The theoretical amount of flux required to dissolve a 100 Å oxide film is about 0.02 g/m2
(1 Å = 10-10 m = 0,1 nm). For a 400 Å film, still only 0.08 g/m2 flux is required. These do not take into account losses to moisture, oxygen or poisoning of the flux by Mg alloy additions.
In practice however, the recommended loading for fluxing is 5 g/m2, uniformly distributed on all active brazing surfaces. This is more than 250 times the theoretical amount required for oxide dissolution. To visualize what 5 g/m2 flux loading might look like, think of a very dusty car. As the heat exchange manufacturer gains experience with his products, he may find that a little more is required for consistent brazing or that he can get away with a little less flux.
Too little flux will result in poor filler metal flow, poor joint formation, higher reject rates, and inconsistent brazing. In other words, the process becomes very sensitive.
Too much flux will not affect the brazing results. However there will be pooling of flux which can drip on the muffle floor, the surface of the brazed product will be gray and there will be visible signs of flux residue. Furthermore, flux will accumulate on fixtures more rapidly which then requires more frequent maintenance. More importantly yet, using too much flux will increase the process costs.
In some cases, heat exchanger manufacturers use higher than recommended flux loadings to mask furnace atmosphere deficiencies. This should be viewed as a short-term solution and the furnace problems should be addressed.
See also: How to evaluate flux load?