), and bismuth (Bi). The content of these alloying elements is 0.2 mass% or less, and preferably 0.1 mass% or less, of the total amount of the silver alloy.

[0050] The content of the alloying elements is preferably 0.003% by mass or more, more preferably 0.01% by mass or more, in order to achieve both intergranular degradation resistance and high strength. Among these, the silver alloy wire preferably contains one of nickel (Ni) and copper (Cu). In this case, the content of nickel (Ni) is preferably 0.01% by mass or more and 0.1% by mass or less, relative to the total amount of the silver alloy, and the content of copper (Cu) is preferably 0.01% by mass or more and 0.1% by mass or less, relative to the total amount of the silver alloy. The higher the purity of silver (the ratio of silver content to the total amount of the silver alloy), the better the intergranular degradation resistance and high strength can be achieved even with a smaller content of alloying elements. Furthermore, the amount of alloying elements is preferably equal to or less than the amount of inevitable impurities in the silver alloy.

[0051] The silver alloy wire of this embodiment may contain inevitable impurities in addition to silver and the above-mentioned alloying elements. Examples of inevitable impurities include Ca, Gd, La, P, Ge, Fe, Pb, Sc, Si, Rh, and Ir. The content of these inevitable impurities is usually 160 mass ppm or less, preferably 150 mass ppm or less, and more preferably 100 mass ppm or less. Note that these inevitable impurities have very little effect on the strength and grain boundary degradation resistance of the silver alloy wire.

[0052] The content ratios of elements contained in the silver alloy wire of this embodiment are generally measured by chemical analysis such as inductively coupled plasma (ICP) optical emission spectroscopy, but are not limited thereto. For example, the content ratios can also be measured by secondary ion mass spectroscopy (SIMS), glow discharge mass spectroscopy (GDMS), or energy dispersive X-ray spectroscopy (EDX).

[0053] The wire diameter of the silver alloy wire of this embodiment is, for example, 15 μm or more and 2000 μm or less, and may be 50 μm or more, 75 μm or more, or 100 μm or more, or may be 1500 μm or less. The wire diameter of the silver alloy wire is the diameter if the cross-sectional shape is a perfect circle, and the length of the major axis if the cross-section is other shapes (such as an elliptical, oval, polygonal, or polygon-like cross-section as shown below). However, if the cross-section is a polygonal or polygon-like shape, the length may be measured as width or thickness. FIG. 3 schematically shows an example of the cross-sectional shape of a silver alloy wire. As shown in FIG. 3, the cross-sectional shape of the silver alloy wire may be a circle (FIG. 3a), an ellipse (e.g., FIG. 3b), an oval (e.g., FIG. 3c), a square (e.g., FIG. 3e), a triangle (e.g., FIG. 3d), or a polygon-like shape (e.g., FIG. 3f, g). Furthermore, for example, when a plate material is obtained by rolling a silver alloy wire having a circular, elliptical or oval cross section and slitting the cross section of the obtained plate material, or by cutting a large plate-shaped silver alloy, the plate material may have a shape as shown in Fig. 3h or Fig. 3i.

[0054] (Method for manufacturing silver alloy wire) Next, a method for manufacturing a silver alloy wire of this embodiment will be described. The method for manufacturing a silver alloy wire of this embodiment can be mainly divided into the following four steps, depending on the cross-sectional shape of the silver alloy wire. (1) Casting step (2) Wire drawing step (3) Rolling step (in the case of a silver alloy wire plate material, etc.) (4) Heat treatment step

[0055] (1) Casting Process: An Ag alloy material can be obtained by melting Ag or an Ag alloy of a predetermined purity together with alloying elements in amounts sufficient to achieve the desired composition. A heating furnace such as an arc heating furnace, a high-frequency heating furnace, a resistance heating furnace, or a continuous casting furnace can be used for melting. The molten Ag alloy material can be pulled down through a die from the heating furnace while solidifying to a predetermined wire diameter to obtain a rod-shaped ingot (continuous casting method), or the molten Ag alloy material can be cast into a mold to obtain an ingot (tilting method). Of these, continuous casting is preferred from the viewpoints of yield and productivity.

[0056] In the manufacturing method of this embodiment, the Ag alloy material is preferably melted by maintaining a vacuum or an inert gas atmosphere such as argon or nitrogen above the molten silver in the heating furnace to prevent oxygen from entering from the atmosphere. Vacuum melting, in which an inert gas atmosphere is maintained above the molten silver in the heating furnace, is more preferable. When molten silver comes into contact with oxygen at 973°C and 1 atmosphere, it absorbs approximately 20 times its own volume of oxygen. In contrast, the solubility of gases (including oxygen) in solid silver is approximately 0.6%. Therefore, molten silver releases the absorbed oxygen during solidification. However, if some of the oxygen in the molten silver is not released to the outside during the solidification process and remains inside the solid silver, it will remain inside the silver alloy wire without being released to the outside even during the wiredrawing process and heat treatment process described below. The oxygen remaining inside the silver alloy wire is mainly present at the grain boundaries. Research by the present inventors has revealed that this oxygen present at the grain boundaries can cause internal defects or react with easily oxidized elements in the silver alloy, contributing to grain boundary degradation.

[0057] In order to improve resistance to grain boundary degradation, it is preferable to suppress the occurrence of processing strain in the silver alloy during each manufacturing process. Therefore, in continuous casting, it is preferable to pull down the silver in a semi-molten or semi-solidified state. It is preferable to simultaneously control both the pulling down temperature and the pulling down rate. Specifically, it is preferable to perform the pulling down at a temperature of 1000 to 1200°C and a pulling down rate of 200 to 500 mm / s, followed by rapid cooling with cooling water. When the pulling down rate in continuous casting is 200 mm / s or higher, silver is less likely to solidify during the pulling down process, and frictional resistance between the silver and the inner surface of the die during pulling down is likely to be low. This makes it possible to suppress tensile strain within the ingot, cracks on the ingot surface, or voids within the ingot, which are factors contributing to grain boundary degradation. By setting the pulling down rate to 500 mm / s or less, the molten state of silver can be appropriately maintained during the pulling down process. This makes it possible to suppress the occurrence of unevenness on the ingot surface and molten metal leakage while maintaining low pulling down resistance.

[0058] In addition, in order to improve resistance to grain boundary degradation, it is also effective to reduce the total processing rate up to the final wire diameter in the subsequent wiredrawing process. Therefore, a small ingot diameter is preferable. On the other hand, if the ingot is too thin, the casting time becomes long, which tends to reduce productivity. Therefore, the ingot diameter is preferably 5 mm to 20 mm, and more preferably 8 mm to 10 mm.

[0059] (2) Wiredrawing Process In the wiredrawing process, the ingot obtained in the (1) casting process is drawn. A reduced area ratio in the wiredrawing process is effective in suppressing the <111> orientation ratio to 60% or less. A weak reduction in area ratio is also effective in controlling the grain boundaries of the silver alloy wire. In the wiredrawing process, it is preferable to use multiple diamond dies to gradually reduce the wire diameter. In this case, the area reduction rate (processing rate) per diamond die is preferably 5% or more and 25% or less. Using dies with an area reduction rate of 12% to 25% for intermediate-thick wire processing and dies with an area reduction rate of 5% to 15% for processing close to the final wire diameter is effective in controlling the special grain boundary length ratio and crystal orientation. Silver alloy wires with non-circular cross-sectional shapes can be obtained by drawing the wire using one to three irregular shaped dies that approximate the final cross-sectional shape when the wire approaches the final wire diameter.

[0060] (3) Regarding the rolling process: When producing a plate-shaped silver alloy wire, the rolling process is performed after the (2) wiredrawing process. In the rolling process, it is preferable to roll the silver alloy wire in stages using a pair of rotating rolls. When producing a silver alloy wire with a circular cross section, the rolling process is not performed. In the rolling process, wiredrawing is performed using a deformed die as the final die of the (2) wiredrawing process, so that the processed shape of the die is the same as the cross-sectional shape of the silver alloy wire, or a silver alloy wire with a circular cross section is obtained by the (2) wiredrawing process, and this silver alloy wire is rolled down from the surface in the thickness direction to obtain a silver alloy wire plate material of the final shape. When performing the rolling process, a low reduction ratio is effective for high strength and high resistance to intergranular degradation, and a reduction ratio of 20% to 85% is preferred. However, the lower the reduction ratio, the more man-hours required to obtain the final shape and the higher the production costs. The rolling reduction is the rate of reduction in thickness (in the rolling direction) from the cross-sectional diameter of a round wire having a circular cross section before and after rolling in one reduction.

[0061] (4) Heat Treatment Step It is preferable to perform a heat treatment on the silver alloy wire of the final wire diameter obtained through the steps (1) to (3) or the silver alloy wire plate material rolled to the final shape (heat treatment step). In the heat treatment step, it is effective to perform the heat treatment once or multiple times. When performing multiple heat treatments, it is effective to reduce the processing rate of the (2) wiredrawing step and the (3) rolling processing rate of the rolling step. The processing rate of the (2) wiredrawing step is the rate of change in cross-sectional area from the wire diameter in the heat treatment immediately before the final wire diameter (the wire diameter in the last intermediate heat treatment) to the final wire diameter.

[0062] The heat treatment methods in the heat treatment step include a running line heating heat treatment method in which a silver alloy wire is heated while being continuously running, a running line current heat treatment method in which a silver alloy wire is heat treated by applying a voltage while being continuously running, and a batch treatment method in which the silver alloy wire is housed in a constant temperature heating furnace and heated. For controlling the special grain boundary length ratio and orientation ratio of the silver alloy wire of this embodiment, low temperature, long time heat treatment is effective. For the running line heating heat treatment method, the heat treatment conditions are preferably 300°C to 900°C and a running line speed of 10 to 200 m / min. For the running line current heat treatment method, the heat treatment conditions are preferably a voltage of 5 V to 20 V and a running line speed of 10 to 200 m / min. In addition, nitrogen (N ２ ) gas, or nitrogen (N ２ ) gas contains traces of hydrogen (H ２ It is preferable to carry out the heat treatment in a gas atmosphere containing a mixture of the above gases.

[0063] (Tensile Test) The maximum stress (MPa) is obtained by pulling both ends of the evaluation sample with a tensile tester and dividing the maximum yield strength (N) at break by the cross-sectional area of ​​the evaluation sample. This maximum stress is automatically calculated by converting the tensile force into an electrical signal in the tensile tester. The elongation is obtained by dividing the elongation at break by the length of the sample before the test. Furthermore, the 0.2% yield strength (N) obtained from the tensile test is divided by the cross-sectional area of ​​the evaluation sample to obtain the 0.2% stress (MPa). The tensile test can be performed in accordance with JIS Z2241. Furthermore, the tensile test is performed on five evaluation samples, and the maximum stress and elongation can be calculated as the average values ​​of the five measured values. Furthermore, the stress (MPa) is 1 kgf / mm ２ = 9.8 MPa. The cross-sectional area of ​​the plate material is calculated by multiplying the width by the thickness.

[0064] (Applications of Silver Alloy Wire) The silver alloy wire of the present invention can be used for the same applications as all industrial wires conventionally using copper-based materials and aluminum-based materials. Specifically, it can be suitably used as a conductive wire, a bonding wire, a semiconductor wire, a structural wire, etc. Since the cost of silver bullion is high, the silver alloy wire of this embodiment is particularly suitable for applications that prioritize high functionality, high reliability, and high added value over cost.

[0065] Examples of conductive wires include electric wires and cables. Examples of joining wires include wires for connecting electrodes of lithium ion batteries to bus bars in lithium ion battery modules installed in electric vehicles. Examples of semiconductor wires include bonding wires that join components that form electrical circuits within semiconductor modules, such as between semiconductor chips, between semiconductor chips and external electrodes, between circuit patterns on a semiconductor substrate, between circuit patterns and semiconductor chips, or between circuit patterns and external electrodes. Examples of structural wires include meshes and nets for current collectors used in batteries, and coils (windings) for small generators and motors used in medical and other devices. In addition, the silver alloy wire of the embodiment can be suitably used as a stranded wire material for the above-mentioned conductive, joining, and structural purposes.

[0066] More specifically, examples of conductive wires (conductive wires) include overhead transmission lines, optical fiber composite overhead ground wires (OPGW), underground wires, submarine cables, and other power wires, telephone cables, coaxial cables, and other communication wires, cables for wired drones, cab tire cables, EV / HEV charging cables, twisted cables for offshore wind power generation, elevator cables, umbilical cables, robot cables, electric wires for equipment such as train overhead lines and trolley wires, transportation wires such as automotive wire harnesses, marine wires, and aircraft wires, bus bars, lead frames, flexible flat cables, lightning rods, antennas, connectors, terminals, and cable braids.

[0067] (Wire for Semiconductor) Next, a power semiconductor device will be described as an example in which the silver alloy wire of the above-described embodiment is used as a wire for semiconductor.

[0068] (Power Semiconductor Device) Fig. 4 is a diagram schematically showing a semiconductor device 103 using the silver alloy wire of the embodiment. The configuration of the semiconductor device 103 using the silver alloy wire of the embodiment will be described with reference to Fig. 4.

[0069] As shown in FIG. 4, the semiconductor device 103 includes a semiconductor element 1, a metal film 2, wires 3, a circuit pattern 41, a metal pattern 42, an insulating member 43, a heat dissipation member 5, a bonding material 6, a case 7, terminals 8, and a sealing material 9.

[0070] The semiconductor element 1 is, for example, a power semiconductor used as a semiconductor for power supply. Examples of the semiconductor element 1 include a metal oxide semiconductor field effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT).

[0071] The semiconductor element 1 is formed by laminating, in this order, an electrode 11, a substrate portion 13, and a back electrode 12. The electrode 11 is, for example, an aluminum (Al)-silicon (Si) electrode, and the substrate portion 13 is, for example, a silicon (Si) substrate, a silicon carbide (SiC) substrate, a gallium nitride (GaN) substrate, or the like.

[0072] The metal film 2 is provided on the surface of the electrode 11 opposite the substrate portion 13 so as to cover the surface of the electrode 11. The metal film 2 is a nickel (Ni) film, copper (Cu) film, titanium (Ti) film, tungsten (W) film, or the like, and is a film formed by electroplating, electroless plating, vapor deposition, sputtering, or the like.

[0073] The wire 3 is made of the silver alloy wire of the above-described embodiment, and has the same configuration and characteristics as described above. The wire 3 is bonded to the surface of the metal film 2.

[0074] Within the semiconductor device 103, a semiconductor circuit is formed by the semiconductor element 1, wires 3, terminals 8, circuit patterns 41, and metal patterns 42. Within the semiconductor device 103, the wires 3 are bent multiple times, and these bent portions are used to bond to the semiconductor element 1, terminals 8, circuit patterns 41, etc.

[0075] In the semiconductor device 103, a bonding material 6, a metal pattern 42, an insulating member 43, a circuit pattern 41, a bonding material 6, and a semiconductor element 1 are stacked in this order on the surface of a heat dissipation member 5. The bonding material 6 bonds the heat dissipation member 5 to the metal pattern 42, and the circuit pattern 41 to the back electrode 12 of the semiconductor element 1. The insulating member 43 is an insulating substrate or the like.

[0076] The case 7 is a ring-shaped housing having an internal space, and is provided to surround the outer periphery of the heat dissipation member 5. The internal space of the case 7 accommodates the semiconductor element 1, the metal film 2, the wires 3, the circuit pattern 41, the metal pattern 42, the insulating member 43, the bonding material 6, and the sealing material 9 described above.

[0077] The terminals 8 function as connection terminals for connecting to external devices. The terminals 8 are provided on the upper surface of the case 7, and are arranged so that one end protrudes into the internal space of the case 7 and the other end protrudes outside the case 7. The internal space of the case 7 is filled with a sealing material 9 that encloses the semiconductor element 1, metal film 2, wires 3, circuit pattern 41, metal pattern 42, insulating member 43, and bonding material 6. The sealing material 9 is a gel-like sealing resin, a hardened mold resin, or the like.

[0078] The semiconductor device 103 shown in FIG. 4 may have a plurality of semiconductor elements 1, electrodes 11 on the plurality of semiconductor elements 1, a plurality of circuit patterns 41, and a plurality of terminals 8. The semiconductor device 103 preferably has an electrode-circuit pattern bonding structure including one of the electrodes 11 on the plurality of semiconductor elements 1, one of the plurality of circuit patterns 41, and a wire 3 connecting the electrode 11 to the circuit pattern 41. The semiconductor device 100 preferably has an electrode-terminal connection structure including one of the plurality of terminals 8, one of the plurality of electrodes 11, and a wire 3 connecting the terminal 8 to the electrode 11. The semiconductor device 100 preferably also has a circuit pattern-terminal connection structure including one of the plurality of terminals 8, one of the plurality of circuit patterns 41, and a wire 3 connecting the terminal 8 to the circuit pattern 41. The semiconductor device 100 includes one or more electrode-circuit pattern bonding structures, one or more electrode-terminal connection structures, and one or more circuit pattern-terminal connection structures, and preferably includes two or more. That is, the silver alloy wire of the embodiment can be used for connecting the electrode 11 and the circuit pattern 41, for connecting the terminal 8 and the electrode 11, or for connecting the terminal 8 and the circuit pattern 41.

[0079] The semiconductor device 103 may have a plurality of circuit patterns 41, and preferably includes a circuit pattern bonding structure including two adjacent circuit patterns among the plurality of circuit patterns 41 and a wire 3 connecting the two adjacent circuit patterns. The semiconductor device 103 includes one or more circuit pattern bonding structures, and preferably includes two or more. That is, the silver alloy wire of the embodiment can be used to connect the circuit patterns 41 to each other.

[0080] The semiconductor device 103 may further have a substrate (semiconductor element upper substrate) on the semiconductor element 1, and in this case, it is preferable that the semiconductor device 103 has a bonding structure including the semiconductor element upper substrate, a circuit pattern 41, and a wire 3 connecting the semiconductor element upper substrate and the circuit pattern 41.

[0081] In the semiconductor field, development of silver alloy wires for ball bonding has been progressing. For example, Patent Document 1 discloses a composite silver wire for ball bonding that avoids the problem of copper oxidation, is suitable for mass production, and has low manufacturing costs. The composite silver wire contains 4% to 8% by weight of gold and 2 to 4% by weight of palladium. Patent Document 2 discloses an alloyed silver wire having an average diameter in the range of 8 to 80 μm, containing 3 to 6 wt% of palladium, 0.2 to 2 wt% of gold, and other metals. Patent Document 3 discloses a method for manufacturing a silver-palladium alloy wire. Patent Document 4 discloses a silver alloy wire containing 94.1 to 98.5 wt% of silver and 1.5 to 5.9 wt% of palladium.

[0082] Silver alloy wire for ball bonding is bonded to the electrodes of semiconductor devices using a method called ball bonding. Ball bonding is performed as follows: The tip of the silver alloy wire is oriented vertically, and an arc discharge is formed between the wire and a discharge torch using the electron flame-off (EFO) method. The discharge current then inputs heat to the wire tip. This heat input heats and melts the wire tip. The molten metal rises along the wire due to its surface tension, forming a spherical molten ball at the wire tip. The molten ball solidifies to form a free-air ball (FAB). The electrode of the semiconductor device is then heated to approximately 140–300°C while ultrasonic waves are applied, and the free-air ball is crimped onto the electrode, bonding one end of the wire to the aluminum electrode. The bonded electrode is typically sealed with a thermosetting epoxy resin to protect it from external environmental factors such as heat, dust, moisture, and light.

[0083] When resin-sealed, an intermetallic compound is generated between the silver alloy wire ball and the aluminum electrode (called the bonding interface). Because this metal compound is unstable, it easily reacts with moisture, sulfur, and other components contained in the resin, which can lead to a decrease in bonding strength and an increase in electrical resistance when used in a high-temperature, high-humidity environment. The decrease in bonding strength and the increase in electrical resistance result in a decrease in bonding reliability. The above-mentioned prior art proposes a silver alloy wire with a silver purity of 99% by mass or less to improve the bonding reliability of the wire bonding interface.

[0084] In the conventional silver alloy wire, the reliability of the bonding interface has been confirmed to be improved by the effects of the High Accelerated Stress Test (HAST) based on JESD22-A110 and the Thermal Humidity Bias Life Test (THB) based on JESD-A101. However, these conventional silver alloy wires have improved the reliability of the bonding interface, but have not improved the performance of the silver alloy wire itself, making them unsuitable for processing. In addition, the low purity of the silver alloy impairs the excellent electrical conductivity and excellent thermal conductivity of silver itself.

[0085] In contrast, the silver alloy wire of the above-described embodiment has a higher silver purity than conventional silver alloy wires, resulting in excellent electrical conductivity and thermal conductivity. Furthermore, it is possible to achieve both excellent electrical conductivity and thermal conductivity and good strength. Furthermore, since grain boundary degradation is less likely to occur, it is less likely to break even when a predetermined load is continuously applied, and it is easy to maintain strength even at high temperatures, such as in industrial wires. For these reasons, the silver alloy wire of the embodiment is extremely suitable for high-performance, multi-functional products using advanced industrial technology.

[0086] When the silver alloy wire of the above-described embodiment is used as a wire for a semiconductor, the cross section of the silver alloy wire is preferably circular, and its diameter (wire diameter) is preferably 15 μm to 700 μm, more preferably 80 μm to 600 μm. When the cross section of the silver alloy wire is elliptical or oval, the length of the major axis is preferably 0.3 mm to 4 mm, and the length of the minor axis is preferably 0.05 mm to 0.5 mm. When the cross section of the silver alloy wire is rectangular, the length of the long side is preferably 0.3 mm to 4 mm, and the length of the short side is preferably 0.05 mm to 0.5 mm. In particular, when the silver alloy wire of the above-described embodiment is used as a wire for a power semiconductor, its diameter (wire diameter) is preferably 80 μm to 600 μm, more preferably 100 μm to 500 μm. When the cross section of the silver alloy wire is rectangular, the length of the long side is preferably 0.5 mm to 2 mm, and the length of the short side is preferably 0.1 mm to 0.3 mm.

[0087] (Joint Wire) Next, a case where the silver alloy wire of the embodiment is applied as a joint wire to connect a bus bar of a lithium ion battery module of an electric vehicle to an electrode of a lithium ion battery will be described.

[0088] FIG. 5 is a schematic diagram showing a portion of a lithium-ion battery module. The lithium-ion battery module shown in FIG. 5 includes a lithium-ion battery 51, a cathode bus bar 52, an anode bus bar 53, and a silver alloy wire 50 electrically connecting the lithium-ion battery 51 and the cathode bus bar 52. The lithium-ion battery module shown in FIG. 5 also includes another silver alloy wire 50 electrically connecting the lithium-ion battery 51 and the anode bus bar 53. The silver alloy wire 50 is the same as that of the embodiment described above. The lithium-ion battery module shown in FIG. 5 has an anode and a cathode disposed on the top and bottom surfaces, respectively, of a columnar lithium-ion battery, and is typically mounted on a vehicle with the anode facing up and the cathode facing down.

[0089] FIG. 6 is a schematic diagram illustrating a portion of another type of lithium-ion battery module. The lithium-ion battery module shown in FIG. 6 includes a lithium-ion battery 51, a cathode bus bar 52, an anode bus bar 53, and a silver alloy wire 50 electrically connecting the lithium-ion battery 51 and the cathode bus bar 52. The lithium-ion battery module shown in FIG. 6 also includes another silver alloy wire 50 electrically connecting the lithium-ion battery 51 and the anode bus bar 53. The silver alloy wire 50 is the same as that of the above-described embodiment. The lithium-ion battery module shown in FIG. 6 has both the anode and cathode disposed at the center and outer edge of the top surface of a columnar lithium-ion battery, respectively, and is typically mounted on a vehicle with the anode facing up.

[0090] The silver alloy wire of the above-described embodiment is suitable for bonding the busbar and silver alloy wire in this lithium-ion battery module and for bonding the electrodes and silver alloy wire of the lithium-ion battery (wire bonding). The silver alloy wire of the above-described embodiment can combine excellent electrical and thermal conductivity with good strength. Furthermore, since grain boundary degradation is resistant, it is less likely to break even under a constant load and easily maintains its strength even at high temperatures used in industrial wires. For these reasons, applying the silver alloy wire of the embodiment to a lithium-ion battery module can contribute to the high performance and multi-functionality of the lithium-ion battery module. Furthermore, since it is less likely to break even under a constant load and easily maintains its strength even at high temperatures used in industrial wires, it is highly safe. Since it has low electrical resistance (high electrical conductivity), it is less likely to generate heat and can further improve the safety of lithium-ion batteries that are at risk of explosion at high temperatures (e.g., 80°C or higher).

[0091] When the silver alloy wire of the above-mentioned embodiment is used as a joining wire, if the silver alloy wire is a round wire, its wire diameter (long diameter) is preferably 80 μm or more and 600 μm or less, and more preferably 100 μm or more and 500 μm or less. The cross section of the silver alloy wire is preferably a rectangular plate material (also called a ribbon material), and in this case, it is preferable that the length of the long side is 0.5 mm or more and 2 mm or less, and the length of the short side is 0.1 mm or more and 0.3 mm or less.

[0092] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, but includes all aspects encompassed by the concept of the present invention and the scope of the claims, and can be modified in various ways within the scope of the present invention.

[0093] Next, examples will be described, but the present invention is not limited to the following examples.

[0094] <Silver Alloy Wire (Round Wire)> The silver alloy wires of the examples were produced as follows. High-purity silver bullion with a purity of 99.99% by mass or more was prepared. Alloying elements were added to this silver bullion, which was then vacuum-melted under an inert atmosphere and continuously cast to obtain a silver wire material (ingot with a diameter of 8 mm). The ingot was drawn in two stages: to an intermediate wire diameter of 0.9 mm, and then from the intermediate wire diameter to the final wire diameter. In the drawing process, the area reduction rate (working rate) per die to the intermediate wire diameter was 12% to 25%, and the area reduction rate (working rate) per die from the intermediate wire diameter to the final wire diameter was 5% to 15%. The final wire diameters of the silver alloy wires of the examples are shown in Tables 2 to 4. The silver alloy wires drawn to the final wire diameter were subjected to a final heat treatment in an electric heating furnace. The final heat treatment was performed under the conditions of an electric current heating voltage of 5 V to 20 V, a distance between electrode terminals of 800 mm to 1300 mm, and a wire running speed of 50 m / min to 200 m / min. After the final heat treatment, the silver alloy wire was rewound on a spool by a rewinding machine every 100 m. This resulted in a silver alloy wire (round wire) having a circular cross section.

[0095] The silver alloy wires of the comparative examples were produced as follows. In the same manner as in the examples, silver bullion with a purity of 99.99% by mass or more was prepared, and alloying elements were added to this silver bullion to obtain a silver wire material. The silver wire material was then subjected to a wiredrawing process. In the wiredrawing process, the manufacturing conditions, such as the heat treatment temperature and time at the intermediate wire diameter and the final wire diameter, the working ratio from each wire diameter to the next wire diameter, the cooling rate after the intermediate heat treatment, and the area reduction rate of each die, were changed outside the range of the manufacturing conditions in the above examples to produce the silver alloy wires of the comparative examples.

[0096] For example, the silver alloy wire of Comparative Example 2 was produced with an area reduction rate per die of less than 12% up to the intermediate wire diameter, an area reduction rate per diamond die of more than 15% from the intermediate wire diameter to the final wire diameter, and a running speed in the final heat treatment that was higher than that used in the examples. The silver alloy wire of Comparative Example 1 was produced with an area reduction rate per die of less than 12% up to the intermediate wire diameter, an area reduction rate per die of less than 5% from the intermediate wire diameter to the final wire diameter, and a voltage value in the final heat treatment that was lower than that used in the examples. The silver alloy wire of Comparative Example 5 was produced with an area reduction rate per die of less than 12% up to the intermediate wire diameter, an area reduction rate per die of less than 5% from the intermediate wire diameter to the final wire diameter, and a running speed in the final heat treatment that was lower than that used in the examples. The silver alloy wire of Comparative Example 6 was produced with an area reduction rate per die of more than 25% up to the intermediate wire diameter, an area reduction rate per die of more than 15% from the intermediate wire diameter to the final wire diameter, and a voltage value for the final heat treatment that was higher than the range used in Examples. The silver alloy wire of Comparative Example 12 was produced with an area reduction rate per die of more than 25% up to the intermediate wire diameter, an area reduction rate per diamond die of less than 5% up to the final wire diameter, and a running speed for the final heat treatment that was lower than the range used in Examples.

[0097] (Measurement of the special grain boundary length ratio, crystal orientation, and average crystal grain size in the cross section of the silver alloy wire) The special grain boundary length ratio and crystal orientation in the cross section of the silver alloy wire of the examples and comparative examples were measured as follows. Several evaluation samples were prepared by cutting the silver alloy wire into lengths of several centimeters. The evaluation samples were attached straight and flat to a metal plate (an Ag-plated frame was used, but any metal plate capable of fixing the evaluation sample was acceptable) while taking care not to stretch or sag. The evaluation sample and the metal plate were then placed in a cylindrical mold with the metal plate at the bottom of the cylinder. An embedding resin was then poured into the mold, and a curing agent was then added to harden the resin. The cured cylindrical resin containing the evaluation sample was then roughly polished with a polisher to expose the cross section of the silver alloy wire. The cut surface was then finished by final polishing, and subsequently, residual strain on the polished surface was removed by ion milling to obtain a smooth surface. The ion milling device was finely adjusted so that the cut surface of the silver alloy wire was perpendicular to the longitudinal direction of the silver alloy wire.

[0098] The silver alloy wire of the evaluation sample was attached to the sample stage of a field emission scanning electron microscope (FE-SEM, JEOL Ltd., JSM-7800F) so that the cross section (i.e., the polished surface of the evaluation sample) was parallel to the sample stage. The FE-SEM was set to a magnification of 200x, an acceleration voltage of 15 keV, a measurement area of ​​approximately 420 × 420 μm, a measurement interval (Step Size) of 0.7 μm, and a standard phase of silver, and information on the crystalline structure, such as the crystal orientation and grain size of the cross section, was obtained. The EBSD measurement data thus obtained was analyzed using dedicated analysis software (TSL's OIM analysis). In this example and comparative example, the software was set to recognize a single crystal grain when the misorientation was 15 degrees or less and five or more pixels were connected.

[0099] Furthermore, in the analysis of EBSD measurement data, there may be regions where orientation data, etc., cannot be measured due to the roughness of the measurement surface, residual strain due to polishing, contamination, the presence of an oxide film, etc. For this reason, the cleanup processing function provided in the analysis software was used to replace the orientation data, etc., of points where measurement was not performed properly with data from surrounding pixels where measurement was performed correctly, thereby complementing the incomplete measurement areas. This method is effective for removing scattered points where measurement was not performed properly. However, excessive cleanup processing increases noise in the projected image. For example, to recognize crystal grains, the orientation difference was set to 15 degrees or less and five or more pixels were connected, and the grain dilation method was performed once, and the grain CI standardization method was also performed once. Furthermore, even if orientation data, etc., could be measured, the reliability of the measured data may be low. Therefore, a reliability level was set and analysis data was acquired based on this reliability level. In other words, analysis data was obtained by excluding areas where orientation data, etc., could not be measured, or areas where measurement was possible but the reliability was low, etc. Here, the reliability may be measured using parameters provided in the analysis software, and several parameters, such as the Confidential Index (CI value) and Image Quality (IQ value), can be used to select judgment criteria depending on the sample condition, the purpose of analysis, etc.

[0100] (Measurement of the ratio of special grain boundary length) From the data analyzed above, the Σ data of the CSL grain boundary (Coincidence Site Lattice) is obtained, and the sum of the grain boundary lengths of Σ3 to Σ29 is taken as the special grain boundary length. The "Total Length" of the CSL grain boundary data is used as the total grain boundary length, and the ratio of the special grain boundary length is determined. Similar measurements are performed on a total of three randomly selected cross sections of the same sample, and the average value of the ratio of the special grain boundary length is determined.

[0101] (Measurement of <111> Orientation Ratio) The <111> orientation ratio was calculated from the analyzed data.

[0102] (Method of Measuring Crystal Grain Size) From the analyzed data, the average crystal grain size in the cross section of the sample was calculated by the circle approximation method (diameter).

[0103] The ratio of the special grain boundary length, the ratio of the <111> orientation having an angle difference of 15 degrees or less with respect to the longitudinal direction, and the average crystal grain size in the cross section (transverse cross section) perpendicular to the longitudinal direction of each of the silver alloy wires of the Examples and Comparative Examples, which were determined as described above, are shown in Tables 2 to 4 below.

[0104] (Measurement of Alloy Element Concentration) The concentration of alloy elements (other than unavoidable impurities) in each silver alloy wire in the Examples and Comparative Examples was measured as follows. Approximately 1 to 3 g of the prepared silver alloy wire was placed in dilute nitric acid and dissolved, and then the solution was collected. Hydrochloric acid was added to this solution, and a constant volume solution was prepared with ultrapure water. When the silver alloy wire contained Au, the residue remaining after filtration was dissolved in dilute aqua regia and then a constant volume solution was prepared with ultrapure water. The concentration of each element in these constant volume solutions was determined by high-frequency inductively coupled plasma atomic emission spectroscopy (Shimadzu Corporation, ICPE-9000). The alloy elements measured here were Pt, Ti, Zn, Au, Cu, Mg, Ni, Al, Sn, Pd, Cd, In, W, Nd, and Bi, and each is expressed as a concentration (mass%) relative to the total amount of silver alloy wire. The results are shown in the table below. The compositions of the obtained silver alloy wires in the Examples and Comparative Examples are shown in Tables 2 to 4.

[0105] The silver alloy wires of the examples and comparative examples were evaluated for their properties under the following conditions and methods. The results are shown in Tables 2 to 4.

[0106] (Tensile Test) Each silver alloy wire in the Examples and Comparative Examples was cut to a length slightly longer than 100 mm to obtain an evaluation sample. The evaluation sample was subjected to a tensile test at room temperature of 15 to 28°C to measure its tensile strength, i.e., maximum yield strength (N). The maximum yield strength was calculated as the maximum value at break when the evaluation sample was continuously pulled at a speed of 20 mm / min and a load cell rating of 100 N using a tensile testing machine (a universal testing machine manufactured by Shimadzu) with a measurement length of 100 mm. The maximum yield strength is the force applied to the evaluation sample pulled at the above speed, and is usually calculated automatically by converting the tensile force into an electrical signal using a load cell. Taking into account the variability in the measurement results, the maximum yield strength was calculated as the average value of three samples. The maximum yield strength was calculated by dividing the maximum yield strength by the cross-sectional area of ​​the evaluation sample to obtain the maximum stress (MPa). Furthermore, the elongation at break (breaking elongation, %) and 0.2% yield strength were calculated from the results of this tensile test. The breaking elongation obtained in the tensile test is referred to as "elongation A."

[0107] The tensile strength of the silver alloy wire was evaluated as follows. Wires with a maximum stress of more than 180 MPa and less than 250 MPa were evaluated as having excellent strength for industrial use and were designated with the symbol "A." Wires with a maximum stress of more than 160 MPa and less than 180 MPa were evaluated as having good strength for industrial use and were designated with the symbol "B." Wires with a maximum stress of more than 140 MPa and less than 160 MPa were evaluated as having no practical problems and were designated with the symbol "C." Wires with a maximum stress of 140 MPa or less were evaluated as having the risk of practical problems and were designated with the symbol "D." Wires with a maximum stress of more than 250 MPa were evaluated as having extremely high strength, but were therefore highly brittle and unsuitable for secondary processing and were designated with the symbol "(A)." Silver alloy wires rated "(A)" are prone to chip damage when used as semiconductor bonding wires.

[0108] (High-Temperature Creep Test) Each silver alloy wire of the Examples and Comparative Examples was cut to a length slightly longer than 100 mm to obtain an evaluation sample. The high-temperature creep test was performed using a tensile testing machine (Universal Testing Machine: Shimadzu AGS-X), with a measurement length of 100 m, a temperature of 200°C, and a load of approximately 0.9 times the 0.2% proof stress of each Example and Comparative Example (i.e., load ≒ 0.2% proof stress × 0.9). The time until fracture (time to fracture) and the elongation at fracture (elongation to fracture, %) were measured. The elongation to fracture obtained in the high-temperature creep test was referred to as "elongation B."

[0109] (Calculation of Deterioration Rate) Using the elongation rate A obtained in the tensile test and the elongation rate B obtained in the high-temperature creep test, the deterioration rate was calculated according to the following formula (2). A larger deterioration rate indicates that the grain boundary deterioration due to high temperatures is more likely to progress. A deterioration rate of 0.3 or less was evaluated as having excellent resistance to grain boundary deterioration and was indicated by the symbol "A". A deterioration rate of more than 0.3 and less than 0.7 was evaluated as having good resistance to grain boundary deterioration and was indicated by the symbol "B". A deterioration rate of more than 0.7 and less than 1 was evaluated as having slightly poor resistance to grain boundary deterioration but at a level that is not problematic in practical use and was indicated by the symbol "C". A deterioration rate of more than 1 was evaluated as having poor resistance to grain boundary deterioration and was indicated by the symbol "D". Deterioration rate = "Elongation rate B" / "Elongation rate A" (2)

[0110] (Overall Evaluation) Based on the evaluations of strength and intergranular degradation resistance obtained above, an overall evaluation was made as follows. When both evaluations of strength and intergranular degradation resistance were A, it was deemed extremely excellent and was indicated with an "S." When one of the two evaluations was A and the other was B, it was deemed excellent and was indicated with an "A." When one of the two evaluations was A and the other was C, or when both were B, it was deemed good and was indicated with a "B." When one of the two evaluations was B and the other was C, or when both were C, it was deemed pass and was indicated with a "C." When one of the two evaluations was D, it was deemed fail and was indicated with a "D." To summarize the above, the evaluations in parentheses are in no particular order as follows: Overall rating S: Two ratings are (A, A) Overall rating A: Two ratings are (A, B) Overall rating B: Two ratings are (A, C) or (B, B) Overall rating C: Two ratings are (B, C) or (C, C) Overall rating D: Two ratings are (D, D), (C, D), (B, D), (A, D)

[0111]

[0112]

[0113]

[0114] <Evaluation of Rolled Silver Alloy Sheet Materials> Silver alloy wires having the same composition and final wire diameter as the silver alloy wire examples and comparative examples shown in Tables 2 to 4, produced under the same processing conditions, were rolled into sheets with a single reduction to achieve the final thicknesses shown in Tables 5 to 7, to obtain silver alloy sheets of Examples 101 to 138 and Comparative Examples 101 to 112. After rolling, final heat treatment was performed under the following conditions. The conditions for the electrical heating in the final heat treatment of the examples were a voltage of 5 V to 20 V, a distance between electrode terminals of 800 mm to 1300 mm, and a wire running speed of 50 m / min to 200 m / min. The conditions for the electrical heating in the final heat treatment of Comparative Examples 101 to 104 and 107 to 111 were a voltage of 5 V and a running speed of 210 m / min. The conditions for the electrical heating in the final heat treatment of Comparative Examples 106 and 112 were a voltage of 20 V and a running speed of 40 m / min. The conditions for energizing the final heat treatment of Comparative Example 105 were a voltage of 21 V and a running speed of 50 m / min. The obtained ribbon-shaped silver alloy wire (plate material) was used to observe and evaluate the crystal structure in the same manner as in the Examples. The results are shown in Tables 5 to 7. In Tables 5 to 7, w represents the width of the plate material, and t represents the thickness.

[0115]

[0116]

[0117]

[0118] (Examples of Bonding Wire and Semiconductor Wire) Using the silver alloy wires of the Examples and Comparative Examples with a wire diameter of 400 μm in Tables 2 to 4, and the silver alloy plate materials of the Examples and Comparative Examples with a w1 x t0.1 mm in Tables 5 to 7, a HESSE bonder BJ935 was used to continuously bond the wires to an aluminum plate measuring 50 mm in length x 50 mm in width x 1 mm in thickness, and the bond shape and bond strength after bonding were evaluated. The bonding conditions were set to two conditions: Duration 50 ms, Power 40 V, and Loads 1500 gf and 2500 gf. For each sample, a combination of first bonding and second bonding was set to n = 1 (1 set), and 20 sets (n = 20) of continuous bonding were performed, and evaluation samples for bond shape and bond strength were prepared. In this example, an aluminum plate was used as the bonding target for evaluation, but the bonding target is not limited to this; for example, a copper plate or a nickel plate may also be used.

[0119] (Method for Evaluating Bond Shape) The evaluation samples prepared above were observed using an optical microscope for the number of bond failures and the number of shape defects. When the number of bond failures and the number of defects were both zero, the result was evaluated as "S," meaning excellent. When the number of bond failures or the number of defects was one, the result was evaluated as "A," meaning good. When the total number of bond failures and defects was between two and three, there was room for improvement, but it was unlikely to cause problems in practical optimization of bonding conditions, so the result was rated as "B." When the total number of bond failures and the number of defects was four or more, the result was evaluated as unacceptable and rated as "C." The results are shown in Table 8. The bond shape was evaluated based on Figure 7. The photograph in Figure 7 is a photograph of the silver alloy wire bond observed from above the bonding surface, with the silver alloy wire arranged on the left and right sides of the page. The same is true for Figure 8. In the photograph in Figure 7, curling of the silver alloy wire around the bond (area surrounded by dotted lines) can be seen. In determining whether a joint was defective, if the curling of the silver alloy wire around the joint was equal to or greater than the area ratio (ratio to the entire joint) shown in Figure 7, the joint shape was determined to be defective, and if the curling was smaller than that shown in Figure 7, the joint shape was determined to be acceptable. Figure 8 is a photograph of an example of an acceptable joint shape. Note that if curling is present around the joint, there is a very high possibility that damage to the chip or substrate will occur when the product is mounted on a semiconductor device or the like.

[0120] (Method for Evaluating Bonding Strength) A pull test was performed on the loop center of the evaluation sample prepared above, and the number of times lift-off occurred at the bonded portion in the pull test was evaluated. When three or more lift-offs occurred in a pull test (n=20), the test was deemed a failure and was marked with a "C." When two lift-offs occurred, although there was room for improvement, the test was marked with a "B" because it was unlikely to cause a practical problem. When one lift-off occurred, the test was marked with an "A," meaning good. When zero lift-offs occurred, the test was marked with an "S," meaning very good. The results are shown in Table 3. Lift-off refers to peeling at the bonded interface between the silver alloy wire and the substrate during the pull test.

[0121]

[0122] Next, the relationship between creep rupture and grain boundary degradation will be explained. As mentioned above, the inventors used the silver alloy wires produced in the examples and comparative examples to previously investigate the relationship between the ratio (%) of the special grain boundary length and the state of the fracture surface after tensile tests and creep tests, as well as the fracture elongation (%) and fracture time (hrs) in the creep tests. Of these, the results of Example 5 and Comparative Example 1 will be mainly explained.

[0123] As shown in Figure 9, the fracture surface of the silver alloy wire of Example 5 in the tensile test is smooth, and no grain boundary cracks are observed. It can be seen that the silver alloy wire of the Example takes a longer time to fracture and has a slower rate of grain boundary deterioration than the silver alloy wire of the Comparative Example. When several other Examples were similarly observed, it was found that the silver alloy wires of the Examples all have relatively large crystal grains on the high-temperature creep fracture surface and fewer grain boundary cracks, and when compared with the silver alloy wires of the same wire diameter, they tend to generally take a longer time to fracture and have a slower rate of grain boundary deterioration than the silver alloy wires of the Comparative Example.

[0124] In contrast, in the fracture surface of the silver alloy wire of Comparative Example 1 in the creep test shown in FIG. 10, small crystal grains and many fine cracks (voids) at the grain boundaries were observed.

[0125] From these results, the inventors have considered that the higher the ratio of the special grain boundary length, the more dominant the special grain boundary with higher bond strength becomes in an environment where a predetermined load is continuously applied at high temperatures, and the slower the deformation rate of the silver alloy wire due to the load, and the less susceptible to grain boundary degradation.On the other hand, the lower the ratio of the special grain boundary length, the more dominant the grain boundary with lower bond strength becomes, and the more likely the silver alloy wire is to undergo grain boundary degradation before recrystallization, leading to grain boundary fracture accordingly.In addition, by using the ratio of the special grain boundary length of the silver alloy wire as an index, the occurrence of grain boundary degradation can be predicted in the early stages of silver alloy wire production, so that failures and serious accidents of parts using the silver alloy wire can be avoided in advance.

[0126] Next, the silver alloy wire sheet will be described. The inventors used the silver alloy sheets produced in Example 113 and Comparative Example 103 to previously investigate the relationship between the special grain boundary length ratio (%) of the cross section of the silver alloy sheet, the <111> orientation ratio (%), the state of the fracture surface after a tensile test or creep test, the fracture elongation (%), and the fracture time (hrs). Note that both Example 113 and Comparative Example 103 have a cross-sectional size of w2 mm x t0.2 mm. The results are shown in Table 9.

[0127]

[0128] In both the silver alloy sheets of Example 113 and Comparative Example 103, grain boundary cracks (voids) were observed on the fracture surface. However, the silver alloy sheets of the Examples took longer to fracture than the silver alloy sheets of the Comparative Examples when the cross-sectional shapes were the same. This shows that the silver alloy sheets of the Examples exhibited a slower rate of grain boundary degradation than the silver alloy sheets of the Comparative Examples. Furthermore, the silver alloy sheets of the Comparative Examples exhibited small grains near the fracture surface of the high-temperature creep test, resulting in many fine grain boundary cracks, whereas the silver alloy sheets of the Examples exhibited large grains near the fracture surface of the high-temperature creep test, resulting in fewer grain boundary cracks. These results confirmed that the relationship between the special grain boundary length ratio and grain boundary degradation is similar to that of silver alloy round wires.

[0129] 103...semiconductor device, 1...semiconductor element, 2...metal film, 3...wire, 41...circuit pattern, 42...metal pattern, 43...insulating member, 5...heat dissipation member, 6...bonding material, 7...case, 8...terminal, 9...sealing material, 50...silver alloy wire, 51...lithium ion battery, 52...cathode side bus bar, 53...anode side bus bar, 54...gel-like substance

Claims

1. A silver alloy wire containing 99.8% by mass or more of silver, characterized in that the ratio of the special grain boundary length defined by the following formula (1) of the crystal grains in a cross section perpendicular to the longitudinal direction of the silver alloy wire is 30% or more, and the ratio of the crystal orientation <111> having an angle difference of 15 degrees or less with respect to the longitudinal direction of the silver alloy wire among the crystal orientations <hkl> in the longitudinal direction of the silver alloy wire measured in the cross section is 10% or more and 60% or less. Special grain boundary length ratio = grain boundary length of Σ3 to Σ29 / total grain boundary length (1) 2. The silver alloy wire according to claim 1, wherein the average crystal grain size in the cross section of the silver alloy wire is 2 μm or more and 15 μm or less.

3. A silver alloy wire as described in claim 1 or 2, which contains at least one element selected from the group consisting of Pt, Ti, Zn, Au, Cu, Mg, Ni, Al, Sn, Pd, Cd, In, W, Nd and Bi, and the concentration of the elements relative to the total amount of the silver alloy wire is 0.003 mass% or more and 0.1 mass% or less in total.

4. A silver alloy wire containing 99.8% by mass or more of silver, in which the ratio of the special grain boundary length defined by the following formula (1) of the crystal grains in a cross section perpendicular to the longitudinal direction of the silver alloy wire is 30% or more, and which contains at least one element selected from the group consisting of Pt, Ti, Zn, Au, Cu, Mg, Ni, Al, Sn, Pd, Cd, In, W, Nd and Bi, and the concentration of the element relative to the total amount of the silver alloy wire is 0.003% by mass or more and 0.1% by mass or less in total. Special grain boundary length ratio = grain boundary length of Σ3 to Σ29 / total grain boundary length (1) 5. The silver alloy wire according to claim 4, wherein the ratio of crystal orientations <111> having an angle difference of 15 degrees or less with respect to the longitudinal direction of the silver alloy wire among the crystal orientations <hkl> in the longitudinal direction of the silver alloy wire measured in the cross section is 10% or more and 60% or less.

6. The silver alloy wire according to claim 4 or 5, wherein the average crystal grain size in the cross section of the silver alloy wire is 2 μm or more and 15 μm or less.

7. A conductive wire using the silver alloy wire according to claim 1 or 4.

8. A joining wire using the silver alloy wire according to claim 1 or 4.

9. A semiconductor wire using the silver alloy wire according to claim 1 or 4.

10. A structural wire using the silver alloy wire according to claim 1 or 4.

Images