A.J. Scarpellino, Jr., Anil Rana, and R. Salazar, Vacco Industries
The analysis of hydrochloric acid, commonly referred to as “free acid”, in used ferric chloride is of consequence for a number of inter-related reasons which help to maintain etching process control. Free acid helps to solubilize the metal that is etched during the etching process. It prevents ferrous chloride from precipitating out of solution, and participates in the oxidation of this material back to ferric chloride using the oxygen in the air. Lastly, free acid prevents the buildup of complex ferrous salts, such as Fe(OH)Cl, the effects of which are not wholly understood but are presumed to be largely detrimental to the etching process.