It is certainly possible to manufacture and reflow surface mount assemblies using Pb-free soldering materials without using nitrogen. As long as we are talking about SAC alloys using Type III or Type IV powder in the paste. Advances in solder paste technology have produced materials that are Pb-free air-reflow compatible that produce no-clean residues that are clear, colorless, and pin testable.

Once the powder sizes go below Type IV and/or with some exotic alloys the use of nitrogen may be required in order to produce reliable, well wetted components. Where we do see a need to implement nitrogen is in the wave soldering process. The use of nitrogen reduces the amount of dross, reduces the chance of bridging and other defects.

If using a selective soldering process, nitrogen is critical for the reasons you mention but also to prevent nozzles from clogging. There are many study's available showing the benefits of nitrogen and the money that you actually save when using it. You save a lot of waste because much less dross is created compared to not using it, and more importantly you save on re-work because of the better flow characteristics. While using nitrogen it is also possible to lower the already high temperature of the Pb-free solder pot, which also helps to eliminate delamination and contamination.

So, you can see with the initial investment of nitrogen, in the long run it can save you money and headaches.

In a one word answer YES.

We have many customers operating lead free processes without Nitrogen.
However we have other customers running lead free processes with Nitrogen.
The Nitrogen discussion continues.

There are certainly benefits to Nitrogen atmosphere in a reflow oven, however your lead free solder paste, components, board material, board finish, and other factors will determine what if any benefit Nitrogen will have for your operation.

In our work we have seen a dramatic improvement on the solderability of lead free solder pastes over the last six years.

The first lead free solder pastes we printed and reflowed six years ago barely moved from their printed shape and position. We have all seen the pictures of lead free solder paste that remained in its printed location after reflow soldering. They certainly reflowed but almost did not spread at all.
The newer lead free solder pastes we have printed and reflowed spread much better. Still not the same spreading capability as tin lead material but much better then the older lead free solder paste products.

As always, work with your materials and component suppliers to identify the correct reflow environment and reflow profile for your operation.

It is possible, but it could also be cost effective to use nitrogen. Depending upon your resources and time, it might be valuable to run an experimental design where you look at rework/scrap rate with and without nitrogen. Depending upon the cost of your product and first pass yields, the reduction in scrap might pay for the cost of the nitrogen.

The quick answer is DEFINITELY!

The Japanese have been running Lead Free without nitrogen since 1999 with great success. Virtually all Camcorders, Cameras, DVD Players, TV's, Computers, Audio Equipment, etc made in Japan are made without nitrogen and WITH Lead Free Paste.

There is a great article in the May issue of Circuits Assembly Magazine that goes into some detail on this subject. And Eli Westerlaken deserves all the credit so I will just forward the link:

http://circuitsassembly.com/cms/content/view/3262/95/

You will need to work with your paste vendor to find a suitable formulation but as you noted that tends to be much less expensive than running nitrogen!