Material Removal
Chemical fabrication defect occurs when the etchant removes copper from beneath the protective dry film or metal resist. Trace etch undercut reduces the width of the conductor at its base, leading to a trapezoidal cross-section rather than a rectangular one. This reduction in volume increases the resistance of the trace and can alter the expected impedance of the circuit.
Controlling the etch rate and the chemistry concentration helps to minimize this effect during volume production.
Aspect Ratio
Geometric limitation defines the relationship between the thickness of the copper and the minimum achievable trace width. As the copper becomes thicker, the impact of trace etch undercut becomes more pronounced because the etchant has more time to move horizontally. For high-density designs, fabricators use thinner copper foils to maintain better control over the trace geometry.
The etch factor is a ratio used to quantify this horizontal removal relative to the vertical depth of the etch.
Process Control
Quality maintenance focuses on the regulation of the conveyor speed and the spray pressure in the etching machine. When trace etch undercut is kept within specified limits, the signal integrity of the board remains predictable. Automated optical inspection systems are used to measure the width of the traces at the top and bottom to verify the etch quality.
Proper maintenance of the chemical bath ensures a consistent etch rate across the entire production lot.
