Light emitting diode (LED) bonding wire and manufacturing method for LED bonding wire

Abstract

Provided are: a silver alloy bonding wire for an LED, the bonding wire realizing high reflectance in relation to blue light and having improved high-temperature strength in a ball neck portion even in ultra-low-loop-shaped bonding; and a manufacturing method thereof. A bonding wire for a light-emitting diode (LED) according to an embodiment is formed of a silver alloy containing silver at 98 mass% or greater and has a high-temperature / normal-temperature strength ratio from 0.6 to 0.99 and a high-temperature elongation percentage of 1.5% or greater.

Description

[Technical Field]

[0001] The present invention relates to a bonding wire for light-emitting diodes (LEDs), particularly a bonding wire for white LEDs used in backlights of LCDs (liquid crystal displays), and a manufacturing method thereof. The present invention also relates to an LED device using this LED bonding wire. [Background technology]

[0002] Light-emitting diodes (LEDs) have come into widespread use in recent years. LEDs are light-emitting elements made by combining P-type and N-type semiconductor materials. Light-emitting elements formed into various packages are also called LEDs. A distinction is sometimes made between packaged elements and light-emitting elements, with LEDs (or LED devices) and LED elements being referred to as LED elements. In this specification, unless otherwise specified, light-emitting elements are referred to as "LED elements" and packaged elements as "LED devices."

[0003] LED elements in red, orange, and yellow-green colors have been commercially available since the 1950s. Taking advantage of their compact size and low power consumption, they have been used in indicators, information displays, and other electronic devices. In 1993, bright blue LED elements were commercialized, and in 1996, pseudo-white LEDs (hereafter simply referred to as "white LEDs") were developed by combining yellow-emitting yellow phosphors with blue LED elements. White LEDs have attracted attention for their use in lighting and are rapidly becoming more widely used, surpassing conventional incandescent and fluorescent lighting fixtures. Because white LED lighting fixtures are expected to offer greater energy savings than conventional lighting fixtures, they have attracted attention as a potential energy-saving solution in the lighting sector, which accounts for a large portion of Japan's electricity consumption, following the increased awareness of energy conservation following the Great East Japan Earthquake in March 2011. Since then, white LEDs, due to their visual brightness, have been widely used in a variety of lighting applications, including bulb-shaped LED lamps for general households, facility and outdoor lighting, and automotive headlights.

[0004] With the spread of white LEDs for lighting, many improvements have been made to LED devices, such as higher luminous flux, smaller size, and lower cost, and as a result, the applications of LED devices have continued to expand. Miniaturized LED devices have begun to be used as backlights for LCDs (liquid crystal displays).

[0005] In recent years, LCDs have been widely used as display devices for a variety of electronic and information terminal devices, including televisions, personal computers (PCs), digital cameras, tablet devices, smartphones, smartwatches, car navigation devices, and drive recorders, and there has been a demand for improvements in a wide range of performance to suit these various applications.

[0006] For example, LCD panels have been developed with brightness levels of 700cd to a maximum of 4000cd, which is much brighter than the 300cd required for indoor use. Such high-brightness LCD panels have high visibility in bright sunlight-filled environments, and are therefore used in shop windows and outdoor advertising display panels. Smartphones and smartwatches have become increasingly popular, and as their usage environments expand, the demand for high visibility in outdoor sunlight has increased. For example, for smartphone applications, a brightness of 520cd / m 2 LCD panels with 100 times the brightness have been developed.

[0007] The environments in which LCD panels are used are diversifying, not only for outdoor applications where they are exposed to wind and rain, but also for indoor use. For example, LED devices used in car navigation systems and drive recorders must be able to withstand the constant vibrations that occur while the car is moving, as well as adapt to harsh environments such as cold and tropical climates, or even driving on asphalt under the scorching sun.

[0008] There has been a notable trend toward smaller and thinner electronic devices and information terminals equipped with LCD panels, and in recent years, the LED devices used in these devices have also been getting smaller and thinner at a rapid pace. For example, in 2013, an LED device for backlighting small and medium-sized LCDs with a thickness (thickness in the light-emitting direction) of approximately 0.8 mm was developed, but in recent years, the thickness of LED devices is approaching approximately 0.6 mm (of which the thickness of the area where wire bonding is possible is approximately 0.3 mm or less).

[0009] Thus, in recent years, there has been a marked demand for LED devices for LCD backlight applications that can simultaneously achieve improved brightness, downsizing, and thinning, and that can be adapted to harsh environments.

[0010] Here, we will explain the principles and configuration of white LEDs, the structure of LED devices for LCD backlights, and the issues they pose. The principle and structure of a typical white LED for lighting or backlighting are as follows: When blue light with a wavelength around 450 nm emitted from a blue LED element is absorbed by a yellow phosphor, yellow light with a complementary wavelength of around 590 nm is emitted. The yellow light emitted from the yellow phosphor and the blue light not absorbed by the phosphor mix together to produce a visually white color. White LED devices have a structure in which the top surface of the LED element is encapsulated in a transparent resin (silicone resin or epoxy resin) containing phosphor. White LEDs are classified into vertical LEDs, in which the pad electrodes of the blue LED element are orthogonal to the lead electrodes on the wiring board, and horizontal LEDs, in which they are parallel. Today, white LEDs for lighting with a variety of color renderings have been developed by combining yellow phosphors, green phosphors that emit green light, and red phosphors that emit red light.

[0011] There are two types of LCD backlights: "direct" and "edge." "Direct" backlights use multiple LED light sources arranged in a plane parallel to the LCD panel, opposite the viewing surface of the LCD panel, so that light from the light sources is incident perpendicularly to the LCD panel's surface. This allows the entire LCD panel to be illuminated, resulting in realistic detail and rich color reproduction. "Edge" backlights use one or more LED light sources arranged near the edge of the LCD panel, so that light from the light sources is incident parallel to the LCD panel's surface, and a reflective or light-guiding structure is used to illuminate the entire LCD panel. While edge backlights offer inferior detail and color reproduction compared to direct backlights, they facilitate the miniaturization and thinning of panels and reduce power consumption. Furthermore, low cost and high brightness (high luminous flux) are important for LED backlight devices, and improved brightness allows for high definition and high visibility of LCD panels.

[0012] The structure of an LED device for an LCD backlight will now be described. FIG. 1 shows a schematic cross-section of an LED device 10, an example of a surface-mount LED device used in conventional edge-mount backlights. The LED device 10 shown in FIG. 1 includes a package substrate 9, an LED element 1 mounted on the package substrate 9, a bank 6 surrounding the LED element 1, a bonding wire 3, and a pair of pad electrodes 21 and 22 mounted on the LED element 1. The LED device 10 also includes lead electrodes 81 and 82. The bank 6 and the lead electrodes 81 and 82 form a recess with the bank 6 as the side surface and the lead electrodes 81 and 82 as the bottom surface. The LED element 1 is mounted on the bottom of the recess via an adhesive layer 7 made of silver paste or the like. The lead electrodes 81 and 82 are made of aluminum or the like and are bonded to the package substrate 9 with silver paste or the like. The pair of pad electrodes 21 and 22 are electrically connected to the lead electrodes 81 and 82 via a bonding wire 3. The LED element is covered with an encapsulant 4. The sealing material 4 is, for example, a thermosetting resin such as an epoxy resin, a silicone resin, an epoxy-modified silicone resin, or a modified silicone resin. The sealing material 4 contains a phosphor (not shown) that converts the wavelength of the light from the LED element 10.

[0013] The LED device shown in Figure 1 is an example of an LED device from before the trend toward smaller and thinner LED devices. In this LED device, the bonding wire loop shape is generally either a "high loop shape" in which the loop height from the electrode is relatively high, or a "special loop shape" in which part of the loop is bent, compared to the LED devices that have become smaller and thinner in recent years. For example, in a high loop shape, the loop height (h) is designed to be at least four to five times the wire diameter. In contrast, LED devices that have become smaller and thinner generally use a "low loop shape" in which the loop height from the electrode is lower.

[0014] Conventionally, gold (Au) bonding wire, which has high electrical conductivity and good adhesion to electrodes, has been used to connect pad electrodes of blue LED elements to lead electrodes on wiring boards. However, as LED devices have become increasingly powerful and efficient, bonding wire made of silver, which has a higher thermal conductivity than gold, has come to be used to improve heat dissipation efficiency. Known Ag alloy bonding wires include silver alloys containing copper (Cu), platinum (Pt), gold (Au), palladium (Pd), nickel (Ni), chlorine (Cl), zinc (Zn), tin (Sn), rhodium (Rh), osmium (Os), rare earth elements, and other elements in predetermined amounts depending on the desired purpose and characteristics (see, for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Patent Publication No. 2012-099577 [Patent Document 2] Patent Publication No. 2013-110410 [Patent Document 3] Patent Publication No. 2014-096403 Summary of the Invention [Problem to be solved by the invention]

[0016] As mentioned above, in recent years, there has been a significant demand for LED devices for LCD backlighting that can achieve both improved brightness and compact / thin design, as well as adaptability to harsh environments. In contrast, LEDs for lighting are used in relatively mild environments, and the demand for compactness is not as strong. To achieve brightness comparable to that of fluorescent lamps, LEDs for lighting are designed to have a high luminous flux of 180 lm to 1800 lm per LED device. To achieve this high luminous flux, it is necessary to take measures such as increasing the area of ​​the LED element, increasing the number of LED elements installed in the lighting fixture, or increasing the current (forward current) that drives the LED element to emit light.

[0017] In contrast, LED devices for LCD backlights are designed to have a lower luminous flux per LED device, at a maximum of around 20 lm, compared to LED devices for lighting. As electronic devices and information terminals become smaller and thinner, the space available for placing LED devices becomes more limited, making it extremely difficult to increase the luminous flux of LED devices by simply expanding the area of ​​the LED element. Furthermore, as electronic devices and information terminals become smaller and thinner, the heat dissipation capabilities of LED devices decrease, making it extremely difficult to increase the luminous flux of LED devices for LCD backlights by simply increasing the forward current.

[0018] Furthermore, there is a limit to how much luminous flux (brightness) and brightness can be increased by improving the LED element itself. Therefore, in order to increase the luminous flux and brightness of LED devices for LCD backlights, measures have been taken to prevent the light emitted from the LED element from attenuating by improving the performance of peripheral components of the LED element, such as encapsulants and bonding wires.

[0019] For example, in LED devices, the bonding wire crosses a portion above the light-emitting surface of the LED element (see Figure 1), and the reflection and absorption of light by the bonding wire reduces the external light output. In particular, high-purity gold (Au) with a purity of 99.99% or more by mass has a low reflectivity of the gold (Au) element itself, which casts a black shadow of the bonding wire, reducing the visual white efficiency and brightness of the LED device. In contrast, bonding wire made of pure silver (Ag) or a silver alloy close to pure silver, which has a high reflectivity, can prevent a decrease in LED brightness. Because it is cheaper than gold, it is expected to be advantageous for realizing low-cost, bright LEDs.

[0020] However, pure silver (Ag) and silver alloy bonding wires are less resistant to temperature and stress changes than gold, and are therefore less durable. For example, in applications such as in-vehicle navigation systems and drive recorders, which are exposed to heat and vibration in harsh operating environments, LED devices must be made smaller and thinner, and must also be shock-resistant, requiring high durability. Making LED devices smaller and thinner requires wiring to be located closer to the LED element, which requires greater freedom in wiring design for bonding wires. However, wires with low resistance to temperature and stress changes are prone to failure due to temperature and stress changes when used in wiring structures with frequent bends in narrow areas.

[0021] Therefore, although wire with a high silver purity can achieve high reflectivity, it poses practical problems for use in LED devices, which are becoming increasingly smaller. However, if pure silver is reduced in purity by adding alloying elements, a decrease in reflectivity is unavoidable. As such, there is a trade-off between resistance to temperature and stress changes and high reflectivity, and this has been found to be a problem that limits the design of white LEDs, which aim to be smaller, thinner, and brighter. The limitations imposed by the unique problems with bonding wire on the expansion of applications for white LEDs, which have immeasurable potential, could even lead to disadvantages in major technological innovations in the future.

[0022] However, this trade-off issue cannot be resolved with conventional Ag alloy bonding wire, as follows: It places technical constraints on the development of LED devices. The negative impact of technological constraints on the development of LED devices is immeasurable, so silver alloy bonding wire for LEDs must be able to withstand temperature and stress changes while maintaining the brightness of white LEDs.

[0023] For example, Patent Documents 1 to 3 attempt to solve the problems associated with conventional high-loop bonding, but do not disclose any means for suppressing fatigue failure of the ball neck portion or increasing resistance to repeated temperature and stress changes, as described below, in silver alloy bonding wires that require ultra-low loops.

[0024] The required characteristics of bonding wire for LEDs differ from those of bonding wire for ICs (integrated circuits) and LSIs (large-scale integrated circuits). As explained below, LED bonding wire requires resistance to specific temperature and stress changes. The thermal expansion coefficient of transparent silicone resins used as encapsulants for LED devices is, for example, 200–400 ppm / K, significantly higher than the thermal expansion coefficient of epoxy resins used as encapsulants for ICs and LSI devices (for example, approximately 6–40 ppm / K). Furthermore, while encapsulants for ICs and LSI devices are typically filled with inorganic micro-silicone (silicon oxide) fillers to reduce the thermal expansion coefficient to a value close to that of the silicon chip, encapsulants for LED devices do not typically contain inorganic micro-silicone fillers. This is because, if inorganic micro-silicone fillers were filled into encapsulants for LED devices, they would reduce the light transmittance of the encapsulant, thereby reducing the brightness of the LED. As a result, encapsulants for LED devices inevitably experience greater thermal expansion changes than encapsulants for ICs and LSI devices. Since the bonding wire embedded in the encapsulant is exposed to repeated stress due to the expansion and contraction of the encapsulant, LED bonding wire is required to have a higher level of resistance to repeated temperature and stress changes (fatigue resistance) than bonding wire for ICs.

[0025] Therefore, in order to obtain a silver alloy bonding wire that can withstand temperature and stress changes while maintaining the brightness of white LEDs, the inventors compared the defects that occur during long-term use of LED devices using conventional high-loop or special-loop bonding with the defects of LED devices using low-loop or ultra-low-loop bonding, which achieves smaller and thinner sizes, along with their structures, and found that there were the following differences in the defects.

[0026] First, the main problem with LED devices using conventional high-loop or special-loop bonding was fatigue fracture at the wedge neck of the silver alloy bonding wire (the part where the wire stands up due to the wedge bonding with the lead electrode). Figure 2 shows a photograph of the wedge neck of a wire where fatigue fracture occurred.

[0027] The inventors' investigations revealed that in LED devices fabricated using conventional silver alloy bonding wire for ultra-low loop bonding, which has become increasingly compact and thin, fatigue failure of the ball neck portion (the portion where the wire stands up at the ball bond with the pad electrode on the LED element) was the main defect, unlike the conventional high loop and special loop LED devices mentioned above. In other words, as the loop became lower, fatigue failure of the wedge neck portion tended to decrease and fatigue failure of the ball neck portion tended to increase. Figure 3 shows a photograph of the ball neck portion of a wire where fatigue failure occurred.

[0028] The inventors investigated the cause of fatigue failure at the ball neck by closely examining the movement and condition of bonding wires during wiring within a narrow area of ​​a small LED device. They found that frequent bending of the wire during wiring tends to cause strain to be biased toward the bend, and that strain is particularly likely to concentrate at the ball neck, which experiences frequent bending. Furthermore, they found that stresses caused by thermal expansion and contraction of the encapsulant due to temperature changes around the LED element are difficult to disperse in an ultra-low loop shape and tend to concentrate at the ball neck. These findings suggest that strain during bending and the concentration of thermal expansion and contraction stresses of the encapsulant at the ball neck are one of the causes of fatigue failure at the ball neck. Furthermore, they found that the miniaturization of LED devices tends to reduce heat dissipation and increase junction temperatures (Tj), exposing the ball neck to high temperatures and promoting fatigue failure. Based on this, the inventors believed that by increasing the heat resistance strength of the ball neck portion, it would be possible to meet the demand for bonding wires that are bonded in an ultra-low loop shape, with a loop height that is even lower than conventional low loops, in applications where LED devices have become significantly smaller and more highly luminous in recent years.

[0029] The inventors also conducted a detailed study on the reduction in brightness of LED devices due to the shadow of the bonding wire, and discovered that by using a wire configuration that maintains high reflectivity for the blue light emitted by a blue LED, it is possible to improve the brightness of the LED device while also improving the heat resistance strength of the ball neck when using a bonding wire.

[0030] Based on these findings, the inventors have developed a silver alloy bonding wire for LEDs that improves the brightness of LED devices by providing high reflectivity for blue light, and that also improves the heat resistance of the ball neck, even in ultra-low loop bonding, thereby increasing the freedom of wiring design.

[0031] In light of the above, an object of the present invention is to provide a silver alloy bonding wire for LEDs that achieves high reflectivity for blue light while improving the heat resistance strength of the ball neck even in ultra-low loop shape bonding, and a manufacturing method thereof. [Means for solving the problem]

[0032] That is, the gist of the present invention is as follows. [1] A bonding wire for a light-emitting diode (LED) made of a silver alloy containing 98% or more by mass of silver, wherein the high-temperature / room-temperature strength ratio shown by the following formula (1) is 0.6 or more and 0.99 or less, and the elongation at break in a tensile test at 150°C is 1.5% or more. High temperature / room temperature strength ratio = (maximum load of tensile test at 150°C) / (maximum load of tensile test at room temperature) (1) [2] The LED bonding wire according to [1], wherein the silver alloy contains copper. [3] The LED bonding wire according to [1] or [2], wherein the reflectance ratio of blue light having a wavelength of 440 nm to 490 nm, as shown in the following formula (2), is 2.6 or more. Reflectance ratio = (reflectance of blue light of the LED bonding wire) / (reflectance of blue light of gold wire with a purity of 99.99% by mass) (2) [4] The LED bonding wire according to any one of [1] to [3], containing copper in a total amount of 0.1 mass % or more and 2.0 mass % or less relative to the total amount of the silver alloy. [5] The LED bonding wire according to any one of [1] to [4], containing copper in an amount of 0.3 mass % or more and 1.0 mass % or less relative to the total amount of the bonding wire for a light-emitting diode. [6] The LED bonding wire according to any one of [1] to [5], wherein the high-temperature / room-temperature strength ratio represented by the above formula (1) is 0.7 or more and 0.97 or less. [7] The LED bonding wire according to any one of [1] to [6], wherein the reflectance ratio of blue light having a wavelength of 440 nm to 490 nm, as shown in the above formula (2), is 2.7 or more.

[0033] [8] A method for manufacturing a bonding wire for a light-emitting diode (LED) made of a silver alloy, the method comprising the steps of: obtaining a silver alloy wire containing 98% or more by mass of silver from a silver alloy material; a wiredrawing step of drawing the silver alloy wire in stages, the wiredrawing step including at least one wiredrawing step with an area reduction rate of 7% or more and 30% or less; and a heat treatment step of applying heat treatment to the silver alloy wire, thereby adjusting the high-temperature / room-temperature strength ratio, as shown in the following formula (1), of the LED bonding wire to 0.6 or more and 0.99 or less, and adjusting the elongation at break in a tensile test at 150°C to 1.5% or more. High temperature / room temperature strength ratio = (maximum load of tensile test at 150°C) / (maximum load of tensile test at room temperature) (1) In this specification, the symbol "~" indicates a numerical range from the value on the left of the symbol to the value on the right of the symbol. [Effects of the Invention]

[0034] According to the present invention, it is possible to provide a silver alloy bonding wire for LEDs that achieves high reflectivity for blue light while improving the heat resistance strength of the ball neck even in ultra-low loop shape bonding, and a manufacturing method thereof. According to the present invention, it is possible to provide an LED device that can be made smaller and brighter by improving the heat resistance strength of the ball neck even in ultra-low loop shape bonding and using a silver alloy bonding wire with high reflectivity for blue light. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating a high loop shaped bonding LED device. [Figure 2] 1 is a photograph showing fatigue fracture at the wedge neck. [Figure 3] 10 is a photograph showing fatigue fracture of the ball neck portion. [Figure 4] FIG. 2 is a cross-sectional view schematically showing the vicinity of a ball neck portion. [Figure 5] 1 is a cross-sectional view schematically illustrating an LED device according to an embodiment. [Figure 6] 1 is a graph showing an SS curve (Stress-Strain curve) illustrating the relationship between stress and elongation in a tensile test, with the vertical axis representing stress σ (MPa) and the horizontal axis representing elongation ε (%). [Figure 7] FIG. 2 is a schematic diagram for explaining locations where reflectance is measured. [Figure 8] 10 is a diagram schematically showing a region R for measuring reflectance of a bonding wire 72. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0036] Next, an embodiment of the present invention will be described. The silver alloy bonding wire for LED of the embodiment is a bonding wire mainly composed of silver, which is used to connect the pad electrode and the lead electrode of an LED element. In other words, the silver alloy bonding wire for LED of the embodiment is a bonding wire for LED made of a silver alloy.

[0037] In the silver alloy bonding wire for LEDs of the embodiment, the ratio of the maximum load in a tensile test at 150°C to the maximum load in a tensile test at room temperature, i.e., the ratio expressed as (maximum load in a tensile test at 150°C) / (maximum load in a tensile test at room temperature), is 0.6 or more and 0.99 or less. This ratio expressed as (maximum load in a tensile test at 150°C) / (maximum load in a tensile test at room temperature) is hereinafter also referred to as the "high temperature / room temperature strength ratio." The maximum load in a tensile test at 150°C is also referred to as the "high temperature strength," and the maximum load in a tensile test at room temperature is also referred to as the "room temperature strength." The silver alloy bonding wire for LEDs of the embodiment has excellent heat resistance because the "high temperature / room temperature strength ratio" is in the above-mentioned range, and therefore can obtain the high heat resistance strength of the ball neck portion required for LED bonding wires used in, for example, LCD backlights.

[0038] In the silver alloy bonding wire for LEDs according to the embodiment, in order to improve the heat resistance strength of the ball neck portion, the high-temperature / room-temperature strength ratio is preferably 0.7 or more and 0.97 or less, and more preferably 0.8 or more and 0.95 or less. If the high-temperature / room-temperature strength ratio is too small, the rate of change in wire strength due to temperature change increases, and the heat resistance strength of the ball neck portion may decrease. On the other hand, if the high-temperature / room-temperature strength ratio is too large, the amount of alloy elements and impurity elements is too large, which tends to decrease the reflectivity of the wire.

[0039] In the silver alloy bonding wire for LEDs of the embodiment, in order to improve the heat resistance strength of the ball neck portion, the high temperature strength is set to 12.0 kgf / mm in terms of the value per unit cross-sectional area (stress value). 2 It is preferable that the resistance is 15.0 kgf / mm or more. 2 More preferably, it is 18.0 kgf / mm or more. 2 It is more preferable that the ratio is 29.0 kgf / mm or more. In this case, the high-temperature / normal-temperature strength ratio tends to be large, and the heat resistance strength of the ball neck portion is easily improved. The high-temperature strength is expressed as a stress value, for example, 29.0 kgf / mm 2 The following is the result.

[0040] In the silver alloy bonding wire for LEDs of the embodiment, in order to improve the heat resistance strength of the ball neck portion, the room temperature strength is set to 20.0 kgf / mm in terms of the value per unit cross-sectional area (stress value). 2 Over 30.0kgf / mm 2 It is preferable that the resistance is 21.0 kgf / mm or less. 2 Over 29.0kgf / mm 2 More preferably, it is 22.0 kgf / mm or less. 2 Over 28.0kgf / mm 2 It is more preferable that the room temperature strength is not too low, which makes it easier to improve the heat resistance strength of the ball neck portion. On the other hand, if the room temperature strength is not too high, it makes it easier to reduce the amount of additive elements, which leads to an increase in the reflectivity of the wire.

[0041] Furthermore, the silver alloy bonding wire for LEDs of this embodiment has a breaking elongation of 1.5% or more in a tensile test at 150°C (also simply referred to as "high temperature elongation"). Because the silver alloy bonding wire for LEDs of this embodiment has a high temperature elongation of 1.5% or more, strain is less likely to accumulate in the ball neck portion even in an environment where the temperature changes from high temperature (about 150°C) to low temperature (about -40°C), and high heat resistance strength can be obtained in the ball neck portion.

[0042] The elongation at break of the silver alloy bonding wire for LEDs of the embodiment in a tensile test at 150°C is preferably 1.8% or more, and more preferably 2.0% or more. The upper limit of the high-temperature elongation is usually 20%. With the high-temperature elongation in the above range, accumulation of strain in the ball neck portion due to temperature changes is small, and excellent heat resistance strength of the ball neck portion can be obtained. It is presumed that a wire having a high-temperature / room-temperature strength ratio and a high-temperature elongation in the above ranges has a metal structure in which the wire itself can withstand sudden temperature changes, as will be explained below.

[0043] The inventors performed ball bonding using the silver alloy bonding wire of the embodiment and a conventional silver alloy wire, and compared the state of the crystal grains near the ball bonding portion as follows. EBSD analysis was performed on a cross section parallel to the wire axis of the bonding wire near the ball bonding portion to obtain a crystal grain mapping image of the cross section. The obtained crystal grain mapping image was used to compare the trends in the size of the crystal grains above and below the ball neck portion. As a result, it was found that with conventional silver alloy wire, in the cross section near the ball bonding point, near the center of the wire (a region one-third of the wire diameter including the central axis of the wire), there were significant differences in the size and shape of the crystal grains above and below the ball neck. In other words, with conventional silver alloy wire, the crystal size was larger in the region below the ball neck, while in the region above the ball neck, crystal grains of different sizes and shapes were mixed. For example, in the region above the ball neck, elongated crystal grains with an aspect ratio (longest length / shortest length of the crystal grain on the image) of 5 or more were observed, and the range of the crystal grain area (maximum area - minimum area of ​​the crystal grain on the image) was large. In contrast, in the silver alloy bonding wire of the embodiment, it was found that there was little difference in the size and shape of the crystal grains above and below the ball neck, and the crystal grains were highly uniform. In the image observed above, no thin crystal grains with an aspect ratio of 5 or more were observed near the center of the wire, and the range of the crystal grain area was smaller than that of conventional silver alloy wire. This is thought to be because, by adjusting the high-temperature / room-temperature strength ratio and high-temperature elongation within the above-mentioned ranges, the strain accumulated in the metal structure of the wire due to the thermal stress caused by repeated heating and cooling is appropriately relieved by recovery or recrystallization during heating. As a result, it is easy to maintain the uniformity of the crystal size in the ball neck, and the occurrence of cracks in the ball neck due to thermal stress is suppressed or the propagation speed of the cracks is slowed.

[0044] As has been studied with conventional silver alloy bonding wires for LEDs, maintaining high reflectivity and improving the heat resistance strength of the ball neck portion have not been achieved by simply alloying silver (selecting or adding alloying elements) or optimizing the tensile strength at room temperature and high temperature. Therefore, the inventors conducted extensive research into the relationship between the strength at room temperature and the strength at high temperature for Ag alloy wires of various compositions. As a result, they discovered that when there is a specific correlation between the strength at room temperature and the strength at high temperature, and the elongation at high temperature is within a specific range, the heat resistance strength of the ball neck portion is significantly improved, even with ultra-low loop bonding.

[0045] Conventionally, in the manufacture of Ag alloy bonding wire with a wire diameter (diameter) of 18 to 25 μm, for example, a final heat treatment is performed to adjust the elongation in a tensile test to about 4% at room temperature (around 25°C). In contrast, it has been found that the maximum load and breaking load in a tensile test when a wire processed under similar conditions is heated to 150°C (high temperature) vary significantly depending on the wire composition. For example, according to experiments conducted by the inventors, in the case of an Ag-20% Au alloy with a wire diameter of 25 μm, the maximum load is about 10.5 gf at room temperature, but becomes about 8.2 gf at high temperature. In addition, in the case of an Ag-2% Pd alloy with the same wire diameter, the maximum load is about 10.0 gf at room temperature, but becomes only about 5.8 gf at high temperature. The high-temperature / room-temperature strength ratio is a numerical index of the difference in maximum load between room temperature and high temperature, and represents the rate of change in wire strength between high temperature and room temperature.

[0046] The maximum load of silver alloy bonding wire for LEDs is calculated as the maximum load at which a 100mm-long bonding wire specimen is pulled continuously at a speed of 10mm / min with a load cell rating of 2N until it breaks. To account for variability in measurement results, it is recommended to calculate the average of at least three specimens. The room-temperature maximum load is the maximum load until the wire breaks during a tensile test at room temperature. Room temperature varies depending on the season and weather, but as long as it is between 15°C and 28°C, direct heating or cooling of the wire is not necessary. The high-temperature maximum load is the maximum load until the wire breaks after the specimen is held in a furnace (49mm long heater) at 150°C (±5°C) for 10 seconds and then tensile tested in the furnace. When conducting a tensile test in a furnace, the wire should be positioned so that the clearance (gap) between the wire being tested and the furnace's soaking zone is 2mm to 3mm, to prevent the wire from contacting the soaking zone of the furnace wall. The elongation at break in a tensile test at 150°C can be calculated as the elongation at which the test piece breaks in the tensile test. It is also desirable to calculate the average of at least three specimens to account for variations in the measurement results.

[0047] The silver alloy bonding wire for LEDs of the embodiment may be composed of an Ag alloy in which alloying elements are added to Ag. One effective method for adjusting the high-temperature / room-temperature strength ratio and high-temperature elongation is to adjust the type and concentration of alloying elements in the Ag alloy. To adjust the high-temperature / room-temperature strength ratio within the above range and achieve high luminous flux in, for example, a backlight, the silver alloy bonding wire for LEDs of the embodiment is preferably composed of an Ag alloy containing copper (Cu), and preferably contains a total of 0.1% by mass to 2.0% by mass of copper relative to the total amount of the silver alloy. This provides the bonding wire with high reflectivity for blue light and improves the brightness of the LED device. The amount of Cu added is more preferably 0.3% by mass to 1.0% by mass, and even more preferably 0.4% by mass to 0.9% by mass. In particular, when the bonding wire contains 0.4% by mass or more and 0.9% by mass or less of copper (Cu), it is possible to impart high reflectivity to blue light, and also to adjust the high-temperature / room-temperature strength ratio and high-temperature elongation rate to suitable values, thereby significantly improving the heat resistance strength of the ball neck portion.

[0048] If the silver alloy bonding wire for LEDs of this embodiment has the above-mentioned composition, it can suppress sulfurization of the bonding wire, and can improve long-term reliability by suppressing a decrease in resistance to such temperature changes and stress changes and a deterioration in reflectivity.

[0049] The reflectance of the silver alloy bonding wire for LEDs of the embodiment to blue light with a wavelength of 440 nm to 490 nm is preferably 2.6 or more, more preferably 2.7 or more, and even more preferably 2.8 or more, as a ratio to the reflectance of a gold wire with a purity of 99.99% by mass (reflectance ratio, described below). By adjusting the high-temperature / room-temperature intensity ratio within the range of the embodiment and controlling the reflectance, it is possible to achieve high reflectance while efficiently improving the heat resistance strength of the ball neck portion with a small amount of added elements, and the wire can be manufactured at low cost.

[0050] The reflectance ratio of the silver alloy bonding wire for LEDs of the embodiment to that of gold wire for light with wavelengths of 440 nm to 490 nm can be measured as follows. After bonding a silver alloy bonding wire for LEDs for evaluation to a bonding evaluation substrate, the bonded wire surface is irradiated with light from a white LED lamp to measure the reflectance. The light reflected from the wire surface can be measured using a spectroradiometer or hyperspectral camera to determine the reflectance for each wavelength. Of these reflectances, the reflectance at the blue emission peak wavelength of the light source is defined as the reflectance of blue light with wavelengths of 440 nm to 490 nm. In addition, a gold wire with the same wire diameter as the silver alloy LED bonding wire to be measured and a gold purity of 99.99% by mass is prepared. After bonding the gold wire to a bonding evaluation substrate, the reflectance of blue light is measured in the same manner as above, and the reflectance at the blue emission peak wavelength of the light source is measured. From these results, the reflectance ratio is calculated using the following formula (2).

[0051] Reflectance ratio = (blue light reflectance of silver alloy bonding wire for LEDs) / (blue light reflectance of gold wire with 99.99 mass% purity) (2)

[0052] In addition, an error of ±5 nm in the wavelength of the blue light emission peak of the light source in the above reflectance measurement is acceptable. The blue light emission peak wavelength of the light source can be determined as the wavelength with the highest reflectance in the blue region when measuring the reflectance of a white board. It is also desirable to calculate the reflectance ratio by taking into account variations in the measurement results and averaging at least three samples.

[0053] In the silver alloy bonding wire for LEDs of the embodiment, the concentration of silver contained in the Ag alloy is 98% by mass or more, preferably 98.5% by mass or more, more preferably 99% by mass or more, and even more preferably 99.1% by mass or more, in order to obtain high reflectivity. Furthermore, in order to obtain high heat resistance strength of the ball neck portion, the concentration of silver is preferably 99.9% by mass or less, more preferably 99.7% by mass or less, and even more preferably 99.6% by mass or less. If the amount of silver is below the above-mentioned upper limit, sulfurization can be suppressed and long-term reliability can be improved.

[0054] The silver alloy bonding wire for LEDs of the embodiment may contain trace additive elements and inevitable impurities in addition to silver and the above-mentioned alloying element (copper). The trace additive elements are Ca, Y, La, Gd, P, Bi, Ge, Al, and Fe, and the amount thereof is preferably 100 mass ppm or less relative to the total amount of the silver alloy. The inevitable impurities are, for example, In, Pb, Si, etc., and the amount thereof, in total with the trace additive elements, is usually 100 mass ppm or less, preferably 50 mass ppm or less, relative to the total amount of the silver alloy. In order to achieve both excellent heat resistance strength and high reflectivity of the ball neck portion, the silver alloy bonding wire for LEDs of the embodiment preferably does not contain any elements other than silver, alloying element (copper), and inevitable impurities. The trace additive elements contained in silver alloy bonding wire for LEDs can be detected as the concentration of the elements contained in the entire bonding wire, for example, by analyzing a liquid obtained by dissolving the bonding wire to be evaluated in strong acid using a high-frequency inductively coupled plasma (ICP) optical emission spectrometer or an ICP mass spectrometer.

[0055] The wire diameter of the silver alloy bonding wire for LEDs of the embodiment is preferably 15 μm or more and 30 μm or less, and more preferably 18 μm or more and 25 μm or less. The thinner the bonding wire diameter, the brighter the LED device. The silver alloy bonding wire for LEDs of the embodiment has excellent heat resistance at the ball neck, even at such a thin wire, making it suitable for thinning. Therefore, it is suitable for use in LED devices that require compactness, thinness, and high luminous flux. For example, when using conventional silver alloy bonding wire, the bonding loop height (h) must be designed to be at least four to five times the wire diameter. However, by using the silver alloy bonding wire for LEDs of the embodiment, the loop height (h) can be designed to be less than three times the wire diameter. Furthermore, even with such an ultra-low loop shape, the yield rate of manufactured LED devices can be increased due to its resistance to heat and impact, which facilitates mass production and is cost-effective. Furthermore, the silver alloy bonding wire for LEDs according to the embodiment can achieve long-term reliability even when used in harsh in-vehicle environments such as car navigation equipment, drive recorders, etc. The loop height (h) is the height from the top 40 of the ball in the ball bond to the highest part 41 of the loop, as shown in Figure 4.

[0056] [Manufacturing method for silver alloy bonding wire for LEDs] Next, a method for manufacturing the silver alloy bonding wire for LED of the embodiment will be described. The method for manufacturing the silver alloy bonding wire for LED of the embodiment includes a step of preparing an Ag alloy material, a step of obtaining an Ag alloy wire from the Ag alloy material, a wiredrawing step of drawing the Ag alloy wire, and a heat treatment step of heat treating the Ag alloy wire.

[0057] (Process for producing Ag alloy material) Ag alloy materials can be obtained by melting Ag or Ag alloys of a predetermined purity together with amounts of alloying elements to achieve the desired composition. Heating furnaces such as arc heating furnaces, high-frequency heating furnaces, resistance heating furnaces, and continuous casting furnaces can be used for melting. To prevent oxygen from entering from the atmosphere, it is preferable to maintain a vacuum or an inert gas atmosphere such as argon or nitrogen above the molten silver in the heating furnace.

[0058] (Process for obtaining Ag alloy wire) The Ag alloy material melted as described above is cast and solidified from a heating furnace to a predetermined wire diameter to obtain an Ag alloy wire, or the molten Ag alloy material is cast into a mold to make an ingot, which is then drawn to a predetermined wire diameter to obtain an Ag alloy wire.

[0059] The Ag alloy wire obtained is drawn to a final wire diameter and heat-treated as necessary to produce an Ag alloy bonding wire. The drawing and heat treatment are preferably performed in stages during the drawing process.

[0060] (Wire drawing process) The drawing rate is determined depending on the final wire diameter and application of the silver alloy bonding wire for LEDs to be manufactured. The drawing rate is preferably 90% or more and 99.99% or less as the total drawing rate until the Ag alloy wire is drawn to the final wire diameter. This drawing rate can be calculated as the reduction rate of the wire cross-sectional area. The drawing process preferably uses multiple diamond dies to reduce the wire diameter in stages. In this case, the area reduction rate (processing rate) per diamond die is preferably 7% or more and 30% or less. Furthermore, since the smoothness of the wire surface affects the reflectivity, it is preferable to use a single crystal diamond die for a specified wire diameter.

[0061] (Heat treatment process) It is preferable to perform a heat treatment after drawing the Ag alloy wire to the final wire diameter. For example, it is preferable to determine the temperature and time of the heat treatment while taking into consideration the properties required for the bonding wire. In addition, the heat treatment may be performed at any stage of wire production (wire drawing process) depending on the purpose.

[0062] Heat treatment is preferably performed while running, in which the wire is passed through a heated atmosphere heated to a predetermined temperature, because the heat treatment conditions are easy to adjust. In the case of while running heat treatment, the heat treatment time can be calculated based on the wire passing speed and the distance the wire passes through the heating vessel. An electric furnace or the like is used as the heating vessel. Heat treatment is preferably performed in a tubular electric furnace under a nitrogen (N2) gas atmosphere or a gas atmosphere consisting of nitrogen (N2) gas mixed with a small amount of hydrogen (H2) gas.

[0063] As a method for manufacturing the silver alloy bonding wire for LEDs of this embodiment, for example, a wiredrawing process is performed in which the area reduction rate is 7% or more and 30% or less during the wiredrawing process, and heat treatment is performed after the wiredrawing process is completed. The heat treatment is performed at a temperature slightly lower than the heat treatment temperature for general silver alloy wire. Specifically, if the electric furnace length (soaking part) is 530 mm, the heat treatment can be performed at a temperature in the electric furnace of 200°C or more and 500°C or less, and the wire feed speed is 30 m / min or more and 80 m / min or less. This makes it possible to obtain a silver alloy bonding wire for LEDs having a high-temperature / room-temperature strength ratio and a high-temperature elongation rate within the above ranges.

[0064] [LED Devices] Next, an LED device using the silver alloy bonding wire for LED of the embodiment will be described. Fig. 5 is a cross-sectional view schematically showing an LED device 50 of the embodiment.

[0065] The LED device 50 shown in FIG. 5 includes a package substrate 59, an LED element 51 provided on the package substrate 59, a bank 56 surrounding the LED element 51, a bonding wire 53, and a pair of pad electrodes 521, 522 provided on the LED element 51. The LED device 50 also includes lead electrodes 581, 582. The bank 56 and the lead electrodes 581, 582 form a recess with the bank 56 as the side surface and the lead electrodes 581, 582 as the bottom surface. The LED element 51 is mounted on the bottom surface of the recess via an adhesive layer 57. The lead electrodes 581, 582 are made of aluminum or the like and are bonded to the package substrate 59 with silver paste or the like. The pad electrodes 521, 522 are made of gold (Au) or the like, and the pair of pad electrodes 521, 522 and the lead electrodes 581, 582 are electrically connected to each other via the bonding wire 53. The LED element 51 is covered with a sealing material 54.

[0066] The bonding wire 53 is made of the silver alloy bonding wire for LED of the above-mentioned embodiment, and the preferred aspects are also as described above. By using the silver alloy bonding wire for LED of the above-mentioned embodiment, it is possible to design the loop height (h) to be less than three times the wire diameter, and the LED device 50 can be made smaller and thinner. The LED element 51 is made of, for example, a nitride-based semiconductor (In X Al Y Ga 1-X-Y N, 0≦X, 0≦Y, X+Y≦1. The LED element 51 emits light in the wavelength range of 380 nm to 485 nm, which is the short wavelength region of visible light, and preferably has an emission peak wavelength (maximum emission wavelength) in the wavelength range of 420 nm to 485 nm, more preferably in the wavelength range of 440 nm to 480 nm.

[0067] The encapsulant 54 is, for example, a thermosetting resin such as epoxy resin, silicone resin, epoxy-modified silicone resin, or modified silicone resin. The encapsulant 54 contains a phosphor (not shown) that converts the wavelength of light from the LED element 51. Examples of phosphors include cerium-activated yttrium-aluminum-garnet phosphors (YAG:Ce); cerium-activated lutetium-aluminum-garnet phosphors (LAG:Ce); nitrogen-containing calcium aluminosilicate phosphors (CaO-AlO-SiO) activated with europium and / or chromium; europium-activated silicate phosphors ((Sr,Ba)SiO); nitride-based phosphors such as β-sialon phosphors, CASN-based phosphors, and SCASN-based phosphors; KSF-based phosphors (KSiF:Mn); sulfide-based phosphors; and quantum dot phosphors. By combining these phosphors with a blue light emitting element or an ultraviolet light emitting element, it is possible to obtain white light emission. [Example]

[0068] Next, examples will be described, but the present invention is not limited to the following examples.

[0069] As raw materials for the bonding wire, high-purity material with a purity of 99.99% or more by mass was used for the main component Ag, and raw materials with a purity of 99.99% or more by mass for the additive elements Pd, Cu, and Au, and materials with a purity of 99.99% or more by mass for the other elements were used.

[0070] An Ag alloy material containing predetermined alloying elements was prepared as described above, and an Ag alloy wire material with a wire diameter of several mm was obtained by continuous casting. The Ag alloy wire material was then subjected to wire drawing by cold die drawing and heat treatment. In the wire drawing process, the area reduction rate (processing rate) per diamond die was 7% to 30%. In the examples, the total processing rate until the final wire diameter was 90% to 99.99%, and single-crystal diamond dies were used from a predetermined wire diameter (e.g., 400 μm). In the comparative examples, the total processing rate until the final wire diameter was 90% or less or 99.999% or more, and single-crystal diamond dies were used from a predetermined wire diameter (e.g., 50 μm). The final wire diameter after wire drawing was 15 to 25 μm.

[0071] Regarding the heat treatment, the final heat treatment temperature was set to 200°C or higher and 500°C or lower in the example, and 350°C or higher and 600°C or lower in the comparative example. The heat treatment atmosphere used was nitrogen gas mixed with a small amount of hydrogen (N2+H2) or nitrogen gas (N2).

[0072] [Analysis of components contained in bonding wire] The concentration of components (other than unavoidable impurities) contained in the bonding wire can be detected as the concentration of elements contained in the entire bonding wire by analyzing a solution obtained by dissolving the bonding wire to be evaluated in strong acid using an ICP optical emission spectrometer or ICP mass spectrometer.

[0073] [Maximum load at room temperature and high temperature] The maximum load will now be described. In a tensile test, which is one of the tests for measuring the mechanical properties of a wire, increasing the force pulling the wire causes the wire to deform. The force pulling the wire in a tensile test is generally referred to as "stress," "σ," "load," "proof strength," "stress," etc. The deformation of the wire is generally referred to as "strain," "ε," "distortion," "elongation," etc. In this specification, the force pulling the wire in a tensile test is referred to as "stress" and is represented by the symbol σ, and the deformation of the wire is referred to as "elongation" and is represented by the symbol ε. A graph showing the relationship between stress and elongation in this tensile test, with stress σ (MPa) on the vertical axis and elongation ε (%) on the horizontal axis, is an SS curve (stress-strain curve). The graph in FIG. 6 shows an example of a graph of an SS curve. In this embodiment, the maximum load of the wire is the tensile force at point S1 on the SS curve graph, and the fracture elongation is the ratio of the elongation between the gauge points of the tensile test piece at point S2 to the original gauge length.

[0074] The maximum load in a tensile test can be measured using a tensile testing device (e.g., Autocom, Model AC-20CT-M, manufactured by TSE Corporation). Using a tensile testing device, a 100 mm long bonding wire is continuously pulled at a speed of 10 mm / min, a load cell rating of 2 N, and a full scale of 10% until it breaks. The maximum load is calculated as the maximum value. The load or stress here refers to the force applied to the wire being pulled at the above speed, and is usually calculated automatically by converting the pulling force into an electrical signal using a load cell. The maximum load was calculated as the average of three wires to account for variations in the measurement results.

[0075] Next, the temperature environment in this tensile test will be described. The room-temperature maximum load (room-temperature strength) of the bonding wire of the present invention is the maximum load until the wire breaks when a 100 mm long sample is tensile-tested at room temperature of 15 to 28°C, as described above. Furthermore, the high-temperature maximum load (high-temperature strength) of the bonding wire is the maximum load until the heated portion of the wire breaks when a 100 mm long sample is heated to 150°C in a furnace, held in the furnace at 150°C for 10 seconds, and then tensile-tested in the furnace. Using the room-temperature strength and high-temperature strength obtained above, the high-temperature / room-temperature strength ratio of the bonding wires of the examples and comparative examples was calculated. The high-temperature / room-temperature strength ratio was calculated by averaging three wires and then rounding down to two decimal places.

[0076] [High temperature elongation] In the tensile test for measuring the maximum load at high temperature, the elongation at which the wire broke was measured and used as the high-temperature elongation. Taking into account the variability of the measurement results, the high-temperature elongation was calculated by averaging three wires and then rounding down to one decimal place.

[0077] [Reflectance ratio] Each bonding wire for evaluation in the Examples and Comparative Examples was bonded to a bonding evaluation board, for a total of 20 wires. After bonding, the bonded wire surface was illuminated with a general-purpose white LED lamp and measured using an NH-6TND hyperspectral camera manufactured by EVA Japan Co., Ltd. to confirm the reflectance at a wavelength of 460 nm, which is the peak wavelength of blue light emitted by the general-purpose white LED lamp. Figure 7 illustrates the location where the reflectance was measured. Figure 7 schematically shows a bonding wire 72 ball-bonded onto a pad electrode 71. The location where the reflectance of the bonding wire was measured was approximately near the center of the portion of the bonding wire 72 that formed a loop parallel to the substrate (near point P in Figure 7). The area near point P was photographed using an optical microscope equipped with an NH-6TND hyperspectral camera manufactured by EVA Japan Co., Ltd., with a 150x objective lens. The measurement area was a 100 x 100 pixel area approximately in the center of the image captured using a dedicated hyperspectral camera application. Figure 8 shows a schematic diagram of the reflectivity measurement area R for the bonding wire 72. In Figures 7 and 8, the arc and double lines on the bonding wire 72 indicate that the lengths of the arc and double lines are equal. The peak wavelength of blue light emitted by the general-purpose white LED lamp was determined as the blue light wavelength with the highest reflectivity in the reflectivity measurement of a white board (standard sample). Similarly, the reflectivity of a gold wire with the same wire diameter and a gold purity of 99.99% by mass was measured, and the reflectivity ratio was calculated using the above formula (2). To account for variations in the measurement results, the reflectivity ratio was calculated by averaging the values ​​of three wires and then truncating to three decimal places. Reflectivity ratios of 2.8 or higher were evaluated as ◎ (excellent), 2.7 to less than 2.8 as ○ (good), 2.6 to less than 2.7 as △ (fair), and less than 2.6 as × (unacceptable).

[0078] [Thermal shock test of ball neck] Forty wires each for evaluation in the Examples and Comparative Examples were bonded to a bonding evaluation board with a loop height three times the wire diameter. After bonding, the bonding evaluation board was subjected to 500 cycles of thermal shock testing using a commercially available gas-phase thermal shock device, with each cycle consisting of a 15-minute hold at -40°C and a 15-minute hold at 150°C. After the 500 cycles of thermal shock testing, a pull test was performed by placing a hook approximately 5-8% of the loop length from the ball bond and pulling the hook until it broke. The inner diameter of the hook used here was 4-6 times the wire diameter, and the pull speed was a constant 250 μm / s. During the pull test, some wires broke at the hook location, but this test counted the breakage rate at the ball neck, where strength is weakened due to frequent bending operations. For each bonding wire used for evaluation in the examples and comparative examples, if the fracture rate at the ball neck was 70% or less, it was evaluated as having good resistance to the thermal shock test, with a ◎; if it was more than 70% but less than 85%, it was evaluated as having sufficient resistance, with a ○; and if it was 85% or more, it was evaluated as ×, with the risk of practical defects.

[0079] [Heat resistance strength test of ball neck] Seventy-five bonding wires for evaluation in the Examples and Comparative Examples were bonded to a bonding evaluation board with a loop height three times the wire diameter. After bonding, the bonding evaluation board was subjected to vibration testing using a vibration tester at a frequency of 30 Hz, a vibration time of 1 minute, and an amplitude twice the wire diameter, followed by heating at 150 °C for 15 minutes. This vibration and heating constitute one cycle. The pull strength of 75 ball necks was measured after 10 cycles and 20 cycles. A breakage rate of 1 g or less was determined as "breakage," and the breakage rate was calculated. For each bonding wire for evaluation in the Examples and Comparative Examples, if the breakage rate after 10 cycles was 20% or more, it was evaluated as "x" due to the risk of practical defects, and further testing was not performed. If the breakage rate after 10 cycles was less than 20%, the number of cycles was increased and the test was continued up to 20 cycles. If the fracture rate after 20 cycles was 5% or less, the heat resistance strength of the ball neck portion was deemed good and rated as ⊚; if the fracture rate after 20 cycles was 20% or less, the heat resistance strength of the ball neck portion was deemed sufficient and rated as ◯.

[0080] [comprehensive evaluation] For each bonding wire used for evaluation in the examples and comparative examples, in the overall evaluation, those with a reflectivity of ◎ or 〇 and with a result of the heat resistance strength test of the ball neck portion of ◎ or 〇 were rated as "A" because they were very good, those with a reflectivity of △ and with a result of the heat resistance strength test of the ball neck portion of ◎ or 〇 were rated as "B" because they had sufficient performance, and all other wires were rated as "C" because they failed.

[0081] The results are shown in Table 1. In the table, the heat treatment gas is represented as "N2H2" for (N2+H2) gas and "N2" for nitrogen gas. Comparative Example 1 is a silver wire with a purity of 99.99% by mass or more, and blanks for the additive elements and trace additive elements indicate that the element is not included.

[0082] [Table 1]

[0083] As shown in Table 1, silver alloy LED bonding wire containing 98% or more by mass of silver, having a high-temperature / room-temperature strength ratio of 0.6 to 0.99 and a high-temperature elongation of 1.5% or more, can achieve high blue light reflectivity while also achieving high heat resistance at the ball neck. With conventional silver alloy bonding wires, in order to maintain sufficient reflectivity for LEDs, it was necessary to design the LED loop height (h) to at least four to five times the wire diameter to prevent damage to the ball neck. In contrast, the silver alloy LED bonding wires of each example have high blue light reflectivity, allowing for high heat resistance at the ball neck while also achieving high reflectivity for LED light. Therefore, the loop height (h) can be designed to be less than three times the wire diameter, further facilitating the miniaturization and thinning of LED devices. Furthermore, the silver alloy LED bonding wires of the examples have excellent heat resistance at the ball neck, allowing for thinner wires, which is also suitable for improving the brightness of LEDs. [Industrial Applicability]

[0084] The present invention can be used for bonding LEDs, particularly for bonding LED devices that are becoming increasingly smaller and thinner.

Claims

1. A bonding wire for a light-emitting diode (LED) made of a silver alloy containing 98% by mass or more of silver, A bonding wire for LEDs characterized in that the high temperature / room temperature strength ratio represented by the following formula (1) is 0.6 or more and 0.99 or less, and the elongation at break in a tensile test at 150°C is 1.5% or more. High-temperature / room-temperature strength ratio=(maximum load of tensile test at 150°C) / (maximum load of tensile test at room temperature) (1) Here, the conditions for the tensile test when determining the high temperature / normal temperature strength ratio and the elongation at break are a sample length of 100 mm, a tensile speed of 10 mm / min, and a load cell rating of 2N.

2. The LED bonding wire of claim 1 , wherein the silver alloy includes copper.

3. The LED bonding wire according to claim 1 or 2, wherein the reflectance ratio of blue light having a wavelength of 440 nm to 490 nm, as represented by the following formula (2), is 2.6 or more. Reflectance ratio=(reflectance of blue light of the LED bonding wire) / (reflectance of blue light of gold wire with a purity of 99.99% by mass) (2)

4. The LED bonding wire according to claim 1 or 2, wherein the silver alloy contains copper in a total amount of 0.1 mass % or more and 2.0 mass % or less with respect to the total amount of the silver alloy.

5. The LED bonding wire according to claim 1 or 2, containing copper in an amount of 0.3 mass % or more and 1.0 mass % or less with respect to the total amount of the bonding wire for a light-emitting diode.

6. 5. The LED bonding wire according to claim 4, wherein the high-temperature / room-temperature strength ratio represented by the above formula (1) is 0.7 or more and 0.97 or less.

7. The LED bonding wire according to claim 4, wherein the reflectance ratio of blue light having a wavelength of 440 nm to 490 nm, as represented by the above formula (2), is 2.7 or more.

8. A method for manufacturing an LED bonding wire made of a silver alloy, comprising: a step of obtaining a silver alloy wire material containing 98 mass% or more of silver from a silver alloy material; a wiredrawing process for drawing the silver alloy wire in stages, the wiredrawing process including at least one wiredrawing process with an area reduction rate of 7% or more and 30% or less; After the heat treatment process, silver alloy wire is heat treated. The manufacturing method of the LED bonding wire, wherein the high temperature / room temperature strength ratio represented by the following formula (1) is adjusted to 0.6 or more and 0.99 or less, and the elongation at break in a tensile test at 150°C is adjusted to 1.5% or more. High-temperature / room-temperature strength ratio=(maximum load of tensile test at 150°C) / (maximum load of tensile test at room temperature) (1) Here, the conditions for the tensile test when determining the high temperature / normal temperature strength ratio and the elongation at break are a sample length of 100 mm, a tensile speed of 10 mm / min, and a load cell rating of 2N.

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