Etching is required for any of the process alternatives within the subtractive process. Panels entering the etch process have been coated with an etch resist, usually a dry film photo-resist. The resist layer selectively protects the circuit areas from etchant, whereas the remaining copper foil is etched away.

Etchant sprayed onto the surface of the panel removes the exposed copper, but cannot significantly dissolve the copper residing under the resist. In this way, a copper circuit is formed. Etching is performed with conveyorized equipment that typically includes a main spray chamber, an etchant flood rinse, and several cascading water rinses. Long conveyorized units that include developing, etching, and film stripping are common only in large production shops. Acidic cupric chloride and alkaline ammoniacal are the most common etchants (sulfuric-peroxide, a common microetchant, is also employed as a primary etchant). Chromic acid and ferric chloride, dominant in the past, are now rarely found. A complex array of issues surround the choice between the remaining chemistries, and the decision is tied to downstream and upstream process material choices as well as economic considerations. For example, cupric chloride is generally incompatible with the metallic resists (tin or tin-lead), which are commonly applied to outer layers, but may be selected for inner layers based on performance (for fine-line etching), waste minimization (ease of on-site regeneration and copper recovery), or other issues. In this example, two etching systems are required, cupric chloride for inner layers and ammoniacal for outer layers. Being a one-step conveyorized process, etching is not often a production bottleneck and two etching systems may be difficult to justify. For this reason, many small shops employ the more versatile ammoniacal etchant for both inner and outer layers.

Cupric chloride etchants consist of cupric chloride (CuCl2) and hydrochloric acid. The simple etch reaction is driven by copper's two oxidation states:

Cu + CuCl2 --> Cu2Cl2

The reaction with cupric chloride etchant is reversible chemically by chlorination, oxidation with peroxide or other oxidizer, or electrolytically. Several regeneration systems have been developed that reoxidize cuprous chloride and maintain total copper content at desirable levels (usually in the 15 to 20 ounce/gallon range). Chlorination, the most common method, is performed in a closed-loop arrangement in which spent etchant is circulated through the chlorinator and back to the etcher sump. Copper oxide waste is produced. Since this etchant is acidic, no attack on the alkaline-sensitive dry film resists occur. Cupric chloride has a similar etch rate to ammoniacal but is not, as mentioned above, compatible with many metal resists.

Ammoniacal etchant is popular due to ease of use and general compatibility with most etch resists. Ammoniacal etchant systems are comprised mainly of ammonium hydroxide and ammonium chloride. Other ingredients are present to a lesser degree and serve a variety of functions. As with cupric chloride etchant, the etching reaction is driven by the cupric (Cu++) ion:

Cu + (Cu(NH3)4)+2 --> (2Cu(NH3)4)+1

Ammoniacal etchants are maintained for continuous operation with a feed-and-bleed arrangement based on baume or specific gravity measurements. In this arrangement, a pump is connected to a baume-activated switch. When the baume of the etchant in the sump rises due to the increasing copper concentration, the pump is switched on. Copper rich etchant is removed from the sump while fresh etchant is introduced. In this way, a steady concentration of copper (critical in maintaining a steady etching rate) is maintained. In the absence of regeneration, the spent ammoniacal etchant stream is usually the largest waste stream shipped off-site by PWB facilities.

Sulfuric-peroxide (i.e., sulfuric acid and hydrogen peroxide) is commonly used as a micro-etchant. This chemistry was reported in use by a small percentage of respondents as a primary etchant. Sulfuric-peroxide has a much lower copper-holding capacity than other etchants (approximately 4-5 oz/gal Cu vs. 15-24 oz/gal Cu or more for ammoniacal and cupric chloride) but is easily regenerated on-site and is compatible with metallic etch resists.