Focus Metrology
Measurement technique that determines surface topography by identifying the sharpest focus of a broadband light source at different vertical positions allows for the capture of complex three-dimensional features. White light focus variation is effective for inspecting the steep sidewalls of deep holes and the rough surfaces of solder joints on a printed circuit board. It combines the small depth of field of an optical system with vertical scanning to reconstruct the geometry of the sample.
Topographic Mapping
Vertical resolution depends on the numerical aperture of the lens and the precision of the z-axis motor that moves the camera. As the system scans through different heights, the software analyzes the local contrast at each pixel to determine the exact point of focus. This data is then used to create a color-coded map showing the height variations across the surface of the board.
High-density designs benefit from this method because it can resolve features that are too small for traditional mechanical probes. The resulting 3D model allows for the measurement of volume and area with high repeatability.
Inspection Benefit
Comparing the results of white light focus variation against the CAD model allows for the rapid detection of assembly errors like missing components or tilted pins. The technique is also used to measure the height of copper traces. Because it uses standard light and does not require a vacuum, it is easily integrated into a production environment for real-time quality control.
