Silver plating film and electrical contact having the same

Abstract

To provide a silver plated film having high conductivity and wear resistance even in a high temperature environment, and an electrical contact having the silver plated film. The silver plated film contains 0.02% by weight or more and less than 1% by weight of bismuth, and has a crystallite size of 230 A or less. The electrical contact has the silver plated film containing 0.02% by weight or more and less than 1% by weight of bismuth, and has a crystallite size of the following.

Description

Technical Field

[0001] This disclosure relates to silver plating, and in particular to silver plating suitable for use as electrical contacts and terminals in connectors, switches, relays, and other similar components. Background Technology

[0002] Silver (Ag) plating is widely used in electronic components due to its high conductivity. In electronic components such as connectors, switches, and relays, electrical contacts and terminals are subject to wear due to insertion, removal, and sliding; therefore, in addition to conductivity, wear resistance is also required. To impart wear resistance, methods such as increasing hardness and reducing friction are mainly used.

[0003] For example, in Patent Document 1, a technology is proposed for silver-plated terminals for connectors: a silver plating layer with large grains is used to coat the surface of a base material made of copper or copper alloy, thereby preventing the increase in contact resistance caused by copper diffusion; in addition, a silver plating layer with small grains is used to coat the outermost surface, thereby improving hardness.

[0004] Furthermore, Patent Document 2 proposes a technique to prevent increased contact resistance while maintaining high hardness by including selenium in the silver plating film. Patent Document 3 describes a method for using a silver plating film containing 0.1 to 2.0% by mass of antimony to improve hardness. Additionally, Patent Document 4 discloses a copper or copper alloy component with a silver alloy layer having an antimony concentration of 0.5% by mass or higher and a Vickers hardness of Hv140 or higher formed on its outermost layer.

[0005] In addition, Patent Document 5 proposes a technology for an article containing a silver-bismuth alloy layer, which achieves a coefficient of friction of less than 1 by containing 1 to 10% by mass of bismuth.

[0006] [Existing Technical Documents]

[0007] [Patent Literature]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2008-169408

[0009] [Patent Document 2] Japanese Patent Application Publication No. 2016-145413

[0010] [Patent Document 3] Japanese Patent Application Publication No. 2005-133169

[0011] [Patent Document 4] Japanese Patent Application Publication No. 2009-79250

[0012] [Patent Document 5] Japanese Patent Application Publication No. 2021-66953 Summary of the Invention

[0013] [The problem the invention aims to solve]

[0014] In electronic components, electrical contacts and terminals, such as connectors, switches, and relays, wear is inevitable due to insertion, removal, and sliding. Therefore, in addition to conductivity, wear resistance is also required. Furthermore, in recent years, with the increasing popularity of electric vehicles, the number of electronic components carrying high currents has also increased. Under high currents, these electronic components generate heat. Therefore, electrical contacts and terminals that undergo repeated insertion, removal, and sliding require high conductivity (low contact resistance) and wear resistance even in high-temperature environments.

[0015] However, in the configurations of Patent Document 1 and Patent Document 2, although the initial hardness of the outermost silver or silver alloy plating can be improved while keeping the contact resistance low, the silver recrystallizes at high temperatures, resulting in a significant decrease in hardness due to the coarsening of the grain diameter.

[0016] Furthermore, in patent documents 3 and 4, if the antimony content in the silver coating increases, the purity of the silver decreases, thus reducing the contact resistance. In addition, under high-temperature environments, there are problems such as antimony thickening on the coating surface due to diffusion and its oxidation, leading to increased contact resistance. Furthermore, there is the issue of antimony's high toxicity to humans.

[0017] Furthermore, while the initial hardness can be expected to be improved and the hardness can be maintained at high temperatures in the configuration of Patent Document 5, it is difficult to reduce the initial contact resistance due to the presence of a large amount of bismuth with high resistivity. Moreover, at high temperatures, there are further problems such as a large increase in contact resistance caused by the oxidation of bismuth.

[0018] In view of the aforementioned problems, the subject of this disclosure is to provide a silver coating that has high conductivity and wear resistance even at high temperatures, and an electrical contact having the silver coating.

[0019] [Methods used to solve problems]

[0020] The main points of this disclosure are as follows.

[0021] [1] A silver coating containing 0.02% by weight and less than 1% by weight of bismuth, and the crystallite size of the silver coating is less than 230 Å.

[0022] [2] The silver coating as described in [1] has a contact resistance of less than 2mΩ.

[0023] [3] The silver coating as described in [1] or [2] has a contact resistance of less than 2mΩ after being heat-treated at 180°C for 100 hours.

[0024] [4] The silver coating according to [1] or [2] has a Vickers hardness of Hv135 or higher.

[0025] [5] The silver coating as described in [1] or [2] has a Vickers hardness of Hv135 or higher after heat treatment at 180°C for 100 hours.

[0026] [6] The silver coating as described in [5] has a Vickers hardness reduction of less than Hv20 after heat treatment at 180°C for 100 hours.

[0027] [7] An electrical contact having a silver coating as described in any one of [1] to [6].

[0028] [Invention Effects]

[0029] According to this disclosure, it is possible to provide a silver coating that has high conductivity and wear resistance even at high temperatures, as well as electrical contacts having the silver coating. Detailed Implementation

[0030] The silver coating according to embodiments of the present invention contains 0.02% by weight or more and less than 1% by weight of bismuth, and the crystallite size is 230 Å or less. By containing 0.02% by weight or more of bismuth (Bi), the decrease in hardness of the silver coating under high-temperature conditions can be suppressed.

[0031] The bismuth content is preferably 0.05% by weight or more, more preferably 0.1% by weight or more, and particularly preferably 0.5% by weight or more. On the other hand, by keeping the bismuth content less than 1% by weight, the increase in contact resistance under high temperature conditions can be suppressed, and high conductivity can be maintained.

[0032] Furthermore, the crystallite size of the silver coating involved in this embodiment is 230 Å or less, thereby further increasing the hardness and improving wear resistance. The crystallite size of the silver coating is preferably 200 Å or less, and more preferably 150 Å or less.

[0033] The increase in contact resistance at high temperatures is due to the thickening and oxidation of foreign elements in the silver coating on its surface. Therefore, it is believed that if a foreign element is difficult to thicken on the film surface and is also difficult to oxidize, the increase in contact resistance at high temperatures can be suppressed. Compared to antimony, bismuth has a higher standard Gibbs energy of oxide formation and is less likely to form oxides.

[0034] Alloying with dissimilar elements, such as antimony, helps to inhibit recrystallization (maintaining hardness) and also increases hardness. However, as mentioned above, dissimilar elements oxidize at high temperatures, increasing contact resistance. Therefore, a small amount of dissimilar elements is needed to inhibit recrystallization and maintain hardness. Bismuth has very low solid solubility in silver at room temperature, so the pinning effect is easily achieved. It can inhibit recrystallization at a small content, and thus age hardening can be expected during heat treatment.

[0035] On the other hand, if the bismuth content is too low, the initial hardness improvement effect caused by the presence of foreign elements, such as solid solution strengthening and precipitation strengthening, is reduced. In the silver coating of this embodiment, by reducing the crystallite size, the effect of crystallization micro-refinement strengthening is increased, and high hardness can be obtained even with a low bismuth content.

[0036] The contact resistance of the silver-plated film involved in this embodiment is preferably 2 mΩ or less. More preferably, the contact resistance is 1.5 mΩ or less, and particularly preferably 1 mΩ or less. If the contact resistance is low, it is useful as an electrical contact with excellent conductivity.

[0037] Furthermore, in this embodiment, the contact resistance of the silver coating after heat treatment at 180°C for 100 hours is preferably less than 2 mΩ. More preferably, the contact resistance after heat treatment is 1.5 mΩ or less, and particularly preferably 1 mΩ or less. If the contact resistance increases significantly due to heat treatment, the heat generated during energization increases, further promoting the increase in contact resistance and softening of the film, leading to a decrease in performance as an electrical contact, which is therefore undesirable.

[0038] The Vickers hardness of the silver coating involved in this embodiment is preferably Hv135 or higher. More preferably, it is Hv150 or higher, and particularly preferably Hv170 or higher. The higher the hardness, the more useful it is as an electrical contact with excellent wear resistance.

[0039] Furthermore, in this embodiment, the Vickers hardness of the silver coating after heat treatment at 180°C for 100 hours (after heat treatment) is preferably Hv135 or higher. The Vickers hardness after heat treatment is more preferably Hv150 or higher, and particularly preferably Hv170 or higher. If the hardness decreases due to heat treatment, the wear resistance decreases, resulting in reduced performance as an electrical contact, which is therefore not preferred.

[0040] The silver coating involved in this embodiment preferably exhibits a Vickers hardness reduction of Hv20 or less after heat treatment at 180°C for 100 hours. More preferably, the Vickers hardness reduction after heat treatment is Hv10 or less, and particularly preferably Hv5 or less. A significant reduction in Vickers hardness would decrease wear resistance and is therefore not preferred.

[0041] The decrease in Vickers hardness is calculated using the following formula.

[0042] (Vickers hardness before heat treatment) - (Vickers hardness after heat treatment) = (Decrease in Vickers hardness after heat treatment)

[0043] It should be noted that increasing hardness through heat treatment is not particularly problematic, therefore there is no particular limit to the amount of increase in Vickers hardness.

[0044] An example of a method for manufacturing a silver plating film according to this embodiment is shown. However, the silver plating film according to this embodiment is not limited to silver plating films obtained by the following manufacturing method. It should be noted that, in order to avoid unnecessarily obscuring the manufacturing method, detailed descriptions of well-known matters are omitted.

[0045] By using the following plating bath to perform electroplating on a substrate (the object to be plated), a silver plating film containing a predetermined amount of bismuth microcrystals can be formed on the surface of the substrate. Electroplating can be performed under the following conditions.

[0046] (An example of plating bath composition)

[0047] Potassium silver cyanide (as silver): 40–120 g / L

[0048] Potassium cyanide: 50–150 g / L

[0049] Bismuth compounds (as bismuth): 0.2–50 g / L

[0050] Carboxylates: 1.5–150 g / L

[0051] Sulfur-based brightening agents: 0.1–10 g / L

[0052] (Conditions for electroplating)

[0053] Current density: 0.5~10A / dm 2

[0054] pH: 11-14

[0055] Liquid temperature: 25℃~55℃

[0056] The composition of the plating bath and the electrolysis method are not particularly limited to those described above, as long as the bath contains a cyanide compound and can produce a finely crystalline film. Examples of bismuth compounds used as bismuth sources include bismuth oxide, bismuth chloride, and bismuth nitrate. Examples of carboxylates used as ligands to stably dissolve bismuth in the bath include tartrates, citrates, gluconates, and lactates. Examples of sulfur-based brighteners used as additives to refine the crystal structure include thiosulfate, mercaptobenzothiazoles, mercaptobenzimidazoles, thiocyanates, and thiourea.

[0057] Furthermore, as long as the additive is one that refines the crystal structure, it is not particularly limited to sulfur-based brighteners, but it is preferable to use additives that do not contain components that increase contact resistance, such as antimony. In addition, the method for refining the crystal structure 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, can also be used.

[0058] [Example]

[0059] Next, embodiments and comparative examples of the present invention will be described. It should be noted that the following embodiments are representative examples, and the present invention is not to be limited to these embodiments, but should be interpreted within the scope of the technical concept described in the specification.

[0060] The various physical properties of the silver coatings in this embodiment and comparative example are evaluated as follows.

[0061] <Coating thickness and Bi content>

[0062] Using a Hitachi High-Tech Science fluorescence X-ray film thickness gauge for 160 hours, the central portion of the sample was measured using 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 A1 as a first-order filter. Kα rays and Lα rays, which are characteristic X-rays of silver and bismuth, were used for measurement, respectively.

[0063] <Measuring Contact Resistance>

[0064] Using a CRS-113-AU type electrical contact simulator (Au wire, φ0.5mm) manufactured by Yamazaki Seiki Research Institute Co., Ltd., the contact resistance of the sample was measured once at 5 points under the conditions of a load of 0.05N, an operating load of 1mm, an operating speed of 1mm / min, and a measurement point of 5 points. The average value of the results at each measurement point was calculated.

[0065] <Determination of Vickers Hardness>

[0066] Using a Mitutoyo HM-221 microhardness tester from Mitutoyo Corporation, the central part of the sample was tested five times with a Vickers indenter under a load of 0.05 N to 0.1 N, and the average value was calculated.

[0067] <Determination of crystallite size>

[0068] The X-ray diffraction apparatus (Smart Lab II) manufactured by Rigaku Corporation was used to measure and calculate the central part of the sample with a scanning step of 0.02°, a scanning range of 30° to 150°, a scanning speed of 40° / min, an entrance slit of 1.00 mm, an open light-receiving slit, and a Hypix-3000 detector. The crystallite size was determined by the diffraction lines of the {200} plane using the Scherrer formula K = 0.94.

[0069] (Examples 1 to 5)

[0070] The square copper plate (2.5cm×2cm) used as the substrate was subjected to alkaline electrolytic degreasing and pickling to clean the surface. Then, it was electroplated with a matte nickel sulfamate bath (nickel sulfamate: 450g / L, liquid temperature: 55℃, pH: 4, current density 2ASD) to form a nickel plating film with a thickness of 1μm on the surface of the substrate.

[0071] Next, in order to improve the adhesion, silver electroplating was performed on the nickel film by electrolysis for 5 to 10 seconds in a silver electroplating bath (potassium silver cyanide: 3.7 g / L, potassium cyanide: 100 g / L, liquid temperature: 30℃, pH: 12, current density 2 ASD).

[0072] Then, on the substrate with the nickel plating and silver contact plating formed, a silver plating film was formed under the above conditions to form a silver plating film containing a predetermined amount of bismuth. The bismuth content and crystallite size in the silver plating films formed in Examples 1 to 5 are shown in Table 1.

[0073] For the silver coating obtained above, Vickers hardness and contact resistance were measured. It should be noted that Vickers hardness and contact resistance were measured before heat treatment (initial values) and after heat treatment (180°C, 100 hours). The results are shown in Table 1. As shown in Table 1, in Examples 1 to 5, the contact resistance after heat treatment was low, and the Vickers hardness also showed high values.

[0074] [Table 1]

[0075]

[0076] (Comparative Examples 1 to 5)

[0077] The square copper plate (2.5cm×2cm) used as the substrate was subjected to alkaline electrolytic degreasing and pickling to clean the surface. Then, it was electroplated with a matte nickel sulfamate bath (nickel sulfamate: 450g / L, liquid temperature: 55℃, pH: 4, current density 2ASD) to form a nickel plating film with a thickness of 1μm on the surface of the substrate.

[0078] Next, in order to improve the adhesion, silver electroplating was performed on the nickel film by electrolysis for 5 to 10 seconds in a silver electroplating bath (potassium silver cyanide: 3.7 g / L, potassium cyanide: 100 g / L, liquid temperature: 30℃, pH: 12, current density 2 ASD).

[0079] Then, a silver coating was formed on the substrate on which a nickel coating and a silver contact coating were formed, under the following conditions. The bismuth content and crystallite size in the silver coatings formed in Comparative Examples 1 to 5 are shown in Table 1.

[0080] (Composition of the plating bath)

[0081] Potassium silver cyanide (as silver): 35–100 g / L

[0082] Potassium cyanide: 65–125 g / L

[0083] Bismuth compounds (as bismuth): 0.2–50 g / L

[0084] Carboxylates: 1.5–150 g / L

[0085] Sulfur-based gloss agents: 1-10 g / L

[0086] Potassium carbonate: 0–20 g / L

[0087] (Conditions for electroplating)

[0088] Cathode current density: 1~10A / dm 2

[0089] pH: 11-14

[0090] Liquid temperature: 25℃~55℃

[0091] For the silver coating obtained above, Vickers hardness and contact resistance were measured. It should be noted that the Vickers hardness and contact resistance were measured before and after heat treatment (atmospheric environment, 180°C, 100 hours). The results are shown in Table 1.

[0092] In Comparative Example 1, since it does not contain bismuth and the crystallite size is as large as 285 Å, the Vickers hardness is as low as Hv95. In addition, the Vickers hardness after heat treatment becomes Hv58, resulting in a large decrease in hardness due to heat.

[0093] In Comparative Example 2, the crystallite size was as small as 170 Å, but because it did not contain bismuth, the Vickers hardness was as low as Hv127. Furthermore, the Vickers hardness after heat treatment became Hv57, resulting in a large decrease in hardness caused by heat.

[0094] In Comparative Example 3, although it contained 0.05% by weight of bismuth, the crystallite size was as large as 236 Å, resulting in a Vickers hardness as low as Hv119 and a Vickers hardness as low as Hv115 after heat treatment.

[0095] In Comparative Example 4, although it contained 0.56% by weight of bismuth, the crystallite size was as large as 232 Å, resulting in a Vickers hardness as low as Hv129 and a hardness of Hv130 after heat treatment.

[0096] In Comparative Example 5, due to the high bismuth content, the contact resistance after heat treatment was 2.7 mΩ, resulting in a large increase in contact resistance.

[0097] [Potential for Industrial Applications]

[0098] The silver plating film described in this embodiment exhibits excellent conductivity and wear resistance even at high temperatures. This silver plating film is suitable for use as electrical contacts and terminals, such as connectors, switches, and relays. It is particularly suitable for use as electrical contacts and terminals in electronic components that generate heat when carrying large currents.

Claims

1. A silver coating containing 0.04% by weight or more and less than 1% by weight of bismuth, wherein the crystallite size of the silver coating is [missing information]. the following.

2. The silver coating according to claim 1, wherein the contact resistance of the silver coating is less than 2mΩ.

3. The silver coating according to claim 1 or 2, wherein the contact resistance of the silver coating after heat treatment at 180°C for 100 hours is less than 2mΩ.

4. The silver coating according to claim 1 or 2, wherein the Vickers hardness of the silver coating is Hv135 or higher.

5. The silver coating according to claim 1 or 2, wherein the Vickers hardness of the silver coating after heat treatment at 180°C for 100 hours is Hv135 or higher.

6. The silver coating according to claim 5, wherein the reduction in Vickers hardness of the silver coating after heat treatment at 180°C for 100 hours is within Hv20.

7. An electrical contact comprising the silver coating as described in claim 1 or 2.

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