Page 21 - PCMI Summer Journal 2021
P. 21
The Chemistry and Control of Etching Ferrous Metals with Ferric Chloride Solutions: The Concept of a Constant Etchant Pool Presented by: David M. Allen, Emeritus Professor of Microengineering, Cranfield University, UK
Thus, addition of HCl will be of advantage in preventing hydrolysis of the ferric chloride to form ferric hydroxide, an intermediate in the formation of ferric oxide (an insoluble material that will result in spray nozzle blockages). It is therefore good practice to add HCl to the etchant prior to the start of etching.
Note that the addition of base (such as NaOH) will only succeed in producing more Fe(OH)3. This is important when it comes to chemical analysis of the HCl content in the etchant by titration against a base. The FeCl3 must be complexed first to prevent hydrolysis and liberation of additional HCl not present as the original free acid of the etchant formulation.
The ferric chloride etchant pool recognising the importance of HCl in the formulation is shown in Figure 2.
Figure 2. Ferric chloride dissolved in water with additional HCl for hydrolysis prevention.
Effects of ferric chloride concentration and the limited capability of °Baumé control
It is usual in the PCM industry to express the density of an etchant in terms of °Baumé (°Bé) where °Bé = 145 (sg -1)/sg and sg is the specific gravity of the solution.
Many PCM companies seem to rely extensively on measurement of °Bé (a physical variable) to control the etchant composition (a chemical variable) and it must be noted that the °Bé value:
• only reflects the total concentration of metal ions in solution.
• is not a measure of the ferric chloride concentration once the etching reaction starts
as its concentration changes with the formation of ferrous chloride by-product.
However, °Bé is a useful indicator of etchant viscosity.
• is dependent on temperature. This temperature dependence is readily illustrated in
Table 1 where it shows that as temperature increases, specific gravity falls and thus
Issue 137 August 2021 PCMI Journal 21
