DUT Locating

Proper PCBA positioning is critical for creating a reliable test fixture. Misaligned boards lead to missed test points, damaged probes, and inconsistent test results. This guide covers the primary methods for accurately locating the Device Under Test (DUT) within a fixture.

Primary Locating Methods#

Spring Guide Pins#

Spring guide pins are the preferred method for accurately locating most DUTs. These conical-tip, spring-loaded pins install into the fixture's probe plate and engage with mounting holes on the PCB.

Specifications:

• Accommodate hole diameters from approximately 2.5mm to 5mm
• Spring loading compensates for board thickness variation
• Development fixtures use generic GP-2UL pins
• Production fixtures use Misumi PGPU pins for enhanced durability

Spring guide pins work well when the DUT has existing mounting holes that can serve double duty for fixture alignment.

Tooling Pins#

For applications needing smaller holes (below 3mm) or side actuation, fixed tooling pins provide an alternative. Sources include Ingun, McMaster, and Misumi.

• Production fixtures typically mount tooling pins directly to the moving plate
• Pins can be press-fit or screw-mounted depending on application
• Consider pin material and hardness for expected cycle life

Custom Cradles#

When DUTs lack suitable mounting holes, custom cradles provide reliable positioning. Cradles can be fabricated from:

• 3D-printed PETG or PLA — Quick turnaround for prototyping
• Machined Acetal (Delrin) — Higher durability for production

Cradles mount to the probe plate using standard guide pin mounting points and capture the board edges or features for precise location.

Design Specifications#

Best Practices#

Obtain Accurate DUT Models#

Before designing the locating system:

• Source accurate STEP models or DXF files of the PCB
• Verify mounting hole locations and tolerances
• Account for PCB manufacturing methods (machined vs. V-grooved edges)

Probe Plate Design#

• Size probe holes to probe tip diameter plus 1mm clearance
• Add chamfered edges to probe holes for easier probe insertion
• Include alignment features that match DUT mounting holes

Consider Board Manufacturing#

Different PCB edge treatments affect cradle design:

• Routed boards — Tighter tolerances possible (0.1mm offset)
• V-grooved boards — Looser tolerances required (0.2mm offset)

The breakaway tabs from V-groove panelization create variable edge conditions that the cradle design must accommodate.

Ease of Use#

• Add chamfered entry features for easier board loading
• Consider operator viewing angle when placing the DUT
• Mark orientation indicators for correct board placement

Verification#

After fixture assembly:

1. Test with several production boards to verify consistent alignment
2. Check probe contact marks on a test board
3. Verify repeatable test results across multiple loading cycles

Related Documentation#

• DUT Presence Detection — Verify board is properly loaded
• Fixture Bases — Base specifications and mounting
• Dev Fixture Guidelines — Development fixture specifics