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.
Target Pad Geometry
Contact Reliability
Board layout for automated testing must incorporate a physical layout feature that defines the shape, size, and mask clearance of the copper areas designated for test probe contact. This layout parameter, known as target pad geometry, determines the likelihood of a spring probe successfully landing on and making electrical contact with a test node. It balances the need for a large surface area to ensure high test reliability against the board space constraints of dense circuits.
Round pads are most common, but square or rectangular pads are sometimes used depending on trace routing requirements and probe tip selection. Properly sized pads prevent the probe from striking the surrounding solder mask, which can cause false failures and probe damage.
Layout Recommendation
Recommended dimensions for these pads depend on the positional accuracy of the test fixture and the board registration mechanism. For standard fixtures, a diameter of thirty-five mils is typical, while precision fixtures with guide pins can tolerate a diameter down to twenty-five mils. The target pad geometry must also specify a solder mask clearance that keeps the mask well away from the probe contact zone.
If the solder mask encroaches on the pad, the probe tip can ride up on the mask and fail to touch the copper.
Solder Interaction
Solder paste is often omitted from these pads during assembly to prevent probe contamination. This ensures that the probe makes direct contact with the clean copper or plated surface.