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4.5 Environmental and Burn-in Testing

Environmental Burn-In Testing is a high-stress, accelerated aging process applied to finished electronic systems to deliberately induce failure in weak components before the product reaches the customer. This process is mandatory for high-reliability products (IPC Class 3) as it filters out Infant Mortality — the high initial failure rate observed early in a product's life due to manufacturing defects or latent component flaws. A successful burn-in ensures a stable, reliable product upon shipment.

4.5.1 Protocol Mandate and Stressors

Burn-in is not a simple functional test; it involves simultaneous application of environmental and electrical stress tailored to the product's class.

A) Mandatory Stressors

  • Thermal Stress: Elevated ambient temperature and periodic thermal cycling (ramps ≤ 5˚C/min).
  • Electrical Stress: Constant dynamic load (near-max duty) and controlled power cycling (typically 5 – 20 cycles) to test inrush and sequencing.
  • Operational Soak: The unit must be functionally exercised: fans running, storage R/W active, I/O periodically checked, and network throughput verified.
  • Brownout Dips: Briefly dropping input voltage to the edge of spec (e.g., -20%) to verify the unit does not latch up or corrupt storage.

B) Typical Profiles (Starter Ranges)

Profiles must be specific to the SKU (Stock Keeping Unit) and scaled based on complexity and environment.

Product Class

Duration

Ambient

Load Profile

Power Cycles

Consumer/Office

2 – 4 h

40 – 45 ˚C

50–80% duty; periodic I/O poke.

3 – 5 cycles

Industrial/Rugged

8 – 24 h

55 – 65 ˚C

Near-max duty; network active.

5 – 10 cycles + thermal ramps.

Mandate: Testing must start aggressively at New Product Introduction (NPI), then taper to a sampling plan only when First Pass Yield (FPY) is stable and field data supports it.

4.5.2 Chamber Integrity and Monitoring

The uniformity of the test environment and the continuous logging of health metrics are critical for valid results.

A) Chamber and Fixture Requirements

  • Chamber Control: The chamber must maintain uniform airflow and monitored temperature (± 2˚C band).
  • Racks and Power: Burn-in racks must have dedicated power distribution with current limits/fusing and cable strain relief.
  • Safety: Racks/chambers require mandatory E-stop, thermal cutouts, and door interlocks. The ESD strap must be OFF during HV tests; rated cable boots must be used in hot zones.

B) Monitoring Mandates

Monitoring must be automated, high-cadence, and logged to the MES (Manufacturing Execution System).

  • Log Rate: Data must be logged at a rate sufficient to capture transient failures (e.g., 1 – 60 second cadence).
  • Monitored Health Metrics:
    • Power: Input current (including inrush peaks), voltage, and power consumption trends.
    • Thermals: Hottest sensor/heatsink ∆T (Delta Temperature); fan RPM and tach faults.
    • System: Watchdog resets, error counters, crash logs, and storage SMART data.

4.5.3 Acceptance Criteria and Failure Response

Acceptance is based on continuous stability and reaching a defined thermal margin, not just simple functionality.

A) Pass/Fail Criteria

  • Stability: Zero watchdog resets, crash logs, or unexpected power spikes. Current must be flat or settling (no rising trend).
  • Thermal Margin: The hottest monitored temperature (Tmax) must remain below the specified limit minus a margin (e.g., Tmax ≤ Spec - 5˚C guard band).
  • Critical Failures: Any smoke/odor, repeated watchdog, or unplanned power draw spike must trigger a fail fast response.

B) Data Traceability and Rework

  • Data Bind: The log must bind the Profile ID (temps, durations, ramp rates), Time-series summaries (max current, min RPM), and Event logs (resets, errors) directly to the unit Serial Number (SN).
  • Failure Response: All burn-in failures trigger immediate Corrective and Preventive Action (CAPA) investigation. The focus must be on finding the process flaw (e.g., incorrect torque application, component batch issue), not just component replacement.
  • Humidity Rework: If using humidity stress, units must be stabilized to room temperature before any Hi-Pot/IR testing to avoid false fails from condensation.

Final Checklist

Mandate

Criteria

Verification Action

Mortality Filter

Unit subjected to thermal and power cycling per profile (e.g., 40˚C to 55˚C).

Test profile ensures product is aged past the early failure phase.

Stability

Zero watchdog resets/crash logs; current flat or settling.

Log current and resets at a high cadence (e.g., 1 – 60 s).

Thermal Guard Band

Hottest temperature Tmax ≤ Spec - 5˚C margin.

Continuous logging of temperatures; log confirms no thermal throttling/over-temp.

Power Stress

Profile includes power cycling and brownout dips.

Inrush peaks captured and verified against the golden limit.

Contamination

If humidity is used, dew point controlled; unit stabilized before safety tests.

Audit prevents false Hipot fails due to condensation/moisture.

Traceability

Time-series summaries (current, temp, cycles) linked to the unit SN.

MES records profile ID and final reason codes.


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