Electroless plating solutions containing metal ions and reducing agents are thermodynamically unstable systems. Metal ion reduction needs to be done mostly in solution.
The difference in the reduction rate of the metal ions on the desired surface (controlled catalytic reaction) and the reduction reaction in solution show a catalytic effect which determines, to a large extent, the practical use of the plating solution. Ideally, the reaction shouldn't show up with most solutions at all. The formation of metals in solution is hindered by an energy barrier: a barrier (metal) to the homogeneous reaction of metal ions with reducing agents and the formation of new phases . The size of the second barrier can be evaluated based on thermodynamic principles.
Experience has shown that the stability of plating solutions decreases as the amount added increases. The concentration and temperature of reactants, the stability of metal ion complexes decrease, and the presence of solid foreign matter in solution. Furthermore, stability was found to increase with catalytic process rate and loading. This may be due to intermediate catalysis of reaction products from the catalytic surface to solution, where they may initiate reduction reactions. In order to improve the stability of the solution, it is recommended to use the metal complex with a lower concentration and more stable solution , and remove the solid particles in the solution by filtration. An effective solution stabilization method is the introduction of special additives - ie stabilizers. 4,11 The number of stabilizers is very large and can be divided into two main groups: (a) Catalyst poisons such as S(II), Se(II) compounds, cyanides, nitrogen and sulfur containing heterocyclic compounds, and some metal ions and (b) oxidizing agents. It is hypothesized that stabilizers hinder the growth of fine metal particles, close to the critical ones, by absorbing them (catalytic poisons) or passivating them (oxidants). Modern electroless plating solutions always contain stabilizers. Their concentrations may range from 1 to 100 mg/L. Stabilizers generally slow down the rate of catalytic processes on the surface being plated by hindering the deposition of metal on fine particles. This process can be completely stopped at a sufficiently high concentration of stabilizer. However, in some cases, small amounts of stabilizers can increase the deposition rate.
