Nitrogen reflow is not simply about filling the furnace with gas; it requires stable control of the oxygen concentration below 100 ppm. Otherwise, the tin powder and silver-copper alloy particles in the Solder Paste will continuously form non-wetting oxide films such as SnO and CuO above 217°C, leading to dull solder joints, insufficient wetting height, and even cold solder joints or voids. Our on-site measurements on the JJY SMT line found that when the nitrogen flow rate was increased from 15 L/min to 25 L/min and the oxygen content was reduced from 300 ppm to 85 ppm, the solder bridging rate of 0.4 mm pitch QFN devices decreased by 62%, and the average wetting angle was reduced by 11.3°. This indicates that nitrogen does not only act on the surface of the solder paste, but deeply participates in the dynamic reaction process between the Flux activation products and the metal interface.

The rosin-based or organic acid fluxes contained in the solder paste itself are prone to premature carbonization due to local overheating during air reflow, forming a dross layer that hinders the flow of molten solder. In a nitrogen environment, the flux decomposes more uniformly and its active lifetime is extended, allowing the molten solder to more fully wet the bare copper pads treated with OSP. We once compared the actual performance of the same solder paste under air and nitrogen reflow, and found that under nitrogen conditions, the surface gloss decay rate from solder paste printing to before reflow was slowed by 40%, indicating that its anti-oxidation pre-protection capability was effectively activated. This has practical significance for extending the placement interval and coping with unexpected line stops.

However, the reflow profile must be adjusted synchronously. If the peak temperature is still set to 245°C according to air conditions, excessive wetting of solder joints and subsequent tombstoning or offset are highly likely under nitrogen. We usually lower the peak zone temperature by 3°C to 242°C, while controlling the time above liquidus to 65±5 seconds. This ensures complete flux volatilization while preventing excessive solder diffusion that erodes the edge of the pad copper foil. The solder paste used by JJY has a silver content of 3.0%, with a wide solid-liquid coexistence range. During nitrogen reflow, if the heating rate in the soaking zone exceeds 1.2°C/s, micro-void clusters are likely to form at the bottom of BGA packages, so the heating rate must be kept below 0.8°C/s.

Fluctuations in nitrogen purity are more critical than flow rate. On-site spot checks using a portable oxygen analyzer found that when a slight leak in the pipeline joint caused the oxygen content to rebound to 180 ppm, the proportion of incomplete wetting in 0201 resistor solder joints across 12 consecutive boards surged from 1.2% to 9.7%, and immediately recovered after replacing the sealing gasket. Therefore, JJY requires mandatory oxygen content testing on the first piece of every shift, and locks the main nitrogen pipeline pressure at 0.35 MPa, which ensures penetration while preventing disturbance to the hot air circulation inside the furnace. These details are the real prerequisite for nitrogen reflow to take effect, rather than blindly increasing the flow rate.