In-circuit Test - Bed of Nails Tester

Bed of Nails Tester

A bed of nails tester is a traditional electronic test fixture which has numerous pins inserted into holes in an Epoxy phenolic glass cloth laminated sheet (G-10) which are aligned using tooling pins to make contact with test points on a printed circuit board and are also connected to a measuring unit by wires. Named by analogy with a real-world bed of nails, these devices contain an array of small, spring-loaded pogo pins; each pogo pin makes contact with one node in the circuitry of the DUT (device under test). By pressing the DUT down against the bed of nails, reliable contact can be quickly and simultaneously made with hundreds or even thousands of individual test points within the circuitry of the DUT. The hold-down force may be provided manually or by means of a vacuum, thus pulling the DUT downwards onto the nails.

Devices that have been tested on a bed of nails tester may show evidence of this after the fact: small dimples (from the sharp tips of the Pogo pins) can often be seen on many of the soldered connections of the PCB.

Typically, four to six weeks are needed for the manufacture and programming of such a fixture. Fixtures can either be vacuum or press-down. Vacuum fixtures give better signal reading versus the press-down type. On the other hand, vacuum fixtures are expensive because of their high manufacturing complexity. The bed of nails or fixture as generally termed is used together with a in-circuit tester.

This technique of testing PCB's is being slowly superseded by boundary scan techniques (silicon test nails), automated optical inspection, and built-in self-test, due to shrinking product sizes and lack of space on PCB's for test pads.