Study On Electroless Silver Plating On Copper Surface Of Printed Circuit Board And Its Transmission Loss

As the carrier of components,printed circuits need to be soldered onto the exposed copper surface of the outer layer of the printed circuit board to complete assembly.At the same time,copper signal transmission lines are also exposed to the air.Therefore,in order to protect exposed copper signal transmission lines from air oxidation and ensure better reliability during component welding,copper surface finishing technology for printed circuits has emerged.The copper surface finishing technology for printed circuit boards refers to the process of covering the exposed copper surface of the printed circuit board with a protective coating through electroplating,chemical plating,or adsorption after the production of the printed circuit board is basically completed.The trend of lightweight,highly integrated,and highly reliable electronic products provides new opportunities for the development of copper surface finishing technology in printed circuits.Therefore,this thesis studies a copper surface chemical silver plating technology for printed circuits,aiming to provide a silver protective layer on the copper surface,reduce surface roughness and signal transmission loss.This thesis mainly starts with the principle of metal activity and determines the copper surface electroless silver plating technology mainly based on reduction reaction,including pre-treatment,preparation of plating and reducing solutions,electroless silver plating,and other processes.It also explores in detail the influence of different factors in the process on the effect of chemical silver plating.After analyzing the full factors experimental design,the basic formula for copper surface electroless silver plating is determined.In this thesis,by means of field emission scanning electron microscope,3D laser confocal microscope,X-ray thickness tester,vector network analyzer and other characterization methods,the single factor experiment method was used to explore the influence of different bath components and working conditions in electroless silver plating on copper surface,and the optimal electroless silver plating formula was determined: silver nitrate 10～15 g/L,ammonia 20～80 m L/L,ethylene diamine 20～25m L/L,ethanol 20～25 m L/L,2,2-nenenebb bipyridine 0.1～0.2 g/L,p H=12,40～50 ℃/10 min。 By using an X-ray diffractometer to analyze the silver coating after electroless silver plating,it was found that the growth formula in this experiment takes Ag(111)crystal plane as the dominant growth crystal plane.The mechanism of the coordination agent(ammonia and ethylenediamine)in the electroless silver plating reaction was explored using electrochemical analysis methods.It was found that it reduces the electrode potential of silver by coordinating with silver,allowing silver particles to deposit at an appropriate rate.This thesis also explores the Galvanic effect in copper surface electroless silver plating and its impact on printed circuit boards.Through field emission scanning electron microscopy,it was found that controlling the electroless silver plating time,micro etching time and reducing the ratio of solder resist ink coating area to electroless silver plating treatment area,can effectively reduce the Galvanic effect,reduce the degree of copper corrosion and ensure the electrical performance of printed circuit boards.And after conducting reliability tests on printed circuit boards using the copper surface electroless silver plating technology,it was found that the technology requires additional post-processing to further improve its sulfurization resistance.Other reliability tests met the application requirements.