When we are using a lead-free process the reflow temperature is relatively higher from the leaded process.in the higher temperature reflows soldering process the risk of oxidation of metal surfaces increases and due to that the strength and performance of solder joints slashed.
The aim of every organization is to establish a process that reduces Costs increases productivity and improves customer satisfaction.
Oxidation is directly related to temperature higher temperatures mean faster rates of Oxidation. When PCBA are soldered in the air, the metal alloy systems used in the electronic assembly process are subject to oxidation. The layer of Solder oxides always create issues and results in a poor wetting The and weak inter-connections joints. Whereas the use of nitrogen as a process gas provides an inert, oxygen-free reflow soldering Atmosphere. By eliminating the formation of solder oxides, wetting is solid and soldering quality will be improved.
Why Nitrogen used as an inert gas
Due to the ternary covalent atoms bonding in nitrogen, it reacts as an inert gas in nature due to that nitrogen is helpful to eliminate the oxygen during the reflow soldering process.it directly helpful in proper solder spread improving wetting angle and force with surface tension.
After analysis of the various design of experiments in different environments, the major outcome was that the spread of solder on printed circuit boards remarkably at lower reflow temperature if the presence of oxygen level is lesser. Sn63/Pb37it was observed that the melting point 183°C rest oxygen 10 ppm the spread of solder done in soak zone up to 205°C only where at the peak temperature 270°C remaining oxygen is 1000 ppm.
From this experiment, it can be concluded that the low oxygen level during reflow soldering will have a positive effect on solder paste spreading properties. In other words, we can say that the good spread significant impact of wetting and solder joint connections.in cases of Sn/Ag/Cu solder alloys, it is recommended to use the use nitrogen if solder paste alloy having poor solderability issues.
Another benefit of nitrogen is that it will help to reduce surface tension due to the inert atmosphere during reflow soldering. In both soldering process reflow and wave soldering nitrogen is recommended to reduce soldering related defects.
Nitrogen in reflow soldering processes: –
The assemblies soldered in nitrogen environment have a clear and healthy visual appearance due to the less activated solder paste is used during soak time during reflow. So, one of the main benefits of nitrogen atmosphere is that the additional cleaning is not recommended that will save the cost of PCBA cleaning equipment.
A study says that if nitrogen is used in reflow soldering process the yield lowered down up 6 to 7%. In other words, we can say the 60 to 70% defects can be reduced if nitrogen is adopted in process.
Lowered pad to molten solder surface tension and improved wettability the problem of cold solder joints and no wettability improved drastically by using nitrogen atmosphere is reflow soldering. The most advantage of nitrogen has become more significant in defect reduction when using fine pitch component or Ball Grid Arrays (BGA, s).
The defects related to poor soldering or oxidation can not 100% detected by inspection instruments, it will create the product reliability issue and customer complaints. So, to reduce aftermarket complains the nitrogen soldering is remarkably beneficial.
In different cases, studies on solder joint quality analysis and laboratories shear force examination approve the nitrogen atmosphere soldering having a positive influence and improved quality of solder connections.
Benefits of N2 in a Reflow Soldering Process
• Flexibility – wider “process window”;
• Implementation of Low Residue, No-Clean Soldering
• Reduction in Labor due to less rework;
• Reduced automatic testing false fails;
• BGA voiding reduction;
• Reduced defects / increased yield;
• Increased joint reliability; and Performance.
• Aesthetic/shiny joints.
To address such challenges the popularity of soldering in the inert medium during reflow soldering was increased in recent years. For inert medium use of nitrogen gas is a cost-effective and easily available option.
The usage of nitrogen highly on depends upon the application and strongly recommended for class 3 products like automotive, Defense & aerospace, Medical types of equipment etc. where human life risk is associated, and high-quality products are mandatory. soldering in nitrogen atmosphere will help to contribute wider process window.
However, the usage of nitrogen adds some cost but considering the long-term saving due to high quality and the sound solder joint with less repair/rework it can be managed and ultimately leads to the profitability of any organization.