Four basic technical parameters of electroless plating solution

The electroless plating solution used in practice contains other substances besides the basic components (metal salt to be deposited and reducing agent). Usually the situation is as follows:

1. Ligands form soluble complexes with metal ions, which is necessary for alkaline solutions. Also, the use of stable complexes sometimes enhances the catalytic effect.

2. Substances that control and maintain a certain pH of the solution are used: the addition of buffers is especially important because during metal reduction, hydrogen ions are formed.

3. The stabilizer slows down the reduction reaction in the solution volume, therefore, can improve the catalysis. Sometimes, medications such as brighteners are also added to the solution.

The basic technical parameters of the electroless plating solution are described in detail below.

deposition rate

The deposition rate is usually expressed in microns per hour (μm/h; or H/h, μμm/h, mg/cm2h). During deposition, if the concentration of reacting species cannot be maintained at a constant level, the rate decreases. The values ​​given in the literature are usually averages and reflect only the initial period.

Such an average rate depends on the ratio of the surface being plated to the solution volume (DM2/L).

The effect of deposition rate (V) on the concentration of reacting species is quite complex. It is usually described by an empirical formula such as:

where k is the rate constant (constant for a given type of system) and L is the concentration of free ligand (not bound to the metal ion in the complex). The indices A and B are usually less than unity, and C is negative (in alkaline solutions, the OH ion concentration is used, in which case the indices are usually positive, O < C < 1). The exponent D is usually close to zero; when ligands are substituted, the deposition rate may change substantially. As the concentration of other solution components increases, the deposition rate decreases when the stability of the metal complex increases (as the concentration of free metal ions decreases), but this relationship is generally not strict.

Under suitable conditions, the chemical deposition rate of most metals is about 2-5 μm/h, and only the electroless nickel plating rate can reach 20 μm/h (which is equivalent to the electroplating process with a current density of 200 μm/2).

service life

The lifetime indicates the maximum duration of solvent usefulness. The onset of reduction of metal ions in solution may terminate their mining. However, in most modern electroless plating baths, there is usually no reduction in bulk volume under normal operating conditions, and the lifetime of the solution is limited by the accumulation of reaction products or impurities. Therefore, it is better to describe the life of the solution than the time, which depends on the development intensity, but by the volume unit of the maximum amount of metal deposited from the solution (g/L or μM/L) or the number of turnovers in the solution Shows how many times the initial amount of metal may be deposited in the formation of the coating. The number could be as large as 10 to 20. After removing the undesired substances accumulated in the solution, it can be used for a longer period of time, just like the electrolyte used in electroplating.

After a long period of development, a certain amount of precipitation will appear in the solution, because even in a completely stable solution, the volumetric reaction may occur within a limited range.

Reductant Efficiency Factor

The efficiency factor of a reducing agent is expressed as the amount of reducing agent (moles or grams) required to deposit one mole or one gram of coating. The required amount (according to the reduction reaction) of reducing agent, which is equal, for example, 2 moles of 1 mole of metal (or reduction of nickel ions by hyposphosphite formaldehyde copper ions) over true electroless plating process due to side reactions occur.

Solution sensitivity to activation

Catalyst The minimum amount of catalyst that needs to be present on the activated surface is dictated by the sensitivity of the solution to activation. This parameter is related to solution stability.

The lower the solution stability, the easier the reaction, even on surfaces with low catalytic activity. High sensitivity of solutions is not always desirable, since metals in such solutions can deposit even on non-activated surfaces; in this case, selective plating becomes impossible. When a palladium compound is used for activation, the lead content per square centimeter of the dielectric surface of nickel and copper, respectively, is not less than 0.01 and 0.03 μg to 0.05 μg. When silver is used as an activator, it is only suitable for some electroless copper plating solutions, and about 0.4 μg of silver is required per square centimeter.