Why Add Nitrogen to Reflow?
Reflow soldering happens at 235-250°C. At these temperatures, copper oxidizes rapidly in air. Solder paste flux fights this oxidation — it removes oxides from pad surfaces and component leads, allowing the molten solder to wet and form a reliable intermetallic bond.
But flux can only do so much. In air (21% oxygen), oxidation competes with flux activation. The result:
Duller, grainier joints — surface oxidation on the molten solder as it solidifies
Solder balls — tiny spheres of solder that separate from the main joint due to oxide skins preventing coalescence
Increased voiding — oxidized surfaces trap flux volatiles that would otherwise escape
Poor wetting on difficult surfaces — OSP (Organic Solderability Preservative) finishes and older components benefit significantly from nitrogen
Nitrogen reflow replaces the air inside the oven with nitrogen gas, reducing the oxygen level to well below 1,000 ppm (ideally<500 ppm="">
How Nitrogen Reflow Works at Superb
Our TEA-1200-DH 10-zone reflow oven is equipped with a nitrogen supply system:
| Parameter | Specification |
|---|---|
| Nitrogen source | Bulk liquid nitrogen tank with vaporizer |
| Oxygen level | <500 ppm="" maintained="" in="" all="" heated="" zones=""> |
| Flow rate | 15-25 m³/hour depending on conveyor speed and board loading |
| Monitoring | Continuous oxygen sensor in each zone; alarm if >1,000 ppm |
The nitrogen is injected into each heating zone through diffusers that create a laminar flow over the board surface. This displaces oxygen-rich air and maintains the inert atmosphere throughout the reflow cycle.
[PHOTO: TEA-1200-DH reflow oven — nitrogen flow meters and oxygen sensor display]
Nitrogen vs. Air Reflow — Measurable Differences
| Parameter | Air Reflow | Nitrogen Reflow |
|---|---|---|
| Wetting angle (SAC305 on Cu) | 25-35° | 15-22° (lower = better wetting) |
| Solder ball count (per board) | 5-50 typical | <5 typical=""> |
| Joint surface appearance | Dull, grainy | Semi-bright, smoother |
| Solder spread (ENIG finish) | Good | Excellent |
| Solder spread (OSP finish) | Adequate | Good (OSP benefits most from N₂) |
| Voiding (BGA, typical) | 5-15% of joint area | 2-8% of joint area |
| Flux residue | Often brown, indicating oxidation | Lighter color, more complete activation |
When Nitrogen Reflow Makes the Biggest Difference
Nitrogen reflow benefits all boards, but the improvement is most significant for:
1. OSP-Finished PCBs
OSP (Organic Solderability Preservative) is a thin organic coating that protects copper pads from oxidation before soldering. It's the most cost-effective PCB surface finish, but it's also the most sensitive to reflow atmosphere. In air, OSP decomposes and copper oxidizes during preheat, before the solder melts. In nitrogen, the OSP lasts longer, protecting the copper until the flux activates and the solder wets.
2. Fine-Pitch Components (<0.5mm pitch="">
Fine-pitch pads have less surface area for wetting. Any oxidation reduces the already-small wettable area. Nitrogen reflow ensures every square micron of pad is available for solder wetting.
3. Ball Grid Arrays (BGAs)
BGA solder balls are fully hidden after assembly. You cannot inspect them visually, and rework is expensive. Nitrogen reflow reduces voiding inside BGA joints — a critical reliability parameter. Lower voiding means less thermal-mechanical stress concentration, which translates to longer thermal cycling life.
4. Second-Pass Reflow (Double-Sided Boards)
When a board is reflowed a second time (bottom side after top side), the first-side joints see a second thermal cycle. Oxidized solder on the first-side joints can degrade. Nitrogen reflow minimizes this oxidation, preserving the quality of first-side joints during second-pass reflow.
5. Lead-Free SAC305 Soldering
SAC305 has inherently poorer wetting than leaded solder (Sn63/Pb37). Nitrogen compensates for this — bringing SAC305 wetting performance close to that of leaded solder in air.
The Cost of Nitrogen — And Why We Include It as Standard
Nitrogen reflow adds cost: the liquid nitrogen supply, the vaporizer, the piping, and the oxygen monitoring system. Some contract manufacturers treat it as a premium add-on — charge extra, reserve it for "high-reliability" orders, and run everything else in air.
At Superb Automation, nitrogen reflow is standard on all production. We made this decision for two reasons:
It's simpler operationally. Maintaining one reflow profile with one atmosphere eliminates the risk of running the wrong profile for the wrong order.
It produces better joints across the board. The incremental cost of nitrogen per board is a fraction of the cost of a single rework event or field failure.
Our position: if nitrogen makes the joint better, we use it. No upcharge. No special request required.
Nitrogen and the SAC305 Reflow Profile
The nitrogen atmosphere does not change the reflow temperature profile — SAC305 still requires the same peak temperature (235-245°C) and time above liquidus (45-75 seconds). Nitrogen improves the chemistry of soldering, not the physics of melting. The profile is developed and verified in nitrogen, so the board experiences exactly the thermal conditions it was qualified for.
