Research On ENEPIG Surface Modification Technology Of High Frequency And High Speed Printed Circuit Board

Gold wire bonding is widely used as the interconnection between semiconductor chip and printed circuit board,which is very critical in integrated circuit packaging process.Gold wire bonding technology should be applied to electroless nickel-palladium immersion gold(ENEPIG)surface modification technology,while electroless nickelpalladium-impregnated gold surface modification technology has made non-palladium activation technology a hot spot in the industry because of its high cost and poor antiinterference ability.At the same time,with the increase of transmission frequency,the effect of the properties of nickel layer on signal transmission is more and more significant in ENEPIG surface modification technology,so studying the influence of nickel layer properties on signal integrity plays a key role in ensuring signal transmission quality.Based on the current problem of palladium activation,this paper uses dimethylaminoborane(DMAB)-sodium hypophosphite double reducing agent system for palladium-free activation technology research.The DMAB electrochemistry and coating properties were analyzed by means of orthogonal optimization experiment and SEM?EDS test.It was found that the content of sodium citrate had the greatest influence on the stability,plating speed and coating property of the bath,and the increase of sodium subphosphite was helpful to improve the deposition efficiency of the coating.In a comprehensive consideration,when 25g/L of sodium citrate,12g/L of sodium hypophosphite,10g/L of ammonium chloride,30g/L of nickel sulfate,and 2g/L of DMAB were taken,nickel coatings with excellent bath stability,good corrosion resistance,uniform surface arrangement,phosphorus content reaching the standard,and high catalytic activity could be obtained.The quality of electroless nickel coating(EN)plays a key role in the whole ENEPIG link.The effects of four sulfur-containing stabilizers(thiourea(THU),4,6-dimethyl-2-mercaptrim(DLMP),2-aminobenzothiazole(NA),tripolythiocyanate(TTCA))on the stability,deposition rate and coating properties of electroless nickel plating solution were investigated by electrochemical measurements and other methods.The results showed that the four stabilizers had significant effects on anodization of hypophosphite,solution stability,deposition rate,and phosphorus content,among which the low concentration of DLMP and TTCA had a significant effect on the stability of the solution.The quantum chemical calculations and molecular dynamics simulations of THU?DLMP?NA?TTCA four sulfur-containing stabilizers show that DLMP and NA have higher reactivity and are more prone to electron transfer;the molecular dynamics simulation results show that DLMP have greater adsorption energy.Combined with quantum chemical calculation and molecular dynamics simulation results,DLMP adsorption on copper surface is easier.The accelerated effect of sulfur-containing stabilizer on nickel deposition is related to the adsorption of sulfur-containing stabilizer on copper surface by quantum chemical calculation and molecular dynamics simulation.To explore the influence of surface modification technology on signal transmission of differential microstrip line,four-layer test board was designed with M4 substrate material,and effects of electroless tin plating,electroless silver plating,electroless nickelpalladium,electroless nickel-palladium immersion gold and OSP on surface performance and signal transmission of differential microstrip lines were investigated by means of SEM test,roughness test,characteristic impedance test and insertion loss test.The results showed that the OSP surface modified transmission line had the most flat surface and the least influence on signal transmission,while the insertion loss of ENIG and ENEPIG was much higher than that of other surface finishes,and the higher the phosphorus content in the nickel coating,the lower the insertion loss.The thicker the nickel coating,the greater the insertion loss.At the same time,it was found that there was no direct correspondence between the characteristic impedance and the insertion loss of the differential transmission line surface of the test board.