It’s expensive, but necessary. How to choose the best option.

I am often asked whether nitrogen is needed for selective soldering. My response is always an emphatic “yes”: Nitrogen is a required and necessary consumable with all selective soldering.

Without nitrogen, solder tends to bridge and form icicles. Nitrogen reduces the solder surface tension, permitting the molten solder to readily break away from the solder site. In an open atmosphere, dross formation will overwhelm the solder pump and nozzle, causing excessive cleaning and pump maintenance. Nitrogen displaces air (oxygen), minimizing oxidation’s effects. The process simply would not be satisfactory without it.

Although larger manufacturers may have an in-plant supply, many small- to mid-size operations generally do not. The most economical way to purchase nitrogen for a “part-time,” single-shift operation is in liquid form. Pressurized gas tanks do not have much capacity and will last only hours. All industrial gas suppliers provide 45 to 50 liter liquid tanks, called Dewars, vessels used for keeping liquids at temperatures differing from that of the surrounding air. A Dewar flask consists of a double wall, with the space between the two walls exhausted to a very high vacuum, to minimize transfer of heat by convection and conduction. When ordering nitrogen in a Dewar, one should specify 99.995% purity or better with same-day delivery. (Praxair and Air Liquide are two of the many suppliers from which to choose.) Tanks are delivered and positioned by the local vendor. These tanks are safe for open floor use. Cost will be approximately $300 per tank, which includes tank rental and delivery/setup. Customers need to purchase a regulator at a one-time cost of about $85.

Consumption will vary with the make and model of the selective soldering machine and the scale of production. Our selective soldering machines require 60-80 psi fed to the machine. The Dewar will supply approximately 3600 cubic feet of nitrogen. Based on an average machine consumption of 20/50 cfh, this is approximately a 2 to 4 week supply of nitrogen for a single-shift operation. It should be noted that regardless of production volume, or nitrogen use, the Dewar tank would be exhausted in 3 to 4 weeks. The tank is constantly venting, as a result of the internal process, and even if the nitrogren is not being consumed by machine operation, it will be vented and eventually exhausted.

In regard to the purity level, we find that for a Pb-free process, when using smaller nozzles, we strongly recommend upgrading nitrogen purity to five “9s” (99.999% pure). This purity helps solder flow through the smaller orifices, and is probably true for many, if not all, makes of selective soldering machines.
If large nozzles are used, or use greater than 30 hours a week is expected from the selective machine, it is more economical to purchase a nitrogen generator. We recommend onsite nitrogen generators. These are the best overall choice for supplying unlimited, pure nitrogen, short of the installation of an outdoor facility-sized storage tank. These generators generally feature a multi-position purity level selector switch and a digital readout of the exact purity.

As often as I am asked about nitrogen use, I am asked about its cost, particularly the comparative costs between the Dewar and the generator. I use the formula that a Dewar tank costs approximately $300 and holds about 3500 cfh. The bottle will last about 100 running hours at a set rate of 35 cfh. (Actually, it is less, as the gas will vent, causing some lost capacity.) Add to that the cost of the inconvenience of the changeover, the possibility of running out of gas during a run and the inconsistent quality from tank to tank. The cost for a 160-hr. run per month approaches $500 for nitrogen from the Dewar, while the cost (on a lease plan) for a nitrogen generator is generally under $400, and the lease of the generator is easily cost-justified. Users we know that use generators rather than Dewars find it a cost-effective alternative, but again, production volume is the factor determining need.  CA

Alan Cable is president of A.C.E. Production Technologies (ace-protech.com); acable@ace-protech.com.

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