Investigation Of Process For HEDP Non-cyanide Copper Electrodeplating

It is of great significance to study the cyanide free copper plating which can replace the highly toxic cyanide copper plating. The HEDP copper plating is widely concerned because of its environmental protection, non-toxic and can be directly plated on the steel substrate. In this paper, electrochemical behavior of copper plating from HEDP-cupric plating solution were studied by cathodic polarization curve and other electrochemical test methods, informed of the reduction process and controlling step of copper deposition. Studied the effect of HEDP/Cu²⁺ molar ratio and pH on the adhesion of the copper layer. The effect and mechanism of auxiliary complexing agent C₄O₆H₄KNa and wetting agent SDS on the layer and plating solution were studied by electrochemical and any other methods, then a low porosity HEDP non-cyanide copper plating process was proposed. The main results are following:This paper presented a copper plating formula as follows:14g/L cupric subcarbonate, 90g/L HEDP, 40g/L potassium carbonate, 2ml/L hydrogen peroxide, 7g/L sodium potassium tartrate and 0.1g/L sodium dodecyl sulfate. The results showed that the cathodic current density range up to 0.2³.58A/dm² when the HEDP/ Cu²⁺ molar ratio was maintained at 3 and 4. The current efficiency at a temperature between 40⁶5℃ are ≥90%, it reached 95.6% at 50℃, higher than cyanide copper plating, pyrophosphate copper plating and tartaric acid-citrate copper plating. The addition of wetting agent SDS can increase the cathodic polarization, at the same time contact angle of the plating solution on the steel substrate and the copper layer is reduced. When added to 0.1g/L SDS while applying cathode movement, layer that about 8μm thickness porosity can decrease to zero. Hull Cell test showed that the amount of C₄O₆H₄KNa added 3¹4g/L is more appropriate. It was observed that C₄O₆H₄KNa increases cathodic overpotential. As a result,the grain size was reduced from 44 nm to 40 nm. It was also found that the nucleation was three—dimensional（3D）and progressive in the electrolyte while as the C₄O₆H₄KNa in the electrolyte increased. There was an increase in the outward growthrate of crystal nuclei and number density of nuclei with the C₄O₆H₄KNa increased. Throwing power rise from 66.42% to 83.87%. After added 0.1g/L SDS leveling ability 9.3% drop to a negative leveling-7.6%. Covering power reaches 100%, better than cyanide copper plating.At the pH9 and 10, the main form of HEDP complex with copper ions is CuL^2-, respectively, accounted for 97.13%, 99.70%. The cathodic process of HEDP copper plating is divided into two steps, and there is a preceding reaction, the possible reduction process is2 2 4Cu L Cu L-（10）-T（10）, Cu^2- 2e Cu（10）（10） T, the cathodic reduction process is an irreversible process. Withing the potential of-1.15V^-1.30 V, the cathodic reaction was preceding reaction of CuL^2-; then electrocrystallization of Cu was main from-1.35V^-1.50 V. The cathode transfer coefficient of the two steps is（αn）1=0.2923 and（αn）2=0.1176 respectively at 55℃, the exchange current density is j10=7.8478×10-7A/cm² and j20=1.0965×10-5A/cm² correspondingly. So the preceding reaction was the controlling step.The higher of pH or HEDP/Cu²⁺ molar ratio, the more stable of HEDP copper complex, as a result it can improve the adhesion of the copper layer. Fe electrode in HEDP-cupric plating solution has a tendency to passivation, but in the pH9 above and the HEDP/Cu²⁺ molar ratio is greater than 3, the adhesion strength of the copper layer is good, and the potential difference of two electrode Δφ≤0.02 V, this may be the reason plated steel parts directly in HEDP copper plating solution. When the concentration of free copper ion less than the critical value that 0.51×10-20mol/L, the layer′s that plating from HEDP-cupric plating solution directly adhesion strength is well.