Au-Co Alloy Compositionally Codulated Electrodeposition in DMH and Citric Acid Contained Electrolyte and Films’ Growth Process

Author:Luo Gong

Supervisor:yuan guo hui


Degree Year:2019





Au-Co alloy can be divided into pure gold,hard gold,Karat gold with different cobalt content.They are widely used in jewelry industry and electronic plating.Gold ions are very hard to form complex compound,due to its strong oxidizing.Cyanide contained electrolyte system is mainly used for gold alloy electroplating production.Along with the demands of market and the ban on environmental protection,cyanide-free Au-Co alloy electrodepositing system is urgently needed in electroplating industries.Based on the theory of alloy codeposition and double complexing,a novel cyanide-free Au-Co alloy modulated electrodeposition system was studied in this work utilizing 5,5’ -dimethylhydantoin(DMH)and citric acid(H3Cit)as ligand.Uniform layer can be obtained from the novel cyanide-free electrolyte contained Au(Ⅰ),Au(Ⅲ)and Co(Ⅱ)ion,respectively.Au(Ⅲ)salt’ s stability is better than that of Au(Ⅰ)salt and Au(Ⅲ)electrolyte can adapt to a wider current density range.So it was better to use Au(Ⅲ)salt in cyanide-free Au-Co electrolyte.Gold salt concentration has greatest influence to electrolytes’ stability and films’ quality;then followed by cobalt salt and DMH concentration,while the influence of citric acid content and pH value were smaller.Cobalt content in films increased with Co(Ⅱ)/Au(Ⅲ)ratio increasing and deposition potential negative shifting.Based on orthogonal experimental study,a desirable homogeneous hard gold layer was obtained from designed electrolyte with 0.81wt.%cobalt content.The film had a similar surface morphology,same color and thickness to PCB bonding finger hard gold film.The hardness values were 6084.37 MPa and 6084.37 MPa,under 50mN and 75 mN load using microhardness tester.In summary,the film had a equivalent performance with PCB hard gold film.And 20nm pure Au and 90nm Au-94.07at.%Co multilayers were synthesized in single-bath by multi-potential valueφand adjusted Co(Ⅱ)/Au(Ⅲ)ratio.XRD analysis showed that Au-Co alloy alloy film presented gold crystal structure at lower cobalt content.Gold diffraction peak became weak when cobalt content increased.Gold characteristic peak disappeared when cobalt content reached20.70wt.%,but cobalt diffraction peaks were not observed in Au-Co alloy alloy films.It meaned that alloy films mainly existed in gold crystal without forming cobalt crystal structure.XPS analysis showed that gold was metal state in alloy films’ surface and internal layer,and cobalt had+3,+2 oxidation and complexed state on films’ surface.Cobalt presented metal and+2 oxidation states in low cobalt internal layer,extra complexed state also existed in high cobalt internal layer.It confirmed that complexe inclusion existed in Au-Co alloy films.Combining with carbon and nitrogen testing results,the complexe inclusion was confirmed to be complexes of cobalt and citric acid.Oxidation state and complexing state of cobalt were found in high cobalt content films,supporting the result that thin alloy films were hard to form cobalt lattice structure.Absorption spectra and electrochemical analysis showed that ions’ complexing make ions’ discharge potential negative shifted.Only gold ion discharged under relatively positive potential.Gold and cobalt ion codeposited under relatively negative potential.The reduction of gold and cobalt ions in the cyanide-free electrolyte were segmented,obeyed to normal codeposition regularity during electrodepositing.Au-Co alloy electroplating displayed instantaneous nucleation under relatively positive potential,while nucleation process shifted between progressive and instantaneous nucleation at more negative potential.Based on the differences between theoretical model and experimental i-t curves during electrodepositing process,limited diffusion process and same discharge probability of ions with equidistance to electrode surface were used to discribe electrodepositing nucleation and its growth process.Classical nucleation growth theory was amended,a cylindric limited diffusion mass transfer film growth model was established.And the current shift formula of nucleation and growth process were calculated.