4.4 Fire safety in thermal processes

Thermal processing equipment—such as reflow ovens, wave soldering machines, and curing ovens—essentially functions as a contained heat source within a manufacturing enclosure. Careful thermal regulation and fuel management are what separate a stable soldering profile from a major facility fire. Teams should be trained to treat flux residue not simply as “dirt,” but as accumulated fuel that can ignite if conditions allow.

Fuel management (flux residue)

Vaporized flux naturally condenses on the cooler metal surfaces within the oven tunnel and exhaust ducting, creating a highly flammable condensate.

Reflow Soldering: A consistent preventive maintenance (PM) schedule should be implemented to mechanically scrape and clean the tunnel entrance and exit zones. The residue depth should never exceed 2 mm.
Wave Soldering: It is critical to remove dross and spent flux daily. Titanium fingers and solder pots operate at temperatures exceeding 250˚C; any accumulated paper or flux debris near the pot presents a significant ignition hazard.
Exhaust Interlocks: If the exhaust velocity drops below 5 m/s, the ductwork should be inspected promptly for physical blockage. Low flow allows volatile organic compounds (VOCs) to accumulate, creating a potentially explosive atmosphere inside the oven.

Thermal runaway protection

Software controls can fail, and relying solely on the PLC to manage critical temperature limits is insufficient for safety.

Primary Control Failure: If the primary control loop fails (for example, if a solid-state relay shorts ON), a dedicated hardware over-temperature switch must automatically cut the main power. This switch needs to be entirely independent of the PLC logic and must break the contactor coil circuit.
Conveyor Failure: If the conveyor stops unexpectedly due to a jam or mechanical motor failure, the internal heaters must automatically shut off. Stationary PCBs resting under active convection heaters can char and ignite within seconds.

Suppression systems

Water sprinklers can destroy sensitive electronics and may cause violent steam expansion if they interact with a bath of molten solder.

Inside the Machine: Only CO₂ or Clean Agent extinguishers should be used. These suppress fire by rapidly displacing oxygen and do not leave conductive residue that could otherwise damage the machine’s internal electronics.
Surrounding Area: Standard building water sprinklers should serve as the final defense for the facility structure itself, not as the primary defense for the machine.

Recap: Fire Safety in Thermal Processes

Parameter	Requirement	Action	Condition
Flux Residue Depth (Reflow)	≤ 2 mm	Scrape & clean tunnel zones	Per PM schedule
Exhaust Duct Velocity	≥ 5 m/s	Inspect for blockage	Upon velocity drop below limit
Primary Control Failure	Independent hardware over-temperature switch	Cut main power	Upon PLC/control loop failure
Conveyor Stoppage	Automatic heater shutdown	Deactivate heaters	Upon jam or motor failure
Internal Machine Fire	CO₂ or Clean Agent only	Use for suppression	Fire inside machine enclosure