Cross Hatched Ground Plane Geometry Optimization for High Frequency Signal Integrity
Cross-hatched ground plane geometry requires balancing mesh pitch, line width, and trace bias angle to prevent impedance elevation and slow-wave phase delay.
Photoresist Undercut
Etch Profile
Lateral erosion happens beneath the protective mask layer during the chemical removal of unwanted copper. This phenomenon, known as photoresist undercut, results from the isotropic nature of liquid etchants which attack the metal horizontally as well as vertically. The resulting profile features a copper trace that is narrower at the top interface than at its base.
Dimensional Change
Design calculations must account for the reduction in conductor width caused by the etchant penetrating under the resist. Precision becomes difficult when the etch factor is inconsistent across the panel. Narrower traces increase the DC resistance of the circuit.
Fine pitch components suffer from reduced pad surface area which leads to weaker solder joints.
Process Control
Spray pressure and chemical concentration determine the severity of the horizontal removal rate. Automated optical inspection identifies traces where the width falls below the minimum specification. Over etching frequently leads to complete separation of thin features from the substrate.
Fabricators adjust the initial artwork dimensions to compensate for the expected lateral loss. Careful monitoring of the bath temperature and conveyor speed ensures that the metal removal remains within the required bounds for high density designs. Maintaining a tight tolerance on the etch process is mandatory to minimize photoresist undercut.