High vacuum brazing and high vacuum soldering

Vacuum soldering and brazing is a heat treatment application for the production of many electronic components, for example devices used in satellites or aircrafts, which have to withstand challenging environments such as vacuum or extremely high temperatures. To manufacture reliable electronic components like these a connection between dissimilar materials is required.

Connecting dissimilar materials with vacuum soldering and brazing

This connection can be metal-to-metal or even insulator-to-metal. It has to be strong, high temperature resistant and suitable for use in vacuum, as outgasing of flux material is not acceptable. The purpose of flux material is to remove remaining oxides and to reduce the surface tension in order to promote wetting of the dissimilar materials’ surfaces. However, if exposed to vacuum or a high temperature environment, the effects of the flux on the electronic component are harmful. The flux material, which contains acid and salts, changes into the gaseous phase due to its high vapor pressure.

The resulting condensation of the flux material on the insulators may produce conductive paths causing a leakage current. This process will destroy the expensive component. Unfortunately, the most active (and therefore corrosive) fluxes also form the strongest connections. Some material properties, for example vacuum resistance, cannot be obtained when manufacturing under conventional atmosphere conditions. One other problem with conventional atmospheres is that gas impurities are always embedded in the connecting surface.

The solution to this problem is high vacuum soldering and brazing. For both processes the connection between the two dissimilar materials is made by a third metallic material, the so called solder or brazing filler material. The exact distinction between soldering and brazing is that in the case of soldering (reversible) adhesion is predominant, whereas brazing (irreversible) produces diffusion of the materials, leading to a much stronger connection. The complete process takes place in a high vacuum (HV) or even ultra high vacuum (UHV) environment. These environments prevent oxidation and allow the usage of a solder made of flux-free material. The requirements for components used in vacuum environment are fulfilled.

Soldering and brazing in a vacuum

To produce components which need to withstand extreme conditions, a furnace with special features is required. The furnace needs to be completely sealed to permit heat treatment in a vacuum environment. Depending on the materials and the solder involved, the temperature has to be adjustable up to approximately 1200 °C with superior temperature homogeneity and stability throughout the entire sample. Data logging is another important factor: the dissimilar materials, for example, must have a certain temperature before the filler turns into a liquid. Therefore, the furnace should permit controllable and reproducible data logging.

All these requirements are fulfilled by CARBOLITE GERO’s HBO top hat furnace, a metallic furnace based on tungsten or molybdenum for vacuum applications (HV or UHV). Depending on the customer’s vacuum requirement the leakage rate can be reduced (down to < 10-3 mbar l/s) and a high vacuum pumping system attached.

In vacuum the heat transfer is only possible by heat radiation (Planck’s radiation law) which yields the best temperature homogeneities, i. e. a temperature gradient in the hot zone of ± 3 °C.

The thyristor-controlled power supplies of the heating zones inside the HBO provide superior temperature stability, i. e. a derivation with respect to time smaller than ± 1 °C. Vibration-free operation is ensured to achieve a bright connection interface free of any distortion.