Acid Recess Depth
Chemical removal of copper from the printed circuit board traces beyond the designed mask boundary during the inner layer etching stage creates an undercut defect that compromises current carrying capacity. Etchant pooling inside the narrow vertical channel beneath the dry film resist proceeds laterally at a predictable rate once vertical breakthrough occurs. Copper foil thickness dictates the horizontal dissolution span because thicker base material requires longer immersion times in the cupric chloride or ammoniacal solution.
Automated optical inspection equipment flags excessive lateral eating when the measured trace width falls below the minimum limit specified in the fabrication drawing.
Conductor Cross Section
Geometric degradation reduces the effective copper area available for electrical transmission and concentrates mechanical stress during thermal cycling. Trapezoidal profile formation replaces the rectangular trace configuration once chemical action eats away the lower boundary adjacent to the laminate interface. Conductor resistance increases proportionally with the loss of metallic volume across the affected span.
Etch Factor Limit
Process engineers control lateral dissolution by adjusting conveyor speed, etchant temperature, and chemical regeneration rates to maintain a minimum ratio between vertical depth removal and horizontal side eating. High performance boards demand tight etch factors to ensure impedance control remains stable throughout multilayer lamination cycles. Vacuum assistance inside spray chambers evacuates spent etchant rapidly from narrow channels to prevent localized concentration gradients from accelerating lateral attack.
Final acceptance depends on cross sectional microsection analysis where metallographic preparation reveals the exact perimeter loss at the copper base.
