Nitrogen Gas
Nitrogen is a common gas found in the atmosphere. Approximately 78
percent of the air we breathe consists of nitrogen. The rest is made
up of oxygen, carbon monoxide and carbon dioxide, as well as traces
of argon and other gases. The fact that animal life survives in air
is an indication that nitrogen is nontoxic. But pure nitrogen does
not sustain life - not because of any inherent toxicity, but rather
because of a lack of oxygen. Hence, when using nitrogen, precautions
must be taken to avoid a situation where nitrogen is present without
the required amount of oxygen.
Nitrogen boils at approximately -320.4°F (-195.8°C), meaning liquid
nitrogen (as it may be delivered) is very cold. Cold nitrogen also
is heavier than air and accumulates in low-lying areas, if given a
chance.
Nitrogen is not manufactured but extracted from air, giving a
zero-sum gain as far as the environment is concerned. The extracted
nitrogen is returned to the air, and the air is neither depleted nor
enriched because of this circular process.
Effects of Nitrogen
In soldering, there are a number of basic phenomena caused by
changing the ambient air to nitrogen. By examining these effects,
including spreading behavior, wetting force, wetting angle and
surface tension, it can be seen that nitrogen provides noticeable
benefits for both wave and reflow soldering.
Comparing wetting force for different fluxes in air and nitrogen
reveals that nitrogen coverage can improve the process. Usually
measured with a wetting balance, wetting force is an indicator of
the quality and shape of the joint. A study3 comparing a number of
fluxes found that, with one notable exception (adipic acid), the
wetting force increases if measured under nitrogen rather than air.
For some fluxes, the gain is substantial.
The surface tension of the liquid solder plays a major role in the
soldering process. It is responsible for the shape of the fillet as
it counteracts wetting and gravity. It is practically impossible to
measure the solder surface tension under air, as a minute amount of
tin oxide in the surface layer will falsify the measurement.
Nevertheless, these measurements
again indicate that the
surface tension is higher under nitrogen than under air.
Nitrogen and Reflow
In reflow flux activity, residues and cleanliness also play a major
role when deciding whether nitrogen should be used. The same
arguments apply that have been used in wave soldering: residues are
less objectionable and, if necessary, easier to clean because they
are not oxidized. One study even indicates that nitrogen reduces
residues for some fluxes by as much as 66 percent compared to their
application in air. This explains why many users switching from
ambient atmosphere to nitrogen.
The most important factors in considering a nitrogen process are
defect levels and joint reliability
Other operations where nitrogen was introduced have shown
improvements in first-pass yield between 5 and 7 percent, which
translates into a reduction in defect levels of 50 to 60 percent.
The fact that not every introduction of nitrogen was equally
successful is explained by differences in layout and pitch.
Admittedly, bad layout cannot be compensated by simply introducing
nitrogen as a cover gas. It seems, however, that those processes
with the narrowest pitch benefit the most from nitrogen use.
Lead-free Soldering
The problem of soldering without lead is an intricate one. Many
different aspects play into it, including toxicity, availability,
price, worldwide distribution, wetting ability and reliability.
Much effort has gone into finding a replacement for 63/37, not only
because of its lead content (most specialists agree that the danger
posed by lead in the solder is rather limited) but because there is
a general need for a "better" solder. But as of yet, no one has been
successful in finding that ideal combination that replaces (as a
drop-in) the solder of 5,000 years ago.
The few potential choices (tin plus some copper; tin plus some
silver; tin plus some copper and some silver) all need higher
process temperatures and thus may not really work without good
inerting. The increased amount of tin in the solders not only makes
them more expensive (lead is cheap), but heightens the tendency to
oxidize, form dross and react with other metals. Although the higher
tin content would lead one to believe that wetting could be
improved, this is not generally true. Only under nitrogen can
similar wetting be achieved.
Conclusion
Nitrogen use in flow and reflow equipment may benefit the process as
well as the quality of the end product. In both cases, joint
reliability may increase and defect levels may drop to improve
first-pass yield. In the case of wave soldering, other benefits may
accrue, such as dross reduction, reduced solder pot maintenance, a
safer operation and substantial flux savings.
A nitrogen process should not be adopted, however, without fully
understanding its benefits and limitations. Nitrogen opens the
process window by forgiving the process flaws, but is the increased
window necessary? As the pressure to reduce cost in the industry
increases, the ultimate decision to use nitrogen should be based on
a solid cost-vs.-benefit analysis that goes beyond the unit cost of
nitrogen itself. Nitrogen inevitably adds cost to the process, but
its overall benefits normally should outweigh the additional
adoption expense.
Despite a continuous improvement in flux chemistries, nitrogen is
here to stay and its usage will increase. The continuous integration
of components, the increased demand for higher quality and
reliability assemblies, as well as the introduction of lead-free
solder, all increase the vulnerability of the process to oxygen and
oxides, enhancing the value of inert soldering.
Source From :
By M. Theriault, P. Blostein & Dr. A. Rahn
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