Nitrogen
12 July 2011
Nitrogen is a natural part of our atmosphere, it is an inert gas which if pure will not allow oxide formation.
Oxygen – a significant part of the atmosphere - will allow oxide formation, particularly at elevated temperatures. The components of our atmosphere consist of 78% nitrogen 21% oxygen and nearly 1% of argon. The rest of the atmosphere is made up of carbon dioxide, neon helium krypton hydrogen xenon and radon.
Traditionally nitrogen is separated from air via a cryogenic distillation method. In this process air is taken, filtered, compressed cooled and purified providing a liquid air feed stock held in a distillation column. The bulk of nitrogen produced is in the form of high purity liquid. The column allows different gases to be drawn off for use.
From the column both gas and liquid may be obtained. After further process stages both gas and liquid may be drawn off for different industrial applications. Most of the nitrogen used in the UK electronics assembly industry is sourced from a liquid product produced cryogenically by an air separation unit.
Depending on the demand, nitrogen may be supplied in a variety of packages. For low volume it may be provided in compressed or liquid cylinders. However most of the nitrogen used in the UK industry for soldering applications is held in bulk storage vessels where the material is stored on site as a liquid before being converted to a gas via ambient temperature vaporisers immediately prior to use in the soldering process.
There has been considerable interest in Japan and the USA about on-site generation. Pressure swing absorption uses a carbon molecular sieve to filter the oxygen, leaving nitrogen feed stock. Membrane separation uses a hollow polymer filament - oxygen and water diffuse through the filament leaving a nitrogen stream. Suppliers like BOC can recommend the most appropriate nitrogen supply method that meets process, quality and cost concentrations.
As part of any installation gas suppliers provide process procedures and environmental analysis for a safe working practice. This provides an optimised process in terms of gas consumption and the levels of oxygen during soldering. As well as process optimisation support, suppliers can also periodically monitor equipment to ensure safe working conditions using oxygen analysis equipment.
The benefits of reflow soldering in nitrogen have been outlined many times. Engineers need to consider the levels of oxygen they can tolerate in their process at the required throughput. Most modern reflow systems can now accommodate the use of air or nitrogen during reflow soldering. Engineers must evaluate the consumption of gas required to maintain the chosen reflow machine at that level. Gas consumption rates can be extremely confusing. User experience is extremely valuable to determine the cost justification for using nitrogen on different processes.
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