Effect of Process Conditions on Deposition Rate of Electroless Tin Coating in Thiourea-Tartaric Acid-Citric Acid Tri-Ligand System

The effect of complexing agent concentration of thiourea mono ligand,thiourea-tartaric acid and thiourea-citric acid bicomponent ligand,and thiourea-tartaric acid-citric acid tricomponent ligand on the deposition rate of electroless Sn plating was investigated.The effects of major salt concentration,reducing agent concentration,plating bath temperature,pH value and deposition time of tri-ligand system on the deposition rate were examined,and the plating parameters were optimized.The properties of the coating obtained under the optimized plating condition were analyzed.Results indicated that the plating rate in the tri-ligand system was greater than those in the dual-ligand and mono-ligand systems.The plating rate initially increased and then decreased with the increase of concentration of thiourea,tartaric acid,citric acid,sodium hypophosphite and SnCl2·2H2O as well as the rise of temperature and pH value.The optimized bath was composed of 20 g/L thiourea,40 g/L citric acid,40 g/L tartaric acid,20 g/L sodium hypophosphite,40 g/L SnCl2·2H2O and 2 g/L hydroquinone.The optimized plating parameters were suggested as bath temperature of 80 ℃,pH value of 0.72 and deposition time of 30 min.Under the optimized conditions,the plating bath had good stability,and the deposition rate of Sn coating was as much as 3.12 μm/ h.Resultant Sn coating was composed of uniform and compact crystals and possessed good corrosion resistance,adhesion to substrate and solderability.