JP7761766B2 - Light emitting diode (LED) bonding wire and LED bonding wire manufacturing method - Google Patents

Description

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

Light-emitting diodes (LEDs) have become widely used 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. To distinguish between the two, packaged elements are sometimes called LEDs (or LED devices), and light-emitting elements are sometimes called LED elements. In this specification, unless otherwise specified, light-emitting elements are called "LED elements" and packaged elements are called "LED devices."

LED elements in colors such as red, orange, and yellow-green have been commercially available since the 1950s. Taking advantage of their compact size and low power consumption, they have been used in indicators, information boards, and displays for 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 LEDs offer greater energy savings than conventional lighting fixtures, they have attracted attention as a viable energy-saving solution in the lighting sector, which accounts for a large portion of Japan's electricity consumption, following the increased energy conservation awareness 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.

As white LEDs for lighting have become more widespread, 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).

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

For example, LCD panels with brightness levels of 700 cd to a maximum of 4000 cd have been developed, which are much brighter than the 300 cd required for indoor use. Because such high-brightness LCD panels offer high visibility in bright sunlight, they are used in shop windows and as display panels for outdoor advertising. With the rapid spread of smartphones and smartwatches, the range of usage environments has expanded, increasing the demand for high visibility in sunlight outdoors. For example, LCD panels with brightness levels as high as 520 cd/ ^m2 have been developed for smartphone use.

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 dashcams must be able to withstand the constant vibrations that occur while the car is in motion, as well as adapt to harsh environments such as cold and tropical climates, or even driving on asphalt under the scorching sun.

There has been a notable trend toward smaller and thinner electronic devices and information terminals equipped with LCD panels, and the LED devices used in these devices have also been getting smaller and thinner at a rapid pace in recent years. For example, in 2013, an LED device for backlighting small and medium-sized LCDs with a 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).

As such, in recent years, there has been a significant demand for LED devices for LCD backlighting that can achieve both improved brightness and smaller, thinner sizes, and that can adapt to harsh environments.

Here, the principles and configuration of white LEDs, the structure of LED devices for LCD backlights, and the problems they pose will be described.
The principles 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 to produce a visually white light. A white LED device has a structure in which the top surface of the LED element is sealed with a transparent resin (such as silicone resin or epoxy resin) containing a 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.

There are two types of LCD backlights: "direct" and "edge." "Direct" backlights are designed with multiple LED light sources positioned on 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. Because light from the light sources illuminates the entire surface of the LCD panel, realistic detail and rich color reproduction are achieved. "Edge" backlights, for example, position one or more LED light sources near the edge of the LCD panel so that light from the light sources is incident parallel to the LCD panel's surface, and use a reflective or light-guiding structure 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 brightness (high luminous flux) are important for LED backlight devices, and improved brightness contributes to high detail and visibility of LCD panels.

The structure of an LED device for LCD backlights will now be described. Figure 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 Figure 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 and the lead electrodes 81 and 82 as the bottom. 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 adhered to the package substrate 9 with silver paste or the like. The pair of pad electrodes 21 and 22 and the lead electrodes 81 and 82 are electrically connected via 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.

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 loop shape of the bonding wire is typically 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 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.

Traditionally, gold (Au) bonding wire, which has high electrical conductivity and good adhesion to electrodes, has been used to connect the pad electrodes of blue LED elements to the lead electrodes on wiring boards. However, as LED devices continue to increase in power and efficiency, bonding wire made of silver, which has a higher thermal conductivity than gold, is now being used to improve heat dissipation efficiency. Known Ag alloy bonding wires include those containing silver with predetermined amounts of 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 depending on the desired purpose and characteristics (see, for example, Patent Documents 1 to 3).

JP 2012-099577 A JP 2013-110410 A JP 2014-096403 A

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, and that can adapt 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 LED element area, increasing the number of LED elements installed in the lighting fixture, or increasing the current (forward current) that causes the LED elements to emit light.

In contrast, LED devices for LCD backlights are designed to have a lower luminous flux per LED device, at a maximum of approximately 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 improve the luminous flux of LED devices by simply increasing 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 improve the luminous flux of LED devices for LCD backlights by simply increasing the forward current.

In addition, improvements to the LED element itself to increase the luminous flux (brightness) and brightness of LED devices are reaching their limits. Therefore, in order to increase the luminous flux and brightness of LED devices for LCD backlights, measures have been taken to prevent attenuation of the light emitted from the LED element by improving the performance of peripheral components of the LED element, such as encapsulants and bonding wires.

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) of 99.99% by mass or higher has a low reflectivity of the gold (Au) element itself, resulting in the creation of a black shadow from 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, and because it is cheaper than gold, it was expected to be advantageous for realizing low-cost, bright LEDs.

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 car navigation equipment and drive recorders, which are exposed to heat and vibration in harsh operating environments, LED devices must be made smaller and thinner, as well as more resistant to impacts, requiring high levels of durability. Making LED devices smaller and thinner requires wiring in areas 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.

For this reason, while 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 mixed with alloying elements to lower the purity of the silver, a decrease in reflectivity is unavoidable. As such, it has been discovered that there is a trade-off between resistance to temperature and stress changes and high reflectivity, and this poses a problem that limits the design of white LEDs, which aim to be smaller, thinner, and brighter. The limitations imposed by the issues specific to bonding wire on expanding the applications of white LEDs, which have immeasurable potential, could even lead to disadvantages in future major technological innovations.

However, this trade-off issue cannot be resolved with conventional Ag alloy bonding wire, as described below, and places technical constraints on the development of LED devices. The negative impact of these technological constraints on the development of LED devices is immeasurable, so silver alloy bonding wire for LEDs needs to be resistant to temperature and stress changes while maintaining the brightness of white LEDs.

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 in 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.

Note that bonding wire for LEDs and bonding wire for ICs (integrated circuits) and LSIs (large-scale integrated circuits) require different characteristics. 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 typically do not contain inorganic micro-silicone fillers. This is because filling encapsulants for LED devices with inorganic micro-silicone fillers reduces the encapsulant's optical transparency, 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 stresses due to the expansion and contraction of the encapsulant, the bonding wire for LEDs is required to have a higher level of resistance to repeated temperature and stress changes (fatigue resistance) than the bonding wire for ICs.

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 devices, along with their structures, and found that there were the following differences in the defects.

First, the failures in LED devices using conventional high-loop or special-loop bonding were primarily fatigue fractures in the wedge neck portion of the silver alloy bonding wire (the portion where the wire rises due to the wedge bond with the lead electrode). Figure 2 shows a photograph of the wedge neck portion of the wire where fatigue fracture occurred.

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 of the wire where the wire rises 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 described above. In other words, as the loop became thinner, 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 in which fatigue failure occurred.

The inventors investigated the cause of fatigue failure in 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 in the ball neck, which experiences frequent bending. Furthermore, they found that stress from thermal expansion and contraction of the encapsulant due to temperature changes around the LED element is difficult to disperse in an ultra-low loop shape and tends to concentrate in the ball neck. These findings suggest that strain during bending and the concentration of thermal expansion and contraction stress of the encapsulant in the ball neck are one of the causes of fatigue failure in the ball neck. Furthermore, they found that as LED devices become smaller, heat dissipation tends to decrease and junction temperatures (Tj) tend to increase, 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 involving LED devices, which have become significantly smaller and more luminous flux-enhancing in recent years.

The inventors also conducted a detailed investigation into the reduction in brightness of LED devices due to the shadow of the bonding wire. They discovered that by using a wire configuration that maintains high reflectivity for the blue light emitted by blue LEDs, 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.

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

In light of the above, the present invention aims 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 method for manufacturing the same.

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% by mass or more of silver, wherein 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)
[2] The LED bonding wire of [1], wherein the silver alloy contains copper.
[3] The LED bonding wire according to [1] or [2], having a reflectance ratio of 2.6 or more for blue light having a wavelength of 440 nm to 490 nm, as shown in the following formula (2):
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.

[8] A method for producing a bonding wire for a light-emitting diode (LED) made of a silver alloy, the method comprising: a step of obtaining a silver alloy wire containing 98% by mass or more 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 process with an area reduction rate of 7% to 30%; and a heat treatment step of applying heat treatment to the silver alloy wire, thereby adjusting the high-temperature/room-temperature strength ratio, as expressed by the following formula (1), in the LED bonding wire to 0.6 to 0.99, 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 "to" indicates a range of values from the value on the left of the symbol to the value on the right of the symbol.

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 have improved brightness by improving the heat resistance strength of the ball neck portion even in ultra-low loop shape bonding and using a silver alloy bonding wire with high reflectivity for blue light.

FIG. 1 is a cross-sectional view schematically illustrating a high loop shaped bonding LED device. 1 is a photograph showing fatigue fracture at the wedge neck. 10 is a photograph showing fatigue fracture of the ball neck portion. FIG. 2 is a cross-sectional view schematically showing the vicinity of a ball neck portion. 1 is a cross-sectional view schematically illustrating an LED device according to an embodiment. 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 (%). FIG. 2 is a schematic diagram for explaining locations where reflectance is measured. 10 is a diagram schematically showing a region R for measuring reflectance in a bonding wire 72. FIG.

Next, an embodiment of the present invention will be described.
The silver alloy bonding wire for LED of the embodiment is a bonding wire containing silver as a main component, 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.

The silver alloy bonding wire for LEDs of this embodiment has a 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., a ratio expressed as (maximum load in a tensile test at 150°C) / (maximum load in a tensile test at room temperature), of 0.6 to 0.99. Hereinafter, this ratio expressed as (maximum load in a tensile test at 150°C) / (maximum load in a tensile test at room temperature) is 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." Because the silver alloy bonding wire for LEDs of this embodiment has a "high-temperature/room-temperature strength ratio" within the above range, it has excellent heat resistance and can achieve the high heat resistance strength of the ball neck portion required for LED bonding wire used in, for example, LCD backlights.

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/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 will be large, and the heat resistance strength of the ball neck portion may decrease. In contrast, if the high-temperature/room-temperature strength ratio is too large, the amount of alloy elements and impurity elements will be too high, which will likely reduce the reflectivity of the wire.

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 strength is preferably 12.0 kgf/ ^mm2 or more, more preferably 15.0 kgf/mm2 or ^more , and even more preferably 18.0 kgf/mm2 or more, in terms of the value per unit cross-sectional area (stress value). In this case, the high-temperature/room-temperature strength ratio tends to be large, making it easier to improve the heat resistance strength of the ball neck portion. The high-temperature strength is, for example, 29.0 kgf/ ^{mm2 or} less, in terms of stress value.

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 preferably 20.0 kgf/mm2 or ^more and 30.0 kgf/mm2 or ^less , more preferably 21.0 kgf/mm2 ^{or more} and 29.0 kgf/mm2 ^or less, and even more preferably 22.0 kgf/mm2 or ^more and 28.0 kgf/ ^mm2 or less, in terms of the value per unit cross-sectional area (stress value),. If the room temperature strength is not too small, it is easy to improve the heat resistance strength of the ball neck portion. On the other hand, if the room temperature strength is not too large, it is easy to reduce the amount of additive elements, which leads to an increase in the reflectivity of the wire.

In addition, the silver alloy bonding wire for LEDs of this embodiment has a breaking elongation in a tensile test at 150°C (also simply referred to as "high-temperature elongation") of 1.5% or more. 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 environments where the temperature changes from high temperature (approximately 150°C) to low temperature (approximately -40°C), and high heat resistance strength can be achieved in the ball neck portion.

The elongation at break in a tensile test at 150°C of the silver alloy bonding wire for LEDs of this embodiment is preferably 1.8% or more, and more preferably 2.0% or more. The upper limit of the high-temperature elongation is typically 20%. With the high-temperature elongation within the above range, there is little accumulation of strain in the ball neck portion even with temperature changes, and excellent heat resistance strength can be obtained in the ball neck portion. Wires with a high-temperature/room-temperature strength ratio and high-temperature elongation within the above ranges are presumed to have a metal structure in which the wire itself can withstand sudden temperature changes, as explained below.

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. A cross section parallel to the wire axis of the bonding wire near the ball bonding portion was subjected to EBSD analysis 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 in the conventional silver alloy wire, in the cross section near the ball bonding portion 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. That is, in the conventional silver alloy wire, the crystal size was larger in the region below the ball neck, and crystal grains of different sizes and shapes were mixed in the region above the ball neck. 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 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 of 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 portion, and the occurrence of cracks in the ball neck portion due to thermal stress is suppressed, or the propagation speed of the cracks is slowed.

As has been considered for conventional silver alloy bonding wire for LEDs, maintaining high reflectivity and improving the heat resistance strength of the ball neck portion were not achieved by simply alloying silver (selecting or adding alloying elements) or optimizing tensile strength at room temperature and high temperatures. Therefore, the inventors conducted extensive research into the relationship between the strength at room temperature and the strength at high temperatures 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 temperatures and the elongation at high temperatures is within a specific range, the heat resistance strength of the ball neck portion is significantly improved, even with ultra-low loop bonding.

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

The maximum load of silver alloy bonding wire for LEDs is calculated as the maximum load at which a 100 mm long bonding wire test piece is pulled continuously at a speed of 10 mm/min with a load cell rating of 2 N until it breaks. It is recommended to calculate the maximum load by averaging at least three samples to account for variations in measurement results. 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 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 holding the sample in a furnace (electric furnace soaking zone (heater) length: 49 mm) at a temperature of 150°C (±5°C) for 10 seconds and then conducting a tensile test 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 measured and the soaking zone of the electric furnace is 2 mm or more and 3 mm or less, to prevent the wire from coming into contact with the soaking zone of the furnace wall. The elongation at break in the 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.

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. Adjusting the type and concentration of alloying elements in the Ag alloy is one effective method for adjusting the high-temperature/room-temperature strength ratio and high-temperature elongation. To adjust the high-temperature/room-temperature strength ratio within the above range and achieve high luminous flux, for example, in 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 mass % or more and 0.9 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.

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 improve long-term reliability by suppressing a decrease in resistance to such temperature changes and stress changes and deterioration of reflectivity.

The reflectance of the silver alloy bonding wire for LEDs of the embodiment for 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 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, high reflectance can be achieved while efficiently improving the heat resistance strength of the ball neck portion with small amounts of added elements, allowing the wire to be manufactured at low cost.

The reflectance ratio of the silver alloy LED bonding wire of the embodiment to gold wire for light with wavelengths of 440 nm to 490 nm can be measured as follows. After bonding the evaluation silver alloy LED bonding wire to a bonding evaluation substrate, the bonded wire surface is irradiated with light from a white LED lamp and the reflectance is measured. 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. Additionally, 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 also measured. From these results, the reflectance ratio is calculated using the following formula (2):

Reflectance ratio = (blue light reflectance of silver alloy bonding wire for LEDs) / (blue light reflectance of gold wire with a purity of 99.99% by mass) ... (2)

Note that an error of ±5 nm is acceptable for the wavelength of the light source's blue emission peak when measuring the reflectance described above. The blue 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.

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.

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 elements (copper). Trace additive elements include Ca, Y, La, Gd, P, Bi, Ge, Al, and Fe, and their amount is preferably 100 mass ppm or less relative to the total amount of the silver alloy. Irresistible impurities include, for example, In, Pb, and Si, and their amount, in total with the above-mentioned trace additive elements, is typically 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 and high reflectivity at the ball neck portion, the silver alloy bonding wire for LEDs of the embodiment preferably does not contain any elements other than silver, alloying elements (copper), and inevitable impurities. The trace additive elements contained in the 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 solution 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.

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. A thinner bonding wire can improve the brightness of LED devices. Even with such a thin wire, the silver alloy bonding wire for LEDs of the embodiment has excellent heat resistance at the ball neck, making it suitable for thinning. This makes it suitable for use in LED devices that require compact, thin, and high luminous flux. For example, while conventional silver alloy bonding wires require a bonding loop height (h) of at least four to five times the wire diameter, the silver alloy bonding wire for LEDs of the embodiment can be designed to have a loop height (h) of less than three times the wire diameter. Furthermore, even with such an ultra-low loop shape, its resistance to heat and impact increases the yield rate of manufactured LED devices, facilitating mass production and providing cost advantages. 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 FIG.

[Method for manufacturing silver alloy bonding wire for LED]
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.

(Step of 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.

(Step of 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.

The resulting Ag alloy wire is drawn to the final wire diameter and heat treated as necessary to produce Ag alloy bonding wire. It is preferable that the drawing and heat treatment be carried out in stages during the drawing process.

(Wire drawing process)
The wiredrawing processing rate is determined depending on the final wire diameter and application of the silver alloy bonding wire for LEDs to be manufactured. The wiredrawing processing rate is preferably 90% or more and 99.99% or less as the total processing rate until the Ag alloy wire is processed to the final wire diameter. This processing rate can be calculated as the reduction rate of the wire cross-sectional area. The wiredrawing process preferably uses multiple diamond dies to gradually reduce the wire diameter. 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 from a predetermined wire diameter.

(Heat treatment process)
It is preferable to perform 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, heat treatment may be performed at any stage of wire manufacturing (wire drawing process) depending on the purpose.

For the heat treatment, a running heat treatment in which the wire is passed through a heated atmosphere heated to a predetermined temperature is preferred because the heat treatment conditions are easy to adjust. In the case of a running heat treatment, the heat treatment time can be calculated based on the wire passing speed and the wire passing distance in the heating vessel. An electric furnace or the like is used as the heating vessel. It is preferable to perform the heat treatment in a tubular electric furnace under a nitrogen ( _N2 ) gas atmosphere or a gas atmosphere in which nitrogen ( _N2 ) gas is mixed with a small amount of hydrogen ( _H2 ) gas.

In one embodiment of the present invention, the silver alloy bonding wire for LEDs can be manufactured by, for example, performing wire drawing with an area reduction rate of 7% to 30% during the wire drawing process, followed by heat treatment after the wire drawing process. The heat treatment temperature is slightly lower than that used for general silver alloy wire. Specifically, if the electric furnace length (soaking section) is 530 mm, the heat treatment can be performed at a temperature of 200°C to 500°C in the electric furnace, with a wire feed speed of 30 m/min to 80 m/min. This allows for the production of a silver alloy bonding wire for LEDs with a high-temperature/room-temperature strength ratio and high-temperature elongation within the above ranges.

[LED Device]
Next, an LED device using the silver alloy bonding wire for LEDs according to the embodiment will be described. Fig. 5 is a cross-sectional view schematically illustrating an LED device 50 according to the embodiment.

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 and the lead electrodes 581, 582 as the bottom. The LED element 51 is mounted on the bottom of the recess via an adhesive layer 57. The lead electrodes 581, 582 are made of aluminum or the like and are adhered 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 bonding wire 53. The LED element 51 is covered with a sealing material 54 .

The bonding wire 53 is made of the silver alloy bonding wire for LEDs of the above-described embodiment, and the preferred aspects are also as described above. By using the silver alloy bonding wire for LEDs of the above-described embodiment, it is possible to design the loop height (h) to be less than three times the wire diameter, allowing the LED device 50 to be manufactured in a smaller and thinner size. The LED element 51 is, for example, a blue LED element including a nitride-based semiconductor (In _x Al _y Ga _1-X-Y N, 0≦X, 0≦Y, X+Y≦1) as a light-emitting semiconductor layer. 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.

The sealing material 54 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 54 contains a phosphor (not shown) that converts the wavelength of the 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-Al ₂ O ₃ -SiO ₂ ) activated with europium and/or chromium; europium-activated silicate phosphors ((Sr,Ba) ₂ SiO ₄ ); nitride phosphors such as β-sialon phosphors, CASN phosphors, and SCASN phosphors; KSF phosphors (K ₂ SiF ₆ :Mn); sulfide phosphors, and quantum dot phosphors. White light can be obtained by combining these phosphors with a blue light-emitting element or an ultraviolet light-emitting element.

Next, examples will be described. The present invention is not limited to the following examples.

For the raw materials used in the bonding wire, the main component Ag is a high-purity material with a purity of 99.99% by mass or more, the raw materials for the additive elements Pd, Cu, and Au are also 99.99% by mass or more pure, and the other elements are all 99.999% or more pure.

An Ag alloy material containing the specified alloying elements was prepared as described above, and then continuous cast to obtain an Ag alloy wire material with a wire diameter of several millimeters. 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 up to the final wire diameter was 90% to 99.99%, and single-crystal diamond dies were used from the specified wire diameter (e.g., 400 μm). In the comparative examples, the total processing rate up to the final wire diameter was 90% or less or 99.999% or more, and single-crystal diamond dies were used from the specified wire diameter (e.g., 50 μm). The final wire diameter after wire drawing was 15 to 25 μm.

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 (N ₂ +H ₂ ) or nitrogen gas (N ₂ ).

[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.

[Maximum load at room temperature and high temperature]
The maximum load will now be described. When conducting 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,""yieldstrength,""stress," etc. The amount of 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 the vertical axis representing stress σ (MPa) and the horizontal axis representing elongation ε (%), is called an SS curve (stress-strain curve). The graph in FIG. 6 shows an example of a graph of an SS curve. The maximum load of the wire in this embodiment is the tensile force at point S1 on the SS curve graph, and the breaking elongation is the ratio of the elongation between the gauge points of the tensile test piece at point S2 to the original gauge point distance.

The maximum load in a tensile test can be measured using a tensile testing device. Using a tensile testing device (for example, Autocom, Model AC-20CT-M, manufactured by TSE Corporation), a bonding wire with a measurement length of 100 mm 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 load or stress here is 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, taking into account variations in the measurement results.

Next, we will explain the temperature environment used in this tensile test. The room-temperature maximum load (room-temperature strength) of the bonding wire of the present invention is the maximum load applied until the wire breaks when a 100 mm-long sample is tensile-tested at room temperature (15-28°C), as described above. Furthermore, the high-temperature maximum load (high-temperature strength) of the bonding wire is the maximum load applied 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 the values of three samples and then rounding down to two decimal places.

[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 defined 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.

[Reflectance ratio]
Each bonding wire for evaluation in the Examples and Comparative Examples was bonded to a bonding evaluation substrate, for a total of 20 wires. The bonded wire surface was then irradiated 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 was 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 where the bonding wire 72 formed a loop parallel to the substrate (near point P in Figure 7). The area near point P was photographed using a 150x objective lens with an optical microscope equipped with an NH-6TND hyperspectral camera manufactured by EVA Japan Co., Ltd. The measurement area was a 100-pixel x 100-pixel area approximately in the center of the image captured using a dedicated hyperspectral camera application. Figure 8 schematically shows the reflectivity measurement region R of 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 blue emission peak wavelength of the general-purpose white LED lamp was determined as the blue emission wavelength with the highest reflectivity in the reflectivity measurement of a white plate (standard sample). Similarly, the reflectivity of a gold wire with the same wire diameter as each example and a gold purity of 99.99% by mass was measured, and the reflectivity ratio was calculated using the above formula (2). Taking into account the variability of the measurement results, the reflectivity ratio was calculated by averaging three wires and then truncating to three decimal places. The reflectivity ratio was evaluated as 2.8 or higher as ◎ (excellent), 2.7 to less than 2.8 as ○ (good), 2.6 to less than 2.7 as △ (passable), and less than 2.6 as × (unacceptable).

[Thermal shock test of ball neck]
Forty 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 500 cycles of thermal shock testing using a commercially available gas-phase thermal shock device, with one cycle consisting of a 15-minute hold period at -40°C and a 15-minute hold period at 150°C. After the 500-cycle thermal shock test, a hook was placed at a position approximately 5-8% of the loop length from the ball bond, and a pull test was performed by 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. In the pull test, some wires broke at the hook placement location, but in this test, the breakage occurrence rate was counted at the ball neck, where the 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 portion 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 O, as it had sufficient resistance; and if it was 85% or more, it was evaluated as ×, as it may be defective in practical use.

[Heat resistance strength test of ball neck part]
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, and the pull strength of 75 ball neck portions was measured after 10 cycles and 20 cycles. When the pull strength was 1 g or less, it was judged as "fracture" and the fracture occurrence rate was counted. For each bonding wire for evaluation in the Examples and Comparative Examples, if the fracture occurrence 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 fracture occurrence rate after 10 cycles was less than 20%, the number of cycles was increased and the test was performed up to 20 cycles. If the fracture incidence 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 incidence rate after 20 cycles was 20% or less, the heat resistance strength of the ball neck portion was deemed sufficient and rated as ◯.

[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.

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 mass% or more, and blanks for the added elements and trace added elements indicate that the element is not included.

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 wire, maintaining sufficient reflectivity for the LED required designing 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 wire of each example exhibits high blue light reflectivity, thereby achieving high reflectivity for LED light while also achieving high heat resistance at the ball neck. 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 wire of the example exhibits excellent heat resistance at the ball neck, enabling thinner wires to be used, which is also suitable for improving the brightness of LEDs.

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

Claims (8)

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. The LED bonding wire according to claim 1, wherein the silver alloy contains copper. 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) The LED bonding wire according to claim 1 or 2, containing copper in a total amount of 0.1% by mass or more and 2.0% by mass or less relative to the total amount of the silver alloy. 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 based on the total weight of the light-emitting diode bonding wire. 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. The LED bonding wire according to claim 4, wherein the reflectance ratio for blue light with a wavelength of 440 nm to 490 nm, as expressed by the above formula (2), is 2.7 or higher. 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.