Definition

A test harness is a custom fixture that interfaces between test equipment and a PCB under test, providing mechanical support and electrical connections for automated testing. Bed-of-nails fixtures use spring-loaded pogo pins that contact test points when the board is pressed down. Cable harnesses connect through production connectors for functional testing. Vacuum fixtures hold boards in position during high-speed testing. For PCB designers, test harness requirements affect layout: provide accessible test points (typically 0.9-1mm pads) on a regular grid where possible, ensure adequate clearance for pogo pins (1.9mm minimum between points), place test points on one side only for single-sided fixtures, and use production connectors that can handle repeated mating cycles. Discuss test strategy with your CM early - fixture design often drives test point placement.