Reaction Process of Two-step Catalysation Pre-treatment for Electroless Plating on Non-conducting Substrates

This paper describes the formation process and chemical state of adsorbates produced during the two-step catalysation pre-treatment prior to electroless deposition. Tin species adsorbed during sensitisation are divided into two types, namely one that adsorbs weakly and one that adsorbs strongly. The former exists in equilibrium with Sn²⁺ in the sensitising solution. It easily desorbed from the substrate during the following HCl immersion, and the loss is recovered during an ensuing sensitising step. The latter is not desorbed during the HCl immersion step. A dry process after sensitisation, which should promote oxidation, induces the transition from Sn²⁺ to Sn⁴⁺. In the activating step, two reactions proceed independently of each other; that is, an oxidation-reduction reaction between Sn²⁺ and Pd²⁺ takes place, which produces Pd⁰ and Sn⁴⁺. Simultaneously, some Sn²⁺ is desorbed from the substrate. XPS analysis confirms that tin oxide (or hydroxide) and metallic palladium are the final products in the two-step catalysation pre-treatment.