The Chemistry of the Cupric Chloride Etching Process
Copper can exist in three oxidation states, the elemental form, 
, the blue colored Cupric form, 
, and the less common Cuprous form, 
. The fact that you can etch Copper using a solution of the Copper itself depends on the fact that you can easily transform from one oxidation state to the other. The general reaction is this:
Reaction 1.
In other words the elemental Copper reacts with the Cupric Copper to form two atoms of the Cuprous Copper, which is the state half way between the two original forms. The regeneration of the etchant converts the Cuprous Copper () back to the Cupric state, so it is ready to etch more metal. This is an overview and simplification of the reaction, the actual reaction mechanism is much more complicated, and must be examined to understand the effects of the chemical variables.
It is common to think that the Cupric Copper exists as Cupric Chloride (
), and that the Cuprous Copper exists as Cuprous Chloride (CuCl) in the etch solution, this is not exactly true. Both actually exist to a large extent as a complex with the Hydrochloric Acid (HCl), and thus the real overall reaction is something more like this:
Reaction 2.
It is important to understand that this reaction consumes HCl. But even Reaction 2 is an abbreviation of the real process, the real steps are these:
Reaction 3.
Reaction 4.
The importance of Reaction 3. is that it produces an insoluble precipitate, CuCl, which stops further etching. Reaction 4. shows how the CuCl reacts further with HCl to dissolve the insoluble CuCl, so that further etching can occur. This reaction is an HCl consuming process, and it would appear that the availability of the HCl is the critical etch rate controlling reagent. But there is further insight that can be gained.
The typical Cupric etchant contains about 180 grams per liter of Copper as Cupric Chloride, which is equivalent to about 3 molar, or in this case 3 Normal also. The etchants are typically also about 1-2 Normal with respect to HCl. Since the following reversible reaction occurs, which reacts the Cupric Chloride and Hydrochloric Acid, it requires 2 moles of HCl for every mole of Cupric Copper:
Reaction 5.
It follows that most of the HCl present is not free, but actually tied up in the Cupric complex 
. The HCl which is tied up in this complex will titrate as if it were free HCl, but it is not as readily available for reaction with the insoluble CuCl as “free”, uncomplexed HCl. Thus the actual available HCl is even lower than what a titration would seem to indicate. In order to satisfy the demand for HCl completely by the , there has to be twice the Normality of HCl as there is Copper, or roughly 6 Normal.
This underscores the fact that the etch reaction is HCl starved, and that the availability of HCl is the etch rate controlling step.
All of this is the reason why the HCl content in the etch is a key factor in raising the etch speed. This is confirmed by research done in the Photo Chemical Milling industry (1).
However, there are always tradeoffs, and the disadvantages to increasing the HCl content are:
- Increased undercut, or etch factor, thus diminishing precision of etching.
- Increased potential for free Chlorine release, if the system is replenished with Sodium Chlorate.
- Potential for attack on Titanium if etchant is over-replenished with oxidizer.
(1) Footnote: D.M. Allan, Principles and Practices of Photochemical Machining.