Fixture Validation

This guide explains how to validate mechanical-only fixture builds, specifically focusing on fixtures equipped with Auto-TPCBs (Automated Test Point Carrier Boards) that route signals from probes to organized connector pins.

What is an Auto-TPCB?#

An Auto-TPCB is a custom PCB interface that routes signals from individual test points to organized connector pins, simplifying the connection to test equipment. Rather than running individual wires from each probe, the TPCB consolidates all signal routing on a manufactured board.

Validating that every probe makes contact and every signal path is complete ensures the fixture will perform reliably in testing.

Two-Step Validation Process#

Step 1: Probe Contact Testing#

Verify that all probes make physical contact with the DUT.

Procedure:

1. Load a fresh (untested) circuit board into the fixture
2. Run through a complete test cycle—close the fixture to engage probes
3. Remove the board and inspect under magnification
4. Document any test points lacking visible contact marks

What to Look For:

• Small indentations or marks from probe tips
• Consistent mark patterns across all test points
• Missing marks indicate non-contacting probes

Contact marks confirm the probe engaged the test point. Missing marks require investigation—the probe may be misaligned, stuck, or the test point may be obstructed.

Step 2: Electrical Conductivity Testing#

Verify complete signal paths from test points through the TPCB to connector pins.

Equipment:

• Digital multimeter with continuity function

Procedure:

1. Set multimeter to continuity mode
2. For each test point:
   • Place one probe on the DUT test point
   • Place the other probe on the corresponding TPCB connector pin
   • Verify continuity (audible beep or low resistance reading)
3. Document any failed paths

This confirms that:

• The probe receptacle connection is good
• TPCB traces are intact
• Connector solder joints are complete

Troubleshooting#

Missing Contact Marks#

No Continuity#

Intermittent Results#

Best Practices#

Documentation#

• Record validation results for each fixture
• Note any deviations or repairs made
• Keep validation records with fixture documentation

Consistent Procedures#

• Use the same validation process for every fixture
• Train operators on proper validation technique
• Create checklists to ensure no steps are missed

Periodic Re-validation#

• Re-validate after any fixture repair or modification
• Consider periodic validation for high-cycle fixtures
• Track validation history to identify wear patterns

Peer Review#

• Have a second person verify validation results
• Cross-check documentation against actual fixture
• Review questionable results together

Equipment Calibration#

• Verify multimeter accuracy periodically
• Use known-good cables for testing
• Replace worn test leads

Validation Checklist#

Before releasing a fixture for production:

• All probes show contact marks on test board
• All signal paths show continuity
• No intermittent connections
• Documentation complete
• Peer review completed
• Fixture labeled and tracked

Why Validation Matters#

Proper validation prevents:

• False test failures — Good boards failing due to missed probe contact
• False passes — Bad boards passing due to incomplete test coverage
• Production delays — Troubleshooting fixture issues during production
• Quality escapes — Defective products reaching customers

Related Documentation#

• TPCB Guide — Test Point Carrier Board design
• Selecting Probes — Probe selection and tips
• DUT Presence Detection — Board presence verification