Chemical Mechanism
Continuous subtractive chemical processing dissolves exposed copper foil from outer printed circuit board layers using an aqueous ammoniacal solution while leaving tin or tin-lead metallic resist intact. Alkaline etching operates in continuous inline spray chambers where ammonium hydroxide and ammonium chloride react with metallic copper to form soluble cupric ammonium complexes. Dissolution occurs as cupric ions oxidize copper metal to the cuprous state, which is subsequently re-oxidized by dissolved oxygen.
The chemical bath maintains a pH range between 8.1 and 8.5 to prevent hydroxide precipitation while preserving the underlying laminate base. Copper dissolution stops at the metallic plating boundary.
Process Control
Automatic dosing equipment maintains steady specific gravity and oxidation-reduction potential inside the spray module. High cupric ion concentrations reduce the etching speed, which requires periodic replenishment with fresh chemistry. Spray pressure settings control solution turnover across circuit tracks.
Differential fluid movement creates variation in track widths between panel centers and edges.
Lateral Overetch
Chemical dissolution undercuts the resist edge as fluid penetrates laterally into the copper sidewall. Copper thickness governs horizontal loss, which engineers quantify through a ratio of vertical depth to lateral attack. Etch factor values improve when spray nozzles operate under balanced pressures.
Excessive lateral attack reduces signal trace widths below electrical requirements.
