Bonding Wire

Abstract

The present invention provides a bonding wire in which a coating layer mainly composed of Au is provided on the surface of a core material mainly composed of Ag, and the bonding wire has good wiredrawability and can be used to form a FAB with a good shape and high sphericity. The bonding wire W of the present invention is a bonding wire W having a core material 10 containing Ag as its main component and a coating layer 12 provided on the surface of the core material 10 and containing Au as its main component, wherein the core material 10 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, Bi, and Sn of 0.0005 mass% or more and 0.5 mass% or less, and the melting point difference ΔTma 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 or more and 105°C or less.

Description

[Technical Field]

[0001] The present invention relates to a bonding wire in which the surface of a bonding wire containing Ag (silver) as a main component is coated with a coating layer containing Au (gold) as a main component. [Background technology]

[0002] Bonding wires used to connect electrodes on semiconductor elements to electrodes on substrates are generally very thin and are made from metal materials that have good conductivity and are easy to process. In particular, bonding wires primarily composed of Au have been widely used due to their chemical stability and ease of handling in air.

[0003] However, bonding wires containing Au as the main component are very expensive because 99% or more of their mass is Au. Therefore, bonding wires containing Ag (silver) as the main component 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 using 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 wire whose main component is Ag, this spitting phenomenon makes it difficult to obtain a spherical FAB when discharging in the air.

[0006] For this reason, when using bonding wire 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 complex.

[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). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 5616165 [Patent Document 2] Patent No. 5529992 [Patent Document 3] JP-A-64-17436 Summary of the Invention [Problem to be solved by the invention]

[0009] In the bonding wire described above, in which the surface of a core material primarily composed of Ag is coated with a coating layer, Au or Pd (palladium) is sometimes added to the core material to improve wire drawability in order to improve wire productivity. However, adding Au or Pd to the core material makes it difficult to consistently form a well-shaped FAB with high sphericity 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. [Means for solving the problem]

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

[0012] [1] A bonding wire having a core material containing Ag as its main component and a coating layer formed 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 (indium), Bi (bismuth), and Sn (tin) 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.

[0013] [2] A bonding wire as described in [1] above, which has a diffusion layer between the core material and the coating layer that has a higher Au content than the core material, 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] Bonding wire 0.2% yield strength is 160N / mm 2 More than 230N / mm 2 The bonding wire according to the above [1] or [2], which is: [Effects of the Invention]

[0015] The bonding wire of the present invention is a bonding wire having a coating layer mainly composed of Au on the surface of a core material mainly composed of Ag, which can improve wire drawing workability and form a FAB with a good shape and high sphericity. [Brief explanation of the drawings]

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

[0017] (1) Composition of 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 exaggerated in some cases 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 ball bonding. 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 no exposed portions are present 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 refining, 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] The core material 10 contains, in addition to Ag, 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 a rod-shaped ingot is drawn 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 including Au and Pd in the core material 10, the melting point Tm1 of the core material 10 becomes higher than the melting point of Ag. By including 0.0005% by 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, allowing for the formation of a FAB with a good shape and high sphericity. Furthermore, by setting the total content of In, Bi, and Sn in the core material 10 to 0.5% by 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 melts later than the melting of the core material 10. 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 will not melt easily 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 for 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 may be made of a gold alloy in which an additive element is added to Au. 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] Diffusion layer 14 is an alloy layer containing Ag and Au formed between core material 10 and coating layer 12 by diffusion of the metal constituting core material 10 and the metal constituting coating layer 12, and contains Ag and more Au than core material 10. In such diffusion layer 14, the Au content 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 region 14a where 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 setting the thickness of the region 14a to 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 setting the thickness of the region 14a to 0.005 μm or less, it is possible to ensure an even greater thickness of the coating layer 12.

[0032] In addition, the bonding wire W has a 0.2% yield strength of 160N / mm 2 More than 230N / mm 2 It is preferable that the resistance is less than 180N / mm 2 More than 200N / mm 2 It is more preferable that the 0.2% yield strength is 160 N / mm or less. 2 If this is the case, wire flow, in which the bonded wire moves due to the flow of molding resin during resin molding after bonding, is unlikely to occur, and the wire flow is 230 N / mm 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 circumferential surface of the bonding wire W is bonded to the electrode after the first bonding). 2 If the strength is above 200N / 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 the depth to the portion where the intensity is half the Au intensity on the surface of the bonding wire W, using the SiO2 equivalent value of the depth profile analysis obtained by Auger electron spectroscopy.

[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 above region 14a in the diffusion layer 14 is the interface width from 16% to 48% when the Auger peak intensity of Au changes from 16% to 48%.

[0037] The 0.2% yield strength is the stress value at which a permanent strain of 0.2% is reached on a stress-strain curve in accordance with the tensile test specified in JIS Z2241 2011.

[0038] (2) Manufacturing method of bonding wire W Next, an example of a method for manufacturing the bonding wire W having the above 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 with 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] Then, the core material 10 on which the coating layer 12 is formed is 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 160N / mm 2 More than 230N / mm 2 The bonding wire W may be subjected to heat treatment during or after the wiredrawing process in order to impart appropriate mechanical properties to the bonding wire W, 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) Effects 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, thereby providing good wiredrawability.

[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, where the 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 coating layer 12 in its solid phase during the first bonding. In other words, the region 14a melts even at a temperature that is lower than the coating layer 12 by a certain amount, 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 that the coating layer 12 has a thickness that can cover the outside of the core material 10 in its solid phase 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 More than 230N / mm 2 If it is below this, 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. [Example]

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

[0049] An Ag raw material with a purity of 99.9% by mass or more was blended with an Ag alloy having the chemical composition shown in Table 1 below, and an ingot was produced by continuous casting. The ingot was then subjected to wire drawing to reduce the diameter to a core material 10 having a diameter of 150 μm, and then a coating layer 12 made of pure gold with an Au content of 99.9% or more was formed on the outer circumferential surface of the core material 10 by electroplating. Thereafter, the core material 10 with the coating layer 12 formed thereon 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, thereby obtaining 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] 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 of the bonding wires of Examples 1 to 16 and Comparative Examples 1 to 10 are as shown in Table 1 below.

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

[0053] (1) Wire productivity (wire drawing processability) Bonding wires with a diameter of 100 μm were produced by repeatedly performing 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 the diameter shown in Table 1. If there was no or one break during the wire drawing process, it was rated as "A", and if there were two or more breaks, it was rated as "D".

[0054] (2)FAB sphericity Using a wire bonder (Shinkawa Co., Ltd., UTC-5000NeoCu), fabricated fabricated aluminum balls (FABs) with diameters 1.9 to 2.1 times the wire diameter in an air atmosphere, and the fabricated fabricated aluminum balls were evaluated for sphericity. To evaluate the sphericity of the fabricated aluminum balls, 100 fabricated aluminum balls were fabricated for each bonding wire of the examples and comparative examples. Then, their appearances were observed using a general-purpose electron microscope (JEOL Ltd., JSM-6510LA), and the lengths of the fabricated aluminum balls in the parallel and perpendicular directions were measured. If the average ratio (X / Y) of the fabricated aluminum balls' length in the parallel direction (X) to their length in the perpendicular direction (Y) was within 100±5%, the balls were judged to have "sphericity" and rated "A." If the average ratio was within the range of 90%≦(X / Y)<92% or 108%<(X / Y)≦110%, the balls were judged to have "no sphericity" and rated "D."

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

[0056] (4) Continuous bonding 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 fabricated bond (FAB), pressing the formed FAB against the electrode (1st bonding), pressing the outer surface of the bonding wire against the other electrode (2nd bonding), and tearing off the bonding wire (tail cutting). If the wire bonder (equipment) did not stop during bonding, it was given an "A," if the equipment stopped once due to peeling of the 2nd bond, it was given a "B," and if it stopped two or more times, it was given a "D."

[0057] (5) Wire flow during resin molding After sealing a bonding sample with a wire length of 5 mm with epoxy resin, the maximum amount of wire flow was measured using an X-ray non-destructive observation device. Measurements were made on 20 samples, and the average measured value 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", but if it was 7% or more, it was considered problematic for practical use and was given a rating of "D".

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

[0059] [Table 2] 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, highly spherical FABs were difficult to form, and the FAB sphericity deteriorated. Also in Comparative Example 8, in which the melting point difference ΔTma exceeded 105°C, highly spherical FABs were difficult to form, and the FAB sphericity 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. [Explanation of symbols]

[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 a main component and a coating layer formed on the surface of the core material and containing Au as a main component, 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, A bonding wire in which 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 diffusion layer having a higher Au content than the core material is provided between the core material and the coating layer; The bonding wire according to claim 1 , wherein the diffusion layer has a thickness of 0.01 μm or less in a region where the melting point is 50° C. or more lower than the melting point of the coating layer.

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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