Silver plating film, and electric contact comprising silver plating film

Abstract

To provide a silver plating film, which has high conductivity even under a high temperature environment and stable abrasion resistance, and an electric contact comprising the silver plating film.SOLUTION: A silver plating film contains bismuth of 0.01 wt.% or more and 0.1 wt.% or less, has a crystallite size with 240 Å or more and 340 Å or less, and a contact resistance of 1 mΩ or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present disclosure relates to a silver plating film. In particular, it relates to a silver plating film suitable as electrical contacts and terminal members such as connectors, switches, and relays.

Background Art

[0002] Silver (Ag) plating films are widely used in electronic components because of their high conductivity. In electrical contacts and terminal members such as connectors, switches, and relays in electronic components, wear occurs due to insertion, extraction, and sliding, so in addition to conductivity, wear resistance is required. To impart wear resistance, mainly means such as improving hardness and reducing friction are used. For example, Patent Document 1 proposes a technique for a silver-plated terminal for a connector, in which the surface of a base material made of copper or a copper alloy is coated with a silver plating layer having large crystal grains to prevent an increase in contact resistance caused by copper diffusion, and further coated with a silver plating layer having small crystal grains on the outermost surface to improve hardness.

[0003] Also, 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 using a silver plating film containing 0.1 to 2.0% by mass of antimony to increase hardness. Furthermore, Patent Document 4 discloses a copper or copper alloy member having a silver alloy layer with a Vickers hardness Hv of 140 or more and an antimony concentration of 0.5% by mass or more formed on the outermost layer. Furthermore, Patent Document 5 proposes a technique for an article including a silver-bismuth alloy layer, in which the friction coefficient is made 1 or less by incorporating 1 to 10% by mass of bismuth.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] In electronic components, electrical contacts and terminal members such as connectors, switches, and relays are worn by insertion, removal, or sliding. Therefore, in addition to electrical conductivity, wear resistance is required. In recent years, with the spread of electric vehicles, the number of electronic components through which a large current flows has increased. Since electronic components generate heat under a large current, electrical contacts and terminal members that involve repeated insertion, removal, or sliding are required to maintain high electrical conductivity (low contact resistance) and wear resistance even in a high-temperature environment.

[0006] However, in the configurations of Patent Documents 1 and 2, although the initial hardness of the outermost silver or silver alloy plating layer can be improved while keeping the contact resistance low, silver recrystallization progresses in a high-temperature environment, accompanied by coarsening of the crystal grain size and a significant decrease in hardness. In addition, in Patent Documents 3 and 4, when the antimony content in the silver plating film increases, the purity of silver decreases, resulting in a decrease in contact resistance. Furthermore, in a high-temperature environment, antimony is concentrated on the plating surface due to diffusion, and when this antimony oxidizes, the contact resistance increases. There is also a problem that antimony is highly toxic to the human body. In addition, in the configuration of Patent Document 5, although an improvement in initial hardness and maintenance of hardness at high temperatures can be expected, since it contains a large amount of bismuth with a large specific resistance, it is difficult to reduce the initial contact resistance. Moreover, at high temperatures, there is a problem that the increase in contact resistance due to the oxidation of bismuth is large.

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

Means for Solving the Problems

[0008] The gist of the present disclosure is as follows. [1] A silver plating film containing 0.01 wt% or more and 0.1 wt% or less of bismuth, 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 Hv100 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 Hv100 or more after heat treatment at 180 °C for 100 hours. [6] The silver plating film according to [4], having a decrease in Vickers hardness of 10 Hv or less after heat treatment at 180 °C for 100 hours. [7] An electrical contact including the silver plating film according to [1] or [2].

Advantages of the Invention

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

Embodiments for Carrying Out the Invention

[0010] The silver plating film according to an embodiment of the present disclosure contains 0.01 wt% or more and 0.1 wt% or less of bismuth. By containing 0.01 wt% or more of bismuth (Bi), in the silver plating film under a high-temperature environment, coarsening of crystal grains due to recrystallization can be suppressed, a decrease in hardness can be suppressed, and stable wear resistance can be obtained. On the other hand, by containing 0.1 wt% or less of bismuth (Bi), a high conductivity comparable to that of a pure silver plating film can be ensured. Further, under a high-temperature environment, oxidation of bismuth can be suppressed (the oxidation amount of bismuth is very small), almost no increase in contact resistance occurs, and high conductivity can be maintained. 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 the present embodiment has a crystallite size of 240 Å or more and 340 Å or less, and a contact resistance of 1 mΩ or less. When a certain amount of bismuth is eutectic with silver, the crystal grains are refined and the hardness is improved. However, under a high-temperature environment, the bismuth in the eutectic product is oxidized and the contact resistance increases. That is, since there is a trade-off relationship between hardness and contact resistance with respect to the eutectic amount of bismuth, the desired characteristics (hardness or contact resistance) differ depending on the application and the like. The present embodiment mainly aims to solve the problem of softening of the silver plating film while ensuring a 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. Further, the contact resistance of the silver plating film is preferably 0.8 mΩ or less, particularly preferably 0.6 mΩ or less. The lower the contact resistance, the more useful it is as an electrical contact with excellent conductivity.

[0012] Bismuth is less likely to form oxides compared to antimony (Sb) described in Patent Documents 3 and 4, so it can suppress the increase in contact resistance due to oxidation even in a high-temperature environment. Also, bismuth has a low solid solubility in silver at room temperature and can suppress recrystallization with a small content rate. Taking advantage of the above characteristics of bismuth, Patent Document 6 discloses a silver plating film containing less than 0.1 wt% of bismuth and having a crystallite size of 230 Å or less. However, the upper limit value of the bismuth content rate in Patent Document 6 is high, and since there are many grain boundaries with the refinement of crystals, an increase in contact resistance cannot be denied. The silver plating film according to the present embodiment has a different technical idea in that even when it contains 0.1 wt% or less of bismuth, it can almost suppress the increase in contact resistance while suppressing the decrease in hardness in a high-temperature environment.

[0013] The silver plating film according to the present embodiment preferably has a Vickers hardness of Hv100 or more. More preferably, the Vickers hardness is Hv110 or more, and particularly preferably, it is Hv120 or more. The higher the hardness, the more useful it is as an electric contact excellent in wear resistance.

[0014] In the present embodiment, it is preferable that the contact resistance of the silver plating film after heat treatment at 180 °C for 100 hours is 1 mΩ or less. Note that simply saying "after heat treatment" means heat treatment at 180 °C for 100 hours. More preferably, the contact resistance after heat treatment is 0.8 mΩ or less, and particularly preferably, it is 0.7 mΩ or less. If the contact resistance increases (the conductivity decreases) due to heat treatment, the amount of heat generated by energization increases in a high-temperature environment, which promotes a further increase in contact resistance and softening of the film, leading to a deterioration in the performance as an electric contact, so it is not preferable.

[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 Hv100 or more can be obtained even after heat treatment. As a result, the hardness will not significantly decrease from the initial hardness of this embodiment, 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 Hv100 or more after heat treatment at 180°C for 100 hours (after heat treatment). More preferably, the Vickers hardness after heat treatment is Hv110 or more, and particularly preferably, the Vickers hardness after heat treatment is Hv120 or more. If the hardness decreases due to heat treatment, the wear resistance in a high-temperature environment will decrease, leading to a decrease in the performance as an electrical contact, which is not preferable.

[0017] The silver plating film according to this embodiment preferably has a decrease in Vickers hardness of within Hv10 after heat treatment at 180°C for 100 hours (after heat treatment). More preferably, the decrease in Vickers hardness after heat treatment is within Hv5, and particularly preferably within Hv3. A significant decrease in the Vickers hardness after heat treatment will impair the stability of the wear resistance in a high-temperature environment. The decrease in Vickers hardness is calculated by the following formula. (Vickers hardness before heat treatment) - (Vickers hardness after heat treatment) = (decrease in Vickers hardness after heat treatment) Note that since an increase in hardness due to heat treatment is not particularly problematic, the increase amount (negative value) of the Vickers hardness is not particularly restricted.

[0018] An example of the manufacturing method of the silver plating film according to this embodiment is shown. However, the silver plating film according to this embodiment is not limited to being obtained by the following manufacturing method. In order to avoid unnecessary obscurity of the manufacturing method, detailed descriptions of well-known matters are omitted.

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

[0020] The components of the plating bath and the electrolysis method are a cyanide bath containing a bismuth compound, and are not particularly limited as long as a silver plating film having a predetermined Bi content and crystallite size according to this embodiment can be obtained. The bismuth compound is used as a Bi source, and examples include bismuth oxide, bismuth chloride, bismuth nitrate, etc. The carboxylate is used as a ligand for stably dissolving bismuth in the bath, and examples include tartrate, citrate, gluconate, lactate, etc. Additives such as sulfur compounds may be used to adjust the crystallite size, but it is preferable to use those that do not contain components that increase the 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, a method based on electrolysis conditions such as pulse plating may be used. Also, by adjusting the stirring speed of the plating bath together with other plating conditions, the crystallite size of the silver plating film can be controlled.

Example

[0021] Next, examples and comparative examples of the present invention will be described. It should be noted that the following examples show typical examples, and the present invention need not be limited by these examples and should be interpreted within the scope of the technical idea described in the specification.

[0022] For the various physical property evaluations of the silver plating films in this example and the comparative example, the following was done. <Bi content> Using a fluorescence X-ray film thickness meter 160h from Hitachi High-Technologies Corporation, the central part of the sample was measured with a measurement diameter of 0.1 mm, tube voltage of 45 V, tube current of 1000 μA, and using A1 as the primary filter, and measured by the thin film FP method. As the characteristic X-rays of silver and bismuth, Kα ray and Lα ray were used for measurement respectively.

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

[0024] <Measurement of contact resistance> Using an electric contact simulator CRS-113-AU type (Au wire, φ0.5 mm) manufactured by Yamazaki Precision Machinery Laboratory Co., Ltd., the contact resistance was measured at three points at the central part 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 from Mitutoyo Corporation, the Vickers hardness was measured at 10 points at the central part of the sample with a load of 0.05 N to 0.1 N using a Vickers indenter, and the average value was calculated.

[0026] (Example 1) As a base material, a rectangular copper plate (2.5 cm × 2 cm) was subjected to alkaline electrolytic degreasing and pickling to clean the surface. Then, electrolytic plating was carried out in a dull nickel sulfamate bath (nickel sulfamate: 450 g / L, bath 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 base material. Next, in order to enhance adhesion, electrolysis was carried out for 5 - 10 seconds in a silver strike bath (potassium silver cyanide: 3.7 g / L, potassium cyanide: 100 g / L, bath temperature: 30 °C, pH: 12, current density: 2 ASD) to apply a silver strike plating on the nickel plating. Thereafter, a silver plating film was formed on the base material on which the nickel plating film and the silver strike plating film were formed under the conditions shown in Table 1 to form a silver plating film containing a predetermined amount of bismuth. Regarding the silver plating film obtained as above, the bismuth content, crystallite size, Vickers hardness, and contact resistance were measured. Then, heat treatment was carried out at 180 °C for 100 hours, and the Vickers hardness and contact resistance of the silver plating film after heat treatment were measured. The results are shown in Table 2. As shown in Table 2, Example 1 was shown to have a low contact resistance even after heat treatment, and also a high Vickers hardness value, having high conductivity and wear resistance in a high-temperature environment.

[0027]

Table 1

[0028]

Table 2

[0029] (Examples 2 - 12) On the base material on which nickel plating film and silver strike plating film were formed in the same manner as in Example 1, silver plating films were formed by changing the plating conditions as shown in Table 1 respectively to form silver plating films containing a predetermined amount of bismuth. For the obtained silver plating film, the bismuth content, crystallite size, Vickers hardness, and contact resistance were measured respectively. Also, the Vickers hardness and contact resistance were 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 any of Examples 2-12, the contact resistance was low even after heat treatment, and the Vickers hardness value was also high, indicating that it has high conductivity and wear resistance in a high-temperature environment.

[0030] (Comparative Examples 1-7) On the base materials formed with nickel plating films and silver strike plating films prepared in the same manner as in Example 1, silver plating films were formed by changing the plating conditions as shown in Table 1 respectively, and silver plating films containing a predetermined amount of bismuth were formed. For the obtained silver plating film, the bismuth content, crystallite size, Vickers hardness, and contact resistance were measured respectively. Also, the Vickers hardness and contact resistance were 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 any of Comparative Examples 1-7, a significant decrease in Vickers hardness was observed after heat treatment. Also, in Comparative Example 4, the contact resistance became high after heat treatment, indicating that the conductivity and wear resistance are inferior in a high-temperature environment.

Industrial Applicability

[0031] The silver plating film according to this embodiment has an excellent effect of having high conductivity and wear resistance even in a high-temperature environment. The silver plating film according to this embodiment is suitable for applications such as electrical contact points and terminal members including connectors, switches, and relays. In particular, it is suitable for applications as electrical contact points and terminal members of electronic components where a large current flows and heat is generated.

Claims

1. A silver plating film containing 0.01 wt% or more and 0.1 wt% or less of bismuth, 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. The silver plating film according to Claim 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 Claim 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 Claim 1 or 2, having 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, having a decrease in Vickers hardness of 10 or less in Hv after heat treatment at 180°C for 100 hours.

7. An electrical contact provided with the silver plating film described in Claim 1 or 2.

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