Bonding wire

Abstract

Provided is a bonding wire in which a coating layer having Au as a main component is provided on the surface of a core material having Ag as a main component, and which is capable of forming an FAB having a good shape and a high sphericity degree. This bonding wire (W) has a core material (10) containing Ag as a main component, and a coating layer (12) provided on the surface of the core material (10) and containing Au as a main component, the core material (10) containing one or more elements selected from Au, Pd, and In, and the ratio of the film thickness of the coating layer (12) to the diameter of the core material (10) is 0.0014-0.0048. The ratio of the intrinsic resistance of the core material (10) to the intrinsic resistance of the coating layer (12) is 85%-115%.

Description

Technical Field

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

[0002] Bonding wires used for connecting electrodes of semiconductor elements to electrodes of substrates, etc. are generally very thin, and thus are made of a metal material having good conductivity and excellent workability. In particular, from the viewpoints of chemical stability and ease of operation in the atmosphere, bonding wires mainly composed of Au have been widely used.

[0003] However, more than 99% of the mass of a bonding wire mainly composed of Au is Au, which is very expensive. Therefore, a bonding wire having Ag as a main component instead of Au has been proposed (for example, Patent Documents 1 to 2 below).

[0004] Although Ag is cheaper than Au, the surface of a bonding wire mainly composed of Ag is more likely to be sulfided than that of a bonding wire mainly composed of Au, and poor bonding with electrodes is likely to occur. In order to suppress poor bonding caused by sulfidation of Ag, a bonding wire in which a surface of a core material mainly composed of Ag is coated with a coating layer mainly composed of Au has been proposed (for example, Patent Documents 3 to 5 below).

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent No. 5616165

[0008] Patent Document 2: Japanese Patent No. 5529992

[0009] Patent Document 3: Japanese Patent Application Laid-Open No. 2016-517623

[0010] Patent Document 4: International Publication WO2013 / 129253

[0011] Patent Document 5: International Publication WO2021 / 205674 Summary of the Invention

[0012] If a coating layer mainly composed of Au is provided on the surface of a core material mainly composed of Ag, it is difficult to stably form a good-shaped gold ball (hereinafter, simply referred to as FAB) having a high sphericity during ball bonding to an electrode, or the shape of the 1st bonding portion formed when the FAB is brought into contact with and crushed against the electrode is difficult to become a good circle.

[0013] The present invention has been completed in view of the above circumstances, and an object thereof is to provide 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 a good-shaped FAB with high sphericity can be formed, and at the same time, the shape of the first bonding portion can be made into a good circle.

[0014] In order to solve the above problems, the bonding wire of the present invention is a bonding wire having a core material containing Ag as a main component and a coating layer containing Au as a main component provided on the surface of the core material, the core material contains one or more selected from Au, Pd (palladium), and In (indium), the ratio of the film thickness of the coating layer to the diameter of the core material is 0.0014 to 0.0048, and the ratio of the specific resistance of the core material to the specific resistance of the coating layer is 85% to 115%.

[0015] In the bonding wire of the present invention, the core material may contain at least one of Au and Pd.

[0016] In the bonding wire of the present invention, the core material contains Au and Pd, and the content of Au may be more than that of Pd.

[0017] In the bonding wire of the present invention, the core material may contain 1 mass ppm to 50 mass ppm of Bi (bismuth).

[0018] In the bonding wire of the present invention, the core material may contain 0.1 mass% to 1.0 mass% of In.

[0019] In the bonding wire of the present invention, in 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, a good-shaped FAB with high sphericity can be formed, and at the same time, the shape of the first bonding portion can be made into a good circle. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a cross-sectional view of a bonding wire according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] (1) Configuration of bonding wire W

[0022] Hereinafter, a bonding wire W according to an embodiment of the present invention will be described with reference to the drawings.

[0023] The bonding wire W of the present embodiment is a bonding wire for connecting an electrode (e.g., an Al alloy electrode, a nickel-palladium-gold coated electrode, an Au coated electrode, etc.) on a semiconductor element in a semiconductor device (power IC, LSI, transistor, BGA (Ball Grid Array package), QFN (Quad Flat Non lead package), LED (light emitting diode), etc.) to a conductor wiring (electrode) of a circuit wiring substrate (lead frame, ceramic substrate, printed circuit substrate, etc.) 2 by a ball welding method. It should be noted that the bonding wire W of the present embodiment can be used as a bonding wire in various ways in addition to being used for semiconductor devices.

[0024] As Figure 1 shown, the bonding wire W of the present embodiment includes a core material 10 mainly composed of Ag and a coating layer 12 mainly composed of Au, and the coating layer 12 covers the entire outer peripheral surface of the core material 10 without an exposed portion.

[0025] Specifically, the core material 10 contains 90% by mass or more, preferably 95% by mass or more of Ag. The Ag constituting the core material 10 may contain impurities such as Fe (iron) inevitably present in purification, and Ag alloy with a purity of 99.9% by mass or more is preferably used to produce the Ag alloy constituting the bonding wire W.

[0026] In addition to containing Ag, the core material 10 contains one or more selected from Au, Pd, and In, and the intrinsic resistance ρ1 of the core material 10 is adjusted. That is, the contents of Au, Pd, and In in the core material 10 are adjusted so that the ratio β (=ρ1 / ρ2) of the intrinsic resistance ρ1 of the core material 10 to the intrinsic resistance ρ2 of the coating layer 12 is 85% to 115%, preferably 90% to 110%.

[0027] If an example of the intrinsic resistance of the core material 10 and the coating layer 12 is given, the intrinsic resistance of the core material 10 can be 1.95 μΩ·cm to 3.26 μΩ·cm, and the intrinsic resistance of the coating layer 12 can be 2.30 μΩ·cm to 2.83 μΩ·cm.

[0028] When the intrinsic resistance of the coating layer 12 is 2.3 μΩ·cm, the core material 10 contains only one of Au, Pd, and In. In order to make the ratio β of the intrinsic resistance ρ1 of the core material 10 to the intrinsic resistance ρ2 of the coating layer 12 be 85% to 115%, it can contain 1.3% by mass to 4.0% by mass of Au, can contain 0.7% by mass to 2.1% by mass of Pd, and can contain 0.2% by mass to 0.6% by mass of In.

[0029] In addition, the core material 10 and the film thickness t of the coating layer 12 are set such that the ratio α (= t / φ) of the film thickness t of the coating layer 12 to the diameter φ of the core material 10 is 0.0014 to 0.0048. The diameter φ of the core material 10 can be set to various sizes according to the use of the bonding wire. For example, the diameter (wire diameter) φ of the core material 10 can be 15 μm to 150 μm. The film thickness t of the coating layer 12 is preferably 0.15 μm or more and preferably 0.50 μm or less.

[0030] It should be noted that regarding the specific resistance of the core material 10, three evaluation specimens are fabricated, and the specific resistance of each of the three evaluation specimens is measured at room temperature using the four-terminal method, and the average value of the measurement results is taken as the specific resistance of the core material 10. In addition, regarding the specific resistance of the coating layer 12, three evaluation specimens are fabricated from a bonding wire made of the same alloy composition as the coating layer 12, and the specific resistance of each of the three evaluation specimens is measured at room temperature in the same manner as for the core material 10 using the four-terminal method, and the average value of the measurement results is taken as the specific resistance of the core material 10.

[0031] In addition, the diameter φ of the core material 10 and the film thickness t of the coating layer 12 are obtained by converting the intensity of the fluorescent X-ray generated by irradiating the X-ray with a fluorescent X-ray film thickness gauge into the film thickness of the coating layer.

[0032] Regarding the bonding wire W, it is preferable that the core material 10 contains Au, and it is also preferable to contain Pd together with Au. When the core material 10 contains both Au and Pd, it is preferable that the content of Au is greater than that of Pd. When the core material 10 contains both Au and Pd, it is preferable that the total content of Au and Pd is 0.6 mass% to 3.0 mass%.

[0033] The core material 10 may contain 0.1 mass% to 1.0 mass% of In. If the core material 10 contains 0.1 mass% or more of In, the corrosion resistance of the 1st joint portion formed when pressing the FAB against the electrode and crushing it is improved, and the long-term reliability can be improved. If the content of In in the core material 10 is 1.0 mass% or less, the specific resistance of the wire is maintained within an appropriate range.

[0034] In addition, in addition to containing one or more selected from Au, Pd, and In, the core material 10 may contain 1 mass ppm to 50 mass ppm of Bi.

[0035] It should be noted that in the present invention, in addition to the above-mentioned Au, Pd, Bi, and In, elements selected from Ca and Cu, one or two kinds, can be further added. If the total content of Ca and Cu is 100 mass ppm or less, the above-mentioned effects achieved by adding Au, Pd, Bi, and In will not be impaired, and the strength of the wire can be improved. When Ca and Cu are added, the total content of Ca and Cu is preferably 1 mass ppm to 100 mass ppm, and more preferably 5 mass ppm to 85 mass ppm.

[0036] The coating layer 12 contains 95 mass% or more, preferably 99 mass% or more, of gold and contains gold as the main component. The coating layer 12 can be composed of pure gold (the content of gold is 99.9% or more), or can also be composed of a gold alloy to which additive elements are added to gold. The gold alloy constituting the coating layer 12 can contain at least one element selected from Ag, Pd, Bi, Pt (platinum), Ni (nickel), Co (cobalt), and Sb (antimony).

[0037] (2) Manufacturing method of the bonding wire W

[0038] Next, an example of the manufacturing method of the bonding wire W configured as above will be described.

[0039] First, elements selected from Au, Pd, and In are added to Ag with a purity of 99.9 mass% or more, and Bi, Ca, and Cu are added as needed and contain 95 mass% or more of Ag, thereby casting an Ag alloy having an intrinsic resistance of 85% to 115% of the intrinsic resistance ρ2 of the coating layer 12, and then a rod-shaped ingot with a specified diameter is produced by the continuous casting method.

[0040] Next, the rod-shaped ingot is drawn, and the diameter is reduced to become a core material 10 with a specified diameter. Then, a coating layer 12 containing Au is formed on the entire outer circumference of the core material 10. The coating layer 12 can be formed by a known method such as electroplating, electroless plating, or vapor deposition.

[0041] Then, the core material 10 formed with the coating layer 12 is further drawn, and the diameter is reduced to reach the specified diameter. Thus, a bonding wire W can be obtained in which the ratio α of the thickness t of the coating layer 12 to the diameter φ of the core material 10 is 0.0014 to 0.0048, and the ratio β of the intrinsic resistance ρ1 of the core material 10 to the intrinsic resistance ρ2 of the coating layer 12 is 85% to 115%.

[0042] It should be noted that the bonding wire W can be heat-treated as needed during or after the wire drawing process. If an example of the heat treatment is given, a continuous annealing treatment is performed at 300 to 1000 °C for 0.1 to 60 seconds in an air atmosphere, a nitrogen atmosphere, or a mixed gas atmosphere of 5% hydrogen and 95% nitrogen.

[0043] (3) Effect

[0044] In the bonding wire W of the present embodiment, a coating layer 12 mainly composed of gold is provided on the outer peripheral surface of the core material 10. Therefore, the corrosion resistance of the bonding wire W can be improved, and at the same time, good bonding strength can be obtained for metals such as Al (aluminum), Au, Ag, and their alloys that form electrodes.

[0045] In the bonding wire W, by making the film thickness t of the coating layer 12 be 0.0014 times or more of the wire diameter φ of the core material 10, the corrosion resistance performance of the bonding wire W can be exerted, ball welding can be performed in the atmosphere, and at the same time, good bonding strength can be obtained for the metals that form electrodes. In addition, by making the film thickness t of the coating layer 12 be 0.0048 times or less of the wire diameter φ of the core material 10, the balance of the surface tension of the bonding wire W dissolved when forming the FAB can be well maintained, and a good-shaped FAB with high sphericity can be formed.

[0046] In the bonding wire W, by making the ratio of the specific resistance ρ1 of the core material 10 to the specific resistance ρ2 of the coating layer 12 be 85% - 115%, when the tip of the bonding wire W is heated by discharge, the discharge current flows uniformly through the tip portion of the bonding wire W, and the tip portion of the bonding wire W dissolves from any direction, and the FAB grows uniformly. Therefore, a good-shaped FAB with high sphericity can be formed, and at the same time, the shape of the 1st bonding portion can be a good circle.

[0047] In the bonding wire W, the core material 10 contains Au. Thus, when forming the FAB at the tip of the bonding wire W by discharge heating or the like, Au in the coating layer 12 is not easily diffused into the core material 10, and a good-shaped FAB with high sphericity can be formed more stably.

[0048] Pd has a larger specific resistance than Au. Therefore, by containing Pd together with Au in the core material 10, the total content of Au and Pd is suppressed and the specific resistance of the core material 10 mainly composed of Ag can be made close to the specific resistance of Au. In addition, when the core material 10 contains two elements of Au and Pd, the content of Au is greater than that of Pd. Thus, a good-shaped FAB with high sphericity can be formed, and at the same time, the shape of the 1st bonding portion can be a good circle.

[0049] In the bonding wire W, if the core material 10 contains 1 mass ppm - 50 mass ppm of Bi, a good-shaped FAB with high sphericity can be formed stably.

[0050] In the bonding wire W, if the core material 10 contains 0.1% by mass or more of In, the corrosion resistance of the 1st joint formed when the FAB is pressed against the electrode and crushed is improved, and the long-term reliability can be improved. If the content of In in the core material 10 is 1.0% by mass or less, the inherent resistance of the wire is maintained within an appropriate range, and a FAB with a high sphericity and a good shape can be formed.

[0051] As described above, the embodiments of the present invention have been described, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other ways, and various omissions, substitutions, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are similarly included in the scope of the invention described in the claims and their equivalents.

[0052] Examples

[0053] Hereinafter, the present invention will be further specifically described by way of examples, but the present invention is not limited to these examples.

[0054] Using an Ag raw material with a purity of 99.9% by mass or more, an Ag alloy having the composition shown in Table 1 below was dissolved, and a rod-shaped ingot was produced by continuous casting. Then, the rod-shaped ingot was subjected to wire drawing and reduced in diameter to a core material 10 having a diameter of 105 μm, and then a coating layer 12 made of an Au alloy having the composition shown in Table 1 below was formed on the outer peripheral surface of the core material 10 by electroplating. Then, the core material 10 having the coating layer 12 formed thereon was further subjected to wire drawing and reduced in diameter to a wire diameter φ of 20 μm and a film thickness t of the coating layer 12 of about 57 nm, and then a continuous annealing treatment (heat treatment) was performed at 400 to 600 ° C for 0.4 to 0.5 seconds in a mixed gas atmosphere of 5% hydrogen and 95% nitrogen to obtain the bonding wires of Examples 1 to 21 and Comparative Examples 1 to 7.

[0055] In addition, except for not coating the outer peripheral surface of the core material 10 with Au, wires composed only of the core materials 10 of the bonding wires of Examples 1 to 21 and Comparative Examples 1 to 7, and wires having the same alloy composition as the coating layers 12 of the bonding wires of Examples 1 to 21 and Comparative Examples 1 to 7 were produced by the same method as described above.

[0056] Moreover, the diameter φ of the core material 10 and the film thickness t of the coating layer 12 of the bonding wires of Examples 1 to 21 and Comparative Examples 1 to 7 were measured by the above measurement method, and the ratio α of the film thickness t of the coating layer 12 to the diameter φ of the core material 10 was calculated based on these measurement results.

[0057] In addition, using a wire composed only of the core material 10 and a wire having the same alloy composition as the coating layer 12, the specific resistance ρ1 of the core material 10 and the specific resistance ρ2 of the coating layer 12 were measured by the above-described measuring method. Based on the obtained specific resistance ρ1 of the core material 10 and the specific resistance ρ2 of the coating layer 12, the ratio β of the specific resistance ρ1 of the core material 10 to the specific resistance ρ2 of the coating layer 12 was calculated.

[0058] The diameter φ of the core material 10, the film thickness t of the coating layer 12, the ratio α, the specific resistance ρ1 of the core material 10, the specific resistance ρ2 of the coating layer 12, and the ratio β of the bonding wires of Examples 1 to 21 and Comparative Examples 1 to 7 are shown in Table 1 below.

[0059] The following evaluations (1) to (4) were performed on the obtained bonding wires of Examples 1 to 21 and Comparative Examples 1 to 7. The specific evaluation methods are as follows.

[0060] (1) FAB sphericity (formability)

[0061] FABs having a size 1.9 times to 2.1 times the wire diameter were fabricated in an atmospheric environment using a wire bonder (Shinkawa Ltd., UTC-5000NeoCu). As an evaluation of FAB sphericity (formability), after fabricating 100 FABs for each bonding wire of the examples and comparative examples, the appearance was observed using a general-purpose electron microscope (JEOL Ltd., JSM-6510LA), and the lengths in the wire parallel direction and the perpendicular direction of the fabricated FABs were measured. If the average value of the ratio (X / Y) of the length X in the wire parallel direction to the length Y in the perpendicular direction of the FAB is within the range of 100 ± 5%, it is judged as "having sphericity" and denoted as "A". If the average value of the above ratio (X / Y) is 92% or more and less than 95% or more than 105% and 108% or less, it is denoted as "B". If the average value of the above ratio (X / Y) is 90% or more and less than 92% or more than 108% and 110% or less, it is denoted as "C". If it is not circular upon visual observation, it is denoted as "D".

[0062] (2) Shape stability of the 1st joint

[0063] Using the wire bonder used in the above (1), 100 fabricated FABs were pressed against the electrodes in an atmospheric gas atmosphere, and bonding was performed such that the diameter of the 1st joint was 1.4 times the diameter of the FAB. The center-to-center distance between the center of the 1st joint and the chamfer diameter of the capillary provided in the wire bonder was measured using a three-dimensional microscope (Olympus small measurement microscope, STM6). Among all 100 FABs, if the center-to-center distance is 2.5 μm or less, it is denoted as "A"; if it exceeds 2.5 μm and the generation rate of the 1st joint that is 5 μm or less is less than 20%, it is denoted as "B"; if it exceeds 2.5 μm and the generation rate of the 1st joint that is 5 μm or less is 20% or more, it is denoted as "C"; if there is even one 1st joint that exceeds 5 μm, it is considered that the roundness is low and it is not suitable for micro-pitch applications, and it is denoted as "D".

[0064] (3) High-temperature storage test (long-term reliability)

[0065] Bonding was performed using the wire bonder used in the above (1) in an atmospheric atmosphere, and then an epoxy resin-sealed semiconductor specimen was fabricated. Using a commercially available constant-temperature bath, the fabricated semiconductor specimen was held at 150 °C for 1000 hours, and then electrical measurements were performed to conduct conduction evaluation. The number of lines for evaluation was 500. When the defect rate was 1% or less, it was denoted as "A"; when it exceeded 1% and was 3% or less, it was denoted as "B"; when it exceeded 3%, since the tolerance was low, it was denoted as "D".

[0066] (4) Comprehensive evaluation

[0067] In the evaluation items of the above (1) to (3), if all are evaluated as A, it is denoted as "A"; if there are 1 to 2 A evaluations, it is denoted as "B"; if there is no A evaluation, it is denoted as "C"; if there is even one D evaluation, it is denoted as "D".

[0068]

[0069] As shown in Table 1, in Examples 1 to 21, good results were obtained in all the evaluations of the above (1) to (4).

[0070] On the other hand, in Comparative Examples 1 to 3 where the ratio α of the thickness t of the coating layer 12 to the diameter φ of the core material 10 was outside the range of 0.0014 to 0.0048, circular FABs were not formed, and the sphericity of the FAB and the shape of the 1st joint deteriorated. In addition, in Comparative Examples 4 to 7 where the ratio β of the intrinsic resistance ρ1 of the core material 10 to the intrinsic resistance ρ2 of the coating layer 12 was outside the range of 85% to 115%, the shape of the 1st joint deteriorated.

[0071] Symbol Explanation

[0072] 10…Core material, 12…Coating layer, W…Bonding wire

Claims

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, the core material contains one or more selected from Au, Pd, and In, the ratio of the film thickness of the coating layer to the diameter of the core material is 0.0014 to 0.0048, the ratio of the specific resistance of the core material to the specific resistance of the coating layer is 85% to 115%.

2. The bonding wire according to claim 1, wherein, The core material contains at least one of Au and Pd.

3. The bonding wire according to claim 2, wherein The core material contains Au and Pd, and the content of Au is more than that of Pd.

4. The bonding wire according to any one of claims 1 to 3, wherein, The core material contains 1 mass ppm to 50 mass ppm of Bi.

5. The bonding wire according to any one of claims 1 to 3, wherein, The core material contains 0.1 mass% to 1.0 mass% of In.

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