Measurement Principle
Non-contact optical profilometry techniques capture surface topography across three dimensions by casting structured sinusoidal light patterns onto an object and recording the phase distortion with digital sensors. Surface mount assembly facilities rely on fringe projection within automated solder paste inspection and post-reflow automated optical inspection systems to determine the height, shape, and volume of wet solder deposits and populated components. Digital light processing projectors display alternating bands of bright and dark lines onto the target circuit board.
When these linear fringe patterns intersect raised solder deposits or component leads, the lines bend and displace proportionally to the physical height of the features. Calibrated cameras acquire multiple images as the fringe patterns shift through fractional wavelength increments, enabling computing processors to calculate absolute coordinate heights across every pixel. The measurement ceases to provide height information over completely mirror-specular surfaces or totally opaque vertical overhangs where reflected light fails to reach the receiving optical sensor.
Phase Processing
Mathematical extraction of topological heights requires phase shifting algorithms combined with geometric phase unwrapping routines. Standard implementations project three or four sinusoidal patterns, each shifted by a defined phase angle such as ninety degrees. A CMOS imaging sensor captures each sequential image frame under constant exposure conditions.
Calculating the arctangent of the intensity differentials at each image coordinate yields a phase map wrapped between negative pi and positive pi. Phase unwrapping algorithms resolve cyclic ambiguities by tracking continuous wave phase changes across adjacent pixels or using multi-frequency fringe patterns to define absolute surface step heights. Triangulation geometry between the known projection angle and the camera sensor optical axis then maps the unwrapped phase value directly into physical micrometric height values.
Production Verification
Solder paste verification algorithms convert the generated surface point cloud into area, height, and volumetric metrics for every printed pad on the assembly panel. Solder bridging, coplanarity deviations, insufficient paste volume, and dog-ear peaks stand out clearly against baseline PCB laminate coordinates. Warpage compensation routines continuously subtract the calculated substrate topography from solder deposit heights, preventing board flex from falsifying solder thickness measurements.
Dual-projection setups illuminate parts from opposing angles to eliminate shadowing effects produced by tall adjacent surface-mount parts. These non-destructive optical readings enable automated solder printing systems to adjust stencil separation speeds, squeegee pressure, and cleaning cycles before defects progress into reflow ovens.
