In Circuit Test Fixture Coverage and Unprobed Assembly Netlist Limits
Unprobed netlists drop structural fault coverage, requiring integrated boundary scan vectors and adjusted warranty reserves to cover escape risks.
Spring Probe Pitch
Mechanical Constraint
In-circuit test fixture design is governed by a physical dimension that specifies the minimum center-to-center distance between adjacent test pins on a printed circuit board. This mechanical constraint, known as spring probe pitch, dictates the density of the test points that a fixture can access during board verification. It determines whether a fixture can utilize standard, rugged probes or must instead deploy fragile, ultra-fine probes to contact dense component leads or microvias.
As board designs shrink, fixture designers must choose probes that match the test point spacing without causing mechanical interference between the probe barrels.
Probe Selection
Standard probe pitches are typically set at one hundred mils or seventy-five mils, but high-density boards frequently require fifty mils or even thirty-nine mils. Smaller spring probe pitch values require thinner probe barrels and more delicate internal springs, which wear out much faster and are more prone to bending or breaking during high-volume testing. These ultra-fine probes are also more expensive and require more frequent maintenance to ensure consistent electrical contact.
Designers must place test points with sufficient spacing whenever possible to avoid these high-maintenance probes.
Structural Integrity
Drilling tolerances for the fixture plates also become critical at narrow probe spacing. A small drilling deviation can cause the probe tip to miss the board pad completely.