Test Fixture Design and Management

Test Fixture Design and Management with TofuPilot

The test fixture is the most underappreciated part of hardware test infrastructure. A worn pogo pin or a loose cable causes more false failures than bad product. TofuPilot helps you monitor fixture health through measurement trends and station-specific analytics.

What Makes a Test Fixture

A test fixture connects the DUT (device under test) to the test instruments. It typically includes:

Monitoring Fixture Health Through Test Data

You don't need dedicated fixture tests. Your production test data already contains fixture health signals.

Contact Resistance as a Health Indicator

If your test includes a continuity or resistance measurement, track it per station.

from tofupilot import TofuPilotClient

client = TofuPilotClient()

# This measurement doubles as a DUT test AND a fixture health indicator
client.create_run(
    procedure_id="BOARD-FUNCTIONAL",
    unit_under_test={"serial_number": serial},
    run_passed=True,
    steps=[{
        "name": "Contact Verification",
        "step_type": "measurement",
        "status": True,
        "measurements": [
            {"name": "probe_contact_mohm", "value": 35, "unit": "mohm", "limit_high": 100},
        ],
    }],
)

Track probe_contact_mohm per station over time:

Station 3's contact resistance is climbing fast. Service the pogo pins before it hits the 100 mohm limit and starts causing false failures.

Measurement Variance as a Health Indicator

Even when measurements pass, increasing variance signals fixture problems.

A voltage measurement that bounces between 3.29V and 3.33V has more variance than one that reads 3.31V consistently. High variance on a specific station means something is intermittent: a loose wire, a dirty probe, or a worn alignment pin.

Cycle Time as a Health Indicator

If a test normally takes 42 seconds and starts taking 55 seconds on one station, something changed:

• Pneumatic actuator slowing down (low air pressure, seal wear)
• Instrument communication timeout (loose cable, aging connector)
• Retry loops from intermittent contacts

Fixture Lifecycle Tracking

Maintenance Schedule Based on Data

Replace calendar-based maintenance with data-driven maintenance.

Condition-based maintenance costs less and catches problems earlier. TofuPilot's measurement trends tell you which fixtures need attention now and which are fine.

Tracking Pogo Pin Life

Pogo pins have a rated cycle life (typically 100,000-1,000,000 cycles). Track cycles per fixture.

# Track fixture cycle count alongside DUT test
client.create_run(
    procedure_id="BOARD-FUNCTIONAL",
    unit_under_test={"serial_number": serial},
    run_passed=True,
    steps=[{
        "name": "Contact Verification",
        "step_type": "measurement",
        "status": True,
        "measurements": [
            {"name": "probe_contact_mohm", "value": contact_r, "unit": "mohm", "limit_high": 100},
            {"name": "fixture_cycle_count", "value": get_fixture_count(), "unit": "cycles"},
        ],
    }],
)

When contact resistance starts rising, check the cycle count. If you're at 80% of rated life, schedule replacement during the next planned downtime.

Fixture Design Best Practices

Golden Board Verification

A "golden board" is a known-good reference board. Run it through the fixture periodically to verify the fixture is working correctly.

# Weekly golden board verification
GOLDEN_SERIAL = "GOLDEN-001"
EXPECTED_VCC = 3.310  # Known value from initial characterization

vcc = measure_voltage()
drift = abs(vcc - EXPECTED_VCC)

client.create_run(
    procedure_id="FIXTURE-GOLDEN-BOARD-CHECK",
    unit_under_test={"serial_number": GOLDEN_SERIAL},
    run_passed=drift < 0.010,  # Less than 10mV drift
    steps=[{
        "name": "Golden Board Verification",
        "step_type": "measurement",
        "status": drift < 0.010,
        "measurements": [
            {"name": "vcc_3v3", "value": vcc, "unit": "V", "limit_low": 3.300, "limit_high": 3.320},
            {"name": "drift_from_reference_mv", "value": drift * 1000, "unit": "mV", "limit_high": 10},
        ],
    }],
)

If the golden board measurement drifts, the fixture or instrument has changed, not the board.

Common Fixture Failures and Detection

Multi-Fixture Management

For production lines with multiple fixtures for the same test:

1. Give each fixture a unique identifier
2. Track which fixture ran each test (station ID in TofuPilot)
3. Compare performance across fixtures
4. Rotate golden board checks across all fixtures

If Fixture A has 98% yield and Fixture B has 94% yield on the same product, Fixture B needs attention. TofuPilot's station comparison view shows this at a glance.