Study On Stability And Maintenance Of Non-cyanide Co-electroplating Bath For Au-Sn Alloy

In order to improve luminescence efficiency and thermal dissipation,high-power LED packaging technology generally use flip-chip structure.While,the bump material of the flip-chip structure is related to the reliability of flip-chip structure.Au-30 at.% Sn eutectic alloy has good thermodynamic properties and excellent mechanical properties,it is suitable as a bump material.Compared with the traditional method of preparing Au-30 at.% Sn eutectic alloy,the electroplating method has the advantages of low cost,simple process and small bump size.However,the stability of cyanide-free gold-tin co-plating solution is poor,which restricts the application of cyanide-free gold-tin alloy co-plating technology in practice.In this paper,the stability of cyanide-free Au-Sn alloy plating solution was studied.The main conclusions are as follows:(1)In the plating temperature of 65°C,non-cyanide Au-Sn co-plating solution remained clear,and no precipitation generated,which is conducive to high-speed plating.With the accumulation of plating time,the Sn content of the coating gradually decreases.In 60 min,the solution can obtain near-eutectic Au-Sn alloy,and the surface of the coating was dense and flattened;after 60 min,the Sn content of the film was deviated from the Au-Sn eutectic composition,and the surface of the coating has large particles formation.(2)The oxidation of catechol and tin salt was studied by electrochemical method.It was found that the oxidation of tin salt was not obvious during 120 min,while catechol was oxidized obviously.In gold-tin alloy plating solution,catechol could play the role of reducing agent and brightener.During the electroplating process,due to oxidation consumption of catechol,the content of catechol in the bath gradually reduced,resulting the Sn content of films decreases with the plating time.(3)The co-plating bath can be stable and Au–Sn eutectic alloy can be obtained by addition of 0.003 mol/L catechol to bath after plating for 20 min.The content of ascorbic acid had no obvious effect on the surface morphology and the Sn content of coatings.Ascorbic acid could replace catechol as a reducing agent.The tin content of gold-tin alloy coating obtained by the electroplating of the new Au-Sn co-plating bath is not obvious with plating time,not more than 1 at.%.The saccharin sodium can be used as a brightener instead of catechol.(4)The optimum concentration ratio of gold and tin in the new alloy plating is 1:3.The concentration of sodium saccharin has a great influence on the surface morphology of the coating.When the concentration of saccharin is 0.01 mol / L,the surface quality of the coating is the best.When the peak current density is less than 10 mA/cm2,the coating becomes smooth with increasing current density;when the peak current density reaches 10 mA/cm2,the coating roughness increases with increasing current density.During the process of peak current change,there is a stable value of the tin content in the coating.When the peak current density reaches 15 mA/cm2,the tin content of film is stable at about 14 at.%,and it is not affected by the peak current density.