Abstract: The deposition rules of Pd were investigated during electroless plating in baths with different amounts of Pd²⁺. It was demonstrated that the Pd deposition rate was controlled by the auto-catalytic activity of the metal layer surface when the deposited Pd layer became thicker than 5 μm even if Pd²⁺ in the plating baths was far from being consumed. When the Pd²⁺ amount in the bath was kept lower than that necessary for reaching the Pd layer thickness (e.g. corresponding to 1–4 μm Pd), the Pd²⁺ conversion in the bath was higher than 90% after reaction for 180 min. Furthermore, the same was found during Ag deposition from silver plating baths with a low amount of Ag⁺, i.e. Ag⁺ conversion was higher than 95% after plating for 120 min in baths containing Ag⁺ sufficient for thick Ag layer of 0.5–2 μm. It was proved by scanning electron microscopy measurements that the higher conversions of Ag⁺ were related to the special growth properties of Ag particles. According to the deposition rules of Pd and Ag, the thickness and metal composition of ultra-thin Pd/Ag membranes prepared by layer-by-layer electroless plating could be pre-designed as well as accurately controlled by preparing relevant plating bath with low amounts of metal.

Key words: palladium, silver, composite membrane, electroless plating, control of membrane thickness, control of metal composition, deposition rule