BACKGROUND OF THE INVENTION

Technical Field

The present invention relates to a tin-plated copper terminal material, a terminal made of the terminal material, and an electric-wire terminal structure using the terminal.

Priority is claimed on Japanese Patent Application No. 2017-096979, filed May 16, 2017, the content of which is incorporated herein by reference.

Background Art

Conventionally, by connecting a terminal crimped to a terminal end of a conductive wire to a terminal provided on the other equipment, the conductive wire is connected to the equipment. The conductive wire and the terminal are generally formed from copper or a copper alloy having high electrical conductivity; a conductive wire made of aluminum or an aluminum alloy is also used for reducing weight and so forth.

For instance, in Patent Document 1, disclosed is an electric wire with a terminal mounted on vehicles such as an automobile, in which a terminal made of copper (a copper alloy) on which tin plating is formed is crimped on a conducting wire made of aluminum or an aluminum alloy.

In a case in which the conductive wire is made of aluminum or an aluminum alloy and the terminal is made of copper or a copper alloy, if water enters between the terminal and the conductive wire; the conductive wire may corrode by galvanic corrosion resulting from an electric potential difference between different metals; and it may arise that an electrical resistance value in a crimped part be increased and a crimping force be reduced.

In order to prevent the galvanic corrosion, in Patent Document 1 for instance, between a substrate layer and a tin layer in the terminal, an anti-corrosion layer made of a metal (zinc or a zinc alloy) having a sacrificial anti-corrosion property against the substrate layer is formed.

An electrical contact material for connectors shown in Patent Document 2 has a substrate made of a metal material, an alloy layer formed on the substrate, and a conductive film layer formed on a surface of the alloy layer. The alloy layer essentially includes Sn (tin), and further includes one or more of an additive element selected from Cu, Zn, Co, Ni and Pd. As the conductive film layer, one including Sn3O2(OH)2(hydroxide-oxide) is disclosed.

As an example of adding Zn in Sn, Patent Document 3 discloses an Sn plating material. This Sn plating material has a ground Ni-plating layer, an intermediate Sn—Cu plating layer, and a superficial Sn plating layer in order on a surface of copper or a copper alloy. In this Sn plating material, the ground Ni-plating layer is made of Ni or a Ni alloy; and the intermediate Sn—Cu plating layer is made of an Sn—Cu alloy in which an Sn—Cu—Zn alloy layer is formed at least at a side in contact with the superficial Sn plating layer: the superficial Sn plating layer is made of an Sn alloy including Zn with 5 to 1000 ppm by mass, and further has a high-concentration Zn layer in which Zn concentration is more than 0.2% by mass to 10% by mass on an outermost surface.

CITATION LIST

Patent Literature

Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2013-218866

Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2015-133306

Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-285729

SUMMARY OF INVENTION

Technical Problem

In a case in which the anti-corrosion layer made of zinc or a zinc alloy is provided as a ground of the tin layer as in Patent Document 1, there is a problem in that an Sn substitution arises when carrying out an Sn plating treatment on the anti-corrosion layer, and adhesiveness between the anti-corrosion layer and the Sn plating is deteriorated.

In a case in which the conductive film layer including Sn3O2(OH)2(hydroxide-oxide) is provided as in Patent Document 2, there is a problem in that the conductive film layer is immediately damaged when it is exposed in corrosive environment or heating environment and durability is low. Moreover, one having the Sn—Zn alloy (the superficial Sn plating layer) laminated on the Sn—Cu type alloy layer (the intermediate Sn—Cu plating layer) and the high-concentration Zn layer on the outermost layer as in Patent Document 3 has a problem in that the Sn—Zn alloy plating is low in productivity, and an anti-corrosion effect cannot be obtained with respect to the conductive wire made of aluminum if copper in the Sn—Cu type alloy layer is exposed on a surface layer.

The present invention is achieved in consideration of the above circumstances, and has an object to provide a tin-plated copper terminal material in which the galvanic corrosion is effectively suppressed, a terminal made of the terminal material, and an electric-wire terminal structure using the terminal.

Solution to Problem

A tin-plated copper terminal material of the present invention has: a substrate made of copper or a copper alloy; an intermediate zinc layer which is formed on the substrate, made of a zinc alloy, and has a thickness of 0.10 μm to 5.00 μm (inclusive); and a tin layer which is formed on the intermediate zinc layer, made of tin or a tin alloy, in which a proportion of a length occupied by low-angle grain boundaries in a length of all crystal grain boundaries is 2% to 30% (inclusive).

In this tin-plated copper terminal material, since the intermediate zinc layer which is made of a zinc alloy having a corrosion potential nearer to that of aluminum than that of tin is provided under the tin layer at a surface and zinc thereof is diffused on the surface of the tin layer; a corrosion potential is lowered at the surface. Thereby, there is an effect of preventing contact corrosion of different metals (galvanic corrosion) when being in contact with a conductive wire made of aluminum and the like having a lower corrosion potential than that of copper.

In this case, if the thickness of the intermediate zinc layer is less than 0.10 μm, the diffusion of zinc to the surface of the tin layer is not enough, so that the effect of reducing the corrosion potential at the surface is not enough and a corrosion current is increased. If the thickness of the intermediate zinc layer is more than 5.00 μm, breakages may arise while a bending work.

The zinc in the intermediate zinc layer under the tin layer is supplied to the surface through the crystal grain boundaries of the tin layer: diffusion speed of zinc is low through the low-angle grain boundaries among the crystal grain boundaries, and it does not contribute to the diffusion of zinc (i.e., reduction of the corrosion potential). Accordingly, by appropriately setting the proportion of the low-angle grain boundaries, the diffusion speed of zinc can be controlled to be desirable. If the length proportion of the low-angle grain boundaries is less than 2%, zinc is excessively supplied, so that whiskers can be easily generated. If the length proportion of the low-angle grain boundaries is more than 30%, zinc is not diffused enough, the effect of reducing the corrosion potential is not enough, and the corrosion current is high.

In the tin-plated copper terminal material of the present invention, it is preferable that corrosion potential with respect to a silver-silver chloride electrode be not more than −500 mV and not less than −900 mV.

In this case, the corrosion current can be reduced low and an anti-corrosion effect is excellent.

In the tin-plated copper terminal material of the present invention, it is preferable that the intermediate zinc layer include one or more additional elements among nickel, iron, manganese, molybdenum, cobalt, cadmium and lead, and content percentage of zinc in the intermediate zinc layer be 65% to 95% by mass (inclusive).

By including these additional elements, zinc is prevented from excessive diffusing and there is an effect of reducing the generation of whiskers. If the content percentage of zinc is more than 95% by mass, the zinc diffusion to the surface of the tin layer is excessive, contact resistance is increased, and the effect of preventing the whiskers is poor. If the content percentage of zinc is less than 65% by mass, the zinc diffusion is not enough, and the corrosion current tends to be high.

In the tin-plated copper terminal material of the present invention, it is preferable that an average crystal grain size of the tin layer be 0.5 μm to 8.0 μm (inclusive).

If the average crystal grain size of the tin layer is less than 0.5 μm, zinc is excessively diffused because density of the crystal grain boundaries is too high, corrosion durability of the tin layer is deteriorated, the tin layer exposed in the corrosive environment corrodes, and the contact resistance may be deteriorated with respect to the conductive wire. If the average crystal grain size of the tin layer is more than 8.0 μm, the zinc diffusion is not enough, and the effect of preventing the corrosion of the conductive wire is poor.

In the tin-plated copper terminal material of the present invention, it is preferable that a surface metallic-zinc layer be provided on the tin layer. In this case, it is possible to prevent generation of the galvanic corrosion resulting from contact with the conductive wire more reliably.

It is more preferable that the surface metallic-zinc layer have zinc concentration of 5 at % to 40 at % (inclusive) and a thickness of 1.0 nm to 10.0 nm (inclusive) in terms of SiO2.

In the tin-plated copper terminal material of the present invention, it is preferable that a ground layer be provided between the substrate and the intermediate zinc layer, that is made of nickel or a nickel alloy, with a thickness of 0.10 μm to 5.00 μm (inclusive) and a nickel content percentage of not less than 80% by mass.

The ground layer between the substrate and the intermediate zinc layer has functions of improving adhesiveness between them and preventing diffusion of copper from the substrate made of copper or a copper alloy to the intermediate zinc layer and the tin layer. If the thickness of the ground layer is less than 0.10 μm, the effect of preventing copper from diffusion is poor: if it is more than 5.00 μm, breakages easily arise while a press process. Moreover, if the nickel content percentage in the ground layer is less than 80% by mass, the effect of preventing copper from diffusing to the intermediate zinc layer and the tin layer is small.

It is preferable that the tin-plated copper terminal material of the present invention have a belt-sheet shape carrier part and elements for terminal connected to the carrier part and disposed with interval in a longitudinal direction of the carrier part.

A terminal of the present invention is a terminal formed from the above-mentioned tin-plated copper terminal material. An electric-wire terminal structure of the present invention is formed by crimping the terminal to a terminal end of an electric wire having a conductive wire made of aluminum or an aluminum alloy.

Advantageous Effects of Invention

According to the tin-plated copper terminal material of the present invention, since zinc is diffused in the tin layer so that the corrosion potential is lowered, there is the anti-corrosion effect with respect to the conductive wire with the low corrosion potential. Moreover, even though the tin layer is disappeared, the galvanic corrosion of the conductive wire is prevented by the intermediate zinc layer, so it is possible to prevent the electrical resistance value from rising and the adhesiveness from deteriorating.

