How does circuit appear on the PCB board? This will be achieved by etching: retaining the copper foil of circuit part firstly in the outer layer, then pre-plating with a solder resist layer, and then corroding chemically the rest copper foil. The copper has two layers, one layer needs etching completely, and remaining the other layer as circuit.
At the beginning of etching, the surface of the metal board is protected by pattern and the rest metal surfaces contact the etchant, where the etching is carried out vertically. When the metal surface is etched to a certain depth, the new metal surface appears at exposed sides. Thus the etching solution also includes the etching of both sides in addition to vertical. As the depth increases, the etching area of the metal surface on both sides is also increasing.
For the part etched for a long time, the depth of etching is also large on both sides, forming serious lateral erosion. The bottom etching time is shorter, so lateral erosion is relatively slight. Lateral erosion can make the lines of the convex surface become smaller, which will lead to the deformation or oversize of the board, or even seriously damaging the whole board.
There are several factors affecting the lateral erosion: the greatest impact on the etch effect is etching solution, and nitric acid-based liquid almost has not side erosion. In addition, the PH value of the alkaline solution cannot be too high and should be controlled below 8.5. As for substrate, you’d better use thin copper foil with the smaller line width, so the time copper foil in the etching solution will be shorter and lateral erosion amount will be less.