Page 23 - 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
The equation also shows that if the diffusion layer thickness (S) is reduced by agitation, the etch rate should increase. A constant etch rate thus also requires constant etchant agitation as well as a constant etchant formulation and temperature (Figure 4).
Figure 4. Application of constant temperature and control of viscosity through monitoring of specific gravity (°Baumé).
Commercially, it is usual to achieve a fast constant etch rate by applying the etchant in a spray etching machine and controlling etchant spray pressure rather than applying the etchant by flowing or stirring, as shown in Table 2.
Table 2. Rates of etch of various materials showing rate of etch is affected more by method of agitation than by etchant temperature [1(a)].
Material
Copper
Copper Mild steel Spring steel AISI 304 stainless steel
Temperature Ferric chloride concentration (°C) for maximum etch rate
45 2.5 M (34°Bé)
25 2.5M (34°Bé) 70.1 2.3M (32°Bé) 50 2.1M (30°Bé) 50 2.3M (32°Bé)
Rate of etch (m/min) and method of agitation 5.6 (flowed)
31 (sprayed)
6.6 (stirred)
32 (sprayed)
38 (sprayed)
Issue 137 August 2021 PCMI Journal 23