Preparation And Performance Of Electroless Gold Plating With 5,5-Dimethylhydantoin

Printed circuit board?PCB?finishing technology is very important surface treatment process in manufacture of electronic circuit.It not only provides corrosion protection for exposed copper line,but also ensure a stable connection between electronic components.Electroless Nickel Electroless Palladium Immersion Gold?ENEPIG?processes in all of PCB finishing technology have very prominent advantages.But gold plating process generally uses cyanide,and it bring serious problems of pollution.Developing other complexing agent replace cyanide is an important direction in electroless gold plating.This paper presents multifunctional ENEPIG process as the research background and develops a new cyanide-free electroless gold plating technology for ENEPIG.In this paper,The system uses NaAuCl₄ as gold source and 5,5-Dimethylhydantoin as main complexing agent,formic acid as reducing agent for gold ions.Changing major components and reaction conditions refer to coating thickness and quality of appearance,stability of the plating solution,the effect of each factors on the process was investigated.and finally determine a without cyanide new plating process for 1.5g/L NaAuCl₄·2H₂O,35g/L 5,5-Dimethylhydantoin?DMH?,15g/L HCOOH,8g/L EDTA-Na₂,30g/L K₂HPO₄,reaction temperature is 60?,pH=8.0,reaction time is 20min.The Gold-plated layer thickness is0.041?m;speed of Plating is 0.00209?m/min.The relative standard deviation of coating thickness is 12.45%.Gold-plated layer is intact and shining.Plating solution is stable and solution of after plating can be placed 25 days.For reducing erosion of gold ions to substrate in common gold plating process.Using formic acid as reducing agent for gold ions in this experiment.Effect of formic acid concentration on gold deposition process was examined by relationship between plating solution with different formic acid concentration to Gold-plated layer thickness and the cathode polarization test.Ability of the Ni,Pd and Au catalyzed oxidation for formic acid was examined by Cyclic Voltammetry.And the concentration of metal ions was examined by AAS front and back.Confirming proportion of replacement gold and reduction gold.Two kinds of mechanism of gold deposition in this system were presented.The results show that the Pd has a strong catalytic oxidation ability for HCOO^-,The Au is weaker than Pd,The Ni has not catalytic oxidation ability for HCOO^-.HCOO^-both can be used as reductant with Au³⁺,also has a coordination effect with Au³⁺.Coordination of Au³⁺is more prominent when HCOO^-is a low content in plating solution,and it can inhibit reduction of Au³⁺.Ability of reducing Au³⁺is more prominent when HCOO^-is a high content in plating solution,it can be a reducing agent.HCOO^-greatly inhibit the substitution reaction.After plating in Electroless Nickel?EN?layer,Electroless Nickel Electroless Palladium?ENEP?surface,ratio of replacement gold is 1.93%and ratio of reduction gold is 98.07%.The morphology and composition of the conversion coating were examined by scanning electronmicroscopy?SEM?,X-raydiffraction?XRD?andenergydispersive spectrometer?EDS?.The results show that the orientation of Au in?111?crystal growth significantly more than solution preparation which does not contain formic acid.EN layer and EP layer have a cellular structure,and its size range 3⁶?m.There are tiny pores that average diameter is about 0.15?m in the surface.On the surface of Au layer,Au is some small granular crystal.If Solution does not contain formic acid,it will bring a strong erosion to sample.Rate of porosity is lots,about 60/100?m²,If Solution contain formic acid,Rate of porosity is few,about 7/100?m².Immersion tin test and neutral salt spray test were used in ENEPIG that prepared by this process.The results show it has good welding performance and can keep stabilization for 24h in neutral salt spray.The corrosion resistance ability of EN,ENEP layer,ENEPIG layer and Electroless Nickel Immersion Gold?ENIG?layer were determined by Tafel and EIS.The results show ENEPIG layer has best Corrosion resistant ability.The corrosion potential is-0.216V and the corrosion current density of the layer is2.24?A·cm^-2.Surface roughness of EN layer,ENEP layer,ENIG layer and both of ENEPIG layers were determined by photoelectric contourgraph.The results show that ENEP layer is the roughest,arithmetic average deviation of contour R_a=463.7nm.Introduction of HCOO^-and Pd layer can reduce roughness of coating.The R_a of ENEPIG layer that Prepared by this process is 183.1nm.