Research Of Electroless Copper Plating Using Sodium Hypophosphite As Reducing Agent

Since its discovery by Narcus in 1947, electroless copper plating has been wildly used in electronics, machinery, aerospace and many other industries. Most of the traditional electroless copper deposition used formaldehyde as reducing agent. Due to this technology had many drawbacks, such as the low deposition rate, bad stability of the solution and the steam of formaldehyde was harmful for human and environment, it was very urgent to look for substitutes for formaldehyde. Electroless copper plating using sodium hypophosphite as reducing agent possessed the advantages of wide operation range, long-life of the electrolyte and without the deleterious steam, and it might replace the formaldehyde copper deposition. Therefore, it is very necessary to do this research. In this dissertation, we explored a new copper deposition system using sodium hypophosphite as reducing agent and sodium citrate as chelating agent, which solve the low deposition rate and the solution stability problems. At the same time, many modern analytical techniques, such as scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), four-probe resistance instrument, electronic pull test-bed, etc, were used to detect the morphology, microstructure, composition and performance of the copper deposit. Electrochemical methods were used to test the anodic and cathodal polarization processes. Finally, we used in-situ FTIR spectroscopy to explore the mechanism of activation nickel ions in electroless copper plating using sodium hypophosphite as reducing agent. Main results of this dissertation were summarized as follow:1. Deposition conditions of electroless Cu-Ni and their influence rulesBase on the experiments, we confirmed the basic technology of electroless copper plating using sodium hypophosphite as reductant, which available temperature was between 60～70℃, the pH value between 8～9, and the NiSO4·6H2O concentration 1～2g/L. The deposition rate will be accelerated with the increasing of temperature, pH value and concentration of nickel ions. The deposit was Cu-Ni alloy with face-center cubic configuration without obvious crystal face preferred orientation, which copper and nickel mass percentage were 87.70% and 12.30%.Influence of temperature, pH value and the concentration of nickel ion on oxidation of sodium hypophosphite and reducing of copper ion were explored by liner sweep voltametry. The results showed that higher bath temperature accelerated both of the electrodic process; while the increasing pH value only promoted the oxidation of sodium hypophosphite but blocked the reducing of copper ion; the nickel ions not only intensively catalyzed the hypophosphite oxidation, but also codeposited with the copper ion to form the Cu-Ni alloy. With regard to its catalytic activity, this alloy enabled the continuation of the electroless copper plating reaction. We made a confirmation that the speed determinate step in this system was the oxidation of sodium hypophosphite. The electrochemical results were consistent with the deposition rate experiments.2. Influence of additives on electroless copper plating2,2ˊ-dipyridine, sodium benzene sulphinate and the methyl orange were effective additives on electroless copper plating using sodium hypophosphite as reducing agent. Their effects on deposition rate, surface morphology, deposit structure, the anodic and cathodal polarization processes of electrolyte were explored. The results showed that electrolyte was stable and the deposit appearance was bright when additives were added to the solution. 2,2ˊ-dipyridine slower the deposition rate; sodium benzene sulphinate and the methyl orange both accelerated the plating rate at a certain concentration while slower the deposition rate at high concentration.Results of liner sweep voltametry indicated that the 2,2ˊ-dipyridine blocked the oxidation of sodium hypophosphite; while a certain amount of sodium benzene sulphinate and the methyl orange promoted the oxidation process. The cathodal process was relatively complex: the present of 2,2ˊ-dipyridine made the reducing peak potential of copper ions moved to negative value compared to no additive in the solution and the current peak increased; sodium benzene sulphinate would promote the reducing of copper ions at low concentration while baffled it at high concentration; the reducing peak potential shifted to a positive value and the peak current reduced when the methyl orange concentration increased. The electrochemical results were inosculated with the experiments of deposition rate. SEM experiment displayed that 2,2ˊ-dipyridine affected the growth of the particulate, and their shape changed from prick to agglomerate, and the surface became denser. Sodium benzene sulphinate and methyl orange had less influence on the deposit morphology. Especially, mixture of two additives had a better effect on the surface morphology compared to single additive. The combination of 2,2ˊ-dipyridine and sodium benzene sulphinate possessed the best effect, and the deposit displayed bright appearance.EDS results showed that additive in the electrolyte redounded to the copper aggradations, and copper percentage in the alloy increased. XRD analysis indicated that the layers were still Cu-Ni alloy with face-center cubic configuration without obvious crystal face preferred orientation, and the concentration of Cu2O was very low.3. Compared with the copper deposit using formaldehyde as reducing agentBased on the contrastive experiments, we found that each technology possessed the advantages and the electrolyte using sodium hypophosphite as reducing agent was much more stable than that using formaldehyde as reducing agent. Electrolyte containing sodium hypophosphite didn't decompose even after seven metal cycles, while the electrolyte of formaldehyde broke up only after three cycles. Besides, the deposition rate of sodium hypophosphite was higher than formaldehyde's.SEM results indicated that the deposit particulate was very small and dense when using formaldehyde as reductant, and the deposit of sodium hypophosphite was agglomerate. The XPS result indicated the copper and nickel element existed in the deposit as metal state, and the mass concentration of phosphorus was less than 0.05%. In the solution containing 2,2ˊ-dipyridine the mass percentage of copper was 93.90% in the deposit using sodium hypophosphite as reducing agent, and the rest 6.10% was metal nickel. Because of the existence of nickel metal, the resistance, tensile strength and ductibility of formaldehyde's deposit were better than that of sodium hypophosphite. Therefore, the deposit using sodium hypophosphite as reductant was more suitable for the industries which needed a relatively lower requirement on conductance.4. Mechanism of nickel ions in electroless cooper platingIt was very necessary to add the re-active agent of nickel ions in order to keep the self-catalysis reaction when using sodium hypophosphite as reducing agent, due to the catalysis of copper was very low to hypophosphite. The nickel ions were reduced to nickel metal to form Cu-Ni alloy so as to catalyze the oxidation of hypophosphite, and the alloy could keep the self-catalysis reaction. The catalysis of pure nickel was stronger than that of Cu-Ni alloy.A new absorb peak was detected in situ FTIR spectroscopy experiment, referred to many reference; we speculated it was the intermediate HPO2-ads. Besides, there was a prepositive electrode reaction in the oxidation process of hypophosphite, which created HPO2-ads and H ads. The mechanism of nickel ions was illuminated and the theoretical model of the electroless copper plating using sodium hypophosphite as reducing agent was established based on the experiments.