Electroless Palladium Plating Process And Its Application In The Surface Treatment Of Circuit Boards

Printed circuit board(PCB)surface treatment technology is a key process in the circuit board manufacturing process.It is necessary to apply another layer of metal that is not easily oxidized on the copper surface to protect the effective connection of the process line.Electroless nickel plating/displacement gold plating(ENIG)process is currently the most widely used Plating treatment process process,and its main problem is the "black disk" phenomenon,"Black disk" refers to the excessive corrosion of the nickel layer caused by the excessive corrosion of the composition of the plating solution in the process of replacement gold plating on the nickel,which causes cracks in the bottom plate.causing cracks on the nickel surface,which seriously affects the subsequent welding performance.In order to solve the "black disk" phenomenon of the ENIG process,the chemical nickel/palladium/gold(ENEPIG)technology is a new type of PCB surface treatment technology developed on the basis of the ENIG process.In ENEPIG process,the palladium layer deposited between the nickel layer and the gold layer acts as a barrier layer to solve the "black disk" problem.At present,the reducing agent used in the ENEPIG process palladium plating solution is mostly sodium hypophosphite,and the palladium plating layer(thickness ~0.1?m)is a Pd-P layer.In the new era,5G information technology and electronic products in precision fields such as military industry put forward higher requirements for the quality of the palladium layer,a pure palladium layer(0.2~0.3?m)thick is required,so the development of new ENEPIG process and technology has important strategic significance.This paper focuses on the research and development of the electroless palladium plating process and its application in ENEPIG surface treatment for the stability of the electroless palladium plating solution and the thickness of the coating in the ENEPIG process.1.Formulation optimization of formic acid reduction palladium plating system and research on palladium plating process: Firstly,it is determined that Pd Cl2 is the main salt and formic acid is the reducing agent,and a series of complexing agents are selected through electrochemical assisted methods;Preliminary screening of formic acid concentration,p H,plating temperature,and complexing agent concentration with a single variable experimental method,Then the orthogonal test method is used to further narrow the concentration range of each component of the plating solution and determine the process conditions,and obtain the best formula and process conditions of the final plating solution: 500 ppm Pd Cl2,10g/L sodium citrate,8g/L sodium acetate,15g/L complexing agent C,10ml/L formic acid,p H=5.5,plating temperature 65?.The high temperature stability test of the plating solution shows that the palladium plating solution can continuously withstand a temperature of 70°C for 72 h.The UV-Vis spectrum test shows that the solution can be stored at room temperature for 30 days after the high temperature resistance.There is basically no change in the absorption spectrum,the plating solution remains transparent,without obvious decomposition,and the stability is good.The thickness of the palladium layer increases with the extension of the plating time.The thickness of the palladium layer plated in this experimental solution is about 0.12?m/10 min,0.19?m/20 min,0.28?m/30 min.The appearance of the palladium plate after plating is silvery white,and the surface smooth.2.Study on the mechanism of palladium deposition process,using cyclic voltammetry curve test,There is only one reduction peak in the curves of different scanning speeds,In the figure,you will find that there is only one reduction peak in the curves of different scan rates.As the reduction peak current density increases,the potential shifts to the left.From the cyclic voltammogram data,the transfer coefficient ? of the palladium deposition process can be calculated,indicating that the palladium deposition in the palladium plating bath Irreversible process.Chronoamperometry obtains the dimensionless chronocurrent curve at different potentials,and compares it with the theoretical curves of instantaneous nucleation and progressive nucleation.,Iit shows that the palladium layer deposited by the palladium plating solution system in this study is consistent with the electrodeposited palladium mechanism mentioned in other documents,and the nucleation of the deposited palladium conforms to the three-dimensional continuous nucleation law.3.Evaluation of the structure and performance of the palladium layer: The microstructure of the palladium layer is observed by scanning electron microscope(SEM).The unit cell of the palladium layer is dense,the surface is flat,and there are no cracks and pores.The X-ray diffractometer(XRD)test showed that the coating crystallinity and crystal phase purity were good,and the SEM matching energy chromatograph(EDS)test of the element content of the coating showed that the coating was a pure palladium layer.Using 3M tape to stick and pull the coating to test the adhesion of the coating,it shows that the adhesion between the palladium coating and the Ni layer is good.The electrochemical method Tafel curve(Tafel)and electrochemical impedance spectroscopy(EIS)were used to test the corrosion resistance of the coating.The two test results together proved that the corrosion resistance of the coating increased after palladium plating on the nickel layer.Increases as the palladium thickness increases.The neutral salt spray test shows that the coating can be maintained for about 16 hours.4.Electroless Palladium Plating in ENEP Surface Treatment Technology Application Research:The test results of the high and low temperature test show that the palladium layer has a better ability to withstand the impact of cold and heat cycles.The experimental description of the stability of continuous plating of palladium bath and the stability of resistance to foreign ions,The palladium plating solution in this study has good tolerance to nickel ions and copper ions,the thickness of the palladium layer can still be close to0.3?m/30 min,indicating that the palladium plating solution has good reliability.The electroless palladium plating process is applied to the surface treatment of printed circuit boards,that is,the replacement gold plating process(ENEPIG)is performed on the PCB board with the palladium layer.The results show that the thickness of the gold layer can reach about 0.05?m/20 min,and the gold layer The performance test is qualified.