Undercutting The etching that occurs on the sidewall of the wire under the resist pattern is called undercutting. The degree of side etching is expressed by the width of the side etching. See Figure 1. Side etching is related to the type and composition of the etching solution and the etching process and equipment used. Etching coefficient The ratio of wire thickness (excluding plating thickness) to the amount of side etching is called the etching coefficient. Etching coefficientu003dV/X The level of etching coefficient is used to measure the amount of side etching. The higher the etching coefficient, the smaller the amount of side etching. In the etching operation of printed boards, it is desirable to have a higher etching coefficient, especially for printed boards with high-density fine wires. Plating widening During pattern electroplating, since the thickness of the electroplated metal layer exceeds the thickness of the electroplated resist layer, the width of the wire increases, which is called widening of the plating. The broadening of the plating layer is directly related to the thickness of the plating resist and the total thickness of the plating layer. In actual production, it is necessary to avoid the broadening of the coating as much as possible. The sum of the broadening of the metal anti-corrosion coating and the amount of side etching is called the plating edge. If there is no widening of the coating, the edge of the coating is equal to the amount of side erosion. See Figure 2 (above). Etching rate The depth of the etching solution to dissolve metal in a unit time (usually expressed in μm/min) or the time required to dissolve a certain thickness of metal (min). ·The amount of copper dissolved The amount of copper dissolved in the etching solution at a certain allowable etching rate. It is often expressed in terms of how many grams of copper are dissolved per liter of etching solution (g/l). For a specific etching solution, its copper dissolving ability is certain.