With the increase of assembly density and the appearance of fine spacing assembly technology, Nitrogen filled reflow welding processes and equipment were created to increase the quality and yield of reflow welding. These improvements are the main drivers for many of the technologies that we have been discussing in our SMT series.
The main purpose of nitrogen reflux welding is to limit the amount of oxygen in the reflow furnace thereby decreasing the oxidation that is common during this process. This process is mainly used to enhance the weld quality where oxidation can affect the ability of components to adhere to the flux. As the name suggests, this is achieved by filling the reflow chamber with nitrogen during the heating process. This is done by pumping nitrogen into the chamber. Since nitrogen is lighter than oxygen, it forces the oxygen to the bottom of the chamber and reduces the exposure of the reflowing parts to oxygen.
Using a nitrogen reflow technique has the following advantages:
(1) Prevents oxidation in components during heating
(2) Improves the wetting power of welding and speed up the wetting speed
(3) Reduces the generation of solder balls, bridging and obtain better welding quality
Most of the furnaces used at present are the forced hot air circulation type. To obtain better welding quality, it is particularly important to use solder paste with less active flux to improve solder joint performance and reduce discoloration. Since different welding products require different nitrogen levels, it's important to choose the right flux to match the process, heat and component tolerances. Today's solder manufacturers are working to develop products with a higher oxygen content that can be well welded without washing the solder paste. This has the benefit of reducing nitrogen consumption during this process.
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