| Electroless gold plating is the key surface treatment process in printed circuit board?PCB?manufacturing process,The main role is to provide a protective layer with excellent conductivity and corrosion resistance.The electroless gold plating process is developing from traditional cyanide system to green cyanide free system.The traditional cyanide gold plating has the advantages of high stability,good appearance and good uniformity of the coating,but the CN-contained in the bath may cause great harm to the human body and the environment.China has published many environmental protection policies,limiting the elimination of gold cyanide gold plating at the end of2014.However,The stability of cyanide free plating bath and the product rates are poor At present,cyanide gold plating system is mainly used in the manufacturing of PCB,so it is urgent to develop a new type of cyanide free chemical plating and used for PCB surface treatment.Electroless nickel/gold process may cause nickel matrix overcorrosion,but the cyanide free gold plating is instability.In this paper,Na3Au?SO3?2 is selected as gold salt,the complexing agent is selected by polarization curve and open circuit potential curve.And the reaction kinetics of chemical nickel gold is determined by orthogonal experiment.The components and process conditions of electroless gold plating were determined by orthogonal experiment.The reaction kinetics and mechanism of electroless gold are also discussed,which provides a theoretical basis and application basis for the application of cyanide free gold plating liquid system to the surface treatment of PCB as soon as possible.The main contents of this paper include the following aspects:?1?Five kinds of collaterals,such as organophosphorus,sulfhydryl carboxylic acid,sulfhydryl sulfonic acid and amino carboxylic acid,were screened by polarization curve and open circuit potential curve,and 13 kinds of complexing agents were screened.The results showed that the addition of S2 and EDTMP·5Na could reduce the initial rate of displacement reaction.?2?Based on the screening of complexing agents,the composition and technological conditions of the plating bath were further determined by orthogonal test.SEM and EDS were used to characterize the microstructure and surface element composition of the coating,and comprehensively evaluate the advantages and disadvantages of the coating.At the same time,the change of the appearance of the bath was evaluated during the plating process to evaluate the stability of the application.The research show that when the concentration of complexing agent is low,the bath will become yellow easily after plating,When the concentration of complexing agent increased to 0.10 mol·L-1,the stability of the bath increased significantly.When the solution was placed at room temperature for 30 days,the appearance of the plating solution did not change significantly.At the same time,the microstructure of the coating showed that by adding sodium alkyl sulfate twelve and sodium alkyl benzene sulfonate,the stress distribution in the coating could be more evenly,and the problem of pinhole in the coating could be solved.After optimization,the composition of gold plating system is:7.5×10-3 mol·L-11 Na3Au?SO3?2,0.15 mol·L-1 S2,4.1×10-22 mol·L-11 C6H8O7,0.12 mol·L-11 Na2HPO4·12H2O,300 mg·L-1 C12H25SO4Na,pH is 6,plating temperature is 75?,plating time is 60 minutes.Under the technological conditions,the appearance of the coating is golden yellow and the microstructure is good,When the bath is placed in room temperature for 30 days,the appearance has no obvious change.The coating thickness can reach 0.05?m by 60 min plating.?3?Adding Na3Au?SO3?2 to S2 solution by dropping,it was found that the absorbance increased with the increase of Na3Au?SO3?2 concentration.Which indicates that Na3Au?SO3?2 may have coordination interaction with the complexing agent.Atomic absorption spectrometry was used to further determine the contents of Au and Ni elements in the bath before and after plating,It is indicated that the process of plating is mainly the replacement of gold plating.The deposition kinetics of the system on the Ni layer was studied by AFM and electrochemical method.The average activation energy of the gold deposition reaction was calculated to be 33.03 kJ·mol-1.Chronoamperometric study of nucleation model indicated that the deposition of Au complex ions on Ni surface is continuous nucleation mode.?4?The system is applied to the PCB surface treatment process.By the salt spray test,the corrosion evaluation of the coating is 9 grade according to the salt spray test rating standard.The results of weldability test shows that the tin coating is full,indicating that the system has good practicability. |
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