2.4 Nitrogen / Vacuum / Exhaust Utilities

These are the "Circulatory and Respiratory" systems of the factory. While power and air turn the machines on, Nitrogen, Vacuum, and Exhaust determine if the process is capable of producing a reliable solder joint.

A fluctuation in Nitrogen pressure opens the door to oxidation. A drop in exhaust static pressure creates a "chimney effect" in the reflow oven, altering the thermal profile and overheating the PCB.

Nitrogen (N2) Distribution

In Section 2.3, we defined the purity (99.99%). This section controls the delivery.

The goal is Laminar Flow. Turbulence in the pipe knocks loose oxidation scales from the pipe walls, sending debris into the reflow oven's gas diffusers.

Engineering Mandates:

• Piping Material:
  • Allowed: Copper (Type L) or Stainless Steel (304/316).
  • Prohibited: Black Iron (rust) or PVC (brittle/static generation).
• Pressure Stability:
  • Target: 5.0 Bar ± 0.2 Bar at the machine regulator.
  • The Risk: If pressure drops < 4 Bar, the oven cannot maintain positive pressure in the tunnel. Oxygen enters, and solder joints oxidize immediately.
• Filter Strategy: Install a 0.01-micron particle filter at the oven inlet to catch pipe scale.

Pro-Tip: Insulate N2 lines running through non-conditioned spaces (like ceiling voids). Cold liquid N2 boil-off can cause condensation on the outside of the pipe, "raining" water onto the equipment below.

Vacuum (House & Process)

Vacuum is the "hand" that holds the product. In manufacturing, we distinguish between Process Vacuum (Holding) and House Vacuum (Cleaning).

1. Process Vacuum (Fixture Hold-Down / ICT)

• Application: Holding PCBs on router beds or Bed-of-Nails testers.
• Requirement: High Flow, Low Vacuum.
• Control: Monitor Hg (Inches of Mercury).
  • If Vacuum < 20 inHg -> Then Interlock machine start. A loose board during routing is a projectile.

2. SMT Nozzle Vacuum

• Most modern SMT machines generate their own vacuum via compressed air venturis (ejectors). However, if your facility uses a Central Vacuum for pick-and-place:
• The Danger: "Cross-talk." If one machine purges a large volume, the pressure drop must not affect its neighbor.
• Architecture: Use a Loop Topology (ring main) for the vacuum header to equalize pressure instantly.

Warning: Never connect "House Cleaning" vacuum ports to the "Process Vacuum" lines. Dust from floor cleaning will migrate back into the clean process lines during pump shutdowns.

Process Exhaust (The "Lungs")

Exhaust is not just about smell; it is a critical thermal control variable. Reflow ovens and Wave Soldering machines rely on specific "draw" (suction) to balance their internal convection heaters.

Categorization:

• Type A (General Heat): Removal of hot air from chillers/compressors.
  • Ducting: Galvanized steel.
• Type B (Solder Fume/Flux): Contains rosins and adipic acid. Condenses into a sticky "honey."
  • Ducting: Stainless Steel or welded ducts. Must have cleanout doors every 3 meters.
  • Risk: Flux residue is highly flammable.
• Type C (VOC/Chemical): Conformal coating/Cleaning solvents.
  • Ducting: Spark-proof fans + Stack height > 3m above roof line.

The "Static Pressure" Rule:

Do not measure airflow in CFM (Volume); measure Static Pressure (Pascal) at the machine collar.

• Standard Target: -250 Pa to -400 Pa.
• Failure Mode:
  • Too High (e.g., -800 Pa): You are sucking the hot air out of the oven. The heaters will run at 100% duty cycle to compensate, burning out elements and wasting energy.
  • Too Low (e.g., -50 Pa): Flux fumes escape into the room. Operator health hazard.

Final Checklist

Utility	Parameter	Critical Control
Nitrogen Purity	99.99% (at Point of Use)	O2 Analyzer < 1000 ppm
N2 Piping	Copper / Stainless	No PVC
Vacuum (Process)	Stability	Loop Topology
Exhaust (Reflow)	Static Pressure	-250 to -400 Pa
Duct Hygiene	Flux Residue	Cleaned Annually
Separation	Process vs. Cleaning Vac	Strictly Segregated