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Fixture Validation ​

The following provides an overview of how we approach validating mechanical-only fixture builds.

How to Validate Fixtures with Auto-TPCBs ​

Overview ​

After assembling a bed-of-nails test fixture with an Auto-TPCB (Automated Test Point Carrier Board), proper validation is critical to ensure reliable functional testing of circuit boards. This validation process consists of two essential steps that verify both physical contact and electrical connectivity.

What is an Auto-TPCB? ​

An Auto-TPCB is a custom PCB that serves as an interface between test points/receptacles on the fixture and connectors. It routes signals from individual test points to organized connector pins, simplifying the connection to test equipment and improving signal integrity.

Validation Process ​

Step 1: Probe Contact Testing ​

Purpose: Verify that all test probes make proper physical contact with the Device Under Test (DUT).

Procedure:

  1. Prepare a Fresh DUT: Use a new or clean circuit board that hasn't been previously tested
  2. Perform Test Cycle: Run the DUT through a complete test cycle on the fixture
  3. Microscopic Inspection:
    • Remove the DUT from the fixture
    • Use a microscope to examine each test point on the DUT
    • Look for contact marks left by the test probes
    • Document any test points that show no contact marks

What to Look For:

  • Clear, consistent contact marks on test points
  • Marks should be centered on the test points
  • All critical test points should show evidence of contact
  • No damage to the DUT test points

Common Issues:

  • Missing contact marks indicate probe misalignment or insufficient pressure
  • Off-center marks suggest mechanical positioning problems
  • Damaged test points may indicate excessive probe pressure

Step 2: Electrical Conductivity Testing ​

Purpose: Verify electrical continuity from each test point through the complete signal path.

Equipment Needed:

  • Digital multimeter with continuity testing ("beep" function)
  • Test probes or appropriate connectors

Procedure:

  1. Test Point to Auto-TPCB Connection:

    • Place one multimeter probe on the test point/receptacle
    • Place the other probe on the corresponding connector pin on the Auto-TPCB
    • Verify continuity (multimeter should beep)
    • Record any open circuits or high resistance connections

  2. Test Point to Feedthrough Plate (if applicable):

    • Test continuity from each test point to its corresponding pin on the feedthrough plate
    • Ensure the signal path is complete through all intermediate connections
    • Verify proper routing matches the design specifications

Documentation:

  • Create a continuity test matrix showing all test points and their corresponding connector pins
  • Note any failed connections for rework
  • Record resistance values if specified in design requirements

Troubleshooting Common Issues ​

No Contact Marks ​

  • Cause: Probe not making contact
  • Solution: Adjust probe height, check for bent probes, verify DUT thickness

Intermittent Continuity ​

  • Cause: Poor solder joints, loose connections, or contamination
  • Solution: Inspect and rework solder joints, clean connections

Wrong Pin Mapping ​

  • Cause: Wiring error or incorrect Auto-TPCB design
  • Solution: Verify against design documentation, check wiring harness

Best Practices ​

  1. Documentation: Maintain detailed records of all validation tests
  2. Consistency: Use the same validation procedure for all fixtures
  3. Regular Re-validation: Re-test fixtures periodically or after repairs
  4. Quality Control: Have validation results reviewed by a second technician
  5. Calibration: Ensure test equipment is properly calibrated

Quality Assurance Checklist ​

  • [ ] All test points show proper contact marks
  • [ ] Continuity verified for all signal paths
  • [ ] Auto-TPCB connections tested
  • [ ] Feedthrough plate connections verified (if applicable)
  • [ ] Documentation completed
  • [ ] Any issues identified and resolved
  • [ ] Fixture ready for use

Conclusion ​

Proper validation of fixtures with Auto-TPCBs is essential for reliable circuit board testing. The two-step process of probe contact testing and electrical continuity verification ensures that the fixture will perform accurately in production. Following these procedures helps prevent false test results and maintains the integrity of the functional testing process.