Characterisation of aqueous ferric chloride etchants used in industrial photochemical machining

Ferric chloride (FeCl3) is the most commonly used etchant for photochemical machining (PCM) but there is a great variety in the grades of the commercial product. This paper aims at defining standards for industrial etchants (that are not chemically pure) and methods by which they are analysed and monitored. In an ideal world, to maintain a constant rate of etching and hence control of part dimensions dependent on etch time, the etchant composition would be constant. Unfortunately, in the real world, the etchant composition changes continuously. As an n-valent metal (M) is dissolved into solution, etchant is consumed and the by-products of ferrous chloride (FeCl2) and metal chlorides (MCln) are generated, i.e.nFeCl3+M→nFeCl2+MCln Thus, for quality control (QC) of PCM, this creates a specific demand for data relating to the composition of the etchant as it changes. The demand covers characterisation of the virgin etchant, its degradation with use and the etchant resulting from its in situ chemical regeneration. There are various parameters that are typically measured in commercial enterprises in order to assess the quality of the etchant. These include: • °Baumé (or specific gravity); • temperature; • free acid (HCl content); • oxidation–reduction potential (ORP); • dissolved metal content. Variations in any of the above can affect amongst other things, the rate of etch (with a resultant change in etched dimensions) and surface finish. The intention behind the work detailed in this paper was to establish whether some of these parameters could be used to determine the etchant quality and thereby determine its fitness for purpose at any given time. Theoretically, by monitoring variations in the parameters, it should be possible to predict the behaviour of the etchant. This in turn would assist in deciding whether to continue etching or replace the etchant or when to regenerate it.