Let's start with the real reason an assembly engineer uses nitrogen in inert soldering: because it is the cheapest gas available that does not react with hot metal surfaces to form an unsolderable film. That's it. Period. People who use nitrogen for reflow are not using it because it has any wonderful properties, they are using it because it has low oxygen and moisture levels, and can purge (dilute) oxygen down to a low enough level to prevent or slow the oxidation of metal surfaces during heating.
To understand any process using inert (unreactive) gases, you have to understand the composition of air; the most abundant gas available to us. Air is around 78% nitrogen, 20.9% oxygen and 0.9% argon, with small amounts of other gases, carbon dioxide and so on, along with varying amounts of water vapor. Water vapor may go up to around 4%, and of course, at this level, it will dilute the other levels of gases by (96/100), just in case you think there as a problem with the math. The oxygen level (20.9%) equates to 209,000ppm (parts per miilion). The ppm unit is a much more useful measure when you are down at low percentage levels, for example 0.01% = 100ppm. It is also important to note that the fractional measure (ppm or %) correlates to the amount by volume and, from the ideal gas equation, also the molar percentage.
I'll cover half of the of the myths now, and half next time.
Myth 1: "Nitrogen removes oxides"
Fact: Nitrogen used at reflow temperatures has no fluxing (oxide-removing) properties whatsoever and does not chemically react with anything at these temperatures. Nitrogen prevents or slows oxidation or (in the case of a flux-cleaned surface) re-oxidation simply because it is not an oxidizing gas. Forming gas (a mixture of hydrogen and nitrogen) is very different, and I will discuss this in a subsequent note.
Myth 2: "Nitrogen improves heat-transfer"
Fact: It has no practical thermal effect on the soldering process. Heat transfer in gases at the same pressure and temperature is governed by the molecular weight of the gases: nothing else. Since nitrogen has a molecular weight of 28, and oxygen almost the same at 32, the difference in heat transfer properties between air and nitrogen is minimal whether you are talking about laminar or turbulent flow.
Myth 3: "If I measure the oxygen level in my incoming nitrogen, then I know the level in the oven"
Fact: Even an apparently well-sealed inerted reflow oven is actually mixing ambient air with your nitrogen to some degree. It does this through simple diffusion (driven by difference in partial pressure of oxygen in air versus inside the oven) or by turbulent mixing of nitrogen with air near an opening. What happens in a real oven is shown in the illustration (above). Putting a low flow rate of nitrogen into the oven will have little or no effect (a), then putting more in will reduce the level, but you will see large variations (b), then finally you will reach a plateau (c) where you have obtained the minimum oxygen level possible, but turbulent mixing is still introducing oxygen from the outside air. As you increase the nitrogen flow rate, you are simply increasing the turbulence, and hence the rate of mixing
Myth 4: "Purer nitrogen will give me better results"
Fact: Standard, cryogenic quality nitrogen has around 2-5ppm of oxygen in it. Even purging a well-sealed oven will not get you down to exactly the same level as the incoming gas. You will see no difference if you are using a nitrogen source at 10ppm or 10ppt (parts per trillion) oxygen. As you can see from the illustration above (c), the effect of the highly pure gas is completely negated by the mixing with air.
Myth 5: "Nitrogen reduces all soldering defects"
Fact: Nitrogen can help with some wetting-related defects, and can often turn a so-so (marginally acceptable) soldering process into an acceptable one. Other Fact: It may not only increase wetting ("wicking") uncontrollably onto leadframes or other surfaces, but may also cause solderspatter and contribute to die tilt (power semiconductor assembly) or tombstoning (SMT).
More next time. Part II of this post can be found here.