Annular Ring Tolerances a Volume Shop Actually Holds
Volume PCB shops hold a real-world minimum annular ring of 0.002 inches for Class 2 designs when artwork grants 0.005 inches of nominal land margin over tool size.
Optical Targets
Fiducial Geometry
Geometric patterns etched onto the surface of a circuit board provide fixed points for automated assembly equipment. These optical targets establish a coordinate system that allows pick and place machines to calculate the precise position of components relative to the substrate. Accurate alignment prevents solder bridges and open circuits during the manufacturing process.
Alignment Reference
Machine vision systems identify these contrast-rich shapes to compensate for substrate expansion or panel deformation during thermal cycles. Recognition software scans for the predefined shape to calculate rotational offsets and positional deviations. Detected errors trigger real-time physical adjustments to the placement head before the component reaches the board.
This correction maintains high yields during high-speed surface mount assembly.
Manufacturing Boundary
Global fiducials typically reside on the panel edge to define the global origin for all mounted parts on the entire board. Local fiducials sit near high pin-count integrated circuits to ensure extreme placement accuracy for fine-pitch leads. Fabrication tolerances dictate the minimum distance between the target and the board edge to guarantee visibility under camera lighting.
Design requirements mandate high contrast between the feature and the background material to ensure reliable recognition by sensors. Precision in target placement reduces the likelihood of rework during the final inspection phase.