Investigation Of Replacing Formaldehyde Reductant In The Electrodeless Copper Plating Of PCB Substrates

It is highly desirable to replace the conventional electroless copper plating solution using formaldehyde as the reductant due to the issues of environmental protection. Two of the most attractive candidates are sodium hypophosphite and glyoxylic acid. However, the major challenge for their wide application is that sodium hypophosphite cannot be self-catalytic, and the nickel in the as-deposited copper layer increases the resistance of the film. Meanwhile, glyoxylic acid is expensive. Therefore, reducing free formaldehyde overflow is a favorable method. In this thesis, we used hydroxyl methyl sulfonic acid sodium and glyoxylic acid as composite reductants, and intensively investigated the plating parameters and the corresponding properties. The results showed that by using composite reductants the deposition rate was high, the bath was stable, and Cu coating with smooth appearance and good adherence to the substrate was obtained.The optimal conditions of electroless copper plating solution were determined. The compositions of the bath used were as follows:copper sulfate (0.032mol), sodium hydroxide (0.375mol/L), formaldehyde (0.0136mol/L), sodium sulfite (0.036mol/L), glyoxylic acid (0.0263mol/L), disodium ethylenediamine tetraacetate (0.046mol/L), tartaric acid potassium sodium (0.028mol/L),2,2-dipyridine (10mg/L), YL-343A (0.4mL/L), YL-343B (0.08g/L). As compared with the plating morphology of the optimized experiment system with sodium hypophosphite system and the general method in the market, it was found that pretty smooth and pore-free coating was obtained in our experiment system.The above optimized solution without adding any additives was measured as the basic solution. On conclusion, the cathodic reduction of copper was promoted by increasing the concentration of copper sulfate and sodium hydroxide; hydrogen evolution reaction of adsorbed hydroxyl methyl sulfonic acid sodium and glyoxylic acid, and oxidation dissolution reaction of anode copper were promoted by increasing sodium hydroxide; the dissolution of the copper was inhibited by increasing the concentration of glyoxylic acid. Controlled experiments have been conducted to investigate the influence of single additive and composite additives. A synergistic effect has been observed between different additives, which improved plating solution performance.The plating bath stability by adding formaldehyde adducts and glyoxylic acid as reductants was evaluated using PdCl2to accelerate the decomposition. The results showed that the composite additives played an important role in accelerating the reaction, stabilizing the plating bath and prolonging the service life.The free formaldehyde content of different reductant systems was analyzed by hydrochloric acid titration method. Free formaldehyde content was reduced by adding sodium sulfite, which suited environmental protection. By comparing commercially available copper solution to the above optimized solution, the feasibility and development of this experimental solution was considered.The copper coating surface roughness and adhesion force were analyzed by gold microscope observation and100grid test technique. The appearance of the coating was shining and the backlight level was nearly8.5, and the adhesion force of the coating accorded very well with Chinese standard of GB5270-85under optimum conditions.