Electroless plating solutions containing metal ions and reducing agents are thermodynamically unstable systems. Reduction of metal ions needs to take place over a large portion of the solution.
The difference in the metal ion reduction rate of the desired surface (controlled catalytic reaction) and the reduction reaction in solution shows the catalysis, which largely determines the practicality of the plating solution. Under satisfactory conditions, most reactions in solution should not occur.
The formation of metals in solution is hindered by an energy barrier: an activation barrier for a uniform reaction between metal ions and reducing agents and an obstacle to the formation of new phases (metals). The size of the second potential barrier can be evaluated based on thermodynamic principles.
According to experience, the stability of the plating solution decreases with the increase of reactant concentration and temperature, the stability of the metal ion complex decreases, and there are solid impurity particles in the solution. In addition, the stability decreased as the catalyst treatment rate and loading increased. This may be attributed to the transfer of intermediate catalytic reaction products from the catalytic surface into solution, where they may initiate reduction reactions. In order to improve the stability of the solution, it is recommended to use a solution with a lower concentration and a more stable metal complex, and to remove solid particles in the solution by filtration. An effective solution stabilization method is the introduction of special additives, ie stabilizers. 4,11 Stabilizers, the number of which are very large, can be divided into two categories: (i) catalytic poisons, such as S(II), selenium(II) compounds, cyanides, nitrogen and sulfur heterocyclic compounds, and some metal ions, and (b) an oxidizing agent. It is assumed that the stabilizer hinders the growth of metal particles, close to the critical, absorbs them (catalyst poison) or passivates them (oxidant).
Stabilizers are always present in modern electroless plating baths. Their concentrations, which may range from 1 to 100 mg/L, hinder the deposition of metals on fine particles and generally slow down the rate of catalytic processes on the surface to be plated. This process can be completely stopped at sufficiently high stabilizer concentrations. However, in some cases, small amounts of stabilizers can increase the deposition rate.
