Optical Window
Optical parameters define the range over which an imaging system maintains a specified level of image sharpness without refocusing. Laser direct imaging and automated optical inspection systems rely on depth of focus to compensate for board thickness variations or slight substrate warpage. When the distance between the lens and the target shifts beyond this threshold, the resulting image blurs and causes false rejects or registration errors.
Focal Range
Resolution and aperture size dictate the axial distance that remains sharp. A high numerical aperture lens increases resolution but shrinks the depth of focus substantially. Extremely flat vacuum tables are required to keep the entire panel surface within the sharp imaging zone.
Inspection Tolerance
Tools for solder mask or primary circuit imaging must balance throughput against the narrow window allowed by the optics. If the substrate topology exceeds the depth of focus, the pattern edges lose definition and lead to undercut or over-etching during subsequent chemical processes. Automated systems often use dynamic z-axis adjustment to stay within this limit across a large panel, but this adjustment increases the total scan time for every unit.
This trade off between speed and precision defines the operational limits of high end lithography equipment.
