Ammonical Etching of Copper

by Rudy Sedlak

This part of the PCB production process is one of the least understood, most subtle, and most important parts of the modern PCB production line. The Copper that is dissolved in solution already is the catalyst that is used to dissolve the copper metal on the board.

Key things to remember to sort this explanation out:

A. The metal is dissolved by being "Oxidized", which means that it loses electrons, which changes it from the yellow/gold metal to a blue compound.

B. The electrons are initially removed from the metal by the copper in solution. This seemingly unlikely idea can only happen because copper can exist in 3 ways, as metal, as"Cupric" salts, which are the blue compounds that everybody associates with copper, and as a third form, which is halfway between the yellow metal, and the blue salts, which is called "Cuprous" salts. Cuprous salts are typically white.

1. Metal - copper with a "correct amount of electrons"
2. Cuprous salts, Cu+(white)- copper missing one electron
3. Cupric salts, Cu++(blue)- copper missing two electrons

The initial metal dissolving reaction is:
Cuo(metal) reacts with Cu++(Blue) to give> 2 Cu+(cuprous)

The cuprous salts are then (supposed to be) immediately oxidized (electrons removed) by the Oxygen in the air, and in the process the oxygen is converted into water in the etchant.

                 2 Cu+ + O(Air) ---> 2Cu++ + H2O

All of this is facilitated by the ammonia etchant, because the cuprous(Cu+) compounds are not soluble except in the presence of the ammonia(NH3). And the overall reactions are these:

            Cu(NH3)4Cl2 + Cu(metal) ---> 2 Cu(NH3)2Cl
(Blue, Cupric)                (White, Cuprous)

Then the air reacts with the Cuprous to change it to the Cupric:

     2Cu(NH3)2Cl + 2NH3 + 2NH4Cl + O ---> 2Cu(NH3)4Cl2 + H2O
(Cuprous)      (Etchant)    (Air)   (Blue, Spent Etchant)

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