Contact Pressure
The mechanical load applied by a pogo pin against a test pad determines the reliability of an electrical connection during automated in-circuit inspection. Probe force constitutes the physical magnitude of this pressure, typically measured in grams, which must overcome oxidation on board surfaces to establish low-resistance signal paths. Designers specify requirements for this interaction based on the plating material, the geometry of the pin tip, and the expected spring fatigue over thousands of cycles.
Excessive weight causes physical damage to the delicate copper or solder finish, whereas insufficient pressure results in intermittent contact that creates false failures during board testing. Adjusting the compression spring rate within the fixture assembly ensures that every point receives consistent contact without deforming the PCB substrate.
Spring Constant
Manufacturers select specific internal springs to maintain probe force within the operational window dictated by the probe manufacturer. This value depends on the travel distance of the pin after initial contact with the target site. Variations in the alignment of the fixture plates alter the deflection of individual pins, changing the load exerted on the pads.
If the assembly depth deviates from the print, the resulting change in displacement shifts the pressure outside the acceptable tolerance. Technicians monitor this variable across the entire fixture to prevent uneven wear on the test head. A stable spring rate allows the test equipment to maintain high signal fidelity across dense interconnect arrays on complex boards.
Reliability Threshold
Engineers establish the minimum probe force necessary to break through solder mask debris or contaminants that migrate onto test pads during reflow processes. Lower limits prevent the probe tip from slipping off small targets during high-speed contact sequences. Once a pin hits the surface, the force must stay high enough to keep the connection closed despite the vibration produced by the test hardware.
High contact loads eventually lead to hole creation in the pad metallurgy or trace lifting on flexible circuits. Proper calibration of this mechanical requirement prevents unnecessary board scrap during the final production stage.
