Extension Boundary
Mechanical compression beyond the initial contact point ensures that all spring probes in a multi-point test head establish a secure electrical connection with the circuit board pads. This intentional compression, known as overtravel displacement, compensates for variations in planarity across the substrate and the tester. The distance is measured in millimeters or thousandths of an inch.
Maintaining this deflection within specified limits prevents damage to the probe assembly and the board.
Deflection Mechanics
Calculation of the spring force generated by the probe pins uses the spring rate and the applied overtravel displacement. As the test fixture closes, the pins compress beyond their touch point to break through surface oxides on the solder pads. This action ensures low contact resistance during the test run.
High levels of displacement can cause probe bending or excessive pad indentation, which creates cosmetic defects or functional damage. Low levels of displacement fail to establish reliable electrical contact, which increases false rejection rates during functional testing.
Wear Mechanism
Prolonged use of the probe fixture leads to spring fatigue and a reduction in the effective overtravel displacement. This physical degradation causes the test pins to lose their pre-load force. Regular maintenance and pin replacement are required to prevent test errors.
When the system operates with degraded probes, the resulting contact instability leads to erroneous test outputs.
