Bonding wire

Abstract

Provided is a bonding wire in which a coating layer having Au as a primary component is provided on the surface of a core material having Ag as a primary component, said bonding wire having good wire drawing processability, and being able to form an FAB that has a good shape with high sphericity.　A bonding wire W according to the present invention has a core material 10 that includes Ag as a primary component, and a coating layer 12 that is provided on the surface of the core material 10 and includes Au as a primary component, wherein the core material 10 is a bonding wire in which the total included quantity of one or two elements selected from the group consisting of Au and Pd is 0.1 mass% to 3.0 mass%, the total included quantity of one or more elements selected from the group consisting of In, Bi, and Sn is 0.0005 mass% to 0.5 mass%, and the melting point difference ΔTtma obtained by subtracting the melting point Tm1 of the core material 10 from the melting point Tm2 of the coating layer 12 is 90°C to 105°C.

Description

Bonding Wire

[0001] The present invention relates to a bonding wire in which the surface of a bonding wire mainly composed of Ag (silver) is coated with a coating layer mainly composed of Au (gold).

[0002] Bonding wires used for connecting electrodes on semiconductor elements to electrodes on substrates are generally very thin and are therefore made of metal materials that have good conductivity and excellent workability. In particular, bonding wires primarily composed of Au have been widely used because of their chemical stability and ease of handling in the atmosphere.

[0003] However, bonding wires mainly composed of Au contain 99% or more of Au by mass and are very expensive. Therefore, bonding wires mainly composed of Ag (silver) instead of Au have been proposed (for example, Patent Documents 1 and 2 listed below).

[0004] In ball bonding, before the first bonding, the tip of the bonding wire is heated and melted by a discharge current or the like, and a spherical free air ball (hereinafter abbreviated as FAB) is created by the surface tension of the molten metal.

[0005] When Ag comes into contact with oxygen in the air above its melting point, it absorbs a large amount of oxygen and releases the absorbed oxygen during solidification, a phenomenon known as spitting. In the case of bonding wires whose main component is Ag, this spitting phenomenon makes it difficult to obtain a spherical FAB when discharged in the air.

[0006] For this reason, with bonding wires whose main component is Ag, it is necessary to form the FAB in a state where oxygen is blocked by flowing an inert gas such as nitrogen toward the tip of the wire, but in order to achieve a state where oxygen is blocked, the bonding equipment becomes very complicated.

[0007] In response to this, a bonding wire has been proposed in which the surface of a core material mainly composed of Ag is covered with a coating layer mainly composed of Au (for example, Patent Document 3 listed below).

[0008] Patent No. 5616165 Patent No. 5529992 Japanese Patent Publication No. 17436 / 1986

[0009] In the bonding wire in which the surface of a core material mainly composed of Ag is coated with a coating layer as described above, Au or Pd (palladium) is sometimes added to the core material to improve wire drawing processability in order to improve wire productivity. However, when Au or Pd is added to the core material, it becomes difficult to stably form a FAB with high sphericity and a good shape when ball bonding to an electrode.

[0010] The present invention has been made in consideration of the above circumstances, and aims to provide a bonding wire having a coating layer mainly composed of Au on the surface of a core material mainly composed of Ag, which has good wiredrawability and can form a well-shaped FAB with high sphericity.

[0011] The present invention includes the embodiments shown below.

[0012] [1] A bonding wire having a core material containing Ag as a main component and a coating layer provided on the surface of the core material and containing Au as a main component, wherein the core material has a total content of one or two elements selected from the group consisting of Au and Pd of 0.1 mass% or more and 3.0 mass% or less, and a total content of one or more elements selected from the group consisting of In (indium), Bi (bismuth), and Sn (tin) of 0.0005 mass% or more and 0.5 mass% or less, and the melting point difference obtained by subtracting the melting point of the core material from the melting point of the coating layer is 90°C or more and 105°C or less.

[0013] [2] The bonding wire described in [1] above, wherein a diffusion layer having a higher Au content than the core material is provided between the core material and the coating layer, and the thickness of the diffusion layer in a region that is 50°C or more lower than the melting point of the coating layer is 0.01 μm or less.

[0014] [3] 0.2% yield strength of bonding wire is 160 N / mm ２ 230N / mm or more ２ The bonding wire according to the above [1] or [2], which is as follows:

[0015] In the bonding wire of the present invention, a coating layer mainly composed of Au is provided on the surface of a core material mainly composed of Ag, and the bonding wire can have good wiredrawability and can form a FAB with a good shape and high sphericity.

[0016] 1 is a cross-sectional view of a bonding wire according to an embodiment of the present invention; FIG. 2 is an enlarged cross-sectional view of a bonding wire showing a main part of FIG. 1;

[0017] (1) Configuration of the Bonding Wire W Hereinafter, a bonding wire W according to one embodiment of the present invention will be described with reference to the drawings. Note that the drawings may be drawn in an exaggerated manner compared to the actual state for the purpose of explanation.

[0018] The bonding wire W of this embodiment is a bonding wire for connecting electrodes (e.g., Al alloy electrodes, nickel-palladium-gold coated electrodes, Au-coated electrodes, etc.) on semiconductor elements in semiconductor devices (power ICs, LSIs, transistors, BGAs (Ball Grid Array packages), QFNs (Quad Flat Nonlead packages), LEDs (light-emitting diodes), etc.) to conductor wiring (electrodes) on circuit wiring boards (lead frames, ceramic substrates, printed circuit boards, etc.) by a ball bonding method. Note that the bonding wire W of this embodiment can be used as a bonding wire in various forms other than semiconductor devices.

[0019] As shown in FIGS. 1 and 2, the bonding wire W according to this embodiment includes a core material 10 containing Ag as a main component and a coating layer 12 containing Au as a main component.

[0020] The diameter of the core material 10 may vary depending on the application of the bonding wire. For example, the diameter (wire diameter) φ of the core material 10 may be 15 μm or more and 150 μm or less. The thickness t of the coating layer 12 may be any thickness that can cover the entire outer surface of the core material 10 so that there are no exposed portions and block oxygen. For example, the thickness t of the coating layer 12 may be 0.01 μm or more and 0.2 μm or less.

[0021] The core material 10 contains 95% by mass or more, preferably 97% by mass or more, of Ag. The Ag constituting the core material 10 may contain impurities such as Cu (copper) and Fe (iron) that are inevitably present during purification, and it is preferable to produce an Ag alloy constituting the bonding wire W using Ag with a purity of 99.9% by mass or more.

[0022] In addition to Ag, the core material 10 contains one or two elements selected from the group consisting of Au and Pd, and one or more elements selected from the group consisting of In, Bi, and Sn.

[0023] The core material 10 contains Au and Pd, which improves wiredrawability. High-purity Ag is prone to breakage when drawing a rod-shaped ingot into wire. However, if the total content of Au and Pd (the amount of Au or Pd when Au or Pd is added alone, or the total amount of Au and Pd when Au and Pd are added in combination) is 0.1% by mass or more, wire breakage is less likely to occur during wiredrawing. If the total content of Au and Pd is 3.0% by mass or less, the content of precious metals is low, which reduces the manufacturing cost of the bonding wire. Therefore, the total content of Au and Pd can be 0.1% by mass or more and 3.0% by mass or less.

[0024] By containing Au and Pd, the melting point Tm1 of the core material 10 becomes higher than the melting point of Ag. By containing 0.0005 mass% or more of one or more elements selected from In, Bi, and Sn in the core material 10, the melting point Tm1 of the core material 10 can be lowered, and a FAB with a good shape and high sphericity can be formed. Furthermore, by setting the total content of In, Bi, and Sn in the core material 10 to 0.5 mass% or less, the capillary holding the bonding wire W is less likely to be contaminated.

[0025] When the melting point Tm1 of the core material 10 is lower than the melting point Tm2 of the coating layer 12, and the melting point difference ΔTma (ΔTma = Tm2 - Tm1) obtained by subtracting the melting point Tm1 of the core material 10 from the melting point Tm2 of the coating layer 12 is within a predetermined range, a FAB with a good shape and high sphericity can be formed.

[0026] In other words, if the melting point difference ΔTma between the melting point Tm1 of the core material 10 and the melting point Tm2 of the coating layer 12 is equal to or greater than a predetermined temperature, when the tip of the bonding wire is heated before the first bonding, the coating layer 12 remains in a solid state even when the core material 10 melts, and the coating layer 12 also melts after the core material 10 melts. When the temperature of the bonding wire drops after the coating layer 12 melts, the coating layer 12 solidifies first, followed by the core material 10. This allows the core material 10 to melt and solidify while blocking oxygen with the solid-state coating layer 12. This makes it difficult for the Ag contained in the core material 10 to absorb oxygen when the core material 10 melts, allowing for the formation of a FAB with a good shape and high sphericity. On the other hand, if the melting point difference ΔTma becomes too large, the coating layer 12 becomes difficult to melt after the core material 10 melts, making it difficult to form a FAB with a good shape and high sphericity.

[0027] Specifically, the contents of In, Bi, and Sn in the core material 10 are adjusted so that the melting point difference ΔTma is 90° C. or more and 105° C. or less. When the melting point difference ΔTma is 90° C. or more and 105° C. or less, the above-mentioned effect allows the formation of a FAB with a good shape and high sphericity.

[0028] The coating layer 12 contains 95% by mass or more, preferably 99% by mass or more, of Au, with Au being the main component. The coating layer 12 may be made of pure gold (Au content of 99.9% or more) or a gold alloy containing Au and an additive element. The Au alloy constituting the coating layer 12 may contain at least one element selected from the group consisting of Ag, Pd, Bi, Pt (platinum), Ni (nickel), Co (cobalt), and Sb (antimony).

[0029] In the present invention, a diffusion layer 14 may be present between the core material 10 containing Ag as the main component and the coating layer 12 containing Au as the main component.

[0030] The diffusion layer 14 is an alloy layer containing Ag and Au formed between the core material 10 and the coating layer 12 by diffusion of the metal constituting the core material 10 and the metal constituting the coating layer 12, and contains Ag and more Au than the core material 10. The Au content of such diffusion layer 14 increases from the center (core material 10 side) to the outside (coating layer 12 side), and the melting point gradually increases.

[0031] The thickness ta of the region 14a of the diffusion layer 14, which is made of an Ag-Au alloy and has a melting point Tm3 that is 50°C or more lower than the melting point Tm2 of the coating layer 12, i.e., the melting point difference ΔTmb (ΔTmb = Tm2 - Tm3), obtained by subtracting the melting point Tm3 of the diffusion layer 14 from the melting point Tm2 of the coating layer 12, is 50°C or more, is preferably 0.01 μm or less, and more preferably 0.005 μm or less (see FIG. 2). By making the thickness of this region 14a 0.01 μm or less, it is possible to ensure the thickness of the coating layer 12 that covers the outside of the core material 10 in a solid phase during the first joining and blocks oxygen. By making the thickness of region 14a 0.005 μm or less, it is possible to ensure a greater thickness of the coating layer 12.

[0032] The bonding wire W has a 0.2% yield strength of 160 N / mm ２ 230N / mm or more ２ Preferably, it is 180 N / mm or less. ２ 200N / mm or more ２ It is more preferable that the 0.2% yield strength is 160 N / mm or less. ２ If the strength is more than 230 N / mm, wire flow, in which the bonded wire moves due to the flow of molding resin during resin molding after bonding, is unlikely to occur. ２ If the 0.2% yield strength is 180 N / mm or less, poor bonding is less likely to occur during the second bonding (when the outer peripheral surface of the bonding wire W is bonded to the electrode after the first bonding). ２ If the strength is more than 200 N / mm, wire flow is less likely to occur. ２ If it is equal to or less than this, poor bonding is less likely to occur during the second bonding.

[0033] In this specification, the chemical compositions of the core material 10 and the coating layer 12 are values ​​analyzed by ICP optical emission spectroscopy using samples sampled from ingots of the Ag alloy constituting the core material 10 and the Au alloy constituting the coating layer 12.

[0034] The melting points of the core material 10 and the coating layer 12 are values ​​measured by a thermogravimetric differential thermal analyzer (TG-DTA) using samples taken in the same manner as the chemical compositions.

[0035] The thickness t of the coating layer 12 is determined by the SiO ２ Using a converted value, it is the depth to the part where the strength of the Au on the surface of the bonding wire W is half.

[0036] The film thickness of the diffusion layer 14 is the interface width from 16% to 84% when the Auger peak intensity of Au changes from 100% to 0% based on the depth direction analysis results obtained by Auger electron spectroscopy, and the thickness ta of the region 14a in the diffusion layer 14 is the interface width from 16% to 48% when the Auger peak intensity of Au changes from 100% to 0%.

[0037] The 0.2% proof stress is the stress value at which a permanent strain of 0.2% is obtained from a stress-strain curve in accordance with the tensile test described in JIS Z2241 2011.

[0038] (2) Manufacturing Method of Bonding Wire W Next, an example of a manufacturing method of the bonding wire W having the above-described configuration will be described.

[0039] First, an element selected from Au and Pd and an element selected from In, Bi, and Sn are added to Ag having a purity of 99.9% by mass or more to cast an Ag alloy containing one or two elements selected from Au and Pd in ​​an amount of 0.1% by mass or more and 3.0% by mass or less, and one or more elements selected from In, Bi, and Sn, and having a melting point difference ΔTma between the melting point Tm2 of the coating layer 12 and the melting point Tm1 of the core material 10 of 90°C or more and 120°C or less, and then a rod-shaped ingot of a predetermined diameter is produced by a continuous casting method.

[0040] Next, the rod-shaped ingot is drawn to reduce its diameter to a predetermined diameter of the core material 10. Thereafter, a coating layer 12 containing Au is formed on the entire outer periphery of the core material 10. The coating layer 12 can be formed by known means such as electroplating, electroless plating, or vapor deposition.

[0041] The core material 10 on which the coating layer 12 is formed is then further drawn to reduce its diameter to a predetermined value, thereby obtaining the bonding wire W having the above-described configuration.

[0042] The 0.2% yield strength of the bonding wire W is 160 N / mm ２ 230N / mm or more ２ In order to impart appropriate mechanical properties to the bonding wire W, the bonding wire W may be subjected to heat treatment during or after the wiredrawing process, such as as follows. As an example of the heat treatment, continuous annealing treatment can be performed at 300 to 1000°C for 0.1 to 60 seconds in an air atmosphere, a nitrogen gas atmosphere, or a mixed gas atmosphere of hydrogen and nitrogen.

[0043] (3) Effect In the bonding wire W of this embodiment, the core material 10, which is mainly composed of Ag, contains 0.1 mass % or more and 3.0 mass % or less of one or two elements selected from the group consisting of Au and Pd, so that the wire drawing processability is good.

[0044] The bonding wire W of this embodiment contains one or more elements selected from the group consisting of In, Bi, and Sn, and the melting point difference ΔTma between the melting point Tm2 of the coating layer 12 and the melting point Tm1 of the core material 10 is 90° C. or more and 120° C. or less. As a result, during the first bonding, the core material 10 can be melted and solidified while blocking oxygen by the solid-phase coating layer 12, so that Ag contained in the core material 10 is less likely to take in oxygen when the core material 10 melts, and a FAB with a good shape and high sphericity can be formed.

[0045] Furthermore, in this embodiment, when a diffusion layer 14 is provided between the core material 10 and the coating layer 12, by setting the thickness ta of the region 14a of the diffusion layer 14, whose melting point Tm3 is 50° C. or more lower than the melting point Tm2 of the coating layer 12, to 0.01 μm or less, the core material 10 is more likely to melt and solidify while blocking oxygen due to the solid-phase coating layer 12 during the first bonding. In other words, the region 14a is melted even at a temperature that is lower than the coating layer 12 by a certain amount, and at which the coating layer 12 does not melt. By setting the thickness of such region 14a to 0.01 μm or less, it is possible to ensure a thickness of the coating layer 12 that can cover the outside of the core material 10 in a solid-phase state and block oxygen during the first bonding.

[0046] In this embodiment, the 0.2% yield strength of the bonding wire W at room temperature is 160 N / mm ２ 230N / mm or more ２ If it is below this value, it is possible to prevent defects from occurring during the second bonding, and also to prevent the bonded wire from moving due to the flow of molding resin during resin molding after bonding.

[0047] Although the embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the inventions described in the claims and their equivalents.

[0048] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0049] An Ag alloy having the chemical composition shown in Table 1 below was blended using an Ag raw material with a purity of 99.9% by mass or more, and an ingot was produced by continuous casting. The ingot was then subjected to wire drawing to reduce the diameter of the core material 10 to 150 μm, and then electroplating was used to form a coating layer 12 made of pure gold having an Au content of 99.9% or more on the outer surface of the core material 10. Thereafter, the core material 10 on which the coating layer 12 was formed was further wire drawn to reduce the diameter to the wire diameter shown in Table 1 below, and then subjected to continuous annealing (heat treatment) at 500 to 700 ° C. for 0.5 seconds in a 100% nitrogen gas atmosphere to obtain the bonding wires of Examples 1 to 16 and Comparative Examples 1 to 10.

[0050] Then, for the bonding wires of Examples 1 to 16 and Comparative Examples 1 to 10, the melting point Tm1 of the core material 10, the melting point Tm2 of the coating layer 12, the film thickness t of the coating layer 12, and the thickness ta of the region 14a formed in the diffusion layer 14 were measured using the above-mentioned measurement method, and the melting point difference ΔTma was calculated from the measurement results of the melting point Tm1 and the melting point Tm2.

[0051] For the bonding wires of Examples 1 to 16 and Comparative Examples 1 to 10, the melting point Tm1 of the core material 10, the film thickness t of the coating layer 12, the melting point Tm2 of the coating layer 12, the melting point difference ΔTma, the thickness ta of the region 14a formed in the diffusion layer 14, the diameter φ of the bonding wire, and the 0.2% yield strength P are as shown in Table 1 below.

[0052] The resulting bonding wires of Examples 1 to 16 and Comparative Examples 1 to 10 were evaluated in the following (1) and (6). The specific evaluation methods are as follows.

[0053] (1) Wire productivity (wire drawing processability) A 100 μm diameter wire was repeatedly subjected to a continuous wire drawing process using 15 to 20 wire drawing dies with an area reduction rate of 8 to 12% for each die, to produce a bonding wire of 50 km length up to the diameter shown in Table 1. If wire breakage occurred 0 or 1 time during the wire drawing process, it was rated as "A", and if wire breakage occurred 2 or more times, it was rated as "D".

[0054] (2) FAB Sphericity FABs with diameters 1.9 to 2.1 times the wire diameter were produced in an air atmosphere using a wire bonder (Shinkawa Co., Ltd., UTC-5000NeoCu), and the sphericity of the produced FABs was evaluated. To evaluate the sphericity of the FABs, 100 FABs were produced for each bonding wire of the Examples and Comparative Examples, and then their appearance was observed using a general-purpose electron microscope (JEOL Ltd., JSM-6510LA). The lengths of the produced FABs in the parallel and perpendicular directions were measured. If the average value of the ratio (X / Y) of the FAB's length X in the parallel direction to its length Y in the perpendicular direction was within 100±5%, it was judged to have "sphericity" and rated "A." If it fell within the range of 90%≦(X / Y)<92% or 108%<(X / Y)≦110%, it was rated "B." If it fell outside of these ranges, it was judged to have "no sphericity" and rated "D."

[0055] (3) Contamination of the capillary When significant contamination was observed at the tip of the capillary after 20,000 bonding cycles, the capillary was rated as "D," and when bonding could be continued without any particular problems, the capillary was rated as "A."

[0056] (4) Continuous Bonding Ability: Using the wire bonder used in (2) above, 30,000 cycles of bonding were performed on a silver-plated copper alloy frame, with the following steps being performed in sequence: forming a FAB, pressing the formed FAB against an electrode for the first bonding, pressing the outer surface of the bonding wire against the other electrode for the second bonding, and tearing off the bonding wire for the tail cutting. The wire bonder was rated "A" if it did not stop during bonding; "B" if it stopped once due to peeling of the second bond; and "D" if it stopped two or more times.

[0057] (5) Wire flow during resin molding After a bonding sample with a wire length of 5 mm was sealed with epoxy resin, the maximum amount of wire flow was measured using an X-ray nondestructive observation device. Measurements were made for 20 samples, and the average value of the measured values ​​divided by the wire length of 5 mm was taken as the wire flow rate. If this wire flow rate was less than 7%, it was given an "A" rating, and if it was 7% or more, it was considered to be problematic in practical use and was given an evaluation of "D."

[0058] (6) Overall Evaluation If all of the above evaluations (1) to (5) were "A", the overall evaluation was "A", if even one was "B", the overall evaluation was "B", and if even one was "D", the overall evaluation was "D".

[0059] The results are shown in Table 2, and Examples 1 to 16 obtained good results in all of the evaluations (1) to (5) above.

[0060] On the other hand, in Comparative Examples 2 and 3 in which the total content of Au and Pd was less than 0.1 mass %, the wire drawability deteriorated.

[0061] In Comparative Examples 1, 4, 5, and 9, in which the melting point difference ΔTma between the melting point Tm1 of the core material 10 and the melting point Tm2 of the coating layer 12 was less than 90° C., it was difficult to form highly spherical FABs, and the sphericity of the FABs deteriorated. Also in Comparative Example 8, in which the melting point difference ΔTma exceeded 105° C., it was difficult to form highly spherical FABs, and the sphericity of the FABs deteriorated.

[0062] In addition, in Comparative Examples 6, 7, 8 and 10, in which the total content of one or more elements selected from the group consisting of In, Bi and Sn exceeded 0.5 mass%, the capillary holding the bonding wire was prone to contamination.

[0063] 10...core material, 12...coating layer, 14...diffusion layer, 14a...region, W...bonding wire

Claims

1. A bonding wire having a core material containing Ag as its main component and a coating layer provided on the surface of the core material and containing Au as its main component, wherein the core material contains one or two elements selected from the group consisting of Au and Pd in ​​a total content of 0.1% by mass or more and 3.0% by mass or less, and one or more elements selected from the group consisting of In, Bi and Sn in a total content of 0.0005% by mass or more and 0.5% by mass or less, and the melting point difference obtained by subtracting the melting point of the core material from the melting point of the coating layer is 90°C or more and 105°C or less.

2. A bonding wire as described in claim 1, wherein a diffusion layer having a higher Au content than the core material is provided between the core material and the coating layer, and the thickness of the diffusion layer in a region that is 50°C or more lower than the melting point of the coating layer is 0.01 μm or less.

3. The 0.2% yield strength of the bonding wire is 160 N / mm ２ 230N / mm or more ２ The bonding wire according to claim 1 or 2, wherein:

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