Printed circuit board and method of manufacturing the same

Abstract

A printed circuit board includes a substrate, and a wiring provided on the substrate. The wiring includes a copper-based metal wire provided on the substrate and a surface-treated layer provided on the copper-based metal wire. The copper-based metal wire includes mainly a copper. The surface-treated layer includes an amorphous layer including oxygen and a metal with a higher oxygen affinity than the copper.

Claims

1. A printed circuit board, comprising: a substrate; and a wiring provided on the substrate, the wiring comprising a copper-based metal wire having an upper surface provided on the substrate, and a surface-treated layer coextensive with the upper surface, and comprising an amorphous layer comprising oxygen and a metal having a higher oxygen affinity than copper the metal consisting of zinc, wherein the surface-treated layer directly contacts the upper surface of the copper-based metal wire.

2. The printed circuit board according to claim 1, wherein the surface-treated layer is provided on the upper surface, or the upper surface and a lower surface, of the copper-based metal wire.

3. The printed circuit board according to claim 1, wherein the amorphous layer further comprises copper diffused from the copper-based metal wire.

4. The printed circuit board according to claim 1, wherein the surface-treated layer further comprises a diffusion layer under the amorphous layer, and wherein the diffusion layer comprises a copper and a second metal with a higher oxygen affinity than the copper, or oxygen, a copper and a second metal with a higher oxygen affinity than copper.

5. The printed circuit board according to claim 1, wherein the surface-treated layer has a thickness of not less than 3 nm and not more than 100 nm.

6. The printed circuit board according to claim 1, wherein the amorphous layer consists essentially of oxygen, the metal consisting of zinc, and copper diffused into the amorphous layer from the copper-based metal wire.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Next, the present invention will be explained in more detail in conjunction with appended drawings, wherein:

(2) FIG. 1 is a schematic perspective view showing a printed circuit board in a first embodiment of the present invention;

(3) FIG. 2 is a cross sectional view taking on line II-II of FIG. 1;

(4) FIG. 3 is a cross sectional view showing a printed circuit board in a second embodiment of the invention and corresponding to FIG. 2;

(5) FIG. 4 is a graph showing the results of Auger elemental analysis performed on a sample in Example 3 of the invention, where the sample after 3600 hours of a constant temperature test (at 100° C.) is analyzed from the surface in a depth direction while repeating sputtering;

(6) FIG. 5 is a graph showing time-dependent change in an oxygen penetration depth from the surface layer (thickness of oxide film) in the constant temperature test (at 100° C.) conducted on samples in Example 3 of the invention and Comparative Examples 1, 4 and 5; and

(7) FIG. 6 is an electron diffraction image showing the result of RHEED analysis performed on the sample in Example 3 of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Configuration of Printed Circuit Board

(8) Printed circuit boards in the embodiments of the invention are each provided with a substrate and a wiring provided on the substrate. The wiring is provided with a copper-based metal wire provided on the substrate and a surface-treated layer provided on the copper-based metal wire. The copper-based metal wire includes mainly a copper. The surface-treated layer has an amorphous layer containing oxygen and a metal with a higher oxygen affinity than copper, and is provided on one or both sides of the copper-based metal wire.

(9) FIG. 1 is a schematic perspective view showing a printed circuit board in the first embodiment of the invention and FIG. 2 is a cross sectional view taking on line II-II of FIG. 1. FIG. 3 is a cross sectional view showing a printed circuit board in the second embodiment of the invention and corresponding to FIG. 2.

(10) A printed circuit board 100 shown in FIGS. 1 and 2 is provided with a substrate 101 and a wiring 10 provided on the substrate 101. The wiring 10 is provided with a Cu-based metal wire 1 (hereinafter, sometimes simply referred as “Cu wire”) having a rectangular cross section and a surface-treated layer 2 provided on a surface of the Cu-based metal wire 1 opposite to the surface arrange to face a surface of the substrate 101.

(11) A material of the Cu wire 1 is not specifically limited as long as it is a copper-based metal material used for printed circuit boards, but it is preferable that the material includes mainly Cu which is contained in an amount of not less than 90 mass %. That is, Cu alone or a Cu alloy containing not more than 10 mass % of impurities is preferable. It is possible to use, e.g., pure copper such as oxygen-free copper or tough pitch copper, or dilute-copper alloys containing 3 to 15 mass ppm of sulfur, 2 to 30 mass ppm of oxygen and 5 to 55 mass ppm of Ti.

(12) The thickness of the Cu wire 1 is preferably from 10 μm to 500 μm, more preferably from 10 μm to 400 μm, and further preferably from 10 μm to 200 μm. It is difficult to ensure conductivity when thinner than 10 μm, while the cost is increased when thicker than 500 μm. A width of the Cu wire 1 is appropriately determined depending on size, etc., of the circuit board.

(13) The surface-treated layer 2 has an amorphous layer containing oxygen and a metal with a higher oxygen affinity than copper. Alternatively, the surface-treated layer 2 may have an amorphous layer containing oxygen, a metal with a higher oxygen affinity than copper and copper diffused from the Cu wire 1.

(14) The surface-treated layer may be a surface-treated layer 3 having an amorphous layer 5 and a diffusion layer 4 which is formed thereunder and contains copper and a metal with a higher oxygen affinity than copper, or oxygen, copper and a metal with a higher oxygen affinity than copper, as is the second embodiment (a wiring 20 of a printed circuit board 200) shown in FIG. 3. The diffusion layer 4 is different from the amorphous layer 5 in that it is a crystalline layer.

(15) The metal with a higher oxygen affinity than copper, which constitutes the surface-treated layer 2 (amorphous layer) and the amorphous layer 5, is preferably zinc. Besides zinc, it is possible to use, e.g., Ti, Mg, Zr, Al, Fe, Sn and Mn, etc. Ti, Mg and Zr, which are easily oxidized and removed at the time of manufacturing copper, are particularly preferable from the viewpoint of recycling. The same applies to the metal with a higher oxygen affinity than copper, which constitutes the diffusion layer 4, and it is preferable to use the same metal as the metal with a higher oxygen affinity than copper, which constitutes the amorphous layer.

(16) The amorphous layer with randomly-arranged elements is considered to have a denser structure than a crystalline layer with regularly-arranged elements. Therefore, copper diffusion to the surface of the surface-treated layer and oxygen ingress into a copper raw material, which cause oxidation of the copper raw material, are suppressed or reduced by the amorphous layer. It is believed that the amorphous layer thereby serves as a barrier layer which inhibits bonding between copper and oxygen.

(17) Oxygen needs to preferentially bond to a metal other than copper so that the amorphous layer is formed. Then, in order to accelerate the formation of the amorphous layer, it is preferable that a metal with a higher oxygen affinity than copper (e.g., zinc) be arranged on the surface of the Cu wire 1.

(18) Different types of elements are in contact with each other at an interface between the surface-treated layers 2 and 3 and another layer and a gradual concentration change is generally seen at such an interface, which makes difficult to define the thickness of the surface-treated layer. Therefore, the thickness of the surface-treated layer in the invention is defined as “a thickness of a layer which contains oxygen, a metal with a higher oxygen affinity than copper and, in some cases, copper, and in which each of the constituent elements is contained in the amount of not less than 2 at % in terms of atomic concentration (at %) as an elemental content ratio”.

(19) The thickness of the surface-treated layer 2 is preferably not less than 3 nm and not more than 100 nm depending on the heat treatment conditions, more preferably not less than 5 nm and not more than 70 nm, and further preferably not less than 6 nm and not more than 50 nm. Meanwhile, the thickness of the surface-treated layer 3 is preferably not less than 6 nm and not more than 100 nm as the total of the thickness of the diffusion layer 4 and the thickness of the amorphous layer 5.

(20) When the diffusion layer 4 is provided, the lower limit of the thickness thereof is not specifically limited as long as the Cu wire 1 is covered, and the lower limit of coating thickness is preferably about 3 nm in effect. In addition, the upper limit of the thickness of the diffusion layer 4 is preferably not more than 80 nm. When the diffusion layer 4 is more than 80 nm, the amorphous layer 5 contributing to develop high corrosion resistance may be stabilized and become less likely to be formed. The thickness of the amorphous layer 5 is not specifically limited but is preferably not less than 3 nm.

(21) The total thickness of the Cu wire 1 and the surface-treated layer 2 or 3 is preferably less than 0.55 mm, more preferably not less than 20 nm and not more than 0.5 mm, and further preferably not less than 20 nm and not more than 0.3 mm. It is difficult to ensure conductivity when too thin, while the cost is increased when too thick.

Method of Manufacturing Printed Circuit Board

(22) Next, a method of manufacturing the printed circuit boards in the present embodiments will be described.

(23) <Manufacturing Method 1> Form Surface-Treated Layer after Forming Wiring on Substrate

(24) A Cu foil is adhered onto the substrate 101 by an adhesive and the Cu wire 1 is then formed by etching. After that, in case that a metal with a higher oxygen affinity than copper is, e.g., zinc, a Zn layer is formed on the surface of the Cu wire 1 by electrolytic plating in size and shape of the finished product. By subsequent heating in the ambient air at a temperature of not less than 30° C. and not more than 300° C. for not less than 5 seconds and not more than 60 minutes, the surface-treated layer 2 (amorphous layer) is formed. The thickness of the Zn layer is preferably not less than 3 nm and not more than 100 nm, more preferably not less than 5 nm and not more than 70 nm, and further preferably not less than 6 nm and not more than 50 nm. A wiring provided with the surface-treated layer 2 having the amorphous layer containing at least zinc and oxygen is thus obtained. In other words, it is possible to form the surface-treated layer 2 (amorphous layer) on the surface of the Cu wire 1 by a simple method in which a zinc cover layer is simply provided and heat-treated under the predetermined conditions.

(25) <Manufacturing Method 2> Form Wiring on Substrate after Forming Surface-Treated Layer

(26) A Cu foil with the surface-treated layer(s) 2 (on one or both sides) formed by the above method is attached onto the substrate 101 by an adhesive, and subsequently, a wiring formed of the Cu wire 1 having the surface-treated layer(s) 2 is shaped by etching. Alternatively, the wiring may be formed by attaching a Cu foil onto the substrate 101, forming the surface-treated layer(s) 2 on the Cu foil and then performing the etching.

(27) In the present embodiments, the cover layer is heat-treated preferably at a temperature of not less than 30° C. and not more than 300° C. for not less than 5 seconds and not more than 60 minutes as described above, more preferably, at a temperature of not less than 50° C. and not more than 150° C. for not less than 30 seconds and not more than 15 minutes. In addition, a plating process can be suitably used for forming the Zn layer. In addition to the plating process, it is possible to use a sputtering method, a vacuum deposition method and a cladding process, etc.

(28) Alternatively, as a manufacturing method in another embodiment, the surface-treated layer 2 (amorphous layer) may be formed by preliminarily plating zinc before processing into the size and shape of the finished product and heat-treating after processing into the size and shape of the finished product.

(29) Meanwhile, the diffusion layer 4 can be formed by, e.g., covering the surface of the Cu wire 1 with zinc before forming the amorphous layer 5 of the surface-treated layer 3 and then heating in the ambient air or holding in an oil bath or salt bath at a temperature of not less than 50° C. Alternatively, the diffusion layer 4 may be manufactured using electric resistance heat. After that, the amorphous layer 5 is formed on the surface of the diffusion layer 4 by the same method as that used for forming the surface-treated layer 2 (amorphous layer).

Effects of the Embodiments

(30) In the embodiments of the invention, oxidation of the copper-based metal wire can be suppressed since the surface-treated layer 2 or 3, which serves as a barrier layer for suppressing or reducing copper diffusion to the surface of the surface-treated layer and oxygen ingress into the copper-based metal wire, is formed on the surface of the copper-based metal wire. Therefore, it is possible to improve solder bondability or wire-bonding properties between semiconductor components, etc., and the wiring.

(31) In addition, in the embodiments of the invention, it is possible to manufacture printed circuit boards at a lower cost since a Ni/Au plating is not required. Furthermore, as compared to the Ni/Au plating which causes a decrease in conductivity due to Ni remaining after ultrasonic bonding and functioning as a resistive layer, the surface-treated layer (amorphous layer) of the invention does not remain after soldering or does not become a resistive layer due to its extremely small thickness and a printed circuit board with high conductivity is thus obtained. In addition, while flexibility of the printed circuit board is decreased in case of the Ni/Au plating process since the Ni/Au plating layer is relatively thick and hard, the surface-treated layer (amorphous layer) of the invention is thin and the printed circuit board is thus excellent in flexibility.

(32) The following examples further illustrate the invention but the invention is not limited thereto.

EXAMPLES

(33) Table 1 shows the configurations of the samples in Examples 1 to 6 and Comparative Examples 1 to 5. Table 1 also shows the evaluation results of the evaluation items described later. In Examples, the evaluated samples are Cu sheets, not Cu wires, so that the evaluation is simplified.

(34) TABLE-US-00001 TABLE 1 Surface-treated layer Evaluation Material Presence of results Sheet thickness amorphous Oxidation material (μm) layer resistance Examples 1 Cu Zn 0.003 present ◯ 2 Cu Zn 0.006 present ⊚ 3 Cu Zn 0.01 present ⊚ 4 Cu Zn 0.05 present ⊚ 5 Cu Zn 0.1 present ◯ 6 Cu Zn 0.1 present ◯ Comparative 1 Cu Zn 1.0 not present X Examples 2 Cu Zn 0.02 not present Δ 3 Cu Zn 0.02 not present X 4 Cu — — not present X 5 Cu—Zn — — not present Δ alloy

(35) The samples in Examples 1 to 6 shown in Table 1 were generally made as follows: a cover layer formed of zinc plating with various thickness (0.002 to 0.08 μm) was formed on a flat sheet formed of tough pitch copper by electrolytic plating and annealing was then performed in the ambient air. The details of Examples 1 to 5 and Comparative Examples 1 to 5 will be described later.

(36) Meanwhile, in Comparative Example 1 for evaluating influence of the thickness of the zinc layer on the characteristics of the Cu-based metal material (Cu sheet), a zinc layer with a different thickness was formed and heat treatment was then performed in the same manner as Example 1. In Comparative Examples 2 and 3 for evaluating influence of the heat treatment conditions on the characteristics of the Cu-based metal material (Cu sheet), the sample was made under a different heat treatment condition (Comparative Example 2) or was made without heat treatment (Comparative Example 3).

(37) Furthermore, as the samples of Comparative Examples 4 and 5, tough pitch copper (Comparative Example 4) and a Cu-30 mass % Zn alloy (Comparative Example 5) were prepared.

(38) Presence of the amorphous layer shown in Table 1 was confirmed by RHEED analysis (Reflection High Energy Electron Diffraction). The samples showing a halo pattern indicating presence of the amorphous layer are indicated by “Present”, and the samples showing electron diffraction spots indicating a crystalline structure are indicated by “Not present”.

(39) Here, oxidation resistance evaluation of the obtained samples shown in Table 1 was performed as follows.

(40) For evaluating “oxidation resistance”, each sample was kept in the ambient air in a constant-temperature oven set at 100° C. for up to 1000 hours and an increase in oxide film was measured after the test. Based on comparison with the initial thickness (before the test) of the oxide film, the samples with the least change were evaluated as “⊚ (excellent)”, the samples with the most significant change and with associated deterioration were evaluated as “X (bad)”, and the samples with the change in-between were evaluated as either “◯ (acceptable)” or “Δ (unacceptable)” depending on the extent of the change. Using comparison results with the initial thickness (before the test) of the oxide film as quantitative criteria, the samples of which oxide film after 1000 hours has a thickness three times or more were all evaluated as “X” regardless of the change in appearance. Here, ⊚ and ◯ are judged as “passed the test” and Δ and X are judged as “failed the test”.

(41) The details of Examples 1 to 6 and Comparative Examples 1 to 5 are described below.

Example 1

(42) A 0.5 mm-thick flat sheet formed of pure Cu (tough pitch copper; hereinafter, described as “TPC”) was prepared, a 0.002 μm-thick cover layer formed of zinc was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 30° C. for 10 minutes, thereby obtaining a sample provided with a surface-treated layer. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.003 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Example 2

(43) In Example 2, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.005 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 50° C. for 1 hour, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.006 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Example 3

(44) In Example 3, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.008 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 100° C. for 5 minutes, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.01 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Example 4

(45) In Example 4, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.04 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 120° C. for 10 minutes, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.05 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Example 5

(46) In Example 5, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.08 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 150° C. for 30 seconds, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.1 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Example 6

(47) In Example 6, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.08 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 300° C. for 5 seconds, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.1 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Comparative Example 1

(48) In Comparative Example 1, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.95 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 100° C. for 5 minutes, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 1 μm-thick surface-treated layer composed of zinc (Zn) and oxygen (O) was formed.

Comparative Example 2

(49) In Comparative Example 2, a 0.5 mm-thick flat sheet formed of TPC was prepared, a 0.01 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating and heat treatment was subsequently performed in the ambient air at a temperature of 400° C. for 60 seconds, thereby obtaining a sample. Based on Auger analysis performed on the obtained sample from the surface in a depth direction, it was confirmed that a 0.02 μm-thick surface-treated layer composed of zinc (Zn), oxygen (O) and copper (Cu) was formed.

Comparative Example 3

(50) In Comparative Example 3, a 0.5 mm-thick flat sheet formed of TPC was prepared and a 0.02 μm-thick Zn layer was then formed on a surface of the flat sheet by electrolytic plating, thereby obtaining a sample.

Comparative Example 4

(51) In Comparative Example 4, a 0.5 mm-thick flat sheet formed of TPC was used as an evaluation sample.

Comparative Example 5

(52) In Comparative Example 5, a 0.5 mm-thick flat sheet formed of Cu-30 mass % Zn alloy (brass) was used as an evaluation sample.

(53) FIG. 4 is a graph showing the results of Auger elemental analysis performed on the sample in Example 3, where the sample after 3600 hours of a constant temperature test (at 100° C.) is analyzed from the surface in a depth direction while repeating sputtering. The horizontal axis indicates a depth (nm) from the surface and the vertical axis indicates an atomic concentration (at %). Then, a solid line shows the atomic concentration (at %) as the ratio of the oxygen content, a long-dashed line shows the atomic concentration of zinc and a short-dashed line shows the atomic concentration of copper. The oxygen penetration depth was about 8 nm from the surface, and the average elemental content of each element particularly in the surface portion at a depth of 0 to 3 nm, which is defined as [the maximum atomic concentration+the minimum atomic concentration (at the depth of 0 to 3 nm)]/2, was 60 at % for zinc (Zn), 33 at % for oxygen (O) and 7 at % for copper (Cu) in Example 3.

(54) In addition, based on the results including other Examples, it was found that the average elemental content of zinc (Zn) was in a range of 35 to 68 at %, that of oxygen (O) in a range of 30 to 60 at % and that of copper (Cu) in a range of 0 to 15 at %.

(55) On the other hand, the sample in Comparative Example 1 contained 33 at % of zinc (Zn), 41 at % of oxygen (O) and 26 at % of copper (Cu), and the sample in Comparative Example 5 contained 5 at % of zinc (Zn), 46 at % of oxygen (O) and 49 at % of copper (Cu).

(56) FIG. 5 is a graph showing time-dependent change in an oxygen penetration depth from the surface layer (thickness of oxide film) in the constant temperature test (at 100° C.) conducted on the samples in Example 3 and Comparative Examples 1, 4 and 5. The oxygen penetration depth was derived by Auger analysis performed on the samples held for various periods of time from the surface in a depth direction while repeating sputtering. In FIG. 5, the horizontal axis indicates isothermal holding time (h) at 100° C. and the vertical axis indicates the oxygen penetration depth (nm). Then, a solid line shows oxygen penetration depth in Example 3 and dashed lines show the oxygen penetration depth in Comparative Example 4 and that in Comparative Example 5. Comparative Example 1 is indicated by a single point.

(57) In Example 3, as shown in FIG. 4, the oxygen concentration in the vicinity of the surface increased after holding 3600 hours but the penetration depth of oxygen hardly changed before and after the test and was about not more than 0.01 μm, which shows that the sample in Example 3 has high oxidation resistance.

(58) Meanwhile, as shown in FIG. 5, a thickness of an oxygen-containing layer before the constant temperature test was about 0.006 μm from the surface in both Comparative Example 4 (tough pitch copper) and Comparative Example 5, which is comparable to the depth before the test in Example 3. However, after the 3600 hours of the holding test, the oxygen concentration in the vicinity of the surface in Comparative Example 4 significantly increased as compared to that before the constant temperature test, and also, the oxygen penetration depth in Comparative Example 4 was about 0.036 μm which is 5 times or more of that before the test and the oxygen penetration depth in Comparative Example 5 was about 0.078 μm which is 13 times that before the test. In addition, in Comparative Examples 4 and 5, reddish brown discoloration was observed on the appearance after the test and it was obvious that a thick oxygen-containing layer was formed. In addition, in Comparative Examples 1 in which a 0.95 μm-thick Zn layer is formed on TPC, the oxygen penetration depth already reached about 0.080 μm after 1000 hours of the holding test.

(59) FIG. 6 shows the result of RHEED analysis performed on the surface of the sample in Example 3 which is excellent in oxidation resistance. A halo pattern is shown in the electron diffraction image and this reveals that an amorphous layer is formed on the surface as shown in Table 1. On the other hand, it was confirmed that the sample in Comparative Example 4 having less oxidation resistance was a crystalline substance composed of copper and oxygen.

(60) In addition, according to Table 1, in Examples 1 to 6 in which surface-treated layers with various thicknesses of 0.003 to 0.1 μm are respectively provided and each have an amorphous structure, good results were obtained from the evaluation of oxidation resistance. Excellent characteristics were exhibited especially when the thickness of the surface-treated layer was 0.006 to 0.05 μm.

(61) From the above results, it was confirmed that, in the structures of Examples 1 to 6, the process of oxidation does not occur and a stable surface condition is maintained even in the constant temperature test at 100° C. for 1000 hours and also under the environment of 85° C. and 85% of humidity.

(62) On the other hand, satisfactory characteristics were not obtained in some cases in Comparative Examples 1 to 3 even though a Zn-based surface-treated layer is provided in the same manner. The evaluation result of oxidation resistance is “Fail” for all of the samples in which an amorphous surface layer was not formed, such as Comparative Example 1 with thick zinc, Comparative Example 2 with excessive heat treatment after plating and Comparative Example 3 with no heat treatment after plating.

(63) From the above results, it was confirmed that the heat treatment is preferably performed in the ambient air at not less than 30° C.

(64) As for the cost performance (economic performance), Examples 1 to 6 do not require, e.g., coating of noble metal excellent in oxidation resistance of material per se but expensive, uses cheap Zn with very small thickness, and are thus extremely excellent in productivity and economic performance.

(65) It should be noted that the invention is not intended to be limited to the embodiments and the examples, and various kinds of modifications can be implemented.