Etch Geometry
Chemical dissolution of copper during the fabrication process leaves lateral voids beneath the photoresist layer that define the trace undercut ratio. This measurement compares the horizontal width of the removed copper underneath the mask to the vertical depth of the etched channel. Fabricators utilize cross sectional analysis of printed circuit boards to determine the extent of this lateral chemical migration.
Proper control of this ratio prevents the formation of fragile overhangs that fracture during subsequent plating or assembly cycles.
Process Stability
High ratios indicate excessive dwell time in the etching chemistry or elevated concentration levels that compromise structural integrity. Automated optical inspection systems identify these deviations by monitoring changes in impedance profiles caused by the altered copper geometry. Maintaining a narrow range for this parameter ensures uniform signal transmission across high frequency circuits.
Variations in board density or chemistry age force adjustments in spray pressure to manage the chemical kinetics at the interface.
Assembly Consequence
Fragile copper geometries created by poor control frequently suffer from pad cratering when components undergo thermal cycling. Designers account for these edge profiles by specifying annular ring allowances that compensate for potential material loss at the trace base. Standard IPC documentation provides the benchmarks for acceptable lateral erosion based on the thickness of the copper foil.
Reliable electrical connectivity depends on the retention of a stable base width after the removal of the masking layers.
