The nitrogen tank plant, consist of a nitrogen cryo-tank and a vaporiser.
The cold fluid nitrogen is supplied by truck and pumped into the cryo-tank.
When nitrogen is being used, the nitrogen vaporises at minus 196 de-gree Celsius inside the vaporiser and become a gas. The nitrogen gas is then transported in copper tubing to the reflow furnace.

Usually the “cold” nitrogen is applied directly into the reflow zone where the temperature could reach temperatures as high as 250 – 270 degrees Celsius. So, the delta T is quite high, when the nitrogen temperature is as low as 10 - 15 degrees Celsius. This can cause some temperature regulation problems and finally results in large variation in soldering temperature. Especially in the winter time, the temperature of the nitrogen applied into the furnace could be very low. To overcome this a pre-heating of the nitrogen will be necessary. Usually it is enough that the supply tubes is installed inside the factory building the last 50 metres. It is also possible to preheat the nitrogen, by letting it run through some heating panels installed in hot spots inside the furnace. Usually the gas supplier can install a temporary tank plant for a period of time. This makes it possible to test the nitrogen effect on your own equipment, PCBs and components. The gas suppliers can help with a lot of the practical things, such as establishing the necessary tubing and applying for the necessary permission to install the tank plant etc.

The amount of nitrogen used is very much depended on the reflow furnace used. If the furnace is an open type the nitrogen consumption will be very high. Maybe as high as 30-40 m 3 /h. But if the furnace is of the closed types, where a curtain only opens when a PCB enters the furnace the nitrogen consumption can be as low as 14-16 m 3 /h.

But the consumption also depend on the nitrogen regulation system. When using a rest oxygen measuring and nitrogen regulation system, only the necessary nitrogen will be applied. However the most important reason to use a rest oxygen measurement system and nitrogen regulation system, is that the oxygen level will be known at all times. This makes process control a lot easier.

So what is the cost benefit of using the nitrogen? This has to be calculated in every individual case. The
money savings due to the reduction in soldering failures can be calculated fairly easy. But the other benefits
is more difficult to calculate. But, it all comes down to holding the savings and other benefits against the
expenses to nitrogen and rent of nitrogen tank plant.

The benefits of using nitrogen in the reflow process, are often discussed and depends on a lot of factors. E.g. PCB solder pad surface, solder paste flux type, solder-ability of the component terminals etc.
Whether or not there will be a real benefit of using nitrogen in the reflow process, it have to be investigated in the actual production facility.
The nitrogen is used to drive out the oxygen from the soldering chamber and thereby prevent further oxidation of the solder land, solder paste particles and the component terminals. So, the short term benefit comes down to reduction of failures found in the visual and electrical tests. This is fairly easy to measure.

However the long term benefit is more difficult to prove. of course an accelerated temperature cycle test can be used to indicate if less oxygen in the soldering process will add life time to the products. An indirect and faster way is to perform some pull off tests of soldered IC terminals and compare the pull off force of components soldered in air atmosphere and in nitrogen atmosphere.
Underneath is a list of benefits divided into the three categories:

• Improved soldering quality,
• Enlarged process window,
• Other benefits.

Improved soldering quality.

1. The higher surface tension and less reoxidation of the IC terminals will increase the wetted surface area of the terminals by up-to 30%. This increases the solder joints strength.
2. The less reoxidation also improves the wetting on NiAu and bare copper solder lands.
3. The higher surface tension minimises solder balling when using fine pitch solder paste containing smaller particles that has a larger surface area. The particles melt together more easily.
4. The visual appearance of the solder joint surface will be smoother and in some cases shinier.

Enlarged process window.

1. The soldering process will be less sensitive to poor solderability of solder lands and component terminals.
2. Makes it possible to use low or medium residue type flux.
3. Larger security when soldering BGA components.

Other benefits.

1. A larger surface tension makes the components "swim" easier into place if placed inaccurate.
2. A larger surface tension can hold larger and heavier components on bottom side when performing double sided reflow.
3. Less reoxidation of presoldered joints on bottom side when performing double sided reflow.
4. The exploitation of the solder paste can in some cases be increased by a higher reuse.

In the following paragraphs; level of rest oxygen, failure registration, visual inspection and solder joint strength test, I will discuss ways to investigate the benefits of using nitrogen in the reflow process, added with my experienced results.

Level of rest oxygen.
Now, what level of rest oxygen is acceptable in the soldering chamber? Again, this depends on PCBs, components and solder paste used. If the solder lands or component terminals have a poor solderability and the flux aggressiveness is low, a lower rest oxygen level will be necessary.

The rest oxygen level, in my opinion, should be in the range of 1000 - 100 ppm depending on the mentioned factors. And if using low residue solder paste the oxygen level possibly should be even lower.
The only way to find the correct rest oxygen level is to investigate the results at different levels. One way is, to solder a number of PCBs during a gradual reduction of oxygen and then inspect them visually. This will give a hint of what the maximum oxygen level should be.

Failure registration.
Since the, impact of using nitrogen is depended on the type of components, PCBs and solder paste used, it is important to perform a full scale production test. If soldering failures are already registered on all products in the production, it is fairly easy to see the results when adding nitrogen.

To get a good picture, it is important to use nitrogen in a test period of at least 3 month. If there is no soldering failure registration available, it is necessary to collect these data for a period of minimum 3 month before adding the nitrogen to the soldering process.
This reference is important to be able to make the correct decision, of whether or not to use nitrogen.
The failure registration I performed, showed an average reduction, in the failure type "soldered but no connection" to between 1/3 - 1/4 compared with the level when soldered in air atmosphere.

Visual inspection.
A visual inspection in a microscope gives a good impression of the impact of a lower oxygen level during soldering. If the oxygen is lowered gradually during testing, it is easy to see the improved wetting of the component leads and solder pads as the oxygen level drops. This is especially easy to see on IC leads, where the alloy wets the bare copper toe and forms a perfect filling.

Photo1. Improved wetting.

The Photo 1, show’s the improved wetting of the component terminals after adding nitrogen to the reflow soldering process. The rest oxygen level in the reflow zone was approximately 300 ppm. As seen the solder alloy flows on to and covers the leads completely. At the heel of the IC lead, the alloy will flow higher and increase the solder joint attachment area and thereby also increase the solder joint strength.

Photo2. wetting.

As comparison, Photo 2, show’s the IC soldered in atmospheric air. It is not only the IC solder joints that are improved with nitrogen. Solder joints at SOT23, SOT89, coils etc. are also improved significantly when using nitrogen to prevent the reoxidation.

Solder joint strength test.
To investigate the long term effect of using nitrogen in the reflow oven, the terminal pull off test can be used as an indirect measurement.
This test is done by pulling the cut free leads vertical of the PCB using a shear force tester. The results measured on nitrogen soldered PCBs is compared with the results measured on air soldered PCBs.

To be able to compare the results, it is important to use the same lots of PCBs, solder paste and components. However to determine for sure that the solder joints have a longer lifetime it will be necessary to perform an accelerated temperature cycle test.
The pull off tests of a QFP 0.65 mm lead pitch showed an increased joint strength of at least 10%. This is of course depending on the component terminals initially solder-ability. However an increase of joint strength is not the same as stating a 10% longer joint lifetime.

• The largest disadvantage of using nitrogen is the cost of nitrogen and rent of the tank plant.
• A higher surface tension can result in a higher number of solder bridges at fine pitch component if the
  solder paste print is inaccurate.
• Larger risk of tomb-stoning and skewing of small chip components with poor solder-ability.

Source unknown.