Page 19 - PCMI Summer Journal 2021

The Chemistry and Control of Etching Ferrous Metals with Ferric Chloride
The Chemistry and Control of Etching Ferrous Metals with Ferric Chloride Solutions:
Solutions: The Concept of a Constant Etchant Pool
The Concept of a Constant Etchant Pool Presented by: David M. Allen, Emeritus Professor of Microengineering, Cranfield University, UK
David M. Allen
Emeritus Professor of Microengineering, Cranfield University, UK (Presented at the PCMI Webinar held on Thursday 15th April 2021)
Abstract
This paper details the chemistry and control of etching carbon steels and stainless
steels with aqueous ferric chloride solution. In general, as material dissolves into solution, the composition of the etchant changes such that the concentration of ferric chloride decreases, the concentration of ferrous chloride increases and the concentrations of dissolved metals increase.
This change in etchant composition results in a change of etch rate and requires strict control to enable high yields of products to be manufactured within dimensional specifications. For controlled PCM, the equivalent of “a sharp cutting tool” is “a constant etchant pool”. Control of the etchant pool entails monitoring of
• etchant concentration changes via the oxidation-reduction potential (ORP),
• free hydrochloric acid content,
• dissolved metal concentrations and, most importantly,
• maintaining constant etchant temperature.
Ideally, these processes are carried out on-line and in real time to allow automated additions of ferric chloride (if required), hydrochloric acid and oxidising agents that recycle the ferrous chloride waste product back to ferric chloride etchant.
It is essential that the chemistry of the control processes is thoroughly understood so that the controls are effective and efficient, resulting in improved product yield and accrued financial and environmental benefits.
Introduction
As all PCM companies know, for process efficiency it is essential that the chemistry of
the etching process is strictly controlled to produce in-specification components in the quickest time possible. They want to produce components “right first time” and this can only be achieved by monitoring and controlling the etchant such that it comprises “a constant etchant pool”; in other words, the equivalent of being a “sharp cutting tool” in conventional machining systems. A constant etchant pool will give a constant etch rate and enable component dimensional specifications to be more readily achieved.
It is, unfortunately, a common scenario in a PCM company to see operators repeatedly running up and down etching machines trying to achieve by multiple passes those difficult component dimensions with tight tolerances.
The objective of this paper is to demonstrate how a constant etchant pool can be achieved by chemistry control technology using a variety of monitoring techniques. As Lord Kelvin (1824-1907) famously stated, “If you cannot measure it, you cannot control it”. Etchers therefore need to measure a set of reliable quantitative parameters of the etchant to enable optimum composition to be controlled proactively.
Issue 137 August 2021 PCMI Journal 19