4.5 Environmental & Burn-in Testing

Environmental and burn-in testing is the stage where hidden weaknesses are forced to show themselves before a product reaches the field. By stressing units with heat, load, power cycling, and sometimes humidity, this process screens out early-life failures that slip past functional and safety checks. Unlike design qualification, its purpose is not to prove new concepts but to validate manufacturing consistency and component reliability under controlled, repeatable conditions. With profiles tied to the SKU and results bound to serial numbers, burn-in replaces guesswork with statistics, turning reliability into a measured outcome.

4.5.1 Why do this?

To flush out early-life failures (Chapter 16’s bathtub curve) and prove the unit survives heat, time, and power cycling before it meets a customer. We’re not qualifying the design (16.3)—we’re screening production and validating build consistency.

4.5.2 What counts as “environmental & burn-in”

Thermal stress: elevated ambient, thermal cycling, controlled ramps.
Electrical stress: constant load, power cycling, brownout/surge profiles.
Humidity (if applicable): steady RH or damp heat (only if parts are rated).
Operational soak: exercise fans, I/O, radios, storage—work while hot.
(Light vibration/transport shake belongs in 21.3 for harnesses; only include if your product spec requires.)

4.5.3 Typical burn-in profiles (pick the lightest that works)

Product class	Duration	Ambient	Load profile	Extras
Consumer/office	2–4 h	40–45 °C	50–80% duty; periodic I/O poke	3–5 power cycles
Industrial indoor	4–8 h	45–55 °C	70–100% duty; PWM/relay chatter	5–10 cycles; brownout dips
Outdoor/rugged	8–24 h	55–65 °C (per spec)	Near-max duty; RF/network active	Periodic thermal ramps (±10 °C), power cycles
Compute/network	8–24 h	45–55 °C rack/flow	CPU/NET stress (≥80%); storage R/W	Throughput check each hour

Start aggressive at NPI/ECOs (100% units), then taper to sampling when FPY is stable and field data supports it.

4.5.4 Chambers, racks & fixtures (keep it safe and uniform)

Burn-in racks: dedicated power distribution with current limit/fusing, airflow management, cable strain relief, interlocks.
Chambers: uniform airflow, monitored temp (±2 °C band), dew-point control to avoid condensation (see 25.5.6).
Loads: electronic loads, fan trays, network generators, RF attenuators/shield box if radios run.
Scripts: station launches the right recipe by SKU/Variant scan—no manual tweaks.

4.5.5 What to monitor (and how to decide pass/fail)

Log to MES by unit SN at 1–60 s cadence (pick a rate that shows trends without swamping storage):

Power: input current & voltage, inrush peaks on each cycle.
Thermals: hottest sensor/heatsink ΔT; fans RPM & tach faults.
System health: watchdog resets, error counters, crash logs, throughput (Eth/USB), storage SMART.
I/O activity: periodic loopbacks, LED/display keep-alive, relay/PWM chatter.
Pass if:
- No trips/resets/crash logs.
- Currents within golden bands; no rising trend.
- Hottest T ≤ spec − margin (set a 5–10 °C guard).
- All scheduled checks pass on the final cycle.
Fail fast: any smoke/odor, repeated watchdog, fan stall, over-temp, or unplanned power draw spike.

4.5.6 Thermal cycling & humidity (do it without causing damage)

Thermal cycling (production screen)

Ramps ≤ 5 °C/min; dwell 10–30 min at highs/lows you can justify (not HALT extremes).
Cycle count: 3–10 within the burn-in window for rugged products.

Humidity (only if the design & labels allow)

40 °C / 85% RH for 2–8 h as a stress screen (not qualification).
No condensation: keep chamber setpoint > dew point of the unit’s guts; avoid rapid door-open shocks.
After humidity segments, let units stabilize to room before any Hipot/IR (25.2) to avoid false fails.

4.5.7 Power cycling & brownouts (catch marginal power paths)

Cycle count: 5–20 over the burn-in; include cold start and hot restart.
Brownout dip: briefly drop input to the edge of spec (e.g., −20%) and recover; unit should not latch up or corrupt storage.
Inrush: capture peaks; compare to golden so swollen caps/failing soft-start stand out.

4.5.8 Safety & ESD in hot environments

Racks/chambers have E-stop, thermal cutouts, and door interlocks.
High temp reduces glove grip—use rated cable boots and strain relief.
Radios on? Follow RF safety and local RF rules (use shield boxes/attenuators).

4.5.9 Data & traceability (what to store)

Bind to SN:

Profile ID (temps, durations, ramp rates), recipe version, rack/chamber ID.
Time-series summaries: max current, max temp, min volt, cycle counts, fan RPM min.
Event logs: resets, errors, throttle events, protection trips.
Final PASS/FAIL and reason codes; any rework ticket ID.

4.5.10 Acceptance cues (fast table)

Area	Accept	Reject
Thermal margin	Hottest T ≤ Spec − 5–10 °C	Near/over spec; thermal throttling
Stability	Current flat or settling	Rising current trend, oscillations
Resets/errors	None	Watchdog/crash logs present
Fans	RPM within window	Stall, noisy bearings, tach error
Power cycles	All clean boots	Latch-ups, long boot drifts
Humidity (if used)	Post-soak IR/Hipot PASS	False fails from condensation; actual shorts

4.5.11 Common traps → smallest reliable fix

Trap	Symptom	First move
“Cook and hope” (no monitoring)	PASS but field returns	Log current/temps/resets; set guard bands
Over-aggressive humidity	Hipot fails, corrosion risk	Control dew point; dry to room before safety tests
Same profile for every SKU	Over/under screen	Profiles by SKU/Variant; scale load/temp
Long test on main line	Takt killer	Side loop racks; ping-pong fixtures
Manual recipe edits	Inconsistent stress	Scan-to-recipe; lock scripts; version in MES
Power cycling via mains switch	Nuisance surges	Use programmable PSU; timed sequences
Hot cables sagging	Intermittents	Strain relief; high-temp cable boots; route supports

4.5.12 Pocket checklists

Before

SKU/Variant scanned → correct profile loaded
Rack/chamber ID recorded; interlocks & E-stop OK
Loads/network scripts ready; fans unobstructed
Ambient/chamber sensors read sane; dew-point margin OK (if humidity)

Run

Start log: V/I/T, RPM, heartbeat up
Hold at temp; run I/O/throughput tasks
Execute power cycles & brownouts per plan
Watch trend tiles; fail fast on trips/resets/over-temp

After

Cooldown to room; post-burn-in FCT re-run
Safety tests (25.2) if required post-soak—after stabilization
Results & summaries to MES by SN; fails to NG-QUAR with reason

When implemented with discipline, burn-in testing raises confidence that shipped hardware will survive real-world use without early surprises. The benefit is fewer field returns, stronger customer trust, and production lines that can scale without carrying hidden risks.