Principle of Etching Solution Regeneration

The typical process of PCB processing is graphic plating. That is to say, a lead-tin anticorrosion layer is pre-plated on the copper foil part (the graphic part of the circuit) which needs to be retained on the outer layer of the board, and then the remaining copper foil is corroded by chemical means, which is called etching.

the etching process, copper on the surface of the plate is oxidized by [Cu(NH3)4]2+ complex ion. The etching reaction is as follows:
Cu(NH3)4Cl2+Cu_2Cu(NH3)2Cl
The complex ion of [Cu(NH3)2]1+is Cu1+and has no etching ability. In the presence of excess NH3 and Cl-ions, it can be rapidly oxidized by oxygen in the air to form etching-capable [Cu(NH3)4]2+ complex ions. The regeneration reaction is as follows:
2Cu(NH3)2Cl+2NH4Cl+2NH3+1/2O2_2Cu(NH3)4Cl2+H2O

It can be seen from the above reaction that 2 grams of ammonia and 2 grams of ammonium chloride are needed for etching 1 gram of molecular copper. Therefore, with the dissolution of copper, ammonia water and ammonium chloride should be added continuously in the etching process, so the mother liquor of etching tank will increase continuously. Because the complex ion of [Cu(NH3)2]1+is Cu1+and has no etching ability, it is necessary to exclude some mother liquor and add new sub-liquor (sub-liquor does not contain copper ion) to meet the etching requirements.