Page 27 - PCMI Summer Journal 2021

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
A further analytical complication is that ORP is affected by the concentration of free HCl in the etchant. It has been clearly shown [3,4] that addition of HCl reduces ORP at all temperatures. Therefore, the two variables should always be recorded simultaneously.
By maintaining a constant ORP at a constant free acid level, the [FeCl3]/ [FeCl2] ratio and etch rate are kept constant (Figure 8).
Figure 8. Maintenance of [Fe3+]/[Fe2+] ratio in the etchant pool.
It should be noted that analyses of fresh industrial etchants supplied to commercial PCM companies were found to vary considerably [5] with ranges (at 20°C) of:
• pH from -1.29 to -0.47
• Conductivity from 13.6 to 36.5 mS/cm
• ORP from 715 to 592 mV.
Increasing process efficiency by etchant regeneration
There is a real need to purchase minimum ferric chloride and maximise the amount of metal etched with ferric chloride purchased to lower environmental impact of PCM [3]. Past research indicates that average industrial etchant usage is < 20% if the solution is not regenerated. This is due to the etch rate being slowed down by the drop in [Fe3+]. The challenge of trying to maintain a fast etch rate therefore also leads to the need for etchant regeneration.
Regeneration can be achieved by a variety of methods including the addition of chlorine gas, sodium chlorate plus HCl, ozone gas or electrolysis as shown in Figure 9 [1(b)]. Each method results in by-products that can also change the composition of the etchant pool (Figure 10).
Issue 137 August 2021 PCMI Journal 27