Printed Circuit Assembly Test Point Access Guidelines
Test point geometry and access density directly dictate structural fault coverage, fixture expense, signal integrity, and field escape liabilities across production batches.
Tooling Hole Tolerance
Dimensional Variation
A dimensional manufacturing limit defines the allowable variation in diameter and location for unplated alignment holes drilled into printed circuit board panels. During automated surface mount assembly and test fixture alignment, tooling hole tolerance governs the mechanical registration precision between the circuit board panel and assembly machinery pins. Dimensional tolerance boundaries stop applying once panels are depanelized into individual board assemblies without alignment features.
Fixture Alignment
Precision machinery relies on unplated tooling holes to locate panels accurately during stencil printing and pick and place positioning. PCB fabrication processes incur slight variations in hole diameter and location due to drill bit wear and laminate shrinkage. Oversized or misplaced tooling holes create mechanical play when panels drop onto fixture alignment pins, causing positional offset across the entire board surface.
Registration shifts during solder paste printing result in paste mis-alignment relative to copper land patterns, producing solder bridging or insufficient fillets during reflow. Test fixture pins failing to align precisely with target tooling holes can damage spring probes or misfire against adjacent trace conductors. Board fabricators specify tight drilling tolerances and enforce optical hole position inspection to maintain mechanical registration across volume manufacturing runs.
Registration Accuracy
Strict tooling hole tolerance ensures precise board registration during automated stencil printing and test fixture loading. Minimizing alignment play prevents component misplacement and solder bridging defects.