Journal of The Surface Finishing Society of Japan

A novel electroless (autocatalytic) gold plating bath containing sodium L-ascorbate as the reducing agent has been developed. A suitable bath composition was formulated based on the results of an electrochemical study in which were determined at a gold electrode the anodic polarization curves of various reducing agents and the cathodic polarization curves of sodium tetrachloroaurate (III) dissolved in solutions of sodium sulfite and/or sodium thiosulfate.
A typical bath containing 0.0125mol/dm³ sodium tetrachloroaurate (III), 0.1mol/dm³ sodium sulfite, 0.1 mol/dm³ sodium thiosulfate, and 0.25mol/dm³ sodium L-ascorbate, was operated under moderate conditions, typically at a pH of 6.0 and 333K. The rate of gold deposition obtained with this bath was in the same range or greater than that obtained with the classical cyanide-borohydride bath. The deposition rate of the new bath depended on both the ascorbate and gold concentrations, and the gravimetrically determined deposition rate was found to correspond well with the rate estimated from electrochemical polarization measurements.

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Several factors associated with the uniformity of electroless copper has been investigated. It was confirmed that the higher the aspect ratio, the lower the uniformity of the deposition. It is because as hole diameter decreases and aspect ratio increases the amounts of hydrogen gas trapped in through-holes in the boad increases. Inclinating or vibrating the boad is an effective way of reducing the amount of trapped gas and improving deposition uniformity. Deposition uniformity was also increased by decreasing the concentration of the dissolved oxygen in the bath. This is related to the surface activation of deposited copper by the dissolved oxygen. The EDTA concentrations in the plating bath also influenced deposition uniformity. Uniform copper obtained when EDTA concentrations were increased.

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A study was conducted on the effects of treatment conditions on the plate-making properties and printability of pre-sensitized plates having an electroformed iron foil support. This iron foil support had excellent bending properties when electroformed at 90-95°C in a bath containing 800g/L ferrous chloride. The surface roughness of the iron foil increased linearly with foil thickness, which needed to be at least 15μm in order to be used as the presensitized plate. The amount of chromium deposited on the iron foil required to improve its corrosion resistance was 5mg/dm². When these chromium-plated iron foils were cathodically treated in a suspension consisting of Al₂O₃ sol and SiO₂ sol, their hydrophilic properties and their adhesion to light-sensitive resins were improved. The amount of aluminum deposit required was 1.0-2.0mg/dm² and the silicon deposit requirement was 0.4-1.0mg/dm². The structure of the hydrophilic film was examined by ESCA and X-ray diffraction and was found to consist mainly of Al(OH)₃ and SiO₂, which are appeared to be amorphous. Pre-sensitized plates supported by these treated iron foil supports were capable of printing 50, 000 sheets without problems of staining, spotting, or scumming.

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A study was conducted concerning the recovery of ZrN fine powders using a combination of acid washing and electrophoretic deposition in order to simply and efficiently separate ZrN fine powders and MgO by-product, prepared by the thermite method. Examinations of the electroadhesion of the ZrN fine powders in the dispersion medium, the concentration of the pertinent dispersion medium, and the appropriate time and voltage of electrophoresis were also made. It was found that the amount of ZrN fine powders deposited could be controlled with ease by changing the concentration of the dispersion medium, the applied voltage and the deposition time. It was also evident that no new addition of electrolyte was necessary to this recovery operation, because the H⁺ ion concentration required for the operation of subsequent deposition was already present on the ZrN particles in the process of acid washing and these particles were consequently charged.

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