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Why is nitrogen used in the reflow process? How to monitor the nitrogen concentration?
Release time：
2025-05-19
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Summary
The use of nitrogen protection and monitoring of its concentration during the reflow process is mainly based on improving welding quality and process stability. The specific reasons and monitoring methods are as follows:
1. The main reason for using nitrogen:
Preventing oxidation reactions: Oxygen in the air at high temperatures will oxidize solder, component pins and PCB pads, resulting in decreased wettability, solder joint voids or cold solder joints. Nitrogen, as an inert gas, can replace oxygen (oxygen concentration must be less than 1000ppm) to reduce the risk of oxidation.
Improving solder wettability: Nitrogen environment reduces the surface tension of liquid solder, enhances its fluidity and wetting ability, especially for lead-free solder (such as SnAg), which can narrow the performance gap with lead solder.
Reducing welding defects:
Reduce the incidence of solder balls, bridging, and tombstoning;
Reduce the void rate of solder joints and improve electrical reliability;
Protect the surface treatment of PCB double-sided welding (such as OSP coating).
Optimizing the process window: Allowing the reduction of the peak welding temperature or shortening the time above the liquidus line to reduce the damage of thermal stress to components.
Improve the appearance and strength of solder joints: The glossiness of solder joints is improved, and rapid nitrogen cooling can also refine the grain structure and enhance mechanical strength.
2. Monitoring method of nitrogen concentration:
Online oxygen content analyzer:
Principle: Real-time collection of gas in the reflow oven, measurement of residual oxygen concentration through electrochemical or zirconium oxide sensors, and conversion to nitrogen purity.
Installation: Set sampling points in key areas of the furnace (such as heating zone and cooling zone) to continuously feedback data.
Two control technologies:
Air doping: Introduce a small amount of air through controlled leakage to maintain the oxygen concentration within the set range (such as 500-1000ppm), without real-time monitoring, suitable for stable production line scenarios.
Nitrogen controlled closed loop system: The oxygen analyzer GC-611 monitors oxygen concentration in real time and dynamically adjusts the nitrogen injection amount to maintain a strict low oxygen environment (such as <100ppm), which is suitable for high-precision welding needs. GC-611 uses zirconium oxide oxygen sensor technology to measure oxygen concentration in the range of 0.01ppm-25%vol, with long-term stability and high precision, which is very suitable for reflow nitrogen control.
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