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Last updated on: Jan. 3, 2024, 1:53 p.m.

Device Under Test Orientation

Typically, a Device Under Test (DUT) has test points constrained to either the bottom or top sides of the Printed Circuit Board (PCB). In most of these cases, the bed of nails test fixture is designed so that the circuit board’s test points are facing downward so they can contact the test probes (eg, pogo pins) mounted on a probe plate.

This results in two orientations for the DUT:

  • right-side up if the test points are on the bottom
  • upside down if the test points are on the top

However, when a DUT requires test points on both sides of the circuit board, ”dual-sided probing” is needed and will require a test fixture that supports top-side probing.

Top Probing

When designing a dual-sided probing bed of nails test fixture, the DUT orientation usually depends on whether there are more test points on one side or the other. Due to mechanical constraints, dual-sided probing is only supported in production-level test fixtures.

In addition to the customary Test Point Carrier Board (TPCB) that is mounted below the probe plate inside the test fixture base, FixturFab designs a custom top-probe plate with additional test probes and a top-located TPCB to facilitate routing all the signals to the primary TPCB board in the base.

Top-sided Probe Assy
Top-side Probe Plate Assembly

As the test fixture’s pressure plate closes, the top-located test probes will contact the test points located on the upward-facing side of the DUT.

Top-sided Probe Stackup
Top-sided Probe Plate Stack-up

Side Probing

Printed Circuit Board Assembly (PCBA) connectors are prime candidates for test point connectivity in bed of nails test fixtures. Oftentimes, auxiliary test points do not (or cannot, due to pitch issues, etc.) provide adequate access to these connectors, so the connectors must be engaged directly.

FixturFab offers a variety of options to satisfy side-connector probing requirements while removing the need for operator interaction with the connection points (automatic engagement/disengagement). We design custom hardware to automate manual connector insertion for various plug types, including hardware and cabling, to meet your probing needs. Side actuation features are only compatible with production-level test fixtures.

FixturFab utilizes a stroke-controlled side approach mechanism (SAM) that converts the vertical stroke movement of the test fixture (operating stroke) into a horizontal movement of the stroke bar on the SAM. Contract Stroke is designed to be a maximum of 16mm with an operating stroke of 7mm from the test fixture.

  • Maximum contact force: 150N
  • Maximum contact stroke: 16mm
  • Maximum operating stroke: 7mm
  • Maximum lever arm length: 60mm
  • Torque: 1.5Nm
  • Weight: 0.14kg
A demo of Ingun’s side approach mechanisms

A custom “side plate” is required to implement test fixture side probing. There are two primary ways that this can be used to implement side probing.

Composites
USB-C Side Actuation Mechanism Mounted in Test Fixture

Side-actuated Test Plugs

Test Plugs exist for most industry-standard PCBA connectors (USB, USB-C, ethernet, HDMI, etc) that are designed to interface with each inlet.

usb-test-plug
Ingun PS-USB xx USB-C Connectors
hdmi-test-plug
Ingun PS-HDMI Connectors

Side-actuated Test Probes

FixturFab can also design custom side plates to contact other kinds of connector test points on the side of the DUT — 0.1” pitch, male right-angle connectors, for example. Using pogo pins instead of the corresponding male test plug can greatly improve the number of test cycles the connection mechanism can sustain.

test-fixture-side-plate-wiring
Side Actuated Pogo Pin Interface
test-fixture-side-probe-plate-render
Test Fixture Side Actuated Pogo Pin to PCBA Pin Header Render

Conclusion

While bed of nails test fixtures can be designed to implement bottom, top, and side probing of test points and PCBA connectors, not all solutions are equal. It is significantly simpler to press a DUT down onto the test probes on the bottom of the test fixture.

The additional tolerances for the mechanical stackup for top and side probing test fixtures can be reliably handled, but this requires additional design time, manufacturing time, and higher quality materials that are only supported by production test fixtures. FixturFab often implements top and side probing, but it should be known that this increases the price and lead time of the test fixture. A good Design For Test rule is to limit the test points to a single side of a PCB. This drastically simplifies the test fixture design and will result in a faster, more affordable solution.

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