Silver plating film and electric contact provided with said silver plating film

Abstract

The present invention addresses the problem of providing: a silver plating film having stable wear resistance and high conductivity even in a high temperature environment; and an electric contact provided with said silver plating film. This silver plating film contains bismuth in an amount of 0.01-0.1 wt%, and has a crystallite size of 240-340 Å and a contact resistance of 1 mΩ or less.

Description

Silver plating film and electrical contacts provided with said silver plating film

[0001] The present disclosure relates to a silver plating film, and more particularly to a silver plating film suitable for use as an electrical contact or terminal member for connectors, switches, relays, and the like.

[0002] Silver (Ag) plating films are widely used in electronic components due to their high electrical conductivity. Electrical contacts and terminal members, such as connectors, switches, and relays, are subject to wear due to insertion, removal, and sliding, and therefore require not only electrical conductivity but also wear resistance. To achieve wear resistance, measures such as improving hardness and reducing friction are primarily used. For example, Patent Document 1 proposes a technology for silver-plated terminals for connectors, in which the surface of a base material made of copper or a copper alloy is coated with a silver plating layer with large crystal grains to prevent an increase in contact resistance due to copper diffusion, and the outermost surface is further coated with a silver plating layer with small crystal grains to improve hardness.

[0003] Furthermore, Patent Document 2 proposes a technique for preventing an increase in contact resistance while maintaining high hardness by incorporating selenium into a silver plating film. Patent Document 3 describes a method for using a silver plating film that contains 0.1 to 2.0 mass % antimony to increase its hardness. Furthermore, Patent Document 4 discloses a copper or copper alloy member having a silver alloy layer with an antimony concentration of 0.5 mass % or more and a Vickers hardness of Hv 140 or more formed on the outermost surface. Furthermore, Patent Document 5 proposes a technique for an article including a silver-bismuth alloy layer, in which the friction coefficient is reduced to 1 or less by incorporating 1 to 10 mass % bismuth.

[0004] Japanese Patent Application Publication No. 2008-169408 Japanese Patent Application Publication No. 2016-145413 Japanese Patent Application Publication No. 2005-133169 Japanese Patent Application Publication No. 2009-79250 Japanese Patent Application Publication No. 2021-66953 Japanese Patent No. 7213390

[0005] Electrical contacts and terminal members in electronic components, such as connectors, switches, and relays, are subject to wear due to insertion / removal and sliding, and therefore require wear resistance in addition to electrical conductivity.In addition, with the recent spread of electric vehicles, the number of electronic components through which large currents flow is increasing, and because electronic components generate heat under high currents, electrical contacts and terminal members that are subject to repeated insertion / removal and sliding are required to maintain high electrical conductivity (low contact resistance) and wear resistance even in high-temperature environments.

[0006] However, while the configurations of Patent Documents 1 and 2 can improve the initial hardness of the outermost silver or silver alloy plating layer while maintaining low contact resistance, they have the problem of silver recrystallization occurring in high-temperature environments, resulting in coarsening of the crystal grain size and a significant decrease in hardness. Furthermore, Patent Documents 3 and 4 state that as the antimony content in the silver plating film increases, the purity of silver decreases, resulting in a decrease in contact resistance. Furthermore, in high-temperature environments, antimony concentrates on the plating surface through diffusion, which then oxidizes, resulting in an increase in contact resistance. Another problem is that antimony is highly toxic to humans. Furthermore, the configuration of Patent Document 5 is expected to improve initial hardness and maintain hardness at high temperatures, but because it contains a large amount of bismuth, which has a high resistivity, it is difficult to reduce initial contact resistance. Furthermore, at high temperatures, the increase in contact resistance due to the oxidation of bismuth is significant.

[0007] In view of these problems, an object of the present disclosure is to provide a silver plating film and an electrical contact including the silver plating film, which have high conductivity and stable wear resistance even in high-temperature environments.

[0008] The gist of the present disclosure is as follows. [1] A silver plating film containing bismuth at 0.01 wt % or more and 0.1 wt % or less, having a crystallite size of 240 Å or more and 340 Å or less, and having a contact resistance of 1 mΩ or less. [2] The silver plating film according to [1], having a Vickers hardness of Hv 100 or more. [3] The silver plating film according to [1] or [2], having a contact resistance of 1 mΩ or less after heat treatment at 180°C for 100 hours. [4] The silver plating film according to [1] or [2], having a crystallite size of 300 Å or less after heat treatment at 180°C for 100 hours. [5] The silver plating film according to [1] or [2], having a Vickers hardness of Hv 100 or more after heat treatment at 180°C for 100 hours. [6] The silver plating film according to [4], wherein the reduction in Vickers hardness after heat treatment at 180°C for 100 hours is within Hv 10. [7] An electrical contact comprising the silver plating film according to [1] or [2].

[0009] According to the present disclosure, it is possible to provide a silver plating film and an electrical contact including the silver plating film that have high conductivity and stable wear resistance even in high-temperature environments.

[0010] The silver plating film according to the embodiment of the present disclosure contains bismuth at a content of 0.01 wt% or more and 0.1 wt% or less. By containing bismuth (Bi) at a content of 0.01 wt% or more, the coarsening of crystal grains due to recrystallization in the silver plating film can be suppressed in high-temperature environments, thereby suppressing a decrease in hardness and achieving stable wear resistance. On the other hand, by containing bismuth (Bi) at a content of 0.1 wt% or less, high electrical conductivity equivalent to that of pure silver plating films can be ensured. Furthermore, oxidation of bismuth can be suppressed (the amount of bismuth oxidation is slight) in high-temperature environments, resulting in almost no increase in contact resistance and maintaining high electrical conductivity. The bismuth content is preferably 0.02 wt% or more, more preferably 0.03 wt% or more, and preferably 0.08 wt% or less, more preferably 0.07 wt% or less.

[0011] The silver plating film according to this embodiment has a crystallite size of 240 Å or more and 340 Å or less, and a contact resistance of 1 mΩ or less. Co-depositing a certain amount of bismuth with silver results in finer crystal grains and improved hardness, but in high-temperature environments, the bismuth in the co-deposit oxidizes, increasing contact resistance. In other words, there is a trade-off between hardness and contact resistance with respect to the amount of bismuth co-deposited, so the desired characteristics (hardness or contact resistance) vary depending on the application. This embodiment focuses particularly on resolving the problem of softening of the silver plating film while maintaining contact resistance equivalent to that of a pure silver plating film. The crystallite size of the silver plating film is preferably 330 Å or less. Furthermore, the contact resistance of the silver plating film is preferably 0.8 mΩ or less, and particularly preferably 0.6 mΩ or less. The lower the contact resistance, the more useful the silver plating film is as an electrical contact with excellent conductivity.

[0012] Compared with antimony (Sb) described in Patent Documents 3 and 4, bismuth is less likely to form oxides, and therefore can suppress an increase in contact resistance due to oxidation, even in high-temperature environments. Furthermore, bismuth has a low solid solubility in silver at room temperature, and even a small content can suppress recrystallization. Taking advantage of the above properties of bismuth, Patent Document 6 discloses a silver plating film containing less than 0.1 wt% bismuth and having a crystallite size of 230 Å or less. However, the upper limit of the bismuth content in Patent Document 6 is high, and the presence of many grain boundaries due to the refinement of the crystals inevitably increases contact resistance. The silver plating film according to this embodiment differs from the previous one in that it can suppress a decrease in hardness in high-temperature environments while almost completely suppressing an increase in contact resistance, even when containing 0.1 wt% or less bismuth.

[0013] The silver plating film according to this embodiment preferably has a Vickers hardness of Hv 100 or more. More preferably, the Vickers hardness is Hv 110 or more, and particularly preferably Hv 120 or more. The higher the hardness, the more useful the electrical contact is as it has excellent wear resistance.

[0014] In this embodiment, the silver plating film preferably has a contact resistance of 1 mΩ or less after heat treatment at 180°C for 100 hours. The term "after heat treatment" simply refers to heat treatment at 180°C for 100 hours. The contact resistance after heat treatment is more preferably 0.8 mΩ or less, and particularly preferably 0.7 mΩ or less. If the contact resistance increases (conductivity decreases) due to heat treatment, the amount of heat generated by current flow increases in a high-temperature environment, further increasing the contact resistance and promoting softening of the film, which undesirably leads to a decrease in performance as an electrical contact.

[0015] The silver plating film according to this embodiment preferably has a crystallite size of 300 Å or less after heat treatment at 180°C for 100 hours (after heat treatment). If the crystallite size is 300 Å or less, a hardness of Hv 100 or more can be obtained even after heat treatment, and as a result, the initial hardness of this embodiment does not decrease significantly, and stable wear resistance can be obtained. More preferably, the crystallite size is 290 Å or less, and even more preferably, 280 Å or less.

[0016] In this embodiment, the silver plating film preferably has a Vickers hardness of Hv 100 or more after heat treatment at 180°C for 100 hours (after heat treatment). More preferably, the Vickers hardness after heat treatment is Hv 110 or more, and particularly preferably, the Vickers hardness after heat treatment is Hv 120 or more. If the hardness is reduced by heat treatment, the wear resistance in high-temperature environments decreases, leading to a decrease in performance as an electrical contact, which is not preferable.

[0017] The silver plating film according to this embodiment preferably has a Vickers hardness decrease of Hv 10 or less after heat treatment at 180°C for 100 hours (after heat treatment). More preferably, the Vickers hardness decrease of Hv 5 or less after heat treatment is particularly preferably Hv 3 or less. A significant decrease in Vickers hardness after heat treatment will impair the stability of wear resistance in high-temperature environments. The decrease in Vickers hardness is calculated using the following formula: (Vickers hardness before heat treatment) - (Vickers hardness after heat treatment) = (decrease in Vickers hardness after heat treatment). Note that an increase in hardness due to heat treatment is not particularly problematic, so the increase in Vickers hardness (negative value) is not particularly important.

[0018] An example of a method for producing a silver plating film according to this embodiment will be described below. However, the silver plating film according to this embodiment is not limited to that obtained by the following production method. In order to avoid unnecessarily obscuring the production method, detailed explanation of publicly known matters will be omitted.

[0019] By electroplating a substrate (object to be plated) using the plating bath described below, a silver plating film of fine crystals containing a predetermined amount of bismuth can be formed on the surface of the substrate. Electroplating can be performed under the following conditions: (Examples of plating bath components) Potassium silver cyanide (as Ag): 90 to 110 g / L Potassium cyanide (KCN): 67 to 77 g / L Bismuth compound (as Bi): 0.25 to 1 g / L Carboxylate: 10 to 20 g / L Potassium carbonate: 5 to 15 g / L (Electroplating conditions) Current density (Dk): 7.5 to 10 A / dm ２ pH: 12.7-13 Liquid temperature: 45-50℃

[0020] The plating bath components and electrolysis method are not particularly limited as long as they are a cyanide bath containing a bismuth compound and can produce a silver plating film with the desired Bi content and crystallite size according to this embodiment. The bismuth compound is used as a Bi source, and examples include bismuth oxide, bismuth chloride, and bismuth nitrate. Carboxylate salts are used as ligands to stabilize bismuth in the bath, and examples include tartrate, citrate, gluconate, and lactate. Additives such as sulfur compounds may be used to adjust the crystallite size, but it is preferable to use additives that do not contain components that increase contact resistance, such as antimony. Furthermore, the method for adjusting the crystallite size is not necessarily limited to using additives in the plating bath as described above; for example, methods based on electrolysis conditions such as pulse plating may also be used. The crystallite size of the silver plating film can also be controlled by adjusting the stirring speed of the plating bath along with other plating conditions.

[0021] Next, examples of the present invention and comparative examples will be described. Note that the following examples are representative examples, and the present invention is not necessarily limited by these examples, and should be interpreted within the scope of the technical ideas described in the specification.

[0022] The physical properties of the silver plating films in the present examples and comparative examples were evaluated as follows. <Bi content> Using a fluorescent X-ray film thickness meter 160h manufactured by Hitachi High-Tech Science Corporation, the central part of the sample was measured by the thin-film FP method with a measurement diameter of 0.1 mm, a tube voltage of 45 V, a tube current of 1000 μA, and an A1 primary filter. Kα and Lα rays were used as characteristic X-rays of silver and bismuth, respectively.

[0023] <Measurement of Crystallite Size> Using an X-ray diffractometer (SmartLab II) manufactured by Rigaku Corporation, the central part of the sample was measured and calculated with a scan step of 0.02°, a scan range of 30 to 150°, a scan speed of 40° / min, an entrance slit of 1.00 mm, a receiving slit of "open", and a detector of Hypix-3000. The crystallite size was measured from the diffraction line of the {200} plane using the Scherrer equation K=0.94.

[0024] <Measurement of Contact Resistance> Using an electrical contact simulator CRS-113-AU type (Au wire, φ0.5 mm) manufactured by Yamazaki Seiki Kenkyusho Co., Ltd., the contact resistance was measured at three points in the center of the sample under the conditions of a load of 0.05 N, an operating load of 1 mm, and an operating speed of 1 mm / min, and the average value of the results of each measurement point was calculated.

[0025] <Measurement of Vickers Hardness> Using a microhardness tester HM-221 manufactured by Mitutoyo Corporation, Vickers hardness was measured at 10 points in the center of the sample with a Vickers indenter under a load of 0.05 N to 0.1 N, and the average value was calculated.

[0026] Example 1: A rectangular copper plate (2.5 cm × 2 cm) was used as a substrate. After alkaline electrolytic degreasing and acid pickling to clean the surface, electroplating was performed in a matte nickel sulfamate bath (nickel sulfamate: 450 g / L, solution temperature: 55°C, pH: 4, current density: 2 ASD) to form a nickel plating film with a thickness of 1 μm on the surface of the substrate. Next, to enhance adhesion, electrolysis was performed for 5 to 10 seconds in a silver strike bath (potassium silver cyanide: 3.7 g / L, potassium cyanide: 100 g / L, solution temperature: 30°C, pH: 12, current density: 2 ASD) to form a silver strike plating on the nickel plating. A silver plating film was then formed on the substrate with the nickel plating film and silver strike plating film under the conditions listed in Table 1, resulting in a silver plating film containing a predetermined amount of bismuth. The bismuth content, crystallite size, Vickers hardness, and contact resistance of the silver plating film obtained as described above were measured. Thereafter, heat treatment was performed at 180°C for 100 hours, and the Vickers hardness and contact resistance of the heat-treated silver plating film were measured. The results are shown in Table 2. As shown in Table 2, Example 1 had low contact resistance even after heat treatment and a high Vickers hardness value, indicating that it had high conductivity and wear resistance in a high-temperature environment.

[0027]

[0028]

[0029] (Examples 2-12) A silver plating film containing a predetermined amount of bismuth was formed on a substrate having a nickel plating film and a silver strike plating film formed by the same method as in Example 1, with the plating conditions varied as shown in Table 1. The bismuth content, crystallite size, Vickers hardness, and contact resistance of the resulting silver plating film were measured. The Vickers hardness and contact resistance were also measured before heat treatment (initial value) and after heat treatment (180°C, 100 hours). The results are shown in Table 2. As shown in Table 2, in all of Examples 2-12, the contact resistance was low even after heat treatment, and the Vickers hardness values ​​were high, demonstrating high conductivity and wear resistance in high-temperature environments.

[0030] (Comparative Examples 1-7) A silver plating film containing a predetermined amount of bismuth was formed on a substrate having a nickel plating film and a silver strike plating film formed by the same method as in Example 1, with the plating conditions varied as shown in Table 1. The resulting silver plating film was measured for its bismuth content, crystallite size, Vickers hardness, and contact resistance. The Vickers hardness and contact resistance were also measured before heat treatment (initial value) and after heat treatment (180°C, 100 hours). The results are shown in Table 2. As shown in Table 2, a significant decrease in Vickers hardness was observed after heat treatment in all of Comparative Examples 1-7. Furthermore, in Comparative Example 4, the contact resistance increased after heat treatment, indicating poor conductivity and wear resistance in high-temperature environments.

[0031] The silver plating film according to this embodiment has the excellent effect of having high conductivity and abrasion resistance even in high-temperature environments. The silver plating film according to this embodiment is suitable for use as electrical contacts and terminal members, including connectors, switches, and relays. In particular, it is suitable for use as electrical contacts and terminal members for electronic components through which large currents flow and generate heat.

Claims

1. A silver plating film containing bismuth at 0.01 wt % or more and 0.1 wt % or less, having a crystallite size of 240 Å or more and 340 Å or less, and having a contact resistance of 1 mΩ or less.

2. The silver plating film according to claim 1, having a Vickers hardness of Hv100 or more.

3. A silver plating film according to claim 1 or 2, which has a contact resistance of 1 mΩ or less after heat treatment at 180° C. for 100 hours.

4. The silver plating film according to claim 1 or 2, which has a crystallite size of 300 Å or less after heat treatment at 180° C. for 100 hours.

5. The silver plating film according to claim 1 or 2, which has a Vickers hardness of Hv100 or more after heat treatment at 180° C. for 100 hours.

6. The silver plating film according to claim 5, wherein the decrease in Vickers hardness after heat treatment at 180° C. for 100 hours is within Hv10.

7. An electrical contact having the silver plating film according to claim 1 or 2.

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