TWI606141B - Electroless copper plating bath and electroless copper plating method for increasing copper plating flatness - Google Patents

Description

Electroless copper plating bath and electroless copper plating method for increasing the flatness of copper plating

The invention relates to a copper plating solution and a copper plating method, in particular to an electroless copper plating solution and an electroless copper plating method.

Electroless plating, also known as chemical plating or auto-catalytic plating, is formed on the surface of a polymer object by chemical redox to form a layer having a thickness of about 0.2 to 2. The micron continuous metal layer enables the surface of the non-conductor object to be turned on for subsequent electroplating operations. The copper plating method has the advantages of uniform plating, low plating porosity, simple operation, plating on non-conductors, etc., and is often performed on plastic, or used for perforated plating of printed circuit boards (PCB), and MID. (Molded Interconnect Device) process.

The so-called MID process is to engrave a 3D three-dimensional line on a non-conductive substrate by laser engraving technology, and then directly plate the above-mentioned engraved recessed line to a thickness of more than 8 μm by electroless copper plating. The copper plating (usually 12 microns) is then coated with a thin layer of gold, silver or copper to prevent oxidation of the copper coating. The MID process has been widely used in the manufacture of antennas in smart phones, IPads or transmission components, and is primarily capable of providing sufficient electrical flux for signal transmission. However, the transmission of mobile phone signals has evolved from the original 2G and 3G to the current 4G. The accuracy of the antenna is becoming more and more stringent. The influence on the signal is the roughness of the coating. The frequency signal is walking on the surface of the conductor. When the surface is rougher or has a bump, noise will be generated, resulting in poor communication quality. In order to achieve low noise requirements, the Ra value needs to be less than 0.35.

Conventional electroless copper plating does not exceed 3 microns in thickness and there is usually no excessive requirement for flatness. However, in order to meet the requirements of the antenna, it is necessary to manufacture a high-thickness copper layer by electroless plating in the MID process, but the conventional electroless copper plating solution often has a problem of excessive surface roughness (Ra value greater than 0.6), which cannot be satisfied. A request. Furthermore, through the published technical literature on electroless copper, the focus is on the gloss of the coating, not the flatness. However, the gloss of the coating refers to the microscopic gloss of the coating crystal, and the flatness refers to the roughness of the surface. The definitions of the two are slightly different. Therefore, according to the existing technical literature, it is still impossible to know how to electroless copper plating. A coating having a high flatness after a thickness exceeding 8 μm was produced.

Accordingly, it is an object of the present invention to provide an electroless copper plating bath which enables a high degree of flatness in the thickness of the electroless copper plating layer at a high thickness.

Thus, the electroless copper plating bath of the present invention contains a solvent, a copper ion, a binder, a reducing agent, and a leveling agent component. Wherein the leveler component contains a first leveling agent. The first leveling agent is selected from the group consisting of polyimine, imidazole quaternary ammonium salt materials, or the like.

The copper ion is derived from a water-soluble copper salt. The water-soluble copper salt is selected from the group consisting of copper sulfate, copper chloride, copper nitrate, copper methanesulfonate, or any combination thereof.

Preferably, the polyimine has a number average molecular weight ranging from 100 to 3,000. The polyimine is for example but not limited to polyethyleneimine (ie polyethylenimine). The imidazole-based quaternary ammonium salt material is, for example but not limited to, an imidazole-based quaternary ammonium salt compound or an imidazole-based quaternary ammonium salt polymer. The imidazole-based quaternary ammonium salt compound is, for example but not limited to, a water-soluble product formed by a quaterisation reaction of a component comprising an imidazole-based compound and a quaternizing agent. The imidazole-based quaternary ammonium salt polymer is, for example but not limited to, a product formed by polymerization of the imidazole-based quaternary ammonium salt compound. The imidazole-based quaternary ammonium salt polymer is, for example but not limited to, an imidazole/epichlorohydrin copolymer (i.e., imidazole/epichlorohydrin copolymer). The imidazole compound may be used singly or in combination, and the imidazole compound is, for example but not limited to, imidazole, 1-methylimidazole, 1-ethylimidazole, 2- 2-methylimidazole, 1,5-dimethylimidazole, 1-ethyl-2-methylimidazole, 1-methoxyimidazole (1-oxymethylimidazole), or 1-vinyl imidazole (1-vinyl imidazole). The quaternizing agent may be used singly or in combination, and the quaternizing agent is, for example but not limited to, epichlorohydrin, monochloroacetic acid, benzyl chloride, chloroacetic acid. Chloroacetoamide, 3-aminobenzyl chloride, dichloroglycerine, methyl iodide, allyl chloride, dichloroethane , or monochloropropane, etc. The quaternizing agent is used in an amount ranging from 1 mole to 1.5 moles, based on the amount of the imidazole compound. The quaternization reaction has an operating temperature in the range of from 40 °C to 100 °C. The quaternization reaction has an operating time ranging from 0.5 to 4 hours. Preferably, the first leveling agent polyimine is polyethyleneimine, and the imidazole quaternary ammonium salt material is an imidazole/epichlorohydrin copolymer.

Preferably, the first leveling agent has a molecular weight of from 100 to 3,000. More preferably, the first leveling agent has a molecular weight of from 200 to 2,000. Still more preferably, the first leveling agent has a molecular weight of from 150 to 1500.

In order to make the plating layer have better flatness and the formation of the plating layer is faster. Therefore, preferably, the concentration of the first leveling agent is from 0.01 to 0.8 g/L (i.e., from 10 to 800 ppm). More preferably, the concentration of the first leveling agent is 0.02 to 0.3 g/L (i.e., 20 to 300 ppm).

When the first leveling agent is a polyimine, the concentration thereof is preferably from 0.01 to 0.2 g/L (i.e., from 10 to 200 ppm), most preferably from 0.02 to 0.15 g/L (i.e., from 20 to 150 ppm). When the first leveling agent is an imidazole-based quaternary ammonium salt material, the concentration thereof is preferably 0.02 to 0.8 g/L (i.e., 20 to 800 ppm), most preferably 0.05 to 0.3 g/L (i.e., 50 to 300 ppm). ).

Preferably, the leveler component of the electroless copper plating bath of the present invention further contains a second leveling agent. The second leveling agent is an alkyl sulfonate compound.

Preferably, the alkyl sulfonate compound of the second leveling agent is selected from the group consisting of bis(3-sulfopropyl)dithio disodium salt (ie: 3,3'-Dithiobis-1-propanesulfonic acid disodium salt , 3-Sulphos-Isothiouronylpropylsulfonate (ie, 3-S-Isothiuronium-propyl sulfonate), Quinaldinium Propane Sulfon Betaine, N,N-dimethyl -Sodium dithiocarbamic acid-3-(sulfopropyl ester) sodium salt, or 3-(benzothiazolyl-2) -Sodium thiosulfonate sodium salt (i.e., (3-(Benzothiazolyl-2-thio)propyl sulfonic acid Sodium Salt).

Preferably, the concentration of the second leveling agent is from 0.0001 to 0.02 g/L (i.e., from 0.1 to 20 ppm). More preferably, the concentration of the second leveling agent is from 0.0002 to 0.01 g/L (i.e., from 0.2 to 10 ppm).

The complexing agent forms a complex with the metal ions, which can effectively improve the stability of the plating solution, adjust the rate of metal precipitation, and improve the properties of the plating layer. Preferably, the complexing agent is selected from the group consisting of tetrasodium ethylenediaminetetraacetate, ethylenediaminetetraacetic acid, N, N, N', N'-tetrakis (2-hydroxypropyl)- Ethylene diamine (i.e., N, N, N', N'-tetrakis-(2-hydroxypropyl)-ethylenediamine), or sodium potassium tartrate (i.e., Potassium sodium tartrate).

When gas is deposited on the surface of the object to be plated, in order to accelerate the discharge of the gas, a wetting agent can be added for improvement. The wetting agent prevents gas and metal deposition from juxtaposed, reducing the formation of pinholes. Preferably, the electroless copper plating bath further contains a wetting agent. More preferably, the wetting agent is selected from the group consisting of polyethers, polyenols or polyphosphates (i.e., Polyphosphoester). The polyether is for example but not limited to polyethylene glycol. The polyphosphate is, for example but not limited to, polyoxyethylene nonylphenyl ether branched phosphate.

In order to stabilize the plating solution, the activity of the plating solution can be controlled by adding a stabilizer to avoid deterioration. Preferably, the electroless copper plating bath further contains a stabilizer. The stabilizer is selected from the group consisting of cyanide (ie, Cyanide) or 2,2'-bipyridine (ie, 2,2'-Bipyridine). The cyanide is, for example but not limited to, Potassium ferrocyanide.

The reducing agent employed in the present invention may be any of the components conventionally known in electroless copper plating. Preferably, the reducing agent is selected from the group consisting of formaldehyde (ie, Formaldehyde), glyoxylic acid (ie, Glyoxylic acid), hydrazine (ie, Hydrazine), sodium borohydride (ie, sodium borohydride), and dimethylamine boron. Alkane (ie: Dimethylamine borane), N,N-diethylamine borane (or N,N-diethylamine borane), or sodium hypophosphite (ie: Sodium hypophosphite).

Another object of the present invention is to provide an electroless copper plating method for increasing the flatness of a copper plating layer which enables the electroless copper plating layer to have a high flatness at a high thickness.

Therefore, the method for increasing the flatness of the copper plating layer of the present invention comprises the steps of: taking a test piece after pretreatment, and then immersing the electroless copper plating solution as described above to grow the copper plating layer.

The thickness of the coating is controlled by first calculating the thickness of the plating growth per unit hour (ie, the growth rate of the coating), and then adjusting the thickness of the coating by the control of the immersion time. Preferably, the plating growth rate is 4 to 6 μm / hr, and more preferably, the plating growth rate is 4.5 to 5.5 μm / hr. The immersion time of the test piece is controlled to be 1 hour or more in accordance with the plating growth rate. Preferably, the soaking time of the test piece is controlled to be 2 to 3 hours.

The temperature of the electroless copper plating bath affects the growth rate of the plating layer. When the temperature is lower than 40 ° C, the growth is too slow, and when the temperature is higher than 60 ° C, there is a problem of energy consumption. Therefore, preferably, the temperature of the electroless copper plating bath is controlled at 40 to 60 °C. More preferably, the temperature of the electroless copper plating bath is controlled at 45 to 55 °C.

The effect of the invention is that by adding the first leveling agent to the electroless copper plating bath, the copper plating layer has an Ra value of less than 0.35 and a flatness after being more than 8 micrometers.

The electroless copper plating solution of the invention comprises a solvent, a copper ion, a binder, a reducing agent, and a leveling agent component. Wherein, the leveling agent component comprises a first leveling agent and a second leveling agent. The first leveling agent is selected from the group consisting of polyimine, imidazole quaternary ammonium salt materials, or the like. The second leveling agent is an alkyl sulfonate compound. The plating layer produced by the electroless copper plating bath of the present invention has high flatness at a high thickness (greater than 8 μm) and an Ra value of less than 0.35.

The invention is further described in the following examples, but it should be understood that these examples are for illustrative purposes only and are not to be construed as limiting.

[ Example 1]

Plating solution preparation

Take 5.29g of copper chloride (ie: containing 2.5g of copper ions) into a 1L beaker, add 0.5L of water, dissolve and add ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na) as a wrong agent Stir until dissolved, and add sodium hydroxide (NaOH) to adjust the pH to above 12.5, then heat to 55 ° C, then add 0.1g of polydecylphenol ethoxylate phosphate (purchased from RHODIA, the product name is RHODAFAC ® RE-610) as a wetting agent, and 0.004g of 2,2'-bipyridyl (purchased from Merck) as a stabilizer, followed by 0.2g (ie: 200ppm) of imidazole/epichlorohydrin copolymer (purchased from BASF, the product name is Lugavan® IZE, hereinafter referred to as IZE) as the first leveling agent. After it is dissolved, add 16g of formaldehyde (Formalin) with a concentration of 24wt% as a reducing agent and add water. The total volume was 1 L in the plating solution, and finally, the copper foil which was put into the test plating was activated by plating for 5 minutes, and the preparation of the electroless copper plating solution was completed.

Sample Preparation

Pre-treatment steps: Take an electrolytic copper foil test piece (length * width 5cm * 5cm, thickness 18.5 microns, Ra = 0.122), immerse in the cathode electrolytic degreasing solution, and apply 4ASD current at 60 ° C temperature The density was degreased for 5 minutes, then the test piece was taken out for washing, and then dilute sulfuric acid having a concentration of 10 v/v% was immersed at room temperature for 2 minutes. After the time was reached, the water was washed out to complete the pretreatment step.

Rate estimation

Taking a pre-processed test piece, immersing it in the prepared electroless copper plating solution, immersing it at a temperature of 55 ° C for 30 minutes, taking out the test piece, washing and drying with water, and then measuring the thickness, Estimate the rate of growth of the copper plating every 30 minutes. The rate of this embodiment is about 5.5 to 6 microns per hour.

Electroless copper

Take the pre-finished test piece, immerse it in the prepared electroless copper plating bath, soak it at 55 ° C, and analyze the water level, copper ion, reducing agent and sodium hydroxide concentration every 30 minutes. The insufficiency is supplemented. After the test piece is immersed for 2 to 3 hours, after the copper plating layer reaches a thickness of 12 μm, the test piece is taken out and washed with water and dried.

test sample

The test piece after the copper plating was taken, and the surface measuring instrument (Profile meter, purchased from Kosaka Laboratory LTD, Japan Otaru Institute, model: Surf corder ET 4000A) was used for the measurement, and the test method was to record the Z-axis with the stylus movement. The surface roughness Ra was measured to be 0.32.

[ Example 2 to 3]

Referring to Table 1, the examples in this series are substantially the same as those in Example 1. The main difference is that the first leveling agent used in the preparation of the plating solution is polyethyleneimine, and the molecular weights are 1400 and 200, respectively. Among them, the polyethyleneimine of molecular weight 1400 is purchased from BASF, the product name is Lugalvan® G 20 (hereinafter referred to as G-20), and the polyethyleneimine of molecular weight 200 is also purchased from BASF, and the product name is Lugalvan® G 35 (hereinafter referred to as G -35). The amount of addition thereof was 0.05 g (i.e., 50 ppm). The surface roughness Ra values were measured to be 0.35 and 0.33, respectively.

[ Comparative example 1]

Referring to Table 1, this comparative example is substantially the same as that of Example 1, and the main difference is that the first leveling agent and the second leveling agent are not added in the preparation of the plating solution. The obtained copper layer had an Ra value of 0.7.

When no leveling agent is added to the electroless plating solution, the thickness of the copper plating layer after reaching a thickness of 12 μm is very rough, and the Ra value is 0.7, but when a flattening agent of the polyimide or imidazole quaternary ammonium salt material series is added, Under high film thickness, the Ra value can be reduced to less than 0.35 (see Examples 1 to 3). Therefore, the addition of the first leveling agent of the present invention does allow the copper plating layer to have a flattening effect at a high film thickness.

[ Example 4]

Referring to Table 1, this example is substantially the same as Example 1. The main difference is that after the plating solution is prepared, after adding the first leveling agent IZE, 0.008 g (i.e., 8 ppm) of benzyl quinone propyl sulfonate is further added. (purchased from HOPAX, the product name is QPS) as the second leveling agent, and then continue to add reducing agent, hydration to 1L and trial plating operation. The plated copper obtained by the plating solution was measured to have a surface roughness Ra of 0.23.

[ Example 5]

Referring to Table 1, this example is substantially the same as Example 4, using a reducing agent of 40% by weight of glyoxylic acid, and a second leveling agent of adding 0.005 g (ie: 5 ppm) of 3- Sodium (benzothiazolyl-2-thio)propanesulfonate (purchased from HOPAX, trade name ZPS). The surface roughness Ra was measured to be 0.26.

[ Example 6]

Referring to Table 1, this example is substantially the same as Example 4, the main difference being that the second leveling agent used in the preparation of the plating solution is added with 0.005 g (i.e., 5 ppm) of bis(3-sulfonylpropyl) Sulfur disodium salt (purchased from HOPAX, product name SPS). The surface roughness Ra was measured to be 0.23.

[ Example 7]

Referring to Table 1, this example is substantially the same as Example 4, the main difference being that the first leveling agent in the preparation of the plating solution is a polyethyleneimine (G-20) having a molecular weight of 1400, and 0.05 g is added (ie: 50 ppm), the second leveling agent is added 0.003 g (ie: 3 ppm) of 3-sulfur-isothiourea sulfonate (3-S-Isothiuronium-propylsulfonate UPS, available from HOPAX). The surface roughness Ra was measured to be 0.28.

[ Example 8]

Referring to Table 1, this example is substantially the same as Example 4, the main difference being that the first leveling agent used in the preparation of the plating solution is a polyethyleneimine having a molecular weight of 200 (purchased from BASF, the product name is Lugalvan® G 35, under G-35), and the second leveling agent uses SPS. The surface roughness Ra was measured to be 0.23.

[ Example 9 to 11]

Referring to Table 1, Examples 9 to 11 are substantially the same as in Embodiment 1, except that the chemicals used for the preparation of the plating solution are different. The components of the electroless copper plating bath are listed in Table 1, and the Ra values are 0.28, 0.27, and 0.25, respectively.

[ Comparative example 2 and 3]

Referring to Table 1, this series of comparative examples is substantially the same as that of Example 1, except that in the preparation of the plating solution, only SPS is added as a leveling agent component or no leveling agent component is added. Only the SPS was added without plating, and the Ra value could not be measured. The Ra value was 0.62 when no leveling agent component was added.

It can be seen from Examples 4 to 11 that when the electroless copper plating solution contains both the first leveling agent and the second leveling agent, the Ra value after the thickness of the plating layer is less than 0.3, and the range is about 0.23. Between 0.28. Therefore, if the leveling agent component further contains a second leveling agent, the Ra value of the plating layer can be further lowered to less than 0.3. It can be seen that at the same time, the first leveling agent and the second leveling agent can further increase the flatness and reduce the Ra value. Only the second leveling agent is added, or no plating is produced, or the Ra value is high. The problem of insufficient plating thickness is that when the first leveling agent is not added, the co-plating of the second leveling agent is fast, causing the copper ions to stop reacting and failing to reach a sufficient thickness. If there is only a second leveling agent in the plating solution, although there is a leveling effect, it is impossible to obtain a plating layer having a sufficient thickness.

Table 1 Note: The wrong agent C1 is ethylenediaminetetraacetic acid tetrasodium salt; the wrong agent C2 is N, N, N', N'-tetrakis (2-hydroxypropyl) ethylene diamine, purchased from BASF, the product name is Lutropur® Q -75; the wrong agent C3 is sodium potassium tartrate; the reducing agent R1 is formaldehyde (Formalin, weight percent concentration: 24% by weight); the reducing agent R2 is Glyoxylic acid (weight percent concentration: 40% by weight); The agent W1 is polyoxyethylene nonylphenyl ether branched phosphate (purchased from RHODIA, the product name is RHODAFAC® RE-610); the wetting agent W2 is polyethylene glycol (polyethylene glycol, molecular weight 6000, Available from Dow under the trade name CARBOWAXTM PEG 6000); stabilizer S1 is 2,2'-bipyridine (2,2'-bipyridine, available from Merck); stabilizer S2 is potassium ferrocyanide (Potassium ferrocyanide) From Merck).

The first leveling agent is made of a polyimine or an imidazole-based quaternary ammonium salt material, is not cracked by the reaction of the aldehyde, and the plating solution is more stable overall, and, when used, the first leveling agent will be treated. The surface of the plating forms a shielding layer which forces the copper ions to be aligned in a more uniform manner and adheres to the surface layer of the object to be plated, and then is reduced, and the shielding layer also suppresses the second leveling. The rate at which the agent (alkyl sulfonate compound) enters the coating allows a suitable sulfur content to be obtained after a plating thickness of 8 microns. Therefore, the electroless copper plating bath of the present invention can solve the problem of flatness.

[ Example 12 to 15]

Examples 12 to 15 were substantially the same as Example 1, except that in the preparation of the plating solution, the difference was in the reagent component (see Table 2), and the operating temperature was controlled at 50 °C. Among them, the Ra values thereof are 0.25, 0.24, 0.26, and 0.24, respectively, and are less than 0.35.

[ Example 16]

The preparation of the plating solution of this embodiment is substantially the same as that of Example 1, except that the components of the reagent are different (see Table 2), and the first leveling agent is mixed with IZE and G-20, and then added to the second whole. Flat agent SPS. Its Ra value is 0.27 and less than 0.35.

Table 2 Note: The wrong agent C1 is ethylenediaminetetraacetic acid tetrasodium salt; the wrong agent C2 is N, N, N', N'-tetrakis (2-hydroxypropyl) ethylene diamine, purchased from BASF, the product name is Lutropur® Q -75; the wrong agent C3 is sodium potassium tartrate; the reducing agent R1 is formaldehyde (Formalin, weight percent concentration: 24% by weight); the reducing agent R2 is Glyoxylic acid (weight percent concentration: 40% by weight); The agent W1 is polyoxyethylene nonylphenyl ether branched phosphate (purchased from RHODIA, the product name is RHODAFAC® RE-610); the wetting agent W2 is polyethylene glycol (polyethylene glycol, molecular weight 6000, Available from Dow under the trade name CARBOWAXTM PEG 6000); stabilizer S1 is 2,2'-bipyridine (2,2'-bipyridine, available from Merck); stabilizer S2 is potassium ferrocyanide (Potassium ferrocyanide) From Merck).

It can be seen from Examples 12 to 15 that the operating temperature of the electroless copper plating bath of the present invention can still form a copper plating layer having a thickness of 12 μm at a lower temperature, and the Ra value thereof is less than 0.35, which can meet the MID process. Requirements for the production of the antenna. The first leveling agent of Example 16 was in a mixed form, and in the group of the second leveling agent, the resulting copper plating layer still had high flatness (Ra value of 0.27).

In summary, the electroless copper plating bath of the present invention enables the copper plating layer to have an Ra value of less than 0.35 and a high flatness after exceeding 8 μm, so that the object of the present invention can be achieved.

However, the above is only the embodiment of the present invention, and the scope of the invention is not limited thereto, and all the equivalent equivalent changes and modifications according to the scope of the patent application and the patent specification of the present invention are still The scope of the invention is covered.

Claims (12)

An electroless copper plating solution comprising a solvent, a copper ion, a binder, a reducing agent, and a leveling agent component, wherein the leveling agent component comprises a first leveling agent, and the first leveling agent is selected from the group consisting of poly A combination of an amine, an imidazole-based quaternary ammonium salt material or a combination thereof, the first leveling agent having a molecular weight of from 100 to 3,000, and the first leveling agent having a concentration of from 0.01 to 0.8 g/L. The electroless copper plating solution according to claim 1, wherein the first leveling agent polyimine is polyethyleneimine, and the imidazole quaternary ammonium salt material is imidazole/epichlorohydrin copolymer. The electroless copper plating solution according to claim 1, wherein the leveling agent component further contains a second leveling agent, and the second leveling agent is an alkylsulfonate compound. The electroless copper plating solution according to claim 3, wherein the alkylating agent of the second leveling agent is selected from the group consisting of bis(3-sulfonylpropyl)disulfide disodium salt, 3-sulfur- Isothiourea propyl sulfonate, benzyl quinone propyl sulfonate, sodium N,N-dimethyl-dithiocarbamic acid-3-(sulfopropyl ester), or 3-(benzothiazolyl) Sodium 2-thio)propane sulfonate. The electroless copper plating solution according to claim 1, wherein the concentration of the first leveling agent is 0.02 to 0.3 g/L. The electroless copper plating solution according to claim 1, wherein when the first leveling agent is a polyimine, the concentration is 0.01 to 0.2 g/L, and when the first leveling agent is an imidazole-based quaternary ammonium salt material The concentration is 0.02 to 0.8 g/L. The electroless copper plating solution according to claim 6, wherein when the first leveling agent is a polyimine, the concentration is 0.02 to 0.15 g/L, and when the first leveling agent is an imidazole-based quaternary ammonium salt material The concentration is 0.05 to 0.3 g/L. The electroless copper plating solution according to claim 3, wherein the concentration of the second leveling agent is 0.0001 to 0.02 g/L. The electroless copper plating solution according to claim 8, wherein the concentration of the second leveling agent is 0.0002 to 0.01 g/L. An electroless copper plating method for increasing the flatness of a copper plating layer comprises the steps of: immersing a test piece after pretreatment, and then immersing the electroless copper plating solution as described in claim 1 to grow a copper plating layer. An electroless copper plating method for increasing the flatness of a copper plating layer as claimed in claim 10, wherein the temperature of the electroless copper plating bath is controlled at 40 to 60 °C. The electroless copper plating method for increasing the flatness of a copper plating layer according to claim 10, wherein the test piece is immersed in the electroless copper plating bath for a time of 1 hour or longer.