Conductive coating film forming bath

Abstract

This invention relates to a conductive-coating bath comprising an aqueous solution containing (A) a copper compound, (B) a complexing agent, (C) an alkali metal hydroxide, (D) a C.sub.2-5 aliphatic polyalcohol compound, and (E) at least one compound selected from the group consisting of reducing compounds having a COOM group, wherein M is hydrogen, an alkali metal, or a NH.sub.4 group, and reducing saccharides having six or more carbon atoms. The present invention provides a composition for forming a conductive coating having excellent properties as a base layer for electroplating, which is effectively used to form a uniform decorative coating having excellent appearance by electroplating on a non-conductive plastic molding.

Claims

1. A conductive-coating bath comprising an aqueous solution containing: (A) a copper compound; (B) a complexing agent; (C) an alkali metal hydroxide; (D) a C.sub.2-5 aliphatic polyalcohol compound; and (E) at least one compound selected from the group consisting of reducing compounds having a COOM group, wherein M is hydrogen, an alkali metal, or a NH.sub.4 group, and reducing saccharides having six or more carbon atoms, wherein the conductive-coating bath has a pH of 10.0 to 14.0.

2. The conductive-coating bath according to claim 1, wherein the bath comprises as component (E) both a reducing compound having a COOM group, wherein M is hydrogen, an alkali metal, or a NH.sub.4 group, and a reducing saccharide having six or more carbon atoms.

3. A method of forming a conductive coating on a non-conductive plastic molding, the method comprising bringing the conductive-coating bath of claim 1 into contact with a non-conductive plastic molding to which a catalyst substance is applied.

4. The method of forming a conductive coating according to claim 3, wherein the conductive-coating bath is in a state in which the amount of dissolved oxygen is increased when brought into contact with the non-conductive plastic molding.

5. The method of forming a conductive coating according to claim 4, wherein the amount of the dissolved oxygen is increased by supplying oxygen-containing gas bubbles or by adding an oxidizing agent to the bath.

6. A method of electroplating a non-conductive plastic molding, the method comprising the steps of: forming a conductive coating according to the method of claim 3; and performing electroplating.

7. A method of forming a conductive coating on a non-conductive plastic molding, the method comprising bringing the conductive-coating bath of claim 2 into contact with a non-conductive plastic molding to which a catalyst substance is applied.

8. A method of electroplating a non-conductive plastic molding, the method comprising the steps of: forming a conductive coating according to the method of claim 4; and performing electroplating.

9. A method of electroplating a non-conductive plastic molding, the method comprising the steps of: forming a conductive coating according to the method of claim 5; and performing electroplating.

10. A method of electroplating a non-conductive plastic molding, the method comprising the steps of: forming a conductive coating according to the method of claim 7; and performing electroplating.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a photograph showing the appearance of a substrate obtained by forming a conductive coating by using comparative bath 2, followed by copper sulfate electroplating.

(2) FIG. 2 is a photograph showing the appearance of a substrate obtained by forming a conductive coating by using bath 2 of the invention, followed by copper sulfate electroplating.

DESCRIPTION OF EMBODIMENTS

(3) The present invention is described in detail with reference to Examples. However, the present invention is not limited to these Examples.

EXAMPLES

Example 1

(4) A flat plate made of an ABS resin (UMG ABS3001M, produced by UNG ABS, Ltd.) measuring 10 cm5 cm0.3 cm (thickness) and having a surface area of about 1 dm.sup.2 was used as a substrate to be treated. A jig for use in plating operation had two contact points for contact with the substrate to be treated, the two contact points being spaced away by 11 cm. The jig was constructed from a stainless steel rod and had a contact point portion with a diameter of 2 mm. The portion other than the contact points in the jig was coated with a vinyl chloride sol by baking.

(5) Degreasing Treatment

(6) First, the substrate to be treated was set in the jig, which was immersed in a solution of an alkaline degreasing agent (Ace Clean A-220, 50 g/L aqueous solution, produced by Okuno Chemical Industries Co., Ltd.) at 50 C. for 5 minutes and washed with water.

(7) Etching Treatment

(8) The resulting substrate was immersed in an etching solution comprising an aqueous solution containing 400 g/L chromic anhydride and 400 g/L of sulfuric acid at 67 C. for 8 minutes to give a rough surface to the resin substrate.

(9) Neutralizing Treatment

(10) Thereafter, the resulting substrate was washed with water, immersed in an aqueous solution containing 50 mL/L of 35% hydrochloric acid at room temperature for 30 seconds to remove the chromic acid from the resin surface, and washed well with water.

(11) Pre-Dipping Treatment

(12) Then, pre-dipping was performed by dipping the substrate into an aqueous solution containing 250 mL/L of 35% hydrochloric acid at 25 C. for 1 minute.

(13) Catalyzing Treatment

(14) The substrate was then immersed in a colloidal solution (pH: 1 or less) containing 83.3 mg/L of palladium chloride (50 mg/L as Pd), 8.6 g/L of stannous chloride (4.5 g/L as Sn), and 250 mL/L of 35% hydrochloric acid at 35 C. for 6 minutes to cause a catalyst to uniformly adhere to the resin substrate.

(15) Conductive-Coating Formation Treatment

(16) Thereafter, the substrate was washed with water and immersed in the conductive-coating bath (bath 1 of the invention) of Example 1 shown in Table 1 below at a bath temperature of 45 C. for 5 minutes to form a conductive coating.

(17) The conductive-coating baths in the Examples may sometimes be referred to as a bath of the invention. The conductive-coating baths in the Comparative Examples may sometimes be referred to as comparative baths.

(18) TABLE-US-00001 TABLE 1 Composition (E) (B) (C) (D) Reducing saccharides (A) Complexing Alkali metal C.sub.2-5 aliphatic Carboxy-containing having six or more No. Copper compound agent hydroxide polyalcohol reducing compound carbon atoms Examples 1 Copper sulfate- Rochelle salt Sodium ethylene glycol (50 g/L) formic acid (10 g/L) 2 pentahydrate (20 g/L) hydroxide 1,2-propanediol (50 g/L) formic acid (10 g/L) 3 (4 g/L) (65 g/L) glycerin (50 g/L) formic acid (10 g/L) 4 erythritol (50 g/L) formic acid (10 g/L) 5 xylitol (50 g/L) formic acid (10 g/L) 6 glycerin (50 g/L) oxalic acid (10 g/L) 7 glycerin (50 g/L) maleic acid (10 g/L) 8 glycerin (50 g/L) glyoxylic acid (0.5 g/L) 9 glycerin (50 g/L) formic acid (50 g/L) 10 glycerin (200 g/L) formic acid (10 g/L) 11 glycerin (200 g/L) formic acid (50 g/L) 12 Copper sulfate- Rochelle salt Sodium ethylene glycol (50 g/L) glucono lactone (0.5 g/L) 13 pentahydrate (20 g/L) hydroxide 1,2-propanediol (50 g/L) glucono lactone (0.5 g/L) 14 (4 g/L) (65 g/L) glycerin (50 g/L) glucono lactone (0.5 g/L) 15 erythritol (50 g/L) glucono lactone (0.5 g/L) 16 xylitol (50 g/L) glucono lactone (0.5 g/L) 17 glycerin (50 g/L) glucose (0.5 g/L) 18 glycerin (50 g/L) sorbit (0.5 g/L) 19 glycerin (50 g/L) cellulose (0.5 g/L) 20 glycerin (50 g/L) cane sugar (0.5 g/L) 21 glycerin (50 g/L) mannitol (0.5 g/L) 22 glycerin (50 g/L) ascorbic acid (0.5 g/L) 23 glycerin (50 g/L) glucono lactone (5.0 g/L) 24 glycerin (200 g/L) glucono lactone (0.5 g/L) 25 glycerin (200 g/L) glucono lactone (5.0 g/L) 26 Copper sulfate- Rochelle salt Sodium ethylene glycol (50 g/L) formic acid (10 g/L) glucose (0.5 g/L) 27 pentahydrate (20 g/L) hydroxide ethylene glycol (50 g/L) formic acid (10 g/L) sorbit (0.5 g/L) 28 (4 g/L) (65 g/L) ethylene glycol (50 g/L) formic acid (10 g/L) cellulose (0.5 g/L) 29 ethylene glycol (50 g/L) formic acid (10 g/L) cane sugar (0.5 g/L) 30 ethylene glycol (50 g/L) formic acid (10 g/L) mannitol (0.5 g/L) 31 ethylene glycol (50 g/L) formic acid (10 g/L) glucono lactone (0.5 g/L) 32 ethylene glycol (50 g/L) formic acid (10 g/L) ascorbic acid (0.5 g/L)

(19) TABLE-US-00002 TABLE 2 Composition (E) (B) (C) (D) Reducing saccharides (A) Complexing Alkali metal C.sub.2-5 aliphatic Carboxy-containing having six or more No. Copper compound agent hydroxide polyalcohol reducing compound carbon atoms Compar- 1 Copper sulfate- Rochelle salt Sodium formic acid (10 g/L) ative 2 pentahydrate (20 g/L) hydroxide oxalic acid (10 g/L) Examples 3 (4 g/L) (65 g/L) maleic acid (10 g/L) 4 glyoxylic acid (0.5 g/L) 5 6 Copper sulfate- Rochelle salt Sodium glucose (0.5 g/L) 7 pentahydrate (20 g/L) hydroxide sorbit (0.5 g/L) 8 (4 g/L) (65 g/L) cellulose (0.5 g/L) 9 cane sugar (0.5 g/L) 10 mannitol (0.5 g/L) 11 glucono lactone (0.5 g/L) 12 ascorbic acid (0.5 g/L) 13 Copper sulfate- Rochelle salt Sodium methanol (50 g/L) 14 pentahydrate (20 g/L) hydroxide ethanol (50 g/L) 15 (4 g/L) (65 g/L) ethylene glycol (50 g/L) 16 1,2-propanediol (50 g/L) 17 glycerin (50 g/L) 18 erythritol (50 g/L) 19 xylitol (50 g/L) 20 diethylene glycol (50 g/L) 21 triethylene glycol (50 g/L) 22 1,3-propanediol (50 g/L) 23 Copper sulfate- Rochelle salt Sodium methanol (50 g/L) formic acid (10 g/L) 24 pentahydrate (20 g/L) hydroxide methanol (200 g/L) formic acid (10 g/L) 25 (4 g/L) (65 g/L) ethanol (50 g/L) formic acid (10 g/L) 26 ethanol (200 g/L) formic acid (10 g/L)
Copper Sulfate Plating Treatment

(20) Then, the substrate was washed well with water and subjected to the subsequent copper electroplating step while held in the jig. A copper electroplating bath was prepared by adding as a brightener 5 mL/L of Top Lucina 2000MU and 0.5 mL/L of Top Lucina 2000A (produced by Okuno Chemical Industries Co., Ltd.) to an aqueous solution containing 250 g/L of copper sulfate-5H.sub.2O, 50 g/L of sulfuric acid, and 50 mg/L of chlorine ions. Using this bath, a copper electroplating operation was performed at a liquid temperature of 25 C. and a current density of 3 A/dm.sup.2 for 5 minutes using a phosphorus-containing copper plate as an anode and the substrate to be plated as a cathode while applying mild air agitation.

Examples 2 to 32 and Comparative Examples 1 to 26

(21) The same substrate and jig as used in Example 1 were used and the same procedures as in Example 1 were performed to the catalyst application.

(22) Thereafter, a conductive coating was formed under the same conditions as in Example 1 by using each conductive-coating bath (baths 2 to 32 of the invention and comparative baths 1 to 26) as shown in Tables 1 and 2 below. Then, each of the resulting substrates was washed with water, and copper electroplating was performed under the same conditions as in Example 1.

(23) The coverage and appearance of each copper-plated coating formed using the above method were evaluated as described below. The surface resistance of each conductive coating after the conductive-coating formation treatment, as well as the surface resistance of each conductive coating after immersion in the copper sulfate plating bath for 5 minutes without applying current, was also measured.

(24) Plating Coverage

(25) Evaluation method: After copper electroplating, the percentage of the copper-plated area on the surface of a test piece was determined.

(26) Appearance after Copper Electroplating

(27) Evaluation method: After copper electroplating, the occurrence of pits and stardust, and the degree of gloss were evaluated by visual inspection.

(28) Surface Resistance

(29) Evaluation method: The surface resistance was measured after the conductive-coating formation treatment and after immersion in the copper sulfate plating bath for 5 minutes without applying current.

(30) For the surface resistance, the value at a 1-cm width of the surface was measured using a simple tester.

(31) Tables 3 and 4 show the results.

(32) TABLE-US-00003 TABLE 3 Surface resistance Surface after the resistance conductive- 5 minutes coating after Electro- film immersion plating formation in copper coverage Plating treatment sulfate No. (%) appearance (k) (k) Examples 1 100 gloss 70-900 65-450 2 100 gloss 70-1,000 65-450 3 100 gloss 70-800 70-400 4 100 gloss 70-1,000 65-550 5 100 gloss 70-900 60-500 6 100 gloss 70-800 65-400 7 100 gloss 70-800 70-400 8 100 gloss 20-900 15-450 9 100 gloss 70-800 65-400 10 100 gloss 70-800 60-450 11 100 gloss 70-800 60-450 12 100 gloss 70-800 60-400 13 100 gloss 70-800 60-450 14 100 gloss 70-850 65-400 15 100 gloss 70-900 65-450 16 100 gloss 70-800 60-450 17 100 gloss 70-800 65-400 18 100 gloss 70-800 65-400 19 100 gloss 70-850 65-400 20 100 gloss 70-800 65-400 21 100 gloss 70-800 70-400 22 100 gloss 20-800 18-400 23 100 gloss 70-850 70-450 24 100 gloss 70-800 70-400 25 100 gloss 70-800 70-400 26 100 gloss 40-300 25-120 27 100 gloss 40-300 30-150 28 100 gloss 40-250 20-110 29 100 gloss 40-310 25-150 30 100 gloss 40-300 30-180 31 100 gloss 40-280 28-150 32 100 gloss 10-280 9-110

(33) TABLE-US-00004 TABLE 4 Surface resistance Surface resistance after the conductive- 5 minutes after Electroplating Plating coating film formation immersion in copper No. coverage (%) appearance treatment (k) sulfate (k) Comparative 1 80 wavy and wrinkled 75-1,500 100-10,000 or more Examples 2 80 wavy and wrinkled 75-1,500 100-10,000 or more 3 80 wavy and wrinkled 75-1,500 100-10,000 or more 4 80 wavy and wrinkled 75-1,500 100-10,000 or more 5 80 wavy and wrinkled 20-1,500 100-10,000 or more 6 80 wavy and wrinkled 70-1,500 100-10,000 or more 7 80 wavy and wrinkled 70-1,500 100-10,000 or more 8 80 wavy and wrinkled 70-1,500 100-10,000 or more 9 80 wavy and wrinkled 70-1,500 100-10,000 or more 10 80 wavy and wrinkled 70-1,500 100-10,000 or more 11 80 wavy and wrinkled 70-1,500 100-10,000 or more 12 80 wavy and wrinkled 18-1,500 100-10,000 or more 13 80 wavy and wrinkled 75-1,500 100-10,000 or more 14 80 wavy and wrinkled 75-1,500 100-10,000 or more 15 80 wavy and wrinkled 70-1,500 100-10,000 or more 16 80 wavy and wrinkled 75-1,500 100-10,000 or more 17 80 wavy and wrinkled 75-1,500 100-10,000 or more 18 80 wavy and wrinkled 75-1,500 100-10,000 or more 19 80 wavy and wrinkled 75-1,500 100-10,000 or more 20 80 wavy and wrinkled 75-1,500 100-10,000 or more 21 80 wavy and wrinkled 75-1,500 100-10,000 or more 22 80 wavy and wrinkled 75-1,500 100-10,000 or more 23 80 wavy and wrinkled 75-1,500 100-10,000 or more 24 80 wavy and wrinkled 75-1,500 100-10,000 or more 25 80 wavy and wrinkled 75-1,500 100-10,000 or more 26 80 wavy and wrinkled 75-1,500 100-10,000 or more

Test Example 1: Plating Coverage

(34) Baths 1 to 32 of the invention all achieved a coverage of 100%.

(35) In contrast, comparative baths 1 to 26 all achieved a coverage of about 80%; i.e., complete coatings were not formed.

Test Example 2: Appearance after Copper Electroplating

(36) As shown in FIG. 2, when baths 1 to 32 of the invention were used, coatings with very excellent gloss appearance were formed, and no flaw was found on the surface of each coating.

(37) In contrast, as shown in FIG. 1, when comparative baths 1 to 26 were used, complete coatings were not formed with pits and stardust being developed, resulting in the formation of coatings with a wavy and wrinkled appearance.

(38) In particular, when comparative baths 23 to 26, which contained an alcohol having one hydroxyl group, such as methanol or ethanol, were used, deterioration of the coating appearance was observed. This indicates that aliphatic polyalcohol compounds having two or more hydroxyl groups are effective.

Test Example 3: Surface Resistance

(39) When baths 1 to 32 of the invention were used, the surface resistance of each conductive coating after the conductive-coating formation treatment was low.

(40) In contrast, when comparative baths 1 to 26 were used, the surface resistance of each conductive coating after the conductive-coating formation treatment was higher than that of each coating obtained with the use of baths 1 to 32 of the invention.

(41) Further, when baths 1 to 32 of the invention were used, the surface resistance of each conductive coating after immersion in a strongly acidic copper sulfate plating bath for 5 minutes was lower than the surface resistance of each conductive coating after the conductive-coating formation treatment, indicating that the conductivity was excellent.

(42) In contrast, when comparative baths 1 to 26 were used, the surface resistance of each conductive coating after immersion in a strongly acidic copper sulfate plating bath for 5 minutes was higher than the surface resistance of each conductive coating after the conductive-coating formation treatment, indicating that the conductivity was deteriorated.

(43) In view of these results, the conductive-coating bath of the present invention is capable of forming a coating with excellent conductivity on a catalyst-applied non-conductive plastic molding, and when each coating is subsequently immersed in a strongly acidic copper sulfate plating solution, the copper oxide of each conductive coating presumably undergoes a disproportionation reaction, thereby forming a dense coating containing metal copper with improved conductivity.

(44) Further, when baths 26 to 32 of the invention containing as the reducing agent (E) both a carboxy-containing reducing compound and a reducing saccharide having six or more carbon atoms were used (Examples 26 to 32), a considerable reduction was seen, in particular, in both the surface resistance of each conductive coating after the formation treatment and the surface resistance of each conductive coating after immersion in a strongly acidic copper sulfate plating bath for 5 minutes, indicating that these coatings had excellent conductivity.

(45) In this manner, baths 1 to 32 of the invention achieved excellent properties in terms of plating coverage, appearance, and surface resistance, compared to comparative baths 1 to 26.

Test Example 4: Relationship Between the Amount of C2-5 Aliphatic Polyalcohol and the Surface Resistance of Conductive Coating

(46) Table 5 shows the surface resistance of each conductive coating obtained using baths 1 and 33 to 38 of the invention. As the C.sub.2-5 aliphatic polyalcohol contained in the conductive-coating bath of the present invention, baths 33 to 38 of the invention contained diethylene glycol or 1,3-propanediol that have three or more carbon atoms between two hydroxyl groups while bath 1 of the invention contained ethylene glycol having two carbon atoms between two hydroxyl groups.

(47) TABLE-US-00005 TABLE 5 Bath 1 Bath 33 Bath 34 Bath 35 Bath 36 Bath 37 Bath 38 of the of the of the of the of the of the of the invention invention invention invention invention invention invention Copper sulfate-pentahydrate 4 4 4 4 4 4 4 Rochelle salt 20 20 20 20 20 20 20 Sodium hydroxide 65 65 65 65 65 65 65 Formic acid 10 10 10 10 10 10 10 Ethylene glycol 50 Diethylene glycol 50 100 200 1,3-propanediol 50 100 100 Surface resistance after 70-900 75-1,500 75-1,500 75-1,000 75-1,500 75-1,000 75-1,000 conductive-coating formation Surface resistance 5 minutes 65-450 10-10,000 70-2,000 65-700 100-10,000 70-2,000 70-700 after immersion in copper sulfate () or more or more Amount: g/L

(48) Bath 1 of the invention containing ethylene glycol having two carbon atoms between two hydroxyl groups achieved excellent conductivity, regardless of the small aliphatic polyalcohol content.

(49) Even when the conductive-coating bath contained aliphatic polyalcohol having three or more carbon atoms between hydroxyl groups, if the aliphatic polyalcohol content was increased from 50 g/L to 100 g/L or 200 g/L, the surface resistance of the coating obtained 5 minutes after immersion in a copper sulfate plating bath was lowered, indicating that an improvement in the conductivity was possible.

Test Example 5

(50) Table 6 shows the relationship between the palladium concentration in the catalyzing treatment and the coverage on the substrate in terms of the conductive coating formed by using each conductive-coating bath of the present invention, followed by the copper electroplating treatment.

(51) TABLE-US-00006 TABLE 6 Palladium concentration (mg/L) 30 50 70 100 Bath 1 of the invention 80 100 100 100 Bath 12 of the invention 80 100 100 100 Bath 31 of the invention 100 100 100 100 Comparative bath 2 50 80 100 100 Comparative bath 11 50 80 100 100 Comparative bath 15 50 80 100 100 Coverage (%) of copper sulfate plating

(52) Bath 1 of the invention, which contained a carboxy-containing reducing compound as the reducing agent (E) (Example 1), and bath 12 of the invention, which contained a reducing saccharide having six or more carbon atoms as the reducing agent (E) (Example 12), achieved a complete coating when the palladium concentration was 50 mg/L.

(53) Bath 31 of the invention, which contained as the reducing agent (E) both a carboxy-containing reducing compound and a reducing saccharide having six or more carbon atoms (Example 31), achieved a complete coating even when the palladium concentration was further reduced to 35 mg/L (Example No. 31).

(54) These results confirm that even when the catalyst amount is reduced, the use of the conductive-coating bath containing as the reducing agent both a carboxy-containing reducing compound and a reducing saccharide having six or more carbon atoms allows to perform electroplating that achieves excellent appearance.

(55) In contrast, comparative baths each containing either a aliphatic polyalcohol compound, a carboxy-containing reducing compound, or a reducing saccharide having six or more carbon atoms required the palladium concentration of 70 mg/L or more to achieve a complete coating of copper sulfate.

Test Example 6: Relationship Between the Amount of Dissolved Oxygen and the Copper Oxide Coating Formation

(56) The relationship between the dissolved oxygen amount in the bath and the formation of copper oxide coating was evaluated using each of the following test baths.

(57) Comparative Bath 11

(58) (A) Copper sulfate-pentahydrate: 4 g/L

(59) (B) Rochelle salt: 20 g/L

(60) (C) Sodium hydroxide: 65 g/L

(61) (E) Gluconolactone: 0.5 g/L

(62) Comparative Bath 15

(63) (A) Copper sulfate-pentahydrate: 4 g/L

(64) (B) Rochelle salt: 20 g/L

(65) (C) Sodium hydroxide: 65 g/L

(66) (D) Ethylene glycol: 50 g/L

(67) Bath 12 of the Invention

(68) (A) Copper sulfate-pentahydrate: 4 g/L

(69) (B) Rochelle salt: 20 g/L

(70) (C) Sodium hydroxide: 65 g/L

(71) (D) Ethylene glycol: 50 g/L

(72) (E) Gluconolactone: 0.5 g/L

(73) Evaluation Method

(74) After the formation of a conductive coating, the substrate was immersed in an acidic copper sulfate plating solution for 5 minutes without applying current, and the copper content in the coating and the surface resistance were measured.

(75) To measure the copper content in the coating, the conductive coating was dissolved in aqua regia, and the copper content in the coating was calculated based on the copper concentration in the aqua regia.

(76) For the surface resistance, the value at a 1-cm width of the surface was measured using a simple tester.

(77) TABLE-US-00007 TABLE 7 Comparative bath 11 Comparative bath 15 Bath 12 of the invention Copper Surface Copper Surface Copper Surface content resistance content resistance content resistance (mg/dm.sup.2) (k) (mg/dm.sup.2) (k) (mg/dm.sup.2) (k) No bubbling 0.06 100-10,000 0.06 100-10,000 0.07 60-400 (dissolved or more or more oxygen: 1.5 mg/L) Air bubbling 0.08 100-7,000 0.08 100-5,000 0.3 30-110 conditions (dissolved oxygen: 4.0 mg/L)

(78) When the amount of dissolved oxygen in the conductive-coating bath was increased to 4.0 mg/L by supplying air bubbles, the copper content in the coating was considerably increased (bath 12 of the invention). As a result, the surface resistance of the coating after being subjected to the copper sulfate plating treatment was considerably reduced, showing excellent conductivity.

(79) Compared to bath 12 of the invention, although comparative baths 11 and 15 showed a reduction in the surface resistance of the coating after being subjected to copper sulfate plating treatment, only a slight increase was confirmed in the copper content.

(80) TABLE-US-00008 TABLE 8 Comparative bath 11 Comparative bath 15 Bath 12 of the invention Copper Surface Copper Surface Copper Surface content resistance content resistance content resistance (mg/dm.sup.2) (k) (mg/dm.sup.2) (k) (mg/dm.sup.2) (k) No bubbling 0.06 100-10,000 0.06 100-10,000 0.07 60-400 or more or more Sodium persulfate 0.24 15-180 0.25 15-170 0.38 15-50 (2 g/L) 30% hydrogen 0.22 25-180 0.21 25-120 0.31 25-70 peroxide solution (5 mL/L)

(81) When the amount of dissolved oxygen was increased by adding an oxidizing agent (sodium persulfate or 30% hydrogen peroxide solution) to the conductive-coating bath of the present invention instead of by supplying air bubbles, it was also clearly shown that bath 12 of the invention achieved an increase in the copper content in the coating, thus achieving significant increase in the conductivity, unlike comparative baths 11 and 15.