Residual oxygen content (O2-content)

For flux-free soldering with reducing gases such as formic acid or hydrogen, the residual oxygen content should be <5ppm. Mathematically, an O2 concentration of 0.2 ppm can be achieved by evacuating twice to 1 mbar and then flooding with nitrogen to atmospheric pressure.

However, oxygen molecules adhere to the vacuum chamber wall and to the products in the chamber. The use of vacuum-compatible materials is therefore extremely important. Porous surfaces such as anodized coatings should be avoided, as should metals with a high saturation vapour pressure. These include zinc (galvanized screws) or magnesium and their alloys. Attention must also be paid to the vacuum suitability of lubricants. A low saturation vapor pressure and high viscosity of the greases are important here. If the materials outgas strongly, the required final pressures of 1 mbar are not reached and therefore the vacuum chamber cannot be completely inerted. Cross-influences on the formic acid or hydrogen are also to be expected.

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