Overview

Etching is one of the major steps in the chemical processing of the subtractive PCB process. By this process, the final copper pattern is achieved by selective removal of all the unwanted copper to retain the desired circuit patterns. The copper which is not protected by an etch resist is removed by the etching process. The following are the commonly used etching methods:

• Chemical etching or chemical machining;

• Electrochemical etching or chemical milling; and

• Mechanical etching (by milling).

9.1 Etching Solutions and Chemistry

Several chemicals are used for etching. The most common etchants are:

• Ferric chloride;

• Ammonium persulphate;

• Chromic acid;

• Cupric chloride; and

• Alkaline ammonia.

The following sections give characteristics, chemistry and process details of these etching methods.

9.1.1 Ferric Chloride

Ferric chloride etching solutions are widely used in the 'print' and 'etch' process in the PCB industry. Ferric chloride has a high etch rate and high copper dissolving capacity. It is used with screen inks, photo-resist (wet film and dry film) and gold plated boards. As the ferric chloride etchant attacks tin, this is not suitable for tin or tin-lead plated boards.

This etchant is an aqueous solution of 28 to 42 per cent by weight of ferric chloride. The solution has a specific gravity of 1.353 to 1.402. It operates over a wide range of concentration, but is most widely used at about 35 per cent. The ferric chloride solution has free acid due to the following hydrolysis reaction:

In order to prevent the formation of insoluble...