Copper Electroless Plating On PCB From Low Formaldehyde Bath

PCB is the key electronic interconnection of electronic products, known as the‘mother of electronic systems products’. With the fast development of PCB industryand electronic industry, the copper electroless plating technology is of significant andin higher demand. At present, the most widely used electroless copper plating processis using formaldehyde as reducing agent, however this processing has big drawbacksbecause formaldehyde-based solution is easy to decomposition and has strong toxiceffect, and it has been limited in many countries. Thus, using sodium hypophosphite asreducing agent instead of formaldehyde is potencial because of its relative safety,harmless efforts, while, sodium hypophosphite electroless copper plating is difficult tobe applied in production due to its low plating rate.This paper aims to research a new electroless copper deposition processing onPCB from low formaldehyde using sodium hypophosphite as major reducing agent anda small quantity of formaldehyde for supporting to solve the problem of porosity ofcoating structure, low plating rate and poor stability. The Cu deposition wascharacterized by scanning electron microscopy （SEM）, energy dispersive X-rayspectrometry （EDS）, X-ray diffraction and adhesion tests. Electrochemical behavior ofsodium hypophosphite electroless copper plating was also studied.Through the study of copper electroless plating on PCB from low formaldehydebath, it can be concluded that the optimal conditions are: CuSO₄·5H₂O9g/L,EDTA·2Na12g/L, sodium potassium tartrate9.6g/L, sodium hypophosphite30g/L,HCHOH3mL/L, nickel sulfate0.8g/L,2,2’-dipyridyl10mg/L, L-arginine0.15mg/L,TEA1mL/L, maleic acid10mg/L, pH was12.5, the temperature was65℃. After20minutes plating, the plating rate is5.30μm/h and the backlight level of coatingachieves10th grade.The XRD test shows that it is copper crystal deposited onsubstrate, the SEM indicates that the coating is uniform and compact, the EDS result shows that there is no second-phase particle in deposite of coating. The stability timeof bath reaches the highest of48h at80℃.The study of electrochemical characteristic on sodium hypophosphite anodicoxidation shows that the peak current density firstly increased then decreased with theaddition of sodium hypophosphite, nickel sulfate, L-arginine, TEA, maleic acid, andfall with the increase of2,2’-dipyridyl. The study of electrochemical characteristic onCu（II） cathodic reduction shows that the peak current density increased firstly thendecreased with the addition of L-arginine, TEA, maleic acid, and rised by the increaseof nickel sulfate, while reduced gradually after the2,2’-dipyridyl is added. With theaddition of mixed-additives, the potential of Cu（II） cathodic reduction shifts activelyand the peak current is decreased. While the addition of mixed-additives resulting inan obviously increasing of the peak current density of sodium hypophosphite anodicoxidation, which give rise to the whole plating rate of sodium hypophosphiteelectroless copper deposition. The suitable range of components in sodiumhypophosphite electroless copper plating were gained by the electrochemical behaviorstudy, which are that sodium hypophosphite is30³5g/L, EDTA·2Na is12g/L, sodiumpotassium tartrate is9.6g/L, CuSO₄·5H₂O is9g/L, nickel sulfate is0.5⁰.8g/L,2,2’-dipyridyl is10¹5mg/L, L-arginine is0.1⁰.15mg/L, maleic acid is10¹5mg/L.Theelectrochemical study results are basically identical with technical study.The above studies have shown that, compared with the currently widely used pureformaldehyde system, the usage amount of formaldehyde in our research of electrolesscopper plating on PCB from low formaldehyde is reduced by80%, meanwhile, it’sproved to be good in solution stability, plating rate and coating quality, which isprobably applied to production.