Introducing nitrogen to the solder process can enhance reliability and yields.

Inert gas soldering systems, which use a controlled nitrogen blanket atmosphere and an entry vacuum oxygen/nitrogen exchange chamber, offer several key benefits:

• Oxide formation. Indirectly through its presence, nitrogen minimizes the formation of oxides during soldering.¹
• Flux selection flexibility. Greater flexibility in choosing fluxes.
• Post-cleaning. Post-soldering cleaning requirements can be reduced or even eliminated.

Figure 1. The presence of nitrogen reduces oxygen in the oven, lessening its impact on solder joints. (Image source: ITW EAE)

Soldering in a nitrogen atmosphere can enhance solder joint quality and manufacturing yields. This process is particularly beneficial for lead-free products soldered at high temperatures, the latter of which can reduce oxidation during assembly of printed circuit boards (PCBs). The cost of installing a nitrogen-based environment, however, must be weighed against the potential improvement in solder joint quality and the cost savings from reduced repair work on manufactured circuit boards.

Reflow Soldering

Because nitrogen is not an oxidizing gas, adding it to the SMT convection reflow oven reduces oxidation on the welding surface and improves wettability.^1,3 Nitrogen, being inert, does not form compounds with metals and isolates oxygen and metals in the air.

In a nitrogen environment, the surface tension of solder is lower than in the atmosphere, improving solder fluidity and wettability.³ Fluxing agents gain efficacy.⁴ Use of nitrogen means the absence of oxygen, and thus oxidation is decreased, particularly during high-temperature soldering and for second-side reflow.²

There are limitations, however. Nitrogen cannot reverse severe oxidation on parts or circuit boards; it has only a remedial effect on slight oxidation.

Other disadvantages include the additional cost of the gas, an increased chance of tombstone formation,⁵ and an enhanced capillary (wicking) effect.

Suitable applications for nitrogen-based soldering include organic solderability preservative (OSP) boards, dual-sided reflow boards, and parts or circuit boards with poor tin wettability, such as QFNs, large packages and high-density BGAs.

Wave Soldering

Inert gas atmospheres can be applied to wave soldering processes either partially or fully. Partial treatment involves only the solder wave area under the inert gas environment, while a more effective but costly tunnel system can operate with adjustable residual oxygen levels between 20ppm and 500ppm in the soldering area, creating an inert atmosphere in the preheat and cooling areas.

Oxidation is more visible in wave soldering processes, and dross removal is necessary. Typically, oxidation speed doubles with every 10K rise in temperature. Tunnel systems and partial inert atmosphere systems can reduce dross,⁶ with tunnel systems offering more significant improvements.

For production processes where dross reduction is critical, residual oxygen levels of about 1000ppm with a partial inert gas atmosphere are adequate. To maximize the benefits of nitrogen wave soldering, residual oxygen levels below 500ppm are required, necessitating a tunnel wave soldering system.⁶

Considering parameters such as layout, temperature profile or solderability of materials, defect rates can be significantly reduced in a nitrogen atmosphere. This environment improves wetting characteristics and surface tension of liquid solder, minimizing defects like incomplete through-hole penetration, insufficient wetting, bridging and solder balls, resulting in higher product quality and reduced repair work.

Selective Soldering

Nitrogen is used in selective soldering machines to prevent excess oxide formation on the solder surfaces of the selective wave and multi-wave, and minimize dross formation during nozzle flushing.

The nitrogen flow around the select wave nozzle prevents oxide layer formation on the wave surface, aiding flux activity.⁶ The required nitrogen purity depends on the process needs, with higher purity providing better assistance and lower consumption. However, nitrogen purity at the soldering position is never the same as the supply due to mixing with surrounding air.

The quality of the solder process dictates the necessary nitrogen purity based on production demands.

Summary

The integration of nitrogen into PCB soldering presents numerous advantages that significantly elevate the quality and efficiency of the assembly process. By minimizing oxide formation and providing greater flux selection flexibility, nitrogen not only simplifies post-soldering cleanup but also enhances solder joint quality and yields. Particularly in lead-free and high-temperature soldering, nitrogen plays a pivotal role in preventing oxidation and contamination, thus reducing repair costs and improving product reliability.

Throughout various soldering methods – be it reflow, selective, or wave soldering – nitrogen consistently demonstrates its ability to improve wettability, reduce defects and enhance solderability, albeit with considerations towards its costs and limitations. The choice to implement a nitrogen-based environment is a strategic one, balancing the initial investment with the long-term benefits of higher quality and efficiency.

References

1. Andy Mackie, Ph.D., “Dispelling 10 Myths About Nitrogen Reflow,” July 2009.
2. Research International, Solder Reflow Technology Handbook, section 4.
3. Mingzji Dong, et al, “Effects of Nitrogen on Wettability and Reliability of Lead-free Solder in Reflow Soldering,” International Conference on Electronic Packaging Technology & High Density Packaging, August 2009.
4. Eric Bastow, “Pros and Cons of Convection Reflow Soldering using Nitrogen,” Apr. 4, 2016.
5. Jeff Kaiser, “Drawbridging during Reflow Soldering in Nitrogen,” ITW EAE Support Center, Apr. 14, 2020.
6. Han-Na Noh, et al, “The Study of the Nitrogen Effect for Wave Soldering Process,” IPC Apex Expo.

Md Imtiaz Uddin is deputy general manager at Napino Auto and Electronics (napino.com); m.imtiaz@napino.com.