The corrosion characteristics and solderability of immersion tin coatings on copper



Immersion tin is widely used as a lead free surface finish in the printed circuit board technology. Tin prevents the underlying copper from corrosion and preserves its solderability during a long storage and lead-free assembly processes. Investigated immersion tin coatings were deposited on copper foil from thiourea-type baths with hydrochloric acid addition (SnHCl coatings) or methanesulfonic acid addition (SnMSA coatings). Obtained coatings were investigated in the as deposited state and after aging (4 h at 155 °C, in air). The scanning electron microscopy studies revealed differences in structure of tin samples deposited from different baths. Results of polarization and impedance investigations indicated that as deposited SnHCl coatings had better corrosion resistance in 0.5 M NaCl solution than SnMSA coatings. The aging resulted in the improvement of the corrosion resistance of thinner coatings (0.2 and 0.5 µm thick SnHCl and 0.3 µm thick SnMSA), which were thoroughly converted into Sn–Cu intermetallic (IMC) phases. In contrary, thicker coatings exhibited some worsening of the corrosion resistance upon aging. The solderability of all as-deposited tin coatings was acceptable, but decreased after aging, especially for thinner coatings, showing the through conversion into Sn–Cu IMC phases.