Pyrophosphate-based bath for plating of tin alloy layers

1. An aqueous cyanide-free electrolyte bath for plating of tin-copper alloy layers on substrate surfaces, comprising:

(i) a tin ion source and a source of copper,

(ii) N-methyl pyrrolidone, and

(iii) a pyrophosphate source,

wherein the bath is prepared by adding the tin ion source, the source of copper and the N-methyl pyrrolidone to water, and

pyrophosphate is present at a concentration which is 3 to 80 times greater than that of tin/copper.

2. The aqueous cyanide-free electrolyte bath according to claim 1 , further comprising an acid.

3. The aqueous cyanide-free electrolyte bath according to claim 2 , wherein the acid is orthophosphoric acid, sulfuric acid or methanesulfonic acid.

4. The aqueous cyanide-free electrolyte bath according to claim 1 , wherein the tin ion source is tin pyrophosphate.

5. The aqueous cyanide-free electrolyte bath according to claim 4 , containing tin pyrophosphate in an amount of 0.5 to 100 g/l.

6. The aqueous cyanide-free electrolyte bath according to claim 5 , containing tin pyrophosphate as the tin ion source in an amount of 10 to 40 g/l and copper pyrophosphate as the source of copper in an amount of 1 to 5 g/l.

7. The aqueous cyanide-free electrolyte bath according to claim 1 , wherein the source of copper is copper pyrophosphate.

8. The aqueous cyanide-free electrolyte bath according to claim 1 , wherein the pyrophosphate source is selected from the group consisting of sodium, potassium and ammonium pyrophosphates.

9. The aqueous cyanide-free electrolyte bath according to claim 8 , containing the pyrophosphate source in a concentration of 50 to 500 g/l.

10. The aqueous cyanide-free electrolyte bath according to claim 1 , containing N-methylpyrrolidone in a concentration of 0.1 to 50 g/l.

11. The aqueous cyanide-free electrolyte bath according to claim 10 , containing N-methylpyrrolidone in a concentration of 0.1 to 4 g/l.

12. The aqueous cyanide-free electrolyte bath according to claim 1 having a pH value of 3 to 9.

13. The aqueous cyanide-free electrolyte bath according to claim 1 , further comprising an antioxidant and/or an organic gloss agent.

14. The aqueous cyanide-free electrolyte bath according to claim 13 , wherein the organic gloss agent is selected from the group consisting of morpholine, 2-morpholine ethanesulfonic acid, hexamethylenetetramine, 3-(4-morpholino)-1,2-propanediol, 1,4-diazabicyclo-[2.2.2]-octane, 1-benzyl-3-carbamoyl-pyridinium chloride, 1-(2′-chloro-benzyl)-3-carbamoyl-pyridinium chloride, 1-(2′-fluoro-benzyl)-3-carbamoyl-pyridinium chloride, 1-(2′-methoxy-benzyl)-3-carbamoyl-pyridinium chloride, 1-(2′-carboxy-benzyl)-3-carbamoyl-pyridinium chloride, 1-(2′-carbamoyl-benzyl)-3-carbamoyl-pyridinium chloride, 1-(3′-chloro-benzyl)-3-carbamoyl-pyridinium chloride, 1-(3′-fluoro-benzyl)-3-carbamoyl-pyridinium chloride, 1-(3′-methoxy-benzyl)-3-carbamoyl-pyridinium chloride, 1-(3′-carboxy-benzyl)-3-carbamoyl-pyridinium chloride, 1-(3′-carbamoyl-benzyl)-3-carbamoyl-pyridinium chloride, 1-(4′-chloro-benzyl)-3-carbamoyl-pyridinium chloride, 1-(4′-fluoro-benzyl)-3-carbamoyl-pyridinium chloride, 1-(4′-methoxy-benzyl)-3-carbamoyl-pyridinium chloride, 1-(4′-carbamoyl-benzyl)-3-carbamoyl-pyridinium chloride, (1′-methyl-naphthyl)-3-carbamoyl-pyridinium chloride, 1-(1-methyl-naphthyl)-3-carbamoyl-pyridinium bromide, 1,1′-(xylenyl)-3,3′-bis-carbamoyl-bis-pyridinium dibromide, 1,1′,1″-(mesitylenyl)-3,3′,3″-tris-carbamoyl-tri-pyridinium trichloride as well as the corresponding bromides, fluorides, iodides and pseudo halogenides of the aforementioned compounds and quaternised N,N-bis-[dialkylamino-alkyl]ureas.

15. A process for electroplating of a glossy and even tin-copper alloy coating, comprising introducing a substrate to be coated into the aqueous cyanide-free electrolyte bath according to claim 1 and electroplating the tin-copper alloy coating on the substrate.

16. The process according to claim 15 , wherein the bath is operated at a current density of 0.01 to 2 A/dm 2 .

17. The process according to claim 16 , wherein the bath is operated at a current density of 0.25 to 0.75 A/dm 2 .

18. The process according to claim 15 , wherein the bath is operated at a temperature of 15 to 50° C.

19. The process according to claim 18 , wherein the bath is operated at a temperature of 25 to 30° C.

20. The process according to claim 15 , wherein the coating on the substrate is electroplated using a frame electroplating method.

21. The process according to claim 15 , wherein membrane anodes are used as anodes.