Sn-plated brass sheet material and its manufacturing method

Abstract

To provide a Sn-plated brass plate material in which a surface layer of a structure in which a Cu layer directly contacting a brass exists is easily assembled with low cost when provided to heat treatment at a temperature where melting of Sn does not occur.SOLUTION: A Sn-plated brass plate material includes a Cu-plated layer with a thickness of 0.28-0.80 μm on a surface of a substrate made of a brass plate, and moreover, a Sn-plated layer with a thickness of 2.8-5.0 μm on the Cu-plated layer. The Sn-plated brass plate material has a property in which a Cu layer derived from the Cu-plated layer in the whole area on the substrate on which the Cu-plated layer and the Sn-plated layer are formed after heat treatment when the heat treatment is performed while being kept at 150°C for 90 minutes in atmosphere.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a Sn-plated brass sheet material in which the surface of a brass sheet is coated with a Sn-plated layer, and a method for producing the same. [Background technology]

[0002] Sn-plated copper-based materials are often used for current-carrying components such as the conductive members and connectors of electronic components. When Sn plating is applied to copper-based materials, a Cu plating layer is often formed first as an undercoat to ensure plating adhesion, etc. In applications where the gloss of the plated surface is important, a reflow process may be performed after Sn plating. However, there are also many applications where reflow processing is not performed, and when electronic components such as connectors made from Sn-plated copper-based materials are embedded in resin, heat treatment (insert molding) in a temperature range where Sn does not melt is sufficient.

[0003] Patent Document 1 shows an example (No. 31 in Table 2) in which a 0.2 μm thick Cu undercoat and a 1 μm thick Sn plating are applied to the surface of a brass material (substrate No. 4), and then a reflow treatment is performed at 500°C.

[0004] Patent Document 2 shows examples (Nos. 27 and 28 in Table 3) in which a 0.45 μm or 0.41 μm thick Cu underlayer and a 0.85 μm or 0.93 μm thick Sn plating are applied to the surface of a brass material (base material m), and then a Ni or Ag metal film is formed on top of that.

[0005] Patent Document 3 describes applying Sn plating to the surface of a brass material via an underlying Ni plating layer and Cu plating layer. It also describes heat treatment at 150 to 170°C to generate Cu6Sn5 at the interface between the Cu plating layer and the Sn plating layer, thereby reducing the insertion force of the terminal. It teaches that without the Ni plating layer, the Cu plating layer reacts quickly with Sn, and is therefore unable to function as a zinc diffusion barrier (paragraph 0035).

[0006] Patent Document 4 describes that in order to prevent stress corrosion cracking of brass caps used in cylindrical fuses, a base Cu plating of 5 μm or more is applied to the surface of the brass material, and then an Sn plating is applied on top of that, with the total thickness of the two layers being 7 to 20 μm. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-156050 [Patent Document 2] Japanese Patent Application Publication No. 8-55521 [Patent Document 3] Japanese Patent Application Publication No. 11-135226 [Patent Document 4] Japanese Patent Application Publication No. 6-267397 Summary of the Invention [Problem to be solved by the invention]

[0008] In conventional Sn-plated brass materials, in which a Sn-plated layer is formed on the surface of a brass material via a Cu-plated layer, heat treatment at a temperature range where the Sn-plated layer does not melt (e.g., insert molding, around 150°C) can cause discoloration of the surface of the Sn-plated layer when the Sn-plated brass material is embedded in a resin to form an electronic component such as a connector. Furthermore, even without heat treatment, similar discoloration can occur during storage before component processing or during use as a component. A discolored Sn-plated layer can also increase surface resistance. According to the inventor's research, the primary cause of this type of discoloration is the diffusion of Zn from the substrate to the surface, resulting in the formation of a Zn-containing oxide on the surface of the material coated with the plated metal. Therefore, to resolve the above-mentioned problems (discoloration and increased surface resistance) specific to brass substrates, measures are needed to prevent Zn from diffusing to the surface.

[0009] In the example of the Sn-plated brass material specifically shown in Patent Document 1 (No. 31 mentioned above), a reflow treatment is performed. However, according to the inventor's investigation, if the Sn-plated brass material (before the reflow treatment) is subjected to a heat treatment at a temperature at which Sn does not melt (for example, around 150°C), the above-mentioned discoloration problem cannot be solved.

[0010] The examples of Sn-plated brass materials specifically shown in Patent Document 2 (Nos. 27 and 28 mentioned above) have a metal film of Ni or Ag formed on the surface. However, according to the inventor's investigation, if the Sn-plated brass material is subjected to heat treatment at a temperature at which Sn does not melt (for example, around 150°C) without forming a metal film of Ni or Ag, the above-mentioned discoloration problem still cannot be solved.

[0011] As mentioned above, Patent Document 3 teaches that in a Sn-plated brass material in which Sn is plated via an underlying Ni and Cu plating layers, if the Ni plating layer is absent, the Cu plating layer will not function as a zinc diffusion barrier. Therefore, even if the technology disclosed in Patent Document 3 changes the two-layer structure of the base consisting of Ni and Cu plating layers to a single-layer structure consisting of a Cu plating layer, the discoloration problem described above cannot be expected to be resolved. On the other hand, if the base is a single-layer structure consisting of only a Ni plating layer, it is believed that the effect of suppressing Zn diffusion can be achieved to some extent. However, since the Ni plating layer tends to harden the material and reduce bending workability, etc., in order to provide a versatile Sn-plated brass material, including for applications where a hard Sn-plated brass material is not desired, it is desirable to solve the discoloration problem by using a method that does not apply a Ni plating layer to the base.

[0012] In the technology disclosed in Patent Document 4, a thick Cu plating layer of 5 μm or more is formed on the brass cap of a cylindrical fuse in order to improve the stress corrosion cracking resistance. However, the method of forming such a thick Cu plating layer on a versatile Sn-plated brass sheet material increases costs and is difficult to adopt.

[0013] According to the inventor's research, it has been confirmed that the Cu layer directly bonded to the brass substrate functions as a barrier layer that prevents Zn from diffusing from the brass. The object of the present invention is to provide a Sn-plated brass sheet material that can be easily and inexpensively constructed with a surface layer structure in which a Cu layer in direct contact with the brass exists when subjected to heat treatment at a temperature that does not cause Sn melting. [Means for solving the problem]

[0014] The above object is achieved by a Sn-plated brass sheet material having a Cu plating layer of 0.28 to 0.80 μm thickness on the surface of a base material made of a brass sheet, and a Sn plating layer of 2.8 to 5.0 μm thickness on the Cu plating layer.

[0015] More specifically, when the above-mentioned Sn-plated brass sheet material is subjected to a heat treatment in an air atmosphere at 150°C for 90 minutes, after the heat treatment, a Cu layer derived from the Cu plating layer remains on the entire surface of the area on the substrate where the Cu plating layer and the Sn plating layer had been formed.

[0016] The present invention also provides a method for producing the above-mentioned Sn-plated brass sheet material, which comprises performing electrolytic Cu plating on the surface of a base material made of a brass plate under conditions whereby a Cu plating layer having an average thickness of 0.28 to 0.80 μm is formed, and performing electrolytic Sn plating on the Cu plating layer formed by the electrolytic Cu plating under conditions whereby a Sn plating layer having an average thickness of 2.8 to 5.0 μm is formed.

[0017] More specifically, by carrying out Cu electroplating and Sn electroplating under the above conditions, a method for producing a Sn-plated brass sheet material is provided in which, when heat treatment is carried out in an air atmosphere at 150°C for 90 minutes, a Cu layer derived from the Cu plating layer remains over the entire surface of the area on the base material where the Cu plating layer and the Sn plating layer had been formed after the heat treatment.

[0018] In this specification, the term "plate material" refers to a sheet-like metal material formed by utilizing the malleability of metal. Thin sheet-like metal materials are sometimes called "foils," and such "foils" are also included in the term "plate material" here. A long sheet-like metal material wound into a coil is also included in the term "plate material." [Effects of the Invention]

[0019] The Sn-plated brass sheet material of the present invention undergoes heat treatment in a temperature range where Sn does not melt (e.g., 150°C ± 30°C), and then develops a surface layer structure in which a Cu layer is present in direct contact with the brass substrate. In Sn-plated brass parts that have undergone such heat treatment, the Cu layer functions as a barrier layer, significantly suppressing the diffusion of Zn from the brass substrate to the material surface, preventing surface discoloration, which is a common problem in insert processing of Sn-plated brass parts. Furthermore, the Sn-plated brass sheet material of the present invention also exhibits excellent discoloration resistance when stored for long periods without heat treatment. The Sn-plated brass of the present invention can be produced at a cost not significantly different from that of conventional products. [Brief explanation of the drawings]

[0020] [Figure 1] 2 is a diagram showing a schematic cross-sectional structure of the surface layer of the Sn-plated brass sheet material of the present invention before and after heat treatment. FIG. [Figure 2] Photographs showing examples of the surface appearance of test pieces (Nos. 1 to 4) in which the Sn-containing layer formed on the surface layer of a heat-treated Sn-plated brass sheet material was peeled off. DETAILED DESCRIPTION OF THE INVENTION

[0021] Brass (Cu-Zn-based copper alloy) is a versatile material widely used in electronic components, electrical components, and the like. The present invention is directed to a Sn-plated brass sheet material using a brass sheet material as a base material. Specifically, the present invention is directed to a Cu-Zn-based copper alloy having a composition range of 28.5 to 40.8 mass% Zn and 99.0 mass% or more of the total of Cu and Zn. This includes alloys designated C2600, C2680, C2720, and C2801 as "brass" in JIS H3100:2000. A more representative composition range is a Cu-Zn-based copper alloy having 28.5 to 31.5 mass% Zn and 99.0 mass% or more of the total of Cu and Zn. This corresponds to the alloy designated C2600.

[0022] FIG. 1 shows a schematic cross-sectional structure of the surface layer of the Sn-plated brass sheet material of the present invention before and after heat treatment. The "before heat treatment" section in the upper part of FIG. 1 represents the Sn-plated brass sheet material of the present invention. A Cu plating layer is formed as an undercoat on the surface of a brass substrate (brass substrate), and a Sn plating layer is formed on the Cu plating layer. The Cu plating layer and the Sn plating layer can be formed by known electroplating methods. It is important to adjust the thickness of the Cu plating layer to 0.28 to 0.80 μm and the Sn plating layer to 2.8 to 5.0 μm. These plating layer thicknesses can be achieved by controlling the plating conditions so that the average deposition thickness (average film thickness) of the plated metal, calculated based on the electroplating conditions and the density (specific gravity) of the metallic Cu or Sn, falls within the above-mentioned numerical range.

[0023] When the Sn-plated brass sheet material according to the present invention shown in the upper part of Figure 1 is subjected to heat treatment in a temperature range where Sn does not melt (for example, 150°C ± 30°C), Cu atoms in the Cu plating layer and Sn atoms in the Sn plating layer interdiffuse to form a Cu-Sn alloy layer at the interface between the two layers. When heat treatment is applied near the above temperature range, for example, by insert molding, which is performed when embedding parts such as connectors in resin, if a Cu layer derived from the Cu plating layer remains, as shown in the "After Heat Treatment" diagram in the lower part of Figure 1, it has been found that this Cu layer functions as a barrier layer that inhibits the diffusion of Zn, significantly suppressing the penetration of Zn in the brass base material into the Sn layer. As a result of various investigations, it was found that if a Sn-plated brass sheet material exhibits the property of leaving a Cu layer derived from the Cu plating layer over the entire area where the Cu and Sn plating layers had been formed on the brass substrate when subjected to a heat treatment experiment in an air atmosphere at 150°C for 90 minutes, it is possible to leave the Cu layer in the usual heat treatment process that is industrially performed when manufacturing electronic components by heating to a temperature range where Sn does not melt, such as the above-mentioned insert molding. Furthermore, a Sn-plated brass sheet material exhibiting the above-mentioned properties also has excellent discoloration resistance when stored for a long period of time without heat treatment, or when a component manufactured using a process without heat treatment is used for a long period of time. In FIG. 1, the thickness of the Cu plating layer and the Cu layer is exaggerated.

[0024] The remaining Cu layer can be effective even if it is very thin, as long as there is no direct contact between the brass substrate and the Cu-Sn alloy layer. For example, when a sample is prepared by stripping the Sn and Cu-Sn layers using a fluorine-based acidic chemical (Sn stripper) that dissolves Sn and Cu-Sn alloy layers but not Cu layers, if no exposed brass substrate is observed, the sample can be considered to have a Cu layer remaining on the surface. Because the surface color of brass is clearly different from that of copper, the presence of the Cu layer can be visually confirmed in samples from which the Sn and Cu-Sn layers have been stripped.

[0025] If the thickness of the Cu plating layer (primary plating) is less than 0.28 μm, it becomes difficult to stably retain Cu when subjected to a heat treatment experiment in which the material is held at 150°C for 90 minutes in an air atmosphere. If a thick Cu plating layer is applied, the function of retaining the Cu layer after heat treatment becomes excessive, which is undesirable from a cost perspective when producing a versatile Sn-plated brass sheet material. Therefore, in the present invention, the thickness of the primary Cu plating layer is specified to be 0.28 to 0.80 μm. A thickness of 0.30 to 0.80 μm is more preferable, and a thickness of 0.40 to 0.80 μm is even more preferable.

[0026] If the thickness of the Sn plating layer is less than 2.8 μm, the Sn layer remaining as the upper layer when the Cu-Sn alloy layer is formed will be thinner, resulting in a significant decrease in solder wettability and whisker formation. On the other hand, if the thickness of the Sn plating layer exceeds 5.0 μm, the cost will increase. Therefore, in the present invention, the thickness of the underlying Sn plating layer is specified to be 2.8 to 5.0 μm, and 2.8 to 4.0 μm is more preferable. [Example]

[0027] Commercially available brass sheet corresponding to alloy C2600 was cut into strips measuring 75 mm in length, 20 mm in width, and 0.3 mm in thickness. Using these sheets as substrates, Cu undercoats of various thicknesses were formed using a conventional Cu electroplating method (using a cyanide bath), and then a 3 μm-thick Sn layer was formed on top of this using a conventional Sn electroplating method (using an organic acid bath), yielding Sn-plated brass sheets. The bath temperature, current density, and plating time, as well as the plating layer thickness (average film thickness) formed under these conditions in the plating bath and plating equipment used, are shown in Table 1.

[0028] Each Sn-plated brass sheet was subjected to a heat treatment in an air atmosphere at 150°C for 90 minutes. The surface of the heat-treated sample was treated with a fluorine-based acidic chemical (TL-105, manufactured by Meltex Inc.) that dissolves Sn-containing layers such as Sn layers and Cu-Sn alloy layers but not Cu layers, thereby removing the Sn-containing layer from the surface. This treatment will be referred to as the "removal treatment" hereafter. The surface of the test piece after the removal treatment was visually observed to check the remaining state of the Cu layer, and the remaining Cu layer after the heat treatment was evaluated according to the following criteria. ◯: The Cu layer remains on the entire surface. △: The Cu layer remains, but the brass substrate is partially exposed. ×: Almost no remaining Cu layer is observed, and the brass substrate is exposed over almost the entire surface. Five tests were conducted on each Sn-plated brass sheet material, and the evaluation result of the test piece with the worst performance among the five test pieces was adopted as the performance of that Sn-plated brass sheet material. Test pieces with an evaluation of ○ were judged to pass. The results are shown in Table 1.

[0029] [Table 1]

[0030] In the examples where the underlying Cu plating layer was formed to a thickness that met the specification of the present invention (0.28 to 0.80 μm), the remaining Cu layer was observed over the entire surface, and the Cu layer remnant property was rated as ○. In these examples, the Cu layer functions as a barrier layer that inhibits the diffusion of Zn, demonstrating high discoloration resistance.

[0031] Figure 2 shows photographs of the surface appearance of the test pieces after the peeling treatment for each example. These are monochrome versions of color photographs. The test pieces rated × exhibit a brass color tone over almost the entire surface, while the test pieces rated ○ exhibit a copper color tone over the entire surface. The difference in color tone between brass and copper can be distinguished visually.

Claims

1. A Sn-plated brass sheet material having a Cu plating layer of 0.28 to 0.80 μm in thickness on the surface of a base material made of a brass plate, and a Sn plating layer of 2.8 to 5.0 μm in thickness on the Cu plating layer, the Sn plating layer constituting the material surface of the sheet material.

2. 2. The Sn-plated brass sheet material according to claim 1, wherein, when subjected to a heat treatment of holding at 150°C for 90 minutes in an air atmosphere, after the heat treatment, a Cu layer derived from the Cu plating layer remains on the entire surface of the region on the substrate where the Cu plating layer and the Sn plating layer were formed.

3. 2. The method for producing a Sn-plated brass sheet material according to claim 1, wherein electrolytic Cu plating is performed on a surface of a base material made of a brass plate under conditions in which a Cu plating layer having an average thickness of 0.28 to 0.80 μm is formed, and electrolytic Sn plating is performed on the Cu plating layer formed by the electrolytic Cu plating under conditions in which a Sn plating layer having an average thickness of 2.8 to 5.0 μm is formed.

4. 2. The method for producing a Sn-plated brass sheet material according to claim 1, wherein the metal coating layer is provided with the Cu plating layer and the Sn plating layer, and the Cu plating layer is provided with the Sn plating layer ... Sn plating layer is provided with the Sn plating layer and the Cu plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Cu plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Cu plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Cu plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Cu plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Cu plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Sn plating layer is provided with the Sn plating layer and the Sn

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