Film-forming metal solution and method of forming metal coating

A method of forming a tin coating, the method comprising:

• disposing a solid electrolyte membrane having sulfonic acid groups between an anode and a substrate serving as a cathode;
• bringing a film-forming metal solution into contact with the solid electrolyte membrane to supply tin ions into the solid electrolyte membrane;
• and
• in conjunction with the supply of the tin ions, applying a voltage between the anode and the substrate while keeping the solid electrolyte membrane in contact with the substrate to deposit tin coating onto a surface of the substrate from the supplied tin ions,
• wherein the film-forming metal solution comprises:
• an aqueous tin sulfate solution,
• a solvent comprising isopropyl alcohol, and
• a nonionic surfactant,
• wherein the nonionic surfactant has a linear hydrophilic group having ethylene oxide units, and a hydrophobic group having a cyclic structure,
• wherein the hydrophobic group of the nonionic surfactant has a minimum length of less than 10 Å, a maximum length of 10 Å or less, and the maximum length is greater than the minimum length,
• wherein the hydrophilic group of the nonionic surfactant has 11 to 15 mol of the ethylene oxide units,
• wherein a concentration of the nonionic surfactant in the film-forming metal solution is 0.01 to 0.08 M,
• wherein an amount of the tin sulfate in the film-forming metal solution is 30 to 50 g/l, and
• wherein the nonionic surfactant has a hydrophilic-lipophilic balance of 13 to 15.