Immersion tin silver plating in electronics manufacture

A method for depositing a whisker resistant tin-based coating layer on a surface of a copper substrate, the method comprising:

• contacting the surface of the copper substrate selected from the group of substrates consisting of copper on a printed wiring board, lead frames in electronic devices, connectors in electronic devices, and die pads in under bump metallization with an immersion tin plating composition comprising the following to thereby form, by displacement plating reaction between Sn2+ ions and Cu metal of the substrate, a tin-based coating comprising at least 80 wt % Sn:
• a source of Sn2+ ions sufficient to provide a concentration of Sn2+ ions between about 5 g/L and about 20 g/L;
• a source of Ag+ ions sufficient to provide a concentration of Ag+ ions between about 10 ppm and about 24 ppm;
• a source of sulfur-based complexing agent sufficient to provide a concentration of sulfur-based complexing agent between about 60 g/L and about 120 g/L;
• a source of hypophosphite ion in an amount sufficient to enhance the rate of deposition, wherein the amount of hypophosphite ion is equivalent to that provided by between 70 g/L and about 100 g/L of sodium hypophosphite;
• in addition to the hypophosphite source, a source of anti-oxidant sufficient to provide a concentration of anti-oxidant between about 30 g/L and about 110 g/L;
• a source of pyrrolidone sufficient to provide a concentration of pyrrolidone of at least about 12 g/L;
• and
• an acid in a concentration sufficient to lower the pH of the composition between about 0 and about 5,
• wherein the tin-based coating layer that is deposited onto the copper substrate surface by the immersion tin plating composition has a thickness between 0.5 micrometers and 1.5 micrometers.