Tin-silver-copper plating solution, plating film containing the same, and method for forming the plating film

1. An electrolytic plating method, comprising the steps of:

preparing a plating solution comprising water which is a primary medium, a sulfonic acid, and tin, copper, and silver ions, and an organic complexing agent, concentrations of the sulfonic acid, and tin, copper, and silver ions being 0.5 to 5 mol/L, 0.21 to 2 mol/L, 0.002 to 0.02 mol/L, and 0.01 to 0.1 mol/L, respectively, a mole ratio of the silver-ion concentration to the copper-ion concentration being in a range of 4.5 to 5.58;

separating a solution around a soluble anode containing 90 percent or more of tin from the plating solution on a cathode side by a non-ionic micro-porous membrane having a pore diameter of 0.01 to 0.05 μm and a thickness of 5 to 100 μm; and

carrying out electrolytic plating on a plating object to be a cathode.

2. The electrolytic plating method according to claim 1 , further comprising:

forming a plating film including 2.6 to 3.4 weight percent of silver, 0.4 to 0.7 weight percent of copper, and a remaining weight percent being tin.

3. The electrolytic plating method according to claim 1 , wherein the solution around the soluble anode separated by the micro-porous membrane contains a surfactant in a concentration higher than that of the plating solution on the cathode side.

4. The electrolytic plating method according to claim 1 , further comprising:

applying a current to both the soluble anode and an insoluble anode.

5. The electrolytic plating method according to claim 4 , further comprising:

applying a part of the current necessary for plating to the soluble anode; and

applying a remainder of the current to the insoluble anode,

the part of the current applied to the soluble anode dissolving tin in an amount obtained by subtracting an amount of tin dissolved chemically from an amount of tin to be plated.

6. The electrolytic plating method according to claim 4 , further comprising:

separating a solution around the insoluble anode from the plating solution on the cathode side with a non-ionic micro-porous membrane having a pore diameter of 0.01 to 0.05 μm and a thickness of 5 to 100 μm.

7. The electrolytic plating method according to claim 4 , further comprising:

providing a circulation path comprising a filtering membrane having a pore diameter of 0.05 to 0.5 μm; and

passing the plating solution through the filtering membrane under a low pressure so as to remove impurities.

8. The electrolytic plating method according to claim 1 , wherein the solution around the soluble anode separated by the micro-porous membrane contains the sulfonic acid in a concentration of 0.5 to 1.5 mol/L.

9. The electrolytic plating method according to claim 1 , wherein the complexing agent is a mercaptan compound.

10. The electrolytic plating method according to claim 9 , wherein the plating solution further comprising an aromatic amino compound.

11. The electrolytic plating method according to claim 9 , wherein the plating solution further comprising a nonionic surfactant.

12. The electrolytic plating method according to claim 1 , further comprising:

employing barrel plating as the electrolytic plating; and

applying a current density of 0.01 to 100 A/dm 2 to the plating object at the time of plating.

13. The electrolytic plating method according to claim 1 , further comprising:

applying a current density of 4 to 11.6 A/dm 2 to the plating object at the time of plating.

14. The electrolytic plating method according to claim 1 , further comprising:

using an electric current with a pulse waveform at the time of plating.

15. The electrolytic plating method according to claim 1 , further comprising:

using an electric current with a direct-current waveform at the time of plating.