Printed Circuit Assembly Test Point Access Guidelines
Test point geometry and access density directly dictate structural fault coverage, fixture expense, signal integrity, and field escape liabilities across production batches.
Test Point Geometry
Landing Pattern
Physical layout and shape of a copper pad are designed specifically for contact by automated test probes. Defining the test point geometry involves choosing a diameter and a surface finish that facilitate reliable electrical coupling during the inspection phase. These pads are usually circular or square and are placed on a specific grid to match the capabilities of a fixture.
The surface must be flat and free of solder mask to prevent the probe from slipping or failing to make contact.
Mechanical Interface
Probes used in bed of nails fixtures exert considerable downward force on the board surface. The test point geometry must be large enough to accommodate the slight positional variations of the pogo pins. If a pad is too small, the probe might hit the surrounding solder mask or the edge of the copper, resulting in a false failure.
Standard designs often use a diameter of thirty five to forty mils for general purpose testing. Smaller pads are possible but require higher precision fixtures and more frequent maintenance of the probe tips.
Solder Mask Clearance
Exposed copper requires a protective opening in the solder resist layer. This clearance area in the test point geometry ensures that the probe tip hits only the metal. Keeping the opening centered is a primary task for the fabrication shop to avoid insulation interference.