Probe Contact Geometry
Tip radius indentation designates the permanent deformation left in a printed circuit board pad after mechanical probing during in-circuit testing. Circuit board fabrication introduces mechanical stress when spring loaded test pins engage test points on copper surfaces during functional verification. Excessive force or improper stylus geometry damages the underlying laminate structure beneath the plated finish.
Pin wear compounds this risk because flattened tips increase the contact area and concentrate pressure differently than a sharp point.
Stylus Force Mechanics
Test fixture manufacturers design probes to overcome surface contamination without puncturing the protective solder mask or underlying copper foil. The depth of the resulting crater depends entirely on spring preload and the physical radius of the stylus tip. High spring rates generate rapid displacement profiles that exceed the yield strength of the copper layer during initial touchdown.
Operators monitor plunger velocity and stroke travel distance to prevent excessive penetration into the substrate material during high volume electrical verification.
Acceptance Criteria
Quality inspectors evaluate residual craters against maximum allowable depth thresholds established by fabrication standards for bare boards and assembled printed circuit boards. Microsection analysis reveals whether the mechanical impact fractured the intermetallic compound layer or cracked the barrel wall of plated through holes. Boards exhibiting fractures near the probe mark fail visual inspection because subsequent thermal cycling causes open circuits in operational environments.
Production facilities replace worn test fixtures immediately upon detecting an upward drift in average indentation depths across sampled production lots.
