JP7302248B2 - Connector terminal materials and connector terminals - Google Patents

Description

TECHNICAL FIELD The present invention relates to a connector terminal material provided with a film and a connector terminal which are useful for connecting electrical wiring in automobiles, consumer equipment, and the like, where microsliding occurs.

2. Description of the Related Art Conventionally, connectors used for connecting electrical wiring in automobiles and the like are known. In this in-vehicle connector (in-vehicle terminal), a contact piece provided on a female terminal contacts a male terminal inserted into the female terminal with a predetermined contact pressure, whereby electrical connection is established. are used with terminal pairs designed to As such connectors (terminals), tin-plated terminals, which are generally obtained by plating a copper or copper alloy plate with tin and performing a reflow treatment, have been widely used. However, in recent years, with the increase in current and voltage in automobiles, the use of precious metal-plated terminals, which are excellent in heat resistance and wear resistance and which allow more current to flow, is increasing.

As a method of plating a vehicle-mounted terminal that requires such heat resistance and wear resistance, there is a method of applying silver plating to the terminal as described in Patent Document 1, for example. In this respect, the hardness of the silver-plated layer decreases due to the increase in silver crystal diameter due to heating. In order to suppress this decrease in hardness, it is conceivable to increase the thickness of the silver plating film, but there is a problem in terms of cost. Moreover, when a nickel plating layer is used as a base, there is also a problem that the silver plating layer peels off.
On the other hand, as in Patent Document 2, antimony may be added to the plating layer to increase wear resistance, but although the initial hardness is high, the hardness decreases due to heating. After concentrating on the surface, it oxidizes and increases the contact resistance. Furthermore, when a nickel-plated layer is used as an underlayer, nickel oxide is generated between the nickel-plated layer and the silver-plated layer by heating, and the nickel oxide may cause peeling of the plated layer. Furthermore, with respect to wear resistance, the hardness of the plating layer is specified, but it was insufficient to evaluate the wear resistance only by the hardness, such as adhesion between the plating layers during sliding.

In addition, in order to prevent deterioration of wear resistance due to heating, as in Patent Document 3, there is a method of sequentially laminating a silver plating layer and a tin plating layer and performing a reflow treatment to alloy the surface. Since only a part is alloyed, if the alloyed part is worn out by sliding, the deterioration of wear resistance cannot be suppressed.
Therefore, as in Patent Document 4, it has been considered to apply silver-tin alloy plating directly to the base material to keep the composition of the entire plating layer uniform.

Japanese Patent Application Laid-Open No. 2008-169408 Japanese Patent Application Laid-Open No. 2009-79250 Japanese Patent Application Laid-Open No. 2017-79143 JP 2015-183216 A

However, in the method described in Patent Document 4, a tin oxide film is formed on the surface in a heating environment, and the contact resistance is lowered by this oxide film.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a connector terminal material and a connector terminal that can improve wear resistance and heat resistance.

The terminal material for a connector of the present invention comprises a substrate having at least a surface made of copper or a copper alloy, a silver-tin alloy plating layer made of a silver-tin alloy coated on at least a part of the surface of the substrate, and the silver-tin a silver-plated layer made of silver having a purity of 99% by mass or more formed on the alloy-plated layer, wherein the silver-tin alloy-plated layer contains Ag in a range of 70 at% or more and 85 at% or less; The silver-tin alloy plating layer has a thickness of 0.5 μm or more and 10 μm or less, and the thickness of the silver plating layer is 0.05 μm or more and 2.0 μm or less.

Since the silver-plated layer is formed on the silver-tin alloy-plated layer in this connector terminal material, the surface is not easily oxidized even in a heated environment, and an increase in contact resistance can be suppressed.
When Ag in the silver-tin alloy plating layer is less than 70 at%, the silver plating layer thereon is not formed sufficiently, and the contact resistance after heating increases. Abrasion resistance decreases due to the increase in the proportion of soft silver in the Examples of silver-tin-based intermetallic compounds include intermetallic compounds of Ag ₃ Sn and Ag ₄ Sn.
If the silver-tin alloy plating layer has a thickness of less than 0.5 μm, the effect of improving wear resistance during use in a connector is poor, and the wear resistance decreases. There is no problem even if the film thickness exceeds 10 μm, but it is preferable to set the film thickness to 10 μm or less from the viewpoint of cost. If the film thickness of the silver plating layer is less than 0.05 μm, it is too thin and is likely to be worn away at an early stage. If the thickness exceeds 2.0 μm, the soft silver plating layer is thick and the coefficient of friction increases.

In one aspect of the connector terminal material, a nickel plating layer made of nickel or a nickel alloy is provided between the base material and the silver-tin alloy plating layer, and the thickness of the nickel plating layer is 0.05. It is good that it is 5 micrometers or more and 2 micrometers or less.

The nickel plating layer has the effect of preventing diffusion of the Cu component from the base material. When the thickness of the nickel layer is less than 0.5 μm, the Cu component diffuses into the silver-tin alloy plating layer from the substrate made of copper or copper alloy in a high-temperature environment, and the resistance value of the silver-tin alloy plating layer increases. If the thickness exceeds 2 μm, cracks may occur during bending.
In addition, since the silver-tin alloy plating layer is formed on the nickel plating layer, the silver-tin alloy and nickel interdiffuse, and tin and nickel form an intermetallic compound. Peeling can be suppressed.

The connector terminal of the present invention is a connector terminal made of the connector terminal material described above, and the surface of the contact portion with the mating connector terminal is the silver plating layer.

ADVANTAGE OF THE INVENTION According to this invention, the abrasion resistance and heat resistance of the terminal material for connectors and the terminal for connectors can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows typically the terminal material for connectors which concerns on one Embodiment of this invention. It is a SIM image (60 degrees of inclination angles) of the cross section of the terminal material for connectors before heating in an Example.

An embodiment of the present invention will be described below with reference to the drawings.
[Configuration of terminal material for connector]
FIG. 1 schematically shows a cross section of a connector terminal material according to one embodiment. This connector terminal material comprises a plate-shaped base material 2 whose surface is at least made of copper or a copper alloy, a nickel plating layer 3 made of nickel or a nickel alloy coated on the surface of the base material 2, and a nickel plating layer 3. A silver-tin alloy plating layer 4 covering at least part of the surface of (in this embodiment, the entire upper surface of the nickel layer 3), and a silver-plating layer 5 formed on the silver-tin alloy plating layer 4 I have.
The composition of the base material 2 is not particularly limited as long as it is made of copper or a copper alloy. Alternatively, it may be composed of a plated material in which a copper-plated layer made of copper or a copper alloy is applied to the surface of the base material. In this case, the base material may be a metal material other than copper.

The nickel plating layer 3 is coated by plating the base material 2 with nickel or a nickel alloy. This nickel plating layer 3 has the function of suppressing the diffusion of the Cu component from the substrate 2 to the silver-tin alloy plating layer 4 coated thereon. The thickness of this nickel plating layer 3 is preferably 0.5 μm or more and 2 μm or less. When the thickness of the nickel plating layer 3 is less than 0.5 μm, the Cu component diffuses into the silver-tin alloy plating layer 4 from the base material 2 made of copper or a copper alloy in a high-temperature environment, and the silver-tin alloy plating layer 4 is formed. The resistance value of is increased, and the heat resistance may be lowered. If the thickness exceeds 2 μm, cracks may occur during bending. The composition of the nickel plating layer 3 is not particularly limited as long as it is made of nickel or a nickel alloy.

The silver-tin alloy plating layer 4 is coated on the upper surface of the nickel plating layer 3 after silver strike plating is applied. This silver-tin alloy plating layer 4 is mainly composed of intermetallic compounds of Ag ₃ Sn and Ag ₄ Sn and contains Ag in a range of 70 at % or more and 85 at % or less. Since it contains such an intermetallic compound, wear resistance is improved.

In addition, the silver-tin alloy plating layer 4 has the effect of supporting the soft silver plating layer 5 formed thereon and improving the lubricity when used in the connector. The thickness of the silver-tin alloy plating layer 4 is preferably 0.5 μm or more and 10 μm or less. If the thickness of the silver-tin alloy plating layer 4 is less than 0.5 μm, the effect of improving slipperiness is poor, and wear resistance is lowered. Although there is no problem even if the film thickness exceeds 10 μm, it is preferable to set the film thickness to 10 μm or less from the viewpoint of cost.

The surface of the silver plating layer 5 is not easily oxidized even in a heating environment, and an increase in contact resistance can be suppressed. This silver plating layer 5 is made of pure silver with a purity of 99% by mass or more, preferably 99.9% by mass or more, and carbon is co-precipitated at a content of 0.1% by mass or more and 0.6% by mass or less. . The remainder contains light elements such as S, N, and H. The reason why the purity is set to 99% by mass or more is that if the Ag concentration of the silver plating layer 5 is less than 99% by mass, a large amount of impurities will be included, resulting in an increase in contact resistance.
In addition, since carbon is co-deposited in the silver plating layer 5, adhesion between the silver plating layers 5 is less likely to occur when sliding as a connector, and the coefficient of friction is lowered. In this case, if the carbon content is less than 0.1% by mass, the effect of reducing the coefficient of friction is poor, and if it exceeds 0.6% by mass, the silver plating layer 5 becomes brittle and workability may deteriorate.
The film thickness of the silver plating layer 5 is 0.05 μm or more and 2.0 μm or less. If the film thickness of the silver plating layer 5 is less than 0.05 μm, it is too thin and is likely to be worn away early. When the film thickness exceeds 2.0 μm, the soft silver plating layer 5 is thick, so the coefficient of friction increases.

Next, a method for manufacturing the connector terminal material 1 will be described. The method for manufacturing the connector terminal material 1 includes a pretreatment step of washing a plate material made of copper or a copper alloy to be the base material 2, a nickel plating layer forming step of forming the nickel plating layer 3 on the base material 2, and a nickel plating layer forming step. A silver strike plating step of applying silver strike plating on the plating layer 3, a silver-tin alloy plating layer forming step of forming a silver-tin alloy plating layer 4 on the nickel plating layer 3 to which the silver strike plating has been applied, and silver plating. and a silver plating layer forming step of forming the layer 5 on the silver-tin alloy plating layer 4 .

[Pretreatment process]
First, a plate material made of copper or a copper alloy is prepared as the base material 2, and pretreatment is performed to clean the surface of the plate material by degreasing, pickling, or the like.

[Nickel plating layer forming step]
At least part of the surface of the base material 2 is plated with nickel or a nickel alloy to form a nickel plating layer 3 on the base material 2 . For example, using a nickel plating solution containing 300 g/L of nickel sulfamate, 30 g/L of nickel chloride, and 30 g/L of boric acid, nickel plating is performed under the conditions of a bath temperature of 45° C. and a current density of 3 A/dm ² to form. be done. The nickel plating for forming the nickel layer 3 is not particularly limited as long as a dense nickel-based film can be obtained, and may be formed by electroplating using a known Watt bath.

[Silver strike plating process]
The surface of the nickel plating layer 3 formed on the base material 2 is subjected to an activation treatment using a potassium hydroxide aqueous solution of 5% to 10% by mass, and then subjected to silver strike plating. This silver strike plating is performed to improve adhesion between the silver-tin alloy plating layer 4 formed on the nickel plating layer 3 and the nickel plating layer 3 . The composition of the plating solution for applying this silver strike plating is not particularly limited as long as it is a non-cyanide bath (a plating bath that does not contain cyanide such as silver cyanide, potassium silver cyanide, sodium cyanide, potassium cyanide, etc.). A bath mainly composed of a silver methanesulfonate bath is desirable.

[Silver-tin alloy plating layer forming process]
Then, a silver-tin alloy plating layer 4 is formed by applying silver-tin alloy plating on the nickel plating layer 3 to which silver strike plating has been applied. A plating bath for this silver-tin alloy plating has a composition containing, for example, methanesulfonic acid, tin methanesulfonate, silver methanesulfonate, and an organic additive containing sulfur. Specifically, a silver-tin alloy plating solution adjusted to have a free methanesulfonic acid concentration of 40 g/L, an Ag concentration of more than 40 g/L but not more than 90 g/L, and an Sn concentration of 5 to 35 g/L is used. good. This silver-tin alloy plating solution does not contain cyanides such as silver cyanide, potassium silver cyanide, sodium cyanide and potassium cyanide. In addition, both Ag and Pt/Ti insoluble electrodes are used for the tin anode, the area of which is at least twice that of the cathode, and the current distribution between Ag and Pt/Ti is set to Ag:Pt/Ti=4:1. is preferred. Furthermore, the bath temperature is set to 40° C. to 60° C. and the current density is set to 1 to 15 A/dm ² to form the silver-tin alloy plating layer 4 .

[Silver plating layer forming step]
The silver-tin alloy plating layer 4 is plated with silver. Although the composition of the plating bath for this silver plating is not particularly limited, for example, potassium silver cyanide (K[Ag(CN) ₂ ]) 30 g/L to 60 g/L, potassium cyanide (KCN) 120 g/L to 160 g /L, 10 g/L to 20 g/L of potassium carbonate (K ₂ CO ₃ ), and an organic additive that is easily incorporated into the plating layer. Examples of organic additives that can be used include thioalcohols such as 2,2-thioethanol, azoles such as benzothiazoles and benzotriazoles, and imidazoles such as imidazole. The addition concentration of this organic additive is preferably 0.1 g/L or more and 10 g/L or less. A non-cyanide bath based on methanesulfonic acid or potassium iodide can also be used.
Then, using a pure silver plate as an anode for this silver plating bath, silver plating is performed for 1 second to 7 minutes under the conditions of a bath temperature of 10° C. or higher and 40° C. or lower and a current density of 1 A/dm ² or higher and 10 A/dm ² or lower. A silver plating layer is formed by applying a certain amount.

In this way, the nickel plating layer 3 is formed on the surface of the base material 2, and press working is performed on the connector terminal material 1 having the silver-tin alloy plating layer 4 and the silver plating layer 5 formed on at least a part of the surface. etc., to form a connector terminal in which a silver plating layer 5 is arranged on the surface of a portion used as a contact.

In this embodiment, since the silver plating layer 5 is formed on the surface, the surface is not easily oxidized even in a heating environment. Therefore, even after heating at 150° C. for 250 hours, the contact resistance is as low as 1 mΩ or less, and the heat resistance can be improved. Since the silver-tin alloy plating layer 4 is formed on the nickel layer 3 via the silver strike plating layer, the separation of the silver-tin alloy plating layer 4 from the nickel plating layer 3 can be suppressed. If the Ag content in the silver-tin alloy plating layer 4 is less than 70 at%, the contact resistance after heating decreases, and if the Ag content exceeds 85 at%, the grain size of the silver-tin alloy plating layer increases, and the wear resistance decreases. do.

The silver-tin alloy plating layer 4 of the present embodiment is obtained by processing the cross section of the terminal material with a focused ion beam device (FIB), and then from the plating surface of the silver-tin alloy plating layer 4 in the cross section toward the nickel plating layer 3. A composition analysis is performed by EPMA on a depth position P1 of 0.3 μm and a depth position P2 of 0.3 μm from the interface with the nickel layer 3 toward the surface side of the silver-tin alloy plating layer 4, respectively. The absolute value of the difference of (P1−P2) when the composition ratio of (Sn) and silver (Ag) is calculated by Ag/(Sn+Ag)×100 (at %) is 5 or less. That is, since the silver-tin alloy plated layer 4 is formed by the above-described plating process, the compositions of the intermetallic compounds of Ag ₃ Sn and Ag ₄ Sn at the positions P1 and P2 are substantially the same. Therefore, it is possible to provide the connector terminal material 1 in which the silver-tin alloy plating layer 4 is excellent in wear resistance and heat resistance (connection reliability).
In addition, a plating solution that does not contain cyanide is used for silver strike plating and silver-tin alloy plating, which facilitates wastewater treatment and reduces environmental impact. If a non-cyanide bath is used when forming the silver plating layer 5, the environmental load can be further reduced.

In addition, detailed configurations are not limited to those of the embodiments, and various modifications can be made without departing from the scope of the present invention. For example, in the above embodiment, the nickel plating layer 3 is provided between the base material 2 and the silver-tin alloy plating layer 4, but this is not limiting and the nickel plating layer 3 is not necessarily included. may That is, the silver-tin alloy plating layer 4 may be formed directly on the base material 2, and in this case, the nickel plating layer forming step and the silver strike plating step may not be performed.
In addition, although a silver-tin alloy plating layer and a silver plating layer were formed on a part of the surface of the terminal material, specifically, on the contact portion of the connector terminal, the silver-tin alloy plating layer and the silver plating layer were formed on the entire surface. It does not prevent forming.

A base material made of a copper alloy and having a thickness of 0.25 mm was plated with nickel to form a nickel plating layer with a thickness of 1.0 μm, and the sample on which the nickel plating layer was formed was added with 5% by mass of water. An activation treatment was performed to clean the surface of the nickel plating layer using an aqueous potassium oxide solution. After this activation treatment, silver strike plating was applied to the base material coated with the nickel plating layer, and then a silver-tin alloy plating layer and a silver plating layer were sequentially formed on each sample (samples 1 to 10). was made. At this time, the amount of Ag (g/L), the amount of Sn (g/L) and the current density in the silver-tin alloy plating solution were the values shown in Table 1.

For comparison, a silver-tin alloy plating layer was formed on the nickel plating layer without forming the silver-tin alloy plating layer (Sample 11), and a silver-tin alloy plating layer was formed on the nickel plating layer. There was also a sample in which a 2 μm-thick silver plating layer was not formed (Sample 12), and a 2 μm-thick silver alloy plating layer to which antimony was added was formed on a nickel plating layer (Sample 13). made.
In addition, the conditions of each plating were as follows.

<Nickel plating conditions>
・Plating bath composition Nickel sulfamate: 300 g / L
Nickel chloride: 30g/L
Boric acid: 30g/L
・Bath temperature: 45℃
・Current density: 3 A/dm ²

<Silver strike plating conditions>
・Plating bath composition Dyne Silver GPE-ST manufactured by Daiwa Kasei Co., Ltd.
・Anode: IrO ₂ /Ti insoluble anode ・Bath temperature: 25°C
・Current density: 1 A/dm ²

<Silver-tin alloy plating conditions>
・Plating bath composition Free methanesulfonic acid: 40 g / L
Tin methanesulfonate: 20 g/L
Silver methanesulfonate: 60 g/L
Organic additive: 5mg/L
・Bath temperature: 50℃
・Current density: 10 A/dm ²

<Silver plating conditions>
・Plating bath composition Potassium silver cyanide: 55 g / L
Potassium cyanide: 130g/L
Potassium carbonate: 15g/L
Nonionic surfactant: 1 g/L
2,2 thioethanol: 5 g/L
・Anode pure silver plate ・Bath temperature: 25℃
・Current density: 5 A/dm ²

The antimony-containing silver alloy plating layer of Sample 13 was produced by bright silver plating using Nissin Bright N bath containing antimony (manufactured by Nisshin Seisaku Co., Ltd.). A standard composition of the plating bath was used at a bath temperature of 25° C. and a current density of 1 A/dm ² , and a pure silver plate was used as the anode to form a silver alloy plating layer (AgSb alloy layer) having a thickness of 2 μm.

The Ag concentration (at%) in the silver-tin alloy plating layer, the film thickness of the silver-tin alloy plating layer, the Ag concentration (mass%) in the silver-plating layer, and the film thickness of the silver-plating layer were measured for the obtained sample. , abrasion resistance and contact resistance, and heat peeling resistance were evaluated.

[Ag concentration in silver-tin alloy plating (at%)]
The plated material is processed with a focused ion beam device (FIB) to prepare a cross-sectional sample, and the Ag concentration in the silver-tin alloy plating is measured using an electron beam microanalyzer: EPMA (model number JXA-8530F) manufactured by JEOL Ltd. The cross section of each sample was measured with an acceleration voltage of 10 kV and a beam diameter of 30 μm.
[Ag concentration in silver plating layer (% by mass)]
Using a glow discharge mass spectrometer (Astrum) manufactured by Ametech Co., Ltd., measurement was performed under the following conditions.
Integration time: 160msec/Ch
Discharge current: 2.0mA
Discharge voltage: 1.0 kV
Discharge gas: Ar (>99.9999)
Preliminary discharge: 20min

[Film thickness of each plating layer]
A cross-sectional sample was prepared by processing the plated material with a focused ion beam device (FIB), and the cross-sectional surface was observed and measured with a scanning ion microscope (SIM).
[Abrasion resistance]
Each sample was cut into a test piece of 60 mm×10 mm, a flat plate sample was used as a substitute for a male terminal, and a sample obtained by subjecting the flat plate sample to convex processing with a radius of curvature of 2.5 mm was used as a substitute for a female terminal. In the sliding test, a friction wear tester (UMT-Tribolab) manufactured by Bruker AXS Co., Ltd. was used, and the convex surface of the female test piece was brought into contact with the male terminal test piece installed horizontally, and a load of 5 N was applied. Then, slide the male terminal test piece horizontally at a moving distance of 5 mm and a sliding speed of 1 Hz, and check the wear depth after sliding 50 times. determined by no. At this time, those with a wear depth of less than 2.5 μm (the base was not exposed after the sliding test) were rated as good “A”, and those with the base exposed after the sliding test were rated as unsatisfactory “B”.

[Contact resistance]
Each sample was cut into a test piece of 60 mm×10 mm, and the flat plate sample was used as a substitute for a male terminal, and a sample obtained by subjecting the flat plate sample to convex processing with a radius of curvature of 2.5 mm was used as a substitute for a female terminal. The contact resistance was measured before heating and after heating at 150° C. for 250 hours. Using a friction wear tester (UMT-Tribolab) of Bruker AXS Co., Ltd., the convex surface of the female test piece is brought into contact with the male terminal test piece placed horizontally, and the male terminal test piece is loaded at a load rate of 1/15N/ A contact resistance value at 10 N when a load was applied from 0 N to 10 N in sec was measured by a four-probe method.

[Heat peelability]
In the heat-resistant peeling test, after heating at 150 ° C. for 1000 hours in an atmospheric heating furnace, the test was performed by the cross-cut method described in JISK5600-5-6. A sample that was peeled off was rated as "B".

As is clear from Tables 1 and 2, the silver-plated layer was formed on the silver-tin alloy plated layer, and the Ag concentration of the silver-tin alloy plated layer was 70 at% or more and 85 at% or less. Samples 1 to 4, 6, and 7 with a thickness of 0.5 μm or more and 10 μm or less and a thickness of the silver plating layer of 0.05 μm or more and 2.0 μm or less had a contact resistance of 1 mΩ even after heating at 150 ° C. for 250 hours. It was as small as below, and both wear resistance and heat-resistant peelability were good. FIG. 2 is a cross-sectional SIM image of sample 3 at an inclination angle of 60°. , a silver plating layer (denoted as Ag) is formed. All nickel plating layers had a film thickness of 1.0 μm. Moreover, all of the silver plating layers were silver plating layers containing 99% by mass or more of silver.
On the other hand, in sample 5, since the film thickness of the silver-tin alloy plating layer was as small as 0.4 μm, the wear resistance was inferior.
In sample 8, since the Ag concentration in the silver-tin alloy plating layer was 65 at %, the deposition of the silver-tin alloy plating layer was rough, and a silver plating layer could not be formed thereon.
Sample 9 had a high Ag concentration of 95 at % in the silver-tin alloy plating layer, and thus was inferior in wear resistance.
In sample 10, since the film thickness of the silver plating layer was as small as 0.02 μm, the contact resistance after heating increased.

1 Terminal Material for Connector 2 Base Material 3 Nickel Plating Layer 4 Silver Tin Alloy Plating Layer 5 Silver Plating Layer

Claims (3)

A base material having at least a surface made of copper or a copper alloy, a silver-tin alloy plating layer made of a silver-tin alloy covering at least a part of the surface of the base material, and a silver-tin alloy plating layer formed on the silver-tin alloy plating layer and a silver plating layer made of silver with a purity of 99% by mass or more, wherein the silver-tin alloy plating layer contains Ag in a range of 70 at% or more and 85 at% or less, and is mainly composed of a silver-tin-based intermetallic compound. , the film thickness of the silver-tin alloy plating layer is 0.5 μm or more and 10 μm or less, the film thickness of the silver plating layer is 0.05 μm or more and 2.0 μm or less,
Further, the terminal material for a connector, wherein the silver plating layer contains carbon co-deposited at a content of 0.1% by mass or more and 0.6% by mass or less . A nickel plating layer made of nickel or a nickel alloy is provided between the base material and the silver-tin alloy plating layer, and the thickness of the nickel plating layer is 0.5 μm or more and 2 μm or less. The connector terminal material according to claim 1 . 3. A connector terminal made of the connector terminal material according to claim 1, wherein a surface of a contact portion with a mating connector terminal is formed of the silver plating layer.

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