Study Of Non-Cyanide Gold Plating Process Using Hydantoin As Complexing Agent

Gold coating has high corrosion resistance, high ductibility, high electric conductivity and chemical stability. Gold plating is widely used to prepare the surfaces of electrical contacts and wire bonding pads of semiconductor devices and also for decorative use. The traditional cyanide-type gold plating bath has good stability, high current efficiency, and good throwing power. Due to problems related to safety, environment protecting and disposal concerns, cyanide-free plating baths are now strongly in demand. The interest in developing non-cyanide gold electrolytes, has grown rapidly in recent years.In this research, 5,5-dimethyl hydantoin was choosed as the gold complexing agent by comparative study on three non-cyanide gold plating baths. Accelerate additives and brightener of the bath were studied in order to improve the disadvantages of the process. The influences of bath composition and operation conditions on coating appearance, deposition rate, surface morphology were disscussed by single factor study.The results showed gold content, 5,5-dimethyl hydantoin content,temperature and current density have more effects on surface morphology and deposition rate. The optimal electrolyte composition and operating conditions were:gold chloride 10g/L, 5,5-dimethyl hydantoin 100g/L, sodium phosphate 75g/L, temperature 40℃, pH value 9, current density 3A/dm~2. The coatings can be obtained with high adhesion strength, good appearance under optimum plating conditions. Pulse plating process was also studied. Orthogonal test was used to obatin the optimum pulse plating parameters. The effect of pulse frequency, pulse occupy and pulse current density on gold coatings and deposition rate were studied and the preferred pulse plating parameters were:the pulse frequency 3kHz, pulse occupy 60%, the average current density 3A/dm~2. The surface morphology of the gold coatings were observed by SEM, AFM. The crystal texture was tested by XRD. The bath has good stability and throwing power. Cyclic voltammetry, cathodic polarization curves and chronoamperometric method were used to learn the electrodeposition process of gold from 5,5-dimethyl hydantoin electrodeposition bath and the influence of the additives on deposition mechanism of electrocrystallization process. The results of cyclic voltammogram curves indicated that 5,5-dimethyl hydantoin is stable in the range of electrodeposition region with no additional reaction. The gold electrodeposition process was irreversible. The additives reduce the cathodic current density in the region of the peak and move to the negative direction. The cathodic polarization curves also showed the additives can increase the cathodic polarization. Chronoamperometric results showed that the electrodeposition process of gold at glassy carbon electrode generally belongs to the three-dimensional progressive nucleation mechanism and the additives didn't change the nucleation mechanism.