2.1 HVAC Monitoring, Alarms & Control Limits

The manufacturing environment is not just "air"; it is a critical process ingredient. In electronics assembly, invisible atmospheric shifts cause visible failures. A 10% drop in humidity can spike ESD defects by 200%, while a 5°C rise destroys solder paste rheology.

This chapter defines the Atmospheric Envelope 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 fought, not just monitored.

The Physics of Failure:

• If RH < 30% (Dry):
  • ESD Risk: Air loses dielectric strength. Insulators charge rapidly.
  • Solder Paste: Solvents evaporate prematurely. Paste becomes dry/tacky, leading to clogged stencils and "insufficient solder" defects.
• If RH > 60% (Wet):
  • MSD Risk: Moisture Sensitive Devices absorb water like a sponge. During reflow, this water turns to steam, causing the package to explode ("popcorning").
  • Solder Paste: Paste absorbs moisture (slumping). This causes bridging 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 non-Newtonian fluid. Its viscosity—and therefore its ability to print correctly—is dictated by temperature.

Thermal Constraints:

• Target: 22°C is the industry baseline.
• Tolerance: ± 3°C.
• The Gradient Rule: The facility must not swing more than 1°C per hour. Rapid cooling/heating cycles cause thermal expansion mismatches in high-precision pick-and-place machines, leading to placement drift.

Zoning Logic:

• Hot Zones: Reflow ovens generate massive heat loads. HVAC returns must be positioned directly above oven exhaust vents to capture this heat at the source.
• Cold Zones: Loading 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 must 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 pause SMT printing.

Final Checklist