During the nitrogen-shielded reflow process, volatiles from lead-free solder balls primarily originate from the decomposition of organic components and certain metal compounds in the flux. The nitrogen atmosphere effectively inhibits excessive oxidative decomposition of organic compounds at high temperatures by isolating them from oxygen. However, components such as rosin, activators, and solvents in the flux will still volatilize from the molten state, generating waste gases primarily composed of volatile organic compounds (VOCs). If these volatiles are not properly controlled, they can impact the production environment, equipment, and operator health. Therefore, dual control is required through process optimization and waste gas treatment technologies.
Nitrogen shielding affects volatiles in two ways: first, it reduces the oxidation reaction rate, thereby reducing the generation of metal oxide particles (such as SnO₂) and complex organic decomposition products caused by high-temperature oxidation. Second, by maintaining an inert atmosphere, it allows the organic components in the flux to volatilize in a more controlled manner, avoiding violent combustion or decomposition. For example, during conventional air reflow, rosin may generate harmful substances such as polycyclic aromatic hydrocarbons (PAHs) due to localized overheating. However, in a nitrogen environment, its decomposition pathway is more inclined to produce single-carbon-chain volatile organic compounds (VOCs), which are easier to handle.
Key process parameters for controlling volatiles include nitrogen purity, furnace oxygen concentration, temperature profile, and conveyor speed. Nitrogen purity must be maintained above 99.99%, and furnace oxygen concentration is typically controlled below 500 ppm to minimize oxidation reactions. The temperature profile design must balance flux activation and volatile generation: a slow temperature increase (2-3°C/s) during the preheating phase can minimize rapid solvent volatilization. The peak reflow temperature should be adjusted based on the lead-free solder ball composition (such as SAC305) to avoid excessive flux decomposition caused by excessive temperatures. A rapid temperature reduction (>3°C/s) during the cooling phase can shorten the exposure time of the liquid solder, reducing intermetallic compound growth and secondary volatile generation. The conveyor speed must be matched to the temperature profile to ensure the PCB spends a reasonable amount of time in each temperature zone, avoiding local overheating or insufficient cooling.
Exhaust gas collection and treatment are key to controlling the release of volatiles. Reflow ovens must be enclosed, with exhaust gases collected centrally via a negative pressure exhaust system. Treatment technologies include multi-stage filtration and adsorption: primary filtration intercepts large particles and solder balls; intermediate wet scrubbers remove acidic gases and some VOCs; and the final stage uses activated carbon or zeolite wheels to absorb remaining organic matter. The purified gases are then discharged into the atmosphere through an exhaust stack. For high-concentration exhaust gases, a catalytic combustion unit can be added to decompose VOCs into CO₂ and H₂O at 250-400°C, achieving harmless treatment.
The choice of flux material is crucial for controlling volatiles. Low-solids, halogen-free fluxes can significantly reduce post-soldering residue and the generation of hazardous gases. For example, while water-soluble flux is easy to clean, potential water pollution from its volatiles is a concern. For no-clean flux, the composition must be optimized to ensure that the volatiles are low-toxic and easily adsorbed in a nitrogen environment. Furthermore, the amount of flux applied must be precisely controlled. Excessive application can lead to a surge in volatiles, while insufficient application can cause soldering defects.
Equipment maintenance and operating procedures are essential for ensuring effective control. Regularly cleaning tin slag and flux residue from the reflow oven walls and conveyor tracks prevents secondary volatilization of volatiles. Nitrogen filters and activated carbon adsorption modules must be replaced according to the manufacturer's recommended cycle to avoid saturation and reduced efficiency. Operators must wear protective equipment, and air purification equipment must be installed outside the enclosed operating room to reduce the risk of long-term exposure.
Controlling volatiles from lead-free solder balls during nitrogen-shielded reflow requires a coordinated approach encompassing process parameter optimization, exhaust gas treatment, material selection, and equipment maintenance. By precisely controlling the nitrogen environment and temperature profile, combined with efficient exhaust gas treatment technology, volatile emission concentrations can be kept below national standard limits, while simultaneously improving welding quality and production safety. In the future, with the development of low-VOC fluxes and intelligent oxygen control technologies, volatile control will become even more refined, helping the electronics manufacturing industry transition towards a green and sustainable future.
