JP2023011169A - Plating method - Google Patents

Abstract

To provide a plating method capable of forming a good quality plating film having ultrathin and uniform thickness and capable of suppressing the occurrence of pinholes or discoloration.SOLUTION: A plating method comprises the steps of: preparing a body 10 to be plated having a first metal surface 11 consisting of first metal and a second metal surface 12 consisting of second metal electrically conducted (continued); applying a voltage to the body 10 to be plated before immersing the body 10 to be plated into a plating solution 14; and immersing a portion to be plated 16 of the body 10 to be plated into the plating solution 14 to coat at least a part of the portion to be plated 16 by coating metal for coating more noble than the first metal.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for plating metal surfaces.

2. Description of the Related Art Conventionally, film formation by a (wet) plating process has been frequently used as one of means for forming a metal film on a metal surface. In the wet plating process, an electroplating method is known as a method of forming a metal film by cathodic reduction of metal ions in an aqueous solution using an external DC power source. On the other hand, as a method for forming a metal film on a base material without using an external DC power supply, there is an electroless plating method, and a displacement type and an autocatalytic method are known. Substitution-type electroless plating is a method of forming a metal film by reducing metal ions in an aqueous solution with electrons generated as metals are eluted from the substrate surface and depositing them on the substrate surface. In the autocatalytic electroless plating method, the reducing agent added to the aqueous solution is oxidized on the substrate surface, and the electrons released at that time reduce the metal ions in the aqueous solution and deposit them on the substrate surface. is a method of forming a metal film.

Metal films formed by plating have various uses, one of which is anti-corrosion and anti-corrosion. A method of forming a metal alloy at a metal contact portion is known (see, for example, Patent Document 1).

JP-A-2006-46619

However, it is difficult to form a film thinly (for example, 3 μm or less) in the first place by electroplating, and generally there is a problem of many pinholes. Pinholes can be filled by increasing the thickness of the film. When it is desired to form a plating film only on a part of a component, it is not desirable to make the plating film thicker than necessary for the sole purpose of burying pinholes.

Also, the electroless plating method can make the film thinner than the electroplating method. However, when plating is performed on an object to be plated that is in contact with dissimilar metals by a conventionally known electroless plating method, discoloration and oxidation (burning) of the formed plating film are likely to occur, resulting in good quality (not oxide). ) It was difficult to deposit the coating metal. For example, when a plating film is used as a base and another film is laminated, if the plating film is not formed in good quality such as discoloration, the adhesion may deteriorate. was desired.

Thus, it has been very difficult or almost impossible to form a good plating film that satisfies these demands with conventional techniques.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a plating method capable of forming a high-quality plating film which is extremely thin, uniform, and which greatly suppresses the occurrence of pinholes and discoloration.

The present invention provides a substrate in which a surface made of a first metal (hereinafter referred to as "first metal surface") and a surface made of a second metal (hereinafter referred to as "second metal surface") are electrically connected. preparing an object to be plated; applying a voltage to the object to be plated before immersing the object to be plated in the plating solution; and coating at least part of the surface of the first metal with a coating metal nobler than the first metal.

In view of such circumstances, the present invention can provide a plating method capable of forming a high-quality plating film that is extremely thin, uniform, and suppresses the occurrence of pinholes and discoloration.

It is a flow chart showing an example of the flow of processing of the plating method of this embodiment. 1A and 1B show an example of an object to be plated according to the present embodiment; FIG. 1B is an external view; and FIGS. BRIEF DESCRIPTION OF THE DRAWINGS It is the schematic which shows an example of the plating method of this embodiment in time series. FIG. 2 is a conceptual diagram for explaining a contact potential difference in bonding dissimilar metals; FIG. 2 is a phase diagram showing the reaction relationship between zinc and copper. FIG. 2 is a conceptual diagram for explaining a contact potential difference in bonding dissimilar metals; It is a figure which compares the state of plating by the conventional plating method, and the state of plating by the plating method of this embodiment. It is a figure which compares the state of plating by the conventional plating method, and the state of plating by the plating method of this embodiment. 4 is a schematic diagram showing a current path of the embodiment; FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, parts with the same reference numerals represent the same configuration.

FIG. 1 is a flowchart showing an example of the processing flow of the plating method of this embodiment. 2A and 2B are schematic diagrams showing an example of the object to be plated 10 of the present embodiment, and FIG. 2A and FIG. (C) to (G) of the figure are schematic cross-sectional views of the object to be plated 10 . FIG. 3 is a schematic diagram showing an example of the plating method of this embodiment in chronological order. In addition, in this figure and subsequent figures, a part of the configuration is appropriately omitted to simplify the drawings. In this drawing and subsequent drawings, the size, shape, thickness, etc. of the members are appropriately exaggerated.

Referring to FIG. 1, the plating method of the present embodiment includes a surface made of a first metal (hereinafter referred to as "first metal surface") and a surface made of a second metal (hereinafter referred to as "second metal surface"). ) is electrically conductive (continuous) (step S01), and a step of applying a voltage to the plated body before immersing the plated body in the plating solution (step S05 ), and a step of immersing at least a portion to be plated on the surface of the first metal in a plating solution to coat the portion to be plated with a coating metal nobler than the first metal (step S07).

The object to be plated 10 of this embodiment will be described with reference to FIG. 2A and 2B are schematic diagrams showing the object to be plated 10. FIG. 2A is an external view of the object to be plated 10 before plating, and FIG. 2B is an external view of the object to be plated 10 after plating. be. (C) is a cross-sectional view taken along the line XX of (B) in FIG. It is a cross-sectional view.

In the plating method of the present embodiment, a plating target portion 16 of an object to be plated 10 as shown in FIG. 16 is covered with a thin film (plating film) 13 made of a coating metal.

Referring to FIG. 2A, the object to be plated 10 of the present embodiment is a structure in which a first metal surface 11 and a second metal surface 12 are electrically connected (continuous). As shown in (C) and (G) of the figure, the first metal surface 11 may be the surface of a first metal material 11M as a base material, or As shown in F), it may be the surface of a first metal material 11M provided covering arbitrary base materials (structures) BD and BD1. Similarly, the second metal surface 12 may be the surface of a second metal material 12M as a base material, as shown in FIGS. As shown in (D) and (E) of the figure, it may be the surface of a second metal material 12M provided covering arbitrary base materials (structures) BD and BD2.

Any matrix BD, BD1, BD2 may be a non-conducting material or a conducting material. In addition, as shown in FIG. 4D, in the case of a configuration in which the first metal material 11M and the second metal material 12M are provided so as to cover one arbitrary base material BD, the arbitrary base material BD is made of a single material. or by bonding, adhering, or laminating a plurality of materials (composite materials may be used). Further, the object to be plated 10 is, for example, as shown in FIG. Another arbitrary base material (second base material BD2) may be continuously provided (for example, by joining), or as shown in FIG. An arbitrary base material (first base material BD1) covered with material 11M and a second metal material 12M (second base material BD2) are continuously provided (for example, by joining). Alternatively, the first base material BD1 and the second base material BD2 may have the opposite configurations.

Further, "the first metal surface 11 and the second metal surface 12 are electrically connected (continuous)" means that the first metal surface 11 and the second metal surface 12 are bonded (atomic bonding), for example. , bonding by melting, etc.), the first metal surface 11 and the second metal surface 12 are electrically connected (continuous) by direct contact. or a configuration in which the first metal surface 11 and the second metal surface 12 are electrically connected (continuously) by connecting them via a conductive member such as another metal. It is not limited to this example, and any structure that allows an electric current to flow between the first metal surface 11 and the second metal surface 12 may be used. That is, as shown in FIG. 1G, the first metal surface 11 (first metal material 11M) and the second metal surface 12 (second metal material 12M) are electrically connected through another metal material m. A conductive (continuous) configuration may also be used.

In this example, the object to be plated 10 is a belt-shaped structure that is long in one direction (long in the horizontal direction in FIG. 2), but the shape of the object to be plated 10 is not limited to this example. As will be described in detail later, at least part of the object to be plated 10 (part to be plated 16) is immersed in the plating solution. In other words, the object to be plated 10 is not limited to the illustrated example as long as it has a shape in which at least the part to be plated 16 can be immersed in the plating solution while a part of the object to be plated 10 is held by a jig or the like.

Further, as shown in FIGS. 2B and 2C as an example, the plating target portion 16 is a part of the object to be plated 10, specifically, the first metal surface 11 (substantially the entire surface). Suppose that That is, the entire outer surface of the first metal material 11</b>M is covered with the plating film 13 . However, the plating target portion 16 is not limited to this example, and may be a portion of the first metal surface 11, or a portion that is continuous with the entire first metal surface 11 and a portion of the second metal surface 12. may

The first metal and the second metal are metals with different ionization tendencies. Specifically, the second metal is a metal that is relatively noble (lower in ionization tendency) than the first metal. Here, as an example, the first metal is a metal containing zinc (Zn) as a main component, and the second metal is a metal containing copper (Cu) as a main component.

Here, "metal containing zinc as the main component" refers to a metal containing 50% or more of pure zinc or a zinc alloy, and when it contains other components different from pure zinc or zinc alloy, other components The type and number of are arbitrary. Also, the metal may contain 100% (or approximately 100%) pure zinc or a zinc alloy. Hereinafter, in this specification, the first metal may be simply referred to as "zinc (Zn)", which means "a metal containing zinc as a main component".

In addition, "metal containing copper as the main component" refers to a metal containing 50% or more of pure copper or copper alloy, and if it contains other components different from pure copper or copper alloy, the type and number of other components is optional. Also, the metal may contain 100% (or approximately 100%) of pure copper or copper alloy. Hereinafter, in this specification, the second metal may be simply referred to as "copper (Cu)", which means "a metal containing copper as a main component".

Also, as an example, the second metal and the coating metal are of the same kind. That is, in the plating method of the present embodiment, as an example, the first metal surface 11 and the second metal surface 12 having different ionization tendencies are electrically continuous in the object to be plated 10. It can be said to be a method of substituting the first metal surface 11 with a relatively noble coating metal and covering the first metal material 11 with the coating metal. This is a method of covering with the same kind of coating metal as 12.

As described above, in the present embodiment, for example, the substitution reaction (oxidation-reduction reaction) between the first metal on the first metal surface 11 and the coating metal proceeds. In other words, the “surface” of the first metal surface 11 or the second metal surface 12 means a portion exposed to the outside, particularly in the case of the first metal surface 11, a substitution reaction of the coating metal with metal ions can proceed. "Part of the first metal surface 11 (second metal surface 12)" means a part in the (visible) plane direction, not in the thickness direction.

Hereinafter, each step of the plating method of this embodiment will be described in detail with reference to FIGS. 1 and 3. FIG. First, the object to be plated 10 described above is prepared (step S01 in FIG. 1, FIG. 2(A), FIG. 3(A)). Also, a counter electrode 15 is prepared. The counter electrode 15 is, for example, an electrode made of the same metal as the film-forming metal, specifically a copper (Cu) electrode (mainly composed of copper), and is immersed in a predetermined plating solution 14 . Also, a predetermined voltage is applied to the counter electrode 15 (connected to one pole (for example, positive electrode) of the power supply E). The plating solution 14 is, for example, a solution containing metal ions (Cu ²⁺ ) of the coating metal. Specifically, for example, it is a solution of a strong acid (for example, about pH=0.5) containing copper sulfate (CuSO ₄ ) and sulfuric acid (H ₂ SO ₄ ) (FIG. 3(B)).

Next, before the object to be plated 10 is immersed in the plating solution 14 (while not being immersed in the plating solution 14), a predetermined voltage is applied to the object to be plated 10 (step S05 in FIG. 1, (C )).

By applying this voltage, the object to be plated 10 is given a predetermined amount of charge. Referring to FIG. 3A, the first metal surface 11 and the second metal surface 12 of the object to be plated 10 are electrically connected (continuously), and a contact potential difference ΔV is generated between them. One becomes positively or negatively charged with respect to the other. Hereinafter, as an example, the potential of the second metal surface 12 (copper) (ground-based potential, the same shall apply hereinafter) will be used as a reference. Due to the potential difference ΔV, the second metal surface 12 is positively charged (potential is +V).

In this step, at least a part of the contact potential difference ΔV of the first metal surface 11 with respect to the second metal surface 12 is canceled in advance by applying a predetermined voltage. In other words, this voltage is the voltage that can set the potential of the object to be plated 10, more specifically the first metal surface 11, within a certain range. The applied voltage and the potential of the first metal surface 11 set thereby will be described in detail later. Specifically, as shown in FIG. It connects (applies a negative voltage) to the pole (for example, the negative pole).

After that, as shown in FIG. 4D, the plating target portion 16 (here, the entire first metal surface 11) is immersed in the plating solution 14 while a negative voltage is applied to the object to be plated 10 (FIG. 1 step S07, FIG. 3(D)). As a result, the first metal surface 11 (zinc) is replaced with copper ions (Cu ²⁺ ) in the plating solution 14 , and copper is reduced and deposited on the surface of the plating target portion 16 . Here, as an example, the application of the negative voltage to the object to be plated 10 is continued until the entire substantial portion of the portion 16 to be plated is immersed in the plating solution 14 . As a result, the entire first metal surface 11 is replaced with the coating metal (copper), and the plating target portion 16 is covered with the plating film 13 .

As described above, in the present embodiment, a voltage is applied to the object to be plated 10 so as to offset at least part of the contact potential difference ΔV of the first metal surface 11 with respect to the second metal surface 12, and then the plating solution 14 is applied. By immersing the object to be plated 10 in the solution, the plating film 13 can be formed by oxidation-reduction reaction (displacement plating). The plated film 13 can be significantly thinner than a plated film formed by a conventional electroplating method, and can also suppress the generation of pinholes.

In addition, when the voltage application to the object to be plated 10 is continued even after the first metal surface 11 (zinc) of the part to be plated 16 is (entirely) replaced with the coating metal (copper), Go (automatically) to electroplating. Therefore, until substantially the entire first metal surface 11 is immersed in the plating solution 14, the voltage is continuously applied to the object to be plated 10, and after substantially the entire first metal surface 11 is replaced with the coating metal, A plating film 13 having a desired film thickness is formed by electroplating, and the plating process is completed (FIG. 3(E)).

Since the good-quality plating film 13 is formed as the lower layer by displacement plating, the plating film 13 can be formed to an arbitrary thickness by the continuous electroplating method. Therefore, the film thickness of the plating film 13 of the present embodiment can be formed to, for example, 3 μm or less, more specifically, about 100 nm to 3 μm, preferably about 300 nm to 2 μm, more preferably about 500 nm to 1.5 μm. Further, since the lowermost layer of the plated film 13 is a film formed by displacement plating, even if the total film thickness is about 500 nm, for example, the occurrence of pinholes can be suppressed.

<Replaceable potential range and applied voltage>
In the plating method of the present embodiment, a predetermined amount of voltage is applied in advance (charge is applied) to the plated body 10 in which different metals are electrically conductive (continuous) to cancel the contact potential difference ΔV. After that, the substrate is immersed in the plating solution 14 to carry out displacement plating.

The voltage applied to the object to be plated 10 is a voltage within a certain range determined based on the materials and compositions of the first metal surface 11 , the second metal surface 12 , the coating metal, the plating solution 14 and the counter electrode 15 .

The applicant of the present application plated copper (Cu) as a coating metal on the first metal surface 11 (zinc side) of the object to be plated 10 in which the first metal is zinc (Zn) and the second metal is copper (Cu). Considered the case. In particular, as the plating film 13, in consideration of the case where a film such as another metal or resin is applied on it, the film thickness is thin and uniform, and the generation of pinholes is suppressed. The purpose was to form a film of (metallic copper, Cu).

In the case of copper (metallic copper) coating, when plating was first performed by a conventionally known displacement-type electroless plating method, although displacement progressed, the formed plating film discolored (copper oxide etc. precipitated). Deposition of copper as a metal has been difficult.

As a result of evaluation under various conditions such as the composition and pH of the plating solution, it was found that the composition of the plating solution and the contact potential difference ΔV between zinc and copper affect the oxidation-reduction reaction between copper and zinc. It was thought that discoloration (precipitation of copper oxide, etc.) was caused.

Referring to FIG. 4, (A) is a schematic diagram showing the work functions of copper and zinc (energy difference between vacuum level VL and Fermi level FL). (B) is a structure in which copper and zinc are electrically connected (continuous) (for example, a structure in which the surface of copper and the surface of zinc are in direct contact (hereinafter referred to as a copper-zinc contact structure); FIG. 2 is a schematic diagram showing a contact potential difference ΔV generated in the object to be plated 10 of the embodiment).

As shown in FIG. 4A, copper (Cu) has a work function of 4.7 eV, and zinc (Zn) has a work function of 3.6 eV. Due to the work function difference between the two, as shown in FIG. −(3.6 eV−4.7 eV)/e, where e is the elementary charge).

FIG. 5 shows the reaction (e.g., oxidation-reduction reaction) between zinc (the first metal surface to be immersed and replaced in the plating solution 14) and copper (the plating solution 14, the coating metal contained in the plating solution 14). ) is a diagram showing the relationship of The diagram (graph) shown on the right side of the figure shows the reaction (eg, oxidation-reduction reaction) between copper and zinc in an aqueous solution system (Zn—H ₂ O system and Cu—H ₂ O system) calculated by the applicant of the present application. is a general phase diagram showing the potential-pH diagram of the Zn—H ₂ O system when the activity of dissolved zinc ions is 10 ⁻⁶ , and the Cu 1 is a potential-pH diagram (referred to herein as a “potential-pH diagram (synthetic)”) in which a potential-pH diagram of a —H ₂ O system is superimposed. In this case, the vertical axis indicates the equilibrium electrode potential of the aqueous solution based on the standard hydrogen electrode (vs. NHE (vs. SHE)). Also, the horizontal axis is the pH of the plating solution 14 .

The equilibrium electrode potential in the potential-pH diagram (synthetic) was calculated based on the activities of zinc ions and copper ions using the Nernst equation for the equilibrium electrode potential shown in Equation A below. Here, zinc is a component that dissolves into the aqueous solution from the solid object to be plated 10 ^, and originally does not exist in the aqueous solution. ⁶ . On the other hand, copper is a component that precipitates on the plated object 10 from ions in the aqueous solution, and originally exists in a large amount in the aqueous solution.

In the figure, the boundary line a indicates the equilibrium electrode potential (redox potential) when the activity of dissolved zinc ions in the zinc half-cell reaction (formula B) below is 10 ⁻⁶ , and the value is −0.94 (V).

Zn ²⁺ + 2e ⁻ = Zn (Formula B)

Boundary line b indicates the equilibrium electrode potential (oxidation-reduction potential) of the following copper half-cell reaction (formula C), and the value when the activity of dissolved copper ions is 1 is +0.34 (V ).

Cu ²⁺ + 2e ⁻ = Cu (Formula C)

In this potential-pH diagram (synthesis), the region (colored region) where the corrosion region of zinc and the insensitive region (non-transformation region, inert state region, stable region) of copper (film metal) overlap is the zinc is ionized and copper is deposited, that is, the zinc surface of the copper-zinc contact body (the first metal surface 11 of the object to be plated 10 is replaced with copper (coating metal), hereinafter, this region is referred to as "replaceable region SR".

That is, when the potential of the surface (first metal surface 11) of the copper-zinc contact (the side of the zinc to be replaced) and the pH of the plating solution 14 satisfy the conditions included in the replaceable region SR, zinc is eluted. Copper can be deposited. The potential range of the replaceable region SR (the potential at which the first metal surface 11 is replaced with the coating metal; hereinafter referred to as "replaceable potential range VR") varies depending on the pH of the plating solution 14. In the example of , +0.34 ≥ VR ≥ -0.94 at the maximum (in the case of a strong acid of about pH < 3), and -0.047 ≥ VR ≥ -0.94 at the minimum (in the case of a weak alkali of about pH = 8.1) 0.94. In this embodiment, as an example, a case where a strong acid (about pH<3) is used as the plating solution 14 and the substitutable potential range VR is +0.34≧VR≧−0.94 will be described.

By the way, the zinc side (first metal surface 11) of the copper-zinc contact body (body to be plated 10) faces the copper side (second metal surface 12) as shown in the left diagrams of FIGS. It is in a positively charged state (+1.1V=-(3.6eV-4.7eV)/e, where e is the elementary charge), and the potential of the replaceable region SR in the potential-pH diagram (synthesis) It is greatly out of (exceeds) the upper limit of the range (substitutable potential range VR).

Therefore, the applicant of the present application forcibly sets the potential of the first metal surface 11 to the potential range for realizing replacement, that is, the replaceable potential range VR, before immersion in the plating solution 14 (before the replacement reaction starts). , the substitution reaction between zinc and copper proceeds well.

That is, a voltage (such as to offset the charged state) is applied to the plating solution 14 so that the potential of the object to be plated 10, which is positively (or negatively) charged by the contact potential difference ΔV, falls within the replaceable potential range VR. We decided to apply it to the object to be plated 10 before it was applied.

More specifically, from a theoretical and practical point of view, the plating solution 14 is a solution containing copper ions (Cu ²⁺ ) and sulfate ions (SO ₄ ²⁻ ), which is easy to handle, and pH=0. Adjusted to 5. As a result, the substitutable potential range VR becomes +0.34≧VR≧−0.94.

Then, to the object to be plated 10, a voltage is applied that cancels at least part of the charge (+1.1 V with respect to the second metal surface 12) charged on the first metal surface 11. FIG. Hereinafter, the voltage that cancels at least part of the charges on the first metal surface 11 (charges with the second metal surface 12 as a reference) due to the contact potential difference ΔV will be referred to as "charge pump voltage CP".

The charge pump voltage CP of the present embodiment is a voltage capable of canceling at least a portion of the charges on the first metal surface 11, preferably the potential of the first metal surface 11 of the object to be plated 10. It is a voltage within the possible potential range VR (+0.34≧VR≧−0.94). Specifically, the charge pump voltage CP is, for example, a negative voltage of −0.76 V (0.34 V−1.1 V)≧CP≧−2.04 V (−0.94 V−1.1 V).

Here, the counter electrode 15 is desirably an electrode capable of continuously releasing metal ions of the coating metal into the plating solution 14, for example, and an example using a copper electrode is shown here.

Then, while maintaining the application of the charge pump voltage CP, the object to be plated 10 is immersed in the plating solution 14 adjusted to pH=0.5. As a result, the first metal surface 11 of the object to be plated 10 and the plating solution 14 are placed under the conditions of the replaceable region SR, and the first metal surface 11 (zinc) is replaced with the coating metal (copper).

This substitution reaction proceeds while zinc constituting the first metal surface 11 exists under the conditions of the substitutable region SR. In other words, the application of the charge pump voltage CP is continued at least from the start of immersion of the portion 16 to be plated until the entire substance thereof is immersed in the plating solution 14 .

The film metal (plating film 13) deposited on the surface of the object to be plated 10 by the substitution reaction can greatly suppress the generation of pinholes and the film thickness can be greatly reduced as compared with the case of electroplating. . When the charge pump voltage CP continues to be applied even after the entire first metal surface 11 is replaced with the coating metal, the electroplating of the object to be plated 10 is continuously and automatically performed. The thickness of the film 13 increases (for example, the voltage when depositing copper by electroplating is generally about 1 to 5 V in an acid bath (pH=0.5)).

Even if electroplating is continuously performed, the underlying layer of this plated film 13 is a good film (layer) formed by displacement plating. That is, even if the electroplating causes pinholes, there is no problem in terms of covering the base material BD, and the electroplating can be completed when the desired thickness of the plating film 13 is formed. is.

That is, the charge pump voltage CP is applied from before the object to be plated 10 is immersed in the plating solution 14 until at least the plating target portion 16 (for example, substantially the entire first metal surface 11) is immersed in the plating solution 14. Continue to apply voltage for a period of time. When the coating metal is deposited on the plating target portion 16 and the plating film 13 reaches a desired thickness (by continuing electroplating for an appropriate time), the application of the charge pump voltage CP is terminated. Alternatively, the object to be plated 10 is taken out from the plating solution 14 .

In this manner, according to the plating method of the present embodiment, it is possible to form a high-quality plated film 13 by displacement plating at least as the lowermost layer. Specifically, the plated film 13 can be formed from a coating metal (for example, metallic copper), suppresses the occurrence of pinholes, and can be formed to have a thin and uniform film thickness. Further, since the electroplating is continuously performed after the displacement plating is completed, the film thickness of the plating film 13 can be controlled by the application time of the charge pump voltage CP while being immersed in the plating solution 14 .

As a result, the film thickness of the plating film 13 can be controlled at an arbitrary thickness, and for example, a thin film of 3 μm or less, which was difficult due to the generation of pinholes in the prior art, can be formed. More specifically, for example, about 100 nm to 3 μm, preferably about 300 nm to 2 μm, more preferably about 500 nm to 1.5 μm can be formed. can be suppressed. For example, when forming the plating film 13 with a thickness of 1 μm or less, electroplating is performed by continuously applying the charge pump voltage CP for 1 to 2 minutes after immersing the plating target portion 16 in the plating solution 14 . Furthermore, the plating solution 14 can be an easy-to-handle solution containing copper sulfate and sulfuric acid.

As described above, the plating method of the present embodiment is applied to the base metal (first metal) side of the object to be plated 10 having the first metal surface 11 and the second metal surface 12 that are electrically conductive (continuous). A method of coating the surface of the (coating with a metal (non-metal oxide) film) with a relatively noble coating metal (coating with a film of metal (non-metal oxide)), wherein the first metal A charge pump voltage CP is applied that cancels at least a portion of the contact potential difference ΔV of the first metal surface 11 with respect to the second metal surface 12 that occurs between the surface 11 and the second metal surface 12. be.

Specifically, the contact potential difference ΔV between the first metal surface 11 and the second metal surface 12, which is the plating target portion 16 of the object to be plated 10, the first metal surface 11 and the film metal (film present in the plating solution 14 Based on the potential-pH diagram (synthesis) showing the oxidation-reduction reaction of the metal ion of the metal used), the potential of the first metal surface 11 in the plated body 10 is the potential of the substitutable region SR in the potential-pH diagram (synthesis) The charge pump voltage CP is determined so as to be set within the range of .

In other words, when the potential range in which the first metal is ionized and the first metal surface 11 is replaced with the coating metal is defined as the replaceable potential range VR, the application of the charge pump voltage CP causes the first metal surface 11 is set within the replaceable potential range VR.

Then, the object to be plated 10 is immersed in the plating solution 14 while applying the charge pump voltage CP.

<Application example>
Again, the object to be plated 10 of the present embodiment may be any object as long as the first metal surface 11 and the second metal surface 12 are electrically connected (continuous). For example, as shown in FIGS. 2(E) to 2(G), the base material BD or the object to be plated 10 may be a dissimilar metal joint.

For example, as an example of a dissimilar metal joined body, there is a structure in which copper and aluminum are directly joined by pressure welding or the like. A dissimilar metal bonded body of copper and aluminum (copper-aluminum bonded body) has a wide range of applications as various parts due to ease of handling of materials, cost, and the like. However, even in this case, it is well known that a contact potential difference between dissimilar metals occurs, which causes galvanic corrosion.

In addition, the surface of aluminum has low environmental stability, such as the formation of an oxide film easily, or when a copper-aluminum bonded body is used as a base material and another film is laminated on its surface, the difference in the base material makes it difficult to achieve uniformity. There is also a problem that it is difficult to laminate a thin film.

Therefore, the entire copper-aluminum bonded body is coated with a single metal film, or the aluminum portion is continuously coated with a copper film from the copper portion of the copper-aluminum bonded body (including the junction between the two). The problem can be solved by coating. Also in such a case, the plating method of the present embodiment can be adopted.

By the way, although aluminum is a less noble metal than copper, it is difficult to replace the surface of aluminum directly with copper. Specifically, the oxide film formed on the surface of aluminum is difficult to dissolve in an acidic copper plating solution for copper replacement (plating solution for copper replacement), and alkaline copper plating does not progress. The liquid requires a complex and a reducing agent to generate copper ions, and is toxic, making it difficult to carry out.

Therefore, a film (layer) of zinc is formed on the surface of the aluminum portion of the copper-aluminum bonded body by zincate treatment. Thereby, in the copper-aluminum bonded body, the first metal surface 11 of zinc covering the aluminum serving as the base material BD and the second metal surface 12 of copper (second metal material 12M) are electrically connected (continuously). It becomes the object to be plated 10 (see FIG. 2(F)), and the first metal surface 11 can be replaced with copper, which is the coating metal, by the plating method of the present embodiment. In this case, the zincate treatment can be performed by a known method, so the details are omitted. apply. The number of times of zincate treatment is arbitrary. As a result, a copper-zinc contact body (body to be plated 10) in which an aluminum portion is covered with a zinc metal film (first metal surface 11) is obtained. Plating is performed by

FIG. 6 is a schematic diagram showing the contact potential difference ΔV of the object to be plated 10 in which the aluminum surface of the copper-aluminum bonded body is zincate treated. FIG. 1A is a schematic diagram showing the work functions of copper, aluminum and zinc, and FIG.

For example, even in the case of a structure in which copper and zinc are electrically continuous after a zincate treatment (regardless of whether or not copper and zinc are atomically bonded or directly bonded, etc.) ), and if they are electrically continuous, for example, via aluminum, a contact potential difference ΔV is generated.

The work function of aluminum (Al) is 4.1 eV. That is, in this case, the contact potential difference ΔV of the object to be plated 10 is +1.1 V (=−((3.6 eV−4.1 eV)+(4.1 eV−4.7 eV)) on the zinc side when the copper side is used as a reference. )/e, where e is the elementary charge). When considering the contact potential difference ΔV between electrically conducting (continuous) copper and zinc in this way, the work function of the metal intervening between them (regardless of the ionization tendency of the metal) is canceled. . In other words, in this case as well, the object to be plated 10 can be treated as a copper-zinc contact object (see FIGS. 4(F) and 5), and the metal copper plating film 13 of good quality can be obtained by the plating method of the present embodiment described above. can be formed.

FIG. 7 is a diagram showing an example of the result of plating by the conventional plating method and the plating method of the present embodiment, and is a photograph of the appearance of objects a to d to be plated. Each of the objects to be plated a to d is a structure obtained by zincating the aluminum portion of the copper-aluminum bonded body, and the first metal surface 11 of zinc and the second metal surface 12 of copper are electrically continuous. It is a zinc contact body, and the plating treatment was performed by changing the conditions by using a part of the first metal surface 11 as the plating target portion 16 . (A) is the result for the object to be plated a, (B) is the object to be plated b, (C) is the object to be plated c, and (D) is the result for the object d to be plated.

Also, FIG. 8 shows the results of examination of the plating films formed on the four samples (objects to be plated a to d) shown in FIG. 7, and FIG. It is the measurement result of the measurement (reduction method), and (B) of the same figure shows the film thickness (Å) of copper oxide for four samples (plated bodies a to d) based on the measurement result of (A) of the same figure. It is a calculated table. For comparison, FIG. 8 also shows the measurement results of a copper substrate (without plating) and the film thickness of copper oxide on the surface of the copper substrate.

In the electrochemical measurement (reduction method) shown in FIG. 8, the surface of the sample was electrochemically reduced with an aqueous borate buffer solution, and the change in the reduction potential over time was measured. The vertical axis of FIG. 1A is the reduction potential (V vs. AgCl), and the horizontal axis is the energization time (sec). The energization time corresponds to the time required for reduction, that is, the thickness of the oxide film formed by plating. In addition, in FIG. 4B, the film thickness is calculated assuming that cuprous oxide (Cu ₂ O) among copper oxides is reduced in the range of −0.35 V to −0.6 V. The film thickness was calculated assuming that cupric (CuO) is reduced in the range of -0.6V to -0.85V.

Referring to FIG. 7, the plating solution used for the plating treatment of the object to be plated a shown in FIG. 0.5. In addition, the plating solutions used for the plating treatment of the objects to be plated b to d shown in FIGS. , pH=0.5).

FIG. 1(A) shows an object to be plated a in which a portion 16 to be plated on a first metal surface (zinc) 11 is subjected to electroless plating by a conventional method. In this case, the plating target portion 16 was formed with a film that was severely discolored to dark brown. Referring to FIG. 8, as a result of measurement by electrochemical measurement (reduction method), a film containing cupric oxide (CuO) as a main component and further containing cuprous oxide (Cu ₂ O) was formed. I found out. Since cupric oxide (CuO) is dark brown and cuprous oxide (CuO) is reddish brown, the plating target portion 16 is composed of a mixture of these, and as a result, a dark brown film was obtained. be done.

FIG. 1(B) shows the object to be plated b in which the portion 16 to be plated on the first metal surface (zinc) 11 is subjected to electroless plating by a conventional method. In this case, the plating target portion 16 turned reddish brown, and as a result of measurement by electrochemical measurement (reduction method), it was found that a film mainly composed of cuprous oxide (Cu ₂ O) was formed ( See Figure 8).

FIG. 1C shows the object to be plated c (object to be plated 10) on which the plating film 13 is formed by the plating method of this embodiment. No discoloration was observed in the plated film 13 formed on the plating target portion 16 of the first metal surface 11 (comparable to the second metal surface 12 (copper)), and the electrochemistry shown in FIG. The measurement results of the measurement (reduction method) show a profile similar to that of the copper base material, and from the table in _FIG . It was found that a plating film 13 of a high metallic copper was formed.

FIG. 4D shows the plated body d when −3.2 V is applied as the charge pump voltage CP outside the replaceable potential range VR in the plating method of the present embodiment. The composition of the plating solution 14 is a mixed solution of copper sulfate and sulfuric acid, and its pH is 0.5. The plating target portion 16 was darker than the plated body B in FIG. 7(B), and a copper-colored plating film was obtained. The measurement by the electrochemical measurement (reduction method) in FIG. 8A was interrupted due to the long reduction time of cuprous oxide (Cu ₂ O). This plated film is presumed to be composed mainly of cuprous oxide (Cu ₂ O).

Thus, it was presumed that the substitution reaction via the oxide occurred in the plating methods of FIGS. 1A, 1B, and 1D. On the other hand, in the plating method of the present embodiment, by appropriately selecting the pH of the plating solution 14 and the charge pump voltage CP based on the substitutable region SR, it can be said that a good metallic copper plating film 13 can be formed.

Thus, according to the present embodiment, at least the aluminum portion of the copper-aluminum bonded body (dissimilar metal bonded body) can be covered with the plating film 13 of metallic copper. Further, the film thickness is, for example, 3 μm or less, more specifically, about 100 nm to 3 μm, preferably about 300 nm to 2 μm, more preferably about 500 nm to 1.5 μm. Further, since displacement plating is used, even if the film thickness is about 500 nm, for example, the generation of pinholes can be suppressed.

It is difficult to make the plating film formed by the electroplating method thin (for example, 5 μm or less). There was a problem that affected the external shape (size) of the product.

According to this embodiment, it is possible to form a plated film 13 that is much thinner than the case of electroplating, is uniform, and has almost no pinholes. Reliable galvanic corrosion countermeasures can be taken without significantly impairing the

In addition, by coating the aluminum portion with the copper plating film 13, substantially the entire outer surface of the copper-aluminum bonded body (dissimilar metal bonded body) can be composed of the same metal (copper in this case). Therefore, when another film (a metal film or a resin film) is laminated or coated using this as a base material (underlayer), the stability of film formation and adherence can be improved.

Thus, in the plating method of the present embodiment, if the first metal surface 11 and the second metal surface 12 having different work functions are electrically continuous, they are not in contact (bonded). may be, and another conductive material (other metal) may be interposed between them,

Furthermore, as the plating method of the present embodiment, the first metal surface 11 is zinc, the second metal surface 12 is copper, and the coating metal is copper, but these metals are not limited to the above examples. That is, the step of preparing the object to be plated 10 in which the first metal surface 11 and the second metal surface 12 are electrically continuous; applying a voltage (charge pump voltage) to the object to be plated 10 so as to cancel at least part of the contact potential difference ΔV of the first metal surface 11 with reference to the second metal surface 12; a step of immersing the part in the plating solution 14 and performing displacement plating on at least a part of the object to be plated 10 .

For example, the first metal surface 11 may be a nickel (Ni)-based metal surface or an iron (Fe)-based metal surface.

FIG. 9 is a diagram schematically showing a current circuit that is a prerequisite for the plating method of this embodiment. The plating solution 14 (metal ions for coating), the counter electrode 15, the power supply, the second metal surface 11, the first metal surface 11, and each element of the replaceable potential range VR have a ground-based potential, and the current shown in the figure is It can be said that it constitutes a circuit. Therefore, the plating method of the present embodiment can be adopted for any metal as long as the current circuit can be constructed (satisfying the potential relationship). Specifically, each concept and numerical values of the plating method of this embodiment are determined as follows, and it can be said that the plating method of this embodiment has the following steps.

(1) Contact potential difference ΔV: The contact potential difference ΔV between the first metal surface 11 and the second metal surface 12 (for example, the second metal surface) is calculated on the other side (the part to be plated 16 side).

(2) Potential-pH diagram (synthesis): A phase diagram is created by synthesizing the potential-pH diagram of the first metal and the potential-pH diagram of the film-forming metal (plating solution 14 containing film-forming metal ions). The coating metal need not be of the same type as the second metal.

(3) Replaceable region SR: In the potential-pH diagram (synthetic), determine the region where the corroded region of the first metal and the dead region of the coating metal overlap.

(4) Substitutable Potential Range VR: Identify the potential range (potential range of the substitutable region SR) in which the first metal is ionized and the first metal surface 11 is substituted with the coating metal.

(5) Charge pump voltage CP: At least part of the contact potential difference ΔV is offset (adjusted, corrected) so that the potential of the plated object 10 (first metal surface 11) charged by the contact potential difference ΔV falls within the replaceable potential range. ) to calculate the voltage (positive voltage or negative voltage). It is not necessary to cancel all of the contact potential difference ΔV, and a part of the contact potential difference ΔV may be cancelled.

(6) Plating solution 14: The components and pH are appropriately selected according to the materials of the first metal surface 11 and the film-forming metal and the substitutable range.

(7) Counter electrode 15: Adopting any electrode material capable of releasing metal ions of the coating metal, immersed in the plating solution 14 and applying voltage (connected to one electrode of the power source as the counter electrode of the object to be plated 10) do.

(8) Object to be plated 10: Before being immersed in the plating solution 14, the charge pump voltage CP is applied (connected to the other pole of the power supply), and while the application is continued, the object to be plated 10 is immersed in the plating solution 14 to perform displacement plating. conduct. Electroplating is continuously performed as necessary, and after forming the plated film 13 with a predetermined film thickness, the plating process is terminated (voltage application is terminated, removal from the plating solution 14, etc.).

In addition, it can be said that the contact potential difference ΔV in the plating method of the present embodiment is a value that satisfies the following relationship (Equation 1).

ΔV = -(WF1 - WF2)/e (Formula 1)
here,
ΔV: Contact potential difference between the first metal (surface) and the second metal (surface) relative to the second metal (surface)
WF1: work function of the first metal (relatively base metal)
WF2: work function of the second metal (relatively noble metal)
e is the elementary charge, and "/e" means to divide by the elementary charge e in order to convert the work function unit eV (energy) to the voltage unit V.

Further, it can be said that the substitutable potential range VR in the plating method of the present embodiment is a value that satisfies the following relationship (Equation 2).

V2≧VR≧V1 (2)
here,
VR: replaceable potential range
V1: Equilibrium electrode potential of the first metal (activity is 1E-6)
V2: Equilibrium electrode potential of metal for film (activity is 1)
is.

Also, it can be said that the charge pump voltage CP in the plating method of the present embodiment is a value that satisfies the following relationship (Equation 3).

V2-ΔV≧CP≧V1-ΔV (3)
here,
CP: charge pump voltage
ΔV: contact potential difference
V1: Equilibrium electrode potential of the first metal (activity is 1E-6)
V2: Equilibrium electrode potential of metal for film (activity is 1)
is.

In addition, for example, when the coating metal and the second metal are the same metal, the plating film 13 may be formed continuously on the first metal surface 11 and the second metal surface 12 .

In step S03 of FIG. 1, the immersion of the counter electrode 15 in the plating solution 14 and the application of the voltage are in no particular order.

It should be noted that the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made without departing from the gist of the present invention.

The plating method of the present invention can be used for plating structures in which dissimilar metals are electrically conductive (continuous).

REFERENCE SIGNS LIST 10 object to be plated 11 first metal surface 12 second metal surface 13 plating film 14 plating solution 15 counter electrode 16 target portion CP charge pump voltage SR replaceable region VR replaceable potential range

In the present invention, a surface made of a first metal (hereinafter referred to as a "first metal surface") and a surface made of a second metal (hereinafter referred to as a "second metal surface") are both exposed and electrically conductive. and applying a voltage to the first metal through the second metal before the object to be plated is immersed in the plating solution, the first metal canceling at least a portion of the potential generated on the first metal surface due to the contact potential difference between the second metal and the second metal; immersing in a liquid to coat the portion to be plated with a coating metal nobler than the first metal.

In this step, by applying a predetermined voltage, at least part of the potential generated on the first metal surface 11 due to the contact potential difference ΔV of the first metal surface 11 with respect to the second metal surface 12 is canceled in advance. In other words, this voltage is the voltage that can set the potential of the object to be plated 10, more specifically the first metal surface 11, within a certain range. The applied voltage and the potential of the first metal surface 11 set thereby will be described in detail later. Specifically, as shown in FIG. It connects (applies a negative voltage) to the pole (for example, the negative pole).

As described above, in the present embodiment, the object to be plated is prepared in advance so as to offset at least part of the potential generated on the first metal surface 11 due to the contact potential difference ΔV of the first metal surface 11 with respect to the second metal surface 12 . By immersing the object to be plated 10 in the plating solution 14 after applying a voltage to 10, the plating film 13 can be formed by oxidation-reduction reaction (displacement plating). The plated film 13 can be significantly thinner than a plated film formed by a conventional electroplating method, and can also suppress the generation of pinholes.

<Replaceable potential range and applied voltage>
In the plating method of the present embodiment, a predetermined amount of voltage is applied (charge is applied) to the plated body 10 in advance in the plated body 10 in which different metals are electrically conductive (continuous), and the metal by the contact potential difference ΔV In this method, displacement plating is performed by immersing the substrate in the plating solution 14 after offsetting the potential generated on the surface .

Then, to the object to be plated 10, a voltage is applied that cancels at least part of the charge (+1.1 V with respect to the second metal surface 12) charged on the first metal surface 11. FIG. Hereinafter, the voltage that cancels out at least part of the charge fluctuation on the first metal surface 11 due to the contact potential difference ΔV will be referred to as "charge pump voltage CP".

The charge pump voltage CP of the present embodiment is a voltage capable of canceling at least part of the charge fluctuation on the first metal surface 11 due to the contact potential difference ΔV , preferably the potential of the first metal surface 11 of the object to be plated 10 is within the substitutable potential range VR (+0.34≧VR≧−0.94). Specifically, the charge pump voltage CP is, for example, a negative voltage of −0.76 V (0.34 V−1.1 V)≧CP≧−2.04 V (−0.94 V−1.1 V).

As described above, the plating method of the present embodiment is applied to the base metal (first metal) side of the object to be plated 10 having the first metal surface 11 and the second metal surface 12 that are electrically conductive (continuous). A method of coating the surface of the (coating with a metal (non-metal oxide) film) with a relatively noble coating metal (coating with a film of metal (non-metal oxide)), wherein the first metal A charge pump voltage CP is applied to cancel at least part of the potential generated on the surface 11 based on the contact potential difference ΔV of the first metal surface 11 with respect to the second metal surface 12 .

The work function of aluminum (Al) is 4.1 eV. That is, in this case, the contact potential difference ΔV of the object to be plated 10 is +1.1 V (=−((3.6 eV−4.1 eV)+(4.1 eV−4.7 eV)) on the zinc side when the copper side is used as a reference. )/e, where e is the elementary charge). When considering the contact potential difference ΔV between electrically conducting (continuous) copper and zinc in this way, the work function of the metal intervening between them (regardless of the ionization tendency of the metal) is canceled out. be. In other words, in this case as well, the object to be plated 10 can be treated as a copper-zinc contact object (see FIGS. 4(F) and 5), and the metal copper plating film 13 of good quality can be obtained by the plating method of the present embodiment described above. can be formed.

Furthermore, as the plating method of the present embodiment, the first metal surface 11 is zinc, the second metal surface 12 is copper, and the coating metal is copper, but these metals are not limited to the above examples. That is, a step of preparing the object to be plated 10 in which the first metal surface 11 and the second metal surface 12 are electrically continuous; 1 applying a voltage (charge pump voltage) to the object to be plated 10 so as to cancel at least part of the potential generated by the contact potential difference ΔV on the metal surface 11; and performing displacement plating on at least part of the object 10 to be plated.

(5) Charge pump voltage CP: At least part of the potential generated by the contact potential difference ΔV is offset so that the potential of the plated object 10 (first metal surface 11) charged by the contact potential difference ΔV falls within the replaceable potential range. A voltage (positive voltage or negative voltage) to be (adjusted, corrected) is calculated. It is not necessary to cancel all of the potential generated corresponding to the contact potential difference ΔV, and it may be partially canceled.

In the present invention, a surface made of a first metal (hereinafter referred to as a "first metal surface") and a surface made of a second metal (hereinafter referred to as a "second metal surface") are both exposed and electrically conductive. and applying a voltage to the first metal through the second metal before the object to be plated is immersed in the plating solution, the first metal setting the potential of the first metal surface to a predetermined potential range by canceling at least part of the potential generated on the first metal surface due to the contact potential difference between the and the second metal; a step of immersing at least a portion to be plated on the surface of the first metal in the plating solution while the voltage is applied, and coating the portion to be plated with a coating metal nobler than the first metal ; is a potential range in which the first metal is ionized and the surface of the first metal is replaced with the coating metal .
Further, in the present invention, both a surface made of a first metal (hereinafter referred to as "first metal surface") and a surface made of a second metal (hereinafter referred to as "second metal surface") are exposed and preparing an electrically conductive object to be plated; and applying a voltage to the first metal through the second metal before immersing the object to be plated in the plating solution, canceling at least part of the potential generated on the surface of the first metal due to the contact potential difference between the first metal and the second metal; immersing in the plating solution to form a displacement plating film on at least the portion to be plated, wherein the displacement plating film is formed by ionizing the first metal so that the surface of the first metal is The plating method is characterized in that the plating film is substituted with a more noble coating metal.
Further, in the present invention, both a surface made of a first metal (hereinafter referred to as "first metal surface") and a surface made of a second metal (hereinafter referred to as "second metal surface") are exposed and preparing an electrically conductive object to be plated; and applying a voltage to the first metal through the second metal before immersing the object to be plated in the plating solution, setting the potential of the first metal surface to a predetermined potential range by canceling at least part of the potential generated on the surface of the first metal due to the contact potential difference between the first metal and the second metal; immersing at least a portion to be plated on the surface of the first metal in the plating solution while a voltage is applied, and coating the portion to be plated with a coating metal nobler than the first metal; The predetermined potential range is the potential range of the region where the potential of the first metal surface of the object to be plated overlaps the corrosion region of the first metal and the dead region of the coating metal in a potential-pH diagram. The present invention relates to a plating method characterized by:
Further, in the present invention, both a surface made of a first metal (hereinafter referred to as "first metal surface") and a surface made of a second metal (hereinafter referred to as "second metal surface") are exposed and preparing an electrically conductive object to be plated; and applying a voltage to the first metal through the second metal before immersing the object to be plated in the plating solution, canceling at least part of the potential generated on the surface of the first metal due to the contact potential difference between the first metal and the second metal; immersing in the plating solution to coat the portion to be plated with a coating metal nobler than the first metal, wherein the first metal is a relatively base metal, and the second When the metal is a relatively noble metal and the contact potential difference is ΔV, the work function of the first metal is WF1, the work function of the second metal is WF2, and the elementary charge is e, the contact potential difference is A potential that satisfies the following formula (1) “ΔV = −(WF1 − WF2)/e (1)” and at which the first metal is ionized and the surface of the first metal is replaced with the coating metal The range (hereinafter referred to as "substitutable potential range") is VR, the equilibrium electrode potential of the first metal (activity is 1E-6) is V1, the equilibrium electrode potential of the coating metal (activity is 1) is When V2, the replaceable potential range satisfies the following formula (2) "V2 ≥ VR ≥ V1 (2)", and when the voltage is CP, the voltage satisfies the following formula (3) The plating method is characterized by satisfying "V2 - ΔV ≥ CP ≥ V1 - ΔV (3)".

Claims (11)

Prepare an object to be plated in which a surface made of a first metal (hereinafter referred to as "first metal surface") and a surface made of a second metal (hereinafter referred to as "second metal surface") are electrically conductive. and
applying a voltage to the object to be plated before immersing the object to be plated in the plating solution;
a step of immersing at least a portion to be plated on the surface of the first metal in the plating solution to coat the portion to be plated with a coating metal nobler than the first metal;
A plating method characterized by: continuing to apply the voltage until the entire substance of the plating target portion is immersed in the plating solution;
The plating method according to claim 1, characterized in that: The application of the voltage cancels at least a portion of the contact potential difference of the first metal surface with respect to the second metal surface that occurs between the first metal surface and the second metal surface;
The plating method according to claim 1 or 2, characterized in that: The above-mentioned applying a voltage,
The plating method according to any one of claims 1 to 3, characterized in that: When the potential range in which the first metal is ionized and the surface of the first metal is replaced with the coating metal is defined as the replaceable potential range,
setting the potential of the first metal surface to the replaceable potential range by applying the voltage;
The plating method according to any one of claims 1 to 3, characterized in that: The second metal is a metal more noble than the first metal,
The plating method according to any one of claims 1 to 5, characterized in that: the first metal is a relatively base metal,
the second metal is a relatively noble metal;
When the contact potential difference is ΔV, the work function of the first metal is WF1, the work function of the second metal is WF2, and the elementary charge is e, the contact potential difference is expressed by the following formula (1):
ΔV=-(WF1-WF2)/e (1)
The filling,
The potential range in which the first metal is ionized and the surface of the first metal is replaced by the coating metal (hereinafter referred to as "potential range of replacement") is VR, the equilibrium electrode potential of the first metal (activity 1E-6) is V1, and the equilibrium electrode potential of the coating metal (activity is 1) is V2, the substitutable potential range is given by the following formula (2)
V2≧VR≧V1 (2)
The filling,
When the voltage is CP, the voltage is expressed by the following equation (3)
V2-ΔV≧CP≧V1-ΔV (3)
satisfy the
The plating method according to claim 3, characterized in that: The second metal and the coating metal are the same metal,
The plating method according to any one of claims 1 to 7, characterized in that: The film thickness of the plating film of the film-forming metal is 3 μm or less.
The plating method according to any one of claims 1 to 8, characterized in that: The first metal is a metal containing zinc as a main component, and the second metal is a metal containing copper as a main component,
10. The plating method according to any one of claims 1 to 9, characterized in that: The object to be plated is a dissimilar metal joint,
The plating method according to any one of claims 1 to 10, characterized in that:

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