Effect of concentration of thiourea and ammonium citrate, electrodeposition stirring speed and current density on Sn-Ag-Cu coating for small size substrate

The deposition potentials of Sn, Ag, and Cu ions differ greatly, which makes it difficult to realize co-deposition. Application of appropriate complexing agents and process parameters is beneficial to depositing Sn-Ag-Cu alloy. In this paper, Sn-Ag-Cu ternary alloy coating was electrodeposited on small-sized copper substrates to prepare micro-bumps solder joints. The effects of additives on the plating morphology and alloy composition ratio were studied using scanning electron microscope and energy dispersive spectroscopy. Results show that Sn-Ag-Cu ternary alloy in the near eutectic composition range can be obtained by selecting a suitable plating solution. In the solution, stannous methanesulfonate, copper methanesulfonate, and silver methanesulfonate were used as the main salts, and 0.5 mol/L thiourea and 0.3 mol/L ammonium citrate were used as complexing agents. Moreover, when the current density is 0.5 A•dm^-2 and stirring rate is 400 rpm, the roughness and microstructure uniformity of the coating can be improved effectively. Furthermore, the micro bumps with good performance can be obtained when the SnAg-Cu alloy coating was reflowed at 280 °C for 60 s.