2.1 HVAC Monitoring, Alarms & Control Limits

The manufacturing environment is not just "air"; it is a critical process variableingredient. In electronics assembly, invisible atmospheric shifts cause visible failures. A 10% drop in electronics assembly. Deviations in temperature and humidity directlycan correlatespike toESD defectdefects generationby in200%, while a 5°C rise destroys solder paste printingrheology.

(viscosity changes), component reliability (moisture absorption), and electrostatic discharge (ESD) events.

This chapter establishesdefines the non-negotiableAtmospheric atmospheric windowsEnvelope required to maintain process capability (Cpk > 1.33) for high-reliability PCBA production.

The Humidity Window (40% – 60% RH) 

Relative Humidity (RH) is the single most volatile variable in the factory. It must be actively regulatedfought, betweennot 40%just andmonitored.

60%.

The Physics of Failure:

• LowIf HumidityRH Risk< 30% (<30%)Dry):
  • ESD Risk: DielectricAir breakdownloses strengthdielectric ofstrength. air decreases, causing exponential increases in electrostaticInsulators charge generation. Solder paste dries out on the stencil, reducing tackiness and causing component release failures during placement.rapidly.
  • HighSolder HumidityPaste: RiskSolvents evaporate prematurely. Paste becomes dry/tacky, leading to clogged stencils and "insufficient solder" defects.
• If RH > 60% (>60%)Wet):
  • MSD Risk: Moisture Sensitive Devices (MSDs) absorb ambientwater like a sponge. During reflow, this water turns to steam, causing the package to explode ("popcorning").
  • Solder Paste: Paste absorbs moisture beyond(slumping). safeThis limitscauses (per J-STD-033), leading to "popcorning" (delamination/cracking) during reflow. Solder paste absorbs water (hygroscopic), causing voidingbridging and solder balling.

Control Logic:

• If RH drops below 35% -> Then Inject Steam/Ultrasonic Humidification immediately.
• If RH rises above 65% -> Then halt the opening of new MSD vacuum bags until levels stabilize.

Pro-Tip: Do not trust the wall thermostat. Place independent data loggers inside the SMT line, specifically near the Screen Printer. This is where the process is most sensitive.

Temperature Stability (22°C ± 3°C) 

Solder paste is a thixotropicnon-Newtonian fluid;fluid. Its viscosity—and therefore its viscosityability to print correctly—is dictated by temperature.

Thermal Constraints:

• Target: 22°C is highlythe temperature-dependent.industry Fluctuationsbaseline.
• Tolerance: >5°± 3°C.
• The Gradient Rule: The facility must not swing more than 1°C duringper ahour. shiftRapid altercooling/heating thecycles printingcause definition,thermal expansion mismatches in high-precision pick-and-place machines, leading to bridgingplacement ordrift.

insufficient

Zoning coverage.Logic:

• GradientHot Control:Zones: Reflow ovens generate massive heat loads. HVAC systemsreturns must preventbe localizedpositioned "hotdirectly spots"above nearoven reflowexhaust ovensvents orto "coldcapture spots"this nearheat loadingat docks.the source.
• Monitoring:Cold Zones: IndependentLoading docks are thermal leaks. Install air curtains or rapid-roll doors to preserve the facility envelope.

Filtration & Cleanliness (ISO Class)

Dust is the enemy of fine-pitch interconnection.

Most electronics facilities target ISO Class 8 (100k) or better.

Filter Maintenance Strategy:

1. Pre-Filters (G4/F7): The "sacrificial" layer.
   • Inspect: Monthly.
   • Replace: When pressure differential (∆P) > 250 Pa.
2. HEPA Filters (H13/H14): The "final defense."
   • Inspect: Annually via particle counter test.
   • Replace: Only when filter integrity fails or airflow drops below spec.

Pro-Tip: Never change filters while the HVAC is running. You will suck the dislodged dust cake directly into the cleanroom ductwork.

Alarms & Escalation

A silent alarm is useless. HVAC data loggersmust be integrated into the central BMS (Building Management System) with tiered alerts.

• Level 1 (Warning): Parameter drifts to limit (e.g., Temp 24.5°C).
  • Action: Email notification to Facilities Team. Check setpoints.
• Level 2 (Critical): Parameter breaches limit (e.g., RH < 30%).
  • Action: Strobe light / Siren on the production floor. Production Shift Leader must recordpause Temp/RHSMT at 15-minute intervals. Alarms must trigger if limits are breached for >1 hour.printing.

Final Checklist

Parameter	Specification	BreachCritical ConsequenceState
Relative Humidity	40% – 60% (Non-Condensing)RH	ESDStop (Work if <40%) 30% or Popcorning> (>60%)70%
Temperature	22°C ± 3°C	StablePaste viscosity(< changes1°C / Print defectsshift/hour)
Data Logging	ContinuousInterval (≤ 15 min interval)mins	AuditActive failure& /Backed Root cause blindnessUp
RecoveryFilter Change	<Based 2on hours∆P (Pressure Drop)	Logged
Overpressure	Production Halt> Corridor	Positive (+5 to 15 Pa)