Etching Defect
Chemical copper removal processes can erode trace sidewalls beneath the protective etch resist during printed circuit board fabrication. This over-etching behavior leads to a foot undercut, which narrows the base of the copper conductor where it meets the board substrate. If the etchant acts too long or at an incorrect temperature, the trace profile changes from a trapezoid to a mushroom shape.
This defect reduces the contact area between the copper trace and the underlying dielectric material. Consequently, it weakens the mechanical strength of the trace, posing a risk of separation during the thermal shocks of the assembly process.
Mechanical Reliability
Trace adhesion depends heavily on the width of the conductor base bonded to the substrate. When foot undercut occurs, the trace becomes vulnerable to peeling under thermal stress or during manual soldering. Quality inspectors use cross-sectional analysis to measure the extent of this copper loss.
Electrical Impedance
High-frequency signals require precise trace geometries to maintain consistent electrical impedance throughout the transmission line. Erodence from a foot undercut alters the cross-sectional area of the conductor, resulting in impedance mismatches and signal reflections. Fabricators control this issue by optimizing chemical spray pressures and etch factor compensation during stencil production.
